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.

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 noncentrosymmetric 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 nonlocal 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.

Invariant pattern recognition based on centroids.

A new method for pattern recognition that is invariant under changes of position, orientation, intensity, and scale is presented. The centroids of objects provide unique points that are related to the energy distribution. For obtaining more such unique points a conformal transform can be used to rearrange the energy distribution of the object. By means of the conformal transform many different centroids can be produced from the same object. A useful pattern-recognition and object-registration method that yields a position-, rotation-, intensity-, and scale-invariant feature vector based on these centroids can be created.

Waveguide grating coupler with a tailored spectral response based on a computer-generated waveguide hologram.

We propose a novel method that, for the first time to our knowledge, makes it possible to tailor the spectral response of an input grating coupler. This method is based on the idea of computer-generated waveguide holography. Grating couplers with different spectral responses have been designed and fabricated, and experimental results are presented that demonstrate the effectiveness of the proposed method.

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 cylinder's central axis. Experimental confirmation of the cylinder's behavior is presented and confirms the suitability of the enhanced ray-optics approach to modeling micrometer-scale objects in optical-trap environments.

Modified white-light Mach-Zehnder interferometer for direct group-delay measurements.

A modified white-light Mach-Zehnder interferometer based on a single beam splitter is described for direct group-delay measurements. The arms of the interferometer are folded in such a manner that a single beam splitter can be used to split the incoming beam and combine the outgoing beams. This method offers a twofold advantage: The measuring range of the interferometer is twice as large as that of the Michelson interferometer, and the systematic error that is associated with the beam splitter is minimized because of the configuration. We report the results of measurements on various optical components performed in the 555-630-nm spectral region and propose a scheme for the processing of the experimental data. We present a comparison of the data analyzed by the proposed processing scheme along with the theoretical calculations.

Magnified grating images used in displacement sensing.

Highly magnified images of a periodic object placed in the close vicinity of the rectangular grating have been obtained by incoherent illumination. The imaging behavior has been explained quantitatively on the basis of the optical transfer function of the system. By advantageously using the high magnification involved, this phenomenon has been discussed for the purpose of precise displacement measurements. Furthermore, an explanation of Lau imaging has also been included as a spatial case of the phenomenon under consideration.

Imaging with photographic diffusers.

A photographically made amplitude diffuser has been shown to produce in its close vicinity a system of multiple images of objects placed in front of it at a certain distance. A qualitative explanation of the formation of such images has been given in terms of pinhole imaging and boundary diffraction wave theory. A report on several interesting features of the imaging with photographic diffusers is presented.

Autocorrelation method for measuring the transfer function of optical systems.

An unconventional autocorrelation method is described for measuring the transfer function of optical systems. The interference takes place between the scattered waves obtained from two laterally sheared correlated partial diffusers. The output of a detector responding only to an extremely-narrowband of spatial frequencies is proportional to the autocorrelation of the system pupil function. An automatic display of the transfer function is obtained by continuously varying the shear between the diffusers. We present the theory and some experimental results of this simple and inexpensive device. A study of various parameters affecting the performance of the instrument is also given.

Ground glass differential interferometer.

With the help of a simple ground glass and a photographic plate the phase variations of a transparent object can be studied. The object is interposed between the source and the ground glass. The photographic plate, kept behind the ground glass, is exposed successively twice, with and without the object. When properly illuminated the negative gives the variation in the derivative of the phase of the object, and this observation can be made in white light.