Self-Induced Modulational-Instability-Assisted Frequency Comb Generation Using a Passive Fiber Ring Cavity Having Varying Normal Dispersion

We experimentally generate an optical frequency comb with a repetition rate of ~43 GHz using an all-passive fiber cavity operating in varying normal dispersion regime and pumped by a multimode laser diode. Coupling of four fiber segments with different dispersion coefficients and core diameters along a ring cavity enhances the spectral broadening at a low threshold pump power. Self-induced modulation instability is experienced due to cross phase modulation that results from the interaction of co-propagating optical signals inside the cavity. Stabilization of the two fundamental modes that are phase matched with higher order modes enables the amplification of the weakly modulated stokes and antistokes modes through dissipative four wave mixing, which results in the formation of a frequency comb of constant mode spacing of 0.136 nm. Phase locked frequency modes consisting of ~250 comb lines covering a spectral range from 966 to 1000 nm centered at 978.512 nm are synthesized and reported for the first time.