Diffraction-limited determination of optical thicknesses with an optical feedback interferometric microscope

Abstract

We design an optical feedback interferometric microscope (OFIM) for measuring micron- sized optical thicknesses at microscopic spatial (transverse) resolution. OFIM employs confocal imaging in a Michelson interferometer and utilizes the sensitivity of the semiconductor laser (SL) output to external optical feedback. The optical thickness t of a transparent sample is placed in one of the optical arms thereby altering the amount of feedback intensity that is returned to SL. The optical feedback is maximum in the absence of a sample. We are specifically interested in measuring t values that are less than the depth-of-field (DOF) of the imaging objective lens L1. Surface visualization is accomplished by scanning the sample with a stationary focused beam. 
The t value is established by comparing the corresponding SL output power (photodiode output) against a calibration interferogram that is produced by axially-moving mirror M1 relative to mirror M2 in the absence of a sample. The periodicity of the interferogram depends on the comparative characteristics of lenses L1 and L2. When L1 and L2 are identical, t values as small as 0.2 microns can be distinguished.