Dual-comb generation based on Raman gain modulation for optical vector analysis

A dual comb is generated based on Raman gain modulation (RGM) for optical vector analysis (OVA), in which the proposed scheme is investigated via numerical simulation. In the method, the dual comb based on electro-optical modulation in the 1550 nm band is employed as the pump source. After being amplified by the erbium-doped fiber amplifier (EDFA), the pump comb is co-launched with a single-frequency seed light at 1645 nm into the highly nonlinear fiber (HNLF), where the energy and structure of the pump comb are coherently transferred to the 1645 nm band by RGM, so that the mutually coherent and high-power dual comb is generated at 1645 nm. Here, a Fabry-Pérot (FP) etalon is used as the optical device under test (ODUT); its amplitude and phase responses can be obtained by multi-frequency heterodyne interferometry. A frequency shift of 35 MHz is applied to one comb via an acousto-optic modulator (AOM) to resolve beat-note degeneracy. Compared with the conventional dual-comb OVA schemes, the dual-comb source at 1645 nm can be generated by RGM, so the problems that the optical devices at the 1645 nm band and methane absorption spectroscopy cannot be measured are avoided. Moreover, the mutual coherence and high power of the dual-comb source are simultaneously preserved, significantly improving the measurement accuracy of the ODUT. In the numerical simulation, the dual comb is generated at 1645 nm with an average power of 7.06 W, and the corresponding radio-frequency (RF) comb exhibits a linewidth of 2 Hz. Compared with the independent-seed OVA scheme, the amplitude and phase deviations are reduced from 9.43% and 1.49% to 0.12% and 0.02% in the proposed OVA scheme, respectively. Our proposed OVA scheme opens the door to the characterization of the frequency responses of optical devices at the 1645 nm band and methane absorption spectroscopy.