Tunable optical filters are required for selecting channels in wavelength-division-multiplexed optical communication systems. In addition to being used to select channels, they play an important role in filtering out noise in amplified wavelength-division-multiplexed systems incorporating optical amplifiers.
When used in these applications, the tunable optical filters should have specific desired characteristics such as a large tunability, a narrow passband, low insertion loss; be able to be locked to the transmitter signal easily, be reliable, be cost-effective, and the like.
At the present time there are various optical filters which have some or most of the above noted desired characteristics, such as, for example: A) a fiber Fabry-Perot interferometer; B) an angle-tuned etalon filter, C) a liquid-crystal etalon filter; D) a Ti:LiNbO3 electro-optic wavelength filter, and E) DFB or DBR active optical filter.
This list, which is not complete, is representative of the many different types of optical filters that are presently known. While some of them may have many of the desired characteristics identified above, not one of them satisfies all of the desired characteristics. The characteristics of these filters are compared in detail in other references (for example, "Angle-tuned etalon filters for optical channel selection in high-density wavelength division multiplexed systems", J. Lightwave Technol., vol. 7, pp. 615-624, 1989).
This invention is an optical filter which has all of the desired characteristics noted above.