There are provided a method and a system for characterizing the CMRR of an ICR under test, which employ highly coherent light from two continuous-wave (CW) single-frequency lasers whose respective optical frequencies mutually differ by an offset defining an “Intermediate Frequency” (fIF) in the rf electrical baseband. The method involves the coherent mixing of light from these two lasers in the ICR under test. A “tone” in the rf electrical baseband at frequency fIF is generated by the beating of light from the two single-frequency lasers as they interfere on the photodetectors of the ICR. The resulting tone at frequency fIF in the output electrical signals of the ICR is then detected and analyzed to characterize the CMRR of the ICR.