Mass stranding of live whales has been explained by proposing many natural or human-related causes. Recent necropsy
reports suggest a link between the mass stranding of beaked whales and the use of naval mid-frequency sonar.
Surprisingly, whales have experienced symptoms similar to those caused by inert gas bubbles in human divers. Our goal
is to develop a compact optical sensor to monitor the consumption of the oxygen stores in the muscle of freely diving
whales. To this end we have proposed the use of a near-infrared phase-modulated frequency-domain spectrophotometer,
in reflectance mode, to probe tissue oxygenation. Our probe consists of three main components: radiofrequency (RF)
modulated light sources, a high-bandwidth avalanche photodiode with transimpedance amplifier, and a RF gain and
phase detector. In this work, we concentrate on the design and performance of the light sensor, and its corresponding
amplifier unit. We compare three state-of-the-art avalanche photodiodes: one through-hole device and two surface-mount
detectors. We demonstrate that the gain due to the avalanche effect differs between sensors. The avalanche gain near
maximum bias of the through-hole device exceeds by a factor of 2.5 and 8.3 that of the surface-mount detectors. We
present the behavior of our assembled through-hole detector plus high-bandwidth transimpedance amplifier, and
compare its performance to that of a commercially available module. The assembled unit enables variable gain, its phase
noise is qualitatively lower, and the form factor is significantly smaller. Having a detecting unit that is compact, flexible,
and functional is a milestone in the development of our tissue oxygenation tag.
We describe the implementation of a non-linear grating-based angular filter for the assessment of turbid media with
ballistic photons. A monochromatic source incident on a ruled grating, positioned at grazing diffraction, followed by a
narrow slit conform the proposed system. We validate the angular amplification experimentally, with values ranging on
the order of 10-20X. In addition, similar values of transversal beam size reduction, provide an efficient ~100X filtering
scheme. We address the plausible application of the angular filter to perform ballistic transillumination of turbid media,
such as biological tissue. Non-linear angular amplification and beam width reduction are employed to separate ballistic
photons from forward-scattered ones. Preliminary experimental results of the technique are encouraging, as compared to
those of traditional transillumination.
We present the mathematical foundation and the experimental validation of a technique that utilizes pass-through (ballistic) photons in a partial coherence interferometric transillumination setup for biomedical analyses. We demonstrate that the implementation depends closely on tissue under test, incident power, spatial and spectral characteristics of the radiation source, and detection electronics. With the aid of the complex material coherence function concept, we foresee tissue characterization and diagnostic imaging as potential applications for the technique. We propose a normalization procedure for in vitro and in vivo measurements, where nontissue-related quantities are canceled out. The validation of the proposal is achieved by obtaining the sample coherence function of a tissue phantom. The expected exponential attenuation is confirmed, and the corresponding scattering coefficients are determined. A good agreement between theory and experiment, for the initial set of samples, serves to establish that pass-through photon-based transillumination is feasible for selected biomedical applications.
KEYWORDS: Interferometers, Biomedical optics, Modulation, Signal detection, Stochastic processes, Interferometry, Signal to noise ratio, Tissues, Beam splitters, Dynamical systems
We introduce the concept of tuning in the low-coherence trans-illumination interferometer to enhance its overall
applicability (i.e., to enable selective diagnosis from different tissue regions). Modulation artifacts of the tuned
interferometer are restricted to the reference arm. Displacements in this system, for pass-through photon-based modality,
must be inverted. Tuning of specific radiation depends on modulation parameters and coherence-time gate. We propose
to use Recurrence Plots and Recurrence Quantification Analysis (RQA) as a robust platform to identify different tuning
states in the trans-illumination experiment. To the best of our knowledge, this is the first time that recurrence analyses
are employed in trans-illumination studies. We suggest the quantitative metric of Determinism as a reference to assess
the degree of tuning of the instrument. Theoretical results confirm that RQA may be useful for discriminating between
different tuning states, including photon isolation for pass-through photon-based biomedical trans-illumination.
We elaborate further the partial coherence model for the trans-illumination interferometric experiment, by including a
phase-locked device at detection plane. We demonstrate that the detected isolated signal, and thus the quality of the
optical properties depend on the coherence characteristics of the source. We introduce a specific measure of coherence
time for discrimination purposes, such that a photon arriving at a greater time will be efficiently filtered. The
quantification of the coherence study is presented by computing the analytical interferograms for different radiation
sources (Broadband source, SLED, LED, and Laser). We show that the use of a low-coherence source improves the
detection of ballistic photons, at the coherence length of the source, by orders of magnitude. As a result of this study, we
suggest the use of a low-coherence source, with reduced bandwidth (coherence lengths ranging from 1 - 100 μm) and
compact spatial distribution, for tissue characterization purposes. For imaging of tissues, a low-coherence source with
greater bandwidth is preferable, because such source effectively isolates pass-through and single scattered photons.
KEYWORDS: Interferometers, Biomedical optics, Modulation, Signal detection, Stochastic processes, Interferometry, Signal to noise ratio, Tissues, Beam splitters, Dynamical systems
We introduce the concept of tuning in the low-coherence trans-illumination interferometer to enhance its overall
applicability (i.e., to enable selective diagnosis from different tissue regions). Modulation artifacts of the tuned
interferometer are restricted to the reference arm. Displacements in this system, for pass-through photon-based modality,
must be inverted. Tuning of specific radiation depends on modulation parameters and coherence-time gate. We propose
to use Recurrence Plots and Recurrence Quantification Analysis (RQA) as a robust platform to identify different tuning
states in the trans-illumination experiment. To the best of our knowledge, this is the first time that recurrence analyses
are employed in trans-illumination studies. We suggest the quantitative metric of Determinism as a reference to assess
the degree of tuning of the instrument. Theoretical results confirm that RQA may be useful for discriminating between
different tuning states, including photon isolation for pass-through photon-based biomedical trans-illumination.
KEYWORDS: Tissues, Monte Carlo methods, Signal to noise ratio, Scattering, Tissue optics, Dental caries, Interferometry, Photon transport, Stochastic processes, Diagnostics
We present a Monte Carlo (MC) program for determining the temporal behavior of radiation in turbid media, in general,
and a tissue, in particular. An object-oriented MC program with flexibility for parallel implementation, and for
performing stochastic analysis, is described. We determine the temporal probability distribution functions for three
tissues: I ) a lipid-based tissue phantom, II ) a healthy dental tissue, and III ) a dental tissue with caries. The expected
time of flight for transmitted radiation is calculated. Signal-to-noise ratios (SNR) are then obtained for several temporal
thresholds. By restraining the time threshold, a two orders of magnitude increase in SNR is predicted. A multivariate
analysis is finally proposed to complement the discrimination process, necessary for trans-illumination interferometry.
We compare the detected signal in the radiometric transillumination experiment for two sources: a gas laser with high
coherence, and an LED with low coherence. Detected signal is significantly improved when a low-coherent light source
is utilized, allowing us to clearly separate ballistic photons from those that undergo multiple scattering. The preliminary
experimental results confirm the feasibility of the concept, requiring precise matching of the source coherence with the
scattering behavior of the tissue (phantom) under study.
We propose a polar discretization as a flexible mathematical model for describing extra-solar radiation. The analysis permits us to predict both: the spatial and the frequency responses of optical systems pointing to an extra-solar star. The spatial response provides information of the misalignment between extra-solar star and optical system. We propose a polar representation in the convolution scan followed by a space transformation to compute the frequency content of the incoming radiation. To the best of our knowledge, the spatial frequency content has not been determined thus before. We demonstrate that the circularly concentric discretization (i.e., complex polar array of radiation sources) predicts an increment on spatial information, and a reduction on the frequency content with respect to conventional stellar models.
We propose a novel method to reconstruct Transmission Profile Function (TPF) information based on visibility measurements. In specific occasions, V<<1, the TPF is proportional to the visibility squared. Extremely low visibilities (~10-8) hinder the capacity of the technique to recover information. We evaluate three biomedical cases to confirm TPF reconstruction. First, we assess a three-dimensional multilayer skin model to demarcate limits on the reconstruction process. A three-dimensional female breast phantom with spherical tumors is evaluated next. We concentrate on technique performance to describe tissue morphology. Reconstructed TPFs with different resolutions are presented. Agreement between modeled and recovered information is demonstrated. Finally, a healthy and metastatic liver is compared. We demonstrate that a reconstructed TPF comparison enables (potentially) the diagnosis of alien tissue.
We derive the alignment condition for the detection of binaries with an interferometer. We present 2-D and 3-D simulations containing the phase, Q, and cos(Q) as a function of the misalignment deviations (qx and qy) and position at the aperture. We generate patterns that enable us to determine, qualitatively, the misalignment degree of our optical system.
A pair of thin prisms is used to deviate the light beam without changing the image orientation in a vectorial shearing interferometer. The relative angle between prisms determines the position of the wave front and its tilt. This system is employed to control the displacement of a sheared wave front as a vector quantity and to introduce a controlled amount of tilt in a novel interferometric shearing system. The predicted performance of this wave-front director is confirmed experimentally.
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