Mid-infrared optical frequency combs are widely used in precision spectroscopy, optical frequency metrology, instrument calibration, and other fields. Fiber-type dual-arm structure difference frequency generation (DFG) mid-infrared combs based on mode-locked fiber lasers are currently the primary technology for generating mid-infrared combs. The spectral tuning range and spectral bandwidth are two key indicators of DFG mid-infrared combs. The spectral tuning range is ensured by the wide tuning range of the fundamental frequency pulse, and the spectral bandwidth is associated with the crystal phase-matching acceptance bandwidth and the spectral width of the fundamental frequency pulse. Generally, the fundamental frequency pump pulse is generated by directly amplifying the oscillator output pulse, whereas the fundamental frequency signal pulse is obtained by amplifying and compressing the output pulse of the oscillator and then pumping a highly nonlinear fiber (HNLF) to generate long-wave frequency shift solitons. Although many reports on wide-tunable DFG mid-infrared combs exist, the bandwidth of two-color fundamental frequency pulses is narrow, owing to the limitation of the gain bandwidth of fiber amplifiers, and thus limits the bandwidth of the generated DFG mid-infrared combs. Therefore, the generation of a fundamental frequency pulse with a wider spectrum to obtain DFG mid-infrared combs with larger bandwidths and tuning ranges as well as the design and development of a practical light source device requires further research.

A fully polarization-maintaining 9-cavity fiber laser was used as the pulse source, and the repetition frequency was locked to the rubidium atomic clock through a servo feedback loop. The output of the oscillator was filtered and shaped and further divided into two paths using an optical coupler (OC) after erbium-doped fiber amplification (EDFA-1). It was then amplified by self-similarity fiber amplifiers EDFA-2 and EDFA-3. The EDFA-3 output pulse after being compressed serves as fundamental frequency pump pulse, the EDFA-2 output pulse after being compressed was used to pump HNLF to generate a supercontinuum (SC), and the frequency-shifted solitons were extracted as the fundamental frequency signal pulse. The two-color fundamental frequency pulses were output through the collimator (Co) collimation space, and the polarization state was adjusted by half-wave plates. The mirrors (M) of M1 and M2 were added to the collimator-2 output port to form a time delay line for adjusting the time synchronization of the two-color fundamental frequency pulses. After the two-color fundamental frequency pulses were combined by a dichroic mirror (DM), they were focused on a GaSe crystal by a lens (L1) with a 40 mm focal length to generate a DFG mid-infrared comb. The comb output by L2 collimation after the fundamental frequency light was filtered by a long pass filter (LPF) (Fig. 1). The integration and packaging of the optical combs were performed using a photoelectric separation method.

The average power of the fundamental frequency pump pulse is 485 mW, the center wavelength is 1.57 μm [Fig. 4(b)], and the pulse width is 45 fs [Fig. 4(a)]. The central wavelength of the fundamental frequency signal pulse is 1.85 μm, and the bandwidth is 250 nm [Fig. 5(a)]. The optical comb system was integrated and packaged by photoelectric separation packaging, and a prototype was prepared (Fig. 6). The measured center wavelength of the difference frequency light was continuously tuned in the 8.0‒10.5 μm range. The bandwidth of each tuning band obtained is greater than 1 μm, and the bandwidth of the 9.5 μm band reaches 2.43 μm, indicating that the wider fundamental frequency signal pulse expands the spectral tuning range and bandwidth of the DFG comb. The average power of each tuning band is greater than 240 μW, and the average power of the band with an 8 μm central wavelength reaches 470 μW [Fig. 7(a)]. The average power fluctuation is less than 1.5%, indicating that the power stability of the optical comb is excellent [Fig. 7(b)].

We independently designed and developed a stable broadband and wide tuning range DFG infrared comb. The fiber link was designed with full polarization-maintaining fiber. By locking the repetition frequency of the pulse source and using technologies such as self-similar fiber amplification, soliton compression, and SC generation, the two-color fundamental frequency pulses with center wavelengths of approximately 1.57 μm and 1.85 μm were obtained. An adjustable time delay line was used to precisely control the time synchronization of the two-color fundamental frequency pulses, and the spatial overlap of the two-color fundamental frequency pulses was strictly regulated. Using a GaSe nonlinear variable frequency crystal, the DFG mid-infrared comb output was obtained through the DFG process. The integrated and packaged instrumented mid-infrared comb has a spectral coverage of 7‒13 μm and a maximum spectral bandwidth of 2.43 μm. The design and development of the DFG mid-infrared optical comb offers a base for the development of optical combs for practical applications such as wavelength calibration and multi-component gas detection.