Fiber-based 1.5 μm lidar vibrometer in pulsed and continuous modes

A fiber-based 1.5 μm heterodyne lidar that is easily switched between pulse-pair and cw modes is described. In laboratory experiments using well-controlled vibrating targets, and in computer simulations, the performance of the two modes is compared given the same average laser power. The accuracy of Doppler frequency (target velocity) estimates, and the signal-to-noise ratio in spectrally resolved plots of vibrational features, are evaluated. When the target-induced frequency modulation is wideband, pulse-pair often has clearly higher carrier-to-noise. But its advantage in signal-to-noise is smaller because combining the more numerous cw measurements improves the estimates of vibration frequencies and amplitudes. They are combined here through autocorrelation-based demodulation, one of several methods that can outperform phase-differencing.