In this paper, Conkey et al. have come up with a method to dynamically compensate for wavefront distortions while imaging through a strongly scattering medium. In this method, a phase mask encoded by a computer generated hologram is created on a Deformable Micromirror Device (DMD) that can be updated at high data rates, allowing high speed wavefront corrections using what is known as a transmission matrix method. DMDs are semiconductor-based light switching arrays of thousands of individually addressable and tiltable mirrors whose speed, precision and broadband capability makes them very attractive for many applications.
The optical transmission matrix of a scattering medium can be measured using a spatial light modulator and a full-field interferometric measurement, and can be used in focusing and detection experiments. In their method, the authors use three phase masks to recover the optical field, obtaining a 25% improvement over the earlier works. This was achieved by employing the high frame rate of a DMD to create binary amplitude holograms, otherwise known as computer-generated holograms, to modulate and control the phase of a wavefront.
The focusing of light through a turbid medium, in this case a ground-glass plate, was tested by creating a binary amplitude hologram on the DMD, thereby, achieving faster measurements of the transmission matrix via a photodiode signal. The transmission matrix of N input modes is mapped onto a single output mode and a phase conjugate mask is calculated in order to maximize the intensity of light at the photodiode. This method provided signal-to-background improvements of 164 and 454 with input modes of N = 256 and N=1024 respectively, as claimed by the authors.
Such a method of “high speed wavefront optimization for focusing through turbid media using a DMD with off-axis binary amplitude holography for phase control” is going to have a significant impact in imaging through highly scattering media, particularly for visualizing biological materials in biomedical imaging applications.
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