Chander P. Grover

Automated single-cell sorting system based on optical trapping

We provide a basis for automated single-cell sorting based on optical trapping and manipulation using human peripheral blood as a model system. A counterpropagating dual-beam optical-trapping configuration is shown theoretically and experimentally to be preferred due to a greater ability to manipulate cells in three dimensions. Theoretical analysis performed by simulating the propagation of rays through the region containing an erythrocyte (red blood cell) divided into numerous elements confirms experimental results showing that a trapped erythrocyte orients with its longest axis in the direction of propagation of the beam. The single-cell sorting system includes an image-processing system using thresholding, background subtraction, and edge-enhancement algorithms, which allows for the identification of single cells. Erythrocytes have been identified and manipulated into designated volumes using the automated dual-beam trap. Potential applications of automated single-cell sorting, including the incorporation of molecular biology techniques, are discussed.

A photorefractive organically modified silica glass with high optical gain

Photorefractive materials exhibit a spatial modulation of the refractive index due to redistribution of photogenerated charges in an optically nonlinear medium. As such, they have the ability to manipulate light and are potentially important for optical applications including image processing, optical storage, programmable optical interconnects and simulation of neural networks. Photorefractive materials are generally crystals, polymers and glasses with electro-optic or birefringent properties and non-centrosymmetric structure. Here we report the photorefractive effect in both non-centrosymmetric and centrosymmetric azo-dye-doped silica glasses, in which refractive index gratings that are spatially phase-shifted with respect to the incident light intensity pattern are observed. The effect results from a non-local response of the material to optical illumination, and enables the transfer of energy between two interfering light beams (asymmetric two-beam coupling). Although the writing time for the present grating is relatively slow, we have achieved a two-beam coupling optical gain of 188 cm -1 in the centrosymmetric glasses, and a gain of 444 cm -1 in the non-centrosymmetric structures. The latter are fabricated using a corona discharge process to induce a permanent arrangement of azo-dye chromophores.

Interrogating fiber Bragg grating sensors by thermally scanning a demultiplexer based on arrayed waveguide gratings

We evaluate a wavelength interrogation technique based on an arrayed waveguide grating (AWG). Initial results show that the Bragg wavelength of fiber Bragg grating (FBG) sensors can be precisely interrogated by thermally scanning an AWG-based demultiplexer. The technique potentially offers a low-cost, compact, and high-performance solution for the interrogation of FBG distributed sensors and multisensor arrays.

EXPERIMENTAL CONFIRMATION OF THE OPTICAL-TRAPPING PROPERTIES OF CYLINDRICAL OBJECTS

A sophisticated modeling program was used recently to predict the trapping and the manipulation properties of elongated cylindrical objects in the focal region of a high-intensity laser beam. On the basis of the model, the cylinders should align their longest diagonal dimension with the propagation axis of the laser beam and follow the beam when it is displaced transverse to the cylinders central axis. Experimental confirmation of the cylinders behavior is presented and confirms the suitability of the enhanced ray-optics approach to modeling micrometer-scale objects in optical-trap environments.

A comparison of wavelength dependent polarization dependent loss measurements in fiber gratings

We have measured wavelength dependent Polarization Dependent Loss (PDL) in chirped fiber Bragg gratings for dispersion compensation, grating filters for wavelength add/drop multiplexing and long period gratings for EDFA gain flattening. The PDL is measured in devices used in reflection and in transmission by applying the Jones matrix method, the Mueller matrix method and the polarization scanning method. A comparison of the experimental results and an analysis of the sources of errors are presented.

Preparation and Optical Characterization of Thick-Film Zirconia and Titania Ormosils

Crack-free films with thicknesses of up to 30 μm were prepared by the sol-gel process using the dip-coating technique. Thick films were obtained from various starting solutions based on two, three or four components, with particular emphasis on ternary systems. The ternary systems were composed of two tetraalkoxy precursors (a silicon tetraalkoxide and a zirconium or a titanium tetralkoxide ) and a trialkoxysilane with a non-hydrolyzable group. By using these trialkoxysilanes, the tendency of the films to crack during the drying process is reduced because of the stress absorption by the network structure. The use of zirconia or titania allows for control of the refractive index of the films. Optical parameters of the films including refractive index, thickness, surface roughness and UV-Vis and IR transmission spectra were determined for each composition and the structural characteristics of the films were inferred from the IR spectra. UV cut-off and antireflective properties were also studied for some compositions.

Information coding and retrieving using fluorescent semiconductor nanocrystals for object identification

The spectral features, i.e., wavelength and intensity, of fluorescence generated from semiconductor nanocrystals (quantum dots) can be used for coding information. Unlike the 1-D and 2-D barcodes, the information carrier is applied to a very small area and hardly visible. The information retrieving by a fluorospectrometer is not subjected to the changes of rotation and scale. A de-convolution-based algorithm is used to separate the overlapped spectral profiles. This technology can be applied to small products labeling, document security and object identification.

A tunable multiwavelength fiber ring laser for measuring polarization-mode dispersion in optical fibers

We describe and demonstrate a tunable triple-wavelength erbium-doped fiber ring laser along with a method of measuring polarization-mode dispersion (PMD) in optical fibers based on a broad-band orthogonal-pump four-wave mixing in a semiconductor optical amplifier. The measured PMD results for optical fibers are in good agreement with values measured by means of commercial PMD testing equipment.

Waveguide shaping and writing in fused silica using a femtosecond laser

Fused silica waveguides were written and reshaped with an 800-mn amplified femtosecond laser in a lateral writing configuration. The asymmetry of the waveguide shape and the index profile was partially corrected through multiple scans shifted by 1 or 2 /spl mu/m. Following the careful adjustment of both laser pulse energy and writing speed, the resulting waveguides exhibited substantial reduction in propagation and coupling losses by more than 50% in the C-band.

Cryptography based on the absorption/emission features of multicolor semiconductor nanocrystal quantum dots

Further to the optical coding based on fluorescent semiconductor quantum dots (QDs), a concept of using mixtures of multiple single-color QDs for creating highly secret cryptograms based on their absorption/emission properties was demonstrated. The key to readout of the optical codes is a group of excitation lights with the predetermined wavelengths programmed in a secret manner. The cryptograms can be printed on the surfaces of different objects such as valuable documents for security purposes.