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Results for “Refractometry”

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At least 1,261 records · Page 70Linked to original sources
PubMed2005

Evanescent field-based optical fiber sensing device for measuring the refractive index of liquids in microfluidic channels.

We report a simple optical sensing device capable of measuring the refractive index of liquids propagating in microfluidic channels. The sensor is based on a single-mode optical fiber that is tapered to submicrometer dimensions and immersed in a transparent curable soft polymer. A channel for liquid analyte is created in the immediate vicinity of the taper waist. Light propagating through the tapered section of the fiber extends into the channel, making the optical loss in the system sensitive to the refractive-index difference between the polymer and the liquid. The fabrication process and testing of the prototype sensing devices are described. The sensor can operate both as a highly responsive on-off device and in the continuous measurement mode, with an estimated accuracy of refractive-index measurement of approximately 5 x 10(-4).

Complex Mixtures↗
PubMed2005

Heterodyne detection of multiply scattered monochromatic light with a multipixel detector.

A new technique is presented for measuring the spectral broadening of light that has been multiply scattered from scatterers in motion. In our method the scattered light is detected by a heterodyne receiver that uses a CCD as a multipixel detector. We obtain the frequency spectrum of the scattered light by sweeping the heterodyne local oscillator frequency. Our detection scheme combines a high optical etendue (product of the surface by the detection solid angle) with an optimal detection of the scattered photons (shot noise). Using this technique, we measure, in vivo, the frequency spectrum of the light scattered through the breast of a female volunteer.

Breast↗
PubMed2005

Surface-sensitive polarized Raman spectroscopy of biological tissue.

In a two-layer diffusing medium, polarized light directly backscattering off the superficial layer will partially retain its sense of polarization, whereas deeper-probing light will be increasingly depolarized by diffusion. This effect has been studied in both elastic scattering and fluorescence contexts. We apply this method to Raman scattering in two two-layer models with a highly diffusing lower layer of glucose powder and an upper layer of either clear plastic or chicken skin. We employ detection of orthogonal polarization states to generate a Raman spectrum of only the superficial layer by combining the orthogonal signals.

Animals↗
PubMed2005

Common-path phase-shift interferometry surface plasmon resonance imaging system.

Surface plasmon resonance (SPR) and common-path phase-shift interferometry (PSI) techniques are integrated in a biosensing imaging system for measuring the two-dimensional spatial phase variation caused by biomolecular interactions on a sensing chip without the need for additional labeling. The common-path PSI technique has the advantage of long-term stability, even when it is subjected to external disturbances. Hence the system meets the requirements of the real-time kinetic studies involved in biomolecular interaction analysis. The proposed SPR-PSI imaging system demonstrates a detection limit of a 2 x 10(-7) refractive-index change, a long-term phase stability of 2.5 x 10(-4) pi rms for 4 h, and a spatial phase resolution of 10(-3) pi with a lateral resolution of 100 microm.

Biosensing Techniques↗
PubMed2005

Role of spatial coherence in polarization tomography.

We analyze an experimental setup in which a quasi-monochromatic spatially coherent beam of light is used to probe a paraxial optical scatterer. We discuss the effect of the spatial coherence of the probe beam on the Mueller matrix representing the scatterer. We show that, according to the degree of spatial coherence of the beam, the same scattering medium can be represented by different Mueller matrices. This result should serve as a warning for experimentalists.

Computer Simulation↗
PubMed2005

Image of a straight edge in confocal self-interference microscopy.

An image of a straight edge in confocal self-interference microscopy (CSIM) is analyzed. Simulations of edge images based on a two-dimensional imaging equation are presented that show a 103% increase in edge gradient and a 43.1% decrease in the 10-90% width. The first experimental results, to our knowledge, for CSIM are presented and show good agreement with the simulation results and a 23% decrease in the 10-90% width.

Algorithms↗
PubMed2005

Picosecond-pulse-induced two-photon fluorescence enhancement in biological material by application of grating waveguide structures.

We report enhancement of two-photon fluorescence (TPF) excitation in fluorescent dyes and fluorescently labeled biomolecules by exploiting the optical properties of double grating waveguide structures (DGWSs). Picosecond laser pulses generate a large evanescent field based on the guided mode phenomenon in the resonant DGWSs, which induces strong TPF signals from fluorescent dyes at the waveguide surface. By recording enhanced TPF signals of Rhodamine B and Lucifer Yellow under resonance conditions, a detection sensitivity of concentrations of approximately one dye molecule per 0.1 microm2 was achieved. For the first time to our knowledge, enhanced TPF signals of a Lucifer Yellow-labeled biomolecule (human self-peptide) in an aqueous environment are demonstrated. These results strongly encourage the use of DGWSs as enhancement platforms in modern biophysics and biotechnology for investigations of biological membranes and cells.

Equipment Design↗
PubMed2005

Polarimetric laser radar target classification.

Imaging laser radar (ladar) systems have been developed for automatic target identification in surveillance systems. Ladar uses the range value at the target pixels to estimate the target's 3-D shape and identify the target. For targets in clutter and partially hidden targets, there are ambiguities in determining which pixels are on target that lead to uncertainties in determining the target's 3-D shape. An improvement is to use the polarization components of the reflected light. We describe the operation and preliminary evaluation of a polarization diverse imaging ladar system. Using a combination of intensity, range, and degree of polarization, we are better able to identify and distinguish the target from other objects of the same class.

Algorithms↗
PubMed2005

Three-dimensional vision with dual acousto-optic deflection encoding.

We report a new method of sensing a three-dimensional (3D) object surface with an arbitrary geometric shape. In this approach, the first-order beams diffracted from two acousto-optic deflectors (AODs) interfere with each other to form a spatial carrier that is used to encode the depth information from the 3D object surface. A direct digital synthesizer is utilized to control two AODs to generate sequentially spatial carriers with different spatial frequencies so that a modified temporal phase-unwrapping technique can be applied for decoding the shape information of the test surface. Preliminary experimental results are presented to demonstrate the effectiveness of this method.

Algorithms↗
PubMed2005

Four-color fluorescence correlation spectroscopy realized in a grating-based detection platform.

We have developed a filterless multicolor detection unit for fluorescence correlation spectroscopy (FCS). This grating-based setup is continuously tunable for multicolor separation and is thus a powerful alternative to the classical cascade of dichroic mirrors and filters. Our tailored platform allows for accommodation of up to 15 detection channels covering the entire visible spectral range. As a proof of principle, we successfully demonstrate simultaneous FCS of four distinct fluorescent quantum dot species being mixed in aqueous solution. Grating-based detection allows for spectral high-resolution FCS in a stable and compact setup and is a feasible tool for quantitative investigation of complexbiomolecular dynamics on a single molecule level.

Equipment Design↗
PubMed2005

Mie theory interpretations of light scattering from intact cells.

We present a Mie theory-based analysis of angularly resolved light scattering from intact cells that involves the product of a particle size distribution rho and the scattering cross section sigma. The analysis shows that the ability to size intracellular organelles depends strongly upon the assumed functional form of the particle size distribution; however, the product sigma rho extracts the dominant light scatterers regardless of this choice. We examine goniometer measurements from our own and another laboratory and find that this analysis reconciles seemingly conflicting results and confirms that mitochondrial-sized organelles dominate scattering from intact cells in the angular range 50 degrees - 90 degrees.

Algorithms↗
PubMed2005

Elastic backscattering spectroscopic microscopy.

The spectral properties of elastic light-scattering signals have been shown to provide a wealth of information on nanostructures and microstructures. We present elastic backscattering spectroscopic microscopy that allows simultaneous acquisition of microscopic images and backscattering spectra at each pixel. Within a single homogeneous micrometer-scale particle we observe two distinct and highly localized spectral oscillation features that arise from different optical paths: (1) surface waves (e.g., the ripple structure) and (2) a not previously reported anomalous ripple structure that is due to the interference of waves scattered from front and back surfaces at the particle's center. We also demonstrate that the spectroscopic data can provide nanoscale structural information beyond what conventional microscopy reveals.

Equipment Design↗
PubMed2005

Phase retrieval of optical fringe patterns from the ridge of a wavelet transform.

A new method for phase retrieval of optical fringe patterns is presented. This method is based on a wavelet transform and is capable of extracting the full 2D phase distribution from a single fringe pattern. An important conclusion that the phase of the optical fringe pattern is equal to the phase of its wavelet transform on the ridge of the wavelet transform is theoretically clarified. The method is compared with the Fourier transform and the integration methods. A numerical simulation and an experimental example of phase retrieval are shown.

Algorithms↗
PubMed2005

Compact multireference wavefront sensor design.

We present a compact optical design for a multireference Shack-Hartmann-based wavefront sensor (WFS) for multiconjugate adaptive optical systems. The key component of this WFS design is a field lenslet array that separates the exit pupil images in the sensing plane for all reference sources. An analytical method for WFS optical design is presented, and the optimal strategy for selecting optical components from a discrete set is outlined. The feasibility of the WFS design has been demonstrated for a prototype WFS system in a laboratory setup with five reference sources and two deformable mirrors representing a wavefront-distorting medium.

Algorithms↗
PubMed2005

Optimal design of optical reference signals by use of a genetic algorithm.

A new technique for the generation of optical reference signals with optimal properties is presented. In grating measurement systems a reference signal is needed to achieve an absolute measurement of the position. The optical signal is the autocorrelation of two codes with binary transmittance. For a long time, the design of this type of code has required great computational effort, which limits the size of the code to approximately 30 elements. Recently, the application of the dividing rectangles (DIRECT) algorithm has allowed the automatic design of codes up to 100 elements. Because of the binary nature of the problem and the parallel processing of the genetic algorithms, these algorithms are efficient tools for obtaining codes with particular autocorrelation properties. We design optimum zero reference codes with arbitrary length by means of a genetic algorithm enhanced with a restricted search operator.

Algorithms↗
PubMed2005

Adaptive monogenic filtering and normalization of ESPI fringe patterns.

A technique is presented for filtering and normalizing noisy fringe patterns, which may include closed fringes, so that single-frame demodulation schemes may be successfully applied. It is based on the construction of an adaptive filter as a linear combination of the responses of a set of isotropic bandpass filters. The space-varying coefficients are proportional to the envelope of the response of each filter, which in turn is computed by using the corresponding monogenic image [Felsberg and Sommer, IEEE Trans. Signal Process. 49, 3136 (2001)]. Some examples of demodulation of real Electronic Speckle Pattern Interferometry (ESPI) images patterns are presented.

Algorithms↗
PubMed2005

High-refractive-index measurement with an elastomeric grating coupler.

An elastomeric grating coupler fabricated by the replica molding technique is used to measure the modal indices of a silicon-on-insulator (SOI) planar waveguide structure. Because of the van der Waals interaction between the grating mold and the waveguide, the elastomeric stamp makes conformal contact with the waveguide surface, inducing a periodic index perturbation at the contact region. The phase of the incident light is changed to match the guided modes of the waveguide. The modal and bulk indices are obtained by measuring the coupling angles. This technique serves to measure the high refractive index with a precision better than 10(-3) and allows the elastomeric stamp to be removed without damaging the surface of the waveguide.

Elastomers↗
PubMed2005

Subfemtomole detection of small molecules with microsphere sensors.

We investigated the feasibility of using a silica microsphere sensor for detection of small molecules. Using the silica molecules (60 Da) at the sphere's surface as a model system, we measured the spectral shifts in the whispering-gallery modes (WGMs) when the sphere size was decreased by a hydrofluoric acid (HF) solution. The results demonstrate that our sensor is capable of detecting a 4 pm (or 0.01 layer of silica) decrease in sphere radius, corresponding to a change of 0.4 fmol silica molecule. These results suggest that small molecules can be detected in trace quantities at the surface of an optical microsphere sensor.

Algorithms↗