Effect of pulse duration fluctuation on the photodetection of a train of pulses from an optical frequency comb

Microwave signals with unprecedented ultra-low phase noise can be generated by photo-detecting stabilized optical pulse trains from femtosecond laser. However, excess noise introduced by photodetection limits the phase noise performance of generated microwave signals. Here, we demonstrate a new limitation that arises from the pulse duration fluctuation of ultrashort optical pulses. Such fluctuations, although minute compared to the pulse duration itself, convert to phase fluctuations in the microwave electrical waveforms generated by photodetection in a fast InGaAS pin photodiode, due to nonlinear processes. We quantitatively characterize the pulse duration to phase conversion coefficient under various experimental configurations (bias voltage, optical power, etc). Furthermore, by combining this quantitative characterization with experimental microwave phase noise measurement under conditions where this new effect is the dominant source of noise, we have characterized quantitatively, to our knowledge for the first time, the pulse duration noise that is present at the output of an Erbium doped fiber based femtosecond laser and its optical amplification chain.