Phase Noise Reduction of an Optical Frequency Comb Using a Feed-Forward Heterodyne Detection Scheme

A novel feed-forward phase noise reduction scheme is introduced to reduce the phase noise of an optical frequency comb. The phase noise of a single comb line is measured using a heterodyne technique, and the inverse of this signal is then applied to the entire comb. The method is capable of simultaneously reducing the phase noise of multiple lines and is limited only by the correlation of phase noise across the comb. Experimentally, we demonstrate a reduction in the optical linewidth of a 16-mode optical frequency comb derived from a mode-locked semiconductor laser from around 20 MHz to less than 300 kHz, with 11 of the 16 output spectral modes possessing optical linewidths less than 100 kHz.