Optical frequency comb stabilized to a fiber delay line

We have developed a simple and practical optical frequency comb (OFC) stabilization method for comb linewidth reduction in this Letter. Two comb modes of an Er-fiber optical frequency comb (OFC) have been phase-locked to a kilometer-long fiber delay line, while narrow linewidth lasers are not required as optical references. The fractional frequency stability of the 1542-nm comb mode in the Er-fiber OFC reaches 9.13 × 10−13 at 12.8 ms average time, and its short-term linewidth is 580 Hz, which is compressed by a factor of ∼170 compared to the free-running condition. The whole stabilization scheme gets rid of nonlinear progress, which can be an alternative approach for OFC stabilization, especially in ultra-high repetition-rate combs, e.g., electro-optic combs, quantum cascade laser combs, and micro-combs with low pulse energy.