Microwave photonics instantaneous frequency measurement receiver based on a Sagnac loop.

A microwave photonics instantaneous frequency measurement receiver was conceived and practically demonstrated. The system is based on phase modulation inside a Sagnac loop and therefore needs no bias voltage, resulting in no DC bias drifts. The system provides a DC output whose amplitude is a function of input signal frequency, and thus no high-frequency photo-detector is required. Furthermore, the system exhibits a very good sensitivity without the aid of any sort of electrical noise cancellation techniques, such as lock-in amplification. This makes the system response faster, which would be another benefit for electronic warfare applications. Less than 6% measurement error was achieved over a 0.01-40 GHz frequency range for -30  dBm RF level. Also, a 10% error was achieved at -51  dBm RF power level.