Thermal hysteresis exhibited by a germanium Fabry–Perot interferometer heated by laser radiation

It is demonstrated theoretically that thermal hysteresis should occur in a Fabry–Perot interfermoeter because of a change in the refractive index of the interferometer material when it is heated by laser radiation. The results are given of an experimental study of the temperature polystability of a germanium interferometer illuminated with CO₂ laser radiation. Regions of stability of the interferometer temperature are found for different temperatures of the ambient medium and abrupt changes in this temperature by various amounts are reported. It is concluded that similar effects appear also in the light transmitted by the interferometer.