Slab-Waveguide Interferometer for Sensing Ammonia in Wet Air

John H. Glenn Research Center

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.

This **Slab-Waveguide Interferometer** provides an indication of the ammonia-induced change in the index of refraction of the doped PMMA film.

At opposite ends of the waveguide, the wide rectangular facets of triangular input and output coupling prisms are pressed against the surface of the PMMA film. The prisms are made of gallium gadolinium garnet. Light from a He-Ne laser (wavelength of 633 nm) is chopped and sent through a single-mode optical fiber to input focusing optics. The design of the input focusing optics and the input coupling prism is such that two transverse magnetic (TM) modes — the ones of zeroth and first order (TM₀ and TM₁, respectively) — are excited simultaneously in the slab waveguide. After traveling along the waveguide, the waves propagating in the two modes encounter the output coupling optics, which are similar to the input coupling optics except that their role is to focus light in the two modes onto an end face of a multimode optical fiber. Because the two modes are coherent, they give rise to an interference pattern. With proper design of the optics and proper placement of the multimode optical fiber, the spatial period of the fringes is several times the diameter of the fiber.

The multimode optical fiber leads to a photodetector. The output of the photodetector is processed through a lock-in amplifier synchronized with the chopper. When index of refraction of the doped-PMMA waveguide film changes upon exposure to wet ambient air containing ammonia, the resulting change in its index of refraction causes a change in the differences between the phases of the TM₀ and TM₁ modes at the output end of the waveguide. This change in phase difference causes the interference fringes to shift with respect to the input face of the multimode optical fiber, thereby further giving rise to a change in the intensity of light arriving at the photodetector. In an experiment, the sensitivity of this device was found to be a phase-difference change of 2π radians (one full oscillation of the intensity of light arriving at the photodetector) per approximately 200 parts per million of ammonia.

The design of this device has not yet been optimized with respect to the laser wavelength, choice of waveguide polymer, and concentration of dopant. In addition, it may be necessary to add a reference interferometer arm isolated from the ambient air to provide temperature compensation, because the single-arm version described above is highly sensitive to temperature (of the order of 2π radians of phase-shift change per 1°C change in temperature).

This work was done by Sergey Sarkisov of Alabama Agricultural and Mechanical University for Glenn Research Center.

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