A technique for measuring small optical loss using an oscillating spherical mirror interferometer

The measurement of very small optical losses (the order of a few per cent) by conventional methods becomes very difficult because of the extreme accuracy required. This article shows that both high mirror reflectances and low transmission losses can be readily measured using an oscillating mirror interferometer as a frequency spectrum analyzer. The theory developed shows that when this type of interferometer is excited by a continuous gaseous laser, the total optical loss is proportional to the frequency resolution or the finesse. The theory also shows that the first-order velocity effect produced by having the mirror move at a velocity of one foot per hour can be large if the total optical loss is about 0.25 per cent. For the velocities and optical losses we have encountered so far in our measurement system, the first-order minor velocity effect can be neglected. The range of reflectance of mirrors we have measured is from 94 to 99.5 per cent, and the measurements for the optical transmission loss range from 0.2 to 3 per cent. The accuracy to which a 1 per cent loss can be repeated is 1.0 ± 0.1 per cent. It was found that the transmission loss through an optical grade of fused quartz (Homosil) at 6328 A is about 1 db per meter, and that for Plexiglas II is about 2 db per meter.