Ultra-precise optical phase-locking approach for ultralow noise frequency comb generation

Optical frequency comb (OFC) with low phase noise enables more stringent metrology in classical and quantum systems. To eliminate phase noise, it is essential to extend the feedback bandwidth of the carrier-envelope phase and the repetition rate. Here, we present an approach to the construction of an ultralow-noise OFC. By exploiting different electro-optic modulators as fast actuators, this method can extend the feedback bandwidth to more than 150 kHz at the repetition rate phase locking and carrier-envelope offset phase locking We obtain a residual phase noise of 21.8 mrad (18.1 as) and 86.1 mrad (71.3 as) for the stabilised optical beat signal (f_beat) and carrier-envelope offset frequency (f_ceo), respectively. We validate this architecture by a measurement the relative linewidth between two combs, which reveals an in-loop beat fractional instability of less than 1.18 × 10⁻¹⁸ and an out-of-loop beat fractional instability of less than 2.31 × 10⁻¹⁵ at a 1-s averaging time. The out-of-loop beat phase noise is 145 mrad (120 as). These results indicate that the combination of Er-fiber laser technology and active feedback with a high bandwidth ensures coherent tracking of an optical reference with ultralow noise. The ultralow-noise OFC offers a universal tool for accurate, high-resolution optical spectroscopy.