Fourier transform methods applied to an optical heterodyne profilometer

Abstract

In this work, theory and experiment describe the performance of a surface profile measurement device based on optical heterodyne interferometry are presented. The object and reference beams propagating through the interferometer are obtained by single-pass through an acousto-optic modulator. The diffraction orders 0 and the Doppler-shifted +1 (object and reference beams, respectively) are manipulated to propagate collinearly towards the interferometer output where a fast photodetector is placed to collect the irradiance. The modulated optical signal is Fourier transformed using a data acquisition card and RF communications software. The peak centered at the acousto-optic frequency in the power spectrum is filtered and averaged. The irregularities on the surface of the reflective sample are proportional to the height of this peak. The profile of a reflective blazed grating has been sketched by translating laterally the sample using a nanopositioning system. Experimental results are compared to the measurement done with a scanning electron microscope. There has been found a good agreement between both methods. © 2013 SPIE.