Frequency combs derived from optical microresonators are required to reach an octave in span. This is required for self-referencing a comb. Presently, the frequency comb span produced by whispering gallery microcavities and other types of cavities is limited mostly by total cavity dispersion.

A finite element model of a microstructured cavity mode. Red traces show lines of equal optical intensity.
To expand the comb towards an octave span, a new class of optical resonators was developed. The characteristic and distinct feature of these new cavities is the presence of a microstructure that defines an optical waveguide and a substrate that shares the same axial symmetry with the waveguide.

The resonator is a waveguide, e.g. a ridge waveguide, that is formed on a cylindrical substrate. The waveguide and a substrate are made with the same transparent dielectric. The waveguide is not limited to a rectangular ridge waveguide; however, any microscopic structure can be used and is essential as the shape of this microstructure defines the geometrical dispersion of the resonator along with the quality factors of its optical modes and the set of supported modes. The important feature is that the scale of a shape-microstructure is the same as the scale of the cross-section of the optical mode field supported by the resonator.

A state-of-the-art fabrication technique was used. Primarily, computer-controlled diamond turning and microstructuring with some hand polishing were used. The steps can be automated, and future fabrication techniques can be developed. There is a potential for on-chip heterogeneous integration as the cavity can be made 20 to 100 micrometers thick.

A new imaging technique, a lithographic photography, was used to image the cross-sections of the microstructures. Previously, similar cross-sections were obtained by cutting a hole in a resonator using ion milling, and using an electron scanning microscope to obtain an image. The new technique is nondestructive.

The new cavity architecture is distinct from all known and demonstrated optical resonators. It shares axial symmetry and mode confinement features with the regular and “single mode” whispering gallery mode (WGM) resonators. However, none of the demonstrated WGM resonators had a microstructured shape that enables dispersion control in the new cavities. The shape and dimensions of the protrusion are critical in defining the cavity dispersion, which has not been emphasized before. The new cavity resembles the conventional ring resonator; however, the key difference is that the substrate shares the axial symmetry of the ring.

This work was done by Ivan S. Grudinin and Nan Yu of Caltech for NASA’s Jet Propulsion Laboratory. For more information, contact iaoffice@ 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-49527