Two-beam Coupling Modules for Photorefractive Optical Circuits.

Modules that perform photorefractive two-beam coupling operations have been built, characterized, and tested. These portable modules, interconnected by fiber optics, dispense with the need for repeated alignment and greatly facilitate the prototyping of complex signal- or image-processing photorefractive circuits. To evaluate the performance of the modules in a photorefractive circuit, we interconnected them in the feature extractor configuration: a ring configuration composed of two modules that selects the strongest signal within the signals presented on its input. With two signals at the input, an output contrast ratio of 45.4 dB is obtained for an input contrast ratio of 5 dB.

Geometry-induced fluorescence resonances in small lossy capillaries.

We report the first experimental demonstration of cavity-induced resonances that occur without total internal reflection. We obtained these resonances by observing the fluorescence spectrum from the molecules enclosed within a small, lossy capillary of the type used for capillary zone electrophoresis. Even with weak reflections from the capillary inner walls, enough feedback exists to superimpose resonances on the otherwise smooth fluorescence spectrum. These resonances, commonly called morphologydependent resonances or whispering-gallery modes, have typically been observed only in cavities in which total internal reflection is present.

Angular distribution of fluorescence from dye-filled capillaries.

We studied the angular distribution of fluorescence from a small, lossy capillary filled with a laser-dye solution. We found that the fluorescence is isotropic for the liquid core and that, far from the liquid-solid phase transition, this isotropy shows no temperature dependence. This result, an extension of studies with solid cylinders, is at variance with theoretical expectations for solids as well as with previous reports by other investigators but is explained by the motion of the molecules in the liquid. Therefore the optimal viewing angle for capillary zone electrophoresis experiments is near 90° because the elastic scattering of the incident laser light is at or near a minimum for these small capillaries. This reduces contamination of the fluorescence signal as a result of stray laser light in the optical system.

Light-scattering Mueller matrix for a rough fiber.

The light-scattering Mueller matrix for an r approximately 2.0 microm radius, rough quartz fiber contains phase information different from that of a perfectly cylindrical fiber of the same optical constants and radius. The rough surface creates higher-frequency, smaller-amplitude oscillations that mask the lowerfrequency oscillations indicative of a perfect cylinder. Roughness also causes scatter outside the plane of incidence.

Light scattering by micron-sized conducting fibers: an experimental determination.

The entire sixteen-element light scattering matrix is experimentally determined for a circular cross-sectioned conducting fiber illuminated with lambda = 632.8-nm radiation at normal incidence. The radius (1.552 +/- 0.007) microm, determined by comparison to Mie theory modified for cylinders, indicates that micron-sized fiber radii can be determined within a few nanometers by polarized light scattering techniques. We discuss changes that occur in matrix elements as a quartz fiber is coated with aluminum to form a conducting fiber. Mueller matrices can be used to study time-varying processes, such as water vapor buildup on small particles and surfaces.

Fluorescent angular scattering emissions from dye-filled fibers.

Fluorescent angular scattering from laser dye-filled small-core-diameter fibers shows increased backscattered fluorescent emissions as the core diameter decreases below 23 microm. The fluorescent angular scattering from Coumarin 7 laser dye-filled hollow-core quartz fibers were measured at three different fluorescent wavelengths and compared with the elastic scattered incident radiation at 442 nm.

Light-scattering Mueller matrix from a fiber as a function of MgO contamination.

The light-scattering Mueller matrix for an r = 0.345-microm-radius quartz fiber, illuminated at lambda = 0.4416 microm, is examined as a function of contamination with MgO crystals. When the MgO contamination is low, the matrix elements resemble those of a fiber of slightly larger radius. The MgO contamination creates higher-frequency, smaller-amplitude oscillations in the matrix elements that mask the lower-frequency oscillations indicative of a perfect cylinder. The contamination also causes scatter outside the plane of incidence.

Probing a standing 0.6328-microm electromagnetic wave in a laser ring cavity.

We used sub-microm-sized quartz fibers to probe a visible light, standing electromagnetic wave inside a laser ring cavity. By observing variations in the cavity's irradiance decay lifetime, we observed the longitudinal shape of the standing wave.

Scattering from geometrically perturbed perfect quartz fibers.

Light scattered from a perfect cylindrical quartz fiber was studied as a function of fiber rotation, bend, bend and rotation, and tilt. These pure geometrical manipulations are basic and easily controlled perturbations that affect both the total intensity and geometrical distribution of the scattered light. The scattering patterns from these perfect fibers are more complex than first suspected.

Polarized light scattering matrix elements for micron-sized rectangular aluminum lines on reflecting optical surfaces.

The entire sixteen-element Mueller scattering matrix has been experimentally determined for several small rectangular cross-sectioned aluminum lines whose known dimensions are of the order of the incident light (wavelength lambda = 4416 A). Each line was fabricated on top of a smooth reflecting aluminum surface using electron beam lithography techniques. The Mueller matrix of the surface plus the line was measured as a function of the increasing size (height and width) of the line for various angles of illumination alpha. The results are compared to scattering and diffraction data from a single slit and opaque circular cylinder of equal dimensions.

Polarized light-scattering matrix elements for select perfect and perturbed optical surfaces.

The angular distribution of scattered light depends on the electromagnetic properties (refractive index, absorptivity), the geometrical properties (size, shape, and distribution) of the scatterer(s), as well as the polarization and illumination angle of the incident light. To study the total information content, we measured the entire experimental sixteen-element Mueller scattering matrix for a smooth reflecting aluminum surface illuminated with lambda = 4416-A light at various angles of incidence alpha. In comparison, we also measured the scattering matrix for a degraded surface of identical material. This paper discusses the experimental procedure and compares the scattering results obtained from these two types of surface.

Light diffraction studies of single muscle fibers as a function of fiber rotation.

Light diffraction patterns from single glycerinated frog semitendinosus muscle fibers were examined photographically and photoelectrically as a function of diffraction angle and fiber rotation. The total intensity diffraction pattern indicates that the order maxima change both position and intensity periodically as a function of rotation angle. The total diffracted light, light diffracted above and below the zero-order plane, and light diffracted into individual orders gives information about the fiber's longitudinal and rotational structure and its noncylindrical symmetry.

Single fiber light scattering matrix: an experimental determination.

The entire light scattering matrix is experimentally determined for two fused silica cylinders illuminated at normal incidence (lambda441.6 nm). Radii of 0.960 +/- 0.002 and 3.091 +/- 0.003 microm are determined by comparison with Mie theory indicating that micron-sized fiber radii can be determined to within a few nanometers by polarized light scattering techniques. The single fiber provides a good calibration source for polarization nephalometers, being free from problems inherent with spheres in solution. The fiber is a well-known geometry which is amenable to perturbation toward irregular shapes, and as such it provides the starting point for studying scattering from irregular particles.

Light scattering from fibers: a closer look with a new twist.

Scattering from an apparently perfect fiber, placed perpendicular to a laser beam, produces an out of plane scattering pattern containing an internal structure not predicted by simple fiber scattering theory. A photographic study of light scattered from twisted fibers shows that the effect is due to periodic disturbances along the fiber axis that act as coherent scattering centers.

Interpretation of light scattering associated with prolonged neural activity and temperature changes.

Changes in light scattering from lobster giant axon which accompany the action potential were observed during periods of prolonged stimulation and as a function of temperature. At an initial temperature of 10 degrees C most (more than 90%) axons produced positive light scattering signals which increased in amplitude when the temperature was lowered. At 2 and 5 degrees C approximately half of the axons produced positive scattering signals. The remaining half produced negative scattering signals which became positive when the temperature was raised to 10 degrees C. The amplitude of the negative signals followed sigmoid transition to positive values as a function of time. The time and temperature dependence of the signal are interpreted in terms of differential changes between the indices of refraction of the membrane matrix and the open or closed early activation channel.

Light scattering and excitation in lobster giant axon. Effects of ion substitution.

Changes in the light scattering signal from single giant axons of lobster were observed during the propagation of the action potential in order to correlate membrane excitability with possible structural changes reflected in the optical properties of the axolemma. Substitution of guanidine and aminoguanidine for sodium resulted in a decreased action potential amplitude to 69 and 50% of control values, respectively. The amplitude of the light signal was, however, not significantly changed by these substitutions and is, therefore, reported to be independent of the transmembrane potential and current. The venom of the scorpion Leiurus quinquestriatus caused a marked prolongation of the action potential and the light scattering signal without significantly altering their amplitudes. A two-state model of the early (sodium) activation channel is suggested, in which the light scattering signal is correlated with a possible difference in the scattering efficiency between the states of the channel.

Application of polarization effects in light scattering: a new biophysical tool.

We demonstrate that a newly developed instrument which measures all polarization and intensity information contained in differentially and elastically scattered light has valuable applications in biology. The polarization states of light scattered differentially from suspensions of biological scatters are shown to contain structural information about those systems. The scatterers are discussed in the context of a 16 component matrix which completely characterizes the scattering process. The instrument and method are described in terms of the corresponding matrix algebra. We also discuss the use of the instrument as a device for distinguishing between closely related structural systems and as a tool for following time-dependent structural changes.