Full field, low frequency heterodyne interferometry using CMOS and CCD cameras with online phase processing

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.