Photonic Doppler frequency shift measurement without ambiguity based on cascade modulation

Abstract

A novel scheme based on the cascade structure of a dual-polarization Mach–Zehnder modulator (DPol-MZM) and a phase modulator (PM) is proposed which can simultaneously identify the value and direction of Doppler frequency shift (DFS). A low frequency reference signal and an echo signal are modulated by the DPol-MZM into orthogonal polarization states. The transmitted signal is modulated onto the reference signal by the PM and then one side band of the modulated signal is suppressed after a filter. The value of DFS is obtained by mixing the echo signal and the transmitted signal and the direction is distinguished by mixing the echo signal and the modulated reference signal. The system has only one channel and DFS measurement only needs a low frequency electrical spectrum analyzer (ESA). The measurement error is less than 0.1 Hz. Because of the cascade modulation structure and the direct frequency mixing of the transmitted signal and the echo signal, the operation frequency range of the system is not limited by the carrier frequency and the scheme has high measurement accuracy.