Posts with the tag: interference
In the last few years, several papers have looked at how it might be possible to use a multimode fibre as an ultra-narrow endoscope (see this post and this post for a bit of background). The most common approach is to use a spatial light modulator to shape the wavefront entering the fibre. If this is done in precisely the right way, interference between light coupled into the different modes of the fibre will result in a focused spot at the far end. By adjusting the input wavefront it’s then possible to scan the spot in two dimensions, allowing point-by-point imaging. Of course, we need to know what wavefronts to use, making it necessary to perform a calibration which requires access to the far end of the fibre. Unfortunately, this calibration is highly dependent on the configuration of the fibre – if the fibre is bent then the calibration changes. This means the technique is only applicable to rigid probes, greatly limiting the scope of potential applications. Now, in a paper published in Nature Photonics, Tomáš Čižmár and colleagues from the University of Dundee have suggested a possible solution to this problem.
Optical microscopy can only penetrate a few hundred microns into thick tissue, a limit imposed by scattering. High resolution imaging requires single-scattering events, so when we have multiple-scattering from particles above and below the focal plane, the resolution and signal to noise ratio quickly degrade. The thicker the tissue (i.e. the deeper the plane of interest) the more the multiple-scattering events dominate over single scattering. Techniques such as optical coherence tomography (OCT) enhance the penetration depth by rejecting multiple-scattered light using what is effectively a time-of-flight measurement. This works because light that has been scattered multiple times will tend to have travelled further than light that has been scattered only once. However, even with this technique, the penetration depth seldom exceeds 1-2 mm, as some multiple-scattered photons will (by chance) have a time of flight close to that of the single scattered photons. As we try to go deeper, these events will begin to dominate again. Now, a group mainly from Korea University in Seoul have suggested an additional method of discriminating between single and multiple-scattered photons, using a technique they call “collective accumulation of single-scattered waves”.
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.
Optical Coherence Tomography (OCT) has a lot of advantages over confocal microscopy, especially for applications where it’s useful to have a large working distance between the probe and the tissue. But a big limitation is that it can only detect reflected light, and so can’t be used with fluorescent stains. Fluorescence is often preferred in conventional microscopy because it allows us to visualise structures that we can’t easily identify in reflectance images. So the race is on to find a way to make OCT work with fluorescent emission as well as reflected light.