PD2-1 An In-Flight Tuning Method for the NASA. ER*"*2 Wsfc^iY* Vsrvw* TidcLnv Sv&fc^sni W.M. Hall*, I. Taback** R.L. Kenimer**, J.J. Degnan***, E.V. Browell* *NASA Langley Research Center **The Bionetics Corporation Hampton, Virginia 23665 Hampton, Virginia 23666 ***NASA Goddard Space Flight Center Greenbelt, Maryland 20771 NASA is designing a tropospheric water vapor differential absorption lidar (DIAL) instrument for the ER-2 aircraft. The program, called LASE for Lidar Atmospheric Sensing Experiment, is an outgrowth of current airborne lidar activities 1 * 2 . Overall program management resides in NASA's Office of Space Science and A p p l i c a t S T Langley Research Center is providing project management, experiment definition, system integration, and wavemeter development. Goddard Space Flight Center will develop the tunable Alexandrite lasers. The LASE in-flight tuning scheme must meet experiment requirements, accommodate an initial laser wavelength uncertainty, and be automatic. This paper, after providing an overview of the experiment reguirements and IASE system, describes a tuning scheme which potentially reduces the wavemeter from a multiple interferometer unit to a single interferometer unit. The DIAL measurement of H2O profiles throughout the vertical extent of the troposphere requires the use of various H2O absorption lines of different line strengths. Some of the selected lines are shown in Figure 1. Absorption line cross sections range from 4.37 X 10~ 24 to 70.1 X 10"" 24 cm 2 , corresponding to LOG S values of -4.73 to -3.19 on the plot (S in cm" 2 atnf 1 ). The widths of all these lines are about 10 pm FWHM at standard temperature and pressure (STP). The on-line laser wavelength centroid must be tuned to within 0.5 pm (7 parts in 10 7 ) of the absorption line center. The DIAL measurement also requires that :»99 percent of the total laser energy reside within a bandwidth of 1 pm and that the spectral distribution be measured to within 0.25 pm in absolute wavelength. The off-line laser must be tuned with an error <2 pm, within 70 pm from the selected H2O line, and in a region that has negligible atmospheric absorption. The IASE system uses two identical 5 Hz, 150 mJ Alexandrite lasers which are line-narrowed to 1 pm by three stepper motor driven intracavity tuning elements (Table 1 ) . The air-gapped etalons use low expansion coefficient spacers, are thermally stabilized to + 0.1°c and are enclosed in a pressurized head. The lasers can maintain an absolute open-loop tuning error (without wavemeter data) of <170 pm across shutdown and all mission environments. In addition, after thermal stabilization the relationship between laser wavelength and stepper motor settings is repeatable to about 0.1 pm. This level of repeatability is not an important error in the proposed in-flight tuning scheme. For closed-loop tuning, a fraction of each laser's beam is input to an integrating sphere to interface with the wavemeter. This assures that all spectral oompanents of the multimode © 1985 OSA/ASSL 1985