Quantum optical coherence tomography

Optical coherence tomography (OCT) has become a versatile and useful biological imaging technique. It is an interferometric scheme that makes use of a light source of short coherence time (broad spectrum) to carry out axial sectioning of a specimen. A particular merit that sets OCT apart from other optical-sectioning techniques, such as confocal and two-photon microscopy, is the decoupling of the axial resolution from the lateral resolution. In microscopy both resolutions, axial and lateral, depend on the numerical aperture (NA) of the lenses and the wavelength of the light used. The axial resolution of OCT is limited by the coherence length of the light source. Thus OCT can maintain high axial resolution at depth where high lateral resolution is not achievable. The axial resolution of OCT is enhanced by increasing the spectral bandwidth of the source. However, as the bandwidth is increased the effects of group-velocity dispersion (GVD) become increasingly deleterious.; In this work, we introduce a quantum version of OCT, quantum optical coherence tomography (QOCT), that makes use of an entangled twin-photon light source. A particular merit of QOCT is that it is inherently immune to the GVD effects of the medium by virtue of the frequency entanglement associated with the twin-photon pairs. For sources of the same bandwidth, the entangled nature of the twin photons provides a factor of two enhancement in resolution relative to OCT. Moreover, QOCT permits us to directly determine the GVD coefficients of the interstitial media between the reflecting surfaces of the sample, a measurement that is not directly accessible in regular OCT. In this Ph.D. work we have developed the theory for the QOCT technique and carried out many experiments that demonstrate the above-mentioned advantages of QOCT over OCT.; The axial resolution of QOCT is governed by the bandwidth of the entangled twin-photon light source. The entangled twin photons may be conveniently generated via spontaneous parametric down-conversion (SPDC). In this process a monochromatic laser beam of angular frequency op, serving as the pump, is sent to a second-order nonlinear optical crystal (NLC). A fraction of the pump photons disintegrate into pairs of down-converted photons. In this work, we also present a method for generating an ultra-broadband spectrum of SPDC that is independent of the thickness of the nonlinear crystal, thereby yielding down-converted photons of high flux. We show that the bandwidth amplification inherent in the SPDC process is maximized at a wavelength where the group-velocity dispersion is minimal. Experimental results demonstrating the broad spectrum produced by this technique are presented.