Method for Measuring Small Optical Losses Using a He–Ne Laser

The output power or threshold for oscillation, respectively, of a He–Ne gas laser are severely influenced by the presence of small losses in the resonator. Small losses between about 0.01 and 1% may be measured by substituting the unknown loss of a smaple for linearly polarized light by the known and adjustable reflection losses due to a plane parallel glass plate near the Brewster angle. A measurement apparatus based on this principle was built and the influence of various sources of error was evaluated. One source of error is the deviation from ideal linear polarization found in the output of gas lasers with Brewster angle windows. Measurements are given of the residual single pass transmission loss of doubly antireflection coated glass plates.