Doppler frequency shift measurement and direction discrimination based on a bidirectional phase modulator and a Sagnac loop

Abstract

A photonic approach to implement Doppler frequency shift (DFS) measurement and direction discrimination is proposed and demonstrated. In the proposed approach, a bidirectional PM and a Sagnac loop are used to obtain the orthogonally polarized optical signals. Combined with a wavelength division multiplexer (WDM), the upper and lower sidebands of the orthogonally polarized signals are separated. After two balanced photodetectors (BPDs), I/Q balanced detection is realized. Subsequently, the value and direction of DFS are estimated at the output ports of the electrical low-frequency 90° hybrid coupler (90° HC) simultaneously. In the proposed approach, using BPD can suppress some spurious signals and improve resolution. The use of double-sideband improves spectrum utilization. Meanwhile, the polarization drift of polarization controller (PC) and the phase offset of 90° HC have little influence on the discrimination of the DFS direction. That is to say, the proposed system for DFS measurement is stable.