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Researchers at the College of Engineering have added a new twist to a three-dimensional diagnostic imaging technique known as optical coherence tomography (OCT). This technology is widely used in ophthalmology and in creating cross-section images of biological tissue for noninvasive optical biopsy.
By replacing the broadband light source used in traditional OCT with pairs of entangled photons, Bahaa Saleh, Malvin Teich, and Alexander Sergienko, professors of electrical and computer engineering, and Ayman Abouraddy (PhD '03), and graduate student Magued Nasr in electrical and computer engineering, have increased the axial resolution of the resulting images by a factor of five.
The investigators produce photon pairs by passing laser light through a nonlinear optical crystal, in this case a krypton-ion laser beam directed at a crystal of lithium iodate. The twin photons that emerge continue to be linked even as they are directed along different paths — one toward the sample under investigation, the other toward a mirror. Both ultimately reach photon detectors. The differences in the amount of time that it takes for the photons in the pair to reach the detector are used to generate a highly accurate image of the interior of the sample under investigation.
The researchers used both techniques to image a piece of fused silica buried beneath a slab of zinc selenide, a highly dispersive material. The improved resolution of the new technique, dubbed QOCT (quantum optical coherence tomography), results from enhanced sensitivity of the photon pairs as a depth probe and from the elimination of dispersion effects created by the wider source bandwidth needed in conventional OCT.
This research has been a centerpiece of the National Science Foundation Engineering Research Center CenSSIS (Center for Subsurface Sensing and Imaging Systems) and earned Nasr the Berman Future of Light Award at Science and Technology Day 2003.
This research was reported in the August 2003 issue of Physical Review Letters. For more information about quantum imaging, visit the Quantum Imaging Laboratory's website.
Image: Monochromatic light (violet) from a laser that has passed through a nonlinear crystal and been split into pairs of entangled photons with longer wavelengths (shown here as red and green). The daughter photons then are sent to the QOCT device. |
