A cascaded optical modulator topology for detecting the phase difference of two microwave signals is presented. It operates based on mixing of two microwave signals. The phase difference of two microwave signals can be obtained from the system output DC voltage. The phase detector is designed to eliminate the problems of DC offset and phase offset, which are present in reported structures. Experimental results demonstrate phase detection of two microwave signals with a 0° to 180° phase difference, less than ±3° phase detection errors and less than 1 mV DC offset. Results also demonstrate the phase detector can be operated over a wide frequency range.