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Residual phase noise measurements of long microwave optical links and photodiodes using a two-tone method

A novel approach for measuring the residual phase noise (RPN) of microwave optical links (MOLs) and individual components in the MOL is proposed in this paper. Compared with the previous RPN measurement methods, the two-tone correlation-based method suppresses the noise contributions of the uncorrelated noise sources except that of the device under test (DUT). By this method, the RPN of externally modulated long microwave optical links (MOLs) is measured, 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. In addition, the RPNs of the photodiode (PD) in different operation conditions were also measured by this method, the experiments reveal that the nonlinearity of PD can deteriorate its RPN.