Investigation of Automatically NPE Mode-locking Femtosecond Laser Frequency Comb

Fiber optical combs based on nonlinear polarization evolution （NPE） mode-locking technology have many advantages such as narrow pulse width, wide spectral range, high repetition rate and low phase noise, but due to the problem of mode-locking stability, they are gradually replaced by nonlinear amplifying loop mirror （NALM） and semiconductor saturable absorber mirror mode-locking technology in engineering applications. In order to expand the engineering application region of NPE fiber optical comb, an NPE erbium-doped fiber optical comb with automatically mode-locking function was developed and the automatic locking of fiber laser oscillator was realized by electronically controlled polarization controller driven by automatic search circuit for mode locking, with a mode-locked pulse width of 90 fs and a repetition rate of 101.5 MHz. The 40 dB carrier-envelope offset frequency f_ceo signal was measured by collinear f-2f self-reference technology, and the f_ceo was locked to 20 MHz by combining up-mixing and divide-by-64 with a locking accuracy of 1.5 mHz. By locking the 40th higher harmonic, the repetition rate f_rep lock is realized by the self-developed repetition rate feedback mechanism, and the frequency accuracy after locking is 199 μHz.