As the main pumping light source of solid state laser, fiber laser, and fiber amplifier, 976 nm diode laser has been widely used in industrial processing, medical treatment, communication, and other fields. As an important pumping light source of erbium-doped fiber amplifier, a 976 nm fundamental transverse mode diode laser can achieve high-efficiency coupling with fiber, improve the output performance of the fiber amplifier, and effectively reduce the cost of the fiber amplifier. It plays a very important role in improving the application of erbium-doped fiber amplifier in fiber communication and other fields. However, since the ridge waveguide in the ridge diode laser uses the weak refractive index guiding mechanism to suppress the higher-order transverse mode, it will be greatly affected by the lateral diffusion of carriers and the self-heating effect and eventually lead to the decline of the ridge waveguide mode guiding and the increase of the far-field angle. To further improve the coupling efficiency of diode lasers in fiber laser pumping applications and reduce the application cost of fiber lasers, it is still important to realize low far-field divergence angle and low power consumption of fundamental transverse mode ridge diode lasers.

Using InGaAs/GaAsP material as the strain-compensated quantum well structure, and GaAsP with high bandgap width as the barrier material can effectively reduce the carrier leakage effect of quantum well, provide strain compensation for InGaAs compressive strain quantum well, and improve the epitaxial growth quality. To achieve low loss, we optimize high output optical power and low far-field angle, the thickness of waveguide layers by using asymmetric large optical cavity epitaxial structure. The doping concentrations of the epitaxial layer materials are optimized to reduce the series resistance of the device, to achieve high power, high conversion efficiency, and low far-field output of the ridge diode laser. To achieve fundamental transverse mode output, we use the effective refractive index method to design and study the width and depth of the ridge waveguide and map the optical field distribution inside the device. Finally, according to the technological conditions, the ridge waveguide structure is selected with a width of 5 μm and a depth of 0.85 μm.

After the laser chip is designed and prepared, the output performance of the device is tested at 25 ℃. The device threshold current is about 51.2 mA, and a maximum continuous output power of 422 mW can be obtained at 550 mA injection current, with a maximum electro-optic conversion efficiency of 53.6% (Fig. 3). The peak wavelength is 973.3 nm at 550 mA injection current, and the corresponding spectral line width (FWHM) is 1.4 nm. When the injection current is 500 mA, the vertical and horizontal far-field distribution diagrams of the device are drawn (Fig. 5), and the corresponding vertical and horizontal far-field divergence angles (FWHM) are 24.15° and 3.9°, respectively. This indicates that the prepared ridge diode laser has a good fundamental transverse model property, which is conducive to improving the coupling efficiency between the diode laser and the fiber. Subsequently, we analyze the temperature characteristics of the device at the operating temperature of 15-35 ℃ and obtain a relatively high characteristic temperature of about 194 K. This is because GaAsP material with high band gap width is added to both sides of the InGaAs quantum well as the barrier layers, and there is a larger band level between the two materials, which can better suppress carrier leakage in the quantum well. The injection current utilization, the luminous efficiency, and the temperature stability of the laser device are improved. Similarly, the horizontal far-field changes little at 15 ℃, 25 ℃, and 35 ℃, and the corresponding horizontal far-field divergence angles are 3.45°, 3.90°, and 3.90°, respectively, which is conducive to increasing the coupling efficiency in optical pumping applications (Fig. 7).

We design and fabricate a 976 nm fundamental transverse mode ridge diode laser. To improve the conversion efficiency of the device, we improve high-bandgap GaAsP materials on both sides of the InGaAs compressive strain quantum well as a tensile strain barrier to improve the internal gain of the device, inhibit the carrier leakage in the quantum well, and improve the current utilization rate. In addition, we optimize the waveguide layer thickness and doping concentration in the device epitaxial structure to reduce the far-field divergence angle and achieve high-efficiency output by using an asymmetric large optical cavity epitaxial structure design. A 976 nm strain-compensated low far-field fundamental transverse mode ridge diode laser with a ridge width of 5 μm and a cavity length of 1500 μm is fabricated. At the operating temperature of 25 ℃, the maximum continuous output power of 422 mW can be obtained, the peak wavelength is 973.3 nm, and the spectral line width (FWHM) is 1.4 nm. When the injection current is 500 mA, the vertical and horizontal far-field divergence angles (FWHM) are 24.15° and 3.90°, respectively. In the operating temperature range of 15-35 ℃, the far-field divergence angle of the ridge diode laser is tested and analyzed. It is found that the far-field distribution of the device changes little with the increase in the test temperature, and the far-field divergence angle can be kept small.

Based on the light-matter interaction, optical tweezers generate strong force on micro and nano-sized particles by momentum transfer, which is non-contact and non-damage. In bioscience, optical tweezers have been applied to capture bacteria and non-invasive manipulation of organelles within a single living cell and become an effective way to detect and control micro- and nano-scale objects. However, optically capturing and manipulating individual particles smaller than the light wavelength remains a major challenge. To overcome the light diffraction limitation, the local surface plasmons (LSPs) in the metal nanostructure can effectively focus and confine the propagating light within the nanometer scale, with better spatial locality and higher local field intensity. A plasma well is generated by the coupling of electromagnetic waves at the interface of the metal-dielectric layer by surface plasmon excitation. This unique electromagnetic pattern can limit light beyond the diffraction limit, causing the electromagnetic field to decay exponentially from the metal-dielectric interface. These two properties are crucial for optical capture applications: the former significantly reduces the volume of the captured object, and the latter enhances the production of optical forces due to the field intensity gradient. Meanwhile, we study the distribution of the electric field and Poynting vector diagram under different polarization modes and the distribution of light force and potential well generated by the interaction between nanoparticles and the scattering near the field of coaxial structure. Finally, it provides a new way for capturing and manipulating micro and nano particles and other living cells in a low-concentration solution.

The coaxial structure consists of silicon concentric ring and a silver layer. The coaxial structure of the laser source illuminates vertically from the bottom, and the electric field distribution of online polarization, circular polarization, and different coaxial apertures under the light source and Poynting vector diagram are calculated by finite-difference time-domain (FDTD) method. Additionally, the Maxwell stress tensor method is adopted to calculate the light force generated by the interaction of dielectric particles with a 10 nm radius with the structure in free space. The optical trapping performance of the structure in two light modes is studied. The optical trapping force and potential well distribution of particles in the x-y and y-z planes under different light source modes are calculated. The force analysis of the nanoparticles shows that the positive force F_x and negative force F_-x in both the x-y and y-z planes cause the coaxial structure to produce the trapped particles in the center of the potential well.

The coaxial structure is coupled with the optical field to enhance transmission and local electromagnetic field (Fig. 1). The transmission characteristic curve shifts to the right as the height of the coaxial structure h increases. When h=150 nm, the transmission spectrum has two peaks at the wavelength of 540 nm and 750 nm. These peaks are transmitted by light waves into a coaxial aperture of finite thickness and are fully reflected, which brings the Fabry-Perot resonance mode. Under the action of the circularly polarized light field, the coaxial plasma structure generates a vortex light field of spin energy flow (Fig. 6). The resulting vortex field affects the spin angular momentum carried by the circularly polarized light and makes it pass through the coaxial aperture, and the orbital angular momentum is transmitted in the near field through the spin-orbit interaction of the electromagnetic field. The optical trapping force and potential well distribution of particles in the x-y and y-z planes under different light source modes are calculated respectively (Figs. 7 and 8). The dual trapping potential well is generated to expand the trapping region, overcome the Brownian diffusion of nanoparticles in a low-concentration solution, and improve the trapping efficiency, providing a new way for capturing and manipulating micro-nano particles and other living cells in a low-concentration solution.

The distribution of electric field and Poynting vector diagram under linear polarization and circular polarization light sources are calculated by the FDTD method, with the optical trapping performance of nanoparticles under two light modes. The results show that the transmission value reaches the maximum at 750 nm, and the depth of the potential well reaches 17kBT under the incident light intensity of 1 μW/μm². Meanwhile, the circularly polarized light forms a potential well depth of 8kBT vortex field above the structure, which overcomes the Brownian diffusion of nanoparticles in a low-concentration solution and improves the capture efficiency. The results can be employed both as optical tweezers for manipulating nanoparticles and as semiconductor structures for laser emission.

Quantum cascade laser (QCL) is a semiconductor laser based on sub-band electronic transition, which results in a broad emitting wavelength covering from 3 to 300 μm. QCL is an ideal light source in the fields of gas sensing, environmental monitoring, medical diagnosis, and photoelectric countermeasures. However, the relatively low output power (1-3 W) of the single transverse mode QCL is a major limitation for its applications. Laser beam combining technology is an effective way to enhance the output power. At present, the power beam combining of mid-infrared and long-wave infrared QCLs is heavily limited by the low-loss optical materials and component preparations. Beam combining with high efficiency and low loss is challenging, and few research results have been reported. Therefore, the fiber combining of long-wave infrared QCLs in the 7.6-7.8 μm wavelength band was studied in this paper. The laser power was combined with a 4-in-1 single-mode hollow-core fiber bundle.

In order to realize the high-efficiency single-mode fiber coupling of QCLs, the optical fiber coupling system was designed. The optical fiber system was composed of a QCL collimator and a fiber coupler. Due to the large QCL emitting area and large divergence angle, an aspheric collimator with a large numerical aperture was designed and fabricated. During the optical design and optimization, the QCL was assumed to be an extended light source. Using the optimized collimator, a fiber coupling efficiency of 88.9% was obtained. To combine the laser beams from individual QCL, a 4-in-1 fiber combiner was fabricated using AgI/Ag single-mode hollow-core fiber, which had a high damage threshold and low transmission loss. During the preparation, the outer protective layer of the fiber was stripped away, and the four fibers were tightly arranged in the sleeve and fixed. Finally, the fiber was protected by metal armor. The input terminals of the fiber combiner were four independent SMA905 fiber connectors, and a unified SMA905 connector was made at the output end.

The optical fiber coupling experiments are conducted using the designed optical fiber coupling system and the prepared long-wave single-mode hollow-core fiber combiner. When the QCL output power is 642 mW, the laser power throughout the single-mode fiber is 438 mW. The corresponding fiber coupling efficiency is 68%. In addition, we experimentally compare the coupling efficiency using a point-source collimator and an extended-source collimator. Using the extended-source collimator with a large numerical aperture, the fiber coupling efficiency is increased from 59% to 68%, as shown in Fig. 10. An infrared camera is used to observe the collimated QCL spot and the beam spot out of the single-mode fiber. In addition, the beam propagation quality factor M² after the fiber coupling is calculated. After the fiber coupling, a symmetric Gaussian distribution is observed, and the beam quality is improved to 1.2, compared to the M² in Table 7. On the basis of the single-channel optical fiber coupling experiment, the optical fiber combining experimental setup of a four-channel long-wave infrared QCL is built. When the total input power from four QCLs is 2.27 W, the fiber combining power is 1.5 W. The corresponding combining efficiency is 66%. In order to evaluate the beam quality of the combined beam, the lens is used to focus the output light, and the intensity distribution of the output spot of the beam combiner is measured within two times Rayleigh distance. The results are shown in Figs. 14 and 15. The transmission quality factors of the combined beam are calculated as MX2=2.67 and MY2=2.56, which meant a good beam quality.

In this paper, the long-wave infrared QCL beam combining technology based on single-mode hollow-core fiber is studied. Considering the large emitting area and big divergent angle of the fundamental transverse mode long-wave infrared QCL, a QCL collimator with a large numerical aperture is used. During the design, the QCL is treated as an extended light source. To obtain the optimized collimation result, both surfaces of the collimator are aspheric. A 4-in-1 fiber combiner is fabricated using the AgI/Ag single-mode hollow-core fiber. The fiber has no end face reflection loss and low transmission loss. The experimental results show that the single-mode fiber coupling efficiency is 68%. After the fiber coupling, the beam propagation quality factor M² is 1.2. In addition, the power combining of four QCLs in the wavelength band of 7.6-7.8 μm is realized. When the input power is 2.27 W, the combined output power is 1.5 W. The beam combining efficiency is 66%. The transmission quality factors of the combined beams are MX2=2.67 and MY2=2.56. The low-loss working band of the fiber combiner ranges from 7 to 15.5 μm. The output optical power can be further increased by increasing the number of QCLs in the beam combining, which provides an effective way to expand the output power and wavelength range in the long-wave infrared wavelength band.

Ultra-fast pulse fiber lasers are extensively employed in fields such as fiber optic communication, medicine, and precision material processing due to their compact structure and high beam quality. Lock mode technology is an effective method for achieving ultra-short pulses. Actively mode-locked lasers introduce active modulation devices into the laser cavity and adopt external modulation signals to change the optical signal characteristics, achieving a laser mode-locked pulse output. They feature flexibility, controllability, and stable output pulses. In recent years, graphene has been extensively studied due to its excellent electro-optical properties. Research has shown that an external electric field can alter the Fermi level of graphene to achieve light absorption modulation. Therefore, graphene-based electro-optic modulators have the potential to achieve actively mode-locked lasers. We present the construction of an efficient and high-speed graphene all-fiber mode-locked device, which achieves high-speed adjustment of graphene's optical performance with low modulation power consumption and high modulation efficiency.

The device is composed of graphene, single-mode optical fiber, polydimethylsiloxane (PDMS), and silver (Ag) film, forming a capacitive device (GCD) structure. By selecting glass as the substrate and leveraging magnetron sputtering technology to deposit 50 nm silver on the glass as the bottom electrode, silver has sound conductivity, which is beneficial for reducing the device resistance. The insulation layer is a spin-coated 200 nm PDMS layer, and the thinner insulation layer can effectively reduce the capacitance value of the device. Meanwhile, hydrofluoric acid (HF) with a concentration of 20% is utilized to corrode standard single-mode optical fibers to 15 µm with a corrosion length of 5 mm, and the corroded single-mode fiber is transferred to PDMS. The device is placed in a UV ozone cleaning machine (multi-frequency, CCI UV250-MC) for 10 minutes to improve the hydrophilicity of the insulation layer PDMS. There is graphene dispersion with a selected concentration of 0.1 mg/mL (Nanjing Xianfeng Nanomaterial Technology Co., Ltd., XFZ20 dispersion). Graphene solution is dropped onto the optical fiber, dried, and then inkjet printed with a silver electrode layer on the device using a microelectronic printer (Power Supply Technology Co., Ltd., MP1100). Finally, the prepared device is connected to the circuit board using silver wire. The GCD device is connected between the isolator and polarization controller by fusion, and a spectrum analyzer, digital oscilloscope, power meter, autocorrelator, and spectrum analyzer are adopted to record the locked pulse signal, including spectrum, pulse width, repetition rate, and output power.

The GCD device is connected to the fiber laser system and a pump power of 80 mW is employed for actively mode-locked experiments. At a pump power of 80 mW, the AC signal amplitude increases from 0 V to 5 V, and the average output power decreases from 1.328 mW to 1.130 mW. After calculation, the insertion loss of the device increases from 1.54 dB to 2.46 dB (Fig. 4). Subsequently, a periodic AC signal (12.2 MHz) that is consistent with the resonant frequency of the laser cavity is applied to the graphene device. Under low voltage amplitude, the control of the graphene Fermi level is limited, resulting in a limited range of dynamic changes in the absorption of graphene devices. Therefore, unstable mode-locked pulse signals are observed. When the voltage amplitude increases to 5 V, the most stable mode-locked pulse signal is observed to achieve active mode locking of the laser. The narrowest pulse width of the mode locking signal is 298 ps (Fig. 5). Meanwhile, by increasing the modulation frequency to twice the resonant frequency of the laser cavity (24.4 MHz), the optical signal inside the cavity undergoes frequency doubling oscillation under the graphene device control, leading to harmonic mode-locked operation. The mode-locked pulse signal is slightly unstable, possibly due to the insufficient response speed of graphene to support the migration time of charge carriers at higher modulation rates, resulting in modulation depth changes in the device. This is also the reason for the wider pulse width of 315 ps, corresponding to a frequency of 24.4 MHz, which achieves active repetition frequency control in mode-locked lasers.

We introduce an actively mode-locked fiber laser based on a graphene all-fiber structure. It combines single-mode optical fibers that have undergone lateral corrosion treatment with graphene capacitor structures and utilize the evanescent wave coupling effect of optical fibers to interact with graphene, achieving efficient modulation. Meanwhile, by utilizing the inherent high carrier mobility of graphene, high-speed adjustment of optical performance can be achieved with lower modulation power consumption. The experimental results show that under a modulation signal of ±5 V, the fiber laser obtains controllable repetitive mode-locked pulses at a fixed pump power of 80 mW, with frequencies of 12.2 MHz and 24.4 MHz respectively. The corresponding mode-locked pulse widths are 298 ps and 315 ps respectively, and the laser center wavelength is 1558 nm. Meanwhile, by changing the amplitude of the AC signal (0-5 V), the average output power of the laser can be adjusted within the range of 1.328 mW to 1.130 mW. The research results provide references for achieving low-power and integrated actively mode-locked lasers, and have practical significance for developing efficient and integrated actively mode-locked laser systems.

After decades of development, mode-locked fiber lasers can provide laser pulses with high coherence, high pulse energy, and controllable pulse width and repetition rate. Mode-locked pulsed lasers can play a key role in some specific research areas. For instance, in biomedicine, lasers are used as light sources to perform coherent tomography imaging and the information of the samples under test can be collected and recorded at the same time. However, in the process, the signals of some substances with similar excitation wavelengths can interfere with each other, thus affecting the measurement results. Therefore, the development of wavelength tunable mode-locked lasers to improve spectral resolution is of great significance to the research in this field. We study the rapid tuning of the center wavelength of a narrow-spectrum passive mode-locked ytterbium fiber laser based on fast acousto-optic filtering technology. Combining fast acousto-optic filtering technology, we obtain a stable mode-locked pulse with a center wavelength tuning function. To investigate the reconstruction process of laser pulses during intracavity filtering and confirm the reliability of this technology, we record the real-time reconstruction process of laser pulses during the tuning of the center wavelength. We hope that our research can provide a reliable solution for applications requiring high spectral resolution.

The laser consists of a laser cavity and a two-stage amplifier. The fiber cavity consists of a semiconductor saturable absorption mirror (SESAM), a wavelength division multiplexer (WDM), a 40 cm long ytterbium-doped fiber (CorActive Yb406, YDF), a 90∶10 fiber coupler (90∶10 OC), a collimator, and a λ/2 waveplate (HWP). It is composed of acousto-optic tunable filter, reflect mirror, and piezoelectric ceramic transducer (PZT). The piezoelectric ceramic is combined with a mirror to lock and stabilize the output laser repetition rate by adjusting the length of the phase-locked loop feedback. The phase-locked loop is composed of a photodetector (PD), an RF amplifier, a bandpass filter, a mixer, a signal source, a low pass filter, and a proportional integral derivative (PID). The voltage intensity of the externally modulated signal can alter the intracavity pumping energy. The rising edge of the modulated signal can be recognized by the acousto-optic tunable filter driver and used to switch the filter wavelength. The arbitrary waveform generator drives the acousto-optic tunable filter and laser semiconductor with the edited modulation signal, such that the center wavelength of the laser can be tuned at high speed while maintaining the mode-locked state. To explore the pulse conversion process in the cavity during wavelength switching, a part of the laser after the first stage amplification is fed into the dispersion compensation fiber, and the stretched optical signal is converted into an electrical signal through a photodetector and transmitted to a high-speed oscilloscope. The real-time observation of the laser pulse reconstruction process can be realized by generating signals through the external arbitrary signal generator, and simultaneously modulating the pump working current of the cavity and the wavelength switching of the acousto-optic tunable filter.

The parameters of the laser are tested (Figs. 2 and 3), and the wavelength tuning ability and frequency stability of the laser are verified (Fig. 4). The phase noise and time jitter of the locked pulse are significantly improved. The time jitter of the locked laser is 9.58 ps, and the phase noise at 10 Hz is -72 dBc/Hz. The information on the pulse reconstruction process of the laser in the state of high pump power and the operation of the single pulse after adjusting the external modulation signal is recorded (Figs. 5 and 6). The information shows the pulse reconstruction time and spectrum of the wavelength tuning process. The spectral stability and the highest wavelength tuning speed can be defined. Also, the result of the dispersive Fourier transform test proves that by editing the external modulation signal to change the internal pump energy of the laser cavity and the filtering band of the acousto-optic tunable filter, a reliable mode-locked fiber laser with high-speed tuning of the center wavelength can be obtained.

We study the rapid tuning of the center wavelength of a narrow-spectrum passive mode-locked ytterbium fiber laser based on fast acousto-optic filtering technology. The narrow-spectrum mode-locked fiber laser system has an output power of 200 mW, a pulse width of 5.87 ps, a repetition rate of 40.874 MHz, and a spectral bandwidth of 0.15 nm. By programming the RF signal to drive the acousto-optic tunable filter, a stable mode-locked pulse with a center wavelength tunable in the range of 1016-1042 nm can be obtained. To investigate the reconstruction process of laser pulses during intracavity filtering, we employ the dispersive Fourier transform technology to visualize the real-time reconstruction process of laser pulses during the tuning of the center wavelength, and the results confirm that the highest central wavelength tuning frequency of the laser is about 5 kHz.