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Investigation of phase conjugation for medical imaging

Ivan Sytcevich

Published Jan 1, 2017• 0 citations
OpenAlex

Abstract

With the development of extremely precise spectral filters based on spectral hole burning and slow light effects, the idea of their implementation in an Ultrasound Optical Tomography (UOT) medical imaging technique arose. Simulations have shown that the resulting Contrast-to-Noise (CNR) ratio is fairly good at depths up to 5 cm but decreases dramatically when probing deeper in tissue. One of the possible enhancements of this technique is to include a phase conjugator after the filter to reverse and amplify ultrasound-tagged photons and send them back to the area of interest. This project tries to investigate the possibilities of using rare-earth-ion-doped crystals as potential phase-conjugating elements in the UOT and is specifically aimed at studying the properties of an optical phase conjugation process in these media. Experimental work was done on two crystals doped with Praseodymium and the phase-conjugated signal was studied as a function of different parameters with results discussed and presented in the respective section. In particular, efficient phase conjugation was observed in a 6-mm thick Pr:YSO crystal with reflectivity values reaching up to 124 \%. Further outlook and possible applications are given at the end of the thesis.

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With the development of extremely precise spectral filters based on spectral hole burning and slow light effects, the idea of their implementation in an Ultrasound Optical Tomography (UOT) medical imaging technique arose. Simulations have shown that the resulting Contrast-to-Noise (CNR) ratio is fairly good at depths up to 5 cm but decreases dramatically when probing deeper in tissue. One of the possible enhancements of this technique is to include a phase conjugator after the filter to reverse and amplify ultrasound-tagged photons and send them back to the area of interest. This project tries to investigate the possibilities of using rare-earth-ion-doped crystals as potential phase-conjugating elements in the UOT and is specifically aimed at studying the properties of an optical phase conjugation process in these media. Experimental work was done on two crystals doped with Praseodymium and the phase-conjugated signal was studied as a function of different parameters with results discussed and presented in the respective section. In particular, efficient phase conjugation was observed in a 6-mm thick Pr:YSO crystal with reflectivity values reaching up to 124 %. Further outlook and possible applications are given at the end of the thesis.
With their optical wavelength in the near infrared (790--800 nm) and their unique spectroscopic properties at cryogenic temperatures, thulium-doped crystals are at the center of many architectures linked to classical signal processing and quantum information. In this work, we focus on Tm-doped YSO, a compound that was left aside in the mid-1990s due to its rather short optical coherence lifetime. By means of time-resolved hole-burning spectroscopy, we investigate the anisotropic enhanced nuclear Zeeman effect and demonstrate deep, sub-MHz, persistent spectral hole burning under specific magnetic field orientation, and magnitude. By estimating the experimental parameters corresponding to a real-scale ultrasound optical tomography setup using Tm:YSO as a spectral filter, we validate Tm:YSO as a promising compound for medical imaging in the human body.
The main aim of the present thesis is to demonstrate the efficient quantum memories for light employing coherent processes in rare-earth-ion-doped crystals. Rare-earth (RE) ions between lanthanum, with atomic number 57, and lutetium, with atomic number 71, have a partly filled 4f shell, which is spatially inside the full shells 5s, 5p, and 6s. Therefore, 4f-4f transitions are shielded from environmental noise. Less noise means longer lifetime and coherence times and, when the crystals are cooled down to about 4 K, these can be about 8 orders of magnitude longer than what is typical for electronic transitions in solids. In quantum information processing, we are looking for physical systems which are isolated from the environment and consequently have good coherence properties. The long coherence time of both optical and spin transitions, is the main attraction of rare-earth ions for quantum information processing. In the present thesis, we map a quantum state of light employing a 4f-4f transition of praseodymium doped in yttrium silicate. We mainly employed the ensemble approach and the atomic frequency comb (AFC) protocol for quantum memory demonstrations. Among several parameters that define quantum memory performance, we mostly focused on quantum memory efficiency. The overall memory efficiency is simply defined as the energy of the pulse recalled from the memory divided by the energy of the pulse sent in for storage in the memory. To maximize quantum memory efficiency, most protocols require high optical depth. This is a limitation for materials with rather good coherence properties but low optical depth. In the research described in the present thesis, we combined the use of a low finesse cavity and the ensemble approach and designed an impedance-match cavity to obtain the advantages of both approaches and thereby to implement an efficient quantum interface in a weakly absorbing solid-state medium. Based on the cavity configuration, we obtained 56% storage and retrieval efficiency, which is the highest storage and retrieval efficiency based on the AFC protocol reported in the research so far. In the present thesis, we demonstrated a narrow-band (about 2 MHz) spectral filter with a >30 dB intensity absorption/transmission ratio, which is based on the large (6 orders of magnitude) ratio between the inhomogeneous and homogeneous broadening of Pr3+ : Y2SiO5. Slow-light effects due to the strong dispersion created by the spectral filter improved the suppression of the filter by also utilizing the time domain. As an application, we detected weak diffuse light shifted by ultrasound from the carrier about 2 MHz, after a 9 cm thick biological tissue. The coupling between light and matter is directly related to the relative direction of the light polarization and transition dipole moment of atoms. I developed a simulation program that was able to investigate the polarization direction and ellipticity of polarization of optical radiation propagating through the crystal. This might open up new opportunities for the investigation of interesting physics regarding propagation effects and for the improvement of spectral engineering of RE ions in the future. In order to deal with the sharp homogeneous transitions of the RE ions, the frequency of our dye laser was stabilized against a highly stabilized Fabry-Perot cavity using the Pound-Drever-Hall locking scheme.