Remote Acoustic Detection Using a Coherent Laser Radar

摘要

A coherent lidar was used to detect sound remotely in an unseeded wind tunnel. The system is able to reconstruct acoustic temporal waveforms by detecting the motion of natural aerosols. The acoustic motions were measured in the presence of much larger motions caused by the ambient fluid-dynamic turbulence. Unlike conventional microphones, remote acoustic sensing is immune to wind-induced self-noise and avoids disturbing the aerodynamic flow. By forming a virtual microphone array it is possible to make the lidar system directional, thereby rejecting acoustic reflections from wind tunnel walls. The system employs a continuous-wave, single-frequency, 1.55 μm fiber laser in a bistatic configuration and a heterodyne receiver with a 200 MHz intermediate frequency. The acoustic waveform was impulsive, characteristic of acoustic signatures of helicopters. There is close agreement between the lidar and conventional microphone signals.