Designs for Coherent X-Ray Lasers,
LAWRENCE LIVERMORE NATIONAL LAB CA
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The property of coherence is important for many suggested applications of x-ray lasers. For x-ray holography, the longitudinal coherence length is required to be larger than the pathlength difference between the object beam and the reference beam, estimated to be about 5 microns in a Fourier transform holography system. The longitudinal coherence length is related to the line width by L sub l lamba squareddelta lambda, where lambda is the laser wavelength. Using the thermal Doppler width for an estimate we get L sub l 37 microns for a 45 A laser. This estimate is supported by a measurement at 206 A. We therefore expect that longitudinal coherence will not be a problem. The transverse coherence length must be larger than the extent of the sample, typically approx. 10 microns, and a certain amount of energy must irradiate the sample in order provide a high signal to noise hologram. The coherence length is obtainable straightforwardly by placing a pinhole in front of the laser, or by placing the object far from the laser. The combined requirement of energy and coherence is however, much more difficult, requiring efficient production of coherent radiation.
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