Laser And Nonlinear Optical Materials For Laser Remote Sensing
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Laser And Nonlinear Optical Materials For Laser Remote Sensing
Norman P. Barnes NASA Langley Research Center Hampton, VA 23681
ABSTRACT NASA remote sensing missions involving laser systems and their economic impact are outlined. Potential remote sensing missions include: green house gasses, tropospheric winds, ozone, water vapor, and ice cap thickness. Systems to perform these measurements use lanthanide series lasers and nonlinear devices including second harmonic generators and parametric oscillators. Demands these missions place on the laser and nonlinear optical materials are discussed from a materials point of view. Methods of designing new laser and nonlinear optical materials to meet these demands are presented.
INTRODUCTION Measurements on the state of health of Planet Earth are required to serve as the basis for rational policy decisions. Among the measurements needed are: green house gasses, ozone, tropospheric winds, and ice cap thickness. Some possible policy decisions, such as limiting green house gas emission, can have huge economic impacts. The necessary measurements can be made on a global basis employing lidars deployed in polar orbiting satellites. The laser transmitters for these lidar systems are usually highly specialized. As an example the wavelength of the laser must coincide with the peak absorption of the atmospheric constituent of interest to within a part in 106. Engineering and selection of the best laser and nonlinear optical materials for these applications can be facilitated by compositional tuning.
SOLID STATE LASER MATERIALS Solid state laser materials can be compositionally tuned to achieve the requisite wavelengths to remotely sense atmospheric variables such as water vapor and green house gasses. For space deployment, lanthanide series lasers are usually preferred over transition metal lasers. Lanthanide series lasers involve 4f electronic transitions. Because the 4f electrons are shielded from the electric field of the crystal lattice by the 5s and 5p electrons, the transitions are usually narrow. On the other hand, transition metal lasers involve 3d electronic transitions. These electrons are not shielded from the electric field of the crystal lattice and the transitions are usally quite wide. The product of the emission cross section, lifetime, and bandwidth is limited. Wide transitions of the transition metal lasers imply small emission cross sections, short lifetimes or both. The latter qualities are incompatible with diode pumping and efficient operation. Conversely, lanthanide series lasers are compatible with efficient diode pumping. However, the narrow band of wavelengths associated with a laser transition between 2 energy levels must
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nearly match the absorption feature of the atmospheric constituent of interest. Compositional tuning is a method of achieving this. A review of the physics of lanthanide series lasers points out the degree of tuning possible. Energy levels of the 4f electrons depend both on the particular lanthanide series atom and th
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