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Spotlight on Optics


  • January 2012

Optics InfoBase > Spotlight on Optics > High-speed scattering medium characterization with application to focusing light through turbid media

High-speed scattering medium characterization with application to focusing light through turbid media

Published in Optics Express, Vol. 20 Issue 2, pp.1733-1740 (2012)
by Donald B. Conkey, Antonio M. Caravaca-Aguirre, and Rafael Piestun

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Spotlight summary: Seeing through scattering media is a fundamental problem in optics. Scattering in inhomogeneous media are of particular interest in many areas such as acoustics, radiofrequency communications and microscopy. In optics, scattering causes wavefront distortions that degrade image quality and limit the focusing ability. Adaptive optics is one of the ways to control wavefront aberrations and is used in microscopy, free space optical communications, and in astronomy for imaging through atmosphere. However, the optical distortions that occur in strongly scattering media such as biological tissue are significantly harder to control.

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.

--Shakil Rehman

Technical Division: Information Acquisition, Processing, and Display
ToC Category: Adaptive Optics
OCIS Codes: (110.7050) Imaging systems : Turbid media
(290.4210) Scattering : Multiple scattering
(110.1080) Imaging systems : Active or adaptive optics

Posted on January 20, 2012

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