A tunable third-order band-pass optical filter has been constructed as an assembly of three coupled, tunable, whispering-gallery-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.
The filter operates at a nominal wavelength of 1,550 nm and can be tuned over a frequency range of ±12 GHz by applying a potential in the range of ±150 V to the electrodes. The insertion loss (the loss between the input and output coupling optical fibers) was found to be repeatable at 6 dB. The resonance quality factor (Q) of the main sequence of resonator modes was found to be 5 × 106, which corresponds to a bandwidth of 30 MHz. The filter can be shifted from one operating frequency to another within a tuning time ≤30 μs. The transmission curve of the filter at frequencies near the middle of the passband closely approximates a theoretical third-order Butterworth filter profile, as shown in Figure 2.
This work was done by Anatoliy Savchenkov, Vladimir Iltchenko, Lute Maleki, and Andrey Matsko of Caltech for NASA’s Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
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Refer to NPO-40873.
This Brief includes a Technical Support Package (TSP).

Tunable Filter Made From Three Coupled WGM Resonators
(reference NPO-40873) is currently available for download from the TSP library.
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