A new method for the measurement of thickness d and index of refraction n of transparent slabs is presented. The method is based on the interferometric measurement of the optical path-length read by a laser beam upon passing through the slab. Optical path-length is a function of angle of incidence , and we measure it by rotating the slab of =±60 deg respect to normal incidence. By fitting the function with an analytical expression so as to minimize a suitable error function, we are able to determine d in a range from a few micron to 1000-micron with typical accuracy ±2%, and n in a range 1.4 to 2.2 with typical accuracy ±0.05. The measurement is carried out by a Self-Mixing interferometer using the delayed optical feedback of light propagated through the sample, up to a remote reflector and back, by retracing the rays to the laser source. The monitor photodiode on the rear mirror of the laser senses the interferometric signal. A variant of method is replacing the remote reflector with a photodiode, whose front surface acts as the reflector while it detects the propagated beam superposed to a double reflected contribution at the slab walls, thus supplementing the self-mixing interferometer with a conventional, forward-going shear interferometer. This simplify the signal processing and improves accuracy.
Application of Delayed Optical Feedback to the Simultaneous Measurement of Index of Refraction and Thickness of Optical Slabs
DONATI, SILVANO;MARTINI, GIUSEPPE;
2011-01-01
Abstract
A new method for the measurement of thickness d and index of refraction n of transparent slabs is presented. The method is based on the interferometric measurement of the optical path-length read by a laser beam upon passing through the slab. Optical path-length is a function of angle of incidence , and we measure it by rotating the slab of =±60 deg respect to normal incidence. By fitting the function with an analytical expression so as to minimize a suitable error function, we are able to determine d in a range from a few micron to 1000-micron with typical accuracy ±2%, and n in a range 1.4 to 2.2 with typical accuracy ±0.05. The measurement is carried out by a Self-Mixing interferometer using the delayed optical feedback of light propagated through the sample, up to a remote reflector and back, by retracing the rays to the laser source. The monitor photodiode on the rear mirror of the laser senses the interferometric signal. A variant of method is replacing the remote reflector with a photodiode, whose front surface acts as the reflector while it detects the propagated beam superposed to a double reflected contribution at the slab walls, thus supplementing the self-mixing interferometer with a conventional, forward-going shear interferometer. This simplify the signal processing and improves accuracy.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
