A wavelength-scanning heterodyne interference confocal microscope quickly accomplishes the simultaneous measurement of the thickness and the refractive index of a sample by detection of the amplitude and the phase of the interference signal during a sample scan. However, the measurement range of the optical path difference (OPD) that is obtained from the phase changes is limited by the time response of the phase-locked loop circuit in the FM demodulator. To overcome this limitation and to improve the accuracy of the separation measurement, we propose an OPD detection using digital signal processing with a Hilbert transform. The measurement range is extended approximately five times, and the resolution of the OPD is improved to 5.5 from 9 μm without the electrical noise of the FM demodulator circuit. By applying this method for simultaneous measurement of thickness and the refractive index, we can measure samples 20–30-μm thick with refractive indices between 1 and 1.5.
© 2002 Optical Society of America
(040.2840) Detectors : Heterodyne
(110.6960) Imaging systems : Tomography
(120.3180) Instrumentation, measurement, and metrology : Interferometry
(120.5050) Instrumentation, measurement, and metrology : Phase measurement
(180.1790) Microscopy : Confocal microscopy
Yuuki Watanabe and Ichirou Yamaguchi, "Digital Hilbert Transformation for Separation Measurement of Thicknesses and Refractive Indices of Layered Objects by Use of a Wavelength-Scanning Heterodyne Interference Confocal Microscope," Appl. Opt. 41, 4497-4502 (2002)