Birefringence Compensation Research Articles

Scanning laser polarimetry of nerve fiber layer thickness in normal eyes after cataract phacoemulsification and foldable intraocular lens implantation

To assess the effect of cataract phacoemulsification and intraocular lens (IOL) implantation on retinal nerve fiber layer (RNFL) thickness using scanning laser polarimetry (SLP). Eye Clinic, Trieste University, Italy. Forty-eight eyes were evaluated prospectively the day before and 30 days after cataract phacoemulsification and foldable IOL implantation. In each eye, lens opacity grading according to the Lens Opacities Classification System III (LOCS III), and axial length (AL) measurements were performed. Retinal nerve fiber layer thickness was quantified at baseline by means of SLP and anterior segment birefringence compensation was evaluated acquiring macular retardation map (MRM). Acrylic and silicone IOLs were implanted randomly. After surgery, RNFL thickness was reevaluated, and MRM was reacquired. Macular retardation map pattern variations regarding baseline profile were classified into 3 groups: no variation, bow-tie profile enhancement, or attenuation. Distribution of IOL power, AL, and cataract type in the 3 groups was assessed, as were presurgery and postsurgery SLP parameters with mean values (+/-SD) compared by paired t test. Twenty-two eyes (Group 1, 45.8%) showed no MRM variation, 14 (Group 2, 29.2%) an enhancement, and 12 (Group 3, 25%) an attenuation. In Group 1, no significant RNFL thickness variation occurred. In Group 2, variation 10% to 15% was measured, whereas thickening a 8% to 15% thinning appeared in Group 3. Variations occurred irrespective of IOL material, AL, or cataract type. Cataract surgery with IOL implantation was associated with an MRM profile change and RNFL thickness variations in 54.2% of eyes. Variations are probably related to opacified lens removal. A new baseline SLP reading is mandatory after cataract surgery.

Comparing Measurements of Retinal Nerve Fiber Layer Thickness Obtained on Scanning Laser Polarimetry with Fixed and Variable Corneal Compensator

To compare retinal nerve fiber layer (RNFL) thickness measurements obtained on scanning laser polarimetry (SLP) with commercially available instruments coupled with fixed (FCC) and variable corneal compensator (VCC). Forty-two eyes of 42 patients underwent a complete ophthalmologic evaluation and achromatic automated perimetry (24-2 program, SITA standard strategy). Nineteen eyes were healthy (average mean deviation: -0.12 dB +/- 2.26) and 23 glaucomatous (average mean deviation: -4.92 dB +/- 6.49). All patients underwent SLP with both FCC and VCC. Adequate compensation of corneal birefringence on FCC-SLP was checked acquiring macular retardation map (MRM). RNFL thickness was evaluated considering superior and inferior maximum (SM, IM), average thickness and ellipse average (AT, EA), and superior and inferior average (SA, IA). Mean values (+/-SD) for each parameter measured by the two polarimeters were compared and linear regression calculated. The ability of each parameter to discriminate between normal and glaucomatous eyes was evaluated on both polarimeters calculating area under ROC curve. A significant linear correlation for all parameters was noted (r range: 0.65-0.78). VCC produced slightly higher thickness values than FCC, both in normal and glaucomatous eyes. On both polarimeters, area under ROC curve for all parameters discriminated adequately healthy from glaucomatous eyes (range: 0.68-0.81). In a highly comparable and selected group of normal and glaucomatous eyes, FCC-SLP and VCC-SLP showed considerable concordance in measuring peripapillary RNFL thickness, both for sectorial and global parameters. Proper corneal birefringence compensation provided separation of normal from glaucomatous eyes on both polarimeters.

P-162: Off-Axis Compensation of 90TN0 LCoS Mode

In projection applications, as the liquid crystal device size continues to shrink, more demand is placed on the illumination optics to increase lumens per unit area on the device. One method to achieve this is to use smaller f-numbers (larger cone angles); however, as the f-number of the optical system reduces the contrast invariably suffers. In this paper, we examine a twisted nematic liquid on silicon device (90TN0, LCoS) and explore the trade-off between contrast and system f-number. This mode is nominally self-compensating, however additional compensation of the off-axis birefringence is required to achieve higher contrast, especially at lower f-numbers (<2.4). Device birefringence characterization was measured as a function of the applied voltage, wavelength and angle of incidence. Both uncompensated and compensated devices were examined. Contrast as a function of f-number was measured in a conoscope system based upon a wire grid polarization beam splitter (PBS). Substantial improvement in contrast was measured when the c-compensator was added, suggesting that contrast in excess of 1000:1 can be achieved for all primaries at F/1.8. Final measurements in a rear-projection system utilizing a wire-grid PBS, operating at F/2.4, provided an increase in the contrast from 1000 to 1350:1 using the additional compensator.

Nd:YAG ring laser with 213 W linearly polarized fundamental mode output power

We present a high power fundamental mode ring oscillator with four longitudinally diode-pumped laser rods and effective birefringence compensation as a laser source for the next generation of gravitational wave detection. With this setup a linearly polarized output power of 213 W in a nearly diffraction limited beam with an M2 value of 1.15 was demonstrated.

Retinal nerve fiber layer measurements do not change after LASIK for high myopia as measured by scanning laser polarimetry with custom compensation

Scanning laser polarimetry (SLP) estimates retinal nerve fiber layer (RNFL) thickness through measurement of retardation of polarized light passing through the birefringent RNFL and cornea. A compensation method is incorporated to eliminate the anterior segment contribution to the total birefringence measured. LASIK is a technique that corrects myopia by ablating corneal tissue. This study evaluated RNFL measurements before and after LASIK as determined by SLP with a new custom compensation device set for the individual cornea before and after ablation. Interventional case series. Patients underwent SLP measurements before and 30 days after LASIK for high myopia with customized compensation for anterior segment birefringence. Postoperative measurements were obtained with both the same compensation as was used preoperatively and with newly determined customized compensation. Standard RNFL parameters obtained before and after ablation with customized compensation were compared by use of paired Student's t tests with Bonferroni's correction for multiple comparisons. Postoperative measurements were also compared with those obtained preoperatively with the same corneal compensation measures as used preoperatively. Retinal nerve fiber layer parameters as determined by SLP before and after LASIK with 2 custom cornea compensator settings postoperatively. Fifty-seven eyes of 29 patients with myopia > -5.00 diopters underwent LASIK (average central ablation depth 101+/-11.3 mum). None of the 13 parameters showed statistically significant differences between preoperative and postoperative values with individually determined corneal compensation. When the preoperative corneal compensation was used postoperatively, 10 of 13 parameters were significantly changed. Comparison of the compensator settings before and after LASIK showed substantial differences in some individuals. On the average, there was a statistically significant change in the slow polarization axis of the cornea, with no statistically significant change in the magnitude of the birefringence, with some eyes showing increases and others decreases. The change in an individual cornea could not be predicted. Changes in RNFL measurements by SLP observed after LASIK in patients with high myopia are due to changes in corneal birefringence. These RNFL changes are not seen when customized compensation is applied for the cornea. Thus, LASIK does change the corneal birefringence but does not affect the RNFL.

High birefringence control and polarization insensitive Bragg grating fabricated in PECVD planar waveguide with UV polarized irradiation

A method for rapid compensation of high intrinsic birefringence in plasma-enhanced chemical vapor deposition (PECVD) based planar waveguides is presented. The intrinsic waveguide birefringence is compensated by a large UV induced opposite birefringence generated with polarized UV ArF excimer laser irradiation (oriented normal to the waveguide axis) and enhanced further through hydrogen loading. The fabrication of polarization insensitive Bragg gratings in PECVD planar waveguides with a large intrinsic birefringence of 6.3×10−4 is demonstrated by trimming it to zero birefringence with a low fluence blanket UV polarized exposure. After annealing for removal of the hydrogen, the polarization insensitive nature of the Bragg grating is unchanged.

Scanning laser polarimetry retinal nerve fiber layer thickness measurements after LASIK

To compare retinal nerve fiber layer (RNFL) thickness measurements before and after LASIK. Cohort study. Twenty participants undergoing LASIK and 14 normal controls. Retinal nerve fiber layer thickness was measured before LASIK and approximately 3 months after surgery in one eye each of 20 patients using a scanning laser polarimeter (GDx Nerve Fiber Analyzer) with fixed corneal compensation (FCC), one with variable corneal compensation (GDx VCC), and optical coherence tomography (OCT). Fourteen normal controls also were tested at baseline and approximately 3 months later. Retinal nerve fiber layer thicknesses measured with the GDx FCC, GDx VCC, and OCT. At baseline, mean (95% confidence interval [CI]) RNFL thicknesses for the GDx FCC, GDx VCC, and OCT were 78.1 microm (72.2-83.9), 54.3 microm (52.7-56.0), and 96.8 microm (93.2-100.5), respectively. In both LASIK and control groups, there were no significant changes between baseline and follow-up examinations in GDx VCC and OCT RNFL thickness measurements globally or in the superior and inferior quadrants (mean change, <5 microm for each instrument). In the control group, there also was no significant change in GDx FCC measurements between baseline and follow-up. In LASIK patients, significant reductions were observed in GDx FCC RNFL measurements. Average absolute values of the mean (95% CI) change in thickness were 12.4 microm (7.7-17.2), 15.3 microm (9.6-20.9), and 12.9 microm (7.6-18.1) for GDx FCC RNFL measurements superiorly, inferiorly, and globally, respectively (all Ps < or = 0.001). LASIK does not seem to change RNFL thickness. Reduction in GDx FCC RNFL thickness measurements after LASIK is a measurement artifact and is most likely due to erroneous compensation for corneal birefringence. With scanning laser polarimetry, it is mandatory to compensate individually for change in corneal birefringence after LASIK to ensure accurate RNFL assessment.

P‐91: TFT Substrate Birefringence Induced Asymmetrical Viewing Angle and Compensation Methods

AbstractThin‐film transistor (TFT) processes for small pixel geometries can induce optical birefringence effects and change the viewing angle from symmetrical to asymmetrical. We experimentally identified the TFT substrate as the source of the birefringence and developed four methods to compensate the added optical birefringence effects to restore the symmetrical viewing angle. We also developed two special wide viewing angle polarizers with Fuji Film wide viewing angle film, and special compensation films to compensate the TFT substrate's birefringence.

Negative Birefringence Films From Non-Traditional Materials

Polymer films possessing negative birefringence are used for the compensation of residual positive birefringence of liquid crystal displays in order to improve their viewing characteristics. We present several types of materials suitable for the fabrication of negative birefringence films and controllable distribution of optic axis. Films featuring a splayed optic axis can be made of discotic materials as a result of different anchoring forces acting at the film boundaries. Films with a planar optic axis can be made by shearing of thermotropic and lyotropic discotics and as a result of irradiation of photosensitive polymers with polarized light. Homeotropic alignment of optic axis can occur as a result of film shrinkage during solvent evaporation or photo-induced rotation of disk planes.

High power fundamental mode Nd:YAG laser with efficient birefringence compensation

Experiments on a high-power end-pumped Nd:YAG rod laser with an efficient birefringence compensation will be presented. A linearly polarized output power of 114 W with an M2-value of 1.05 was realized. Furthermore, the from our best knowledge highest injection-locked singlefrequency output power of 87 W in a nearly diffraction-limited beam was demonstrated.

Analytical method for design a TEM 00 mode resonator of a dual-rod Nd:YAG laser with full birefringence compensation

Analytical formulae are derived for design of a TEM 00 mode dual-rod Nd:YAG laser resonator with full birefringence compensation. These formulae are expressed in a similar notation to those of Magni et al. [Appl. Opt. 25 (1986) 107] for a conventional single rod laser. The analytical method proposed here provides a simple design procedure. Furthermore, expressions are derived for the misalignment sensitivity of the laser rods and the intracavity lens to transversal displacement. Resonators in a recent publication are also analyzed by our method.

Visualization of localized retinal nerve fiber layer defects with the GDx with individualized and with fixed compensation of anterior segment birefringence

Purpose To compare the visualization of localized retinal nerve fiber layer (RNFL) defects in GDx images with fixed and with individualized compensation of anterior segment birefringence (FC and IC, respectively) with their visualization in red-free fundus photographs. Design Observational case series. Participants Eight eyes of six glaucoma patients with localized, wedge-shaped RNFL defects in red-free fundus photographs with matching visual field defects. Methods We imaged all eyes with a GDx equipped with a variable corneal compensator (VCC). The VCC replaced the standard fixed compensator and could be set to compensate for birefringence of up to 120 nm at any axis. Individual anterior segment birefringence was estimated from a macular retardation profile that resulted from the interaction between birefringence of the anterior segment and that of Henle’s fiber layer. Measurements of RNFL retardation were made with the GDx with FC (60 nm of retardation with a slow axis of 15° nasally downward) and with IC. Maps of retardation measurements with FC and IC were superimposed on red-free fundus photographs. Main outcome measures Visualization of localized RNFL defects. Results Localized RNFL defects were visible in GDx retardation maps obtained with IC. The defects closely matched those observed in red-free fundus photographs. With FC, however, the GDx retardation images did not correlate well with red-free fundus photography. Conclusions An individualized anterior segment compensation in the GDx improves the visualization of localized glaucomatous loss.

Faraday plasma current sensor with compensation for reciprocal birefringence induced by mechanical perturbations

A Faraday fiber-optic current sensor was employed to measure the tokamak plasma current. In order to decrease the influence of mechanical perturbations on the sensor sensitivity, a two-pass optical scheme with a variable Faraday mirror at the fiber end is proposed. A decrease, by two orders of magnitude, in the influence of the linear birefringence produced by an external piezoceramic fiber modulator was experimentally observed.

Planar and twisted lyotropic chromonic liquid crystal cells as optical compensators for twisted nematic displays

We have explored the use of lyotropic chromonic liquid crystals (LCLCs) as promising materials for optical compensators for liquid crystal displays. We used aqueous solutions of disodium cromoglycate in the range of concentrations that yield a chromonic N phase. The N phase is aligned by rubbed polyimide layers giving planar orientation. Two different types of LCLC-based elements are described: (a) a uniformly aligned planar N phase that forms an optically uniaxial plate with a negative birefringence; (b) twisted cells of LCLC obtained by doping the N phase with chiral amino acids. Both planar and twisted N* phase cells were used for compensation of the positive birefringence of twisted nematic (TN) displays; the TN devices were appreciably improved. We achieved achromatic dark state and contrast ratio up to 50:1 at all directions within a 40° cone of viewing angle.

Elimination of a narrow unstable zone inside the stable region of a two-rod resonator with a thermal birefringence compensator

We propose and numerically investigate a method to remove a narrow unstable zone invaded in the stable region of a two-rod resonator with a thermal birefringence compensator. The method is based on the modification of the two thermal focal lengths of a laser rod for radial and tangential polarizations using an additional lens. This is done by installing the additional lens at the center of the plane-parallel symmetric resonator. We find that the wide stable region without the narrow unstable zone is achieved when the focal length of the additional lens is approximately equal to the half length of the resonator.

Scanning laser polarimetry with variable corneal compensation: identification and correction for corneal birefringence in eyes with macular disease.

In scanning laser polarimetry with variable corneal compensation (SLP-VCC), the macula is used as an intraocular polarimeter to calculate and neutralize corneal birefringence based on an intact Henle's layer. The purpose of this investigation was to validate this strategy in eyes with macular structural disease. A nerve fiber analyzer was modified to enable the measurement of corneal polarization axis and magnitude so that compensation for corneal birefringence was eye specific. Normal subjects and patients with a variety of pathologic macular conditions underwent complete ocular examination, SLP-VCC, and direct measurement of the corneal polarization axis (CPA), with a slit-lamp-mounted corneal polarimeter. Macular birefringence patterns were classified as well defined, weak, or indeterminate bow ties. A new "screen" method is described that determines the anterior segment birefringence without relying on the presence of macular bow-tie patterns. Forty-seven eyes (20 normal, 27 with maculopathy) of 47 patients (mean age, 59.0 +/- 19.0 years; range, 24-88) were enrolled. The correlation between CPA measured with corneal polarimetry (CPA by P(IV) [fourth Purkinje image]) and SLP-VCC was less in eyes with macular disease (R(2) = 0.22, P = 0.024) compared with normal eyes (R(2) = 0.72, P < 0.0001). Eyes with macular disease had significantly (P = 0.007) more indeterminate macular bow ties (8/27; 29%) than did normal eyes (0/20). The magnitude of difference between CPA by P(IV) and CPA by SLP-VCC was significantly (P = 0.0007) greater in eyes with indeterminate bow-tie patterns than in weak and well-defined patterns. Although no relationship was observed between CPA and 12 retardation parameters obtained with SLP-VCC in normal eyes (P > 0.05), eyes with macular disease showed a significant association between CPA and average thickness (R(2) = 0.27, P = 0.005), ellipse average (R(2) = 0.24, P = 0.0085), superior average (R(2) = 0.24, P = 0.009), inferior average (R(2) = 0.28, P = 0.004), and superior integral (R(2) = 0.37, P = 0.0008), suggesting incomplete corneal compensation. Greater correlation between CPA by P(IV) and CPA derived by SLP-VCC was found by using the screen method (R(2) = 0.83, P < 0.0001) compared with the bow-tie method (R(2) = 0.22, P = 0.024) in eyes with maculopathy. Macular strategies for neutralization of corneal birefringence using SLP-VCC can fail if Henle's layer is disrupted by macular disease. The screen method provides a more robust measure of the anterior segment birefringence in some eyes with macular disease.

Effect of individualized compensation for anterior segment birefringence on retinal nerve fiber layer assessments as determined by scanning laser polarimetry

Purpose Scanning laser polarimetry estimates retinal nerve fiber layer (RNFL) thickness through measurement of retardation of a polarized laser light passing through the naturally birefringent RNFL and cornea. The commercial instrument, the GDx Nerve Fiber Analyzer (Laser Diagnostic Technologies, Inc., San Diego, CA), uses an anterior segment compensator of fixed magnitude and slow polarization axis to eliminate the contribution of the cornea to the total signal. Previous studies have shown up to 30% of patients are not adequately compensated by this method. The aim of this study was to determine the effect of individualized anterior segment compensation using a newly designed variable compensator on estimates of retinal nerve fiber layer thickness compared with those as determined with the fixed compensator in the commercial device. Design Comparative, observational case series. Participants Twenty-eight eyes from 14 normal participants and 24 eyes from 12 patients with bilateral glaucoma. Methods Using information derived from a scan of the macula, a newly designed variable anterior segment compensator for the GDx was set to neutralize anterior segment birefringence. Normal participants and patients with glaucoma underwent RNFL measurements using the standard (fixed) compensator and the variable compensator. The results were compared using Hotelling’s generalized means test and Bonferroni’s adjustment for multiple comparisons. Main outcome measures Standard GDx modulation and thickness parameters as determined with the fixed and variable compensators. Results All thickness values were statistically significantly lower as determined with the variable compensator, with no discernible differences in any of the modulation parameters. Conclusions Individualized anterior segment compensation lowers the RNFL thickness values as determined by scanning laser polarimetry compared with those determined with the standard fixed compensator. This may narrow the normal range and increase the discriminating ability of scanning laser polarimetry between normal and disease. However, modulation is less affected, and the modulation parameters may thus prove more useful for distinguishing between normal and glaucoma.

Scanning laser polarimetry with variable corneal compensation and optical coherence tomography in normal and glaucomatous eyes

Purpose To evaluate the relationship between visual function and retinal nerve fiber layer (RNFL) measurements obtained with scanning laser polarimetry with variable corneal compensation (SLP-VCC) and optical coherence tomography (OCT). Design Cross-sectional analysis of normal and glaucomatous eyes in a tertiary care academic referral practice. Methods A commercial GDx nerve fiber analyzer was modified to enable the measurement of corneal polarization axis and magnitude so that compensation for corneal birefringence was eye specific. Complete examination, SLP with fixed corneal compensation (FCC) and variable corneal compensation (VCC), optical coherence tomography (OCT) imaging of the peripapillary RNFL, and automated achromatic perimetry were performed in all subjects. Exclusion criteria were visual acuity less than 20/40, diseases other than glaucoma, and unreliable perimetry. Results Fifty-nine patients (59 eyes; 29 normal, 30 glaucomatous) were enrolled (mean age, 56.7 ± 20.3 years, range, 20–91). All eyes with glaucoma had associated visual field loss (average mean defect, −8.4 ± 5.8 dB). Using SLP-FCC, nine of 12 retardation parameters (75%) were significantly less in glaucomatous eyes. Using SLP-VCC, 11of 12 retardation parameters (92%) were significantly less in glaucomatous eyes. Multiple regression models constructed for each retardation parameter with visual field demonstrated that the following VCC parameters were statistically significant whereas FCC parameters were not: ellipse average (FCC, P = .28, VCC, P = .001), superior average (FCC, P = .38, VCC, P < .001), inferior average (FCC, P = .10, VCC, P = .008), average thickness (FCC, P = .30, VCC, P = .031), and superior integral (FCC, P = .43, VCC, P = .001). Similar results were obtained for multiple regression models constructed with OCT-derived RNFL thickness: ellipse average (FCC, P = .99, VCC, P = .002), superior average (FCC, P = .90, VCC, P < .001), inferior average (FCC, P = .61, VCC, P = .007), and superior integral (FCC, P = .92, VCC, P < .001). Conclusions Compared with fixed compensation, mean-based SLP parameters generated with SLP-VCC have greater correlation with visual function and RNFL thickness assessments obtained with OCT.

Correction of thermally induced birefringence in double-rod laser resonators – comparison of various methods

We compared various compensation methods for thermally induced birefringence in two-rod Nd:YAG laser resonators. The concept that lay behind these methods was 90° polarization rotation between two birefringent elements. In the first method, polarization rotation was done between laser rods, and in the second method, polarization rotation was done between passes through both rods. Both methods were tested with and without relay imaging between aberration planes. Thermal focusing was adaptively controlled by use of a variable radius of curvature mirror (VRM) or with an imaging VRM (IVRM). Best results were achieved with birefringence compensation between passes through relay-imaged rods and with the IVRM.

A birefringence compensation method for mechanically induced long-period fiber gratings

We present a new method of compensating linear birefringence always present in mechanically induced long-period fiber gratings (MLPFGs). With the proposed compensation method the MLPFGs became polarization insensitive and the polarization mode dispersion of the MLPFGs was substantially reduced.