Tunable dual-frequency coherent Doppler lidar using bi-directional electro-optic modulation in a Sagnac loop

Abstract

A novel tunable dual-frequency coherent Doppler lidar (DF-CDL) for velocity detection is proposed and experimentally demonstrated. In the laser source, although dual-frequency laser light can be generated by an electro-optic intensity modulator biased in the double-sideband suppressed-carrier mode, a slight drift in the direct-current (DC) bias voltage leads to a non-ideal carrier suppression. To address this problem, bi-directional electro-optic phase modulation in a Sagnac loop is employed. Due to the use of electro-optic phase modulators, no bias voltage is needed, resulting in no bias drifts. In the signal processing module, since the envelope of the Doppler signal is a cosine function of the differential Doppler shift, envelope-demodulation is employed to directly derive the differential Doppler shift from the analog Doppler signal. Due to the low-frequency characteristic of the differential Doppler shift, the sampling rate request of the ADC is reduced. In the experiments, the performance of the DF-CDL is validated under the conditions of different dual-frequency spacings and different target speeds. A speed measurement of 10.01 mm/s with a relative error of 0.10% is achieved when the dual-frequency spacing is 40 GHz and the target speed is 10 mm/s.