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.

Figure 2. The Measured Transmission Spectrum of the filter was fitted with a Butterworth profile function γ6/[(ν)6+γ6], where γ = 29 MHz and ν is the laser frequency detuning (the difference between the laser frequency and the peak-transmission frequency).
Figure 1 depicts the optical layout of the present filter comprising an assembly of three coupled, tunable WGM resonators. Each WGM resonator is made from a disk of Z-cut LiNbO3 of 3.3-mm diameter and 50-µm thickness. The perimeter of the disk is polished and rounded to a radius of curvature of 40 µm. The free spectral range of each WGM resonator is about 13.3 GHz. Gold coats on the flat faces of the disk serve as electrodes for exploiting the electro-optical effect in LiNbO3 for tuning. There is no metal coat on the rounded perimeter region, where the whispering- gallery modes propagate. Light is coupled from an input optical fiber into the whispering-gallery modes of the first WGM resonator by means of a diamond prism. Another diamond prism is used to couple light from the whispering-gallery modes of the third WGM resonator to an output optical fiber.

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.

Savchenkov, Vladimir Iltchenko, Lute Maleki, and Andrey Matsko of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free online at www.techbriefs.com/tsp under the Physical Sciences category.

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Refer to NPO-40873, volume and number of this NASA Tech Briefs issue, and the page number.