Showing papers on "Birefringence published in 1984"

Polarization holography. 1: A new high-efficiency organic material with reversible photoinduced birefringence.

Abstract: A new organic material for polarization holographic recording--azo-dye methyl-orange, introduced in polyvinyl alcohol (PVA)--is fabricated and investigated. It possesses all the good characteristics of the known polarization-sensitive materials--high sensitivity, reversibility, etc.--but excels them substantially in the magnitude of the photoinduced birefringence: delta n > 10(-3). This makes possible the recording of reversible polarization gratings with diffraction efficiency over 30%. Depending on the conditions of production and subsequent mechanical and thermal treatments the layers of methyl orange/PVA may also have intrinsic birefringence. In this paper the results of experimental investigations into the properties of this new material are reported.

Refractive properties of magnesium fluoride

Abstract: The refractive indices of a commercially available specimen of single-crystal MgF2 were determined for both the ordinary and extraordinary rays at selected wavelengths from 0.2026 to 7.04 μm. Measurements were made by means of the minimum-deviation method on a precision spectrometer near 19°C. The experimentally determined index values for each polarization were fitted to a three-term Sellmeier-type dispersion equation of the form n2-1=∑Ajλ2(λ2-λj2), where n is the refractive index, λ is the wavelength of interest, λj is the calculated wavelength of an oscillator, and Aj is the strength of the oscillator at λj. The birefringence was computed as a function of wavelength from the calculated index values obtained for the two polarizations. The dispersion coefficients were also determined for the 0 ray and for the birefringence. The results of this study are compared with previously reported work on the refractive properties of MgF2.

Birefringence measurements of liquid crystals

Abstract: A convenient and accurate technique for measuring the birefringence of liquid crystals at discrete wavelengths or as a continuous function of wavelength in the ultraviolet, visible, or infrared spectral regions is described. The method is based on determination of the phase differences which occur when monochromatic polarized light propagates through a medium with an anisotropic refractive index. Birefringence measurements at 0.6328 μm for two liquid crystal materials, BDH-E7 and ZLI-1132, and a continuous birefringence spectrum of ZLI-1132 from 2 to 16 μm are reported. Additionally, a liquid crystal based phase retardation plate which can be voltage tuned and calibrated to provide any degree of phase shift from 0 to 2π over a wide wavelength range (0.4–16 μm) is discussed.

The rotation of the polarization in low birefringence monomode optical fibres due to geometric effects

Abstract: Monomode optical fibres of very low intrinsic linear and circular birefringence have been developed for use in polarimetric optical fibre sensors, most notably the fibre optic current sensor. The polarization of light travelling along such fibres is known to be modified by external stress applied to the fibre. In this paper it is shown that the polarization state may also be rotated if the path of the fibre is bent in a nonplanar curve. A theory is developed which enables the effect to be quantified and the results are shown to agree with some measurements on a fibre bent into a helix.

Magneto-chiral birefringence and dichroism

Abstract: It was pointed out recently that the absorption coefficient of a chiral molecule should be shifted slightly in a magnetic field parallel to a light beam in any polarization state. This suggestion is developed further by considering an analogous refractive index shift and by discussing these magneto-chiral phenomena in the unified context of effects generated by the ‘magnetic’ (time-odd) parts of the complex optical activity tensors. Explicit expressions, in terms of molecular property tensors, are derived for the difference in refractive index and absorption coefficient of a chiral molecule in a magnetic field parallel and antiparallel to the light beam, and magneto-chiral analogues of the Faraday A-, B- and C-terms introduced. A rough estimate of the magneto-chiral birefringence indicates that it should be observable using a modified Rayleigh refractometer. The feasibility of observing magneto-chiral dichroism A-, B- and C-terms in different types of chiral molecules is also considered. The magneto-chira...

Polarization holography. 2: Polarization holographic gratings in photoanisotropic materials with and without intrinsic birefringence.

Abstract: Results are reported of experimental investigations on the diffraction efficiency and its polarization dependence of holographic phase polarization gratings in a photoanisotropic medium methyl orange/PVA with intrinsic birefringence. A comparative study of layers with different intrinsic birefringence was conducted. The conditions—initial birefringence and a type of polarization recording—were found to have high diffraction efficiency (>30%) and strong dependence on the direction of polarization of the linearly polarized readout beam. The results are in agreement with the theoretical considerations on such gratings.

Focusing by electrical modulation of refraction in a liquid crystal cell

Abstract: The creation of a field-controlled variation of the index of refraction in a liquid crystal cell has been analyzed and experimentally verified. To obtain a spherical lens utilizing a simple electrode structure and capable of focusing arbitrary incoming polarizations requires four flat nematic liquid crystal cells. With electrodes fabricated well within the current capability of photolithography, near diffraction-limited performance in terms of the optical transfer function is predicted. The focusing capability of a liquid crystal lens was demonstrated using a single cell with linear transparent electrodes. A plano-convex cylindrical lens for a single incoming polarization was thus created. While the cell had a crude electrode structure, it affirmed all the major qualitative predictions. The fringing along the edge of the electrodes required for eventually obtaining near diffraction-limited performance was observed.

Linear optical birefringence of magnetic crystals

Abstract: The study of changes in the refractive indices (with consequent changes of birefringence) of a transparent magnetic crystal which accompany changes in the magnetic order is becoming more popular. The authors review why this is. The first reason is that birefringence can be measured very accurately: the different experimental arrangements are reviewed. The second reason is because a birefringence measurement is an integrational spectroscopic technique and therefore it is studied both experimentally and theoretically as a branch of magneto-optics and hence gives information on the detailed energy level structure of the solid. The third reason is that in a number of interesting systems the birefringence is proportional to the magnetic energy over a wide temperature range and it is often a more convenient method of obtaining the magnetic specific heat than direct specific heat measurements; this is particularly true in magnetic crystals which show low dimensional ordering. The last reason is that in all magnetic crystals the birefringence change should vary like one of the thermodynamic critical exponents near to the phase transition. They review in detail the reasons why birefringence studies have become so successful for measuring critical exponents in pure and particularly mixed crystals.

Optical device including birefringent aromatic amino carboxylic acid polymer

Abstract: Optical devices including a molecularly oriented highly birefringent polymer are disclosed. The devices include molecularly oriented polymers comprising recurring units which exhibit a distribution of high electron density about the long axes of the polymer and the recurring units thereof. Transparent birefringent polymers comprising a plurality of recurring units having a substantially cylindrical distribution of electron density about the long axis of such units and the chain-extended polymers are included in optical devices and articles. The polymers exhibit high birefringence and simulate in a polymer the optical properties of uniaxial crystal.

Optical device including birefringent polyhydrazide polymer

Abstract: Optical devices including a molecularly oriented highly birefringent polymer are disclosed. The devices include molecularly oriented polymers comprising recurring units which exhibit a distribution of high electron density about the long axes of the polymer and the recurring units thereof. Transparent birefringent polymers comprising a plurality of recurring units having a substantially cylindrical distribution of electron density about the long axis of such units and the chain-extended polymers are included in optical devices and articles. The polymers exhibit high birefringence and simulate in a polymer the optical properties of a uniaxial crystal.

Infrared birefringence of liquid crystals

Abstract: The first measurements of infrared birefringence of liquid crystals are reported. Continuous birefringence spectral data were calculated from measurements of phase differences which occur when monochromatic polarized light propagates through a medium with an anisotropic refractive index. It was found that molecular absorption bands can provide significant resonant enhancement of the refractive indices of liquid crystals, and this effect causes the birefringence of these materials to be relatively large throughout the spectral region from 2 to 16 μm. The birefringence and absorption spectra of two particular liquid crystal mixtures indicate that liquid crystals will be useful for electro‐optic applications in the infrared region.

Acousto-optic frequency shifting in birefringent fiber

Abstract: Single-sideband frequency shifting has been accomplished using traveling acoustic waves to couple the orthogonal polarizations of birefringent fiber. The light coupled from one polarization to the other is shifted in frequency by the frequency of the acoustic wave. An upper sideband is produced in one polarization, a lower sideband in the other. Both surface and bulk acoustic waves have been used. Sideband suppression of better than 30 dB and carriersuppression of better than 20 dB below the desired sideband have been achieved.

In-line fiber-polarization-rocking rotator and filter.

Abstract: An in-line polarization rotator has been built into a single-mode birefringent fiber. The rotator utilizes periodic twists of the fiber’s principal axes, which were formed by rocking the preform as the fiber was drawn. The polarization conversion between the principal axes is wavelength dependent, with a bandwidth inversely proportional to the number of twist periods. The bandwidth of the present rotator was 4.8 nm for 100% conversion in a fiber length of 170 cm.

Single-polarization operation in highly birefringent optical fibers.

Abstract: Single-polarization characteristics in highly birefringent fibers have been investigated by considering the photoelastic effect in the entire cross section of the fiber. It has been shown by stress analysis that the stress-induced birefringence between the two orthogonal polarization modes in the cladding is much smaller than that in the core and the vicinity of the core–cladding interface, which causes differential bending loss for the two polarization modes. A bending loss formula, which takes into account the photoelastic effect, has been derived for each polarization mode. Differential attenuation characteristics and extinction ratios have been investigated for several bending radii using the bending loss formula.

High-birefringence optical fibers by preform deformation

Abstract: High-birefringence optical fibers have been fabricated using a preform deformation technique in which an initially round preform is locally heated and squeezed from two sides. This technique has been used to make both polarization-preserving and single-polarization fibers. A novel feature of these new fibers is their rectangular shape which facilitates location of the principal axes and increases resistance to polarization breakdown from external perturbations. These fibers have circular cores, low loss, and excellent polarization holding.

Subpicosecond Electro-Optic Sampling Using Coplanar Strip Transmission Lines

Abstract: A sampling technique using the electro-optic effect capable of characterizing subpicosecond electrical signals has recently been demonstrated. In this system 100 fs pulses from a colliding pulse mode-locked laser were used to generate the electrical test signal via a Cr:GaAs photoconductive switch. A second beam of pulses was used to probe the birefringence induced by the electrical pulses as they propagate down a balanced stripline fabricated on LiTa03. Signal averaging allowed submillivolt signals to be recovered. The best temporal response of the system was achieved with the velocity matched geometry, which was obtained for a particular angle of incidence of the probe beam on the electro-optic crystal. In this configuration a risetime of 500 fs was obtained. During this work it was observed that strong dispersion effects were taking place when the wavelength of the electromagnetic signal approached the cross-sectional dimensions of the transmission line. Temporal resolution is improved and dispersion effects are reduced as the dimensions of the stripline are reduced. In order to further improve the temporal response of the sampler in this configuration, the thickness would have to be reduced to the order of 10 um, which presents severe fabrication difficulties.

Polarization properties of phase-conjugate mirrors: Angular dependence and disorienting collision effects in resonant backward four-wave mixing for Doppler-broadened degenerate transitions

Abstract: We present the theory of resonant backward degenerate four-wave mixing in gaseous media for Doppler-broadened degenerate two-level systems. The degeneracy of the atomic levels as well as the influence of the polarization of the incident fields are taken into account within the framework of a tensorial formalism. The amplitude, line shape, and polarization of the reemitted phase-conjugate field are calculated up to the third order in the incident fields. The velocity average is performed in the approximation of large Doppler linewidths. One considers in detail the influence of depolarizing relaxation processes and the effects of pump-probe angular separation. In particular, the importance of the residual Doppler effect (shortening of the lifetime of the optically induced gratings by the atomic motion) associated with the pump-probe angle is emphasized. We show that the polarization of the reemitted fields is governed by very simple laws as soon as the residual Doppler effect overcomes the lifetimes of the atomic levels. Phase-conjugate mirrors can be simply characterized by a linear dichroism (parallel pump polarizations) or a birefringence (cross-polarized pumps). Vectorial phase conjugation is also analyzed, in the case of counter-rotating circular polarizations of the pumps. Finally, the case of two-photon transition is studied and compared with the case of a resonant transition.

Phase-matched four-photon mixing processes in birefringent fibers

Abstract: We describe several new phase-matching processes for four-photon mixing in polarization-preserving birefringent fibers. The frequency shifts and polarization conditions of the waves involved in this nonlinear interaction are specified for a single-mode fused silica birefringent fiber. Our calculations include the effect of dispersion in the birefringence of the fibers.

Degree of polarization in jointed fibers: the Lyot depolarizer.

Abstract: Analytic equations for the degree of polarization of the output lightwave from jointed fibers are derived taking into account the coherence time of light and polarization mode dispersion in the fibers. On the basis of the results, the depolarizing conditions and the principles of optics of the Lyot depolarizer are discussed.

Birefringence calculations in elliptical-core optical fibres

Abstract: It is shown that in the useful single-mode region, the birefringence properties of elliptical-core fibres match very well with those of a rectangular-core waveguide having the same core area, same aspect ratio and same core and cladding refractive indices. This gives a simple method of obtaining the birefringence and V-value for maximum birefringence in ellipticalcore fibres.

Optical fiber having in-line polarization filter

Abstract: A device using a birefringent optical fiber having periodic integral perturbations with the period equal to the birefringence beat length being useful as, for example, a polarization rotator and an optical filter.

Electric-field-induced critical phenomena at the nematic-isotropic transition and the nematic-isotropic critical point

Abstract: A convenient method for measuring the dc Kerr effect on the stage of a polarizing microscope is described. Two novel illustrations of this technique are presented: a low-field determination of the pretransitional Kerr effect in the isotropic phase of a nematic having a negative dielectric anisotropy and measurements of the birefringence on both sides of the nematic-isotropic ($N\ensuremath{-}I$) phase boundary as the $N\ensuremath{-}I$ critical point is approached by applying very high electric fields to a nematic with positive dielectric anisotropy.

Fundamental limits to the transmission of linearly polarised light by birefringent optical fibres

Abstract: Experimental results are presented which show that the fundamental mode in highly birefringent fibres is not plane-polarised, as is normally assumed, but has significant orthogonal field components. These components limit the maximum measurable polarisation intensity extinction ratio to ~40 dB. Implications for polarisation measurements and fibre gyros are outlined.

Ferroelectric domains, birefringence and absorption of single crystals of BiFeO3

Abstract: The domain structure of BiFeO3 has been studied by transmission (4–800K) and reflection (300–1100K) microscopy. The rhombohedral phase transforms at ∼ 1083K to a non-cubic twinned phase, stable up to the decomposition at 1185K. Owing to a strong negative temperature shift (∼ -5. 9 cm-1K-1) of an absorption edge BiFeO3 turns opaque upon heating. At 15600cm-1 the 6Alg → 4T2g crystal field transition of Fe3+ occurs. The maximal spontaneous birefringence of the rhombohedral phase is very high (Δnγ, α = 0. 58 at Λ = 644nm and R. T.) and has been measured between 10 and 800K.

Response of moderately concentrated xanthan gum solutions to time-dependent flows using two-color flow birefringence

Abstract: A new optical arrangement for the flow birefringence experiment, based on a two‐color laser, is described. Using this system, flow birefringence measurements can be performed in transient flow conditions with the birefringence Δn and orientation angle χ of the sample being obtained simultaneously in time. Unlike previous designs which have been devised to accomplish this capability, only a single experimental run is necessary. Measurements using a quarter‐wave retarder indicate that the experimental accuracy is 2.2% for the birefringence and 0.15 ° for the orientation angle. The minimum birefringence detectable by this system is estimated to be 2×10−7. Using this technique. Δn and χ of moderately concentrated xanthan solutions were measured following the inception and cessation of simple shear. The effects of increased ionic strength by adding NaCl to the xanthan solution were also examined. A qualitative comparison of measurements between the no salt and 0.1M NaCl solutions indicates that the macromolecu...

A molecular theory of linear birefringence induced by crossed electric and magnetic fields

Abstract: A symmetry method is used to confirm the existence of a previously reported but as yet unobserved linear birefringence, which is induced in a fluid by the simultaneous application of an electric field E and a magnetic field B perpendicular to each other and to the light beam. For weak fields the effect is proportional to EB. Also presented is a quantitative theory which relates the birefringence in an ideal gas to various multipole polarizability tensors of its molecules. The expression obtained is identical to that for another but essentially different linear birefringence, which is induced in a fluid by parallel electric and magnetic fields acting at right angles to the light path. For reasons relating to the fast and slow axes of these two birefringences, that induced by the crossed fields offers a more convenient means to the available molecular information. The existence in ferroelectric crystals of birefringence induced by and linear in an applied magnetic field is inferred from the molecular result...

Single mode optical fiber polarimetric stress sensor having optical common mode rejection

Abstract: An instrument for measuring stress is arranged to be insensitive to stresses caused by conditions other than that of the phenomenon to be measured. The instrument employs two equal lengths of optical fibers of the kind exhibiting birefringence when stressed. Polarized light is directed into one of the optical fibers along its longitudinal axis. The two optical fibers are arranged in series with respect to the longitudinal transmission of the polarized light through those fibers. A polarization rotator is situated in the light path between the two optical fibers. The polarized light transmitted through the polarization rotator is rotated by an amount causing stresses imposed equally on the two optical fibers to have equal and opposite effects of the transmitted polarized light. The invention may be embodied to permit the measurement of acceleration, pressure, fluid flow rate, magnetic field gradient, magnetic field strength, electric field strength, electric field gradient, or any other phenomena that can be made to produce stress on an optical fiber. The invention is especially useful in differential measurement because the device measures the difference in stress imposed on the two optical fibers.

Method of fabricating high birefringence fibers

Abstract: The present invention relates to a technique for producing polarization-preserving and single polarization optical fibers. As disclosed, high birefringence is introduced into the preform by deforming the fiber preform such that a cladding layer becomes flat and highly conformable, while the core remains hard and substantially round. In particular, a cladding layer with a relatively low melting point is utilized such that when the preform is heated the cladding becomes liquified while the core remains solid. The preform may then be deformed so that the cladding layer is substantially flattened. Standard drawing techniques may then be utilized to form polarization-preserving fibers and single polarization fibers from the preform.