Laser ranging with analog all-optical coherent pulse compression using a frequency shifting loop

Abstract

In radar technology, the term pulse compression refers to a set of techniques that allows achieving the resolution of short and high peak-power pulses by transmitting long signals with relatively low peak powers [1] . This result is possible thanks to the use of modulated or coded waveforms with an enlarged time-bandwidth product, which can be easily amplified without the onset of nonlinearities. After these waveforms are sent to a remote target, the cross-correlation (CC) of the detected back-reflections with a reference signal (in a process called matched filtering) produces narrow pulses, enabling a range resolution set by the bandwidth of the transmitted signal. So far, the concept of pulse compression, when extended to the the optical counterpart of radar (lidar), has been mostly limited to incoherent optical waveforms coded in intensity. In this paper, we present a scheme for laser ranging by all-optical coherent pulse compression. It consists in a bidirectional frequency shifting loop (FSL), composed of a couple of acousto-optic frequency shifters (AOFS1 and AOFS2), an optical amplifier (EDFA) and a tunable bandpass filter (TBPF), see Fig. 1a [2] . The system is configured so that the recirculation of the light inside the loop produces two optical frequency combs (OFCs) with quadratic spectral phases, and whose frequency spacings differ by a small quantity δf. By a precise adjustment, the FSL generates two trains of optical chirps with similar chirp rate but slightly different period. The interference of these quasi-continuous waveforms on a photodetector (PDcc), followed by a low-pass filter, produces the matched filtering in a purely analog manner. Simultaneously, the dual-comb interference enables to reduce by orders of magnitude the detection electronics bandwidth.