Cylindrical lenses can be used to increase the efficiency of coupling of light between (1) beams with circular or nearly circular cross sections propagating in single-mode optical fibers and (2) free-space beams with noncircular cross sections and with different radii of curvature of wavefronts on mutually perpendicular meridional planes. Beams of the second type are generated by semiconductor optoelectronic devices (e.g., diode lasers and amplifiers) that contain stripe waveguides; the waist-cross-section aspect ratios of such beams can be as large as 3:1.

Figure 1. A Cylindrical Lens is cemented to the end faces of a fiber-optic connector and the fiber held by the connector.

Heretofore, it has been common practice to use anamorphic prism pairs to change aspect ratios of light beams entering and leaving single-mode optical fibers. It is also possible, in principle, to change beam aspect ratios by use of cylindrical lenses mounted within diode laser packages. Both of these approaches entail disadvantages: Anamorphic prism pairs are bulky and introduce insertion losses of about 10 percent, and it is often not practical to modify diode laser packages to incorporate cylindrical lenses. However, it is practical to mount cylindrical lenses on the ends of optical fibers.

The cylindrical lens for a typical application is made of a suitable glass and is designed to be glued to the end face of a connector that holds the optical fiber (see Figure 1) by use of an ultraviolet-curable cement that matches the index of refraction of the fiber, the lens, or both. In conjunction with the indices of refraction of the fiber and lens glasses, the diameter of the lens and its nominal small distance from the end face of the fiber are chosen to obtain the required conversion of wavefront curvature in the affected plane. The design provides that any gap between the lens and the end face of the fiber be filled with the cement.

Figure 2. These Are Cross-Sectional Images of Light Beams at a wavelength of 852 nm emerging from the end of a polarization-maintaining optical fiber of aspect ratio 0.8, without and with a cylindrical lens. The lens converted the aspect ratio to 2.9. The insertion loss in the presence of the lens was only 3 percent.

In preparation for mounting the cylindrical lens on the end of the fiber, light of the wavelength of interest is coupled into the fiber at one end and a video camera sensitive to light at that wavelength is positioned to obtain a cross-sectional image of the beam of light emerging from the end of the fiber. The cylindrical lens is placed on a three-dimensional translation stage and moved to approximately the desired position near the end of the fiber. The gap between the cylindrical lens and the end face of the fiber is filled with cement, but not with so much that the cement could overflow and cover the free-space side of the lens. Then by use of the translation stage, the lens is maneuvered precisely into position on or near the end face of the optical fiber. The maneuvers are continued until the image exhibits the required aspect ratio and is centered on the optical axis of the fiber (see Figure 2). Then ultraviolet light is applied to cure the adhesive in place.

This work was done by Herbert Pickett and Christopher Mackay of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com 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

Technology Reporting Office
JPL
Mail Stop 122-116
4800 Oak Grove Drive
Pasadena, CA 91109
(818) 354-2240

Refer to NPO-20637, volume and number of this NASA Tech Briefs issue, and the page number.


This Brief includes a Technical Support Package (TSP).
Coupling Light Between Optical Fibers and Noncircular Beams

(reference NPO20637) is currently available for download from the TSP library.

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