Using the spectral asymmetry of TEA CO_2 laser pulses to determine the Doppler-shift sign in coherent lidars with low frequency stability

Abstract

We develop a method for determinating the relative positions of the lidar transmitter (LT) and the local oscillator (LO) frequencies in Doppler CO2 lidars. It uses the weak spectral asymmetry of TEA CO2 laser pulses, defined by a number of secondary peaks at the high-frequency side of the main spectrum peak. Depending on the sign of the beat frequency, these peaks may appear in the demodulated spectrum at either the high- or the low-frequency side. Each laser pulse spectrum is compared with reference spectra with two types of asymmetry, with the cross-correlation coefficients used as criteria. The performance of the method at different values of signal-to-noise ratio is analyzed numerically. The method is also applied to raw data from the lidar reference channel and demonstrates good performance at noise levels lower than the secondary peaks in the pulse spectrum or at a signal-to-noise ratio of > or = 20 dB. Application of the pulse spectrum asymmetry for lidar frequency stabilization is analyzed. Lidar operation without frequency stabilization is considered as well. The method offers a simple Doppler lidar hardware for the creation of low-cost coherent lidars, velocimeters-rangefinders, etc.