Birefringence Distribution Research Articles

Mechanism of heat-modification inside a glass after irradiation with high-repetition rate femtosecond laser pulses

Accumulation of thermal energies by highly repeated irradiation of femtosecond laser pulses inside a glass induces the heat-modification whose volume is much larger than that of the photoexcited region. It has been proposed that the heat-modification occurs in the region in which the temperature had overcome a threshold temperature during exposure of laser pulses. In order to understand the mechanism of the heat-modification, we investigated the temperature distribution during laser exposure and the threshold temperature by analyzing the volume of the modification based on a thermal diffusion model. We found that the threshold temperature becomes lower with increasing laser exposure time. The dependence of the threshold temperature on the laser exposure time was explained by the deformation mechanism based on the temperature-dependent viscosity and viscoelastic behavior of a glass under a stress loading by thermal expansion. The deformation mechanism also could simulate a tear-drop shape of a heat-modification by simultaneous double-beams’ irradiation and the distribution of birefringence in a heat-modification. The mechanism proposed in this study means that the temperature-dependence of the viscosity of a glass should be essential for predicting and controlling the heat-modification.

사출 및 사출/압축 성형품 내에서의 복굴절 구조 변화에 관한 연구

It is still needed to study the effect of process conditions on the final properties of injection-molded parts for producing precision optical products. Especially, the optical anisotropy, i.e., birefringence, is a significant factor to affect the function of many optical components. In the present study we have focused on the effect of holding and compression processes on the birefringence remaining in the transparent disc by examining the gap-wise distribution of birefringence and extinction angle. As a result, two extra birefringence and extinction peaks near the center in thickness direction showed the effect of holding pressure, which came from the flow in packing stage. However, more uniform birefringence distribution than injection-only cases could be found in injection/compression cases. Depends on the process condition even the flow reversal could be found from the distribution of extinction angle. Finally, graphical representation of optical indicatrix was added for better understanding the final structure of injection-only and injection/compression cases.

In vivo 3D human vocal fold imaging with polarization sensitive optical coherence tomography and a MEMS scanning catheter

We present in-vivo 3D human vocal fold images with polarization sensitive optical coherence tomography (PS-OCT). Characterizing the extent and location of vocal fold lesions provides useful information in guiding surgeons during phonomicrosurgery. Previous studies showed that PS-OCT imaging can distinguish vocal fold lesions from normal tissue, but these studies were limited to 2D cross-sectional imaging and were susceptible to sampling error. In-vivo 3D endoscopic imaging was performed by using a recently developed 2-axis MEMS scanning catheter and a spectral domain OCT (SD-OCT), running at 18.5 frames/s. Imaging was performed in the operating room with patients under general anesthesia and 3D images were acquired either by 2D scanning of the scanner on the sites of interest or by combining 1D scanning and manual sliding to capture whole length of the vocal fold. Vocal fold scar, polyps, nodules, papilloma and malignant lesions were imaged and characteristics of individual lesions were analyzed in terms of spatial distribution and variation of tissue structure and birefringence. The 3D large sectional PS-OCT imaging showed that the spatial extent of vocal fold lesions can be found non-invasively with good contrast from normal tissue.

Effects of Crystal-Induced Optical Incoherence in Electro-Optic Field Sensors

Electro-optic (EO) sensors, used for the nonperturbative detection of electric fields, are strongly influenced by inhomogeneities and deviations of birefringence within the nonlinear crystal. These effects can reduce optical phase coherence in the probe beam, reducing sensor responsivity. The effect is most pronounced in photorefractive materials, where distributions of birefringence are likely to occur. Our analysis suggests that a standard deviation of birefringence as little as 10−4 can reduce sensitivity by two orders of magnitude. This indicates that optically stable materials with low photorefractivity and low birefringence are ideally suited for EO sensor applications.

Two-dimensional birefringence measurement system with a polarization modulator and a rotating analyzer

The measuring system of two-dimensional birefringence distribution with a polarization modulator and a rotating analyzer using a rotating analyzer method is presented. The system simply consists of a laser diode, two polarizers, two quarter wave plates and a CCD camera. Thanks to an imaging system, the mechanism to scan a sample is unnecessary and two-dimensional measurement is possible and easy. The birefringence of retardation and orientation of the optic axis can be determined by the azimuth angle of an elliptically polarized light passing through a sample. The azimuth angle is obtained by applying the phase shifting algorithm to measure the polarization properties; by improving the calculation algorithm, wide range measurement is accomplished. The measurement procedure and a number of results are described.

Dynamic polarization states and birefringence distributions measurements in spatial elliptical polariscope using Fourier analysis method

A new method to measure the light polarization state and the birefringent media parameters is proposed. We have used the setup described previously, consisting of two pairs of the linear Wollaston and circular compensators which form a set of two spatially modulated elliptical compensators. We have modified this setup introducing some carrier frequencies in all compensators and assuming that the second linear one would introduce the frequency which is a multiplicity of the basis frequency of the first linear compensator. Both of these modifications allow calculating all polarization parameters of polarized light or birefringent medium from only one measured intensity distribution of the light outcoming the described setup. They allow measuring not only the parameters of homogeneous beams/mediums but also x,y-distributions of all desired parameters, like azimuth and ellipticity angles of the light or first medium eigenvector and the phase difference introduced by this medium. The proposed calculation method comprises of Fourier analysis of obtained intensity distribution with some manipulation of coordinate system and filtration of obtained data. This method is claimed to be simple and fast enough to be treated as a real-time method.

Fluorescence Measurements in Contact Lenses With a Novel Confocal Microscope

The spatial distribution of macular birefringence was modeled to examine the contribution from the foveal Henle fiber layer, particularly cone axons. The model was tested in 20 normal subjects, age 17–55 yr. Phase retardance due to Henle fibers was modeled for rings increasing in radius around the fovea, using a sinewave of two periods (2f). The 2f sinewave amplitude increased linearly with eccentricity for each individual, (p < 0.004) in 19 of 20 subjects. A good fit to linearity implies regular cone distribution and radial symmetry, and the uniformly excellent fits indicate no effect of age in our sample. The peak of the 2f sinewave amplitude varied across subjects from 1.06 to 2.46 deg. An increasingly eccentric peak with increasing age would indicate a relative decrease of cone axons in the central fovea, but the location of the peak was not associated with age for our sample, which did not include elderly subjects.

Optical Vectorial Vortex Coronagraphs using Liquid Crystal Polymers: theory, manufacturing and laboratory demonstration

In this paper, after briefly reviewing the theory of vectorial vortices, we describe our technological approach to generating the necessary phase helix, and report results obtained with the first optical vectorial vortex coronagraph (OVVC) in the laboratory. To implement the geometrical phase ramp, we make use of Liquid Crystal Polymers (LCP), which we believe to be the most efficient technological path to quickly synthesize optical vectorial vortices of virtually any topological charge. With the first prototype device of topological charge 2, a maximum peak-to-peak attenuation of 1.4x10(-2) and a residual light level of 3x10(-5) at an angular separation of 3.5 lambda/d (at which point our current noise floor is reached) have been obtained at a wavelength of 1.55 microm. These results demonstrate the validity of using space-variant birefringence distributions to generate a new family of coronagraphs usable in natural unpolarized light, opening a path to high performance coronagraphs that are achromatic and have low-sensitivity to low-order wavefront aberrations.

高分子水溶液の微小円柱まわりの流れ中における流動複屈折分布(&lt;小特集&gt;非ニュートン性,機能性を示す複雑流体の流動と応用)

Flow-induced birefringence distribution and velocity field were measured in a slit flow containing a tiny cylinder, which was 550μm in diameter and 200μm in height. Xanthan gum solution, which has a non-Newtonian property owing to network structure in the liquid, was used for a test fluid. The laser beam was collected, resulting beam diameter was 6μm, and the birefringence distribution was measured around the tiny cylinder in every 100μm step in space. Consequently, in the upstream region of the cylinder, birefringence became small as the fluid particle approached the cylinder. This is because the flow is decelerating upstream of the cylinder and the fluid particle is elongated in the direction perpendicular to the flow. Furthermore, just upstream of the cylinder the birefringence became large and the molecular orientation was aligned in the direction perpendicular to the flow. On the other hand, downstream of the cylinder the flow is accelerating, thus the fluid particle is elongated in the flow direction. As a result, the birefringence becomes large on the centerline behind the cylinder. This result shows that the strain of the fluid particle generated by flow behind the cylinder is not disappeared even at the fully downstream region.

Numerical simulation of residual stress and birefringence in the precision injection molding of plastic microlens arrays

Microlens arrays are critical components in the assembly of display devices used in various telecommunication products. The optical quality of these plastic lenses is very sensitive to the presence of residual stress. In this paper the distribution of residual stress and birefringence of these microlens arrays has been investigated. It is found that maximum residual stress always occurs in the regions near the gate. The predicted trends and patterns of the residual stress and birefringence distribution agree well with the experimental results. The effects of six main processing parameters on maximum value of residual stress are studied. The noticeable discrepancy between the simulation and the experimental results are discussed.

Distributed Measurement of Birefringence by P-OTDR Assisted with Piezoelectric Polarization Controller

We propose a new polarization sensitive optical time domain reflectometry (P-OTDR) setup assisted with a piezoelectric polarization controller (PPC). The input state of polarization can be changed by varying the voltage of PPC without any rotatable instrument, and only one optical receiver is used to detect the backward beam. We measure a single mode fibre and get the distribution of birefringence along the SMF.

A polarized light field microscope with fast polarization switcher and microlens array reveals the 3-D birefringence distribution of complex anisotropic structures

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

In vivo Three-Dimensional Birefringence Analysis Shows Collagen Differences between Young and Old Photo-Aged Human Skin

Polarization-sensitive optical coherence tomography (PS-OCT) permits non-invasive visualization of dermal birefringence, mainly due to collagenous structures. The purpose of this study is to use PS-OCT to assess intrinsic-age-related and photo-age-related differences in three-dimensional dermal birefringence. We measured dermal birefringence of the cheek skin and photo-protected interior upper arm skin from old and young volunteers. The algorithm that we used automatically produces the transversal dermal birefringence map from the polarization-sensitive OCT volume. This allowed quantitative comparison and visualization of the transverse distribution of the dermal birefringence. We found that dermal birefringence of the cheek skin was significantly smaller in the old group than in the young group (young group, 0.295+/-0.037 degrees microm(-1); old group, 0.207+/-0.03 degrees microm(-1); P=0.003), whereas the interior upper arm showed no age-dependent difference. The transversal map of the cheek showed a heterogeneous decrease in dermal birefringence due to photoaging. The maps suggested that the peripheral regions of some infundibula were surrounded by a strong collagen network. Three-dimensional analyses of dermal birefringence using PS-OCT help to quantify the diagnosis of photoaging.

Spatial distribution of macular birefringence associated with the Henle fibers

The spatial distribution of macular birefringence was modeled to examine the contribution from the foveal Henle fiber layer, particularly cone axons. The model was tested in 20 normal subjects, age 17–55 yr. Phase retardance due to Henle fibers was modeled for rings increasing in radius around the fovea, using a sinewave of two periods (2f). The 2f sinewave amplitude increased linearly with eccentricity for each individual, ( p < 0.004) in 19 of 20 subjects. A good fit to linearity implies regular cone distribution and radial symmetry, and the uniformly excellent fits indicate no effect of age in our sample. The peak of the 2f sinewave amplitude varied across subjects from 1.06 to 2.46 deg. An increasingly eccentric peak with increasing age would indicate a relative decrease of cone axons in the central fovea, but the location of the peak was not associated with age for our sample, which did not include elderly subjects.

Study of process parameters on optical qualities for injection-molded plastic lenses

Numerical simulations for mold-flow analysis and experimental measurements of injection-molded plastic lenses have been conducted for investigation of optical qualities, residual birefringence, and form accuracy resulting from various pertinent process conditions. First, residual birefringence distributions on the lens have been predicted and verified experimentally. Furthermore, full-scale factorial design of experiments was conducted to comprehend the influences of qualities, such as shear stresses, form accuracy, and volumetric deviation, on the measured primary or Seidel aberrations. In conclusion, residual birefringence induced by stresses represented by photoelasticity measurements agrees well with the numerical predictions and the experimental results indicate that the residual birefringence is mainly generated during the mold-filling stage. In addition, spherical aberration of the injection-molded plastic lenses is more sensitive to the pertinent qualities as compared to coma and astigmatism.

2D Polarization Imaging of Turbid Media

Cancer normally tends to result in the decrease of tissue elasticity; i.e., the cancerous region is more rigid than the normal surrounding areas. This would appear as differences in the distribution of internal birefringence that could be used to improve image contrast between the cancerous and normal tissue structures. Different filtering techniques are used to enhance the image to help us identify, locate, and diagnose an "object," such as a tumor inside a biological tissue.

Stress Birefringence in Photonic Crystal Fibers Used in a Novel Field Installable LC Type Optical Connector

Photonic crystal fibers (PCFs) have recently attracted a great deal of interest because of their unique characteristics and many controllable features. They include a wide range of single mode operation, highly birefringent characteristics (~10-3), high-power light transmission, etc. The field-installable LC connectors tend to be of the larger connector types in the fiber-to-the-home (FTTH) distribution units. In these designs, fibers are clamped inside a splice assembly and are stressed within the connector. In this research, we first developed a novel cam-type mechanism for field-use LC connector. The fiber stress was analyzed via a commercial available finite element program. The stress birefringence distributions of single mode fiber and air-silica based PCFs were calculated by stress photoelastic effect. We further studied the effect of air-hole size on the stressinduced birefringence of PCFs. The results indicate that stress-induced birefringence decreases as air-hole size increase. On the other hand, the birefringence increases as applied clamping force increases on the single mode fiber.

Mechanical Characterization Measurement of Polymer Material under Stress by Birefringence Microscope

Some polymer materials under tensile stress provide us interesting information. Internal structure changes of polymer materials must be observed by external stress. Birefringence shows the orientation characteristic in molecular orientation. A microscopic measurement system for birefringence distribution is proposed to analyze the relation between applied stress and birefringence distribution of an internal structure. Birefringence distributions of gelatin such as phase difference and azimuthal angle are shown in cases of loading and unloading stress as a demonstration. The phase difference increased nonlinearly with each process, and change of birefringence in the elastic domain was different from that in the plastic domain.

Deformation corrected real-time terahertz imaging

The authors have developed a method of real-time terahertz imaging in which image deformation due to nonuniformity of residual birefringence in the electro-optic (EO) sampling crystal is corrected. Real-time terahertz imaging using intense terahertz pulses and two-dimensional EO sampling can suffer from birefringence nonuniformity of the EO crystal since the birefringence is explicitly used for the linear detection of the terahertz field. In the proposed method, the distribution of the residual birefringence of the EO crystal is measured and used for image correction. Deformation-free images of the spatial profile of a focused terahertz pulse were obtained.

Prediction of birefringence distribution for optical glass lens

Abstract Based on the viscoelastic theory, simulations were carried out on glass-lens forming processes. In addition, the birefringence distribution was calculated by stress-optic relation. First, to verify this viscoelastic simulation, FEM analysis was performed with the same material properties and process conditions used in the Bruckner's experiment. The result of the analysis was compared with Nadai's and Bruckner's results. The simulation results were in good agreement with the deformation and birefringence distribution in the experimental results. Finally, the FEM simulation technique was applied to actual LD-lens. The simulation was performed for each different cooling rate and the results were compared to each other in terms of birefringence. To investigate the effect of initial preform shape to optical property, a simulation was performed by using the new-proposed-shape-preform and the results were compared to the present shape in terms of birefringence.