Polymer single-arm optical waveguide interferometer for detection of toxic industrial materials

Abstract

We report a novel single-arm double-mode double-order waveguide interferometer being used as a chemical sensor for detection f toxic industrial materials such as ammonia in air. The sensor is based on thin films of polymers poly(methyl methacrylate) and polyimide doped with indicator dyes bromocresol purple and bromothymol blue. These dye- doped polymer materials exhibit a reversible optical absorption in a band near 600 nm being exposed to ammonia in wet air. The rise of absorption is accompanied by the change of the refractive index in near IR region out of the absorption band. The distinguished feature of the sensor is that is uses for reading the change of the refractive index of the dye-doped polymer film the interference of two propagation waveguide modes of different orders. The modes TM0 and TM1 are simultaneously excited in the light- guiding polymer film with a focusing optics and a prism coupler. The modes are decoupled from the film and recombined producing an interference pattern in the face of an output optical fiber. The sensitivity of the sensor to ammonia is 200 ppm per one full oscillation of the signal. We analyze effects of various factors such as polymer composition, light wavelength, ambient humidity and atmospheric pressure on the performance of the sensor. Various design and fabrication issues are also discussed. The problems of particular interest are reduction of losses and sensitivity improvement.