Coupling Integrated Optical Polarizers To Optical Fiber Sensors And Systems

Abstract

Coupling Integrated Optical Polarizers to Optical Fiber Sensors and SystemsJ. P. G. BristowAllied Amphenol Products1925 Ohio Street Lisle, Illinois 60532AbstractThe extinction ratio of integrated optical stripe waveguide polarizers fabricatedwith silicon monoxide /aluminum overlays has been measured. Values in excess of 50 dBwere recorded. The effect on fiber waveguide misalignment has been determined for arange of waveguide polarizer widths and the effect of waveguide width on extinction ratioinvestigated. For a given polarizer system, an optimum waveguide width is found to exist.IntroductionWaveguide polarizers are important for a number of optical fiber systems, includingoptical fiber sensors (ref 1). Much work has been reported in the literatureinvestigating the performance requirements for various forms of optical fibergyroscopes. In order to realize cost effective mass production of the required signalprocessing optics, it is likely that integrated optical technology will be used.Integrated optical stripe waveguide polarizers having high performance have beenreported, however, little investigation has been made of the effect of measurements onassembled fiber -waveguide packages (ref 2), while others have reported measurementsobtained with bulk optical techniques (ref 3). In addition, little work has beenreported concerning the effect of stripe waveguide width on polarizer performance: Thiswill be of concern when integrating polarizers with other components (eg. modulators) onthe same substrate.In this paper, results will be presented for integrated optical polarizers having arange of waveguide widths. In addition, the effect of waveguide fiber misalignment onthe insertion loss of such systems will be reported for a range of waveguide parameters.Several different types of integrated optical polarizers have been reported in theliterature. These include birefringent crystal overlays (ref 4), the use of protonexchange techniques (ref 3), and dielectric -metal overlays (ref 5). Of the varioustechniques, the latter appears to be the most successful. The mechanism for theoperation of such devices is the excitation of a surface plasma wave (SPW) at thedielectric -metal boundary by the quasi TM modes in the waveguide. Demonstration of themechanism for planar waveguides has been reported (ref 6). Correct selection ofwaveguide parameters is essential to ensure maximum coupling to the SPW.Waveguide polarizers are characterised by a differential attenuation between the twomodes in the device, and a loss for the desired mode. The two parameters are termed theextinction ratio and excess loss respectively.TheoryModelling of stripe waveguide polarizers is difficult. Many results have beenpresented for slab waveguide (ref 7, 8), with either step or graded index profiles. Fromthese, the operation of the devices may be understood, but no prediction of the effect oflateral confinement is available. To date, the best agreement between theory andexperiment has been obtained by Ctyroky (ref 2), using a perturbation method (ref 9).Slab models have also given reasonable predictions of the variation in extinction ratiowith buffer thickness for stripe waveguides (ref 10).The attenuation for both TE -like and TM -like modes of a four -layer slab waveguide wascalculated by solving the complex transcendental equation for the system. The opticalconstants of the lithium niobate were calculated from the formulae presented in ref 11,while the optical contants of the metals investigated were interpolated from those of ref12, using a cubic spline interpolant. The metals investigated includes gold, silver andaluminum.