Natural Birefringence Research Articles

Anomalous birefringence of swollen lamellar phases : blue smectics

The birefringence of a lyotropic lamellar phase is calculated as a function of dilution. It is found to vanish and change sign, provided the natural birefringence of surfactant bilayers is positive. Dispersion is calculated about the point of zero birefringence. These predictions are illustrated with experiments of spectrophotometry: the intensity of light transmitted between crossed polarizers through several lamellar samples vanishes as expected at some particular wavelength. The dependence of the transmitted light on wavelength and cell thickness is consistent with theory

Phase-matching techniques and frequency-conversion efficiency in optically active crystals.

Many materials of potential interest for nonlinear optical applications exhibit optical activity and this is generally not taken into account in the standard theoretical treatments of nonlinear optical effects. In this paper we examine the conditions under which optical activity significantly alters the nonlinear optical properties of a material and show how this mechanism might be used in designing a nonlinear optical device. First, an algorithm is developed to calculate the exact optimum phase-matching angles and conversion efficiency for three-wave interactions in optically active biaxial crystals. Corrections to the standard theoretical expressions for birefringent phase-matching angles and conversion efficiency are obtained for application to biaxial crystals with large natural birefringence. Second, a generalized quasi-phase-matching scheme based on the presence of optical activity is formulated for three-wave interactions in second-order nonlinear crystals. It is shown that quasi-phase-matching via optical activity can be achieved by way of an effective harmonic modulation of ${\mathit{d}}_{\mathrm{eff}}$, the effective second-order nonlinear coefficient. For both these effects, it is found that optical activity makes a negligible change in the nonlinear optical properties of a material such as ${\mathrm{KTiOPO}}_{4}$, but they may become important for liquid crystals or polymers with high optical rotation and small birefringence.

Temperature measurements of a birefringent substrate (LiNbO3) in a radio frequency discharge by laser interferometry

Temperatures of a birefringent substrate (LiNbO3) have been measured by laser interferometry in a 13.56 MHz rf discharge. The measurement method is based on monitoring the variation of natural birefringence with temperature. The transient temperature rises and equilibrium values are investigated in nitrogen plasma at rf powers of ∼86 W. Owing to the large heat capacity per unit area of the substrate, the time constant of temperature rises is about ten times as large as the previously reported value of a polymer substrate (PMMA).

Polarization instability in a twisted birefringent optical fiber

The fast-axis polarization instability arises in a weakly birefringent fiber as a result of competition between the natural fiber birefringence and the nonlinear ellipse rotation. Direct observation of the fast-axis polarization instability is reported. A full theoretical development of the polarization instability in a twisted, birefringent optical fiber is presented. The theory includes the derivation of and full solutions for the evolution of light in a twisted fiber as well as stability analysis and phase-plane representation of the solutions. The experiment is described in detail; good agreement is obtained between theory and experiment. As a result of the instability, very small variations in either the input power or the input polarization to the fiber result in large changes in the output polarization. A crossed polarizer at the fiber end converts the polarization variation into intensity information. Thus the modulation depth of an input pulse has been increased from 15% to 100%. Modulation gains of as much as 10 times are theoretically possible.

An LiNbO/sub 3/ integrated coherence modulator

The authors describe the principle of operation and the performance of a newly integrated LiNbO/sub 3/ modulator specially designed to generate optical delays of several millimeters. This is achieved by using the simultaneous propagation of the TE and TM modes in a waveguide and by taking benefit of the high natural birefringence of LiNbO/sub 3/. This modulator is well-suited to coherence modulation of light in fiber-optic transmissions. When powered by a short coherence source, the large optical delays induced by a cascade of such modulators can be used as information carriers to transmit several signals simultaneously. A demonstration is reported using a single source and two modulators in cascade to transmit signals simultaneously. >

Electro-optic measurements by thermo-optic compensation

Abstract Each method for the measurement of the electro-optic (E/O) coefficients needs, at least, two distinct measurements. During experiments, the variation of temperature Δ can lead to a change in the natural birefringence, i.e. the thermo-optic effect, which can be of the same order of magnitude as the electro-optic effect, even if Δ is very small. We present an original method for E/O measurements using the thermo-optic (T/O) effect as a reference of induced phase shift.

Medial Collagen Organization in Human Arteries of the Heart and Brain by Polarized Light Microscopy

The mechanical properties of collagen as a biopolymer ensures that collagen has a significant influence on the mechanical behavior of the host tissue. Structural organization is a key to that influence. We have assessed this relationship quantitatively in the tunica media of arteries from the heart and brain, using the polarizing light microscope and Universal stage. Arteries from 22 autopsies were isolated, cannulated and fixed with 10% buffered formalin, at a distending pressure spanning normal values in vivo. We prepared the tissue for light microscopy, with paraffin embedding, sectioning at 7 microns, and staining with picrosirius red to enhance the natural birefringence of medial collagen. Individual measurements, 30 to 50 per arterial section, referenced against the central axis of the vessel segment, revealed a coherent organization, with an average orientation which was within 1 to 2 degrees of being perfectly concentric for all artery segments. Analysis was done with Lambert projections and circular statistics. We calculated the circular standard deviation, which was 5.2 degrees for 27 brain arteries (S.D. 1.9 degrees) and 5.6 degrees (S.D. 2.1 degrees), for 5 coronary arteries sectioned at less than 15 degrees. Our interpretation is that medial collagen can be strained even though highly aligned, revealing a mechanical property which contrasts that of type I collagen.

Electronic polarizabilities of ions in KDP and ADP from natural birefringence data

The electronic polarizabilities of ions in potassium dihydrogen phosphate (KDP) and ammonium dihydrogen phosphate (ADP) crystals are evaluated from the data on the natural birefringence of these crystals. An accurate calculation of electronic polarizabilities of ions is indispensable in the understanding of problems regarding the piezo-optical and electro-optical behaviour of these crystals. The positive and negative ions are treated as point dipoles placed at the appropriate lattice sites. The Lorentz field is then calculated at (0, 0, 0) due to different sites on the lines suggested in Kinase e l al. [1]. The field at a lattice site (0, 0, 0) due to a dipole #~ at (x, y, z) with electric vector of light being in the X-direction is given by

Light diffraction by a sound beam in circularly birefringent crystals with extreme acoustooptic coupling

A solution is obtained for the self-consistent problem of Bragg acoustooptic interaction (AOI) of circularly polarized light waves in transparent gyrotropic crystals with extreme acoustooptic coupling or a long interaction zone. It is shown that the polarization parameters of the diffracted field differ very little from the corresponding parameters of the incident light wave in AOI with identical circular polarization vectors, and the natural circular birefringence of the medium for the diffracted light wave is compensated in the case of interaction between waves with different circular polarizations. It is confirmed that the laws governing the variation of the polarization azimuth of a given wave coincide with the corresponding AOI laws in optically isotropic media. The diffracted field and the corresponding polarization characteristics are determined for Raman-Nath AOI in gyrotropic crystals.

Optical Kerr effect in glass fiber waveguides with weak and strong birefringence

A birefringence induced by the optical Kerr effect in single-mode glass fiber waveguides was investigated. A study was made of the dependences of the process of inducing the birefringence on the natural birefringence, on the orientation of the polarization of the radiation, and on the pump radiation power. Oscillations of the intensity of a probe wave were observed under certain conditions of excitation of these fiber waveguides with pump radiation.

Spatial-dispersion induced birefringence in cubic crystals: CuBr and CuI

A natural birefringence in single crystals of copper halides is observed and measured in the exciton part of the spectrum. This intrinsic birefringence is due to the spatial dispersion of the dielectric constant ϵ(ω, q). Its spectral dependence and strong enhancement near the energy gap can be explained only, if we take account of the strong excitonic contribution in these compounds. This effect is related to the valence band anisotropy (warning) and thus allows a direct determination of the Luttinger band parameters | γ2 - γ3 | for these compounds.

Stress optical behaviour of ammonium pentaborate under linear and hydrostatic stresses

Ammonium pentaborate (APB) is an orthorhombic crystal of mm2(C 2v ) class. In the present investigation the authors present the results on the stress birefringence of APB under linear and hydrostatic stresses. It is shown that from stress birefringence measurements under linear stresses alone, the effect of hydrostatic pressure on the birefringences of an orthorhombic crystal can be obtained. A comparison is drawn between the natural birefringence and stress birefringence under linear and hydrostatic stresses.

Some characteristics of the polarization properties of single-modeW-type waveguides

An experimental investigation was made of the natural birefringence and of the spectral dependence of the maximum degree of polarization of light traveling in three-layer single-mode W-type fiber waveguides. The linear and circular birefringence was determined. An analysis of the spectral dependence of the maximum degree of polarization revealed a coupling between the dominant and leaky modes in the investigated fiber waveguide. The results obtained were used to estimate the coupling coefficient.

Changes in crystal size and orientation of acidic glycosaminoglycans at the fracture site in fractured necks of femur.

The aim of this study was to try to elucidate the increased susceptibility of the neck of femur to fracture. Quantitative polarised light microscopy has been applied to fresh, undecalcified sections of samples of bone taken from the site of fracture, in specimens taken at operation from patients with fractures of the femoral neck or osteoarthritic femoral heads or from the equivalent site from otherwise normal subjects at necropsy. In all 21 specimens of fractured necks of femur, but in none of the other specimens, relatively large crystals (up to 2.5 X 0.5 micrometres) were found close to the site of fracture; the properties of these crystals were compatible with their being apatite. Measurement of the natural birefringence of the collagen showed no difference in the orientation of the collagen in all three types of specimen. However, the orientation of acidic glycosaminoglycans, measured by the birefringence of alcian blue bound to these moieties, was 45 per cent lower in the specimens from fractured necks of femur than in the other specimens, even though the total content of acidic glycosaminoglycans was unchanged. Although the decreased orientation was most marked close to the site of fracture, it was still apparent 15 millimetres from that site. These changes were unlikely to be simply the sequelae of fracture since they were not found in traumatic fractures of other bones. Thus it is conceivable that changes in the orientation of the ground substance allow formation of relatively large crystals of apatite and that such crystals, in the microcrystalline mass of apatite, are the cause of the increased fragility of such bones.

Linear Electro-Optic Effect of AgGaSe2

Single crystals of AgGaSe2 with sufficient transparency to measure the electro-optic effect have been prepared by the Bridgman technique. The linear electro-optic coefficients are measured by a phase modulated method which is devised to remove the inaccuracy induced by the existence of natural birefringence. The coefficients at constant stress, γ63 and γ41 are found to be 3.9×10-12 and 4.5×10-12m/V respectively at a wavelength of 1.15 µm. The influence of optical activity on the observation of the electro-optic effect is also discussed.

Effect of solvents on dynamic birefringence of polymer solutions

The effect of birefringence of solvents on the birefringence of a solution of given viscosity η with a given flow velocity gradient g is calculated by multiplying shear stress gη by a previously determined value of the natural birefringence of the solvent ( Δn/ gη). It was verified experimentally that the latter value is independent of the fact whether viscosity changes as a result of a temperature variation, or as a result of adding an isotropic polymer to the solution.

Optical Damage in LiTaO3

Optically-induced refractive index inhomogeneities (optical damage) in LiTaO3 are studied in terms of the variations of light extinction. A simple relation between the laser-induced natural birefringence and light extinction is presented. The effects of the melt stoichiometry, the poling procedures and the oxygen-annealing on the optical damage in LiTaO3 are examined from the measurements of extinction ratios. Furthermore, the observation of the thermal decay of extinction ratio indicates that the thermal activation energy is 1.1∼1.3 eV based on the diffusion theory of Amodei. By an improved technique and treatments a LiTaO3 crystal which is made resistant to optical damage by an Ar-ion laser (4880 Å) with power density as high as 14 kW/cm2 is successfully obtained.

An Automatic Optical Bias Control for Laser Modulators

A technique is presented whereby the temperature variation of the natural birefringence of electro-optic modulators can be compensated automatically in an optical modulation system. A description is given of a 80 M bits He–Ne PCM communication system that uses this concept.

Traveling-wave light-intensity modulators using the method of polarization-rotated reflection

A reflection-type traveling-wave light-intensity modulator was constructed and tested over a 1-to-2-GHz bandwidth. The modulator is based on a novel technique named the method of polarization-rotated reflection. This method makes use of a modified optical reflection-type operation thereby canceling the temperature-dependent natural birefringence. Stable operation of the modulator was obtained without any temperature control. It is also shown that, if a microwave is reflected back as well as the light beam, the effective crystal length is doubled. This paper describes the principle of the proposed method, the modulator design, and the experimental results.

Electrooptic and ferroelectric properties of lithium tantalate single crystals as a function of melt composition

Electrooptic, electric, ferroelectric and optical properties of lithium tantalate single crystals are investigated as a function of the melt composition for Li-Ta mole ratios ranging from 0.8 to 1.2. The Curie temperature depends on the melt composition, but the dependence is not linear. Specific resistivity increases as Li concentration in the melt increases. Homogeneity of the natural birefringence becomes higher with increasing Li concentration. Optical loss decreases in crystals grown from Li-rich melts. The crystals become much more resistant to optical damage after annealing in an oxygen atmosphere at about 1000°C.