Proceedings Article | 15 December 2000
Richard Campbell, William Edwards, James Barnes, Mark Storm, Larry Petway, Waverly Marsh, Songsheng Chen
KEYWORDS: Ultraviolet radiation, Transmitters, Laser development, Ozone, Laser applications, Quantum efficiency, Optical parametric oscillators, Laser systems engineering, Photons, Solid state lasers
Development of a UV laser transmitter capable of operating from a space platform is a critical step in enabling global earth observations of aerosols and ozone at resolutions greater than current passive instrument capabilities. Tropospheric chemistry is well recognized as the next frontier for global atmospheric measurement. Moreover, global measurement of tropospheric ozone with high vertical resolution (2.5 km) from space were endorsed for the EX-1 Mission by NASA's Post2002 Mission Planning Workshop. At this workshop, held in Easton, Maryland, in August 1998, it was recognized that a space-based UV Differential Absorption Lidar (DIAL) system was necessary in order to obtain this high- resolution capability for measurements of ozone and aerosols. The results of this workshop can be found at http:llwww.hq.nasa.gov/office/ese/nra/RFldodgelPanelrev.html. For the EX-1 Mission, the UV DIAL measurement would be complemented with passive measurements of ozone precursor gases and pollutant tracer species. Langley Research Center (LaRC) and the Canadian Space Agency (CSA) have jointly studied the requirements for a satellite based, global ozone monitoring instrument. The study, called Ozone Research using Advanced Cooperative Lidar Experiment (ORACLE) has defined the DIAL instrument performance, weight and power, and configuration requirements for a space based measurement. In order to achieve the measurement resolution and acceptable signal-to-noise from lidar returns, 500mJ/pulse (10 Watts average power) is required at both, 308nm and 320nm wavelengths. These are consecutive pulses, in a 10 Hz, double-pulsed format. The two wavelengths are used as the on- and off-lines for the ozone DIAL measurement.1 5 NASA Langley is currently developing technology for a UV laser transmitter capable of meeting the ORACLE requirements; this development effort is focused on improving the efficiency of converting 1im laser energy to the 308 and 320nm energies needed for the DIAL measurement. Our approach includes making maximum use of existing, space-qualified optical components to reduce risk, cost and development time. Our experimental efforts to date have shown that our UV generation scheme is viable, and that energies greater than lOOmJ/pulse are possible. Future work will focus on improving efficiency and on addressing reliability, size and scalability issues. Our goal is to improve the optical conversion efficiency from the current state of the art, currently at 5%, to a minimum of 12%. We will accomplish this by using OPO/OPA and sum frequency mixing technology to generate the required UV wavelengths. The technology being developed has undergone an extensive peer review and down-select process from 20 possible UV generation schemes through in-house and industry trade studies and by experimental investigations. By using the selected technique and a diode pumped laser, a wall plug efficiency (electrical to optical) of greater than 2% is expected. In this paper, we will briefly discuss the study effort to date, the overall system design, and the down select process for the proposed laser design. We will describe UV laser technology that minimizes the total number of optical components (for enhanced reliability) as well as the number of UV coated optics required to transmit the light from the laser (for enhanced optical damage resistance). While the goal is to develop a laser that will produce 500 mJ of energy, we will describe an optional design that will produce output energies between 100-200mJ/unit and techniques for combining multiple laser modules in order to transmit a minimum of 500mJ of UV energy in each pulse of the on- and off-line pulse pairs. This modular laser approach provides redundancy and significantly reduces development time, risk and cost when compared to the development of a single, 500mJ double-pulsed laser subsystem. Finally, we will describe the common source for seeding the OPO's such that the absolute wavelength and linewidth of each transmitter module will be controlled and summarize the laser development effort to date, including results that include the highest known UV energy ever produced by a solid-state laser operating in this wavelength region.
