Mid-infrared (MIR) optical frequency combs exhibit potential advantages in many applications; however, owing to various limitations, it is difficult to directly generate optical combs in the MIR band. The near-infrared-band optical frequency comb, combined with nonlinear frequency transfer technology, can transfer the comb frequency to the mid-infrared band, which is widely used to generate mid-infrared-band optical frequency combs. Most nonlinear frequency transfer techniques only convert the operating wavelength of the pump comb source in the near-infrared wavelength band to a specific mid-infrared spectral window without broadening the optical comb bandwidth. This leads to local spectral expansion and noise accumulation in regions far away from the pump frequency, causing significant degradation of the comb structure. Thus, it is difficult to realize a coherent optical frequency comb with continuous coverage of the mid-infrared wavelength bandwidth. Nonlinear spectral broadening using nonlinear media, especially high nonlinear fiber (HNLF), is an effective method for improving the comb line; simultaneously, the comb spectrum can be continuously expanded to the mid-infrared band.

In this study, a highly nonlinear fiber based on highly doped germanium quartz is used as a nonlinear medium, relying on intra-pulse Raman scattering to induce frequency shifts. The output comb spectral coverage can be expanded from 1400 nm to 1700 nm. Experiments verify the importance of the dispersion characteristics of optical fibers and the importance of nonlinear effects occurring within different dispersion zones for high-quality broadening of comb spectra. The can be achieved in the near-zero-flat dispersion of HNLF. The Raman soliton frequency shift in HNLF with near-zero flat dispersion broadens the comb spectrum above 1700 nm. MIR band optical frequency combs have been hindered by the generation of broadband combs with high mutual coherence and wide bandwidth in this frequency region, which provides an effective technological approach for generating MIR band optical frequency combs with non-overlapping comb bottoms, spectrally continuous and wide bandwidth coverage, and high flatness in the MIR band.

In this study, we continuously optimize the comb flatness by cascading electro-optical modulators and integrally varying parameters such as individual radio frequency (RF) power amplifier gains and intensity modulator bias. The comb spectrum is output after cascade modulation. Furthermore, 3-dB bandwidth of the comb spectrum is 1.74 nm, and the top flat part of the spectrum contains 13 comb lines with the same frequency spacing between the newly generated comb lines, all of which are 18 GHz, with a pulse width of 7.36 ps. Subsequently, the dispersion is compensated using a 1000-m long single-mode fiber, which is the closest to the limiting pulse width of the Fourier transform, and the negative dispersion of single-mode fibers provides a linear negative chirp for compensating the Gaussian pulse. The linear positive chirp at the center of the Gaussian pulse compresses the central part of the pulse, and the compressed pulse width is 3.34 ps. The average power of the compressed pulse is increased to 2 W using a single-mode preamplifier and two single-mode main amplifiers, which correspond to a peak power of approximately 30 W. The frequency components of the spectrum are still relatively clean, the comb flatness remains unchanged after power amplification, and the 3-dB bandwidth of the spectrum begins to exhibit a certain degree of broadening. Furthermore, the nonlinear optical loop mirror (NOLM) 1 is then injected into NOLM2 for time-domain pulse compression. The precisely designed NOLM transmits the high-power portion at the center of the pulse while reflecting the low-power portion of the pulse, including the parasitic parietal valve, which increases the region of the pulse containing the linear chirp and provides optimal conditions for time-to-frequency conversion. After shaping and compression using two nonlinear circular mirrors, a near-ideal Gaussian pulse with a width of 0.9 ps is obtained. The experimental setup is used as a seed source for subsequent comb spectral broadening in a nonlinear medium. The output of the seed source is passed through a fiber amplifier and injected directly into the fiber through an isolator to complete the nonlinear frequency conversion. Light pulses with different peak powers are incident on the highly nonlinear fiber to explore the evolution of the spectral broadening process, analyze and select the most suitable length of the HNLF, and expand the comb-tooth spectral coverage from 1400 nm to 1700 nm by relying on the frequency shift caused by intra-pulse Raman scattering.

In this study, a 100-femtosecond broadband optical frequency comb is generated using a cascaded electro-optic modulator, with two fiber-optic ring mirrors employed to filter and shape the comb via pulse regeneration. The resulting output realizes a repetition rate of 18 GHz, an average power of 0.75 W, a pulse width of 230 fs, and a peak power of 170 W. This serves as a seed source, producing a comb spectrum covering 1500?1600 nm with a top 10 dB bandwidth of 26.86 nm, encompassing 215 relatively flat comb lines. The experimental results demonstrate the critical role of fiber dispersion characteristics and nonlinear effects in different dispersion regions for realizing high-quality comb spectrum broadening. By utilizing Raman soliton frequency shifting in highly nonlinear fiber with near-zero flat dispersion, the comb spectrum is successfully extended beyond 1700 nm.