A Technique for Measuring Small Optical Loss Using an Oscillating Spherical Mirror Interferometer

In developing a long-distance optical communication system employing a large number of components, it is essential to be able to measure accurately the optical transmission loss of each component. These components may include mirrors with reflectances in the order of 99 per cent, Brewster angle output windows, various lenses, and other passive elements with optical transmission losses of 1 per cent or less. For such small losses, the conventional measuring techniques become increasingly difficult because of the extreme accuracy required. 15(53 1564 T H E BELL SYSTEM TECHNICAL J O U R N A L , J U L Y 1904 In conventional measuring systems, the loss (or reflectance) is calculated by comparing the electrical output of a photodetector for two different optical conditions, first with the unknown in the system, and then with the unknown removed. As the magnitude of the optical loss decreases, this comparison becomes more and more inaccurate, as it requires the measurement of a small difference between two relatively large photodetector outputs. In these measuring systems, a number of methods have been developed to minimize these errors, which are principally optical. 1 However, it is believed that measuring techniques to be described below will more readily measure veiy small optical losses. The proposed method uses a frequency spectrum analyzer at optical frequencies. Such an analyzer can be obtained by using a Fabry-Perot type of interferometer as a transmission clement between an optical source and a photodetector.