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Most full-field heterodyne interferometry systems are based on complex electro-mechanical scanning devices. In this
study, however, we present an alternative non-scanning approach based on a low frequency heterodyne interferometer
employing standard CCD and CMOS cameras. Two frequency locked acousto-optical devices were used to obtain two
laser beams with an optical frequency difference as low as 3 Hz. The interference of those beams generated a suitably
low frequency carrier signal that allowed the use of a common 25 frame/second CCD camera. Using a digital CMOS
camera and acquiring a limited number of randomly accessible pixels, measurements with much higher carrier
frequencies were also possible.
The advantages of the heterodyne technique with respect to common phase-stepping methods are the shorter response
time and lower sensitivity to sources of uncertainty such as drift, vibrations and random electronic noises. In order to
directly compare the heterodyne and phase-stepping techniques experimentally, the same interferometer was used for
both methods. The switching between operation modes was achieved by simply altering the electronic driving signals of
the acousto-optical devices where for the phase-stepping mode, the frequency difference of the driving signals was set
to zero. The phase steps were obtained by a piezo-driven mirror. Comparing the phase difference between two pixels in
an image, approximately 0.01 radian of standard deviation, corresponding to a resolution of λ/628, was achieved by
heterodyne technique, as compared to 0.06 radian by the phase-stepping method.
The interferometer with the CMOS camera was applied to monitor the refractive index variation across a micro-channel
where two liquid flows were mixed. Also, the capability for fast, time-resolved full-field optical refractive index
measurements was demonstrated. The examples presented show how the high sensitivity of the heterodyne technique
allows the study of a number of sources of uncertainty that were not otherwise easily quantifiable using standard full field
methods.
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