<title>Low-Scatter, Low-Loss Mirrors for Laser Gyros</title>

Abstract

AbstractDielectric mirrors with scatter <0.03% and loss <0.1% were developed for laser gyros.This paper discusses the current performance of laser gyro mirrors which are manufacturedat Optical Coating Laboratory, Inc., and the test and diagnostic instruments which areused in their production and measurement.IntroductionIn order to reduce the frequency synchronization between oppositely traveling waves ina ring laser, mutual coupling of energy between the two waves must be reduced. Thedominant source of the coupling is the scattering of energy from each of the contra -rotating beams into the direction of the other'. In the ring laser, backscattering sourcescan include specular reflections of energy back into the cavity, reflection from opticalcomponents located in the cavity, reflections from dust and impurity particles in the gas,and non - specular reflection of light which is scattered from the dielectric mirrorcoatings.Another source of coupling is spatially varying losses over the cavity length whichgive rise to loss lock -in2. In a ring laser, the losses are predominantly caused byimperfections and absorption in the dielectric mirrors.The lock -in problem has presented a formidable obstacle during the 15 year period oflaser gyro development. The solution to this problem has been the use of a combination ofone of several biasing techniques' such as mechanical dithering of the laser cavity, andthe lowering of scatter and loss in the dielectric mirrors. Included in the mirrordevelopment is improved lifetime of dielectric coatings directly exposed to a plasma.Optical Coating Laboratory, Inc. has a dedicated program to provide the laser gyroindustry with low- scatter, low -loss dielectric mirrors of high quality and reasonable cost.As a result of this commitment, the company has developed new processes for substratefabrication and polishing, cleaning, coating, and instruments for testing and measurement.The processes have resulted in scatter levels less than 100 ppm (4 Arms) and losses less