Frequency-domain immersion technique for accurate optical property measurements of turbid media

We demonstrate that absorption coefficient micro(a) and reduced scattering coefficient micro(s)(?) of a small turbid object can be measured to high accuracy with a frequency-domain immersion technique. For this technique the sample is immersed in a calibrated scattering medium and the optical properties are obtained from a differential measurement. Compared with conventional approaches, the immersion technique improves accuracy, minimizes variations owing to probe coupling and motion, reduces the effects of boundary conditions, and offers simple and rapid measurement once the immersion medium is calibrated. Accuracy tests of immersion-based measurements of micro(a) and micro(s)(?) agree with reference values to within 3.6% and 2.6%, respectively. These tests are limited by the accuracy of the reference samples rather than by the accuracy of the immersion medium or the precision of the immersion approach. We demonstrate the in vivo capabilities of the technique through time-resolved measurements of micro(a) and micro(s)(?) for a human hand during cuff occlusion on the upper arm.     

Coherence gating in optical heterodyne detection measurements of scattering and absorption in highly scattering media

Laser-Induced Fluorescence (LIF) from the S₁ state of acetone and 3-pentanone was studied as a function of temperature and pressure using excitation at 248 nm. Additionally, LIF of 3-pentanone was investigated using 277 and 312 nm excitation. Added gases were synthetic air, O₂, and N₂ respectively, in the range 0–50 bar. At 383 K and for excitation at 248 nm, all the chosen collision partners gave an initial enhancement in fluorescence intensity with added gas pressure. Thereafter, the signal intensity remained constant for N₂ but decreased markedly for O₂. For synthetic air, only a small decrease occurred beyond 25 bar. At longer excitation wavelengths (277 and 312 nm), the corresponding initial rise in signal with synthetic air pressure was less than that for 248 nm. The temperature dependence of the fluorescence intensity was determined in the range 383–640 K at a constant pressure of 1 bar synthetic air. For 248 nm excitation, a marked fall in the fluorescence signal was observed, whereas for 277 nm excitation the corresponding decrease was only half as strong. By contrast, exciting 3-pentanone at 312 nm, the signal intensity increased markedly in the same temperature range. These results are consistent with the observation of a red shift of the absorption spectra (9 nm) over this temperature range. Essentially, the same temperature dependence was obtained at 10 and 20 bar pressure of synthetic air. It is demonstrated that temperatures can be determined from the relative fluorescence intensities following excitation of 3-pentanone at 248 and 312 nm, respectively. This new approach could be of interest as a non-intrusive thermometry method, e.g., for the compression phase in combustion engines.     

Simultaneous measurements of three-dimensional reflectivity distributions in scattering media based on optical frequency-domain reflectometry

A new type of tomography system based on optical frequency-domain reflectometry is presented. Using an area sensor, the system can simultaneously measure three-dimensional reflectivity distributions in scattering media without the need for mechanical scanning. In preliminary experiments we demonstrate that a target (resolution chart) placed behind biological tissue can be imaged with high depth resolution (47 microm) for a short measurement time.     

Nonlinear optical technique for precise retardation measurements

We present a highly sensitive nonlinear optical technique to measure optical retardation. The technique is based on second-harmonic generation from thin films using two beams at the fundamental frequency. The sensitive polarization dependence of the process allows measuring optical retardation very precisely. The technique relies on fundamental symmetry principles and does therefore not require complicated experimental arrangement or data analysis. The technique was demonstrated by determining the retardation of a nominal half-wave plate to a precision and repeatability better than lambda/10(4).     

Differential optical spectroscopy for absorption characterization of scattering media

Reflectance techniques are commonly used to characterize the optical properties of tissues. However, the precise determination of local chromophore concentrations in turbid media is usually difficult because of the nonlinear dependence of light intensity as a function of scattering and absorption coefficients. A technique is presented to easily determine absorbent compound concentration ratios in a turbid media from three optical reflectance spectra, in the visible range, measured for source-detector distances less than 1cm. The validity of the method is experimentally established, in cases of sets of diluted milk containing absorbent inks, over a relatively wide range of absorption (0.05-0.5 cm(-1)) and reduced scattering (10-20 cm(-1)) coefficients.     

Time-resolved optical backscattering model in highly scattering media

A time-resolved backscattering model, which combines a single large-angle scattering with multiple small-angle scatterings, is used to produce a scattered-light profile about a medium. Inhomogeneity of the medium is included in the model. Some multidimensional integrals can be evaluated analytically.     

Phase-referenced Doppler optical coherence tomography in scattering media

We present a fiber-based, low-coherence interferometer that significantly reduces phase noise by incorporating a second, narrowband, continuous-wave light source as a phase reference. By incorporating this interferometer into a Doppler OCT system, we demonstrate significant velocity noise reduction in reflective and scattering samples using processing techniques amenable to real-time implementation. We also demonstrate 90% suppression of velocity noise in a flow phantom.     

Frequency-swept ultrasound-modulated optical tomography of scattering media

A novel frequency-swept ultrasound-modulated optical tomography technique was developed to image scattering media. A frequency-swept ultrasonic wave was used to modulate the laser light passing through a scattering medium. The modulated light was received by an optical detector and was heterodyned with a reference frequency sweep. The heterodyned signal was recorded in the time domain and was then analyzed in the frequency domain to yield a one-dimensional image along the ultrasonic axis. Multiple one-dimensional images obtained at various positions perpendicular to the ultrasonic axis were combined to yield a two-dimensional tomographic image of the medium.     

An optimized technique for backscatter attenuation measurements in optical fibres

A technique which gives maximum range and sensitivity for estimating fibre attenuation from the backscattered light has been evolved. With this technique stability problems in the source and receiver are overcome by taking two different time samples for each pulse launched into the fibre, the attenuation being derived from the ratio of the two samples. An analysis indicates that an optimum operating strategy exists in terms of the choice of system parameters such that the range over which attenuation can be measured is maximized. Results indicate that local loss measurements can be performed on fibre sections which are remote from the source by a distance corresponding to a fibre attenuation of 27 dB. Furthermore, preliminary experimental results are presented to demonstrate the viability of the technique.     

Time-reversed ultrasonically encoded optical focusing into scattering media

Light focusing plays a central role in biomedical imaging, manipulation, and therapy. In scattering media, direct light focusing becomes infeasible beyond one transport mean free path. All previous methods1-3 to overcome this diffusion limit lack a practical internal "guide star."4 Here we proposed and experimentally validated a novel concept, called Time-Reversed Ultrasonically Encoded (TRUE) optical focusing, to deliver light into any dynamically defined location inside a scattering medium. First, diffused coherent light is encoded by a focused ultrasonic wave to provide a virtual internal "guide star"; then, only the encoded light is time-reversed and transmitted back to the ultrasonic focus. The TRUE optical focus-defined by the ultrasonic wave-is unaffected by multiple scattering of light. Such focusing is especially desirable in biological tissue where ultrasonic scattering is ~1000 times weaker than optical scattering. Various fields including biomedical and colloidal optics can benefit from TRUE optical focusing.     

Multi-sample loading technique for comparative physical property measurements in the diamond-anvil cell

ABSTRACT

We present a method to perform improved measurements of the effects of chemical variability on physical properties of single-crystal samples in the diamond-anvil cell by employing a multi-sample approach. By customizing the sizes and shapes of the samples using a focused ion beam machine the simultaneous loading of relatively large crystals into a single sample chamber becomes feasible. To illustrate the potential of this approach, elastic properties of four single crystals of ringwoodite with different chemical compositions have been measured at high pressure. Our results suggest that the multi-sample approach allows for the quantification of small effects of chemical variations, such as iron and hydrogen incorporation, on physical properties. Furthermore, we discuss the possibility of using the multi-sample approach to load several crystals with different crystallographic orientations of the same material into one sample chamber in order to map out the direction dependence of anisotropic physical properties.     

Absolute scattering cross-section measurements by a ratio technique

Calculations and measurements are presented which show that absolute scattering cross sections (“Rayleigh ratios”) can be determined by measuring the ratio of the depolarized intensity of doubly scattered light to the polarized intensity of singly scattered light. This technique should be useful in determining the susceptibilities of fluids and the molecular weights of macromolecules.     

On the technique of absolutization of diffuse scattering intensity measurements based on thermal diffuse scattering measurements

Features of microdefect (MD) formation in GaAs(Si) single crystals grown by horizontally oriented crystallization were studied by X-ray diffuse scattering (XRDS). Measurements were performed at room temperature (∼298 K) and near the liquid nitrogen evaporation temperature (∼85 K) using an open-flow cooling nitrogen cryostat. A practical technique for measuring XRDS using a triple-axis X-ray diffractometer was developed and applied to separate scattering on defects and thermal diffuse scattering. For a crystal with n = 2.0 × 10¹⁸ cm⁻³, the radius of detected nonspherical MDs was determined as ∼0.2 μm; thermal diffuse scattering (TDS) was experimentally separated. For a crystal with n = 3.9 × 10¹⁸ cm⁻³, nonspherical MDs ∼0.5 μm in radius were detected; TDS was found to be a negligible fraction of total XRDS. At the same time, in the case of coinciding crystal orientations and identical experimental conditions, TDS measurement data for one crystal can be used for other GaAs(Si) crystals with the same orientation.     

Selection of optical radiation in scattering in partially ordered disperse media

Results of theoretical studies of the interaction between optical radiation and partially ordered disperse media are reported. In terms of the amplitude-phase screen model consideration is given to the concentration effects of whitening and darkening in random close-packed systems of optically soft particles. The concentration dependence of transmission of close-packed systems of coarse particles is described with the use of a small-angle solution of the stochastic finite-difference transfer equation. The effects of coherent reirradiation occurring in close-packed monolayers of highly refracting particles are analyzed using a quasicrystalline approximation of the theory of multiple wave scattering and the radial particle distribution function obtained from a solution of the Percus-Yevick equation. This approach extended to multilayer systems is used to describe formation of forbidden photon zones in transmission spectra of one- and three-dimensional disperse systems with a high degree of ordering. Results of quantitative calculations are shown to agree well with experimental data. The possibility of using established regularities for optimization of spectral characteristics of selective elements based on spatially ordered disperse systems with different structural organization is discussed. Institute of Molecular and Atomic Physics of the National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 5, pp. 721–733, September–October, 1998.     

Measurement of optical constants for dense media by low-coherence dynamic light scattering

We perform the optical constants measurements for different absorption dense media by low-coherence dynamic light scattering (DLS) technique. The estimated particle size is used to calculate the scattering coefficient of particles suspended in dense media. The path-length resolved intensity distributions of light backscattered from the absorbing dense media are investigated experimentally by virtue of path-length resolved performance in low-coherence DLS measurements. The absorption coefficient can be obtained by applying the measured path-length resolved intensity distributions to the modified Lambert-Beer law. As a result, we proposed a new low-coherence DLS technique in simultaneous measurement of the scattering and absorption coefficients of absorbing dense media.     

Nonstationary two-flux model of radiation transport for optical tomography of scattering media

A nonstationary two-flux model is formulated for the transport of radiation in an inhomogeneous scattering medium and is applied to the situation where such a medium is irradiated by the narrow beam of a pulsed laser. It is shown that when the time distribution of the transmitted photons is measured it is possible simultaneously to reconstruct the two spatial functions (the coefficients of absorption coefficient and of scattering of the radiation by the medium) by means of an inverse Radon transformation and the solution of a system of nonlinear differential equations on the projection lines. An analytic solution is obtained in quadratures for these differential equations. The results constitute a method of solving problems in optical tomography in an inhomogeneous scattering medium Zh. Tekh. Fiz. 67, 61–65 (May 1997)     

Spectral measurements of the emission from highly scattering gain media

The spectral properties of the emission of a colloidal suspension of titanium dioxide particles in Rhodamine 640 perchlorate were studied systematically. The particle density rho, the dye concentration c, and the energy of the pump pulse were varied over several orders of magnitude; the dependence of the spectra on these three parameters is discussed. We discovered a strong correlation between the particle density and the dye concentration. The amplification by stimulated emission was found to be strongest when the absorption length l(a) and the mean-free-path length l(*) had approximately the same magnitude. The respective l(*) values were determined by a pulsed transmission experiment.     

Analyzing statistical errors for measurements of Brillouin scattering by the edge technique

The signal estimate and statistical uncertainty in the measurements of Brillouin shift by the edge technique are analyzed in detail. A signal to noise parameter factor is introduced and is used to discuss the statistical uncertainty in the measurements. The effect of signal averaging and the effect due to background noise are analyzed. Some helpful conclusions are predicted. PACS 42.68.Wt; 42.79.Qx; 78.35.+c     

Reflection 4f coherent imaging technique for measurements of optical nonlinearity

A one-laser-shot measurement technique, which can characterize the nonlinear optical properties of the film prepared on the opaque substrates conveniently and exactly, is presented. This method is based on the reflection 4f coherent imaging system with phase objects. The proposed technique is examined in the case of a novel ultrathin film [CuPc(COONa)₄]/PDDA coated on an opaque silicon substrate with 532 nm picosecond laser pulses. By measuring the reflected intensity, but not the transmitted one, we verify both nonlinear absorption and refraction of the coated thin film. This progress obviously ensures that nonlinearities caused by reflection of opaque surfaces are measurable.     

Time-domain pulse property of stimulated Raman scattering in a multimode optical fiber

The time-domain pulse property of quasi-steady stimulated Raman scattering (SRS) in a multimode optical fiber, pumped by an A-O Q-switched Nd: YAG laser, is studied both experimentally and theoretically. Good agreement has been achieved between theory and experiment. Discussion shows that the variation of the Stokes pulse width (FWHM)t _s depends on the shape of input pump pulse.