# Double-Peaked Mid-Infrared Generation Based on Intracavity Difference Frequency Generation
## 2 Experimental Setup and Methods
By analyzing the spectra, if special spectral modulation techniquesare used, the intensity at the central wavelength of the ASE source is reduced before its power amplification. Because of the different absorption coefficients of Yb particles in the PM YDF, the spectrum after power amplification may generate a flat or multi-peaked spectrum due to the modulation depth and bandwidth difference. When the modulation reaches a specific depth, a double-peaked pump source will be generated. A tilted fiber Bragg grating (TFBG) was chosen as the spectral modulation device after comparing various fiber passive devices for gain suppression. With the maturity of fiber grating inscription technology in recent years, TFBG has become an indispensable passive device for many laser systems due to its excellent gain flatness and nonlinear suppression. It provided suitable conditions for realizing broadband pumping sources in this paper. Figure 2a shows the experimental setup of the TFBG-modulated ASE fiber source. By analyzing the ASE source spectrum and power, the transmission spectrum depth of TFBG is adjusted to 16 dB, and the central wavelength is adjusted to 1070 nm. The customized TFBG transmission spectrum is shown in Figure 2c, and the generated ASE source spectrum is shown in Figure 2d, with a double-peaked spectrum and a lower intensity in the short wavelength direction compared to the long wavelength direction. The TFBG-modulated double-peaked ASE source has the advantage of generating two nonlinear frequency conversions and a bandwidth of both peaks, which ensures that the two idler beams can be combined into a double-peaked idler beam. The OPO cavity must also satisfy the signal beam at the same wavelength to generate a double-peaked mid-infrared laser. Among the existing PPLN OPOs, few experimental platforms satisfy the above conditions simultaneously. However, a scheme still exists to meet the experimental requirements: the intracavity difference frequency generation under the combined signal beam. The intracavity DFG can generate a high-conversion-efficiency dual-wavelength mid-infrared laser and combine with a Raman laser to generate a high-power mid-infrared laser, which has unique advantages and prospects in nonlinear frequency conversion technology. Moreover, the intracavity DFG has only one signal beam involved in the parametric conversion process, which is also very consistent with the double-peaked mid-infrared laser generation model. Figure 1: (**a**) ASE fiber source experimental setup. (**b**) The spectrum of ASE source before power amplification. (**c**) The spectrum of ASE source after power amplification. (ISO: isolator; PM YDF: polarization-preserving ytterbium-doped fiber; LD: laser diode). The intracavity DFG utilizes a nonlinear frequency conversion process between a narrow-linewidth 1018 nm beam and a pumped beam in the OPO. By tuning the phase-matching state of the OPO cavity, the parametric conversion process of the two lasers in the OPO cavity is in the region where the signal beams are combined. The OPO and intracavity DFG processes are realized simultaneously at the signal-combining point of the two incident laser beams. A high-conversion-efficiency mid-infrared laser is generated. Figure 3 shows the experimental schematic diagram of the intracavity differential frequency of the double-peaked ASE fiber source. The first part is the pump source. The seed source, the spectral modulation device (TFBG), and the power amplification system are inserted with an ISO to prevent the return beam. The second part is the DFG connection system. The ASE fiber source and a narrow-linewidth high-power 1018 nm laser are injected into the wavelength division multiplexer (WDM). Moreover, an optical collimator (OC) is utilized to inject the combining beam into the OPO system. The other output port of the WDM is connected to a spectrometer that monitors the spectrum of the ASE fiber source with the 1018 nm laser to prevent nonlinear effects from causing damage to the OPO. The third part is the intracavity DFG system.