Tunable Filter Made From Three Coupled WGM Resonators

A tunable third-order band-pass optical filter has been constructed as an assembly of three coupled, tunable, whisperinggallery- mode resonators similar to the one described in “Whispering-Gallery- Mode Tunable Narrow-Band-Pass Filter” (NPO-30896), NASA Tech Briefs, Vol. 28, No. 4 (April 2004), page 5a. This filter offers a combination of four characteristics that are desirable for potential applications in photonics: (1)wide real-time tunability accompanied by a high-order filter function, (2) narrowness of the passband, (3) relatively low loss between input and output coupling optical fibers, and (4) a sparse spectrum. In contrast, prior tunable band-pass optical filters have exhibited, at most, two of these four characteristics described in several prior NASA Tech Briefs articles, a whispering- gallery-mode (WGM) resonator is a spheroidal, disklike, or toroidal body made of a highly transparent material. It is so named because it is designed to exploit whispering-gallery electromagnetic modes, which are waveguide modes that propagate circumferentially and are concentrated in a narrow toroidal region centered on the equatorial plane and located near the outermost edge.