One possible design forms an optical cavity with extremely low thermal drift using two materials, one of which exhibits a negative temperature coefficient of optical path length.
Mandeep Singh, Dick Neily, William Eccleshall and Bill McCreath
Because of the stringent requirements for frequency stability in dense wavelength division multiplexing (DWDM) systems, demand is continuing to grow for passive optical telecommunications components with high thermal stability. One class of device common in these systems is the etalon, which is essentially an optical cavity that induces multiple beam interference between two reflecting surfaces. Prevalent variants include Fabry-Perot and Gires-Tournois configurations.
The devices are common subcomponents in wavelength filters (tunable and fixed), laser wavelength lockers, interleavers and chromatic dispersion compensators. Such optical subassemblies typically operate in environments with thermal excursions of several tens of degrees Celsius, which can result from ambient temperature fluctuations or thermal contact between the etalon and a device such as a thermally tunable laser diode. The temperature change induces a change in the cavity's optical path length (with the degree of change dependent on cavity material), which could also produce unacceptable changes in the optical characteristics of the subsystem. At present, the available option for an etalon with extremely low thermal drift is the gas (usually air or nitrogen) cavity etalon…