This work presents a combined modeling and experimental study of radiation-induced loss in thulium-doped fiber amplifiers (TDFAs) under gamma-ray irradiation. Within the experimental dose range of 0-5 krad(Si), the attenuation of both pump and signal lights increases approximately linearly with total dose, with the pump light exhibiting greater sensitivity. Consequently, the output power decreases approximately linearly with dose. Good agreement between the modeled and measured output power is achieved using key parameters extracted from experiments. Furthermore, fibers with higher Ge/Al molar ratios exhibit enhanced radiation resistance, guiding the design of radiation-hardened thulium-doped fibers.

In this work, a high average power and high single pulse energy Ho:YAG master oscillator and power amplifier laser system with a bulk Tm:YLF dual-pump structure was demonstrated at room temperature. At pulse repetition frequency of 3 kHz, as high as 260 W average power, corresponding to a single pulse energy of 86.7 mJ, was experimentally produced with optical-to-optical efficiency of 43.3% and pulse width of 28.1 ns. In addition, the laser central wavelength is ∼2090.4nm, and the beam quality factor M 2 is less than 1.8.

The self-Q-switching (SQS) laser performance on Nd-doped crystal at 1.3 mu m has been reported for the first time, as far as is known. On Sr0.7Nd0.05La0.25Mg0.3Al11.7O19 (Nd:ASL) disorder crystal, a SQS dual-wavelength laser at 1339.9 and 1370.3 nm with output power up to 1.65 W was obtained under an absorbed pump power of 10.13 W with slope and optical-to-optical efficiencies of 22.3 % and 16.3 %, respectively. Furthermore, an on-surface optical axis quartz birefringent filter (BRF) was inserted in the V-folded cavity to tune the laser wavelength. Lasers at 1306.4, and approximately 1340, 1370, or 1391 nm were obtained. The experimental results indicated that sigma polarization direction Nd:ASL is capable of producing dual-wavelength lasers at 1339.9 and 1370.3 nm, which was potential to be employed as the source of THz radiation. Besides, Nd:ASL crystals are enable to generate tunable lasers near 1370 and 1391 nm.

To investigate the origin and suppression mechanisms of relaxation-oscillation spikes (ROS) in quasi-continuous-wave thulium-doped fiber lasers (QCW-TDFLs), we develop a steady-state rate-equation model including cross-relaxation, amplified spontaneous emission (ASE), and bidirectional signal propagation. We numerically simulate the ROS by varying key parameters, including active fiber length, doping concentration, output coupler reflectivity, pump power, and duty cycle. Corresponding experiments confirm the simulation trends. Experimental results show that longer and higher-doping-concentration active fiber, higher output coupler reflectivity, and shorter pump low-level duration effectively mitigate the ROS. Further suppression of relative intensity noise is achieved by incorporating an active fiber outside the oscillator cavity.

The detailed spectroscopic properties and tuning lasers of Nd:GYSAG ((Gd0.6Y0.4)3Sc2Al3O12) crystal were reported for the first time. According to the spectroscopic properties, the discrete tunning laser operation (1048-1120 nm) with a birefringence filter was conducted in a three-mirror folded cavity. Due to the mixing of Y3+ ions giving rise to a greater disorder, the spectra of Nd:GYSAG were inhomogeneous broadened, resulting in similar emission cross-sections at different separate peaks, which enables multi-wavelength laser performance more efficient. In comparison to Nd:GSAG crystal, the higher output power and efficiency with more tunable wavelengths laser were obtained in Nd:GYSAG. The results indicated that Nd:GYSAG was an available laser gain media to emerge multiple wavelengths.

The large-mode-area thulium-doped fiber (LMA-TDF) with a nested-ring doping profile has a smaller doped area and a lower overlap with LP01 mode, which can effectively suppress long-wavelength (>1940 nm) amplified spontaneous emission (ASE) combined with fiber bending techniques. Thulium-doped fiber oscillators (TDFOs) based on this nested-ring doping scheme have advantages in achieving high signal-to-noise ratio (SNR) short-wavelength laser output with low heat load. This paper focuses on optimizing the configuration parameters of the nested-ring doping scheme. It evaluates its impact on the output characteristics of the oscillator in the presence of a backward-propagated laser. Results show that using a nested-ring doping scheme of Delta(r) over bar = 0.1, (r) over bar (c) = 0.95 (Delta(r) over bar and (r) over bar (c) represent the normalized width and the center-normalized radius of the doping ring, respectively) with a high thulium dopant concentration can achieve the best balance between output power, SNR, and thermal management. Under 200 W co-pumping, the oscillator based on this doping scheme achieved 111 W signal power output, a maximum SNR of 63 dB and the lowest maximum heat load of 27 W m(-1), while exhibiting the minimum 1908 nm power drop ratio of 0.0037 W mW(-1) when faced with 2000 nm backward-injected laser. An output power exceeding 1 kW at 1908 nm can be theoretically expected by employing the oscillator based on this optimal doping scheme.

2-3 mu m mid-infrared lasers have important applications in medical, environmental monitoring, and military fields. The Cr & ratio; ZnSe and Cr & ratio; ZnS laser crystals have the advantages such as wide absorption and emission spectrum, large absorption and emission cross-sections at room temperature, and their applications in 2-3 mu m mid-infrared lasers have received widespread attention. The characteristics of Cr & ratio; ZnSe and Cr & ratio; ZnS laser crystals and the research progress of their corresponding mid-infrared lasers are introduced, and the perspectives and future development are also proposed.

Objective A high-power thulium-doped fiber laser with a wavelength in the range of 1900-2100 nm has broad application prospects in many fields, including absorption spectrum diagnosis, biomedical treatment, LIDAR, plastic processing, and mid-infrared laser pump source. Specifically, thulium-doped fiber laser systems have achieved a high power output in the kilowatt class using 793 nm laser diode (LD) pumps. However, the low optical conversion efficiency of 793 nm LD pumping mode causes problems such as serious thermal effects and refractive index distortion during high-power operation, which affect the beam quality of the output laser. Therefore, it is important to investigate the beam-quality variation characteristics of high-power thulium-doped fiber lasers under different conditions. Methods In our previous study, a beam quality prediction model was established based on the finite-difference beam propagation method. A complete simulation link, from solving the rate equation to predicting the output beam quality of a fiber laser, was realized. The model was applied to thulium-doped fiber laser systems with different power levels reported by other researchers and compared with the experimental measurement results; thus, the prediction accuracy of the model was verified. In this study, the model was used to further simulate and analyze the influence of different parameters, such as the input pump power, forward and backward pump ratio, active fiber absorption coefficient, and seed power, on the output beam quality of the laser. Results and Discussions The theoretical simulation and experimental results show that: the beam quality of the laser deteriorates with an increase in input pump power; the backward and forward pump ratio of 1:1 can effectively suppress the internal thermal effect of the laser and is conducive to obtaining the laser output with high beam quality (Fig. 4 and Fig. 5); the active fiber with low absorption coefficient can reduce the temperature inside the fiber and improve the beam quality of the output laser (Fig. 6 and Fig. 7); with the increase of seed power, the beam quality of laser output deteriorates slightly (Fig. 9); and compared with increasing the seed power, changing the pumping ratio has a more obvious effect on the refractive index, and thereby, on the beam quality (Fig. 5 and Fig. 9). The variation in the beam quality predicted by the theoretical model is in good agreement with the experimental result, and the average deviation is approximately 10 degrees o. Based on the aforementioned results, a 15 W seed and 1:1 forward-to-backward pumping ratio in the amplification stage were employed to experimentally verify the beam quality characteristics at higher laser power levels. At a wavelength of 1940 nm, under an input pump power of 1131 W, a laser output power of 513 W with a beam quality factor M-2 of 3.04 is experimentally achieved, and the deviation from the theoretically predicted M-2 value 2.8 is approximately 8.6 degrees o (Fig. 12 and Fig. 14). Conclusions The characteristics of the laser power output, refractive index change, and beam quality change under different conditions are analyzed via theoretical simulation. Theoretical simulation results show that the beam quality of the laser deteriorates with an increase in the input pump power and exhibits a nearly linear growth trend. A backward-to-forward pump ratio of 1:1 can better suppress the internal thermal effect of the laser and is conducive to obtaining a laser output with a high beam quality. Under the condition of the same total gain, an active fiber with a low absorption coefficient can reduce the temperature inside the fiber and improve the beam quality of the output laser. With an increase in the seed power, the laser beam quality deteriorates slightly. Compared with increasing the seed power, changing the pump ratio and using an active fiber with a low absorption coefficient have more obvious effects on the refractive index and thus on the beam quality. Based on a 200-W thulium-doped fiber laser system developed by us, the variation in beam quality with the aforementioned parameters is revealed by experiments. The experimental results are in good agreement with the predicted law of beam quality, with a deviation of approximately 10 degrees o. Based on the aforementioned findings, to obtain the output of high-power and high-beam-quality thulium-doped fiber lasers, the oscillator output with higher power should be preferred as the seed light, a 1 & ratio;1 forward-to-backward pump ratio should be adopted, and an active fiber with a low absorption coefficient should be selected. Based on the aforementioned theoretical simulations and experimental results, the beam-quality characteristics at higher laser power levels are verified. A 15 W seed light is selected, a 1 & ratio;1 forward-to-backward pump ratio is selected for the amplifier stage, and a pedestal thulium-doped fiber with a core ratio of 25/400 mu m and an absorption coefficient of 4.2 dB/m is adopted. Under a total pump power of 1131 W, laser output with a wavelength of 1940 nm, average power of 513 W, and beam quality M-2 of 3.04 is experimentally achieved. The measured M-2 of 3.04 shows an approximate deviation of 8.6 degrees o from the theoretical prediction value of 2.8.

In order to meet the urgent need of infrared search and track applications for accurate identification and positioning of infrared guidance aircraft, an active-detection mid-wave infrared search and track system (ADMWIRSTS) based on "cat-eye effect" was developed. The ADMWIRSTS mainly consists of both a light beam control subsystem and an infrared search and track subsystem. The light beam control subsystem uses an integrated opto-mechanical two-dimensional pointing mirror to realize the control function of the azimuth and pitch directions of the system, which can cover the whole airspace range of 360 degrees x90 degrees. The infrared search and track subsystem uses two mid-wave infrared cooled 640x512 focal plane detectors for co-aperture beam expanding, infrared and illumination laser beam combining, infrared search, and two-stage track opto-mechanical design. In this work,the system integration design and structural finite-element analysis were conducted, the search imaging and two-stage track imaging for external scenes were performed, and the active-detection technologies were experimentally verified in the laboratory. The experimental investigation results show that the system can realize the infrared search and track imaging, and the accurate identification and positioning of the mid-wave infrared guidance, or infrared detection system through the echo of the illumination laser. The aforementioned work has important technical significance and practical application value for the development of compactly-integrated high-precision infrared search and track, and laser suppression system, and has broad application prospects in the protection of equipment, assets and infrastructures.

Mid-infrared lasers in the range of 3-5 mu m have wide applications in fields such as medical, national defense, and sensing. Optical parametric oscillation (OPO) technology is currently one of the main technological approaches for achieving mid-infrared laser output. Pulsed polarization-maintaining thulium-doped fiber lasers have become one of the parametric oscillation technology based on their pumping of ZnGeP2 (ZGP) crystal are introduced, and the summary and prospect are given.

As one of the most critical applications, high peak power, all-fiberized nanosecond Tm-doped fiber amplifiers (TDFA) are extensively used as the pump source for ZnGeP2 optical parametric oscillator (ZGP-OPO) system due to their compact structure and flexibility. However, amplified spontaneous emission (ASE) and modulation instability (MI) are the main constraints on the power scaling of nanosecond TDFAs. In this work, a method of modulating the content of the MI from the seed is proposed to suppress the ASE in the 2043 nm amplifier. Experimental observations indicate that with consistent MI noise content in the seed spectrum, an increase in the seed laser power correlates with stronger ASE in the amplifier. To address this, reducing the fiber link length before the main amplifier is conducted to enhance the spectral purity of the seed laser rather than its power, thereby effectively mitigating ASE and MI in the amplifier. The authors simplified and designed a single-stage amplification configuration that employs a high spectral purity seed laser to verify this approach. The singlestage 2043 nm amplifier achieved pulsed output with 186 ns pulse width and 1 mJ energy at a 50 kHz repetition rate. The signal-to-noise ratio (SNR) is more than 45 dB. Compared to the two-stage amplifier with higher seed laser power, the single-stage amplifier exhibits a similar slope efficiency, and shows an improvement of 3 dB in the signal-to-ASE noise ratio (SANR) and 10 dB in the signal-to-MI noise ratio (SMNR) at maximum output. This novel ASE suppression approach potentially facilitates the achievement of high energy and SNR output in nanosecond TDFAs using a compact all-fiber configuration.

This study realizes a fiber laser based on a ring cavity, directly pumped by a 793 nm laser diode (LD) and operating in the 2 mu m waveband with tunable wavelength and narrow linewidth. A tunable fiberized Fabry-Perot (F-P) filter is inserted into the ring cavity to obtain wide wavelength tuning. By using an unpumped polarization-maintaining thulium-doped fiber as a saturable absorber, a dynamic narrowband grating is generated in the cavity to achieve narrow linewidth laser output. After optimizing the lengths of gain fiber and saturable absorber fiber, the obtained output power of the laser reaches up to 70 mW and the wavelength tuning range is as high as 98 nm, and the tuning range is 1950-2048 nm. The average spectral linewidth of the narrow linewidth laser is 0. 86 GHz in the wavelength tuning range. This laser can serve as a seed source for fiber amplication to obtain a high-power 2 mu m laser with narrow linewidth and wide tunable wavelength.

A Ho:YAG amplifier in-band pumped by a home-built high power and narrow-linewidth thulium-doped fiber laser (TDFL) at 1907.5 nm is demonstrated. Using a single-end-pumped configuration with the TDFL, a maximum output power of 135 W is achieved at 2090.48 nm of the Ho:YAG amplifier. The amplifier delivers a maximum pulse energy of 67.5 mJ with a pulse width of 30 ns at a repetition rate of 2 kHz, corresponding to a peak power of 2.25 MW. The slope efficiency relative to the absorbed pump power reaches 61.9

A Cr & ratio; ZnS main oscillation power amplifier (MOPA) laser experimental setup was designed and constructed. The oscillator adopted a four mirror folded cavity structure and achieved a continuous laser output with a power of 350 mW and a central wavelength of 2393 nm at a pump power of 3. 4 W. By using a reflective grating, a tunable laser was obtained with a tuning range of 550 nm from 2075 nm to 2625 nm. The spectral full width at half maximum was 1. 2 nm at the peak wavelength of 2450 nm. The oscillator was used as a seed source, the amplification characteristics of domestically produced Cr & ratio; ZnS crystals with different doping concentrations were experimentally studied and contrastive analysis. Among them, the Cr & ratio; ZnS crystal with the absorption coefficient of 3. 3 cm(-1) and the crystal dimensions of 2 mmx 4 mmx 8 mm exhibited the best gain output, achieving an amplification factor of 3.3. By using a two-stage amplifier, the small-signal tunable seed source was amplified. A maximum output power of 51. 7 mW was achieved with a central wavelength of 2275 nm.

Mid-infrared (MIR) lasers have important applications in atmospheric detection, materials processing, and laser medicine. However, the robustness of high-power MIR lasers is constrained by the low laser-induced damage resistance of optical coating components. This study, investigated laser-induced damage mechanisms of hightransmittance (HT) and percent-transmittance (PT) coating components susceptible to optical damage in highpower MIR lasers under 2.1 mu m laser irradiation. Both HT and PT components showed the damage induced by significant thermal effects. Different damage morphologies were identified on the PT and HT coating surfaces of 2.1 mu m PT components, characterized by dome-shaped bulges and funnel-shaped pits due to nodule defects and nano-absorptive precursors, respectively. The analysis of substrate effects on MIR multiband HT coatings (HT 1.9-2.2 & 3.5-4.8 mu m) damage revealed that CaF2-based coatings showed higher susceptibility to mechanical interfacial damage compared to fused silica-based coatings, primarily attributed to higher coefficient of thermal expansion (CTE) of CaF2, which induced significant coating-substrate interfacial stress mismatch. These findings provide critical insights for the fabrication of durable MIR optical coatings, paving the way for advancing highpower MIR lasers with enhanced robustness.

The space radiation tolerance characteristic of rare earth ion-doped gain fibers(also known as active fiber)is one of the main factors restricting the space application of fiber lasers.The study designs and develops a multi-parameter online measurement system for space radiation tolerance characteristics of batch gain optical fibers,which can realize the simultaneous measurement of four gain fiber samples with two different core diameters combining data processing.The measurement parameters include absorption spectrum/absorption coefficient,emission spectrum,optical-to-optical efficiency,laser power,beam quality,and so on.The proposed system adopts a distributed structure,which eliminates the influence of performance changes of other optical components in the fiber laser on the laser output characteristics under ground-based simulation space radiation environment,and improves the measurement accuracy and the efficiency of evaluation work.

High-power thulium-doped fiber lasers have various promising applications. Resonant pumping by 1.9 μm laser is a hopeful method to power scale a 2 μm thulium-doped fiber laser (TDFL) to surpass kilowatt level due to low quantum defect and high optical efficiency. Because of the acrylate coating absorption of 1.9 μm light, resonant pumping is usually realized by all-fiber core pumped or free-space pedestal pumped configurations, which leads to other power constraint challenges. In this work, we ingeniously engineer what we believe to be a novel all-fiber resonant pumping configuration comprising a mode-field matched signal combiner, a piece of pedestal-matched transition passive germania-doped fiber (GDF), and a homemade high refractive fiber Bragg grating (FBG) on the core of TDF. This architecture confines pump light within the pedestal waveguide while maintaining signal laser propagation through the core, effectively resolving 1.9 μm cladding leakage-induced thermal effect. A streamlined validation experimental setup was developed to obtain up to 80.9% slope efficiency at 2.02 μm by directly injecting a 1.94 μm laser into the 30 μm pedestal of a 10/30/130 μm TDF. This all-fiber pedestal resonant pump structure can be utilized to further establish a high-power monolithic oscillator at 2 μm waveband for flexible applications.

The damage effect and mechanism of laser irradiation on long-wave focal plane array (FPA) GaAs/AlGaAs quantum well infrared photodetector (QWIP) were preliminarily explored by using numerical simulation and experiment methods. Piecewise functions were employed to simulate the complex boundary structure of the QWIP, enabling the numerical simulation investigation of mono-pulse, nano-second, long-wave infrared laser irradiation damage effect on the QWIP. The highest QWIP temperature, the highest surface temperature and the maximum circumferential thermal stress were analyzed in relation to laser energy density. The pulse average energy density thresholds of thermal decomposition damage, melting damage and thermal stress-induced damage were theoretically obtained. Preliminary experiments were then conducted by using a mono-pulse, nano-second, 7.2 mu m all-solid-state long-wave infrared laser. The experimental results revealed a point-shape damage in QWIP response measurement after the irradiation by a laser pulse of average energy density 1.30 J cm-2, due to the decomposition of GaAs. At a higher average energy density 5.42 J cm-2, both melting and stress-induced damages appeared, with the damage morphology predominantly influenced by stress-induced damage, resulting in the occurrence of blind pixels or the losing of pixels. Furthermore, at laser pulse average energy density 12.48 J cm-2, line-shape damage of the QWIP was observed.

We reported on tunable laser operations in a disordered Nd-doped strontium-lanthanum aluminate (Nd:ASL) crystal, corresponding to the formula Sr1-xNdyLax-yMgxAl12-xO19. We conducted tunable laser experiments in a V-fold cavity with a birefringence filter within the resonator for the first time. The Nd:ASL laser operating at three peak wavelengths (1050, 1062, and 1074 nm) was demonstrated. The results indicated that the wavelength tuned in two separate ranges: 1049.59-1054.43 nm and 1059.71-1078.18 nm. The multi-wavelength feature was also observed in the experiments.