In this Letter we report on an all optical-fiber approach to the synthesis of ultralow-noise microwave signals by photodetection of femtosecond laser pulses. We use a cascade of Mach–Zehnder fiber interferometers to realize stable and efficient repetition rate multiplication. This technique increases the signal level of the photodetected microwave signal by close to $18\mathrm{dB}$ . That in turn allows us to demonstrate a residual phase-noise level of $-118\mathrm{dBc}/\mathrm{Hz}$ at $1\mathrm{Hz}$ and $-160\mathrm{dBc}/\mathrm{Hz}$ at $10\mathrm{MHz}$ from a $12\mathrm{GHz}$ signal. The residual noise floor of the fiber multiplier and photodetection system alone is around $-164\mathrm{dBc}/\mathrm{Hz}$ at the same offset frequency, which is very close to the fundamental shot-noise floor.