High Speed Frequency Locking Module for Lidar Based Remote Sensing Systems

A basic requirement for all Differential Absorption Lidar (DIAL) systems is wavelength switching of the probe laser on and off of an absorption line of the species of interest. For most trace gas species switching accuracy on the order of 10 MHz is also required. Further complications for many DIAL systems are that the platform moves (airborne or space craft) and that the lasers are often high peak power, pulsed lasers. The combination of a moving platform, pulsed laser, and the requirement that the online and offline measurements be made in essentially the same volume implies that the online/offline switching time be less than ~ 1 ms, and many cases even shorter. To date, most lasers used in DIAL systems rely on piezo-electric (PZT) mechanisms for the cavity length changes needed for the frequency switching. Typically this limits wavelength switching speeds to a few hundred Hz. This relatively slow frequency switching prevents researchers from fully exploiting DIAL systems utilizing the high efficiency, multi-kHz lasers or the lower repetition rate, dual pulse lasers systems that are now available. In Phase I, Fibertek demonstrated a brassboard version of a high speed, non-mechanical frequency locking module that allowed shot to shot frequency switching of a 1645.5 nm Er:YAG laser at >1 kHz with a spectral purity of 1,000:1. Our approach to the proposed locking module was an innovative synthesis of all electro-optic (EO) based switching and locking, a compact and efficient EO driver design that reduces voltage requirements by 4x over conventional designs, a novel EO voltage profile that eliminates electrochromic darkening, and a larger off-set locking capability that eliminates the requirement for an additional phase shifter in the cavity. In Phase II we will advance the TRL of the key technology components and incorporate a hardened version of the locking module into a 1645.5 nm Er:YAG laser that is being developed for a methane lidar being built at NASA Langley.