  The generation of microwaves from optical signals suffers from thermal and
shot noise inherent in the photodetection process. This problem is more acute
at lower pulse repetition rates where photodiode saturation limits the
achievable signal-to-noise ratio. In this paper, we demonstrate a 10-15 dB
reduction in the 10 GHz phase noise floor by multiplication of the pulse
repetition rate. Starting with a 250 MHz fundamentally mode-locked
erbium(Er):fiber laser, we compare two different approaches to repetition rate
multiplication: Fabry-Perot cavity filtering and a cascaded, unbalanced
Mach-Zehnder fiber-based interferometer. These techniques reduce the phase
noise floor on the 10 GHz photodetected harmonic to -158 dBc/Hz and -162
dBc/Hz, respectively, for Fourier frequencies higher than 100 kHz.
