The Laser Remote Sensing Group is also currently developing frequency-agile compact solid-state laser technology at eye-safe wavelengths near 2 µm. The relatively mature neodymium-doped solid-state lasers operate at shorter wavelengths, which can be transmitted through the eye and focused onto the retina, requiring severe constraints in remote sensing applications due to eye-safety considerations. This lidar technology is currently under investigation for possible application to the measurement of global-scale wind fields from earth orbit.
Coherent Doppler lidar measurement of the atmospheric wind field on a global scale with the required horizontal (~100 km) and vertical (~1 km) spatial resolutions requires a pulse energy of at least several hundred mJ and an average output power of at least 2W from a single-frequency, diffraction-limited transmitter. High electrical-to-optical conversion efficiency is also critically important due to the limited power resources available from candidate spacecraft bus options.
The Tm,Ho:YLF laser medium has emerged as a promising candidate material due to its eye-safe 2-µm wavelength and relatively favorable laser properties. The efficiency of the Tm,Ho laser process is maximized by optically pumping the medium with 790-nm emission provided by laser diodes. Sustained progress in frequency-agile local oscillator design during the past few years has resulted in the achievement of high CW efficiency and continuous single-mode tunability of >8 GHz at output powers in excess of 30 mW. Development of this demonstration device (shown below) was supported by the Coherent Lidar Group of the NASA Marshall Space Flight Center and was conducted in collaboration with the JPL Optical Communications Group.
This laser technology was selected for application on SPARCLE (SPAce Readiness Coherent Lidar Experiment), a Doppler wind lidar which was intended to fly on the Space Shuttle in 2001 as the NASA New Millenium Program EO-2 Mission.
Although the functionality of the breadboard system approaches that required for space-based coherent lidar implementation, compared to diode laser technology this device is mechanically complex and electrically inefficient. An alternative monolithic semiconductor laser reference oscillator offers superior resistance to environmentally induced alignment degradation and generally longer lifetime. In addition, the semiconductor laser option has the potential for considerably more rapid tuning capability, rendering feasible a wider variety of lidar pointing/scanning strategies.
Such a development is currently under way with support from NASA's Advanced Technology Initiatives Program for Earth Sciences. The approach under investigation for this work is the implementation of corrugation pitch modulated distributed feedback structures in InGaAsSb/AlGaAsSb strained quantum well lasers.
This development is being carried out in collaboration with the Photonics Technology Group of the JPL Microdevices Laboratory (MDL) and the University of Illinois Electro-Optic Systems Laboratory at Urbana-Champaign.
References
B. T. McGuckin, R. T. Menzies, and H. Hemmati: Efficient energy extraction from a diode-pumped Q-switched Tm,Ho:YLiF4 laser. Appl. Phys. Lett., 59(23), 1991, 2926-2928.
B. T. McGuckin and R. T. Menzies: Efficient CW Diode-Pumped Tm,Ho:YLF Laser with Tunability Near 2.067 µm. IEEE J. Quantum Electron., 28(4), 1992, 1025-1028.
B. T. McGuckin, R. T. Menzies, and C. Esproles: Tunable Frequency Stabilized Diode Laser Pumped Tm,Ho:YLF Laser at Room Temperature. Appl. Opt., 32(12), 1993, 2082-2084.