A stabilized master laser system for differential absorption LIDAR.

Abstract

In this thesis, we present a prototype water vapour DIfferential Absorption Lidar (DIAL) instrument with accurate and precise wavelength control of master diode lasers. This stabilization system design has a number of novel elements that work towards a robust and low-cost autonomous DIAL observatory. With two continuous wave optical wavelengths stabilized, a pulse is formed using an Acousto-Optic Modulator (AOM) to switch light out of each control system to form the transmitted pulse. The control systems employ synchronous reference signal detection that suppresses system perturbations due to the optical switching, facilitating the use of deep dither modulation that aids in accurate stabilization to weak absorption lines. Furthermore, ratiometric detection in the control loop suppresses interference caused by back reflections in optical fiber components, as well as amplitude modulation of the laser diode due to injection current. In our system, the first laser is stabilized to an absorption line of a water vapour cell, while the second is beat-frequency stabilized relative to the first using a passive 16 GHz bandpass filter. This technique can be expanded to stabilize any number of reference lasers with respect to each other and to an absolute optical standard. The prototype DIAL uses a Tapered optical Amplifier (TA) to form 1 μs 500 mW optical pulses with a repetition rate of >3 kHz for atmospheric transmission. Fourteen observation experiments were conducted over two years, with water vapour measurements obtained using a calibrated humidity sensor, using three saturated salt solutions as humidity references. The measured pulse extinction was used to calculate the effective absorption cross-section of the transmitter, and therefore used to calculate quantitative water vapour measurements from the DIAL observation data. It is hoped that this work will be useful to the further development and commercialization of this unique and powerful remote sensing technique.