# Double-Peaked Mid-Infrared Generation Based on Intracavity Difference Frequency Generation
## 1 Introduction  
The 3-5 \(\upmu\)m mid-infrared laser covers the absorption peaks of many atoms and molecules. It can detect and quantify molecules effectively [1] and with significant application prospects in physical analysis [2, 3], environmental management [4, 5, 6], and infrared countermeasures [7]. Mid-infrared lasers mainly include rare-earth, ion-doped, solid-state, or fiber lasers; gas lasers; quantum cascade lasers (QCLs); and nonlinear frequency conversion techniques [8, 9, 10, 11, 12]. The nonlinear frequency conversion-based mid-infrared optical parametric oscillator (OPO) utilizes a nonlinear crystal to generate a near-infrared and a mid-infrared beam [13, 14, 15]. The near-infrared laser is called the signal beam, and the mid-infrared generation is called the idler beam. The advantage of a nonlinear frequency conversion is to realize the tunable wavelength and conversion efficiency improvement of the mid-infrared generation. It has become one of the essential research directions on mid-infrared generation in recent years [16]. In 2018, [PERSON] et al. [17] demonstrated a high-power CW mid-infrared OPO. This module weighs only 2.5 kg and has a maximum output power of 10.4 W at 2.7 \(\upmu\)m. Meanwhile, the nonlinear frequency conversion and the fiber laser technology make the system optimized in structure and significantly improve output performance [18, 19]. DIAL is one of the essential techniques for detecting the distribution of trace gases and other meteorological elements. DIAL utilizes a transmitter unit to generate two laser beams at different wavelengths and a receiver unit to receive the backscattered echoes of the two laser beams. The critical information, such as the gas concentration, is calculated based on the different absorption coefficients of the gas molecules by the twolaser beams that are located at different wavelengths. DIAL is based on the absorption spectrum of the gas to be measured. Although the structure is more complex and requires high stability and tunability of the laser, the detection sensitivity is high. It is one of the significant development directions in the current gas analysis field, and it is widely used internationally [20; 21]. The OPO is also widely used in DIAL as a mid-infrared generation with a wide range of tuning [22]. In 2020, [PERSON] et al. [23] used a solid-state OPO as lidar to emit laser pulses tunable within 2.5-4 \(\upmu\)m with a repetition frequency of 500 Hz. The monitoring of NO\({}_{2}\) and SO\({}_{2}\) was performed with high efficiency. Moreover, a broadband, multi-wavelength, or multi-peaked laser emission significantly contributes to the development of DIAL [24; 25]. [PERSON] et al. [26] developed a broadband DIAL for methane monitoring. [PERSON] et al. [27] used a multi-wavelength DIAL (MW-DIAL) to simultaneously measure multiple pollutants in the atmosphere, which improved the accuracy of the technique and achieved high efficiency. Since the spectrum of most gas molecules has high absorption lines in the mid-infrared band, it is essential to apply the tunability and compact structure of the OPO to DIAL [28; 29; 30]. There are several techniques to generate a broadband or multi-peaked mid-infrared OPO. The first technique is to utilize nonlinear crystals with unique structures, such as cascaded or double-periodic, to simultaneously fit the phase-matching conditions for two or more pump beams to initiate and generate a multi-wavelength mid-infrared laser [31]. In 2020, [PERSON] et al. [32] generated a mid-infrared laser with instantaneous bandwidth covering 2.8-3.9 \(\upmu\)m by cascading crystals. The multi-peaked idler beams generated by multiple crystals were also simulated. The second technique is designed in the cavity structure. The cascaded OPO or intracavity DFG is designed to convert the signal beam or the idler beam from the OPO into a secondary nonlinear frequency, generating multiple wavelengths of the mid-infrared laser [33]. In 2022, [PERSON] et al.