Investigation of multilayer planar waveguide parameters using m-line spectroscopy method for biological liquid sensing

A generalized m-line spectroscopy enables an evaluation of the parameters (the refractive index, n and the thickness, W) of sub-guiding films deposited on a planar waveguide. The method is based on a coupling of a laser beam with a prism coupler and studying of light distribution of the beam decoupled by the same prism. Additionally to the beam spot reflected geometrically from the prism base the light distribution takes a form of discrete lines - m-modes. This image can be observed but only for some discrete values of the coupling angles. The refractive index, n and thickness, W of the waveguide are evaluated by the numerical solution of the dispersion equation with a set of the coupling angles. A sub-guiding film (not reaching the guiding effect) when deposited on the existing planar waveguide influences the present guiding system and effects its guiding properties. From the previous and actual coupling angles the thin film parameters can be evaluated. In the paper an application of the generalized m-line spectroscopy to biological samples is presented. Some initial measurements reveal an effect of the presence of the films on the coupling angles of the structures. The presence of glucose has been revealed in secretions of patients suffering diabetes. Testing of its presence with proposed method can be performed with a trace amount of the patient secretion.