High sensitivity mapping of refractive index across a surface using a dual beam differential reflectance technique

Rivers, P.E., Rutt, H.N. and Asal, R. (1998) High sensitivity mapping of refractive index across a surface using a dual beam differential reflectance technique. Measurement Science and Technology, 9 (1), 86-94. (doi:10.1088/0957-0233/9/1/011).

Abstract

With any waveguide device accurate knowledge of the optical characteristics is necessary for device design and specification; hence it is important from a practical and theoretical point of view to establish an efficient and precise method for measuring the refractive index profile of optical waveguides. For the majority of waveguide applications accurate knowledge of the absolute refractive index is rarely required but in contrast the difference between two adjacent regions is essential. The technique presented here addresses this second requirement. Measurements can be performed at any wavelength on the basis of intensity measurements of reflected signals from two adjacent points on the surface of a material and, since it is a non-contact measurement, it is also possible to record reflectance changes in situ. Samples can be heated, illuminated or processed in other ways, with a constant record of the refractive index changes which occur or are being made, mapped across an entire surface to an accuracy in Δn of 8 x 10^-5. The spatial mapping of Δn is limited only by the focused spot diameters and the need for a separation between them, the surface quality and the noise associated with the combined detector and PSD electrical noise. We used CaF₂ and ZnSe at various temperatures to calibrate and measure small refractive index changes.

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