# Performance Studies of High-Power Optical Parametric Oscillators Pumped by a Pulsed Fiber Laser
## 1 Introduction  
Due to excellent penetration ability in atmospheric and strong molecular absorption, high-power mid-infrared (MIR) laser plays a major role in scientific and technological applications, such as remote sensing [1], mass spectrometry imaging [2], light detection and ranging (LIDAR) system [3], medical diagnostics [4] and environmental monitoring [5]. Compared with quantum-cascade lasers [6] and pumping metal-ion-doped crystal technology [7], optical parametric oscillators (OPOs) are more convenient for realizing high output power and broad wavelength tuning. However, their captivity of high output power is limited by the photorefractive damage threshold of the nonlinear crystals and the Manley-Rowe relations [8]. A frequently used nonlinear crystal is periodically poled lithium niobate (PPLN), which has a high effective nonlinear coefficient [9]. Its damage threshold can be significantly enhanced by doping magnesium-oxide (MgO) [10]. Periodically poled magnesium-oxide-doped lithium niobate (MgO:PPLN)-based OPO with tens of watts of MIR power [11, 12] and hundreds of milli joules of MIR energy [13] have been demonstrated. Compared to its excellent performance in the shorter MIR band, the MgO:PPLN-based OPO's capacity for high output power at a longer wavelength range (\(>\)3700 nm) is much more challenging. This is due to the increasing absorption coefficient of MgO:PPLN with wavelength increase. Combined with intra-cavity difference frequency generation (DFG) through a monolithic a periodically poled magnesium oxide doped lithium niobate (MgO:APLN), the OPO realized the highest idler laser power exceeding 4 W at \(\sim\)3800 nmunder the pump power of \(\sim\)25 W from a linearly polarized, pulsed ytterbium-doped fiber laser (YDFL) [14; 15]. The crystal without periodical poling limits the wavelength tunning at a narrow range of 3.76-3.82 \(\upmu\)m because cascaded parametric conversion is realized at a specific temperature range. Using two optical parametric amplifiers (OPAs), the MIR laser at 3800 nm with a maximum output power of 4.69 W and beam quality M\({}^{2}\) factor of 1.82 was obtained [16]. Both the two amplification experiments require cavity mirrors, temperature controllers, and crystals. The complex cavity increases costs and reduces reliability, which limits the popularization of OPOs in the fields of engineering and scientific research. With the rapid development of high-power pump source radiating at \(\sim\)1 \(\upmu\)m, tens of watts power of OPOs can be easily realized. Using a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser with a power of 104 W, the output power of 16.7 W at \(\sim\)3.8 \(\upmu\)m can be obtained. The M\({}^{2}\) factors were 2.03 and 5.89 in the parallel and perpendicular directions, respectively [17]. Furthermore, the output power was able to reach 22.6 W by increasing the pump power to 150 W, and the M\({}^{2}\) factors in the two axes were 1.74 and 4.86, respectively [18]. However, there is a huge difference of the M\({}^{2}\) factors between the two axes because the MgO:PPLN was pumped by an elliptical beam. In addition to the hundred-watt pump power, the high output power in Refs. [17; 18] can be attributed to the high optical-optical conversion efficiency. The conversion efficiency can be improved significantly by using pump-double-pass configuration because the pump beam participates in frequency conversion two times. Using the configuration and optimizing the output coupler curvature radius, pump beam waist radius, and cavity length, a miniaturized idler-resonant OPO could afford a maximum power of 5.84 W at 3.76 \(\upmu\)m (conversion efficiency \(>\) 14.0%) with M\({}^{2}\) factors of 1.57 and 1.49 along the horizontal and vertical directions, respectively [19]. Furthermore, after optimizing the parameters including the pump repetition rate, pulse width, and waist diameter, a signal-resonant and pump-double-pass OPO generated 10.82 W at 3.754 \(\upmu\)m with a conversion efficiency of 13.68% [20].