Showing papers on "Birefringence published in 1997"

Two-dimensional birefringence imaging in biological tissue by polarization-sensitive optical coherence tomography.

Abstract: Using a low-coherence Michelson interferometer, we measure two-dimensional images of optical birefringence in bovine tendon as a function of depth. Polarization-sensitive detection of the signal formed by interference of backscattered light from the sample and a mirror in the reference arm give the optical phase delay between light that is propagating along the fast and slow axes of the birefringent tendon. Images showing the change in birefringence in response to laser irradiation are presented. The technique permits rapid noncontact investigation of tissue structural properties through two-dimensional imaging of birefringence.

Polarization properties of vertical-cavity surface-emitting lasers

Abstract: Polarization-state selection, polarization-state dynamics, and polarization switching of a quantum-well vertical-cavity surface-emitting laser (VCSEL) for the lowest order transverse spatial mode of the laser is explored using a recently developed model that incorporates material birefringence, the saturable dispersion characteristic of semiconductor physics, and the sensitivity of the transitions in the material to the vector character of the electric field amplitude. Three features contribute to the observed linearly polarized states of emission: linear birefringence, linear gain or loss anisotropies, and an intermediate relaxation rate for imbalances in the populations of the magnetic sublevels. In the absence of either birefringence or saturable dispersion, the gain or loss anisotropies dictate stability for the linearly polarized mode with higher net gain; hence, switching is only possible if the relative strength of the net gain for the two modes is reversed. When birefringence and saturable dispersion are both present, there are possibilities of bistability, monostability, and dynamical instability, including switching by destabilization of the mode with the higher gain to loss ratio in favor of the weaker mode. We compare our analytical and numerical results with recent experimental results on bistability and switchings caused by changes in the injection current and changes in the intensity of an injected optical signal.

Quantitative measurements of linear birefringence during heating of native collagen.

Abstract: Background and Objective Linear birefringence is an anisotropic property of rat tail tendon, which is largely composed of collagen. Our goal is to show that the dynamic range and sensitivity of the linear birefringence loss of collagen during heating are sufficient for kinetic modeling of the reaction. Study Design/ Materials and Methods: The linear birefringence loss was quantified for tendon denatured via both a heated-isotonic-saline bath and a heated stage. All measurements were made with a polarizing transmission microscope equipped with a Berek compensator. Results The data show that the loss of linear birefringence is a first-order kinetic reaction. The native rat tail tendon birefringence, Δn = 3.0 ± 0.6 × 10−3 (mean ± std. err.), is lost after denaturation occurs (Δn = 0). Application of the Arrhenius equation to the linear birefringence data yields the activation energy (Ea = 89 ± 1 kcal/mole), pre-exponential coefficient (A = e130±1 s−1), enthalpy (ΔH = 88 ± 1 kcal/mole) and entropy (ΔS = 197 ± 2 cal/°K·mole). Conclusion This study shows that dynamic changes in linear birefringence can be used to monitor thermally induced changes in collagen. Lasers Surg. Medicine 20:310–318, 1997. © 1997 Wiley-Liss, Inc.

Performance of a Novel Optical Compensation Film Based on Negative Birefringence of Discotic Compound for Wide-Viewing-Angle Twisted-Nematic Liquid-Crystal Displays

Abstract: The performance of a novel optical compensation film utilizing a negatively birefringent discotic compound was optimized by both simulation and experiment to achieve wide-viewing-angle characteristics of twisted-nematic liquid-crystal displays (TN-LCDs). It was shown that the film remarkably enlarges the viewing angle of TN-LCDs when the discotic compound layer has hybrid molecular orientation which is a similar structure to that of on-state liquid crystals of TN-LCDs.

Design, fabrication, and characterization of form-birefringent multilayer polarizing beam splitter

Abstract: Polarizing beam splitters that use the anisotropic spectral reflectivity (ASR) characteristic of high-spatial-frequency multilayer binary gratings have been designed, fabricated, and characterized. Using the ASR effect with rigorous coupled-wave analysis, we design an optical element that is transparent for TM polarization and reflective for TE polarization at an arbitrary incidence angle and operational wavelength. The experiments with the fabricated element demonstrate a high efficiency (97), with polarization extinction ratios higher than 220:1 at a wavelength of 1.523 m over a 20 angular bandwidth by means of the ASR characteristics of the device. These ASR devices combine many useful characteristics, such as compactness, low insertion loss, high efficiency, and broad angular and spectral bandwidth operations.

Achromatic quarter-wave plates using the dispersion of form birefringence

Abstract: We propose achromatic quarter-wave plates of a subwavelength grating structure. When the period of the grating structure is smaller than the wavelengths of the incident light, the structure is considered to be an optically anisotropic medium. The effective refractive indices strongly depend on the wavelengths, especially when the period is close to the wavelength. Using this feature, we can design a grating quarter-wave plate whose phase retardation is maintained at pi/2 for a wide wavelength range. A design method using the effective medium theory is described, and the wave plates designed were evaluated by numerical calculation with a rigorous electromagnetic grating theory. The calculation results led to the possibility of an achromatic quarter-wave plate whose retardation errors are smaller than 3 degrees for a +/-10% change in wavelength.

Light diffraction at mixed phase and absorption gratings in anisotropic media for arbitrary geometries

Abstract: The coupled wave theory of Kogelnik [H. Kogelnik, Bell Syst. Tech. J. 48, 2909 (1969)] is extended to the case of moderately absorbing thick anisotropic materials with grating vector and medium boundaries arbitrarily oriented with respect to the main axes of the optical indicatrix. Dielectric and absorption modulation with common grating vector and of arbitrary relative phase shift is considered. Solutions for the wave amplitudes, diffraction efficiencies, and angular mismatch sensitivities are given in transmission and reflection geometries. The main difference of the new results with respect to the expressions valid for isotropic media arise due to the walk-off between the wave-front and energy propagation directions. The difference is particularly important in materials with large birefringence, such as organic crystals, ordered polymers, and liquid crystalline cells. The special case of Bragg diffraction and two-beam coupling at holograms recorded in optically inactive photorefractive crystals is analyzed in detail. It is found that the two-beam coupling gain is influenced substantially by an absorption anisotropy.

Viscoelastic flow past a confined cylinder of a low-density polyethylene melt

Abstract: The capabilities of the exponential version of the Phan-Thien-Tanner (PTT) model and the Giesekus model to predict stress fields for the viscoelastic flow of a low density polyethylene melt around a confined cylinder are investigated. Computations are based on a newly developed version of the discontinuous Galerkin method. This method gives convergent results up to a Deborah number of 2.5 for the falling sphere in a tube benchmark problem. Moreover, the specific implicit-explicit implementation allows the efficient resolution of problems with multiple relaxation times which are mandatory for polymer melts. Experimentally, stress fields are related to birefringence distributions by means of the stress optical rule. Three different fits, of equal quality, to available viscometric shear data are used: two for the PTT model and one for the Giesekus model. Comparison of computed and measured fringes reveals that neither of the models is capable of describing the full birefringence pattern sufficiently well. In particular it appears difficult to predict both the birefringent tail at the wake of the cylinder that is dominated by elongational effects and the fringe pattern between cylinder and the walls where a combined shear-elongational flow is present.

Birefringence and dichroism of the QED vacuum

Abstract: We use an analytic form for the Heisenberg - Euler Lagrangian to calculate the birefringent and dichroic properties of the vacuum for arbitrarily strong wrenchless fields.

Linear dichroism and birefringence effects in magnetic fluids

Abstract: The purpose of the paper is to explain the linear birefringence and dichroism observed in magnetic fluids when subjected to a magnetic field. Linear aggregates of nanosize near spherical particles, which are considered to be responsible for these phenomena, have been analyzed in terms of the classical oscillating dipole model where each particle is treated as a simple dipole under the action of the electric field of a light beam. The analysis is produced in terms of the number of particles in the aggregate for both spherical and slightly nonspherical particles. Optical anisotropy factors and expressions for the magnetic-field dependence of the birefringence and dichroism due to preexisting aggregates are given. Numerical calculations have been carried out for Fe3O4 particles. Finally, the formulae are applied to experimental data where a close fit is obtained.

Fault-tolerant optical routing switch

Abstract: An electromagnetically controlled optical switch (100) having a plurality of optical inputs (101). An optical signal at each input (101) is spatially decomposed into two coparallel, orthogonally polarized beams by a birefringent element (130). An array (140) of field-controlled polarization rotators are positioned in the paths of the decomposed light beams so that the polarization of the emergent beam pairs are set to be at one of the orthogonal states (i.e., both beams are either vertical or horizontally polarized). A following birefringent element (150) spatially routes the light beam pairs based upon their polarization. A second array (160) of polarization rotators follows the second birefringent element (150) and is set to be in the opposite state from the first array (140), such that the emergent beams are orthogonal to their input states before they entered the first polarization rotator (140). The beams are combined by a firefringent element (170) to form a plurality of combined beams that are coupled to output ports (102, 103).

Light scattering and form birefringence of parallel cylindrical arrays that represent cellular organelles of the retinal nerve fiber layer

Abstract: The retinal nerve fiber layer (RNFL) comprises bundles of unmyelinated axons that run across the surface of the retina. The cylindrical organelles of the RNFL (axonal membranes, microtubules, neurofilaments, and mitochondria) as seen by electron microscopy were modeled as parallel cylindrical arrays in order to gain insight into their optical properties. Arrays of thin fibrils were used to represent organelles that are thin relative to wavelength, and the model took into account interference effects that may arise from spatial order. Angular and spectral light-scattering functions were calculated for the backscattering hemisphere. Scattering was much larger from axonal membranes than from microtubules or neurofilaments. Spectra from 400 to 700 nm show that scattering increases at shorter wavelengths for both axonal membranes and microtubules. At 560 nm, scattering from mitochondria modeled as thick cylinders was approximately the same as that from microtubules but showed little wavelength dependence. The model reveals differences in backscattered polarization ratios that may permit experimental discrimination between microtubule and membrane mechanisms for the RNFL reflectance. Calculated backscattering exceeds measured values by at least 1 order of magnitude, but calculated form birefringence for microtubule arrays is approximately the same as measured birefringence.

Polarization locking in an isotropic, modelocked soliton Er/Yb fiber laser

Abstract: A modelocked fiber laser, operating in the soliton regime without any explicit intracavity polarizers, is observed to spontaneously lock its output polarization for certain values of the intracavity birefringence. For other settings of the intracavity birefringence the output polarization undergoes pulse-to-pulse evolution. The dependence of the output polarization evolution on intracavity birefringence outside of the locking regions can be understood with a simple model. The locking behavior exhibits several surprising aspects and is not completely understood.

Propagation of solitons in fibers with randomly varying birefringence: effects of soliton transmission control

Abstract: Propagation of solitons in fibers with randomly varying birefringence is studied with and without including the effect of soliton transmission control. A simple expression for the decay of soliton energy due to the birefringence is derived, which will be useful in predicting soliton-pulse broadening in randomly birefringent fibers with their polarization-mode dispersion parameter specified. It is also shown that transmission control of solitons effectively removes dispersive radiations generated by the birefringence.

Optical attenuator using polarization modulation and a feedback controller

Abstract: An optical power regulator employs a variable optical attenuator having a first birefringent element that spatially separates the input optical beam into two orthogonally-polarized beams. Both beams pass through a polarization modulator (e.g., a liquid crystal material) that rotates their polarizations to an extent determined by the control voltage applied across the polarization modulator. A final birefringent element spatially separates both beams exiting the polarization modulator into two pairs of orthogonally-polarized beams (i.e., two horizontally-polarized and two vertically-polarized components). The thicknesses and optical properties of the birefringent elements are selected so that two of the four beams are combined by the final birefringent element to exit at the output port of the regulator, while the remaining two beams are blocked. As a result, the degree of attenuation is determined by the degree of polarization rotation by the polarization modulator, which in turn is a function of the control voltage applied to the polarization modulator. Preferably, the liquid crystal material used in the polarization modulator has a high optical birefringence and a low dielectric anisotropy, which results in a relatively shallow attenuation curve as a function of applied voltage. The intensity of the optical signal is measured by a photodetector and used by a controller to output the control voltage applied to the liquid crystal material to maintain a desired optical power level at the output port of the regulator.

Optical elements comprising photonic crystals and applications thereof

Abstract: The present invention describes the use of photonic crystals to form optical elements which function in optical apparatus in frequency ranges outside photonic band-gaps. Such optical elements may apply such optical properties as dispersion, anisotropy, and birefringence (all of which are exhibited by photonic crystals outside photonic band-gaps). A variety of optical apparatus, including spectrometers, radiation sources, and lasers are enabled by such optical elements.

Fluorescence imaging system

Abstract: A fluorescence imaging system for epi-illumination wherein the usual dichroic beamsplitter is replaced by a polarizing beamsplitter (PBS). The sample is excited with light that is linearly polarized to a significant degree, and fluorescent emission light is collected in the orthogonal linear polarization state. Excitation light that is scattered or reflected by the sample is rejected by the PBS, while the desired emission light is captured for imaging by a detector. By eliminating the usual dichroic beamsplitter member, the imaging system removes the barriers to multi-spectral imaging that such members conventionally impose. A wavelength-selective birefringent network may also be interposed between the beamsplitter and the sample for converting the polarization of either the excitation or the emission light to the orthogonal state without defeating this desirable rejection property, thus permitting measurement of the sample emissions in either or both linear polarization states for assessing fluorescence polarization.

Polarization Evolution and Polarization Instability of Solitons in a Birefringent Optical Fiber

Abstract: Ultrashort optical solitons with different states of polarization are used to map the polarization evolution in a fixed section of a weakly birefringent fiber. Soliton polarization evolution was compared to linear propagation. A significant change in the polarization behavior between the solitons and linear regime is observed. Although solitons contain a continuum of instantaneous intensities, they transform their polarization as a unit, demonstrating the particlelike nature of solitons.

Demonstration of ultrafast all-optical wavelength conversion utilizing birefringence in semiconductor optical amplifiers

Abstract: High-extinction-ratio wavelength conversion has been demonstrated in a semiconductor optical amplifier using a mixture of cross-gain and cross-phase modulation in a polarization rotation technique. 40-ps-wide pulses at 1311 nm have been generated using 47-ps, 5.7-mW peak-power input pulses at 1300 nm. Excellent agreement with experiment has been obtained using a computer model.

Variable polarization beam splitter, combiner and mixer

Abstract: Collimated light from two input fibers are directed against the interface of a beam splitter cube formed by two right-angle prisms. Light transmitted through, or reflected by, depends upon the polarization of the incoming light. A collimator subassembly for an output fiber receives light from the beam splitter cube. Polarization of the incoming light is controlled by orientation of the optical axes of one or two liquid crystal cells placed between each input fiber and beam splitter cube. By controlling the optical axes of the liquid crystal cells light from the two input fibers can be sent to the output fiber in any desired ratio. By rearranging the optical fibers, right-angle prisms and liquid crystal cells, a variable polarization beam combiner can be formed. A combination of the variable polarization beam combiner and beam splitter provides a variable polarization beam mixer, in which two input light signals can be combined and then split into two different output signals by control of liquid crystal cells. Finally, birefringent crystals and laminated beam splitter plates may also be used for the right angle prisms.

Polarization behavior of birefringent multitransverse mode vertical-cavity surface-emitting lasers

Abstract: An analysis has been undertaken of the effect of birefringence on the selection of polarization states of a weakly index guided vertical-cavity surface-emitting laser (VCSEL) supporting both a fundamental and a first-order transverse mode. It is shown that for small index steps polarization switching due to spatial-hole burning effects can occur. For larger index steps it is found that higher order modes can emerge which are orthogonally polarized to the dominant polarization of the fundamental mode.

Experimental investigation of linear polarization in high-birefringence single-mode fibers

Abstract: We report on measurements of linear polarization ratio in high-birefringence fibers as a function of wavelength. Comparison with standard step-index and with dispersion-shifted fibers reconfirms that the modes of polarization-maintaining fibers are not linearly polarized.

Low-loss fiber-matched low-temperature PECVD waveguides with small-core dimensions for optical communication systems

Abstract: Plasma-enhanced chemical vapor deposition (PECVD) offers a simple way of fabricating (doped) silica layers on silicon. A new design of the waveguide core allows low-loss fiber matched waveguides with low birefringence without high-temperature annealing. The increased loss of doped plasma deposited silica due to hydrogen incorporation is overcome by reducing the core dimensions and increasing the refractive index contrast. The waveguides can easily be fabricated using standard PECVD technologies and resist masked reactive ion etching (RIE) etching. Integrated optical devices such as 1/spl times/8 power splitters, 1300/1550-nm wavelength multiplexers and thermooptical switches were successfully fabricated and tested.

A compound high-order polarization-independent birefringence filter using Sagnac interferometers

Abstract: Polarization-independent fiber-optic filters are presented that replace the polarizers in traditional birefringence filters with a Sagnac interferometer. The performance of these filters is discussed when Solc and Lyot type birefringence combinations are used. A high-order, compound filter combining these two filters is presented, along with experimental verification.

Dynamic Processes of Optically Induced Birefringence of Azo Compounds in Amorphous Polymers below Tg

Abstract: The growth and decay processes of the optically induced birefringence in an azobenzene-functionalized polymer, DR19FPOZ (full name given in the text), and in another azobenzene guest/host polymer system have been studied. The temperature dependence of the birefringence signal is studied, and possible mechanisms associated with the growth and decay segments of the signal are analyzed. Rate constants deduced from the optically induced birefringence signal are also investigated as a function of the laser excitation power and film thickness. The rate constants associated with the growth and decay processes and activation energies are obtained for the polymer system below the glass transition temperature.

Magnetic Orientation of Isotactic Polystyrene

Abstract: Magnetic orientation of isotactic polystyrene during the isothermal crystallization at 210 °C in the magnetic field of 6 T is studied by using the magnetic birefringence, X-ray diffraction, and infrared measurements. The orientation starts in the vicinity of the end of the induction period, that is, before the crystal growth is observed by the wide angle X-ray diffraction This suggests that some anisotropic structures formed in the induction period are responsible for the magnetic orientation. Crystals formed are aligned with the c-axis, perpendicular to the magnetic field. Crystallization process is accelerted under the magnetic field.

Refractive index of nematic liquid crystals in the submillimeter wave region.

Abstract: Large electro-optic effects of liquid-crystal materials are attractive in applications to various optical devices in a wider wavelength region. Fundamental optical properties in the submillimeter wave region, such as refractive indices and transmission losses for some cyanobiphenyl nematic liquid crystals, have been investigated for the first time, to our knowledge, with a submillimeter laser. Refractive indices of the liquid crystal materials for ordinary and extraordinary rays are a little larger than those in the visible region, and a larger birefringence comparable with the visible region can also be obtained. Although the loss level is larger by ∼2 orders of magnitude than that of quartz plate, which is an excellent window in the submillimeter wave region, the transmission of the liquid crystal cell is high enough.