A diffraction grating consisting of a periodic gradient in the index of refraction of a thin surface layer has been shown to be effective as a means of far-field coupling of monochromatic light into or out of the "whispering-gallery" electromagnetic modes of a transparent microsphere. This far-field coupling can be an alternative to the near-field (evanescent-wave) coupling afforded by prism- and fiber-optic couplers described in the immediately preceding article. Far-field coupling is preferable to near-field coupling in applications in which there are requirements for undisturbed access to the entire surfaces of microspheres. Examples of such applications include (1) a proposed atomic cavity in which cold atoms would orbit in a toroidal trap around a microsphere and (2) a photonic quantum logic gate based on coupling between a high-Q (where Q is the resonance quality factor) microsphere and trapped individual resonant ions.
From the measurement data, the maximum grating coupling efficiency was calculated to be 14 percent. The grating loaded the resonance sufficiently to decrease the Q of the microsphere to a value in the range of (0.2 to 2) × 106. [The initial Q (without the grating) was 1.2 × 108.] Higher Q could be obtained by reducing the strength of the grating. Efficiency of coupling could be increased by optimizing the exposure to ultraviolet light, improving the grating profile, and minimizing scattering losses. Parasitic coupling to low-Q higher-order modes in the microsphere could be prevented by decreasing the diameter of the microsphere.
This work was done by Vladimir Iltchenko and Lute Maleki of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category.
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-20618, volume and number of this NASA Tech Briefs issue, and the page number.
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Surface Gratings for Optical Coupling with Microspheres
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Overview
The document presents a novel technique for coupling light to high-Q whispering-gallery (WG) modes in optical microsphere cavities, developed by Vladimir Iltchenko and Lute Maleki at Caltech for NASA's Jet Propulsion Laboratory. This method utilizes a UV-induced diffraction grating on the surface of a silica microsphere, enhanced with a germanium-doped photosensitive layer, to facilitate far-field coupling of light. This approach serves as an efficient alternative to traditional evanescent wave couplers, such as total internal reflection prisms and optical waveguides, which can disrupt the symmetry and introduce stray fields that affect the energy levels in applications involving trapped atoms or ions.
Whispering-gallery modes are characterized by their narrow resonances, making them suitable for various applications, including fiber-optic filters, compact narrowband lasers, and optical/microwave oscillators. Previous methods for exciting these modes relied on bulky near-field couplers, which required precise adjustments to the incident light angle or waveguide propagation constants. However, these methods can compromise the integrity of the microsphere's surface and the overall system.
The proposed technique allows for undisturbed access to the entire surface of the microsphere, which is crucial for applications such as atomic cavity designs where cold atoms are trapped in a toroidal configuration around the sphere. The document details the process of writing the grating pattern onto the microsphere's surface using UV light, which improves the grating profile and minimizes scattering losses. Additionally, it mentions that reducing the microsphere's diameter can help prevent parasitic coupling to low-Q higher-order modes.
The findings are supported by test results and materials included as an exhibit in the report. The document concludes by emphasizing the flexibility and effectiveness of this new coupling technique, which could significantly advance the field of optical microsphere applications in quantum optics and photonics.
For further inquiries regarding commercial use of this invention, the document provides contact information for the Technology Reporting Office at JPL. Overall, this work represents a significant step forward in the development of advanced optical technologies, with potential implications for various scientific and engineering fields.