Investigation of noise in Lightwave Synthesized Frequency Sweeper seeded LIDAR anemometers from leakage through the Acousto Optic Modulators

Lightwave Synthesized Frequency Sweepers (LSFS) have potential use as lightsources in lidar anemometers. In this paper noise due to leakage in the acousto optic modulators in an LSFS is investigated. Theoretical expressions describing the build-up of noise in the LSFS due to leakage are derived. Two types of leakage are measured. First the non-shifted leakage through a closed AOM is measured and good agreement with the expected value is found and secondly the non-shifted and multiple-shifted leakage through an open AOS is measured in heterodyne setup. In LSFS driven Frequency Stepped Pulse Train (FSPT) modulated lidars the leakage will give rise to rapidly growing noise in the bins which corresponds to the signal from low radial wind velocities. It is likely that noise canceling techniques similar to those used for RIN removal has to be deployed for measurements of low wind velocities.