Fibre-optic focus-detection system for non-contact, high-resolution thickness measurement of transparent tissues

Abstract

A low-cost, high-resolution, fibre optic system for non-contact thickness measurement of ocular tissues in cadaver eyes was developed. The optical system uses direct modulation at 100 kHz of a 670 nm laser diode delivered to a single-mode fibre coupler. The output of the fibre coupler is focused onto the tissue using a high numerical aperture aspheric lens mounted on a motorized translation stage. Light reflected from the sample is collected by the fibre coupler and sent to a photoreceiver. Peaks in the power signal are detected when the focal point of the aspheric lens coincides with the tissue boundaries. The thickness is proportional to the distance between successive peaks. The optical system has a sensitivity of 52 dB, which corresponds to a detectable change in the refractive index of 0.005 at a boundary-assuming specular reflection. With a focusing numerical aperture of 0.68, the resolution is 4.0 µm and the precision is ± 0.2 µm. The optical system was used to measure the corneal layers and lens capsule thickness of cadaver monkeys eyes. The system was able to measure the thickness of the epithelium, stroma and Descemet's membrane of the cornea, as well as the lens capsule and lens epithelial cell layer.