Floyd E. Hovis

Airborne High Spectral Resolution Lidar for profiling aerosol optical properties

A compact, highly robust airborne High Spectral Resolution Lidar (HSRL) that provides measurements of aerosol backscatter and extinction coefficients and aerosol depolarization at two wavelengths has been developed, tested, and deployed on nine field experiments (over 650 flight hours). A unique and advantageous design element of the HSRL system is the ability to radiometrically calibrate the instrument internally, eliminating any reliance on vicarious calibration from atmospheric targets for which aerosol loading must be estimated. This paper discusses the design of the airborne HSRL, the internal calibration and accuracy of the instrument, data products produced, and observations and calibration data from the first two field missions: the Joint Intercontinental Chemical Transport Experiment--Phase B (INTEX-B)/Megacity Aerosol Experiment--Mexico City (MAX-Mex)/Megacities Impacts on Regional and Global Environment (MILAGRO) field mission (hereafter MILAGRO) and the Gulf of Mexico Atmospheric Composition and Climate Study/Texas Air Quality Study II (hereafter GoMACCS/TexAQS II).

Qualification of the laser transmitter for the CALIPSO aerosol lidar mission

Space-based missions impose a unique set of requirements on laser designs. The development of the CALIPSO Laser Transmitter Subsystem (LTS) began with a successful lifetime test using the Risk Reduction Laser (RRL), followed with an intensive design validation phase using prototype flight hardware, and completed with formal qualification of the flight hardware. We will describe this process in more detail and review the issues encountered and lessons learned. From this experience we developed a set of guidelines for designing and building future space-based lasers.

Single-frequency lasers for remote sensing

We have designed and built two versions of a space-qualifiable, single-frequency Nd:YAG laser. Our approach to frequency stabilization of the seeded oscillator is a variation of the “ramp and fire” technique. In this design, the length of the pulsed laser cavity is periodically varied until a resonance with the seed laser is optically detected. At that point the pulsed laser is fired, ensuring that it is in resonance with the seed laser. For one of the lasers the resulting single frequency pulses are amplified and frequency tripled. This system operates at 50 Hz and provides over 50 mJ/pulse of single-frequency 355 nm output. It has been integrated into the GLOW (Goddard Lidar Observatory for Winds) mobile Doppler lidar system for field testing. The second laser is a 20o Hz oscillator only system that is frequency doubled for use in the High Spectral Resolution Lidar (HSRL) system being built at NASA Langley Research Center. It provides 4 mJ of single-frequency 532 nm output that has a spectral purity of >10,000. In this paper we describe the design details, environmental testing, and integration of these lasers into their respective lidar systems.

Recent progress on single frequency lasers for space and high altitude aircraft applications

The use of lidars in ground, airborne, and space-based missions can provide earth and planetary science measurements that were previously unavailable. Our approach to the laser transmitters needed for such systems focuses on developing environmentally hardened prototypes whose designs can be validated in fielded ground and airborne remote sensing systems before being used in space-based missions. We are applying this approach to the development of the injection seeded single frequency lasers that will be needed for a number of the next generation of airborne and space-based lidar systems. In this paper we describe our most current version of a single frequency Nd:YAG laser that is designed for use in an unpressurized, high altitude aircraft. It is capable of providing two 20 W 1064 nm output beams at 200 Hz. Temperature controlled ovens that hold the nonlinear crystals needed for frequency doubling and tripling are included in the output path of one of the beams.

High energy, single mode, all-solid-state and tunable UV laser transmitter

NASA is developing state-of-the-art, all-solid-state, conductively cooled, diode-pumped, single longitudinal mode, tunable, short-pulsed, and high energy UV transmitters for ozone sensing measurements based on the Differential Absorption Lidar (DIAL) technique. The goal is to demonstrate output pulse energies greater than 200 mJ at pulse repetition frequencies of 10 Hz to 50 Hz, and pulsewidths in the range of 10 ns to 25 ns at UV wavelengths of 308 nm to 320 nm. The proposed scheme is to utilize the robust Nd:YAG pump laser technology in combination with nonlinear optics arrangement comprising of a novel optical parametric oscillator (OPO) and a sum frequency generator (SFG) to generate required UV wavelengths. In this paper, recent results of the development of Nd:YAG pump laser and UV converter module are presented. At 1064 nm, an output pulse energy of 1020 mJ at 16 ns pulsewidth and 50 Hz PRF yielding greater than 7% wall plug efficiency has been demonstrated. With improved drive electronics, this pump laser has the potential to generate greater than 1.2 J/pulse. The refined OPO module to aid in the generation of >200 mJ/pulse of UV radiation is also presented. The UV transmitters are being designed for DIAL operation under strong daylight conditions from space based platforms.

Conductively cooled lasers for space-based applications

The design of the diode-pumped gain medium is critical to the successful deployment of lasers in space-based missions. We have developed a number of diode-pumped, conductively cooled zigzag slab designs for this application. These designs include both one-sided and two-side pumped and cooled designs. In one of the one-sided pumped and cooled amplifier designs we optimized the efficiency by maximizing the overlap between the extracting beam and the diode pumps at the total internal reflection (TIR) surface, a so-called “pump on bounce” approach. With this approach we achieved an electrical to optical efficiency from the amplifier of over 11% with an output beam M2 of approximately 3. By reducing the size of the extracting beam to reduce diffraction effects in the slab the beam quality could be improved to an M2 of 1.5 but the amplifier electrical to optical efficiency dropped to 6.7%. The other one-sided approach we have investigated is a near Brewster angle slab that incorporates beam propagation parallel to the slab axis and achieves good efficiency by a high overall volume fill factor. In a high beam quality oscillator (M2 = 1.2) we achieved over 6% electrical to optical efficiency with a Brewster angle head design. Modeling of the thermal effects in both approaches has been performed and will be reported on. The final design approach we have investigated is based on two-sided pumping and cooling. Both modeling and preliminary experimental results indicate that this approach will allow scaling to higher average powers while still maintaining beam qualities and extraction efficiencies at least as good as those obtained with the one-sided pumped and cooled approaches. From the results of these tests and analyses, we have developed a design for a space-qualifiable 1 J, 100 Hz laser operating at 1064 nm.

ICESat-2 laser technology development

A number of ICESat-2 system requirements drove the technology evolution and the system architecture for the laser transmitter Fibertek has developed for the mission.. These requirements include the laser wall plug efficiency, laser reliability, high PRF (10kHz), short-pulse (<1.5ns), relatively narrow spectral line-width, and wave length tunability. In response to these requirements Fibertek developed a frequency-doubled, master oscillator/power amplifier (MOPA) laser that incorporates direct pumped diode pumped Nd:YVO4 as the gain media, Another guiding force in the system design has been extensive hardware life testing that Fibertek has completed. This ongoing hardware testing and development evolved the system from the original baseline brass board design to the more robust flight laser system. The final design meets or exceeds all NASA requirements and is scalable to support future mission requirements.

Development and testing of a risk reduction high energy laser transmitter for high spectral resolution lidar and Doppler winds lidar

Spaceborne 3-dimensional winds lidar and spaceborne High Spectral Resolution Lidar (HSRL) for aerosol and clouds are among the high priority future space missions recommended by the recent National Research Council (NRC) Decadal Review. They are expected to provide the important three dimensional winds data and aerosol data critically needed to improve climate models and numerical weather forecasting. HSRL and winds lidar have a common requirement for high energy solid-state lasers with output wavelengths at 1064nm, 532nm and 355nm, which can be achieved with Nd:YAG lasers and 2nd and 3rd harmonic generations. For direct detection winds lidar, only the 355nm output is needed. One of the key development needs is the demonstration of laser transmitter subsystem. Top issues include power and thermal management, lifetime, high energy UV operations, damage and contamination. Raytheon and its partner, Fibertek, have designed and built a space-qualifiable high energy Nd:YAG laser transmitter with funding from Raytheon Internal Research and Development (IR&D). It is intended to serve as a risk-reduction engineering unit and a test bed for the spaceborne HRSL and direct-detection Doppler winds Lidar missions. Close to 900 mJ/pulse at1064nm and a wall-plug efficiency of 6.5% have been achieved with our risk reduction laser. It is currently being characterized and tested at Raytheon Space and Airborne Systems. In this paper, we will discuss the design, build and testing results of this risk reduction high energy laser transmitter.

Development of an airborne molecular direct detection Doppler lidar for tropospheric wind profiling

Global measurement of tropospheric winds is a key measurement for understanding atmospheric dynamics and improving numerical weather prediction. Global wind profiles remain a high priority for the operational weather community and also for a variety of research applications including studies of the global hydrologic cycle and transport studies of aerosols and trace species. In addition to space based winds, high altitude airborne Doppler lidar systems flown on research aircraft, UAVs or other advanced sub-orbital platforms would be of great scientific benefit for studying mesoscale dynamics and storm systems such as hurricanes. The Tropospheric Wind Lidar Technology Experiment (TWiLiTE) is a three year program to advance the technology readiness level of the key technologies and subsystems of a molecular direct detection wind lidar system by validating them, at the system level, in an integrated airborne lidar system. The TWiLiTE Doppler lidar system is designed for autonomous operation on the WB57, a high altitude aircraft operated by NASA Johnson. The WB57 is capable of flying well above the mid-latitude tropopause so the downward looking lidar will measure complete profiles of the horizontal wind field through the lower stratosphere and the entire troposphere. The completed system will have the capability to profile winds in clear air from the aircraft altitude of 18 km to the surface with 250 m vertical resolution and < 3 m/s velocity accuracy. Progress in technology development and status of the instrument design will be presented.

High efficiency laser designs for airborne and space-based lidar remote sensing systems

The increasing use of lidar remote sensing systems in the limited power environments of unmanned aerial vehicles and satellites is motivating laser engineers and designers to put a high premium on the overall efficiency of the laser transmitters needed for these systems. Two particular examples upon which we have been focused are the lasers for the ICESat-2 mission and for the Laser Vegetation Imaging Sensor-Global Hawk (LVIS-GH) system. We have recently developed an environmentally hardened engineering unit for the ICESat-2 laser that has achieved over 9 W of 532 nm output at 10 kHz with a wall plug efficiency to 532 nm of over 5%. The laser has a pulse width of <1.5 ns and an M2 of <1.5. For the LVIS-GH lidar, we recently delivered a 4.2 W, 2.5 kHz, 1064 nm laser transmitter that achieved a wall plug efficiency of 8.4%. The laser has a pulse width of 5 ns and an M2 of 1.1 We provide an overview of the design and environmental testing of these laser transmitters.