Simple-structured, subfemtosecond-resolution optical-microwave phase detector.

Abstract

We demonstrate a simple all-fiber photonic phase detector that can measure the phase (timing) difference between an optical pulse train and a microwave signal with subfemtosecond resolution and -60 dB-level amplitude-to-phase conversion coefficient. It is based on passive phase biasing of a Sagnac loop by the intrinsic phase shift of a symmetric 3×3 fiber coupler. By eliminating the necessity of magneto-optic components or complex radio frequency (RF) electronics for phase biasing of the Sagnac loop, this phase detector has potential to be implemented as an integrated photonic device as well. When using this device for synchronization between a 250MHz mode-locked Er-fiber laser and an 8GHz microwave oscillator, the minimum residual phase noise floor reaches <-154 dBc/Hz (at 8GHz carrier) with integrated root mean square (rms) timing jitter of 0.97fs [1Hz-1MHz]. The long-term rms timing drift and frequency instability are 0.92fs (over 5000s) and 4×10-19 (at 10,000s averaging time), respectively.