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.

Our proposed innovation will improve the performance of the injection-seeded, single frequency lasers that are planned for use in a number of airborne and space-based missions, including the following. 1) Aerosol/Cloud/Ecosystems (ACE): This is an expanded scope follow-on to the highly successful CALIPSO cloud and aerosol lidar mission. Multiple NASA researchers have been developing three wavelength (1064 nm, 532 nm, and 355 nm) High Spectral Resolution Lidar (HSRL) systems as candidate lidars for the ACE mission. 2) 3-D Winds: Space-based measurement of winds has been identified as a critical mission for enhancing both weather and climate modeling. 355 nm airborne demonstrators for this mission include a direct detection wind lidar that was built at GSFC and an Optical Auto-Covariance Wind Lidar (OAWL) being developed at Ball Aerospace. 3) Global Atmospheric Composition Mission (GACM): A scaled up version of the 355 nm pumped Ozone DIAL system being developed at NASA Langley is a strong contender for the GACM mission for global ozone measurements. 4) NASA Langley High Altitude Lidar Observatory (HALO): This is an ongoing lidar development program for airborne measurements of methane and water vapor. Our proposed high speed locking and switching technology is critical for achieving the desired performance for the 1 kHz, 1 m pump lasers and the optical parametric oscillators (OPOs) used to generate the 1.65 um and 1.57 um beams used to measure methane and water vapor.

In addition to our NASA customers, Fibertek does significant lidar development work with various DoD agencies that would benefit from the proposed effort. Two specific examples are briefly described below. 1) Single-frequency blue lasers for use in underwater communications: Since the early 1970's the U.S. Navy has made major investments (>$100M) in attempts to develop single-frequency blue lidar systems for underwater communications. Fibertek has supported these development efforts and anticipates that the successful development of the proposed technology would enhance our chances of winning future business in this area. 2) Doppler Wind Lidar (DWL) systems for precision air drop: An unmet need for the Army is a compact and robust DWL system that could be used to quantify the vector wind fields in the vicinity of regions where supplies will be dropped from medium to high altitude aircraft. The technology proposed here would enhance Fibertek's ability to respond and compete in this business area.

This is early/mid-stage (TRL 5) — the most realistic path in is NASA SBIR/STTR, which funds small businesses and research institutions to develop technology aligned with NASA's needs (equity-free, phased funding). Check whether a current SBIR/STTR solicitation topic overlaps with this project's technology area, or contact the project directly (above) to ask.

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