Excitation of Waveguides for Integrated Optics with Laser Beams

Communications systems using light waves as the carrier of information need some means of signal processing at the end terminals and perhaps also at intermediate repeater points along the transmission lines. It has been suggested1,2 to employ integrated optical circuits for the purpose of filtering, amplifying, pulse regeneration, etc., of the optical signal beam. These introductory remarks make it plausible that the need exists for exciting guided modes in the waveguides used for integrated optical circuits.3 An efficient method of mode excitation utilizes the evanescent field outside of a high-index prism to couple light energy from a laser beam to one of the guided modes of a thin-film waveguide.4 This method is particularly suitable for wide thin-film guides. However, for the very narrow light guides that are likely to be used for integrated optical circuits the prism coupler method may become less 43 44 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1971 efficient since the dielectric optical waveguide may easily be narrower than the laser beam. A very narrow beam must be highly focused and thus has the disadvantage of a high-beam divergence contrary to the requirements of the prism coupler. An alternate method of exciting the guided modes of dielectric waveguides is by shining the laser beam directly at the end of the guide. It has been shown theoretically that the conversion efficiency obtainable by this method is as high or higher than that of the prism coupler.5,6 Light injection by direct excitation with a laser beam is usually employed with cladded round optical fibers.