Analysis of noise mechanisms limiting the frequency stability of microwave signals generated with a femtosecond laser

Excess phase noise is observed in the spectrum of the microwave signal extracted from a photodetector illuminated by a train of ultrashort light pulses from the femtosecond laser. This noise affects the stability of frequency transfer from optical to microwave domains with the femtosecond laser. Some contributions to the excess phase noise are related to intrinsic beam-pointing fluctuations of the femtosecond laser and optical power fluctuations of the detected light. These factors contribute to excess phase noise at the harmonics of the pulse repetition rate due to power-to-phase conversion in the photodetector, spatially dependent time delays, and photodiode nonlinearities that distort the pulse shape. With spatial filtering of the laser beam and active control of its power, the additional fractional frequency fluctuations of pulse repetition rate associated with the excess noise of the photodetection process were reduced from 6·10^-14 to approximately 3·10^-15 over 1 s of averaging. The effects of other noise mechanisms, such as laser shot noise and phase noise introduced by a microwave amplifier, were also examined but were found to be at a less significant level.