Residual phase noise in long microwave optical links and Mach-Zehnder modulators

A two-tone method 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 MOLs and a Mach-Zehnder modulator (MZM) both are measured for the first time, the RPN of a 6-km MOL is successfully measured to be -130 dBc/Hz at 1-kHz offset and -140 dBc/Hz at 10-kHz offset. Finally, we experimentally investigate the relationship between the RPN and the input RF powers of a LiNbO₃ MZM biased at quadrature point, it appears that in the linear region, the white phase noise decreases proportional to the increasement of the input RF power.