What is a Fabry-Perot etalon?
A Fabry-Perot etalon is an optical filtering device. Only a very narrow bandwidth of light is allowed to pass through the etalon, while the remaining light is reflected back. The Fabry-Perot etalon physically consists of two extremely flat quartz surfaces. Both surfaces are covered with a uniform reflective coating, while the reflectance of each surface plane is of equal magnitude. These two surfaces can be separated by an air gap, a solid quartz gap, or by other means. However, the two surfaces must be kept parallel with a high degree of precision. If an air gap is used, then an active control system may be used to ensure parallelism. Pictured on the right is a solid model of an air-gapped Fabry-Perot etalon. The inner surfaces of the two quartz plates are covered with a reflective coating (dark blue region), and are separated by three posts to maintain parallelism. If the light source entering the etalon is collimated, then the light is reflected back and forth within the etalon, cre
