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A method was developed for relative distance measurements of long optical pathlengths of up to 40 m with good distance resolution up to +/- 5 nm ((lambda) /120 at 0.6238 (mu)) and moderate temporal resolution up to 250 Hz, sufficient to quantify low and moderate frequency piston jitter of a ring resonator. The measurement scheme used two frequency interferometry using laser probe beams and was based upon a fringe counting technique utilizing a modified Hewlett-Packard (HP) Model 5527 A distance measuring interferometer. Modifications included a much different interferometer than that available from HP and a high performance receiver front-end based upon an avalanche photodetector. The interferometric method enabled in-situ resonator piston jitter measurements, internal to the resonator optical train, which were insensitive to optical disturbances external to the resonator. In an experimental demonstration, portions of a ring resonator optical train were installed in a supersonic combustion driven laser test-bed and its vibrational characteristics analyzed using this method. Data was recorded and analyzed during quiescent, vacuum pumps-on and 'cold flow' conditions with variations of number and combination of optical mounts. The data indicated that a high quality state-of-the-art translation stage incorporated in the ring resonator design was not sufficiently stable for the ring resonator design requirements. The data also indicated that the facility disturbances during pumps-on and 'cold flow' conditions significantly increased piston jitter. On the other hand, the device flow disturbances of the laser probe beams crossing the gain flow region during 'cold flows' and disturbances due to table bending and twisting between two optic vacuum optic boxes were minimal. Facility disturbance data measured by accelerometers and tilt disturbances measured by a laser probe beam incident upon a position sensor are also included. A design is presented for an interferometer which can be integral to an operating unstable ring resonator.
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