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.

The paper 1 begins with a theoretical analysis, which concludes that the speckle autocorrelation function (a measure of how much the speckle pattern changes with time) is proportional to the decay in time of the intensity of the spot created by optical phase conjugation.  This result was then verified experimentally using a digital optical phase conjugation experiment. The experiment used off-axis holography to measure the wavefront of a collimated beam after passing through a tissue sample. A spatial light modulator was used to ‘playback’ the wavefront, resulting in a collimated beam back at the other side of the tissue. This was then imaged onto a photodiode, to measure the intensity of the reconstructed beam, and onto a camera to record the speckle pattern.

A tissue phantom was used for initial experiments. As expected, good correlation was obtained between the time constants of the speckle decorrelation and the intensity of the beam. They then repeated the experiment with a dorsal skin flap of a mouse, with different degrees of pinching to limit blood flow. As before, the speckle decorrelation predicted the success of the optical phase conjugation. The decorrelation times ranged from 2 seconds for heavy immobilisation to 50 milliseconds for minimum immobilisation. These times are fairly short when we consider the computational complexity of the calculations needs for digital optical phase conjugation. However, the good news is that a measurable collimated beam was achieved for considerably longer than the time constant. This was because the high initial intensity meant that only a small amount of correlation was sufficient to get at least something through. These results should be interesting for anyone hoping to use optical phase conjugation of live biomedical samples.

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