Low noise erbium fiber fs frequency comb based on a tapered-fiber carbon nanotube design

We report on a low noise all-fiber erbium fs frequency comb based on a simple and robust tapered-fiber carbon nanotube (tf-CNT) design. We mitigate dominant noise sources to show that the free-running linewidth of the carrier-envelope offset frequency (f_ceo) can be comparable to the best reported performance to date for fiber-based frequency combs. A free-running fceo linewidth of ∼20 kHz is demonstrated, corresponding to an improvement of ∼30 times over previous work based on a CNT mode-locked fiber laser [Opt. Express 18, 1667(2010)]. We also demonstrate the use of an acousto-optic modulator external to the laser cavity to stabilize f _ceo, enabling a 300 kHz feedback control bandwidth. The offset frequency is phase-locked with an in-loop integrated phase noise of ∼0.8 rad from 10Hz to 400kHz. We show a resolution-limited linewidth of ∼1 Hz, demonstrating over 90% of the carrier power within the coherent f_ceo signal. The results demonstrate that the relatively simple tf-CNT fiber laser design can provide a compact, robust and high-performance fs frequency comb.