1.5?1.8 μm and 3?5 μm infrared lasers are widely used in free-space optical communication, trace gas monitoring, environmental pollution monitoring, and biomedicine. Infrared lasers can be obtained using quantum cascade, fiber, solid-state, and Raman lasers. Compared with these methods, an optical parametric oscillator (OPO) can be used to achieve an infrared laser with a wider tuning range, higher power, and more stable operation. Based on the resonance of the pump, signal, and idler laser in the OPO cavity, the OPO can be referred to as singly resonant OPO (SRO), doubly resonant OPO (DRO), or triply resonant OPO (TRO). Compared with DRO and TRO, SRO requires only that the signal (or idler) light resonates in the cavity, has a relatively simple design, and no external servo system locking is required to obtain a stable, high-power output. Therefore, the output characteristics of a high-power SRO are investigated in this study.

First, a theoretical analysis is conducted on the influence of the SRO cavity length and nonlinear crystal thermal lens effect on the stability of the resonant cavity under standing-wave and ring-cavity structures. To ensure a stable operation of the SRO within significant changes in the resonant cavity parameters, the cavity length corresponding to |(A+D)/2|=0 is selected when the SRO is designed, that is, the standing wave cavity length is 54 mm, and the ring cavity length is 516 mm. According to the focusing factor selected for the SRO cavity, the waist spot of the signal light at the center of the MgO∶PPLN crystal is calculated to be 59.5 μm. Subsequently, based on the design of the SRO resonant cavity structure, the laser output characteristics of different SRO cavity types are theoretically analyzed. The theoretical analysis reveals that the pump light in the standing-wave cavity SRO passes through the nonlinear crystal twice; therefore the power gain of the signal light during forward and backward transmission must be considered simultaneously, whereas the pump light in the ring-cavity SRO passes through the nonlinear crystal in a single pass, and the parametric interaction during backward transmission is not considered. The threshold pump power and output power of the signal and idler light from the standing-wave cavity and ring-cavity SROs are calculated. Finally, the two mirror standing-wave cavity SROs and four mirror ring-cavity SROs based on the MgO∶PPLN crystal pumped by a high-power continuous-wave single frequency 1.06 μm laser are constructed, and the relationship between the output power of the signal and idler light with the pump power, as well as the power fluctuation and frequency drift of the signal and idler light are studied.

By controlling the temperature of MgO∶PPLN from 30 ℃ to 65 ℃, the signal wavelength can be tuned from 1550.03 nm to 1561.38 nm, and the corresponding idler wavelength can be tuned from 3394.7 nm to 3340.13 nm. When the temperature of the MgO∶PPLN crystal is controlled as 40 ℃, the signal and idler wavelengths of the SRO are 1.553 μm and 3.378 μm, respectively. The threshold pump power of the standing-wave cavity SRO is 3.2 W, and at a pump power of 14.2 W, the signal and idler powers are 5.2 W and 2.2 W, respectively. The threshold pump power of the ring-cavity SRO is 7.2 W, and at the pump power of 25 W, the signal and idler powers are 8.1 W and 3.6 W, respectively. The measured value of the ring-cavity SRO output power is in good agreement with the theoretical prediction result (Fig. 6). When the pump power is less than 15 W, the measured standing-wave cavity SRO output power agrees well with the theoretical prediction result; however, when the pump power is greater than 15 W, the measured standing-wave cavity SRO output power deviates significantly from the theoretical prediction result ( Fig. 6). According to the theoretical analysis, when the pump power is 15 W, the resonant signal power in the standing-wave cavity SRO is 260 W. The thermal lens focal length of the nonlinear crystal is 14 mm, and the corresponding stability parameter of the standing-wave cavity SRO is 0.98. An increase in the pump power results in the stability parameter of the standing-wave cavity SRO to be greater than 1; the SRO cannot operate stably, and the output power of the SRO decreases. To obtain a higher output power from the signal and idler lasers, the ring-cavity SRO is a better choice. In addition, when the signal and idler output powers from the standing-wave cavity and ring-cavity SROs are 5.0 W and 2.0 W, respectively, the power fluctuations in the signal and idler light by the standing-wave cavity SRO within 2 h are better than ±2.76% and ±2.53%, and the power fluctuations in the signal and idler light by the ring-cavity SRO within 2 h are better than ±1.24% and ±1.19%, respectively. The long term frequency drift of the signal is better than ±40 MHz and ±28 MHz, respectively.

The influence of the SRO cavity type on the output characteristics at high pump power is investigated in this study. First, a theoretical analysis is conducted on the influence of the SRO cavity length and nonlinear crystal thermal lens effect on the stability of the resonant cavity under the standing-wave cavity and ring-cavity structures. Notably, the ring-cavity SRO can operate stably within significant changes in the resonant cavity parameters. The output characteristics of the SRO are also theoretically analyzed. Second, a two-mirror standing-wave cavity and a four-mirror ring-cavity SROs based on a MgO∶PPLN crystal are experimentally constructed. At a pump power of 14.2 W, the signal and idler output powers from the standing-wave cavity SRO are 5.2 W and 2.2 W, respectively. At a pump power of 25 W, the signal and idler output powers from the ring-cavity SRO are 8.1 W and 3.6 W, respectively. The measured value of the ring-cavity SRO output power agrees well with the theoretical prediction result. The power fluctuations in the signal and idler light from the standing-wave cavity SRO within 2 h are better than ±2.76% and ±2.53%, and the power fluctuations in the signal and idler by the ring-cavity SRO within 2 h are better than ±1.24% and ±1.19%, respectively. The long term frequency drift of signal light from the standing-wave cavity and ring-cavity SROs are better than ±40 MHz and ±28 MHz, respectively. The research results indicate that to obtain a higher output power from the signal and idler lasers, the ring-cavity SRO is a good choice.