Showing papers on "Birefringence published in 2000"

Giant Birefringent Optics in Multilayer Polymer Mirrors

Abstract: Multilayer mirrors that maintain or increase their reflectivity with increasing incidence angle can be constructed using polymers that exhibit large birefringence in their indices of refraction. The most important feature of these multilayer interference stacks is the index difference in the thickness direction (z axis) relative to the in-plane directions of the film. This z-axis refractive index difference provides a variable that determines the existence and value of the Brewster's angle at layer interfaces, and it controls both the interfacial Fresnel reflection coefficient and the phase relations that determine the optics of multilayer stacks. These films can yield optical results that are difficult or impossible to achieve with conventional multilayer optical designs. The materials and processes necessary to fabricate such films are amenable to large-scale manufacturing.

High-speed fiber–based polarization-sensitive optical coherence tomography of in vivo human skin

Abstract: A high-speed single-mode fiber-based polarization-sensitive optical coherence tomography (PS OCT) system was developed. With a polarization modulator, Stokes parameters of reflected flight for four input polarization states are measured as a function of depth. A phase modulator in the reference arm of a Michelson interferometer permits independent control of the axial scan rate and carrier frequency. In vivo PS OCT images of human skin are presented, showing subsurface structures that are not discernible in conventional OCT images. A phase retardation image in tissue is calculated based on the reflected Stokes parameters of the four input polarization states.

Tunable two-dimensional photonic crystals using liquid crystal infiltration

Abstract: The photonic band gap of a two-dimensional photonic crystal is continuously tuned using the temperature dependent refractive index of a liquid crystal. Liquid crystal $E7$ was infiltrated into the air pores of a macroporous silicon photonic crystal with a triangular lattice pitch of 1.58 $\ensuremath{\mu}$m and a band gap wavelength range of 3.3--5.7 \ensuremath{\mu}m. After infiltration, the band gap for the H polarized field shifted dramatically to 4.4--6.0 \ensuremath{\mu}m while that of the E-polarized field collapsed. As the sample was heated to the nematic-isotropic phase transition temperature of the liquid crystal $(59\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}),$ the short-wavelength band edge of the H gap shifted by as much as 70 nm while the long-wavelength edge was constant within experimental error. Band structure calculations incorporating the temperature dependence of the liquid crystal birefringence can account for our results and also point to an escaped-radial alignment of the liquid crystal in the nematic phase.

The refractive index of AlxGa1−xAs below the band gap: Accurate determination and empirical modeling

Abstract: The refractive indices of AlxGa1−xAs epitaxial layers (0.176⩽x⩽1) are accurately determined below the band gap to wavelengths, λ<3 μm. The layers are grown on GaAs substrates by molecular beam epitaxy metal organic and chemical vapor deposition with thicknesses ranging from 4 to 10 μm. They form improper waveguide structures with the GaAs substrate. The measurements are based on the excitation of the improper waveguide modes with grating couplers at 23 °C. The refractive indices of the layers are derived from the modal propagation constants in the range of 730 nm<λ<830 nm with an estimated uncertainty of Δn=5×10−4. The temperature coefficient of the refractive index is investigated in the same spectral range. From the effective indices of the TE and TM modes, we derive the strain-induced birefringence and the elasto-optic coefficients. High-resolution x-ray diffraction is used to determine the strain of the layers. The layer compositions are obtained with inductively coupled plasma atomic emission spectro...

Influencing intramolecular motion with an alternating electric field

Abstract: Analogues of mechanical devices that operate on the molecular level, such as shuttles, brakes, ratchets, turnstiles and unidirectional spinning motors, are current targets of both synthetic chemistry and nanotechnology. These structures are designed to restrict the degrees of freedom of submolecular components such that they can only move with respect to each other in a predetermined manner, ideally under the influence of some external stimuli. Alternating-current (a.c.) electric fields are commonly used to probe electronic structure, but can also change the orientation of molecules (a phenomenon exploited in liquid crystal displays), or interact with large-scale molecular motions, such as the backbone fluctuations of semi-rigid polymers. Here we show that modest a.c. fields can be used to monitor and influence the relative motion within certain rotaxanes, molecules comprising a ring that rotates around a linear 'thread' carrying bulky 'stoppers' at each end. We observe strong birefringence at frequencies that correspond to the rate at which the molecular ring pirouettes about the thread, with the frequency of maximum birefringence, and by inference also the rate of ring pirouetting giving rise to it, changing as the electric field strength is varied. Computer simulations and nuclear magnetic resonance spectroscopy show the ring rotation to be the only dynamic process occurring on a timescale corresponding to the frequency of maximum birefringence, thus confirming that mechanical motion within the rotaxanes can be addressed, and to some extent controlled, by oscillating electric fields.

Cavity Ring-Down Polarimetry (CRDP): A New Scheme for Probing Circular Birefringence and Circular Dichroism in the Gas Phase

Abstract: Cavity ring-down polarimetry (CRDP), a new, ultrasensitive method for probing circular birefringence and circular dichroism, has been developed by extending the well-established technique of pulsed cavity ring-down spectroscopy (CRDS). The concurrent incorporation of polarization elements into the stable resonator, injection optics, and detection train of a conventional CRDS apparatus is found to permit the quantitative measurement of optical rotation and differential absorption induced by the presence of chiral compounds. The sensitivity of this novel scheme is sufficient to allow (low-pressure) gas-phase species to be interrogated under ambient conditions, a fact highlighted by the direct determination of specific rotation at 355 nm for gaseous samples of α-pinene, β-pinene, cis-pinane, limonene, fenchone, and propylene oxide. Although usually precluded by the signal discrimination limits imposed upon traditional polarimeters, such gas-phase studies of nonresonant optical activity in the visible and nea...

Multiwavelength erbium-doped fibre laser based on a high-birefringence fibre loop mirror

Abstract: A novel multiwavelength fibre laser is proposed, which is based on a fibre loop mirror formed by a high-birefringence (HiBi) fibre, a polarisation controller, and a directional coupler By setting the polarisation controller properly, the HiBi fibre loop mirror acts as a polarisation-independent wavelength filter With this configuration, the wavelength spacing of the laser can be varied accurately by adjusting the length of the HiBi fibre Experimentally, by cooling the erbium-doped fibre with liquid nitrogen to reduce its homogeneous linewidth, stable emission at 16 wavelengths with a spacing of 08 nm and an amplitude variation of < 5 dB was achieved

Statistical characterization of fiber random birefringence.

Abstract: The statistical properties of the random birefringence that affects long single-mode fibers are experimentally evaluated by means of a polarization-sensitive optical time-domain reflectometry. The measurements are in good agreement with theoretical predictions and show, for what we believe is the first time, that the components of the local birefringence vector are Gaussian random variables.

Vacuum deposition of chiral sculptured thin films with high optical activity

Abstract: We present the technique of bideposition to realize thin-film helicoidal bianisotropic mediums (TFHBM’s) that exhibit high optical activity. We show, by experiment as well as by simulation, that the optical rotation produced by these chiral sculptured thin films is roughly proportional to the square of the local linear birefringence. Experimental measurements on bideposited TFHBM’s of titanium oxide yield a typical value of 5°/µm for the effective specific rotation in the short-wavelength regime; the corresponding value determined for the standard unideposited TFHBM’s is 1°/µm. Both types of TFHBM’s are highly optically active in comparison with quartz, fluorite films, and cholesteric liquid crystals. Bideposited TFHBM’s will lend themselves to many different types of optical devices.

Dual polarization modulation: A real-time, spectral-multiplex separation of circular dichroism from linear birefringence spectral intensities

Abstract: A real-time, spectral-multiplex method for the complete separation of circular dichroism (CD) spectra from linear birefringence (LB) spectra is presented. The method, called dual polarization modulation (DPM), involves the introduction of a second source of polarization modulation after the CD sample. The first source of polarization modulation, as in conventional CD spectrometers, is located before the sample. Intensity signals at the detector in phase with each of the two polarization modulation frequencies are demodulated simultaneously in parallel and combined electronically in opposition to eliminate the LB spectrum by real-time cancellation. The accuracy of the cancellation can be adjusted electronically without the need to change the optical alignment of the instrument. The DPM method permits baseline-corrected CD spectra to be measured without the need for a subsequent CD background measurement.

A photorefractive organically modified silica glass with high optical gain

Abstract: Photorefractive materials1 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 applications1 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 structure2. 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 process3 to induce a permanent arrangement of azo-dye chromophores.

Mechanism of intrinsic wavelength tuning and sideband asymmetry in a passively mode-locked soliton fiber ring laser

Abstract: We have experimentally observed continuous-wavelength tuning in a passively mode-locked fiber ring laser. Depending on the polarization setting, two separated tuning ranges are observed. We show that the wavelength tuning is a result of the existence of birefringence in the laser cavity. We have also shown that the same mechanism is responsible for the power asymmetry of sidebands appearing in the soliton spectrum. © 2000 Optical Society of America [S0740-3224(00)01901-9] OCIS codes: 140.4050, 140.7090, 140.3510, 060.5530.

Polarization-locked temporal vector solitons in a fiber laser: experiment

Abstract: We experimentally observe polarization-locked vector solitons in a passively mode-locked fiber laser. The vector soliton pulse is composed of components along both principal polarization axes of the linearly birefringent laser cavity. For certain values of birefringence and pulse energy these components propagate with a constant relative optical phase of ±π/2, and hence the pulse has a fixed elliptical polarization state. The linear birefringence of the cavity is canceled by the nonlinear birefringence created by the unequal amplitudes of the two polarization components. This dynamic equalization of the phase velocities of the components results in the stable propagation of an elliptically polarized vector soliton pulse. Under different conditions we also observe the nonlinear instability of the fast principal axis as an intracavity pulse linearly polarized along the slow axis of the cavity. We present the experimental characterization of both the polarization-locked vector soliton and the fast axis instability and discuss the nonlinear mechanism creating both phenomena.

Experimental and phenomenological aspects of circular birefringence and related properties in transparent crystals

Abstract: Here, we review the history, theory, measurement technique and experimental results on gyrotropic phenomena including optical rotation (optical activity), electrogyration, the Faraday effect and magneto-electrogyration in transparent crystals, including examples of structural phase transitions. Relations to the absolute structure are discussed and model calculations are performed on the basis of electronic polarizability and crystal structure.

Pulse-train uniformity in optical fiber lasers passively mode-locked by nonlinear polarization rotation

Abstract: The generation of uniform soliton pulse trains by additive pulse mode locking has been experimentally demonstrated in a birefringent fiber laser with a passive polarizer. Numerical simulations of pulse propagation around such a fiber loop are presented which reveal that this mode-locking scheme does not result in strictly uniform pulse trains. Rather, the train of output pulses exhibits periodic fluctuations in intensity and polarization. A model for the pulse dynamics is developed which shows that these fluctuations depend on the strength of the fiber birefringence and the alignment of the polarizer with the fast- and slow-polarization axes of the fiber. It is also shown that increased uniformity of pulse trains is achieved with near alignment of the polarizer with the slow axis of the birefringence.

Birefringence free planar optical waveguide made by flame hydrolysis deposition (FHD) through tailoring of the overcladding

Abstract: Stresses developing in a planar waveguide resulting from the different thermal expansion coefficients of the substrate and the three glass layers (buffer, core and cladding) were analyzed using a finite element method. It can be shown that mainly the thermal expansion of the overcladding determines the birefringence in the finished waveguide. Based on that result, recipes for an overcladding made with the flame-hydrolysis-deposit ion (FHD)-process were devised. We demonstrate the absence of birefringence in a commercial waveguide layer overclad with this glass. The high doping levels required for the cladding to have a thermal expansion coefficient sufficient for this raises concerns about the moisture sensitivity of such a glass. We examined the depth dependent composition of the glass using WD-ESCA (wavelength dispersive electron microprobe) and show, that at the surface a layer depleted of dopants is formed during the high temperature sintering process, This layer can serve as a protective coating to isolate the underlying, higher doped layer from the effects of moisture. Analysis of the stresses shows that this does not effect the birefringence behavior of the waveguide.

Piezoelectric optical switch device

Abstract: A piezoelectric optical switch includes a planar Mach-Zehnder optical device having a piezoelectric rib disposed on one or both of the waveguide structures. The piezoelectric rib deforms the waveguide structure creating a strain vector that alters the optical path of the waveguide. The piezoelectric rib is offset from the propagation path in the waveguide. This yields several important advantages. By positioning the piezoelectric rib away from the waveguide, the strain components in the propagation path of the waveguide in the directions perpendicular to the direction of propagation, e.g., in the x-direction and the y-direction, are negligible. Since strains in these directions create birefringence, elimination of these strains will minimize the birefringence. The elimination or reduction of birefringence is greatly desired because birefringence degrades the extinction ratio at the outputs of the Mach-Zehnder device. Thus, the piezoelectric switch of the present invention provides a high extinction ratio and a low power consumption and small switching time expected of piezoelectric devices.

Surface Dynamics in Rubbed Polymer Thin Films Probed with Optical Birefringence Measurements

Abstract: Measurements of the optical birefringence were used to probe the relaxation of rubbed ultrathin polystyrene (PS) films on glass substrates. It was found that the glass transition temperature, Tg, of the films dropped by 15-20 K as the film thickness decreased from 10 Im to 5.8 nm. Experiments on thick films (10 Im) revealed that molecules closer to the polymer-air interface relax more quickly than molecules farther from the interface. These results are explained in the context of past experimental studies of thermal properties of similar ultrathin polymer films.

Images of absolute retardance L.Deltan, using the rotating polariser method

Abstract: Modulation techniques for measuring changes in optical birefringence, such as the rotating-polariser method (Wood & Glazer, 1980, J. Appl. Crystallogr. 13, 217), allow one to determine |sin δ|, δ = 2πLΔn/λ, Δn= double refraction, L = light path and λ = wavelength. However, they generally suffer from not providing absolute values of the optical retardance or are limited to relatively low retardance values. In addition, knowledge of the absolute phase is required when establishing the correct values of optical orientation information. In this paper, it is shown how the phase δ, and thus optical retardance, can be extracted from combining measurements of |sin δ| at different wavelengths. The new approach works on each single point of a 2-D picture without the need to correlate with neighbouring points. There is virtually no limit to the retardance, and the computational efforts are small compared with other methods (e.g. Ajovalasit et al. 1998, J. Strain Analysis 33, 75). When used with imaging techniques, such as the rotating polariser method of Glazer, Lewis & Kaminsky 1996 (Proc. R. Soc. London SeriesA452, 2751) this process has the potential to identify automatically optically anisotropic substances under the microscope. The algorithm derived in this paper is valid not only for birefringence studies, but can be applied to all studies of interfering light waves.

Optical bodies made with a birefringent polymer

Abstract: A optical body has at least two polymeric materials forming a reflective interface for at least one polarization of light. One optical body is a multilayer optical film that includes birefringent first optical layers and second optical layers interleaved with the first optical layers. Each first optical layer is formed using a polymer, such as a copolymer of polyethylene naphthalate. The second optical layers are formed so that they have a lower in-plane birefringence than the first optical layers for 632.8 nm light. Optical bodies can be used as, for example, polarizers and mirrors. The optical bodies can be formed using polymers that provide better index matching and are more easily protected from UV light without coloring the optical body.

Birefringent reflectors using isotropic materials and form birefringence

Abstract: Multilayer thin film reflectors, such as mirrors and reflective polarizers, are described in which form birefringent optical layers are incorporated into a plurality of optical repeat units in the film. The form birefringent layers exhibit birefringence as a result of microscopic structures that have a dimension that is small compared to the wavelength of light but large compared to molecular distances. The optical layers within the optical repeat units have out-of-plane indices of refraction that are tailored to produce desired effects as a function of incidence angle for p-polarized light. The multilayer reflectors can be made by conventional vacuum deposition techniques using known inorganic optical materials, but can also be made entirely with polymeric materials by co-extrusion or other processes.

Electro-optic measurements of the ferroelectric-paraelectric boundary in Ba1−xSrxTiO3 materials chips

Abstract: The combinatorial material chip strategy is used to study the ferroelectric-paraelectric phase boundary of the Ba1−xSrxTiO3 thin film system. The electro-optic (EO) effect at different compositions is measured using a modified direct-current/alternating-current birefringence EO measurement technique. We find that Ba1−xSrxTiO3 thin films exhibit relaxor like behavior with diffused ferroelectric domains existing well past the previously defined ferroelectric-paraelectric boundary (x>0.3).

I Polarimetric optical fibers and sensors

Abstract: Publisher Summary This chapter discusses the polarimetric optical fibers and sensors. The chapter reviews the achievements and efforts in research related to the development of a new generation of polarimetric optical fibers and sensors at both fundamental and applied levels. The chapter provides an overview of the physical origin of the perturbations on the lowest-order mode propagation in highly birefringent (HB) polarization-maintaining fibers, together with their impact on applications in optical fiber sensors and systems. When birefringence is introduced into an isotropic fiber, the circular symmetry of the ideal fiber is broken, thus producing the anisotropic refractive index distribution into the core region. The asymmetry results from either intrinsic birefringence including a geometrical deformation of the core and stresses induced during the manufacturing process or from material anisotropy due to induced (extrinsic) elastic birefringence. The deformation effects in highly birefringent fibers include symmetrical deformation effects, hydrostatic pressure, axial strain, nonsymmetric deformation effect (twist), and twist under pressure/strain. A potential application of polarimetric optical fibers and sensors is to embed them directly inside various ceramic and composite materials, and to measure strain distribution in different structures using the concept of smart skins and structures.

Continuous wave infrared laser action, self-frequency doubling, and tunability of Yb3+:MgO:LiNbO3

Abstract: The laser performance of the nonlinear Yb3+:MgO:LiNbO3 crystal is assessed under Ti:sapphire laser pumping. Spectral properties related to infrared laser action such as the emission and absorption cross sections have been evaluated. Different crystallographic configurations have been used. Laser oscillation in the near infrared region has been observed along the crystallographic c axis and along the type I phase matching direction for infrared to green generation. High slope efficiency in the infrared region (62%) and the possibility of low laser thresholds (14.7 mW) are demonstrated. By using a birefringent filter, broadband tuning from 1030 to 1070 nm is obtained in this system. In addition, 58 mW of green power by self-frequency doubling of infrared laser emission is achieved, revealing the potential of LiNbO3:MgO as a self-frequency-doubling material.

Optical element made from fluoride single crystal, method for manufacturing optical element, method for calculating birefringence of optical element and method for determining direction of minimum birefringence of optical element

Abstract: The present invention provides a method for calculating the birefringence of an optical element and selecting the direction of minimum birefringence in the optical element. A material for manufacturing the optical element is a fluoride single crystal with a birefringence value that is reduced compared to conventional materials. The fluoride single crystal is cut from a fluoride single crystal ingot obtained by crystal growth so that the {111} crystal planes are two parallel planes, after which the optical performance is improved by subjecting this material to a heat treatment. The birefringence of the optical element is calculated by converting known piezo-optical constants in a specified three-dimensional orthogonal coordinate system for the optical material into piezo-optical constants in an arbitrary three-dimensional orthogonal coordinate system. The amount of change in the refractive index Δn 1 of the optical material in a first direction along the direction of one coordinate axis of the arbitrary three-dimensional orthogonal coordinate system, and the amount of change in the refractive index Δn 2 of the optical material in a second direction which is perpendicular to the first direction are calculated using a uniaxial stress that is applied to the optical material along the first direction and the piezo-optical constants in the arbitrary three-dimensional coordinate system. The amount of birefringence as seen from a third direction perpendicular to the first direction and the second direction is determined in the arbitrary three-dimensional orthogonal coordinate system by determining the difference between the amount of change in the refractive index Δn 1 and the amount of change in the refractive index Δn 2.

Optical polarization beam combiner/splitter

Abstract: An optical polarization beam splitter comprises a first optical fiber having an end defining a first optical axis, a second optical fiber having an end defining a second optical axis, and a third optical fiber having an end defining a third optical axis parallel to and spaced apart from the second optical axis. A collimating lens is disposed along the first optical axis positioned to form a collimated optical beam from the first optical fiber. A focussing lens is disposed along a path of the collimated optical beam. A birefringent walk-off crystal has a first face adjacent to the focussing lens and a second face located at a focal plane of the focussing lens and in contact with the ends of the second and third optical fibers. The birefringent crystal is oriented such that and has a thickness between its first and second faces selected such that a first component of the optical beam having a first polarization exits the crystal at its second face and enters the end of the second optical fiber along the second optical axis and a second component of the optical beam having a second polarization orthogonal to the polarization of the first polarization exits the crystal at its second face and enters the end of the third optical fiber along the third optical axis.

Is a birefringence band a shear band

Abstract: The discrepancy between the width of birefringence bands and shear rate bands in the system CTAB/water is addressed using a combination of NMR spectroscopy and NMR velocimetry. In correspondence with birefringence measurements in the annular gap of a cylindrical Couette cell, 2H NMR spectroscopy indicates a separation of fluid into distinct nematic and isotropic phases. By contrast the velocity profile exhibits an extremely narrow shear band, consistent with the predictions of the lever rule. Contrary to previous assumptions, the experimental evidence points to a nematic phase with high viscosity.