Optical thickness profiling using a semiconductor laser confocal microscope

Abstract

A semiconductor laser confocal microscope is developed for measuring the optical thickness of thin transparent samples at high spatial resolution. The optical sectioning capability of a confocal microscope, and the sensitivity of the laser output to optical feedback, are both utilized to detect the presence of a nonabsorbing sample in the path of the probe beam that is initially focused onto a plane mirror. The index mismatch at the sample interface defocuses the beam away from the mirror and reduces the amount of optical feedback. The optical thickness is computed from the amount of axial displacement that the mirror must be given to regain maximum feedback. The laser power output is monitored using the monitor photodiode in the laser package. When the geometrical thickness of the sample is known a priori, the technique can be used to measure its refractive index and vice versa. The smallest and largest measurable sample thickness are determined by the sharpness of the axial intensity response of the microscope and the working distance of the focusing objective, respectively. Distortions in the central spot distributions of the response degrade the precision of the measurement technique. We demonstrate the technique for both the 830 and 780 nm laser output wavelengths, by classifying cover glasses of differing optical thickness (geometrical thickness range: 70–536 μm, refractive index=1.523), as well as to image the sudden index changes found in the phase ridge formed by two closely spaced cover glasses (surface flatness=7 μm) of different geometrical thickness. Our experiments show that at least for objectives with small numerical apertures (≤0.25), spherical aberrations are negligible and our measurements are to within the accuracy set by the glass manufacturer.