High-power SC laser sources have a variety of potential applications including active hyperspectral imaging, long-range environmental sensing, homeland security, and so on. The growing trends of MIR-SC lasers are mainly listed as below.

1) Output power is increasing. With the technological development of near-infrared and short-wave infrared fiber lasers and soft glass fiber processing, the output powers of mid-infrared SC fiber lasers based on fluoride fibers have been continuously improved to 30 W. With the development of mid-infrared fiber laser technology and the demand for SC lasers, the output powers of 3-5 μm mid-infrared SC fiber lasers should be further improved.

2) Power conversion efficiency is gradually improved. In the early research, the inefficient spatial coupling and high fiber transmission loss limit the improvement of power conversion efficiency of SC lasers. With the development of low-loss beam coupling technologies such as end-butting and fusion splicing, the successful development of low-loss optical fibers, and the red shift of pump wavelength, the power conversion efficiency of SC lasers has been gradually improved. It can be expected that with the further reduction of transmission loss in mid-infrared fibers and the further red shift of pump wavelength, there is still room for improvement in power conversion efficiency.

3) Optimization of spectral characteristics is continuing. Due to the limitation of the short wavelength or narrow spectrum of the pump laser, the short-wavelength component of the early high-power mid-infrared SC fiber laser has a high proportion of power, or there is a residual spectral peak at the pump wavelength. After the emergence of SC lasers as pumping sources, spectrally flat mid-infrared SC lasers have been widely studied, and the power ratio of long-wavelength spectral components has been significantly improved. With the development of long-wavelength pump source technology, the spectral characteristics (such as spectral shape and long-wavelength power ratio) have become a clear trend.

4) Pump wavelength is gradually moving in the long wavelength direction. From the development of high-power SC fiber lasers, it can be found that with the development of fiber laser technology, the pump wavelength of mid-infrared high-power SC laser gradually moves in the long wavelength direction, from the 1.5 μm band to the 2 μm band, then to the 2-2.5 μm band, and now to 3 μm band. The increase of the pump wavelength is beneficial to reduce the quantum defect, reduce the heat generation, and improve the power conversion efficiency during the conversion from the pump light to the mid-infrared SC laser. It can be predicted that pumping wavelength would be extended to long wavelength, which is an important trend of mid-infrared SC fiber lasers.

It can be expected that in the near future, high-power mid-infrared SC fiber lasers would move from experimental researches to practical applications, and play a unique role in scientific researches, production, and daily life.

In this article, the advantages and disadvantages of different methods to obtain mid-far infrared lasers as well as the latest research results are first introduced. Second, a detailed introduction of the latest research results on the optical nonlinear frequency conversion technologies in Harbin Institute of Technology is illustrated, including the acousto-optic modulated Ho∶YAG Q-switched lasers with different pulse repetition rates. Third, the Ho∶YAG lasers with different pulse repetition rates and the latest research results in high-power mid-infrared and far-infrared lasers with ZGP, BGSe and CdSe crystals in the nearest three years are also introduced. Finally, with the development of the ultrashort pulsed 2 μm laser, high power ultrashort mid-far-infrared lasers is prospected to become a research hotspot in next few years.

The past decade has seen great progress of mid-infrared mode-locked fluoride fiber lasers, enabling a range of new applications. Different mechanisms, e.g., MSA, NPR, and FSF, have been introduced into rare-earth ions doped fluoride fiber lasers to demonstrate mode-locked operations in the spectral region of 2.73.6 μm and with the time scale of 10 s of fs to ps. However, most of these systems involve some local free-space alignments, weakening the stability, reliability and compactness while preventing pulse narrowing and soliton formation especially at the strong water vapor absorption band. Thus the development towards a robust all-fiber structure is undoubtedly the trend in the future. In addition, the performance parameters of the mode-locked fiber lasers in the mid-infrared fall far short of that in the near-infrared, which needs to be improved by employing new mode-locking techniques and designs. Parallel to the mid-infrared continuous wave fluoride fiber lasers, further extending the wavelength of mode-locked pulses is also an important direction.

Raman fiber lasing is an efficient approach for the generation of high power MIR fiber lasers. Currently, by using tellurite, fluoride or chalcogenide glass fibers as the Raman gain media, the 3.77 μm second order cascaded Raman fiber laser and 24.3 μm tunable Raman soliton fiber laser have been developed. However, limited by the characteristics of infrared glass and high quality fiber & device fabrication technologies, the research on MIR Raman fiber lasers is still in the development stage, and the output power is quite low in the order of watts. Very recently, the authors have developed fluorotellurite glass fibers with good stabilities and high laser damage thresholds, and preliminarily verified their potential for constructing high power MIR Raman fiber lasers. We believe that tunable MIR Raman soliton lasers covering the whole 25 μm region could be obtained by optimizing the parameters of fluorotellurite glass fibers and pump lasers, and hundred-watt-level high power MIR Raman fiber lasers would be achieved by improving the quality of fluorotellurite fibers and with the development of high quality MIR fiber devices (e.g. fiber gratings) in the future.

The BGSe crystal is a new type of wide-bandgap IR NLO material with unique advantages and good application prospects in mid-far IR laser output via the frequency down-conversion process. The future research direction of BGSe mainly focuses on improving the crystal quality, searching for the optimal phase-matching direction, and solving the matching problem between the different types of pump sources and the optical properties of BGSe.

In conclusion, the recent progress of ULMA ChG PCFs shows promise for high-power mid-IR applications at 100 W. Single modes and low bending loss are traded off in ultra-low-NA ULMA ChG PCFs. Finally, it is pointed out that refractive index fluctuation at the order of 10^-5 in the ChG glasses (Fig. 9) is the ultimate barrier responsible for i) increasing the MFD of a ULMA ChG PCF to 200-μm or higher, and ii) mode instability of such a ULMA PCF during high-power operation. To address such a technical issue, additional glass purification and precise thermal management are proposed.

Chalcogenide fibers have been used in laser processing, laser surgery, and homeland security. The development of chalcogenide fibers with low loss and high laser damage threshold has great scientific value and application prospects. In particular, hollow-core micro-structured fibers are a technological race for the next generation of infrared fibers and related applications.

With its recent breakthrough in terms of output power and laser efficiency, the erbium-doped 3 μm laser has become an object of intense scientific research. With the improvement of high-quality low phonon energy laser gain media, especially the development of ceramic gain media, the 3 μm laser performance can be further improved by optimizing the wavelength and spectral linewidth of pump sources, doping concentration of Er-doped laser gain media, and laser cavity parameters. Compared with semiconductor laser and nonlinear frequency conversion technologies, the laser-diode pumped Er-doped laser emitting 3 μm wavelength is very promising, especially in the pulsed laser operation for producing high peak power and large pulse energy.

Herein, we report a high-peak-power Er∶ZBLAN fiber amplifier at 2.8 μm that is seeded by an NPR mode-locked Er∶ZBLAN fiber laser with a pulse width of 240 fs and a peak power of 3.1 kW at 54.3 MHz. A numerical simulation of the model amplifier based on a generalized nonlinear Schrödinger equation is used to analyze its operation and demonstrate the soliton self-compression process. We experimentally obtain the amplified pulses with a pulse width of 110 fs and a peak power of 151 kW. Our results show that self-compression amplification is a reliable method for producing high-peak-power mid-infrared ultrashort pulses at 2.8 μm. In future, higher-peak-power pulses are expected from the Er∶ZBLAN fiber amplifier through dispersion management and system optimization.

In summary, an ultrabroadband SC spanning from 1.5 μm to 14.3 μm with a high coherent property is obtained in a 10 cm long ANDi T-DCF pumped at 6 μm. For comparison, the SC broadening in an ANDi T-SIF under the same pumping conditions is also measured. Our results show that double clad leads not only to improve the mechanical strength but also lower the energy loss in the transition region of the fiber taper for ultra-broadband SC generation, which has a considerable practical potential in various applications.

The mid-infrared tunable lasers have a wide range of applications in military, medical, biological imaging, and scientific research fields. As the core working medium of mid-infrared tunable lasers, the gain medium plays an important role. In order to solve the bottleneck problem of Er³⁺ , Ho³⁺ , and Dy³⁺ ion activated mid-infrared laser crystals and obtain broadband fluorescence emission in the ~3 μm band, which is beneficial to realize a broadband tunable output, this work proposes a relevant optical performance optimization control scheme. Based on the existence of coupling interaction between rare earth ion levels, three coupling schemes of Er³⁺ /Ho³⁺ /Pr³⁺ ∶PbF₂, Yb³⁺ /Er³⁺ /Dy³⁺ ∶PbF₂ and Yb³⁺ /Ho³⁺ /Dy³⁺ ∶PbF₂ are proposed, and the optical properties are studied systematically. At the same time, an enhanced and widened 3 μm fluorescence emission is obtained, indicating that these kinds of crystals are expected to be gain materials for broadband tunable lasers.

In summary, we have demonstrated the design of a three-stage ultrafast mid-IR fiber laser system, which can directly deliver 10 W level average power, hundreds of nJ pulse energy, and hundreds of fs pulse duration. We believe that the design and analysis demonstrated here can provide useful information to the construction and optimization of real high-power, ultrafast, mid-IR fiber laser systems.

This study presented the preparation and characterization of an all-solid-state PbSe QD-doped glass fiber with tunable broadband MIR emission. Based on the selection and optimization of the basic glass system, a size-controllable PbSe QD-doped glass was prepared and achieved broadband tunable emission in the MIR regions of 1.82.8 μm. The core glass was a PbSe QD-doped borosilicate glass, and the cladding glass was a PbSe QD-doped borosilicate glass. The PbSe QD-doped glass fiber was successfully prepared using the melt-in-tube melting, and it was then characterized using EPMA. There was no evidence of elemental migration in the glass fibers, which had a complete core-cladding structure. When excited by an 808 nm laser, PbSe QD-doped glass fibers produced broadband tunable emission within 1.82.8 μm, which was expected to be used as a broadband tunable MIR light source.

In this study, we use the femtosecond laser line-by-line method and a 50 × dry objective to study the influence of laser pulse energy on grating fringes. The pulse energy of 150 nJ, the inscribe speed of 100 μm/s, the line length of 50 μm, and the period pitch of 1.994 μm are selected as writing parameters. A mid-infrared FBG with narrow bandwidth and high reflectivity is prepared based on a fluoride fiber. The central wavelength is 2964.34 nm and the reflectivity is up to 99.27%. The mid-infrared FBG with a central wavelength of 3090 nm and a reflectivity of 99.12% is prepared by adjusting the period pitch of this FBG. And it is expected to achieve a long band FBG writing by this method. This work is beneficial to the construction of an " all-fiber" mid-infrared fiber laser, which is of great significance to promote the autonomy of the core device of a mid-infrared fiber laser in China.

QCW TFL was constructed with a center wavelength of PRR of 1939.31 nm, a pulse width of 02000 μs, and a PRR of 02 kHz, respectively. 34.2 W was the maximum average output power. Following that, in vitro lithotripsy experiments were performed using the laser. The results indicate that when the single pulse energy is constant, the average output power of the laser is the primary factor affecting the ablation efficiency of lithotripsy. And when the average output power is comparable, the energy of the single pulse becomes the determining factor. Additionally, the TFL can generate more heat when operating in the long pulse width mode, resulting in a greater rise in water temperature. In conclusion, TFL with a wavelength of 1940 nm has a significant lithotripsy effect. When combined with fiber lasers’ superior characteristics, it is very likely to become the laser source for the next generation of laser lithotripsy.

In this study, ZBYA glass is prepared and tested and its better thermal and chemical stabilities are demonstrated. Accordingly, we fabricate the Ho³⁺ -doped ZBYA glass fiber and achieve a ~2.9-μm laser. The maximum output power is 137 mW, and the slope efficiency is 8.9%. Currently, the background of fiber is still high owing to the purity of raw materials; however, by optimizing the preparation process to reduce fiber loss, it is expected to achieve higher output power and slope efficiency. Furthermore, silica fiber and ZBYA fiber’s fusion splicing technology should be investigated to reduce the complexity of laser systems. The future work should focus on the preparation of double-cladding fibers to increase the maximum pump power and obtain higher power output.

DFG was confirmed to be an effective technique to realize stable mid-infrared pulse output, even when the idler wavelength reached at 3.8 μm. High quality pulse pump laser and signal laser could still be constructed with average power high enough for efficient DFG conversion, even if their wavelengths were situated at the edges of their gain spectra. Conversion efficiency saturation was likely to happen in DFG at long idler wavelength mainly due to material adsorption. Improvement could be expected by using signal source with wider spectrum.

By derivation, we find that when the group velocity difference between the pump wave and the generated idler wave is small, the pump acceptance bandwidth is broad. In a PPLN crystal, when the pump wavelength is 1050 nm and the generated idler wavelength is about 3.4 μm, the group velocity difference between the pump wave and the idler wave is small, and thus the corresponding pump acceptance bandwidth is wide. In addition, we report an ultra-short-pulse-pumped DFG, by using a 1-mm-long PPLN crystal with a poling period of 29.5 μm, and we obtain mid-infrared sources with bandwidths covering 2.724.15 μm and an average power of 7.5 mW. Meanwhile, as indicated in the simulation, when the central wavelengths of the pump wave and the signal wave are 1050 nm and 1525 nm, respectively, increasing the bandwidth of the pump wave can increase the bandwidth of the generated idler wave.

Performance and power scalability of resonantly pumped Tm-doped fiber systems are theoretically investigated. Simulations demonstrate that the 1.9 μm resonant pedestal pump provides high-power Tm-doped fiber systems with a high operation efficiency of >90%, resulting in a controllable heat waste (within 10% of the output). This limited heat greatly relieves the thermal effects inside the active fiber, eliminating transversal mode instability as a major concern for power scaling of Tm-doped fiber systems. For the widely exploited 25/250 double-clad TDF, the 10-kW-level output is attainable with the 1.9 μm resonant pump.

We have proposed and implemented a 2.8 μm synchronously mode-locked pulsed fiber laser pumped by a 976 nm single-mode pulsed laser. The 2.8 μm pulsed laser repetition rate is 6.534 MHz, the center wavelength is 2784.7 nm, the 3 dB spectral width is 0.4 nm, and the maximum output power is 15 mW. Due to the low output power, it is difficult to detect the actual pulse duration through the autocorrelator, but the oscilloscope shows 3.1 ns, which is close to the detection limit pulse width of the photodetector. Next, we will further optimize and improve the quality of the 976 nm pump light, use a fluoride fiber with a thinner core to improve the effect of cross-phase modulation, and select an active fiber with a relatively high doping concentration to increase the output power and achieve ultra-short mid-infrared mode-locked pulse. In short, based on the synchronous pump mode locking technology, there is no need to introduce any active and passive modulation devices in the resonator, and the system has a simple structure, good stability, and easy to achieve entirely monolithic fiber lasers. It provides a solution for realizing stable and reliable mode-locked pulse lasers in the mid-infrared regime.

The traditional quartz polarization-maintaining fiber cannot be used in the mid-infrared due to its limitation of phonon energy. Contrarily, the simulated birefringence of chalcogenide PCF is much higher. However, there is no experimental report about PCF because of its fabrication complexity. Therefore, this study proposes a novel "—" typed suspended-core chalcogenide fiber with a rectangle core, which is optimized based on the elliptical core fiber model. When the aspect ratio of the rectangular core is 3.6, the air hole radius is 28 μm, and the distance between the two air holes is 5.1 μm, the birefringence can reach up to 7.1×10^-3 at 5 μm. Additionally, the fundamental mode losses in the two orthogonal directions reduce to 10^-3 dB/m at 5 μm. We find that the proposed fiber can achieve high birefringence in the mid-infrared. Furthermore, the fiber is fabricated based on chalcogenide glass extrusion technology, and its birefringence can reach up to 4.6×10^-3 at 5 μm. This study reports all simulation and experimental results for such a suspended-core chalcogenide fiber in the mid-infrared and shows the possibility of a high-birefringence fiber device.

The GSA and ESA dual-wavelength pumped Tm: YAP laser emitting in the 2.3 μm region have been realized successfully. The Tm∶YAP crystal has a maximum output power of 2.28 W at 2.3 μm. To the best our knowledge, this is the highest CW output power obtained in a 2.3 μm Tm-doped solid-state laser. Making full use of the broadband emission spectrum of the ³H₄→³H₅ transition of the Tm∶YAP crystal, a watt-level LD-pumped 2.5 μm Tm∶YAP laser is achieved using the dual-wavelength pumping scheme. Our results show that GSA and ESA dual-wavelength pumping is an effective technical means to achieve a high power output of 2.3 μm Tm-doped laser.

In this study, high quality ZGP crystals with diameters of 3.85.0 cm have been grown by the ultralow gradient freezing technique in self-made furnaces with growth yields over 95%. The crystal growth efforts and post processing have resulted in ZGP OPO devices with an ultra-low absorption loss (0.015 cm^-1 @2.09 μm) and a size of up to 30 mm×30 mm×40 mm. A high average output power (>100 W) ZGP OPO with only one low absorption ZGP device has been demonstrated, with no obvious limits to further scaling. The results indicate that the combination of large aperture and low absorption losses makes our ZGP crystals extremely attractive for high average power and high energy applications.

In summary, we have achieved a 4.5 W mid-infrared laser output at 3.1 μm in an acetylene-filled HCF, corresponding to an optical-to-optical conversion efficiency of 14%, which is the currently the maximum power of a fiber gas laser in the mid-infrared region. The experimental results show that fiber gas lasers have the potential to achieve high-power mid-infrared fiber laser outputs.