Significance Attosecond (1 as=10 ^-18 s) light pulses provide new approach to the basic mechanics in the quantum world in its natural time scale. A novel research area called attosecond science was opened up since the first observation of attosecond pulses in 2001. Owing to the advances of ultrafast laser techniques and the in-depth understanding of the attosecond pulse generation mechanism, a world record of 43 as light pulse has been demonstrated in 2017, which is shortest pulse ever obtained by human beings. Nowadays table-top attosecond sources based on high harmonic generation (HHG) have been routinely achieved by many groups worldwide. It is widely applied in the measurements of various ultrafast phenomena like photoionization time delay in atoms, molecules, and solids, electron correlation effects such as Fano resonance, Auger decay, and inner shell ionization, charge migration and dissociation in molecules, and manipulation of dielectrics. Attosecond pulses has achieved impressive progress in different fields such as atomic and molecular physics, condensed matter physics, chemistry, and biology in the past two decades.

Progress The limited photon flux of the attosecond pulses due to low conversion efficiency and phase mismatch of HHG process prevents the potential applications in multi-photon ionization, single shot coherent diffraction imaging, and attosecond pump-probe. HHG driven by TW or even PW high power laser is the straightforward way to generate intense attosecond pulses. Loose focusing geometry is proposed to overcome the over-ionized plasma that will destroy the phase matching process. Attosecond pulse with μJ pulse energy and 10 ¹⁴W/cm ²power density is obtained using loose focusing geometry and adaptive optics. It serves as an alternative to free electron laser with shorter pulse duration and better stability to investigate ultrafast nonlinear phenomena.

Various gating technique is utilized to isolate singleattosecond burst from an attosecond pulse train. Few-cycle driving laser with stabilized carrier envelope phase (CEP) is typically required for isolated attosecond pulse (IAP) generation. Such driving laser with high pulse energy is still challenging even nowadays. The coherent synthesizer consisting of two-color or multi-color laser fields might produce “perfect” waveform to optimize the HHG conversion efficiency as well as relaxing the pulse duration limitation required for IAP gating. Sub-cycle light transients from waveform synthesizer which is ideal for IAP generation has also been demonstrated.

The so-called “water window” wavelength ranging from 2.3 nm to 4.4 nm between the K-edge of carbon and oxygen elements is very important in chemistry and biology. HHG in water window wavelength or even higher photon energy can be obtained using long wavelength driving laser combined with high gas density waveguide and transient phase matching to compensate the unfavorable scaling of HHG efficiency with driving laser wavelength. The world record of light pulse (43 as) is reported using mid-inferred driven HHG in 2017.

High repetition rate attosecond pulses are required to fulfill coincidence counting or to avoid space charge effect in precise photoelectron spectroscopy. According to the HHG scaling principle, tight focusing, and high pressure are needed to generate high harmonics using low pulse energy laser. The high repetition rate, high average power driving laser, and frequency up conversion technique make it an ideal source for high flux HHG.

HHG from solid phase material follows different mechanism with that from gas phase. The intraband HHG is due to the nonlinear radiation of the Bloch oscillation in the conduction band while the interband HHG is resulted from the transition between electron-hole pairs in different bands. It is not only a potential method to generate high efficiency harmonics, but also an important approach to the band structure and electron interaction of the material.

Conclusions and Prospects The frontier of attosecond science has been paved by the advances in the laser technique. 10 μJ attosecond pulse is obtained by loose focusing geometry of the intense driving laser and phase matching optimization. The mid-inferred driving laser enables the high photon energy HHG up to 1.6 keV and sub-50-as short attosecond pulse. The high repetition rate laser source allows >100 kHz attosecond pulse with photon flux as high as 10¹⁵ s^-1 which is ideal for coincidence measurements. Last but not least, the recent progress of HHG in solid state material provides new approaches to both attosecond pulse generation and all optical measurement of laser-matter interaction. All these novel attosecond sources towards the true attosecond-pump-attosecond-probe measurements will give new insight into the microscopic mechanics in their natural time scale.

Significance High-power low-noise continuous-wave (cw) single-frequency all-solid-state lasers (ASSL) have various advantages, including high conversion efficiency, high beam quality, low noise, and long coherence length, and have been widely used in many application fields, e.g. quantum science and technology, cold atom physics, high-precision measurement, high-efficiency frequency conversion, coherent communication, lidar, and optical sensing. This type of laser promotes the development of essential quantum physics research and the practicality of quantum technology. For example, such lasers can be used to prepare a high entanglement, multi-component quantum entanglement source, which is an important technical element in quantum secure communications and quantum computing. In addition, such lasers are basic light sources for high-precision measurement based on laser interferometers. With the observation that gravitational waves from a binary black hole merge, high-precision measurement has attracted significant attention recently. The sensitivity of the gravitational wave device is inversely proportional to the root mean square of the injected laser power, and thus if the injection laser power of the advanced LIGO device is expected to reach 125 W, its detection sensitivity will be 10 times higher than that of the LIGO device.

Progress To satisfy the application requirements of ASSLs in quantum science and technology, cold atom physics, and high-precision measurement, high-power low-noise cw single-frequency mode-hop-free ASSLs with different central wavelengths were investigated and fabricated. First, based on the analysis of the thermal effect of laser crystal in laser-diode end-pumped high-power ASSLs and the improvement measures for the thermal effect of laser crystals, a universal model of sufficient conditions for a stable single-longitudinal-mode operation for high-power cw ASSLs was established by introducing the nonlinear losses of the fundamental wave when a nonlinear crystal was inserted into the cavity, and a high-power cw single-frequency mode-hop-free ASSL was fabricated. Second, the output power of the cw single-frequency ASSL was scaled up under intense pump. A 125W cw single-frequency 1.064μm laser was achieved by a homemade 50.3W cw single-frequency laser, and a two-stage dual end-pumped master-oscillator power amplifier acted as the seed source and amplifier. The measured power stability of the 1.064μm laser over 8h was better than ±0.73%. In addition, a 25.3W cw single-frequency 532nm laser was obtained with an optical-optical conversion efficiency of 32.2%, and the power stability was greater than 0.4% over 8h. Third, the noise characteristics of the cw single-frequency ASSLs were studied and suppressed. The intensity and phase noise of the cw single-frequency ASSLs reached the shot noise level (SNL) for analysis frequencies greater than 5MHz. The intensity noises of the ASSLs were much greater than the SNL in the low analysis frequency range and less than several megahertz. The intensity noises could be manipulated by intra-cavity nonlinear loss, and the resonant relaxation oscillation noise peak of a 1.064μm laser was reduced and shifted toward low frequencies when the nonlinear loss was increased. The intensity noises of the ASSLs were suppressed by controlling the stimulated emission rate in the laser cavity and by a control system based on a Mach-Zehnder interferometer outside the laser cavity. Fourth, the central wavelengths of cw single-frequency ASSLs were extended. 1) A high-power stable low-noise cw single-frequency 540nm and 1.08μm dual-wavelength Nd∶YAP/LBO laser was fabricated. Maximum output powers of 4.5W at 540nm and 1.5W at 1.08μm were achieved simultaneously. 2) Stable low-noise cw single-frequency 473nm and 946nm Nd∶YAG/PPKTP lasers were fabricated. Maximum output powers of 1.01W at 473nm and 1.5W at 946nm were achieved, respectively. 3) A high-power stable low-noise cw single-frequency 671nm and 1.34μm dual-wavelength Nd∶YVO₄/LBO laser was fabricated. Maximum output powers of 3.17W at 671nm and 2.15W at 1.34μm were achieved simultaneously. 4) A stable low-noise cw single-frequency 1.55μm Er,Yb∶YAB laser was fabricated. Maximum output power of 400mW at 1.55μm was achieved. 5) An all-solid-state cw single-frequency Ti:sapphire laser with continuous frequency-tuning was achieved via an intra-cavity locked LiNbO₃ electro-optic etalon. A maximal tuning range of 110nm (760--870nm) was obtained by rotating the BRFs, and a continuous frequency-tuning range of 20GHz was realized after the electro-optic etalon was locked to the oscillating mode of the Ti:sapphire laser. Maximal output power of 2.88W at 795nm was obtained under a pump power of 16.53W. 6) A continuously tunable single-frequency 461nm Ti:sapphire laser was obtained by PPKTP intra-cavity doubling. Output power at 460.86nm was 1.05W under a pump power of 12W, and the continuous frequency-tuning range of the 461nm blue laser was 15.756GHz. 7) A continuously tunable single-frequency 455nm blue laser was implemented by an intra-cavity frequency doubled Ti:sapphire laser with an LBO crystal. The output power at 455nm was 1.0W under a pump power of 13.5W, and the continuous frequency-tuning range of the 455nm blue laser was up to 32GHz.

Conclusion and Prospect Based on the theoretical study and experimental design of lasers, a series of high-power, low-noise cw single-frequency mode-hop-free ASSLs with different central wavelengths were fabricated. Relative to the future development of high-power, low-noise cw single-frequency ASSLs, scaling up the output power and output energy of the ASSLs remains an important topic. In addition, the central wavelengths and linewidths of these lasers should be extended and narrowed, respectively, and the beam quality and directional stability of the laser beams should be improved further. Finally, to improve the sensitivity of high-precision optical measurements, the extra intensity noise of cw single-frequency ASSLs should be suppressed further, and it is the best that intensity noise can reach the SNL in the whole analysis frequency range.

Significance High-power ultrashort pulse lasers have significant applications in the fields of industrial high-end precision machining, high-order harmonic generation, and spectroscopy. Since the 21st century, countries around the world have successively launched their own manufacturing upgrade plans to focus on the development of high-end precision machining. This puts forward higher requirements for ultrashort pulse lasers and makes them toward the direction of high power, compact structure, high stability, low cost, simple operation, and maintenance.

The average power of an ultrashort laser oscillator does not meet the requirements of high-end precision machining. Therefore, it is necessary for further amplification by using the master oscillator power amplifier (MOPA), regenerative amplifier (RA), chirped pulse amplification (CPA), divided-pulse amplification (DPA), coherent beam combining (CBC), etc. An RA provides 60 dB pulse energy amplification, but however, the Pockels cell inside the RA cavity requires high-voltage driving which reduces stability. CBC can be realized by using multiple amplifiers and multiple time delay line, and it is sensitive to the environmental disturbance. As a result, MOPA, CPA, and DPA are often used to obtain low-cost, high-stability, and high-power ultrashort pulse lasers. The gain materials used in the amplification technologies mentioned above can be categorized as fiber, rod or bulk crystal, slab crystal, thin-disk crystal, and single-crystal fiber (SCF).

Ytterbium-doped fiber is widely used in the ultrashort pulse oscillator and amplifier due to its large gain spectrum bandwidth, high optical-to-optical efficiency, and high beam quality. However, the limited core diameter of fiber causes strong nonlinear effects, pulse distortion, and even damage when high-peak power pulses pass through. In order to reduce the nonlinear effect of fiber while maintaining the fundamental transverse mode, two solutions have been investigated. One is to stretch pulse duration and compress pulse duration after amplification, which is a well-known CPA technology; the other is to expand fiber mode field area, which uses a large mode field photonic crystal fiber (PCF). The restriction for single-channel ultrashort pulse amplification is a self-focus nonlinear effect; the threshold is usually lower than 4 MW. CBC technology can avoid self-focus effect and can further increase average power to 10 kW. Nonetheless, the CBC system increases complexity and cost.

The most commonly used crystals for ultrashort pulse amplification are neodymium-doped crystals and ytterbium-doped crystals, for instance, Nd∶YVO₄, Nd∶YAG, and Yb∶YAG. Compared with Nd³⁺ doped crystals, Yb³⁺ doped ones exhibit small quantum defect, wide spectrum bandwidth, and weak concentration quenching effect. Yb∶YAG crystal shaped in slab or thin-disk configuration, pumped by high-power laser diodes, can realize ultrashort pulses with kilowatt average power. However, a slab crystal amplifier contains a signal shaping system with a complex pump light path; a thin-disk crystal amplifier needs multi-pass pump light path and signal light path. Therefore, the high complexity and high cost of these two structures are inevitable.

The SCF amplifier developed in recent years has promising application prospects. Side-polished thin Yb∶YAG crystal is soldered in heat-sink with minimalized void rate, enabling pump light to travel in a waveguide. Large mode area, excellent heat-dissipation, and high-brightness pump improve its optical-to-optical efficiency, average power, and peak power simultaneously. With the continuous improvement of the brightness of fiber-coupled laser diodes, the amplification ability of SCF and rod (bulk) crystals will be further improved. Therefore, it is possible to obtain simple, cost-effective, reliable high-power, and high-energy ultrashort pulse laser by combining the fiber front-end and SCF or rod (bulk) crystal amplifier. This kind of amplification technology can not only be directly used in industrial applications, but also can be used as the front amplification stage for the slab and thin-disk amplifier, which greatly reduces the complexity and the cost.

This article summarizes the domestic and abroad research progress of ytterbium-doped fiber lasers, PCF amplifiers, SCF amplifiers, and rod (bulk) solid-state amplifiers in recent years, highlights our work in the fields of ultrashort pulse fiber lasers, PCF amplifiers, and solid-state amplifiers, and discusses and prospects the future development direction of hybrid amplification technology.

Progress The beginning of ultrashort pulse amplification is a mode-locked oscillator. The stability of the mode-locked pulse train has a significant impact on amplified pulse train. Therefore, a polarization-maintained (PM) mode-locked all-fiber laser with strong resistance to environmental disturbance is preferred. In recent years, a PM all-fiber oscillator has been widely investigated (Fig.1 and Fig.2). Ultrashort pulses generated by an anti-disturbance all-fiber oscillator need to be amplified in order to meet more applications. According to different shapes and materials, amplifiers can be classified as all-fiber amplifier, PCF amplifier, SCF amplifier, rod-shaped crystal amplifier, and fiber-crystal hybrid amplifier. In 2016, Shi's research group from Tianjin University has achieved an average power of 117 W, pulse duration of 11 ps, repetition rate of 15 MHz, and pulse energy of 7.8 μJ by using an all-fiber MOPA (Fig.3). In 2017, We reported an all-fiber picosecond MOPA system with an average power of 225 W at repetition rate of 58.2 MHz (Fig. 5). In order to reduce the impact of nonlinear effect in the process of fiber amplification and increase the output average power and pulse energy, PCF has been developed. In 2017, Lavenu et al. from France presented a high-energy femtosecond ytterbium-doped fiber amplifier delivering 130 fs, 250 μJ laser at 200 kHz. In 2020, we have built a PCF CPA system by using domestic home-made PCF, which achieves 140 W, 167 ps laser at 1 MHz (Fig. 10). Combining fiber amplifier and crystal amplifier is an attractive amplification technology, which not only increases pulse energy and peak power, but also improves the compactness and stability. In 2020, Beirow et al. from University of Stuttgart reported a simple and compact single-stage Yb∶YAG single-crystal fiber amplifier delivering 290 W, 6 μJ laser at 48.5 MHz. With the improvement of pump light brightness, it is possible to achieve an average power of greater than 100 W by using the rod crystal. In 2019, we reported a low-cost hybrid Yb∶YAG thin-rod MOPA laser pumped by high-brightness laser diodes, which delivers 100.4 W, 7 ps laser at 20 MHz (Fig. 21).

Conclusions and Prospects Ytterbium-doped fiber lasers are widely used because of their compact structure, high stability, and simple maintenance. However, the limited core diameter restricts the peak power of ultrashort pulses. The threshold of self-focus nonlinear effect of quartz materials limits the amplified peak power. This limitation can be effectively overcome by using ytterbium-doped crystals. Fiber-crystal hybrid ultrashort pulse amplification effectively combines the high gain of fiber amplifier and the high peak power and high pulse energy of the crystal amplifier. By employing the CBC technology and a high brightness pump source, the amplification efficiency, amplified average power, and amplified pulse energy will be further improved. The development of efficient room temperature heat-dissipation technology is a direction for future investigation.

Significance Due to their narrow spectral range, single-frequency lasers have the advantage of long coherence length due; thus, such lasers are widely used in coherent detection. Recently, the demand for atmospheric aerosol detection, wind field information, atmospheric gas concentration distribution, and coherent imaging have become urgent. A pulsed single-frequency laser source is significant for coherent laser detection. Single-frequency laser pulses with high output pulse energy are required for long-distance coherent detection.

To avoid the scattering of a single-frequency-pulsed laser in the atmosphere from damaging the human eye, the light source of a lidar system must consider the safety of laser irradiation. According to the International Electrotechnical Commission IEC60825 international application standard and laser safety classification method, 1.4-2.6 μm laser irradiation is less harmful to human eyes than 1.06 μm laser irradiation under the same laser pulse energy. Thus, the 1.4-2.6 μm band is called eye safety band. The eye-safe laser is represented by the 1.6-μm band output generated with an erbium-doped gain medium and the 2-μm band output generated with a holmium-doped gain medium. In recent years, with the development of 1.4-1.5 μm and 1.9-μm laser diodes and resonant pumping technology, Er³⁺ doped single-frequency lasers ~1.6 μm and the Ho³⁺ doped single-frequency lasers ~2 μm have been greatly promoted. The 1.6-μm band locates in the communication band and the atmospheric window. The corresponding devices, such as detectors are more mature and efficient. Therefore, the 1.6 μm band is more suitable for long-distance lidar. One output line of Er∶YAG is ~1.65 μm, and there are characteristic absorption peaks of CH₄ gas; therefore, a single-frequency laser at 1.645 μm can be used for differential absorption detection of methane. Tm³⁺ and Ho³⁺ lasers ~2 μm are also located in the atmospheric window and have higher atmospheric transmittance than 1.6 μm lasers. Additionally, Ho∶YLF single-frequency lasers at a wavelength of 2.05 μm can be used for differential absorption detection of CO₂.

Aiming at the application requirements of lidar for single-frequency lasers, this article reviews the research progress of continuous and pulsed all-solid-state single-frequency lasers in the 1.6- and 2-μm bands.

Progress Typically, continuous-wave (CW) operation of a single-frequency laser is realized by inserting a longitudinal mode selection device in a standing wave resonator or a one-direction ring cavity while pulsed single-frequency lasers are usually obtained via a CW narrow linewidth seed laser injected into the driven laser to achieve amplification and single-frequency-pulsed laser output. In the latter case, power is increased through the main oscillation power amplification (MOPA). In this paper, considered the application requirements of single-frequency lasers in lidar systems, the technical developments of CW and pulsed all-solid-state single-frequency lasers are reviewed, and the output characteristics of single-frequency lasers in the 1.6- and 2-μm bands are compared and analyzed. The technical characteristics of different injection locking methods are discussed, and combined with the application requirements of lidar systems, the future development of eye-safe all-solid-state single-frequency lasers is considered.

Conclusions and Prospects Given the requirements of coherent wind measurement lidar and differential absorption lidar, eye-safe single-frequency lasers have improved rapidly in recent years. CW single-frequency laser technology, pulsed laser technology, resonant pump technology, seed-injection locking technology, and MOPA amplification technology have made significant progress. However, eye-safe single-frequency all-solid-state lasers need to be further studied in terms of energy enhancement of the seed-injection regenerative amplifier, MOPA amplification, pulse width control, new type gain media, and laser structure optimization to further improve the characteristics of all-solid-state single-frequency lasers. Further studies are expected to improve the performance of long-range coherent laser wind measurement and differential absorption lidar, for example in terms of detection length and accuracy.

Significance In 2018, Gerard Mourou and Donna Strickland were awarded the Nobel Prize in Physics for their work on chirped pulse amplification (CPA) technology, which provides a reliable concept for improvement in femtosecond-laser energy. Ti∶sapphire femtosecond amplifiers have been developed using CPA technology and are widely used in the fields of attosecond science, and strong-field physics. Due to the limitations of pump power and thermal management, the repetition rate of Ti:sapphire amplifiers is typically less than 100 kHz. However, high-repetition-rate amplifiers at several hundred kilohertz and megahertz are essential for some scientific and industrial applications, such as XUV optical-frequency combing, high-flux high-harmonic generation, angle-resolved photo-emission spectroscopy, and micromachining. Under a fixed pulse energy, work at a high repetition rate means work at a high average power, which causes severe thermal issues.

With the rapid development of diode-pumping technology since the 1990, all-solid-state Yb femtosecond lasers have opened up a new path for the generation of high-power ultrashort laser pulses. Yb-doped lasers are very promising for high-power ultrashort-pulse generation due to their ability to be pumped by readily available high-power diode lasers, their intrinsically high efficiency and narrow pulse width made possible by their simple energy-level diagram, and their broad emission bandwidth and low quantum defect. Moreover, Yb-doped laser materials possess higher gain and thermal conductivity at cryogenic temperatures than at room temperature, enabling much higher output powers. Some Yb-doped materials with emission cross sections broad enough for femtosecond-pulse amplification include Yb∶YAG, Yb∶KGW, Yb∶KYW, Yb∶CaF₂, and Yb∶CGA; these were evaluated and found to have potential for use in high-repetition-rate amplifiers (Table 1). In combination with cryogenic refrigeration and traveling-wave amplification (Fig.5), the average power can reach several hundred watts.

Progress The key technical bottlenecks arise from thermal management of gain media and the gain-narrowing effects that accompany high-repetition-rate femtosecond amplification. Thus, the technical routes and research progress on amplifiers with different gain media are comprehensively summarized with reference to previous research on regenerative and traveling-wave amplifiers. For regenerative amplifiers (Fig. 1), Caracciolo's research group at the University of Pavia, Italy, has made pioneering contributions using crystals of Yb∶Lu₂O₃, Yb∶CGA, and Yb∶CaF₂. Pouysegur's research group from University of Paris-Sud reported on the first sub-100-fs regenerative amplifier based on an Yb-doped bulk gain medium (Fig. 3). For hundred kilohertz and above, traveling-wave amplification is one of the most effective ways to further enhance the power to mJ level and even higher. Thin-disk and slab technologies have proven to be very efficient and enable high output powers in the ultrashort regime with up to several kW of output power because of their thermal-management advantages. However, their inherently complex amplifier setups undermine their stability and wider industrial applicability. Single-crystal fiber is a promising alternative with a compact size, greater simplicity, and lower cost compared with other such technologies for obtaining several hundred watts of output power. In addition, most lasers are dominated by Yb∶YAG crystals; other Yb materials are also used in traditional rod amplifiers. Thermal effect and amplified efficiency of rod amplifiers are outstanding difficulties that need to be solved urgently. Among currently available technologies, diode-pumped cryogenically cooled solid-state amplifiers have emerged as the most promising alternative for achieving an output power of hundreds of watts with a rod amplifier. Zapata at the Center for Free-Electron Laser Science, Deutsches Elektronen Synchrotron increased the output power of the Yb∶YAG rod amplifier to 250 W (Fig.9). Due to the gain bandwidth of Yb∶YAG at low temperature, a femtosecond pulse is still difficult to achieve. Therefore, research on simple and versatile high-power amplifiers for femtosecond-pulsed operation is still ongoing. The problems faced in this field and our ongoing research are discussed.

Conclusions and Prospects Ultrafast pulse-laser amplifiers based on an Yb-doped laser medium can reach new heights of amplification efficiency and output power due to the development of efficient diode-pumping technology and the emergence of various new laser media. In the near future, we expect that the amplification system will serve in both scientific and industrial applications. Combining the current development trend of an all-solid-state ultrashort laser with the research basis of our group, we recognize the prospect of achieving a femtosecond laser with an output power of >100 W using new Yb-doped laser crystals and cryogenic technology.

Signature For the past two decades, ultrafast fiber lasers have become fundamental building blocks in many applications, such as optical communications, biomedical imaging, and industrial processing. Passive mode-locking techniques have been investigated. The nonlinear saturable absorption (SA) effect is the core of the passive mode-locking technology of fiber lasers. Passive mode-locking techniques can be categorized into real and artificial saturable absorbers. The real saturable absorbers consist of semiconductor saturable absorption mirrors (SESAM) and nanomaterials. The artificial saturable absorbers consist of the nonlinear polarization rotation evolution (NPE), nonlinear optical loop mirror (NOLM), nonlinear multimode interference (NLMMI), and Mamyshev regenerator (Mamyshev). The abovementioned passive mode-locking technologies have their advantages and disadvantages. In this study, we briefly illustrate their pros and cons and review their recent development in various types of saturable absorption effects in the application of ultrafast pulse generation.

For real saturable absorbers, rising from the extraordinary physical, optical, and electronic properties of graphene in 2004, layered-dependent nanomaterials have attracted significant attention because of their advantages of cost-effectiveness, broadband optical response, high nonlinear, fast relaxation, and flexible compatibility with other photonics structures. The optical modulation effect of nanomaterials provides a pulse shaping mechanism (i.e., reduced absorption with increasing optical intensity); thus, it can support stable pulse generation and operation in a laser system. Most optical modulators are based on the third-order nonlinear optical response of nanomaterials, such as saturable absorption and optical Kerr effects. SA is a process originating from valance band depletion, conduction band filling, and ultrafast intra-band carrier relaxation exhibited by the input power-dependent optical absorption. Various nanomaterial-based SAs have been demonstrated for pulse generation in fiber lasers operating from the visible to mid-infrared regions. For the artificial saturable absorbers, these passive mode-locking technologies including NPE, NOLM, NLMMI, and Mamyshev could generate ultrashort, high repetition ratio, and high peak power pulses with demand. The optical modulation effect of artificial saturable absorbers originates from the operating optical properties: NPE technique using the optical properties of polarization, NOLM technique using optical nonlinear interference, NLMMI technique utilizing nonlinear multimode interference, and Mamyshev mode-locking technique using nonlinear self-phase modulation. These artificial SAs could largely tune optical modulation depths and enable some types of pulse generation.

The above-mentioned passive mode-locking techniques have progressed considerably. NOLM mode-locking fiber laser has been used in frequency comb applications due to its high repetition ratio feature. Mamyshev mode-locking fiber laser-generated ultra-high peak power pulses could be compared to commercial Ti: Sapphire lasers. NPE mode-locking fiber lasers could largely tune operating pulse states, acting as an ideal seed source for laser amplifier systems. Mode-locking fiber lasers made of nanomaterials have wide application prospects due to their flexible features. With increasing demand for fiber lasers, the above-mentioned traditional passive mode-locking techniques need to improve their performances. Thus, we review the recent advancement of these techniques to illustrate how they overcome their disadvantages.

Progress There are two solutions to improve the stability of the NPE passive mode-locking technique. The first one is to replace traditional fiber with polarization maintaining (PM) fiber (Fig. 3), and the second one is to use an external intelligent algorithm to stabilize laser performance (Fig. 4). All-PM fiber laser cavity construction is not easy since the orthogonal polarization states will induce a walkaway effect when propagating. Szczepanek et al. used the PM fiber segments cross-fused method to solve the intracavity orthogonal polarization walkaway effect. They obtain ultrashort pulses under the condition of all-PM fiber construction. An external intelligent algorithm is a method to actively control fiber laser performance and successfully realize an intelligently controlled fiber laser system. Its advantages include not only stabilizing the laser performance but also switching the pulsed laser operation states. A major advantage of the NOLM mode-locking technique is phase stability. Therefore, a high repetition ratio fiber laser for frequency comb application always employs the NOLM technique (Fig. 7). Besides, NOLM can tolerate larger cavity loss than other mode-locking techniques due to its nonlinear interference mode-locking mechanism. Thus, the NOLM mode-locking technique was the first reported visible waveband mode-locking fiber laser where the cavity loss is very large (Fig. 8). The nanomaterial-based saturable absorption (SA) mode-locking technique has a flexible feature that could extend its application region. However, its mode-locking mechanism is laser intensity-dependent absorption rising from nanomaterials. The mode-locking laser performance mainly depends on the optical properties of nanomaterials. The optical properties of integrated nanomaterials SA cannot be operated. To solve this issue, an external gate-controlled fiber laser can be used to tune the optical properties of nanomaterials, thereby controlling the mode-locking performance (Fig. 11). NLMMI is a novel mode-locking technique inspired by the Kerr effect mode-locking mechanism of a solid-state mode-locking laser. Its advantages are its ultrafast response time and all-fiber structure (Fig. 14). Mamyshev regenerator with self-phase modulation mechanism can generate ultra-high peak power. However, it has certain disadvantages such as the Mamyshev regenerator requires an external laser to excite Mamyshev regenerator operation (Fig. 16). With the above mode-locking techniques, ultrafast fiber lasers can satisfy end-users.

Conclusions and Prospects As the mechanism of saturable absorber effects for mode-locked fiber lasers are clear, researchers will be able to choose appropriate mode-locking mechanisms to satisfy the specific demands of end-users. Finally, the recent progress of ultrafast fiber lasers poses a new challenge; thus, more investigation is required.

Significance High-power all-solid-state ultra-fast lasers are highly efficient, stable, compact, and cost-effective. They have several scientific and industrial applications that benefit from short pulse width and high power. Laser systems based on Ti:Sapphire deliver sub 100 fs pulses easily, but their average power is limited to a few watts. Yb-based laser systems can reach a few hundred watts, even more than a kilowatt, but their pulse widths are restricted to 100 fs due to the gain bandwidths of the laser medium. In order to meet the increasingly demanding requirements of lasers in various fields, it is necessary to develop nonlinear pulse compression technology to obtain higher peak power.

According to the theory of the time-width product of a Fourier-transform limited pulse, spectral broadening is an inevitable step before nonlinear pulse compression. When propagating through nonlinear medium, the spectrum of high energy laser pulses is broadened due to self-phase modulation based on the optical Kerr effect. After subsequent chirp removal of the spectrally broadened pulse, a temporally compressed pulse can be obtained. As a result, the pulse compression relies on nonlinear spectral broadening. Nonlinear pulse compression methods can be distinguished according to the nonlinear medium used for spectral broadening. Different methods are suitable for different pulse energies and peak powers; their compression effects also differ.

In recent years, various methods have been proposed to achieve higher peak power, enhance compression ratio, and improve efficiency. However, these methods still face a series of challenges. Therefore, it is necessary to summarize the current research progress and future prospects of nonlinear pulse compression to guide future development of this field.

Progress The method based on bulk dielectrics is the earliest compression approach for high energy pulses. Rolland et al used bulk silica as the nonlinear medium to broaden the spectrum. They compressed a 100-μJ pulse from 100 fs to 20 fs, but space chirp of the broadened spectrum was severe and the loss of pulse energy was too high. When propagating through a bulk dielectric, an ultrafast laser with high peak power can cause catastrophic self-focusing due to the low critical power of bulk media. The self-focusing critical power of noble gases is much higher than that of dielectrics because of the smaller nonlinearity of gases. Hence, hollow-core fibers with noble gases are effective in the compression of high peak power ultrafast lasers. This approach was proven in 1996 and the disadvantages of compression based on a dielectric nonlinear medium were overcome. Nonlinear pulse compression from 740 fs to 88 fs was achieved using the gas-filled hollow-core fiber approach in 2014 (Fig.3). Pulse energy is limited in this scheme due to the waveguide effect.

Therefore, a different approach for higher peak power, broadening the spectrum with multiple fused silica plates, was developed in 2014 (Fig.4). This approach overcame the limitation of pulse energy and can be applied to higher peak power ultrafast lasers. However, the broadened spectrum of this method is inhomogeneous, thereby limiting the compression efficiency. In order to solve this problem, a new technique using a multi-pass cell (MPC) was presented by Schult et al. in 2016. The MPC technique depends on repeated propagation through a nonlinear medium with a small nonlinear phase for each pass. This ensures homogeneous spectral broadening because the nonlinear phase per pass is chosen to be so small that the impact of propagation is negligible. A compressed pulse with 6.5-MW peak power and 330 W average power from 860 fs to 115 fs was achieved by using fused silica as the nonlinear medium; the conversion efficiency of this method is more than 90% (Fig.6). A noble gas was used as the nonlinear medium, due to the larger self-focusing critical power of gas compared with dielectric material, and they achieved compressed pulses with 530-W average power from 590 fs to 26.5 fs with 58 passes and 3.5 bar Argon (Fig.8). The highest compressed pulse energy achieved by MPC is 18 mJ with a 39 fs pulse duration.

Furthermore, when a certain compression method is insufficient, two-stage nonlinear compression is another effective method that can obtain laser pulses with shorter pulse duration and higher peak power. A compressed average power of 98 W and 166 MW peak power with 27 fs was demonstrated in 2019 by combining a MPC with multiple plates (Fig.10).

Conclusions and Prospects Nonlinear pulse compression is a current research focus in the field of ultra-fast lasers. The approach for pulse compression based on gas-filled hollow-core fibers is still the method primarily adopted to obtain few-cycle pulses with high energy. However, the pulse energy obtained by this method is limited due to catastrophic self-focusing. Multiple fused silica plates is an approach that omits the waveguide, so it can be applied to laser pulses with high peak power. Additionally, the approach based on a MPC ensures homogeneous spectral broadening, reduces energy loss, and improves compression efficiency. Compared with a dielectric medium, such as fused silica, a noble gas has a lower nonlinear index of refraction, therefore a gas-filled MPC is suitable for higher peak power. Furthermore, two-stage nonlinear pulse compression is also an effective option. Nonlinear pulse compression technology enhances the applications of Yb-based and even Ti:Sapphire laser systems. New and efficient methods applicable for higher energy pulses or capable of achieving a higher compression ratio are expected to emerge.

Significance A 2--5 μm mid-infrared (MIR) laser has a broad application prospect and plays a significant role in applications such as remote sensing detection, atmospheric environmental monitoring, medical diagnosis, precision measurement, and photoelectric countermeasures. It covers the so-called “atmospheric window area”, that is, a transmission window with the maximum atmospheric transmittance. It has a strong penetrating ability for fog, smoke, and dust. Moreover, it has been widely used in the field of free-space optical communications. The spectral response range of infrared-guided missile detectors used in military applications is in the 2--5 μm band. As the applications of infrared detectors increase, the development of the corresponding interference technologies increases. A laser light source with a 2--5 μm band for photoelectric countermeasures against infrared seekers is urgently needed. The 2--5 μm band is also called the “molecular fingerprint area”, which covers most gas molecular absorption spectra. It finds important applications in the fields of air pollution monitoring, trace gas detection, precision spectral analysis, and molecular biomedicine. In addition, a 2--5 μm ultrastrong and ultrashort MIR laser can generate high-order harmonics, high-contrast attosecond light pulses, MIR frequency combs, and realize high brightness. It can also be used as an optical parametric oscillator (OPO) pump source for obtaining a 6--8 μm or even longer wavelength MIR laser. Therefore, because of the important application background and huge market demand of 2--5 μm MIR lasers, they have always been a desired topic of research in the field of all-solid-state laser technology.

Periodically-polarized optical superlattice crystals mainly consist of periodically-polarized lithium niobate (PPLN), periodically-polarized lithium tantalate, and periodically-polarized KTP. They possess the advantages of a large nonlinear coefficient, a wide tuning range, diversified wavelength tuning methods, and a compact structure. When applied to a MIR OPO, they can realize wide tuning, narrow linewidths, and high-power MIR lasers. An OPO based on an optical superlattice crystal is the most efficient way to generate MIR laser sources operating within the 2--5 μm wavelength range. We review the recent progress of optical-superlattice-based OPOs operating within the 2--5 μm wavelength band and analyse the structural features, advantages, and development prospects of OPOs operating in the continuous-wave, nanosecond, and picosecond regimes. The development tendency of optical-superlattice-based OPOs is also highlighted, indicating that high power, wide tunability, low power consumption, small size, and light weight are important development directions. Moreover, the optical superlattice crystals with high quality and large size, pump sources with better performances, and a reliable engineering designation are the key techniques for future development.

Progress The PPLN crystal was first prepared by applying the electric field polarization technique in 1993 by Yamada et al. In 1995, Myers et al. developed the room-temperature electric field polarization technique and effectively improved the size and quality of optical superlattice crystals. They realized the effective operation of a PPLN-based single-resonant OPO for the first time. Moreover, they obtained a tunable laser output ranging from 1.66 μm to 2.95 μm, which greatly promoted the development of the nonlinear frequency conversion technology, especially OPOs. At that point, the OPO based on optical superlattice crystals began to appear on the stage of history and shine. Operating mode of an OPO is determined by pump light, including a continuous-wave (CW) nanosecond, picosecond, and femtosecond. The current study introduces the research progress of the 2--5 μm MIR OPO based on optical superlattice crystals in a CW, nanosecond, and picosecond operation regime.

CW widely-tunable 2--5 μm MIR lasers have important applications in precision spectral analysis, optical sensing and detection, gas monitoring, free-space optical communications, and photoelectric countermeasures. Compared with other nonlinear optical crystals, optical-superlattice-based CW OPOs can not only effectively reduce the threshold, but also enhance the conversion efficiency and MIR output power. So far, the maximum output power, the narrowest linewidth of an optical-superlattice-based CW OPO is 71.6 W@2.907 μm and 1 kHz@2.7--4.2 μm. He et al. applied high-power single-frequency (linewidth of about 20 kHz) all-solid-state laser operation at 1064.2 nm as the pump source, and realized a broad tunable (1344.6--5103.2 nm), narrow-wavelength (~10 MHz) CW laser with a four-mirror ring cavity based on two PPLN crystals with periods from 25.5 μm to 32.0 μm and a period interval of 0.5 μm.

Compared with CW lasers, nanosecond pulsed lasers have much higher peak power and are easy to help achieve high-efficiency nonlinear frequency conversion. It is also the most widely studied operation mode since the invention of OPOs. Until now, the maximum output power and the widest tunable range of optical-superlattice-based nanosecond OPOs is 74.6 W@2.68 μm and 2128.6--5076.8 nm. He et al. used a high-power nanosecond 1064-nm laser as a pump source and a PPLN crystal with a thickness of 2 mm, a length of 50 mm, and a period of 32.0 μm. They achieved high-power and high-efficiency degeneracy point OPO with the output power of 33.3 W under the pump power of 60.9 W. The power instability RMS and the light-to-light conversion efficiency are 0.5% and 54.7%, respectively. The beam quality in the horizontal and vertical directions are 1.45 and 1.62, respectively. Using two multi-period PPLN crystals, 2--5 μm wide-tunable MIR OPO with a wavelength range from 2128.6 nm to 5076.8 nm was demonstrated using a nanosecond fiber laser as the pump source.

The 2--5 μm wide-tunable MIR picosecond laser has broad application prospects in laser ranging, Lidar, atomic and molecular dynamics, and time-domain spectroscopy. Compared with CW and nanosecond OPOs, picosecond OPOs need a synchronous pumping mechanism, which requires a fairly precise match of a pump pulse repetition rate and the round-trip frequency of an OPO resonator. Thus far, for the maximum output power of 7.1 W@2.1 μm, the widest tunable range of an optical-superlattice-based picosecond OPO is 2.7--5.3 μm. He et al. used a hybrid-slab amplifier to obtain a high-power picosecond laser with an idle frequency optical tuning range of 3362--4290 nm.

Conclusions and Prospects An OPO based on an optical superlattice crystal is the most efficient way to generate MIR laser sources operating within the 2--5 μm wavelength range. In this study, we mainly review the recent progress of optical-superlattice-based OPOs operating within the 2--5 μm wavelength band. We analyze the structural features, advantages and development prospects of OPOs operating in the CW, nanosecond, and picosecond regimes. The development tendency of optical-superlattice-based OPOs is also highlighted, indicating that high power, wide tunability, low power consumption, small size, and light weight are important development directions. Moreover, the optical superlattice crystals with high quality and large size, pump sources with better performances, and a reliable engineering designation are the key techniques for future development.

Objective The motion and correlation of electrons are the most fundamental physical processes in all systems based on electromagnetic interaction, and their characteristic time is of the order of attoseconds (i.e., 10 ^-18 s). To investigate or control the ultrafast dynamics of electrons, it is necessary to use a tool with the same or even shorter time scale as a reference. The emergence of ultrashort laser pulses provides an ideal means for studying ultrafast phenomena. The changing light field is the fastest physical quantity which can be measured and controlled. Currently, femtosecond pulses shorter than 5 fs or even close to a single optical cycle (the oscillation cycle of an optical field with a wavelength of 800 nm is 2.67 fs) have been obtained, covering the spectrum from infrared to ultraviolet. To obtain attosecond pulses equivalent to the characteristic time of electron motion, the carrier wave spectrum must be shifted to the extreme ultraviolet (XUV) or even soft X-ray whose oscillation cycle is of the order of attoseconds. It requires the interaction of femtosecond laser pulses and gas atoms at a peak intensity that is close to 10 ¹³ W/cm ² or higher to generate high-order harmonics and attosecond pulses in the XUV band. Optical gating techniques based on high-order harmonic generation (HHG) in the XUV band have become crucial to obtain isolated attosecond pulses (IAP). An ultrashort IAP requires an ultrabroad continuous spectrum and its intrinsic chirp (atto-chirp) must be compensated.

Methods In the HHG process, after being ionized, the electrons obtain kinetic energy within half a cycle of the driving laser, and they then return and recombine with the ions to radiate XUV photons. The electrons ionized at different times gain different kinetic energies in the electric field, and they return and recombine at different times, causing the XUV photons radiated to have different energies and wavelengths. This process leads to a relative delay between photons with different energies. Thus, the XUV pulses produced via HHG are chirped, with inherent dispersion called atto-chirp. Based on the three-step model, we calculate the central photon energies and atto-chirps of attosecond pulses produced via the HHG process at specific driving laser intensities (Fig. 1). The atto-chirp of the calculated attosecond pulses are compensated by specific metal foils to obtain an IAP close to 50 as.

Results and Discussions The calculations showed that the material dispersions of zirconium (Zr), molybdenum (Mo), and tin (Sn) foil with specific thicknesses (Figs. 2 and 3) may compensate the atto-chirp of continuous XUV spectra centered at 98, 120, and 170 eV produced by laser-atom interaction, respectively. In the numerical simulation based on the strong-field approximation, we use polarization gating to generate a half-cycle linear polarized electric field by 750 nm, 5 fs laser pulses, and produce a continuous XUV spectrum centered near 120 eV in neon gas at an intensity of 9×10 ¹⁴ W/cm ². Simulation results showed that the chirp of the continuous XUV spectrum may be compensated by 150 nm of Mo foil to produce an attosecond pulse of 38 as (Fig. 4). Based on such a set of laser and material parameters, a beamline for attosecond pulse generation and measurement is designed (Fig. 5). Furthermore, polarization gating is selected to generate IAP, and the laser is focused through a combination of concave and convex mirrors to produce a high-order harmonic continuum. The attosecond pulse is focused on the gas target at the TOF entrance of attosecond streak camera using a toroidal mirror. The attosecond pulse width is measured through the noncollinear optical path, and the flat-field grating is imaged on the XUV spectrometer to measure the spectrum. In addition, the multiple iris and observation optical paths are set in the optical path to facilitate the adjustment of the optical path of the system.

Conclusions This study determines the parameters to produce IAP close to 50 as. Atto-chirp, the intrinsic dispersion of attosecond pulses produced via HHG processes, is usually compensated by various metal foil. Semiclassical calculation and numerical simulation are employed to determine the parameters to produce short attosecond pulses. Calculations show that the material dispersions of Zr, Mo, and Sn foil with specific thicknesses may compensate the atto-chirp of continuous XUV spectra centered at 98, 120, and 170 eV with acceptable transmittance, respectively. A previous study confirmed that a single pulse of 67 as was obtained around 98 eV. The numerical simulation shows that a supercontinuum with the central photon energy of 120 eV is produced and its atto-chirp is compensated by 150-nm Mo foil. This scheme can not only compensate well for the positive dispersion carried by the attosecond pulse as it is generated but also maintain a high transmission. Moreover, the Ti:sapphire laser pulse after spectral broadening and compression can generate high-order harmonics in this photon energy range with reasonable efficiency. Finally, for this scheme, we design the optical and vacuum systems of an XUV beamline, including a noncollinear attosecond streak camera and a flat-field XUV spectrometer, which can perform spectral measurement of high-order harmonics, generation, compression, and characterization of attosecond pulses.

Objective High power and low noise continuous-wave (CW) single-frequency all-solid-state lasers at 1.5 μm have important applications in laser interferometry, coherent Doppler lidar, optical frequency standard, cold atom physics, continuous variable (CV) quantum information, and basic research of quantum optics. 1.5 μm lasers with power more than 500 mW and an intensity noise spectrum down to the shot-noise limit are indispensable in the development of telecom band quantum light sources, e.g., CV entangled states. The diode-pumped all-solid-state CW single-frequency 1.5 μm lasers were mainly developed using the Er ³⁺ and Yb ³⁺ co-doped gain mediums and the longitudinal mode selection techniques, such as intracavity etalon, microchip cavity, or twisted-mode cavity. This kind of laser suffers from the relatively low thermal fracture threshold and significant thermal-induced depolarization, and the deterioration of beam quality and longitudinal mode structure. There is no report on a CW single-frequency all-solid-state laser at 1.5 μm providing low noise output more than 500 mW.

Methods To achieve high power CW single-frequency laser operation at 1.5 μm, the heat deposition inside the gain medium was firstly reduced by a dual-end face-cooling scheme, i.e., using two polished sapphire plates acting as transparent heat spreaders to improve the axial heat conduction in the Er,Yb∶YAB crystal. Secondly, to realize a relatively homogeneous distribution of pump absorption inside the gain medium, and to raise the maximum permitted incident pump power and achieve the best mode-matching, a long depth-of-focus polarized dual-end-pump structure was employed (Fig. 1, Fig. 2). Finally, two additional half-wave plates were used to control the pump polarization. Due to the dependence of absorption on the polarization, the pump power axial absorption can be tuned to be more uniform, and the pump saturation effect can be prevented.

The designed ring resonator was based on the precise measurements of the thermal focal lengths of both the laser crystal and the bismuth iron garnet (BIG) magneto-optical crystal. The former was measured using a knife method, and the latter was measured based on the theoretical analysis of the resonator’s dynamical behaviors depending on the varied thermal focal length of the BIG crystal. Considering the thermal lens of the laser and BIG crystals, the resonator length was optimized by balancing the thermal insensitive region, the mode-matching between the laser and pump beams, and astigmatism (Fig. 5). Figure 1 shows the resonator design of the CW single-frequency 1.5 μm laser based on unidirectional traveling-wave ring cavity.

Compared with the single-end face-cooled Er,Yb∶YAB crystal under σ-polarization single-end-pumping, the dual-end face-cooling scheme combined with the long depth-of-focus tunable polarization dual-end-pump structure reduced the thermal lens effect of the laser crystal significantly. The thermal focal length of Er,Yb∶YAB crystal was lengthened from 45 to 78.2 mm under 4.5 W pumping after the optimizations (Fig. 3). Besides, the functional relationship between the intracavity laser power and the thermal focal length of the BIG crystal was experimentally determined [Figs. 4(a) and (b)]. It can be found that the BIG crystal thermal effect was severe, and the thermal focal length was only 58 mm in the case of 25.3 W intracavity laser power. Based on the developments of the cooling and pumping schemes, using the measured thermal focal lengths to optimized the resonator length, 755 mW CW single-frequency 1.5 μm laser was generated from the ring cavity [Figs. 6(a) and (b)], and the power fluctuation within 2 hours was less than ±1.2% (Fig. 7). The intensity noise of the 1.5 μm laser was measured using a balanced homodyne detection system. The intensity noise reached the shot-noise limit for frequencies above 5 MHz (Fig. 8).

Results and Conclusions A low noise CW single-frequency 1.5 μm laser based on Er,Yb∶YAB crystal and unidirectional traveling-wave ring cavity was demonstrated. By measuring the thermal focal lengths of the sapphire-Er,Yb∶YAB-sapphire laser crystal and the BIG magneto-optic crystal, as well as adopting the long depth-of-focus tunable polarization dual-end-pump structure to reduce the thermal effects of laser crystal and raise the maximum permitted incident pump power, CW single-frequency 1.5 μm laser was realized using the unidirectional traveling-wave cavity technique. The laser power was scaled up to 755 mW with the power fluctuation less than ±1.2%, and the laser intensity noise reached the shot-noise limit beyond the analysis frequency of 5 MHz. This laser source can be used to generate a CV entangled light source in the telecom band. The power of the CW single-frequency 1.5 μm laser can be further scaled up by employing low-doped Er,Yb∶YAB crystal as the gain medium, using the magneto-optical crystals with lower absorption loss, as well as designing a 6-mirror ring cavity with weaker astigmatism.

Objective The near-infrared 1240 nm lasers have been widely used in many scientific research fields such as optical time domain reflectometer (OTDR) and water remote sensing, owing to their intrinsic merits including narrow linewidth, perfect beam quality, higher stability, and lower noise. However, it is impossible to directly obtain the 1240 nm laser by the existing laser gain media because there are not suitable gain media covering the 1240 nm spectrum. In recent years, some groups have successfully obtained the stable single-frequency (SF) 1240 nm lasers by means of the stimulated Raman scattering (SRS) based on Raman gain media. Nonetheless, due to small Raman scattering cross-section of Raman gain media, the threshold power of the attained Raman laser is so high that more incident pump power is necessary to scale up the output power of 1240 nm laser. In this paper, we present a cavity resonance-enhanced watt-level SF 1240 nm Raman laser. After the cavity is stably locked to the incident pump laser, the pump and Raman lasers resonate simultaneously in the designed resonator. In this case, the pump threshold of the Raman laser is effectively decreased, and the stable SF Raman laser is obtained at the same time, which provides an effective and feasible way to obtain stable high power SF Raman laser.

Methods In this study, firstly, a high-quality single-crystal diamond grown by chemical vapor deposition (CVD) is chosen as the Raman gain crystal, which has good optical properties of low-nitrogen, low-birefringence, and so on. And an all-solid-state CW SF 1064 nm infrared laser with good performance is served as the pump source to avoid mode competition in the process of SRS. On this basis, according to the transmission matrix theory of the optical resonant cavity, a symmetrical bow-tie double-resonance cavity for both pump and Raman laser is reasonably designed. Furtherly, based on the SRS process rate equation and the principle of cavity resonance-enhancement technology, the transmissivity of the input coupling mirror for the pump laser and the transmissivity of the output coupling mirror for the Raman laser are optimized as 3.5% and 0.5%, respectively. Then, the H?nsch-Couillaud (H-C) locking system is used to accurately lock the resonating frequency of the cavity to the frequency of the pump laser. In addition, a retro-reflecting device consisting of a plane mirror (M₅) coated with high-reflection film at the wavelength of the Raman laser is used to reflect the backward wave leaking from the output coupling mirror to ensure the unidirectional operation of the Raman laser. Finally, an SF 1240 nm Raman laser with stable unidirectional operation is attained.

Results and Discussions In order to accurately lock the Raman cavity, the cavity length is scanned by changing the voltage loaded on the piezoelectric transducer (PZT), and the transmission peak of the cavity is detected by the photodetector. When the incident pump power is lower than the threshold power, the transmission peak curve is a standard Gaussian curve. However, when the incident pump power exceeds the threshold, the transmission peak curve has a broad detuning range, and the detuning becomes severe with the increasing incident pump power (Fig. 4). In the experiment, the cavity detuning is well compensated by reducing the temperature of the diamond crystal and then the cavity length of the Raman laser can be stably locked at the resonant frequency of the pump laser at high incident pump power. After the cavity is stably locked, the maximal output power of the stable SF 1240 nm Raman laser reaches up to 1.48 W with the incident pump power of 9.17 W (Fig. 5). The threshold pump power is as low as 2.73 W, which indicates that the double-resonance cavity can effectively decrease the threshold of the Raman laser.

Conclusions A watt-level SF 1240 nm Raman laser with low pump threshold is demonstrated in this paper, which is implemented by using an SF 1064 nm laser and a diamond crystal as the pump source and Raman gain medium, respectively. In order to decrease the threshold of the Raman laser, a double-resonance cavity is designed and adopted with assistance of cavity resonance-enhancement technology. After the parameters of the optical resonator are optimized and the resonating frequency of the Raman resonator is locked to the frequency of the incident pump laser by the H-C locking system in the experiment, the pump and Raman lasers resonate simultaneously in the designed resonator. The attained pump threshold is as low as 2.73 W. On this basis, the output power of SF 1240 nm laser reaches up to 1.48 W when the pump power is 9.17 W, and the corresponding slope efficiency is 24.9%. The measured long-term power stability in 30 min and the beam quality M² are better than 1.10% (RMS) and 1.2, respectively. The achieved Raman laser with double-resonance cavity can provide a feasible way to decrease the threshold pump power of Raman laser, and the obtained stable SF 1240 nm laser source can be used in atmospheric monitoring and biomedicine field.

Objective Stable single-frequency and high-energy lasers in the eye-safe band are important light sources for lidars and coherent detection. Currently, many studies have reported a human eye-safe 1645-nm single-frequency pulsed laser output. Er∶YAG crystals and Er∶YAG ceramics are two main types of common-gain media for realizing 1645-nm lasers. Compared with crystalline materials, ceramic materials possess the advantages of short growth time, large-scale production, and flexible doping concentration. In the present study, we report the results of an engineered prototype of a single-frequency pulsed Er∶YAG ceramic laser. The volume of this laser system is reduced, and the frequency stability is improved. An optimized symmetrically pumped double Er∶YAG ceramic structure is designed to solve the problem of performance degradation caused by limited space. Such a single-frequency laser light source with a smaller volume and higher stability is more helpful for a practical application.

Methods The single-frequency Er∶YAG pulsed laser system with seed injection mainly includes three parts: an Er∶YAG master laser, a symmetrically pumped dual Er∶YAG ceramic ring-cavity slave laser, and a detection-control system. To improve the stability of the pulsed laser, a single-frequency continuous Er∶YAG non-planar ring-cavity laser is employed as a master laser. To improve the mode-matching efficiency and reduce the laser volume, the slave-laser cavity is designed with a multiple-folding structure that adopts total reflecting mirrors. This structure is a symmetrical one of dual laser-diode(LD)-pumped dual Er∶YAG ceramics. To simultaneously satisfy the requirements of pulse energy and pulse width for lidars, the total cavity length is set to 2.3 m. By using multiple folding mirrors, the volume of the cavity is reduced, and a space is reserved for the seed laser at the center position to realize a reasonable use of space. The working process of the seed injection and laser output is described as follows: a p-polarized seed light is reflected by the injection mirror and enters the slave-laser cavity. The seed laser is then injected through the path of the first diffraction order of the acousto-optic modulator (AOM). In this manner, resonance is realized in the slave laser, and the AOM shuts off the optical path during this period. The photodetector detects the leaked resonant signal and transmits it to the control system. Simultaneously, the control system loads a triangular wave to the piezoelectric ceramic (lead zirconate titanate) to allow it to continuously scan the cavity length. When the peak of the resonant signal has been scanned, the AOM is stopped from turning off the optical path to establish the laser pulse.

Results and Discussions The realization of the stable single-frequency laser output with seed injection is shown. An experimental study of the injection-locked laser is conducted at a repetition rate of 200 Hz. The energy and pulse duration of the single-frequency laser linearly change with the increase in the incident pump power [Fig. 4(a)]. The obtained maximum pulse energy is 22.75 mJ, and the corresponding pulse duration is 223.1 ns. The envelope of the single-frequency pulsed waveform becomes very smooth [Fig. 4(b)] because only one longitudinal mode is present in the pulse after the seed injection. The heterodyne beat-frequency technique is used to detect the single-frequency characteristics of the obtained pulsed laser. The heterodyne beat frequency is obtained at the maximum output energy [Fig. 5(a)], and a fast Fourier transform is performed on the beat-frequency result [Fig. 5(b)]. The spectrogram shows that the output laser pulse has a single-frequency, and the center frequency of the heterodyne signal is 39.09 MHz, which is similar to the acousto-optic frequency shift of 40.68 Hz. The full width at half maximum of the spectrum is 2.46 MHz, 1.2 times the Fourier transform limit, which corresponds to a pulse duration of 223.1 ns. We measure the energy and frequency stabilities when the output energy of the laser pulse is kept at the highest level. The standard deviation of the energy jitter within 30 min is approximately 0.118 mJ [Fig. 6(a)], and the standard deviation of the frequency drift is approximately 0.578 MHz [Fig. 6(b)]. The spot-radius values of the pulsed laser with the highest energy are recorded using an infrared camera at different positions. Subsequently, the results are fitted and calculated, and the beam quality factors in the x and y directions are 1.16 and 1.15, respectively (Fig. 7).

Conclusions In this study, a single-frequency laser is designed and developed based on the seed-injection technology, in which the dual Er∶YAG ceramic symmetrical structure is end-pumped by a dual 1470-nm LD. Through the theoretical analysis, it is found that the main factors determining the seed-injection effect are the coupling of the seed with oscillating lasers, the power, and the frequency detuning of the seed laser. In the experiment, the obtained maximum average pulse energy is 22.75 mJ at a repetition rate of 200 Hz, and the corresponding pulse duration is 223.1 ns. The beam quality factors of the single-frequency pulsed laser are 1.16 and 1.15 in the x and y directions, respectively. The spectral width of this Q-switched single-frequency laser is 2.46 MHz and is 1.2 times of the Fourier transform limit. The standard deviation of the energy jitter is 0.118 mJ, and the standard deviation of the center frequency drift is 578 kHz. This stable and compact single-frequency Er∶YAG pulsed laser system can be used as a laser source for wind lidar and coherent detection.

Objective The repetition rate of a mode-locked fiber laser pulse is one of the most important defining parameters. On-demand repetition rates vary, serving a wide range of applications. For example, lasers with high pulse repetition rates (between tens of MHz and a few GHz) are not only used to generate optical frequency combs or coherent stacked pulses, but also offer high-precision wavelength calibration for astronomical spectrographs. In contrast, lasers with low pulse repetition rates (below 1 MHz) are highly valued in industrial laser materials processing to keep the characteristics of cooling machining. The mode-locked lasers with high repetition rates can easily be generated by shortening the laser cavity length. However, when it comes to the lasers with low repetition rates, the things get more complex and costly because these lasers require an additional acousto-optic modulator (AOM) or electro-optic modulator (EOM) to pick pulses from a laser source with high repetition rates. Compared with the traditional SESAM (semiconductor saturable absorption mirror) mode-locked fiber lasers, the mode-locked fiber lasers with nonlinear amplifying loop mirrors (NALM) as artificial saturable absorbers demonstrate the advantages of fast relaxing time, high damage threshold, life expectancy, all-fiber structure, and ultralow repetition rates over a relative broadband. These pulses can be generated by elongating the cavity length. After compression, these pulses have a sub-picosecond duration. In this study, we report a sub-picosecond NALM mode-locked fiber laser at different repetition rates varying from 21 MHz to 100 kHz by adjusting the length of the passive fiber at the proper position in the oscillator. All pulses with different repetition rates can be compressed to a sub-picosecond level.

Methods We constructed the NALM mode-locked fiber laser based on an all-fiber structure and an all-polarization-maintaining design. To realize different repetition rates in the mode-locked laser pulse output, we gradually elongated the oscillator cavity length via inserting two additional passive fiber segments into two different locations in the cavity (Fig. 1). In case of pulse repletion rates below 1 MHz, the length and position of the two passive fiber segments were carefully designed to avoid accumulating excessive nonlinear effects, such as stimulated Raman scattering, which undermined the stability of mode-locked pulses and decreased the output pulse energy. In particular, the first passive fiber segment was spliced after the gain fiber, whereas the second one was spliced after the output coupler. Finally, we demonstrated a 20 μm/130 μm Yb-doped double cladding fiber amplifier with an ultralow repetition rate NALM mode-locked fiber laser as seed instead of any pulse-picking device.

Results and Discussions First, we obtained a self-starting mode-locked pulse train with a repetition rate of 21.16 MHz at the main and NALM pump powers of 90 mW and 126 mW. The output pulse was centered at 1030 nm with a 3-dB bandwidth at 9.1 nm, and the pulse duration was compressed from 5.3 ps to 352 fs by a pair of 1379 lp/mm gratings (Fig. 2). Second, we obtained mode-locked pulse trains with repetition rates of 5.92 MHz, 1.28 MHz, 457 kHz, 280 kHz, 181 kHz, and 100 kHz by adjusting the cavity length through the addition of two passive fiber segments. The cavity lengths were 33.8, 156, 438, 714, 1106, and 2000 m, respectively (Table 1 and Fig. 3). While the pulse repetition rate decreased from 21 MHz to 100 kHz, the pulse energy and duration increased by two orders of magnitude, namely, from 1 nJ to 104 nJ and 5.3 ps to 300 ps, respectively. The broadest 3-dB spectrum at 30 nm was demonstrated at a repetition rate of 5.92 MHz, which corresponded to the shortest compressed pulse duration of 177 fs. Importantly, all the pulses were compressed to a sub-picosecond level. Especially for the ultralow repetition rate pulses, it was a good choice for ultralow repetition rate laser system to be front-end seeded without any pulse-picking devices. Finally, a 388-kHz, 62.7-ps, 20.8-nJ NALM mode-locked seed was amplified to 3 μJ after a single 20/130-μm Yb-doped double cladding fiber amplifier stage (Fig. 5), which was further compressed to 537 fs.

Conclusions We report on a mode-locked ytterbium-doped fiber laser with a nonlinear amplifying loop mirror in an all-polarization-maintaining designed cavity. The repetition rates of the laser vary between 21 MHz and 100 kHz by adjusting the length of passive fiber at the proper position of the oscillator. The 5.3-ps mode-locked pulse with a 3-dB bandwidth at 9.1 nm is first obtained at a repetition rate of 21.16 MHz, which is then compressed to 352 fs. The broadest 3-dB bandwidth at 30 nm and the shortest compressed pulse duration of 177 fs are demonstrated at a repetition rate of 5.92 MHz. Limited by the length of available passive fiber, we obtain a pulse with a maximum energy up to 104 nJ and duration of 300 ps directly from the oscillator at the lowest repetition rate of 100 kHz. The pulse is subsequently compressed to 1.053 ps. All the output mode-locked pulses at different repetition rates with a broad spectral bandwidth are characterized and compared with the traditional low repetition rate mode-locked fiber lasers. They are compressible to sub-picosecond, which is different from the dissipative soliton resonance and noise-like pulses. A 388-kHz, 62.7-ps, 20.8-nJ NALM mode-locked laser seed is amplified to 3 μJ after a single-fiber amplifier stage, which could be further compressed to 537 fs, and this whole laser system is very compact without any pulse-picking components or multistage fiber amplifiers.

Objective Ultrastable low-noise ultranarrow linewidth laser light source has a wide range of applications in precision measurement, optical atomic clock, time-frequency transmission, and low-noise microwave generation. Ultrastable cavity Pound-Drever-Hall(PDH) frequency stabilization technology is one of the most important solutions for obtaining such ultrastable lasers. Based on this, the linewidth of the distributed feedback(DFB) single-frequency fiber laser has reached the order of millihertz. In addition to the performances of the ultrastable cavity and servo system, the available linewidth and frequency instability of the ultrastable fiber laser depend on the performances of the linewidth, frequency drift and noise level of the free-running fiber laser, and the laser frequency-tuning mechanism. Although many research institutions in China have developed single-longitudinal-mode fiber lasers with frequency-tuning mechanisms and the laser frequency has been stabilized within a hundred kilohertz for a long time by the saturated-absorption frequency stabilization technology based on fine transition spectral lines of gas molecules, home-made single-frequency fiber laser has not been applied to ultrastable cavity PDH frequency stabilization technology to obtain subhertz-linewidth ultrastable fiber laser source. Therefore, it is very necessary to investigate the PDH frequency stabilization with such home-made single-frequency fiber lasers.

Methods By optimizing the structure parameters of the laser cavity, adopting adiabatic packaging and precision temperature control, and integrating the piezoelectric transducer (PZT) in the cavity that can quickly and widely tune the laser frequency, a free-running DBR fiber laser that can be used to obtain an ultrastable laser via ultrastable-optical-cavity PDH frequency stabilization was developed. The laser power was boosted by a low-noise single-mode polarization-maintaining fiber amplifier. The ultrastable optical cavity used for frequency stabilization was made of ultralow expansion (ULE) glass with a cavity length of 10 cm. The corresponding free spectral range(FSR) was 1.5 GHz and the fineness was 360000. The fiber laser was modulated by an electro-optical modulator (EOM) and then coupled into the optical cavity. The laser with modulated sidebands reflected by the optical cavity was detected with a photodetector and then mixed with the drive signal of the EOM through a mixer to obtain the error signal for laser frequency stabilization. The error signal was processed by the servo system, and the low-frequency component was fed back to control the voltage of the PZT to compensate the low-frequency fluctuations of the laser frequency. The high-frequency component was fed back to the acousto-optic modulator (AOM) drive controller to achieve the high-frequency fluctuation compensation of the laser frequency. To evaluate the effect of PDH frequency stabilization of our fiber laser, two DBR fiber lasers were stabilized to the two adjacent cavity modes of the optical cavity at the same time. The performance parameters of the frequency stabilization fiber laser were then measured by the beat frequency of the two frequency-stabilized lasers.

Results and Discussions The developed DBR fiber laser exhibits stable single-longitudinal-mode oscillation characteristics [Fig. 2(a)]. The relationship between the amount of change in the output laser frequency and the modulation frequency of the tuning voltage when the PZT is applied with different voltage values is given [Fig. 2(b)]. The tuning bandwidth of the laser frequency adjusted by PZT is about 8--10 kHz and the maximum tuning range exceeds 3.2 GHz. The signal-to-noise ratio of the output laser is about 60 dB [Fig. 3(a)] and the 3-dB linewidth of the laser is about 1.25 kHz [Fig. 3(b)]. The error signal is recorded by the oscilloscope when a triangular-wave sweep voltage of 7 V at 20 Hz is applied to PZT [Fig. 5(a)]. The error signal then changes to a straight line after the laser frequency is locked to the reference cavity [Fig. 5(b)]. There is a drift in the frequency of the beat frequency signal. The range of drift within 1 h is less than ±20 Hz [Fig. 6(a)] and the frequency drift of each laser after frequency stabilization is less than ±10 Hz. The frequency instability of the frequency-stabilized fiber laser corresponding to 1 s and 100 s is 6×10 ^-16 and 8×10 ^-15, respectively [Fig. 6(b)]. Fig. 7(a) displays the measured frequency noise power spectrum of the frequency-stabilized fiber laser in the range of 1 mHz--100 kHz. The frequency noise is reduced by more than eight orders in the range of 1 mHz--10 Hz. The frequency noise is reduced to about 8×10 ^-3 Hz ²/Hz especially from 1 to 10 Hz. Using the measured beat frequency data, the laser linewidth after Lorentz fitting is 280 mHz [Fig. 7(b)].

Conclusions We have demonstrated the results of ultrastable cavity PDH frequency stabilization based on a home-made single-frequency DBR fiber laser at 1550 nm. By optimizing the structure parameters of the laser cavity, adopting adiabatic packaging and precision temperature control, and integrating the PZT in the cavity that can quickly and widely tune the laser frequency, a free-running DBR fiber laser that can be used to obtain an ultrastable laser via ultrastable-optical-cavity PDH frequency stabilization is developed. Using an ultrastable optical cavity with a length of 10 cm and a fineness of 360000, the frequency drift of the fiber laser after PDH frequency stabilization is less than ±10 Hz and the frequency instability at 1 s and 100 s is 6×10 ^-16 and 8×10 ^-15, respectively. The frequency noise is reduced to 8×10 ^-3 Hz ²/Hz at 1--10 Hz and the linewidth is narrowed down to 280 mHz. It is shown that the main performances of our lasers can be used to construct subhertz-linewidth ultrastable laser light sources, which can be used in fields such as gravitational wave detection, precision measurement, and time-frequency transmission.

Objective Mid-infrared (MIR) lasers play a role in several applications, including environmental atmosphere monitoring, medical diagnosis, spectral analysis, optoelectronic countermeasures, etc. The MIR band of wavelengths spanning 2--5 μm is called the atmospheric window area, which is the transmission window with the highest atmospheric transmittance. This band can penetrate fog and smoke and is widely used in the field of free space optical communication. A MIR laser with a wavelength near 3 μm is located in the most absorptive zone of water molecules. This laser has a shallow penetration depth in human tissues, leading to little thermal damage to surrounding tissues. This greatly improves the ability of the laser to melt, excise, and vaporize tissues; therefore, it is widely used in biomedical fields. The spectral response range of a military infrared guided missile detector is 3--5 μm. With the yearly increase of infrared detector usage, corresponding jamming technology development is also accelerating and a laser light source of this band is urgently needed for a photoelectric countermeasure of an infrared seeker. Motivated by the demand of miniaturized MIR laser sources, a compact, high-efficiency, and widely tunable magnesium-doped periodically poled lithium niobate (MgO∶PPLN) optical parametric oscillator (OPO) pumped by a nanosecond fiber laser was studied.

Methods Optical parametric oscillation with a superlattice is the most effective method for 2--5 μm MIR laser generation. The pump source in this study was a linearly polarized nanosecond fiber laser with a central wavelength of 1064.2 nm. The maximum output power of the pump was 20 W, the pulse width was 200 ns, the repetition frequency was 50 kHz, and the beam quality factor, M², was less than 1.1. A 1-mm-thick and 50-mm long MgO∶PPLN nonlinear crystal, with polarization periods of 29--31.6 μm, was used. We chose a double-pass single-resonance flat-flat cavity OPO and used the quasi-phase matching method to tune the wavelength by periodic and temperature modulation of the PPLN crystal. First, the period and temperature of the six-period MgO∶PPLN were continuously tuned and the wavelength tuning range realized by the OPO was explored. The relationship between the tuning wavelength and temperature and period was theoretically simulated using the three-wave coupling equation. The wavelength of the signal light was experimentally determined and then the three-wave coupling equation was used to calculate the corresponding wavelength of idle light. Then, the average idle light output power of different wavelengths was investigated. The output power at four particular wavelengths, 2.4, 2.7, 3.8, and 4.0 μm, was explored and analyzed. In addition, we used a beam quality analyzer (Nano Scan by PHOTOH, Inc.) to measure the beam quality of the wavelengths at the highest average output power. Finally, the power stability of each wavelength of interest, under the highest average output power, was tested.

Results and Discussions First, when the period of the MgO∶PPLN was 29 μm, the temperature was controlled at 40 ℃ and the OPO began oscillation when the incident pump light power was 3.9 W. At this time, the longest idle wavelength of the PPLN obtained was 4014.4 nm [Fig. 2(a)]. When the period of the MgO∶PPLN was 31.6 μm, the temperature was controlled at 180 ℃ and the shortest idle wavelength of the PPLN was 2370.8 nm [Fig. 2(b)]. Combined with periodic and temperature modulation, 2.37--4.01 μm widely tunable operation was realized with a multi-period (29--31.6 μm) MgO∶PPLN crystal [Fig. 2(c)]. In order to prevent damage to the superlattice material, the fixed pump power was 9.95 W. The period and temperature of MgO∶PPLN were changed to obtain a scatter diagram of the average idle light output power with wavelength [Fig. 3(a)]. The average output power of wavelengths ranging from 2370.8--3750.0 nm was greater than 1.7 W and the corresponding conversion efficiency was greater than 17.1%. When the wavelength was 3.4 μm, the maximum average output power was 3.68 W and the corresponding maximum photoconversion efficiency was 37.0%. The wavelengths of 2.4, 2.7, 3.8, and 4.0 μm had maximum average power outputs of 2.87, 2.45, 1.87, and 1.22 W, respectively (Fig. 4), with corresponding optical conversion efficiencies of 17.2%, 19.8%, 11.2%, and 8.6%, respectively. At the highest output power, the corresponding pulse widths were 129, 132, 159, and 169 ns, and the corresponding single pulse energies were 57.4, 49.0, 37.4, and 24.4 μJ, for wavelengths of 2.4, 2.7, 3.8, and 4.0 μm, respectively (Table 4). The four wavelengths produced spot distortion at maximum power and the measured beam quality deteriorated (Fig. 5). In addition, the two-hour instability root mean square (RMS) of the four wavelengths at the highest average output power were 2.51%, 2.24%, 2.35%, and 2.60%, respectively (Fig. 6).

Conclusions A miniaturized wide-tuned infrared OPO with nanosecond pulse width was designed in this paper. A nanosecond fiber laser with a central wavelength of 1064.2 nm was used to pump 29--31.6 μm MgO∶PPLN crystals for six cycles. By changing the crystal period and continuously tuning the temperature at 40--80 ℃, the continuous tuning output of idle light at 2370.8--4014.4 nm with a tuning width of 1643.6 nm was realized. Under an incident pump power of 9.95 W, the average output powers were larger than 1.7 W for a wavelength range of 2.37--3.75 μm. The highest average output power of 3.68 W was obtained at 3.4 μm, corresponding to an optical-optical conversion efficiency of 37%. Moreover, the MIR OPO output parameters at 2.4, 2.7, 3.8, and 4.0 μm were investigated in detail. The maximum average output powers were determined to be 2.87, 2.45, 1.87, and 1.22 W with corresponding optical-optical conversion efficiencies of 17.2%, 19.8%, 11.2%, and 8.6%, respectively. Our results provide significant experimental basis for the development of miniaturized and widely tunable MIR laser sources.

Objective In the field of ultrashort pulse laser, green laser diodes (LDs) as the pump source of Ti: sapphire laser have received increasing attention. It has been reported that increasing the power of the green laser-pumping source can further improve the output power of Ti: sapphire lasers. However, compared with gallium arsenide-based materials in the near-infrared band, gallium nitride-based materials with a wider bandgap can radiate green light and are prone to produce defects in the growth process, which reduce the radiation efficiency and output power. The maximum power of a single LD commercially available is 1.5 W. Thus, it is necessary to use a beam combining technology to improve the output power. Presently, the incoherent beam combining methods, such as spatial or polarized combination, are mostly used to improve the output power of LD modules. However, it is found that the combined beam, in most cases, is a rectangular beam array. When such a beam is focused by a coupling lens, the angular filling factor (AFF) is low (AFF is defined in Section 2.1), resulting in wasting part of the angle space. If the beam filling can be performed in this region, the brightness of the fiber output can be improved further. The brightness of the pump source is also the key factor to increase the output power of the titanium sapphire laser. This study proposes a method for increasing the AFF to fully use the angle space by combining beams in a closest-packed structure. Based on the proposed method, a high-brightness fiber-coupled green LD module is demonstrated using ZEMAX.

Methods To solve the problem of low AFF and the inability of the numerical aperture of a fiber to be fully used when the beam in a rectangular array is coupled into the fiber, we propose to arrange the beams according to a closest-packed structure. Based on this arrangement, we design a high-brightness fiber-coupled module of green LDs. First, we divide 14 1.5-W green LDs into two groups of 7 each and placed them on the six vertices and centers of the regular hexagon to form a dense structure. Then, the beams emitted from green LDs are collimated, respectively, by the fast and slow axis collimators to form a series of parallel beams. Afterward, the beams at the vertices are moved toward the center beam through a custom-designed beam reducer. Thus, the dark area between the beams can be eliminated while the beam divergence angle is constant. Second, beams emitted from one of the two groups pass through a half-wave plate, and the polarization direction is rotated 90°, then two groups of beams with perpendicular polarization direction are combined into a beam through a polarization beam combiner. Finally, the beam is focused through an aspheric lens into a fiber with a core diameter of 105 μm and a Na of 0.15.

Results and Discussions After collimation, the output beam of LD is approximately circular symmetry (Fig. 3). The divergence angles in the fast and slow axes are 0.262 and 0.750 mrad, respectively (Table 1), which can meet the design requirements. Then, the combined beam passes through the beam reducer and eliminates the dark area between the beams. The combined beam diameter in the fast and slow axis is 6.88 and 7.83 mm, respectively (Fig. 7). When the beam is coupled into an optical fiber, the AFF is calculated to be 63.80%, which is higher than that of the rectangular beam (Eq (3)). The simulation results show that the power coupled out of the fiber is 19.13 W—corresponding to a brightness of 3.125 MW·cm ^-2·Sr ^-1—and the fiber-coupling efficiency is 93.75%. Notably, to describe the concept of the AFF clearly, we deliberately collimate the output of the LD into an approximate circular spot in the near field, and it result in an asymmetry of the spot focused on the facet of the fiber in Fig. 9. The asymmetry can be improved in practical applications by combining multiple beams into a circular beam, replacing the subspot in Fig. 7.

Conclusions In summary, we propose a design of an optical fiber coupling module based on the closest-packed structure of combined beam array. It can fully use the numerical aperture of fiber and improve beam brightness. The simulation shows that 14 1.5-W LDs can be coupled into an optical fiber with a numerical aperture of 0.15 and core diameter of 105 μm, and a total power of 19.13 W can be obtained with an output brightness of 3.125 MW·cm ^-2·Sr ^-1—corresponding to a fiber-coupling efficiency of 93.75%. In the future, experiments should be performed to provide a better pumping source for Ti: sapphire solid-state lasers.

Objective Low-noise narrow-linewidth single-longitudinal-mode (SLM) lasers have several important applications in many fields, such as gravitational wave detection, atomic and molecular physics, time and frequency transfer, and precision measurement. Currently, SLM fiber lasers are divided into three main types: distributed feedback, distributed Bragg reflection (DBR), and ring cavity laser. Among them, the ring cavity laser, which employs a long cavity design, facilitates increase in cavity gain to improve output power using relatively high-gain fibers. A ring cavity laser can also be used in other applications such as frequency tuning and feedback control by simply inserting optical components into a cavity. Moreover, the Q-value of a fiber-laser resonator can be increased by employing a long ring cavity. This long cavity helps in obtaining a narrow free-running laser linewidth and reduces the relaxation oscillation frequency of the laser. This is useful in suppressing a relaxation oscillation peak through photoelectric feedback control. However, a long ring cavity is easily affected because of external-environment noise disturbances, which leads to SLM instability, e.g., mode hopping. Thus far, the achieved continuous operation time of a free-running compound ring cavity fiber laser without mode hopping is only several hours. Therefore, although in principal, a fiber-laser source exhibits excellent performance, it cannot meet practical application requirements. In this study, a low-noise narrow-linewidth-ring erbium-doped fiber laser operating in a SLM and maintaining its polarization is reported.

Methods A polarization-maintaining(PM) fiber's strong ability to resist environmental disturbances allows the parametric optimization of a compound ring cavity through repeatable experiments in a laboratory. During experimental optimization, Fabry-Perot(F-P) type very narrow resonant transmission peaks of a secondary cavity are used to filter potentially oscillating dominant longitudinal modes from a large number of nearby main-cavity modes. Then, notch bands of the band-notch type secondary cavity are used to suppress excess dominant main-cavity modes, which are filtered using the F-P type secondary cavity. Length of each secondary cavity can be designed and optimized according to the length of the main cavity so that it meets the Vernier-effect requirements to broaden the effective longitudinal mode spacing of a compound ring cavity. During laser optimization, a vibrational acoustic wave and a thermal-isolation design are employed to protect the laser from environmental vibrations and thermal disturbances. Length of each cavity in a compound ring cavity fiber laser can be fine-tuned to strictly meet the Vernier-effect requirements by changing the control temperature of a laser. Thus, mode hopping is efficiently suppressed.

Results and Discussions After optimizing the cavity length and employing the vibrational acoustic wave and thermal-isolation design, the laser is capable of performing the SLM operation at room temperature [Fig. 2(a)]. By fine-tuning the length of each cavity through temperature compensation, the laser is capable of achieving continuous SLM operation without mode hopping for more than 14 h. To the best of our knowledge, this is the longest SLM free-running time ever achieved using the proposed laser type [Fig. 2(b]. By adopting a laser cavity structure that maintains the laser polarization and exhibits a high-Q design, the laser output signal-to-noise ratio measured is up to 80dB [Fig. 3(a)], and linewidth is approximately 400Hz [Fig. 3(b)]. The long-term SLM operating characteristics of the compound ring cavity fiber laser designed by our experiment allowed us to measure its broadband noise characteristics. The relative intensity noise-power (Fig. 4) and frequency noise-power spectra (Fig. 5) of the ring cavity fiber-laser type were measured in the mHz--MHz frequency band for the first time. The noise measurement results demonstrate that the proposed laser exhibits excellent noise characteristics, similar to those of the DBR fiber laser. Moreover, its intensity noise and frequency noise are lower than those of the DBR fiber laser in the 1mHz--10Hz frequency band. Additionally, its relaxation oscillation frequency is lower than that of the DBR fiber laser.

Conclusions A low-noise compound ring cavity fiber laser with stable SLM operation is proposed in this study. Using PM fibers with strong resistance to external disturbances, compound ring cavity parameters were accurately optimized to achieve a broad effective longitudinal mode spacing. After fabricating vibration isolation and thermal insulation packaging, temperature control was adopted to further fine-tune the FSRs of a compound ring cavity. Then, the Vernier-effect requirements were strictly followed to suppress mode hopping. Finally, the proposed laser was capable of achieving long-term SLM stable operation for more than 14 h, which is the longest SLM stable operation time ever achieved by a compound ring cavity fiber laser to the best of our knowledge. Owing to the laser high-Q characteristics, its output signal-to-noise ratio is up to 80dB, and 3-dB linewidth is approximately 400Hz. The long-term SLM-operation characteristics of the compound ring cavity fiber laser allowed us to measure the noise behavior of the laser type. The noise measurement results demonstrate that the compound ring cavity fiber laser exhibits low intensity and frequency noise in the mHz--MHz frequency band. Compared with a typical DBR fiber laser, the proposed laser exhibits lower relaxation oscillation frequency, relative intensity noise, and frequency noise in the 1mHz--10Hz frequency band.

Objective The 2-μm mid-infrared laser is located in the weak absorption band of the atmosphere and the safe region of the human eye; thus, it is widely used in atmospheric environmental gas detection, clinical medical diagnosis, laser surgery, optical communication, and other fields. Recently, the 2-μm laser is a research hotspot in mid-infrared lasers. High-peak-power 2-μm pulse lasers are efficient pump sources for mid-infrared lasers using nonlinear frequency down-conversion. Importantly, it can be used as an ideal pump source for the 2.5-μm mid-infrared solid-state lasers such as Cr∶ZnSe and Cr∶ZnS and the 3-5-, 8-12-μm mid- to far-infrared optical parametric oscillators (OPOs) such as ZnGeP, GaSe, and CdSe. The 3-5-μm band laser can monitor the industrial waste gas and polluted gases and the main light source for optoelectronic countermeasures, which is very important in national military security. The 3-5-, 8-12-μm tunable OPO pump sources produce high-quality tunable mid-infrared band laser, which can be used in petroleum exploitation, atmospheric greenhouse-gas detection, data communication, and laser spectroscopy research. The 3-5-μm band laser mainly uses the acousto-optic Q-switched 2-μm laser as the pump source realized by nonlinear frequency conversion. In this study, we reported a pulsed laser with an output power of 33.2 W, a pulse width of 64 ns, and an adjustable repetition rate, which can help design 3-8-μm OPO pump sources and understand the performance parameters selection for Tm∶YAP crystals application.

Methods We used Tm∶YAP crystal, acousto-optic Q-switch, and U-shaped resonant cavity as the research objects. Using the cavity design software, we designed the U-shaped resonant cavity and the experimental device, as shown in Figure 1. We used three different specifications of slab-shaped Tm∶YAP crystals. We selected and performed a continuous-wave experiment. We obtained the crystal with the highest average output power and the highest slope efficiency and selected the best cavity length. Using the best parameter crystal and the cavity length, we performed the following steps: 1) performing the Q-switching experiment; 2) inserting the acousto-optic Q-switched crystal into the laser resonator; 3) adjusting the output TTL level signal of the function generator; 4) setting the frequency to 10 kHz; 5) connecting the output TTL level signal to the Q-switch driver; 6) setting the adjustable regulated power supply output DC to 24 V; 7) connecting the I < 3 A voltage to the driver; 8) adjusting the laser carefully until the output Q-switched signal is stable; 9) measuring the Q-switched pulse signal frequency and pulse width; and 10) continue adjusting the function generator to 20, 30, and 40 kHz TTL level signal. We repeated the aforementioned experimental steps until the best Q-switching parameters were obtained.

Results and Discussions We performed a continuous-wave experiment, in which we found that the crystal's best doping concentration (atom number fraction) is at 2% and size at 1.5 mm×6 mm×30 mm for the experiment. When the pump power reached 65 W, crystal embrittlement occurs, caused by the heat generated by pumping light to the 1-mm-thick crystal, forming a strong temperature gradient on the crystal's surface and inside it. When the pumping power is high, the stress gradient occurs because the crystal is easy to be brittle along the crystal cleavage direction. Hence, we selected the Tm∶YAP crystal with a doping concentration of 2% and a size of 1.5 mm×6 mm×30 mm. When the M4 radius of curvature R=200 mm and the transmittance of the output mirror T=20%, the laser output characteristic is the best. Figure 4 shows the output characteristic curve. When the pump power reached 130 W, we obtained the maximum output power of continuous-wave at 42.5 W, the light-to-light conversion efficiency of 32.7%, and the slope efficiency of 42.5%. In the Q-switching experiment, the maximum Q-switched pulse average output power and the pulse width at 10-kHz repetition frequency are 33.2 W and 200 ns, respectively. When the repetition frequency is increased to 40 kHz, we obtained the shortest pulse width at 64 ns, the corresponding average power at 30 W, and the center wavelength at 1944 nm. Figure 6 shows the relationship between the pulse width and the pump power at different repetition frequencies. Compared with the results of most Q-switching experiments, our Q-switching experiment does not increase the pulse width with the repetition frequency increase; hence, we found opposite results. For active Q-switched lasers, the narrowest pulse width can be obtained when the modulation period is equivalent to the energy level's life on the laser crystal, which means that the pulse width does not always widen with the modulation frequency. This experimental result showed that the relationship between modulation frequency and pulse width is still in the downward path.

Conclusions In this study, we designed a set of pump sources for mid-infrared OPOs. The experimental results showed that the Tm∶YAP crystal with a doping concentration of 2% and size of 1.5 mm×6 mm×30mm gold-plated lath had the best laser output characteristics: high output efficiency and smooth and stable output power. When the LD pump power reached 130 W, we obtained the maximum output power of continuous-wave at 42.5 W, with the light-to-light conversion efficiency of 32.7%, and the slope efficiency of 42.5%. Furthermore, we studied the output characteristics of the Q-switched laser. When the pump power reached 130 W, we obtained a pulsed laser with a maximum average output power of 33.2 W, a narrowest pulse width of 64 ns, a maximum peak power of 16.6 kW, and a center wavelength of 1944 nm. In the next experiment, it is possible to obtain a Q-switched laser output of 40 W by further optimizing the cavity parameters, increasing the pump power to 160 W, and changing the output mirror's transmittance. The ZGP-OPO pumped by the 2-μm high-power laser compensates for the absorption loss of the 1.06-μm pumped PPLN crystal at 4.3 μm, and the gain is high in this band. The absorption loss of the 1.06-μm pumped PPLN crystal at 4.3 μm is compensated by the 2-μm high-power laser pumped ZGP-OPO, and the gain is high in this band. It can be used as the pump source of mid- to far-infrared OPO laser, and it has broad application prospects in generating 3-5-, 8-12-μm mid-infrared laser pulses.

Objective Angular-resolved photoemission spectroscopy (ARPES) is a powerful and unique technique used to study the electron structure in condensed matter. It can directly obtain the energy band structure and electron self-energy by momentum resolution. A 1024 nm laser can be used as an ARPES source after multiple frequency doubling, such as the measurement of charge density wave gaps of telluride of rare earth elements. The main emission peak of Yb ³⁺ ions is near 1064 nm; hence, the emission of the 1024 nm pulse laser is relatively difficult, requiring the fine control of the filter parameters. Thus far, reports on the 1024 nm laser are mainly concentrated on solid-state lasers. A fiber laser has the advantages of compact structure, low cost, and good stability. The low output power of the fiber laser can be made up by the amplifier, which has a good application value.

Methods In this study, pulse evolution in a fiber laser was numerically simulated by the system of the coupled Ginzburg-Landau equation and solved using the split-step Fourier method. The laser passed through each device in turn in the cavity and repeatedly circulated. The pulse formation and evolution were simulated. Experimentally, the seed source was an all-normal dispersion Yb-doped fiber laser mode-locked by nonlinear polarization rotation (NPR). The oscillator consisted of a 976 nm pump, a 980/1030 nm wavelength division multiplexer (WDM), a piece of 54 cm Yb-doped fiber (YDF), an isolator polarization-independent isolator, an output coupler, and an NPR saturable absorber. A master oscillator power amplifier system based on a tunable fiber filter was built to realize the 1024 nm output laser. The seed pulse was filtered by a tunable filter. Subsequently, preamplification and amplification experiments were conducted. The preamplifier contained a pump, a WDM, a piece of YDF, and an isolator, while the amplifier comprised a pump, a WDM, and a piece of YDF. The laser was output by a collimating isolator. The output laser parameters were measured by a 12.5 GHz photoelectric detector, a 6 GHz oscilloscope, a spectrograph (Yokogawa, AQ6370C), an autocorrelator (FR-103XL), and a frequency spectrograph (Agilent, E4447A). The seed laser formation was the NPR mechanism realized by the artificial saturable absorber formed by the wave plate, band pass filter, and polarization beam splitter. The NPR belongs to the fast saturation absorber mode locking mechanism; thus, it was more conducive for the ultrashort pulse generation.

Results and Discussions The dissipative soliton pulses can be generated when the simulated model parameters are appropriate (Fig. 2). The pulses gradually formed with the accumulation of the intracavity energy. Figures 2(b) and (c) depict the evolution of the spectral and pulse shapes at different locations in the cavity. The output pulse of the oscillator was a dissipative soliton with a steep edge spectrum. The established process of the mode-locked pulse was studied herein. The pulse establishment process included the peak power improvement, high peak power, and stable mode-locked stages. The experimental results were consistent with the theoretical simulation results. The seed laser generated by the oscillator can generate a pulse laser with different central wavelengths through the tunable filter (Fig. 6(a)). The tunable range of the central wavelength was 1022--1030 nm. We set the central wavelength of the tunable filter to 1024 nm. The pulse output power was greatly reduced because of the spectral filtering; hence, a preamplifier was added after the tunable filter. To obtain pulses with more concentrated spectral energy and higher peak power, the pump power of the amplifier was set to 200 mW, and the output power of the preamplifier was set to 42.02 mW. Figure 8 shows the output pulse characteristics of the main amplifier. When the pump power of the amplifier was 5 W, the stable output mode-locked laser was obtained with a central wavelength of 1024 nm, an average power of 1.1 W, a pulse energy of 51 nJ, a repetition frequency of 21.5 MHz, and a signal-to-noise ratio of 67.5 dB.

Conclusions This study theoretically simulated the evolution process of the dissipative soliton pulse in the NPR mode-locked Yb-doped fiber laser based on the Ginzburg-Landau equation. We experimentally obtained the dissipative soliton pulses and studied the establishment process of the mode-locked pulses. The results were basically consistent compared with the theoretical simulation results. A series of pulses with 1020--1030 nm central wavelength were obtained by passing the seed laser pulse through a tunable filter. The laser with a 1024 nm central wavelength was selected as the seed laser for amplification. The laser with 8.55 ps pulse width and 42.02 mW average output power was obtained after the preamplifier. After the main amplifier, when the pump power of the amplifier was 5 W, the output average power was 1.1 W, the pulse energy was 51 nJ, and the central wavelength was 1024 nm. A useless wavelength was found in the spectrum, which was mainly caused by the limited filtering effect of the tunable filter. However, compared with the current experimental research reports, the spectral output width of this amplifier is significantly narrowed, and further experiments on spectral compression are under way. The results are helpful in understanding the dynamic characteristics of dissipative soliton mode locking in the NPR mode-locked fiber laser. Furthermore, the pulse laser at this wave band is expected to be applied in ARPES measurement.

Objective The invention of semiconductor lasers has brought about a huge impetus to the laser industry. Traditional edge-emitting lasers can work with high efficiency and high average power, but the poor beam quality limits their widespread adoption. The invention of vertical cavity surface emitting lasers (VCSELs) improved the output of a semiconductor laser to a circular spot but with limited output power. Subsequently, vertical external cavity surface emitting lasers (VECSELs) emerged featuring a smaller divergence angle. Still, the power scaling capability and beam quality optimization could not be achieved at the same time due to the inhomogeneous electric current pump. Next came the optically pumped VECSELs. For conventional lasers, pumped laser crystals usually have a thermal lens effect. The range of focal length of thermal lens determines the scope of the stable area when the laser is operating. The gain chip of the vertical cavity surface emitting laser is a semiconductor material, so the focal length of the thermal lens cannot be directly calculated by the crystal thermal lens focal length formula. To evaluate the thermal lens effect of the gain chip, we estimate the thermal focal length of the gain chip. In addition, the tunable output range is an important parameter to characterize the output properties of the laser. In this article, we also propose a method for estimating the angle tuning range of the VECSEL by directly measuring the angle-dependent characteristics of the fluorescence spectrum of the gain chip. Furthermore, we conducted an angle tuning experiment to verify this method.

Methods To measure the thermal focal length of the gain chip, we use the stability conditions of the laser cavity to determine the thermal focal length of the gain chip. During the experiment, we ensure that the thermal rollover phenomenon does not occur and the length of the resonator is fixed. The thermal focal length of the gain chip gradually decreases with increasing pump power until the active resonator meets the stability conditions, and the output of VECSEL plunges. When the cavity length of the resonator is reduced, the output power of the shortened one does not decrease at the same pump power. The thermal focal length range of the gain chip at this pump power can be estimated later by simulation calculations. Additionally, the gain chip is pumped without an output coupler to investigate its angle-dependent characteristics of the fluorescence spectra and the angle tuning properties are further confirmed in a resonator.

Results and Discussions The VECSEL in our experiment employs a plane-concave resonator with a 45 mm cavity length. When the pump power is more than 28.36 W, the output power decreases rapidly with increase in pump power(Fig. 3). There are two possible reasons. The first is that the mode size on the gain chip becomes larger, and the laser output decreases rapidly with the increasing of diffraction loss. The second possible reason is the onset of rollover inside the semiconductor device as the pump power increases, which also leads to a rapid decrease in output power. Compared with another set of experiments, when the cavity length is 40 mm and the pump power is 31.3 W, the output power can reach 3.74 W. There is no similar phenomenon where the output power decreases with the increase of pump power(Fig. 4). Therefore, the possibility of thermal rollover causing the previous VECSEL output power to dip can be ruled out. This decline can be considered as the output power change caused by the variation of the thermal focal length under enhanced pump power. From the experimental and calculation results, it can be inferred that when the pump spot diameter is 380 μm and the pump power is 31.3 W, the thermal focal length of the gain chip used in the experiment is between 45.7 mm and 53.6 mm. Based on the observed results which show that the fluorescence wavelength of the gain chip changes with the observation angle, it is inferred that the tuning range of the VECSEL is about 35 nm(Fig. 6).

Conclusions To obtain high power and high beam quality laser through state-of-the-art semiconductor technology, VECSELs were developed. In this paper, the thermal focal length of a gain chip is estimated by the calculation of a laser resonator combined with experiments. The phenomenon of a fluorescence spectrum of a gain chip that varies with the observation angle is reported. A method is proposed to directly estimate the tuning range of a VECSEL by observing the fluorescence spectra of the gain chip at different angles.