Photonic approach to microwave frequency measurement under large-signal modulation

A photonic approach to microwave frequency measurement under large-signal modulation is proposed and demonstrated experimentally. Under the case of large-signal modulation, the microwave signal with its frequency to be measured is applied to externally modulate the optical carrier inside a phase modulator. Then the phase-modulated optical signal is equally divided and sent to two dispersive elements with different chromatic dispersions (e.g., single mode fiber), to perform phase modulation to intensity modulation (PM-IM) conversion. Subsequently, the fundamental component and the DC component at the output of photodetector (PD) are measured and used to establish a relationship between the microwave frequency and the power ratio, while the large modulation index arising from large-signal modulation is completely removed. Therefore, a microwave power-independent microwave frequency measurement has been realized under large-signal modulation. The proposed approach is then verified by a proof-of-concept experiment. Within the range of 7-13GHz, measurement errors less than ±0.3 GHz are achieved.