Measuring Film Thicknesses and Surface Profiles by Frequency-Modulated Laser Beams
Original Publication Date: 1984-Jun-01
Included in the Prior Art Database: 2005-Feb-04
A frequency-modulated laser beam is split into its carrier and sideband components which are used as physically separated measurement and reference beams, respectively, whose phase difference is determined by electronic frequency mixing. In the figure, the output beam of CW-laser 1 with a frequency 10 is modulated by electro-optical modulator 2 generating two sidebands 1+1 = 10 + 1M and 1-1 = 10 - 1M, 1M being the modulation frequency generated in RF generator 15. The modulated beam with its direction of polarization in the drawing plane (arrow 20) passes polarizing beam splitter 3 and an optical phase shifter 4 (g/4 plate), impinging upon Fabry-Perot interferometer 5.