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Optics Letters

Enhancing the axial resolution of quantum optical coherence tomography by chirped quasi-phase matching

Silvia Carrasco, Juan P. Torres, Lluís Torner, Alexander V. Sergienko, Bahaa E. A. Saleh, Malvin Carl Teich

Published Oct 15, 2004• 69 citations
Publisher

Abstract

We show theoretically that a chirped quasi-phase-matching nonlinear crystal structure can significantly enhance the axial resolution in quantum optical coherence tomography by increasing the spectral width of the generated entangled photon pairs. For pulsed pumps we show how the pump-pulse duration affects the maximum resolution attainable.

DOI
10.1364/ol.29.002429

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We show theoretically that a chirped quasi-phase-matching nonlinear crystal structure can significantly enhance the axial resolution in quantum optical coherence tomography by increasing the spectral width of the generated entangled photon pairs. For pulsed pumps we show how the pump-pulse duration affects the maximum resolution attainable.
There is renewed interest on the concept of induced coherence between beams generated by separate biphoton sources, that has led to the demonstration of new schemes for imaging, spectroscopy, microscopy and optical coherence tomography (OCT). These schemes make use of continuous wave pumping in the low parametric gain regime, which reflects the frequency entanglement between the down-converted photons. However, is entanglement a requisite to observe induced coherence? Contrary to some claims, we will show that it is not. This might be an advantage for OCT applications. High axial resolution requires a large bandwidth and for continuous wave pumping this requires the use of short nonlinear crystals. This is detrimental since short crystals generate small photon fluxes. We show that the use of ultrashort pump pulses increases axial resolution even with long crystals.
The state-of-art technology of a quasi-phase matched (QPM) device has enabled the efficient generation of frequency-entangled photon pairs through parametric fluorescence [1, 2]. Harris [3] has proposed the generation of photon pairs which are correlated on a mono-cycle time scale of optical period. In that paper, the poling period of QPM device is linearly chirped (increased) from the entrance to the exit. Thus, each poling period is phase matched at different wavelengths of photon pairs, spanning ultra-broad spectrum. Recent experiment by Mohan et al as its application to quantum optical coherence tomography [4], has realized the parametric fluorescence from chirped-QPM device which is phase matched from 820nm to 1500nm using stoichiometric lithium tantalate (SLT) crystal as a nonlinear medium with 13% chirping (poling period: from 7.500µm to 8.500µm).
We generate ultrabroadband biphotons via the process of spontaneous parametric down-conversion in a quasi-phase-matched nonlinear grating that has a linearly chirped poling period. Using these biphotons in conjunction with superconducting single-photon detectors (SSPDs), we measure the narrowest Hong-Ou-Mandel dip to date in a two-photon interferometer, having a full width at half maximum (FWHM) of approximately 5.7 fsec. This FWHM corresponds to a quantum optical coherence tomography (QOCT) axial resolution of 0.85 ?m. Our results indicate that a high flux of nonoverlapping biphotons may be generated, as required in many applications of nonclassical light.
By making use of the spatial shape of paired photons, parametric down-conversion allows the generation of two-photon entanglement in a multidimensional Hilbert space. How much entanglement can be generated in this way? In principle, the infinite-dimensional nature of the spatial degree of freedom renders unbounded the amount of entanglement available. However, in practice, the specific configuration used, namely, its geometry, the length of the nonlinear crystal, and the size of the pump beam, can severely limit the value that could be achieved. Here we show that the use of quasi-phase-matching engineering allows one to increase the amount of entanglement generated, reaching values of tens of ebits of entropy of entanglement under different conditions. Our work thus opens a way to fulfill the promise of generating massive spatial entanglement under a diverse variety of circumstances, some more favorable for its experimental implementation.
As some of the most promising quantum applications such as high resolution quantum optical coherence tomography (QOCT) [1] with dispersion cancellation as well as quantum key distribution require frequency entanglement. Therefore, frequency entanglement light sources are an essential importance for implement of many quantum optical applications. So Far, the most prominent method used for the entangled photon pair generation is spontaneous parametric down-conversion (SPDC) in nonlinear crystals. Recently, with adopting chirp quasi-phase matching (QPM) PPSLT (periodically poled stoichiometric lithium tantalate) has been used as source of entangled photons with large bandwidth [2]. However, improving the overall system efficiency is still remain challenging.