Forming focusing diffraction gratings for integrated optics

Abstract

Abstract An apparatus is disclosed for making unchirped holographic diffraction gratings that are formed from curved lines in a thin film. The gratings so made focus as well as reflect light that is confined to the film incorporating the grating. SU BACKGROUND OF THE INVENTION Gratings have been incorporated in integrated optics devices for several purposes, including the fabrication of distributed feedback lasers, light-wave couplers, and band-rejection filters. Integrated-optics gratings known to the prior art were composed at straight lines, and therefore could not focus the light being processed. Gratings that combine focusing and diffraction were known to be desirable, but the prior art was unable to produce them. The closest prior art that has come to our attention is a method and apparatus for producing curved-line holographic gratings that have unequally spaced, or chirped, lines (U.S. Pat. No. 3,578,845, issued on May 18, 1971 to K. E. Brooks et al). This reference teaches the production of gratings that focus light that propagates into and out of the plane of the grating. It does not teach the relative orientation of laser beams and focal lines that are required in order to produce curved-line gratings that will function in integrated optics devices. SUMMARY OF THE INVENTION The invention relates to a method and apparatus for producing unchirped, curved-line, holographic diffraction gratings in a thin film, which gratings will focus as well as diffract light that is confined to the film in which the grating is formed. (In integrated optics, the film containing the light is called the optical waveguide, and the waveguide with a grating in it is called a corrugated waveguide.) The gratings are made by forming an interference pattern in a photosensitive material, fixing the interference pattern so formed and then using the fixed pattern as a mask for ion or chemical etching processes of conventional type to form corrugated waveguides. One feature of the invention is the use of two cylindrically focused beams of coherent optical radiation for writing holographic diffraction gratings, the focal lines of which beams are oriented in a predetermined manner with respect to each other and with respect to the grating being written. Another feature of the invention is the use of coplanar focal lines of the two beams oriented so that the plane which contains the focal lines also contains the axis of the grating, thereby providing uniform spacing between the grating lines. Further features and advantages of the invention will become apparent from a study of the detailed description and of the drawings.

Inventors

• 👤 Ping K. Tien

Bibliographic Details