Free-space infrared Mach–Zehnder interferometer for relative index of refraction measurement of gradient index optics

Abstract

The availability of more affordable, high resolution, infrared (IR) detector arrays has opened up the need for a larger set of IR materials. This has created a renewed interest in chalcogenide glasses as their transmission spans from 1 to 14 μm. The Naval Research Laboratory has shown that the chalcogenide glasses can be diffused to make gradient index (GRIN) lenses. GRIN materials are interesting because they can have unique dispersion properties that do not exist in the currently discovered homogeneous materials, a key parameter for optical designers. When fabricating a GRIN material, the ability to test and characterize its properties is essential. This has prompted the need and development of an IR Mach–Zehnder interferometer for relative index of refraction measurements of GRIN materials. These same, more affordable IR detector arrays have also allowed for what was commonly done in the visible to be developed in the IR. Now, two-dimensional change in refractive index can be measured without scanning, and the fringe density becomes less of an issue in the IR, allowing for larger changes in refractive index over a small area to be measured. An axial chalcogenide GRIN with Δn of 0.13 was measured at a wavelength of 3.39 μm.