Objective The 3-5 μm mid-infrared laser plays an important role in atmospheric monitoring, medical treatment, and photoelectric countermeasures. The Optical Parametric Oscillators (OPO) as an important means of realizing 3-5 μm laser, has the many advantages such as compact structure, high optical-optical conversion efficiency and widely wavelength tuning range. At present, the nonlinear crystals which achieves 3-5 μm mid-infrared laser are ZnGeP₂ (ZGP), MgO-doped period-polarized LiNbO₃ (MgO:PPLN), etc. However, the ZGP crystals has a strong absorption of laser below 2 μm wavelength, and it is difficult to use 1 μm laser as a pump source. The MgO:PPLN crystals has the advantages of large nonlinear coefficient, widely laser transmission range, and avoids work-off by using quasi-phase matching technology. In this study, the domestic fiber laser and MgO:PPLN crystal was used to realize the domestic mid-infrared laser output based on MgO:PPLN-OPO technology.

Methods The domestic all-fiber nanosecond pulsed lasers used a master oscillator power amplifier (MOPA) structure (Fig.1). The seed source was a Q-switched pulse fiber laser which the repetition frequency, pulse width and output power were 150 kHz, 78 ns, and 1.1 W; The 1st of the power amplifier used Ytterbium-doped double clad fiber (YDF) with a core diameter of 20 μm and the cladding diameter of 130 μm, with an output power of 9.5 W; The 2nd of the power amplifier used YDF with a core diameter of 30 μm and the cladding diameter of 250 μm. The 1 064 nm laser passed the two-stage power amplifier and outputted by an isolated collimator. The OPO system included polarizing beam splitter (PBS), half-waveplate (HWP), Faraday rotators (FR), coupled lens (M₁ and M₂), resonator mirrors (M₃ and M₄), MgO:PPLN crystals (CTL Photonics), oven and beam splitters (M₅). The crystal size of MgO:PPLN was 50 mm×3 mm×2 mm and the polarization periods was 29.5 μm, respectively. The radius of curvature of the input mirror M₁ and the output mirror M₂ was 300 mm (Fig.2).

Results and Discussions With the increase of LD power of the 2nd power amplification, the output power of fiber pulse laser increased linearly. When the LD power of the 2nd power amplification was 115 W, the output power which after the PBS was 43.7 W. At the highest power, the laser pulse width, repetition frequency and output laser center wavelength were 120 ns, 150 kHz and 1 064.3 nm. The spectral width of 3 dB was 3.47 nm and the beam quality factor M² was 1.62. When the signal laser transmission rate of the output mirror are 0%, 10%, 20% and 30%, the threshold pumping power of 3 817 nm laser were 7.2 W, 12.4 W, 16.3 W and 24.5 W, respectively. Under the maximum pumping power, the output power were 5.93 W, 5.24 W, 5.05 W and 4.06 W, respectively (Fig.7). The corresponding optical-to-optical conversion efficiencies were 13.5%, 11.9%, 11.5% and 9.2%, respectively (Fig.8). And the signal laser pulse width were 94.6 ns, 95.8 ns, 89.8 ns and 92.4 ns, respectively (Fig.9). When the temperature was controlled at 30 ℃, the mid-infrared laser and signal wavelengths were 3 817.2 nm and 1 476.2 nm, respectively (Fig.11). The divergence angle of the output mid-infrared laser was measured by the hole-set method. On the maximum power pumped, the divergence angle was less than 12.3 mrad. At the same time, the 15 min instability root mean square (RMS) was 1.97%.

Conclusions A domestic high power mid-infrared laser based on MgO:PPLN-OPO was studied. When the maximum pump power was 43.7 W, the maximum output power of 3 817 nm laser was 5.93 W, and the optical-optical conversion efficiency was 13.5%. The time-domain characteristics of signal laser and depleted pump laser under high power pumping were experimentally studied and analyzed. It was concluded that the decrease of optical-optical efficiency under high power pumping was caused by the back conversion effect and the crystal thermal effect. On the low power pumped, due to the low power density in the cavity, the optical conversion efficiency and the OPO threshold improved by reducing the transmittance of the output mirror; On the high power pumped, the strong back conversion effect and crystal thermal effect in the cavity became the main reasons which reduced the OPO optical-optical conversion efficiency. The output mirror with large transmittance should be selected to improve the OPO optical-optical conversion efficiency and stability under high power pumped. Compared with previous reports, this study adopted a domestically produced wide-spectrum fiber Q-switched pulse laser and MgO:PPLN crystal, which greatly reduced the technical difficulty, development period and cost, and provided a feasible technical path and experimental reference for the development of a domestically produced high-power mid-infrared laser.