Speckle in optical coherence tomography (OCT) images originates in the high spatial coherence of incident light that enables interference of light backscattered from spatially heterogenous tissue specimens. We report results of a numerical simulation and an experiment to test speckle reduction using a partially spatially coherent source. A Gaussian-Schell model for a partially spatially coherent source is used in the OCT simulation. For the experiment, such a source was generated by a spatially coherent boardband light source and a multimode fiber. The advantage of using a multimode fiber in combination with a broadband source is the large number of photons per coherence volume. To illustrate speckle reduction with a partially spatially coherent source, we record low-coherence interferograms of a scattering surface using single-mode and multimode source fibers. Interferograms recorded using a single-mode source fiber are indicative of those observed using conventional OCT. Speckle in OCT images recorded using a multimode source fiber is substantially reduced.
We present a novel fiber-based polarization sensitive spectral domain optical coherence tomography (SD-OCT) system using a fiber optic spectral polarimetry instrument (FOSPI), which provides measurement of depth-resolved polarization state of backscattered light from a specimen with single optical frequency scanning. By inserting a FOSPI in the detection path of SD-OCT, the full set of Stokes parameters of light backscattered at the specific depth of a specimen can be obtained without any other polarization controlling components in the system and the prior knowledge of the polarization state of the light incident on the sample. The operation principle of the fiber-based polarization sensitive spectral domain optical coherence tomography is demonstrated with a Mica plate as birefringent sample.
Functional imaging of clear and tissue-simulating phantoms using phase-resolved swept-source spectroscopic OCT (PhS-SSOCT) is described. Superior sensitivity of PhS-SSOCT technique to monitor ultra-small changes in sample refractive index is demonstrated using aqueous solutions of glucose and aqueous suspensions of polystyrene microspheres and glucose. Glucose-induced changes in the phase are found to be 0.039 rad/mM and 0.037 rad/mM in the 200 μm-thick cell for clear and turbid media, respectively, that is in good agreement with our previous data obtained using differential-phase time-domain OCT. Obtained results suggest that PhS-SSOCT has a potential for noninvasive, depth-resolved, real-time quantitative monitoring of concentrations of glucose and other analytes with high accuracy.
Speckle noise in OCT images originates in the high spatial coherence that allows interference between light backscattered from the tissue specimen. Speckle reduction techniques on which many investigators have reported all rely on averaging independent intensity images of the speckle field. Recently, we completed an experiment to test speckle reduction by averaging independent intensity images formed by incoherent spatial modes emitted from a broadband light source. Broadband light coupled into a multimode optical fiber produces a number of spatial modes that propagate along the waveguide at distinct velocities. When the spatial modes propagate length, modes become mutually incoherent. When the multimode fiber is placed in the source path of a Michelson interferometer, each spatial mode produces an intensity image of an independent speckle field. To illustrate speckle averaging using independent spatial modes, we recorded low-coherence interferograms of a scattering metallic surface using single mode and multimode source fiber. The interferogram recorded using a single mode source fiber is indicative of that observed by conventional OCT. Speckle noise in the interferogram recorded using the multimode source fiber is substantially reduced compared to the single mode case.
Recent work on Step and Flash Imprint Lithography (SFIL) has been focused on process and materials fundamentals and demonstration of resolution capability. Etch transfer rpocesses have been developed that are capable of transferring imprinted images though 150 nm of residual etch barrier, yielding sub 50 nm lines with aspect ratios greater than 8:1. A model has been developed for the photoinitiated, free radical curing of the acrylate etch barrier materials that have been used in the SFIL process. This model includes the effects of oxygen transport on the kinetics of the reaction and yields a deeper understanding of the importance of oxygen inhibition, and the resulting impact of that process on throughput and defect generation. This understanding has motivated investigation of etch barrier materials such as vinyl ethers that are cured by a cationic mechanism, which does not exhibit these same effects. Initial work on statistical defect analysis has is reported and it does not reveal pathological trends.
We present a fiber optic spectral polarimeter, an instrument that allows measurement of the spectrally resolved Stokes parameters. The instrument consists of a pair of polarization maintaining (PM) fibers spliced at 45 degree(s) with respect to each other in line with a polarizer module whose transmission axis is aligned to the fast axis of the first PM fiber. The generated spectrum is composed of three quasi- cosinusoidal components, which carry information required to reconstruct the spectrally resolved Stokes parameters of incident light. Fourier transform of the measured spectrum provides the significant parameters for determination of the spectrally resolved Stokes parameters of light. The performance of the fiber optic spectral polarimeter is demonstrated by the computer simulation and experiments with input light of known polarization states. A unique characteristic of this instrument is that the spectrally resolved polarization state of incident light can be determined from one spectral measurement without any mechanical movement. Moreover, the instrument can be incorporated into a tomographic imaging system such as a conventional microscopy system. Polarization based microscopy combined with a fiber optic spectral polarimeter will provide measurements that allow better understanding of depolarization process of light passing through biological materials for diagnostic imaging.
KEYWORDS: Energy transfer, Chemistry, Molecules, Laser induced fluorescence, NOx, Quantum physics, Molecular energy transfer, Chemical reactions, Probability theory, Carbon monoxide
The dependence of vibrational energy transfer on vibrational excitation has been studied using the stimulated emission pumping technique to efficiently prepare a large range of specific vibrational states of the nitric oxide molecule in its ground electronic state. Laser-induced fluorescence was used to detect collisionally relaxed NO. The self-relaxation rate constants of NO(v >> 1) were up to two hundred times larger than that of NO(v equals 1). Multiquantum relaxation was found to be important at high energy and was quantified at 3.8 eV. Theoretical explanations of our experimental results were attempted and it is shown that at vibrational energy up to approximately 3 eV the qualitative trends observed in these experiments such as: the mass effect and the linear dependence of the relaxation constant on v can be explained by Schwartz-Slawsky-Herzfeld theory. A simple explanation of the anomalously high NO self-relaxation rate is given. The large acceleration of the vibrational relaxation rate above 3.0 eV is coincident with the energetic onset of high energy (NO)2 isomer-complexes. More theoretical and experimental work is needed to explain the quantitative aspects of these observations.
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