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).

The V→T, R relaxation rate for NH3  (ν2) has been studied from 198 to 398 °K by the method of laser-excited vibrational fluorescence. The self-deactivation rate constant decreases from 4.9×10−11 cm3 molecule−1 sec−1 at 198 °K to 2.7×10−11 cm3 molecule−1 sec−1 at 398 °K. The rate constants for deactivation by He, Ar, N2, and O2 are much smaller and show a weak temperature dependence in the opposite direction. For H2 18O the vibrational relaxation rates of the coupled ν1, ν3 stretching level manifold and of the 2ν2 bending level have been studied from 250 to 400 °K. The ν1, ν3 self-deactivation rate goes from 3.5×10−11 cm3 molecule−1 sec−1 at 250 °K to 2.0×10−11 cm3 molecule−1 sec−1 at 400 °K. For 2ν2 it goes from 1.2×10−10 to 7.8×10−11 cm3 molecule−1 sec−1. The temperature depencence of the deactivation of both levels by He and Ar is much weaker and the rates are several hundred times slower. CO2 deactivates ν1, ν3 about 50 times faster than He or Ar at 293 °K.

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Although thin film coating technology has evolved to the point that damage thresholds of several hundred MW/cm2 can be routinely achieved, sealed laser systems which must be operated for extended times or at elevated temperatures frequently experience failure due to optical damage. This damage, which is frequently due to the build up of gas phase contaminants in the sealed optical compartment, occurs in spite of the fact that the lasers were designed such that the intracavity intensities are only a few tens of MW/cm2. Since much of our work involves designing Q-switched Nd:YAG lasers that operate over extreme environmental conditions, eliminating contamination damage at 1 m is of particular interest to us. In this paper we will describe our current understanding of contamination induced damage at 1 m and give an overview of the processes that can be used to eliminate such damage in fielded systems.Keywords: contamination, optical damage, laser-induced damage 1. BACKGROUND Many commercial and military applications of lasers require a Q-switched laser operating in the vicinity of 1 m which is light (system weight --5-15 lbs.), compact (volume < 2000 cm2), and reasonably priced.Environmental considerations frequently require that the laser be sealed and that it be capable of operation over a wide temperature range. These constraints push laser designers toward the use of polymer in the optical cavity, the bonding of optics with polymer based adhesives, and the placement of electronics in the optical compartment. The consequence of these design approaches is an increased probability of optical damage due to the build up of gas phase contaminants, even though the laser intensities on the optics in the system are <100 MW/cm2.One possible solution to the contamination problem is to design lasers that have only metals and ceramics in the optical cavity and to use only hard seals (i.e. no polymeric 0-ring seals). This approach significantly complicates the job of the design engineer who is trying to simultaneously minimize the cost and weight of the laser. Another approach to the contamination problem is to develop a better understanding of the causes of contamination induced damage. Armed with this better understanding, the designer can then avoid the materials that are the major contributors to contamination damage and choose only materials that have low or no contribution to contamination damage. This is the approach that we have taken and successfully implemented for the laser systems built at LLSD. DAMAGE TEST PROCEDUREThe key to understanding contamination induced damage is to be able to duplicate the damage in a controlled manner. The hardware and method we have developed for duplicating contamination damage is illustrated in Fig. 1. The fixture is an aluminum box of 750 ml internal volume with two 0-ring sealed windows on the ends. The top portion of the fixture is sealed to a flat baseplate with a third 0-ring seal. The test procedure begins by cleaning and certifying the test fixture. After appropriate solvent c...