Compact All-Fiber Polarization Coherent Lidar Based on a Polarization Modulator

A compact all-fiber polarization coherent Lidar is demonstrated to simultaneously measure the velocity and depolarization ratio. In the Lidar system, a dual-polarization light beam is generated in a polarization modulator (PolM). The light beam consists of a linearly polarized optical carrier and an optical sideband with an orthogonal polarization state. If the light is depolarized, the polarization directions of the optical carrier and the sideband are rotated. Both of them can be detected along the polarization direction of the original optical carrier. In the receiver, the backscattered light along the polarization direction of the original optical carrier is selected and beats with a frequency-shifted optical local oscillator (LO) signal at a photodetector. A beat signal consisting of two single-frequency components will be generated. The velocity is extracted from the frequencies of the two single-frequency components, while the depolarization ratio can be extracted from the intensity ratio of the two single-frequency components. An experiment is carried out, in which a 159-MHz signal is applied to the PolM, and an 80-MHz signal is used to produce the frequency-shifted optical LO. Two single-frequency signals at 80 and 79 MHz are generated at the receiver and used to simultaneously measure the velocity and depolarization of a target in real time.