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.