We introduce a theoretical method for simultaneous measurement of refractive index and thickness of multilayer systems using Fourier domain optical coherence tomography (FD-OCT) without any auxiliary arrangement. The input data to the formalism are the FD-OCT measured optical path lengths (OPLs) and properly selected spectral components of FD-OCT interference spectrum. The outputs of the formalism can be affected significantly by uncertainty in measuring the OPLs. An optimization method is introduced to deal with the relatively large amount of uncertainty in measured OPLs and enhance the final results. Simulation result shows that by using the optimization method, indices can be extracted with the absolute error ≤0.001 for transparent biological samples having indices <1.55.
We introduce a theoretical framework for simultaneous refractive index and thickness measurement of multilayer systems using the Fourier domain optical coherence tomography (FD-OCT) system without any previous information about the item under investigation. The input data to the new formalism are the FD-OCT measured optical path lengths and properly selected spectral components of the FD-OCT interference spectrum. No additional arrangement, reference reflector, or mechanical scanning is needed in this approach. Simulation results show that the accuracy of the extracted parameters depends on the index contrast of the sample while it is insensitive to the sample’s thickness profile. For transparent biological samples with smooth interfaces, when the object is in an aqueous medium and has indices <1.55, this method can extract indices and thicknesses with the absolute error ≤0.001.
A new method for using a non-selectively filled hollow-core photonic crystal fiber (HC-PCF) as a sensitive
Raman spectroscopy platform suitable for biosensing applications is presented. A 1550 HC-PCF was
completely filled with ethanol (core and cladding holes). Using a 785nm excitation laser, the Raman spectrum
of ethanol in the fiber core was obtained and compared with the equivalent Raman spectrum of an ethanolfilled
cuvette. Using a relatively short 9.5cm length of HC-PCF, a Raman signal enhancement factor of 40 over
a bulk solution of ethanol was observed under the same excitation conditions. The small sample volume
utilized and longer interaction length provides the potential for compact, sensitive, and low-power Raman
sensing of biological materials
In a step towards the development of improved long-term prognostic indicators for patients with end-stage renal disease, we utilized absorption spectroscopy to determine the baseline status of whole blood in a cohort of 5 clinically-stable hemodialysis patients. The optical absorption spectrum of pre-dialysis and post-dialysis blood samples in the 400-1700nm wavelength range was measured for the cohort over a four-week period. Absorption spectra were consistent over time, with a maximum coefficient of variation (CV) of absorption under 2% (650-1650nm) for any given patient over the four-week period (pre and post-dialysis). Spectra varied by a greater amount across patients, with a maximum CV of 5% in any given week. Analysis of variance indicated a broad spectral range (650-1400nm) where within-patient spectral variation was significantly less than between-patient variation (p<0.001), providing the potential for development of stable baseline blood status indicators. The spectra were investigated using principal component analysis (PCA) including a further set of whole blood absorption spectra obtained from 4 peritoneal dialysis patients. PCA revealed the fingerprint-like nature of the blood spectrum, an overall similarity of the spectrum within each treatment mode (hemodialysis or peritoneal dialysis), and a distinct spectral difference between the treatment modes.
Visible and near infrared transmission and diffuse reflection spectroscopy were used to monitor changes in whole blood resulting from hemodialysis treatment for end-stage renal disease. Blood samples from 8 patients on chronic hemodialysis therapy were measured in the 500- to 1700-nm wavelength range immediately before and after a single treatment. Principal component scores characteristic of each spectrum were derived, and mean pre- and posttreatment scores of the first principal component indicated a significant treatment-dependent change in both optical transmission (P=0.004) and diffuse reflection (P<0.001). Significant treatment-induced change persisted (P<0.05) when the first four principal components were used to account for >97% of the treatment-dependent spectral variation. Some blood spectral changes expressed in terms of difference spectra (posttreatment – pretreatment) were consisent with standard clinical indicators of weight reduction, urea reduction, and potassium change, with probable origins at a molecular level. The results indicate the feasibility of using optical transmission and diffuse reflection spectroscopy to characterize clinically relevant blood changes for the future development of more comprehensive indicators of hemodialysis efficacy and long-term clinical outcomes. Moreover, the optical techniques employed are adaptable for potential online monitoring of blood changes during the hemodialysis treatment.
We have developed a new instrument for non-invasive assessments of biological materials. A new technique was implemented to measure the light-tissue interaction in samples using an efficient light delivery and detection method. The optical properties measured were, transmitted, forward scattered, diffusely reflected and specularly reflected light. Measurements were made using a white light source, as well as with spectrally-resolved signals. Using artificial, human, and rabbit corneas as models, measurements were made to determine correlations of the above optical properties in the different tissues. The instrument repeatability using non-biological controls, was between 0.1% and 0.2% for the measured optical properties. The repeatability was consistent even at low light conditions of 0.01 to 0.05 relative intensity. The instrument repeatability was better than the variability of samples within a test group. For both transmitted and reflected non-specular light, there was an equivalent correlation measured between artificial and human corneas. The instrument also proved useful in tracking time-dependant responses of biological tissues subjected to various insults. This new instrument is a reliable tool for measuring static and dynamic optical properties of various biological tissues. The ability to measure small relative changes in optical properties of tissues make it an invaluable diagnostic tool.
This research project investigates the specifications of phase modulators for the treatment of presbyopia (accommodation loss). Correction of presbyopia is simulated using a pixilated phase modulator directly in front of a human eye model. The results show that maximal phase modulation depth of 17.5l (550nm) is required for a 2 Diopter change in a 6 mm pupil. The same phase retardation provides 7 Diopters of correction for a 3 mm pupil as opposed to 1.2 Diopters for 8 mm pupil.
The impact of diffraction due to pixelation of the phase array on image quality is measured using encircled energy, where a well-focused point is defined as having 75% of its energy within a 15 mm radius. For a 2 Diopter correction in a 6 mm pupil, pixelation effects are important at low pixel density and decrease asymptotically with increasing density, stabilizing at about 51 x 51 pixels. In smaller pupils, the equivalent optical correction requires less pixel density to provide equal image quality.
In conclusion, a phase modulator with a maximal phase change of 17.5l and 64 x 64 pixels could provide up to 2 Diopters of accommodation in a 6 mm pupil and significantly more in a smaller pupil, thus providing an excellent correction.
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