Slab-Waveguide Interferometer for Sensing Ammonia in Wet Air

The figure depicts a single-arm, slabwave- guide interferometer that has been demonstrated to be useable as a means of sensing ammonia in wet air. The slabwaveguide portion of this device comprises a 2-μm-thick film of poly(methyl methacrylate) [PMMA] on a substrate of fused quartz. The PMMA layer acts as a waveguide because its index of refraction is greater than the indices of refraction of both the fused quartz on one side and the ambient air on the other side. The PMMA film is doped with bromocresol purple — an indicator dye that causes the index of refraction of the film to vary with the amount of ammonia that diffuses into the film from the ambient air. The remaining basic features of design and operation, as described below, are devoted to enhancing and measuring the change in an optical phase difference attributable to the change in the index of refraction and thus to the presence of ammonia.