Ultra-low Noise Microwave Generation based on a Free-Running Optical Frequency Comb

We present ultra-low noise microwave (ULNM) signal generation based on a transfer oscillator method using an optical reference and a free-running or only weakly-locked optical frequency comb (OFC). This proof-of-principle demonstration circumvents the requirement for an optical lock of the OFC to the optical reference and the related need of highbandwidth OFC actuators. By electrically removing the phase noise contribution of the OFC from a beat-note signal between the comb and the optical reference, we are able to divide the phase noise of the optical frequency down to a radio-frequency signal without limitation by a locking bandwidth or added servo noise bumps. The measured phase noise of the generated signal at 15 GHz is mainly limited by the phase noise analyzer used in this initial evaluation and by the shot-noise of the photo-detection, leading to a noise floor lower than -150 dBc/Hz at high Fourier frequencies and -60 dBc/Hz at 1-Hz offset frequency obtained with 1,100 cross-correlations. The reported method offers many attractive possibilities, such as the use of OFCs with GHz repetition rates to make the system more compact, of potentially lower cost and even more easily transportable. In addition, a single OFC can be used for the generation of different ULNM signals from distinct optical sources as no optical lock is involved.