Two-Tone Method for Measuring the Residual Phase Noise of Long Microwave Optical Links

The residual phase noise (RPN) of externally modulated long microwave optical links (MOLs) is measured for the first time by means of a two-tone correlation-based method. In the conventional RPN measurement system with a double-balanced mixer, the phase noise of the reference microwave sources contributing to the noise floor will increase with the increasing of the delay of the device under test (DUT). This noise floor component is first suppressed here by correlating the measurement on two microwave tones, so as to improve the sensitivity of the RPN measurement system when measuring large-delay DUTs, such as long MOLs. In the experiments, the link lengths of 1 m, 2 km, and 6 km were employed, respectively. A noise floor of -142 dBc/Hz at 1-kHz offset and -152 dBc/Hz at 10-kHz offset were obtained when using two microwave tones at 9.5 and 9.6 GHz, the phase noises of which were both -124 dBc/Hz at 1-kHz offset and -131 dBc/Hz at 10 kHz. The RPN of a 6-km fiber link was successfully measured to be -130 dBc/Hz at 1 kHz offset and -140 dBc/Hz at 10-kHz offset.