  We report what we believe to be the lowest phase noise optical-to-microwave
frequency division using fiber-based femtosecond optical frequency combs: a
residual phase noise of -120dBc/Hz at 1 Hz offset from a 11.55GHz carrier
frequency. We furthermore report a detailed investigation into the fundamental
noise sources which afflicts the division process itself. Two frequency combs
with quasi-identical configurations are referenced to a common ultrastable
cavity laser source. To identify each of the limiting effects we implement an
ultra-low noise carrier-suppression measurement system, which circumvents the
detection and amplification noise of more conventional techniques. This
technique now allows the suppression of these unwanted sources of noise to very
low levels. In the Fourier frequency range of ~200 Hz to 100 kHz, a fed-forward
technique based on a voltage-controlled phase shifter delivers a further noise
reduction of 10 dB. For lower Fourier frequencies, optical power stabilization
is implemented in order to reduce the relative intensity noise which causes
unwanted phase noise through power to phase conversion in the detector. We
implement and compare two possible control schemes based on an acousto-optical
modulator and comb pump current. We also present wideband measurements on the
relative intensity noise of the fiber comb.
