Dual photo-detector system for low phase noise microwave generation with femtosecond lasers.

Low phase noise microwave signals can be generated by photo-detecting the pulse train of an optical frequency comb locked to a high spectral purity continuous-wave optical reference. Amplitude-to-phase noise conversion is, however, a well-known limitation to this technique. Great care is usually required to overcome this constraint due to its strong dependence on the impinging optical power. Here we demonstrate the combined use of "magic point" operating conditions of photodetectors, pulse repetition rate multipliers, and coherent addition of microwave signals to realize a microwave extraction device largely immune to amplitude-to-phase conversion effects over a large range of impinging optical powers.