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.

Figure 1. Three Coupled, Tunable WGM Resonators constitute a third-order tunable band-pass optical filter.
As 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.

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:

Innovative Technology Assets Management
JPL
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
(818) 354-2240
E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Refer to NPO-40873.

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This Brief includes a Technical Support Package (TSP).
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Tunable Filter Made From Three Coupled WGM Resonators

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Photonics Tech Briefs Magazine

This article first appeared in the April, 2006 issue of Photonics Tech Briefs Magazine (Vol. 30 No. 4).

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Overview

The document is a Technical Support Package for a Tunable Filter made from three coupled Whispering Gallery Mode (WGM) resonators, referenced as NPO-40873 in NASA Tech Briefs. It is part of NASA's Commercial Technology Program, aimed at disseminating aerospace-related developments with potential wider technological, scientific, or commercial applications.

The tunable filter technology is significant for various applications, particularly in telecommunications and sensor systems, where precise frequency control and filtering are essential. The document outlines the principles behind the WGM resonators, which utilize the unique properties of light and sound waves to achieve high sensitivity and selectivity in filtering applications.

The package includes technical details, experimental results, and potential applications of the tunable filter technology. It emphasizes the advantages of using coupled WGM resonators, such as improved performance metrics compared to traditional filtering methods. The document also discusses the design considerations and challenges faced during the development of the tunable filter, including material selection, resonator coupling efficiency, and tuning mechanisms.

Additionally, the document provides information on how this technology can be integrated into existing systems and its potential impact on future innovations in various fields, including telecommunications, aerospace, and medical devices. The tunable filter's ability to adapt to different frequencies makes it a versatile component in advanced communication systems, enhancing signal processing capabilities.

For further assistance and information, the document directs readers to the NASA Scientific and Technical Information (STI) Program Office, providing contact details for inquiries. It also includes a disclaimer regarding the use of the information contained within the document, clarifying that the U.S. Government does not assume liability for its application.

Overall, this Technical Support Package serves as a comprehensive resource for understanding the advancements in tunable filter technology, showcasing NASA's commitment to fostering innovation and collaboration in the aerospace sector and beyond.