A simple reflection-type two-dimensional refractive index profile measurement technique for optical waveguides

We report a new configuration of a reflection-type confocal scanning optical microscope system for measuring the refractive index profile of an optical waveguide. Several improvements on the earlier design are proposed; a light emitting diode at 650 nm wavelength instead of a laser diode or He–Ne laser is used as a light source for better index precision, and a simple longitudinal linear scanning and a curve fitting techniques are adapted instead of a servo control for maintaining an optical confocal arrangement. We have obtained spatial resolution of 800 nm and an index precision of 2 × 10−4. To verify the system’s capability, the refractive index profiles of a conventional multimode fiber and a home-made four-mode fiber were examined with our proposed measurement method.