Optical frequency combs and applications at NPL

The phase noise measured at the “user” end with and without active suppression of the noise introduced in the optical link is shown in Fig. 4. At 1 Hz offset from the carrier the noise is suppressed by more than 10 dB to -101 dBc/Hz. The feedback bandwidth achieved is limited to about 10 Hz by the high voltage amplifier used to drive the fibre stretcher. With improved hardware the feedback bandwidth could be increased towards the theoretical limit of 2 kHz, imposed by the 0.5 ms round-trip travel time of the pulse train.