This invention is related to optical filters which can be tuned for signals at different wavelengths, and more particularly, to tunable optical filters of etalon-type construction.
Optical filters which are tunable are highly desirable components for optical networks and for some optical systems, such as spectrometers, for example. Optical filters typically transmit light at particular wavelengths and block light at other wavelengths. An optical filter which is tunable allows the wavelengths to be selected.
Tunable optical filters can be quite varied in construction. For example, liquid crystal cells, thin film filters, ruled gratings, distributed Bragg reflectors (gratings), and fiber Bragg gratings, a type of distributed Bragg reflector, have been used as the tuning constituents in tunable optical filters. Of course, other parts are required to operate the tuning constituent in a tunable optical filter. The resulting optical performance, reliability, speed of operation, cost and size, among many parameters, of such tunable optical filters vary widely.
One conventional (and simple) structure for an optical filter is the etalon in which two highly reflective, parallel surfaces form a resonating cavity for wavelength selection. This is the familiar Fabry-Perot interferometer. To make the etalon tunable, the optical distance between the two reflecting mirrors must be changed. This has been done conventionally by mechanical means, such as electrical motors, but motors being mechanical are fundamentally unreliable. The moving parts of a motor wear from friction, causing slop in the system, and can ultimately break down. Another drawback is that the mass of the motor parts slows the response time of such a filter. Additionally a motor is comparatively bulky which limits the applications of such tunable optical filters. Piezoelectric materials have been used in place of motors but these require very high voltages to effect the change in optical distance between the mirrors. The high voltages require special high voltage circuits and slow switching speeds and optical performance.
On the other hand, the present invention is directed toward ameliorating some of the disadvantages of an etalon-type tunable optical filter.