Sub-millisecond in situ measurement of the photorefractive response in a self adaptive wavefront holography setup developed for acousto-optic imaging

Interferometric measurements for in-vivo imaging of biological tissues are strongly sensitive to the related speckle decorrelation time tc, whose effect is to reduce the contrast of the speckle pattern at the exit of the sample and thus blur detection. Though acousto-optic imaging is a well suited technique for the case of thick tissues, it has been shown that an acquisition rate in the 1-10kHz range is required for a good efficiency. We have previously built for this purpose an holographic setup that combines a fast but large area single photodetector and a photorefractive crystal, in order to measure a real-time acousto-optic signal by the so-called self-adaptive wavefront holography technique. In such a configuration, one critical point is the time response tPR of the photorefractive effect, which depends on the photorefractive configuration of the setup as well as the light intensity within the crystal. We have developed an original in situ method that determines this time in measuring the acousto-optic response through a combination of an amplitude modulation of the ultrasound and a frequency de-tuning of the reference beam. We can measure precisely this time but also monitor it according to a theoretical model that we have previously described. This offers the possibility to adapt the response of the setup to the decorrelation time of the medium under study, and also to have a measurement of τc.