Low-noise transfer of the spectral purity of an optical comb line using a feedforward scheme

We present a detailed frequency noise analysis of a feedforward scheme used to faithfully transfer the spectral properties of an individual line of an optical frequency comb spectrum to a single-mode laser and in this way indirectly amplify it, which is applicable to any arbitrary comb mode spacing. In contrast to previously reported implementation of the feedforward method for a similar purpose, we present a more thorough noise study, including the measurement of the additive noise of the setup. The reported experimental investigation was performed using two low-noise ultrafast mode-locked lasers with different repetition rates (~?1?GHz and 250?MHz) and show a faithful transfer of the comb mode frequency noise to the auxiliary laser with a sub-radian additive phase noise integrated from 1?Hz to 1?MHz. We discuss the present limitations of the method that is able to transfer optical comb lines with sub-Hz linewidth and propose simple improvements.