Time-domain phase noise measurement in the optical frequency region

We have developed a simple method for measuring relatively large phase noise, which is usually observed in a RF signal down-converted from optical frequency. The noise floor is sufficiently low for observing phase noise of optical frequencies (-120 dBc/Hz) and its dynamic range is practically unlimited. A time-domain phase noise down-converted from optical frequencies is measured by using a high-speed digitizer with sampling rate of 220 MHz. The observed phase noise has difference only within 1 dB compared with that using a conventional method.