Fourier-Based Characterization of Reflective Surface Topography Using a Heterodyne Optical Profilometer

Optical profilometry based on heterodyne interferometry was used to characterize the lateral surface topography of reflective surfaces with microstructures. A 633 nm laser beam is split into an unshifted object beam and a reference beam that is frequency-shifted by 80 MHz using a traveling-wave acousto-optic modulator. The object beam is focused onto the sample through a 100× microscope objective, recombined with the reference beam, and detected using a photodetector with a bandwidth of 150 MHz. At each scan position, a Universal Software Radio Peripheral (USRP) acquires the beat signal, and Fourier analysis isolates the heterodyne component. Electrical tests verified the recovery of a prescribed low-amplitude modulation, whereas translation of a plane mirror through focus identified a monotonic region for displacement-sensitive operation. Reflective silicon microstructures and a nominal 300 lines mm−1 grating were examined. The reconstructed maps reproduced the principal lateral features observed by scanning electron microscopy (SEM). For the grating-period comparison, the SEM-derived mean was 3.545 μm, whereas the heterodyne reconstruction yielded a representative period of approximately 3.50 μm, corresponding to an absolute difference of approximately 0.045 μm (1.3%). The present experiments establish Fourier-domain signal recovery and lateral morphological characterization. Because a traceable axial calibration and an uncertainty budget were not available, the reconstructed vertical coordinate remains qualitative, and the results do not yet establish quantitative surface-roughness parameters.