Chemical Laser In this laser type the energy to molecules is produced by chemical reaction. An example is hydrogen fluoride (HF) laser that has a reaction shown in table 7.2. Another possible chemical lasant is deuterium fluoride (DF). Efficiency for an HF laser is 2.5-3.0% and for a DF laser 3.5- 4.0%. Table 7.2 Chemical Reaction in Hydrogen Fluoride Laser The third reaction shows that a photon stimulates the HF* (energized HF molecule) to emit a second photon of the same frequency. A chemical laser consumes fuel and oxidizer and produces reaction product. The laser is operable only until fuel and oxidizer is consumed. A space chemical laser has to be closed cycled so that fuel and oxidizer can be reproduced from the reaction product. This reproducing can be accomplished by electrolysis or by direct sunlight. Although this laser type is possible in principle, it has not yet been developed. [Walbridge, 1980] Laser Diode Array A Solid State Laser uses semiconductor technology to produce the laser beam. The power conversion and density capability of this type of laser is quite low. Separate diodes can be formed as an array and phase locldng their beams together the output power can be increased. The state of art laser diodes are quite unreliable, inefficient and their life time is low. Technology developments to get pure materials can improve their feasibility. A laser transmitter might then be produced as a diode matrix in a same way as the phase array antenna. If a phase shift between different diodes would somehow be possible the laser beam could be also pointable to different power receiving sites. Transmission Electromagnetic radiation loses energy in plasma through linear (ohmic) and nonlinear (anomalous) absorption. Beverly has studied this absorption in his paper for an intense laser beam propagating in upper atmosphere. In altitudes of 120 km to 340 km the absorption by plasma is 0.17 nW/m3 at maximum using a 100 MW, 5pm laser beam. As plasma has maximum density in altitudes of 200-500 km the absorption due to plasma in space is negligible. [Beverly, 1980b] Laser beaming from space to earth is attenuated mainly by atmosphere. Clear air propagation can be very effective with proper selection of wavelength but haze, fog, clouds and rain can attenuate the beam. Most of the attenuation occurs at altitudes below 0.5 km so reception site should be above this altitude. The attenuation by atmosphere can be either linear or non-linear. Linear attenuation is independent of the beam intensity while non-linear depends on beam intensity. There are some spectral windows on which the attenuation is very low. The molecular absorption is almost negligible for 2 pm and 11 pm wavelength bands at high altitudes. In low altitudes even a light fog and light cloud cover cause an increase on the molecular absorption for 11 pm wavelength. At altitudes of 0.5 km the yearly average transmission efficiency for 9.114 pm (12C18O2) laser is estimated to be 93% and at altitude of 3.5 km some 98%. These values represent clear summer air when laser output wavelength is optimized to molecular absorption. During winter time the air is dryer and these figures may be even slightly better. Hole boring through monodisperse and polydisperse aerosols has been proposed and studied by many authors. These studies have been both theoretical and experimental. The method would be used to generate a clear path for laser beam through certain types of hazes, fogs and clouds. For lasers operating in the 11 pm window, power densities of 100-200 W/cm2 are required. This density is below the weapon-quality densities of 1 kW/cm2 but it is much more than safety
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