Posts from: March 2015

Optical Phase Conjugation is Linked to Speckle Decorrelation

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Optical imaging into thick tissue is generally limited to a penetration depth of only a few millimetres. Beyond this depth, almost every photon is scattered multiple times before reaching the detector, meaning that we no longer have much of an idea where it originated from. It’s possible to sidestep this limit if we can somehow differentiate photons that have come from a single point in the sample (for example by using a guide star or ultrasound to modulate the optical signal), and if we measure the wavefront that emerges from the sample. Then, using a technique called optical phase conjugation, a reversed wavefront can be sent back into the sample. Since this wavefront will undergo the opposite series of scattering events to the outcoming beam, it will be focused back to the original point. This ‘time-reversal’ technique can then allow us to image (by scanning the beam and collecting all the returning light or fluorescence) or to deliver various kinds of laser-therapy. However, the difficulty is that the scattering depends on the positions of all the scattering particles in the tissue; if there is movement then wavefront measurement becomes invalid. An international group of researchers have recently shown that the time-constant of the optical phase conjugation is linked to another property of scattering media – speckle correlation. The paper reports measurements of these time constants, giving an indication of how often the wavefront measurement would be needed.
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