Reconstruction of the refractive index profile in diffused waveguides

A simple method is proposed for measuring the refractive index $n(x)$ at the surface of a diffused waveguide whereby the profile $n(x)$ can be reconstructed uniquely from the mode spectrum of the waveguide. A description is given of a method of measuring effective refractive indices of the waveguide modes using a grating element for coupling radiation into the waveguide. The profile $n(x)$ in single-mode waveguides is discussed.