High Speed Frequency Locking Module for Lidar Based Remote Sensing Systems

A fundamental 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 it is also required that the accuracy of the switching be on the order of 10 MHz. Further complications for many DIAL measurements are that the platform for the system moves, such as an airplane or spacecraft, and that the lasers used 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 switching time between online and off line measurements be less than ~ 1 ms, and many cases even shorter. To date, most lasers used in DIAL systems rely on piezo-electric (PZT) mechanisms to perform the cavity length changes needed for the frequency switching. In practice, 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 order to improve the wavelength switching speeds needed to fully exploit the capabilities of airborne and space-based DIAL systems, Fibertek is proposing to develop a high speed, non-mechanical frequency locking module that allows shot to shot frequency switching of single-frequency lasers at up to 3 kHz with a spectral resolution of <10 MHz. Our approach to the proposed locking module is 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 that's profile that eliminates electrochromic darkening, and a larger off-set locking capability that eliminates the requirement for an additional phase shifter in the cavity.

1. Aerosol/Cloud/Ecosystems (ACE) - This mission will be an expanded scope follow-on to the highly successful CALIPSO cloud and aerosol lidar mission. Researchers at NASA Langley and NASA GSFC have been developing three wavelength (1064 nm, 532 nm, and 355 nm) High Spectral Resolution Lidar (HSRL) systems as candidate technologies for the ACE mission. 2. 3-D Winds - Space-based measurement of tropospheric winds with global coverage 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 requirements for global ozone measurements. 4. The NASA Langley High Altitude Lidar Observatory (HALO) - This is an in progress lidar development program for airborne measurements of methane and water vapor. Our proposed high speed locking and switching technology are critical for achieving the desired performance for both the 1 kHz, 1064 nm pump lasers and the optical parametric oscillators (OPOs) that will be used to generate the 1.65 ?m and 1.57 ?m beams needed to measure methane and water vapor.

1. Single-frequency blue lasers for use in ocean monitoring and 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 a region 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.

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