Detection of carrier-envelope offset frequency of a figure-of-9 erbium-doped fiber laser

We demonstrate the optical part of the optical frequency comb based on a figure-of-9 erbium-doped fiber laser. In this experiment, we embedded an integrated nonreciprocal phase shifter in an all-polarization-maintain fiber oscillator, in order to reduce the mode locking threshold and facilitate self-starting. The repetition rate/ 3 dB spectral bandwidth/ pulse width of the output pulse is 46 MHz/ 15.8 nm/ 527.73 fs, respectively. The pulse trains are injected into the high nonlinear fiber after a two-stage erbium-doped fiber amplifier and a piece of single-mode fiber for compressing the pulse width, then an octave-spanning supercontinuum can be obtained. Finally, the signal of the carrier-envelope offset frequency(f_ceo) with a signal-to-noise ratio of 35 dB is detected by using a collinear self-referenced f-2f interferometer. Constructing a compact system structure with high signal-to-noise ratio fceo signal is critical for long-term stability of optical frequency combs. Our study has laid a foundation for future development of combs applied in space.