Showing papers on "Birefringence published in 2005"

High resolution optical frequency domain reflectometry for characterization of components and assemblies.

Abstract: We describe a technique for polarization sensitive optical frequency domain reflectometry (OFDR) that achieves 22 micrometer two-point spatial resolution over 35 meters of optical length with -97 dB sensitivity in a single measurement taking only seconds. We demonstrate OFDR’s versatility in both time- and frequency-domain metrology by analyzing a fiber Bragg grating (FBG) in both the spectral and impulse response domains. We also demonstrate how a polarization diversity receiver can be used in an OFDR system to track changes in the polarization state of light propagating through a birefringent component.

Infrared refractive indices of liquid crystals

Abstract: The refractive indices of E7 liquid-crystal mixture were measured at six visible and two infrared (λ=1.55 and 10.6μm) wavelengths at different temperatures, using Abbe and wedged cell refractometer methods, respectively. The experimental data of the visible wavelengths fit the extended Cauchy equations well. Using the extended Cauchy equations, we can extrapolate the refractive indices of E7 to IR. The extrapolated results almost strike through the measured data. Thus, the extended Cauchy equations can be used to link the visible refractive indices to infrared, where the refractive index measurements are more difficult.

Numerical modeling of photonic crystal fibers

Abstract: Recent progress on numerical modeling methods for photonic crystal fibers (PCFs) such as the effective index approach, basis-function expansion approach, and numerical approach is described. An index-guiding PCF with an array of air holes surrounding the silica core region has special characteristics compared with conventional single-mode fibers (SMFs). Using a full modal vector model, the fundamental characteristics of PCFs such as cutoff wavelength, confinement loss, modal birefringence, and chromatic dispersion are numerically investigated.

Simple high efficiency optical coherence domain reflectometer design

Abstract: The present invention discloses simple and yet highly efficient configurations of optical coherence domain reflectometry systems. The combined use of a polarizing beam splitter (614a) with one or two polarization manipulator(s) (634a and 646) (e.g. a Faraday rotator or a quarter wave plate) that rotate the returned light wave polarization to an orthogonal direction, enables one to achieve high optical power delivery efficiency as well as fixed or predetermined output polarization state of the interfering light waves reaching a detector (652a) or detector array, which is especially beneficial for spectral domain optical coherence tomography. In addition, the system can be made insensitive to polarization fading resulting from the birefringence change in the sample (620a) and reference (622a) arms. Dispersion matching can also be easily achieved between the sample and the reference arm for high resolution longitudinal scanning.

Optical characterization of the nematic lyotropic chromonic liquid crystals: light absorption, birefringence, and scalar order parameter.

Abstract: We report on the optical properties of the nematic (N) phase formed by lyotropic chromonic liquid crystals (LCLCs) in well aligned planar samples. LCLCs belong to a broad class of materials formed by one-dimensional molecular self-assembly and are similar to other systems such as "living polymers" and "wormlike micelles." We study three water soluble LCLC forming materials: disodium chromoglycate, a derivative of indanthrone called Blue 27, and a derivative of perylene called Violet 20. The individual molecules have a planklike shape and assemble into rodlike aggregates that form the phase once the concentration exceeds about 0.1 M. The uniform surface alignment of the N phase is achieved by buffed polyimide layers. According to the light absorption anisotropy data, the molecular planes are on average perpendicular to the aggregate axes and thus to the nematic director. We determined the birefringence of these materials in the N and biphasic N-isotropic (I) regions and found it to be negative and significantly lower in the absolute value as compared to the birefringence of typical thermotropic low-molecular-weight nematic materials. In the absorbing materials Blue 27 and Violet 20, the wavelength dependence of birefringence is nonmonotonic because of the effect of anomalous dispersion near the absorption bands. We describe positive and negative tactoids formed as the nuclei of the new phase in the biphasic N-I region (which is wide in all three materials studied). Finally, we determined the scalar order parameter of the phase of Blue 27 and found it to be relatively high, in the range 0.72-0.79, which puts the finding into the domain of general validity of the Onsager model. However, the observed temperature dependence of the scalar order parameter points to the importance of factors not accounted for in the athermal Onsager model, such as interaggregate interactions and the temperature dependence of the aggregate length.

Polymeric optical waveguides using direct ultraviolet photolithography process

Abstract: Single-mode polymeric channel waveguides were fabricated using simple direct ultraviolet photolithography process. A cross-linkable negative tone epoxy NANOTM SU-8 2000 polymer was used. Once exposed to ultraviolet light through a photomask, the waveguide stripes were obtained upon development. The polymer has many desirable properties, such as high refractive index, good adhesion to substrate, optical transparency in the infrared wavelength region, and high glass transition and high thermal decomposition temperatures. Properties of the optical waveguides were characterized, and there is an excellent agreement between measured data and theory. The values of dn/dT and waveguide birefringence are -1.87×10-4 /°C and ∼10-4, respectively, and are comparable to those of halogenated acrylate polymers. With an overcladding layer, the propagation losses measured are 0.25 and 0.28 dB/cm at 0.8 μm, 0.62 and 0.77 dB/cm at 1.31 μm, and 1.25 and 1.71 dB/cm at 1.55 μm for TE and TM polarizations, respectively.

A compact silicon-on-insulator polarization splitter

Abstract: A compact directional coupler-based polarization splitter is designed and realized using silicon-on-insulator (SOI) waveguides. Even though silicon does not have any material birefringence, the high index contrast obtained in the SOI platform and reduced waveguide dimensions makes it possible to induce significant birefringence. Polarization splitting is achieved by making use of this geometry-induced birefringence. In this work, we demonstrate polarization splitting in devices as short as 120 /spl mu/m. Even smaller devices can be made using submicron-thick Si waveguides.

All-optical control of microfluidic components using form birefringence.

Abstract: The reflection and refraction of light at a dielectric interface gives rise to forces due to changes in the photon momentum1. At the microscopic level, these forces are sufficient to trap and rotate microscopic objects2,3. Such forces may have a profound impact in the emergent area of microfluidics, where there is the desire to process minimal amounts of analyte. This places stringent criteria on the ability to pump, move and mix small volumes of fluid, which will require the use of micro-components and their controlled actuation4,5,6,7. We demonstrate the modelling, fabrication and rotation of microgears based on the principle of form birefringence. Using a geometric anisotropy (a one-dimensional photonic crystal etched into the microgear), we can fabricate microgears of known birefringence, which may be readily rotated by manipulating the input polarization in a standard optical trap. This methodology offers a new and powerful mechanism for generating a wide range of microfabricated machines, such as micropumps, that may be driven by purely optical control.

Continuously tunable devices based on electrical control of dual-frequency liquid crystal filled photonic bandgap fibers.

Abstract: We present an electrically controlled photonic bandgap fiber device obtained by infiltrating the air holes of a photonic crystal fiber (PCF) with a dual-frequency liquid crystal (LC) with pre-tilted molecules. Compared to previously demonstrated devices of this kind, the main new feature of this one is its continuous tunability due to the fact that the used LC does not exhibit reverse tilt domain defects and threshold effects. Furthermore, the dual-frequency features of the LC enables electrical control of the spectral position of the bandgaps towards both shorter and longer wavelengths in the same device. We investigate the dynamics of this device and demonstrate a birefringence controller based on this principle.

Integrated polarization beam splitter in high index contrast silicon-on-insulator waveguides

Abstract: We design, fabricate, and test a novel transverse-electric/transverse-magnetic (TE/TM) polarization beam splitter which is based on the dependence of birefringence on rib width in silicon-on-insulator waveguides. The polarization beam splitter has high TE to TM extinction ratio, low excess loss and wide operation bandwidth. The simple fabrication method involves only single etch step.

Electromagnetic wave propagation in two-dimensional photonic crystals: A study of anomalous refractive effects

Abstract: We systematically study a collection of refractive phenomena that can possibly occur at the interface of a two-dimensional photonic crystal with the use of the wave vector diagram formalism. Cases with a single propagating beam in the positive or negative direction as well as cases with birefringence were observed. We examine carefully the conditions to obtain a single propagating beam inside the photonic crystal lattice. Our results indicate that the presence of multiple reflected beams in the medium of incidence is neither a prerequisite nor does it imply multiple refracted beams. We characterize our results with respect to the origin of the propagating beam and the nature of propagation left-handed or not. We identified four distinct cases that lead to a negatively refracted beam. Under these findings, the definition of phase velocity in a periodic medium is reexamined and its physical interpretation discussed. To determine the “rightness” of propagation, we propose a wedge-type experiment. We discuss the intricate details for an appropriate wedge design for different types of cases in triangular and square structures. We extend our theoretical analysis and examine our conclusions as one moves from the limit of photonic crystals with high-index contrast between the constituent dielectrics to photonic crystals with low modulation of the refractive index. Finally, we examine the “rightness” of propagation in the one-dimensional multilayer medium and obtain conditions that are different from those of two-dimensional systems.

Silica nanorod-array films with very low refractive indices

Abstract: The refractive-index contrast is an important figure of merit for dielectric multilayer structures, optical resonators, and photonic crystals. This represents a strong driving force for novel materials that have refractive indices lower than those of conventional optically transparent materials. Silica nanorod-array dielectric films with unprecedented low refractive indices of 1.08 are demonstrated and shown to have viable optical properties including enhanced reflectivity of a single-pair distributed Bragg reflector.

Polarization microscopy by use of digital holography: application to optical- fiber birefringence measurements

Abstract: We present a digital holographic microscope that permits one to image polarization state. This technique results from the coupling of digital holographic microscopy and polarization digital holography. The interference between two orthogonally polarized reference waves and the wave transmitted by a microscopic sample, magnified by a microscope objective, is recorded on a CCD camera. The off-axis geometry permits one to reconstruct separately from this single hologram two wavefronts that are used to image the object-wave Jones vector. We applied this technique to image the birefringence of a bent fiber. To evaluate the precision of the phase-difference measurement, the birefringence induced by internal stress in an optical fiber is measured and compared to the birefringence profile captured by a standard method, which had been developed to obtain high-resolution birefringence profiles of optical fibers.

Homogenization of electromagnetic crystals formed by uniaxial resonant scatterers.

Abstract: Dispersion properties of electromagnetic crystals formed by small uniaxial resonant scatterers (magnetic or electric) are studied using the local field approach. The goal of the study is to determine the conditions under which the homogenization of such crystals is possible. Therefore the consideration is limited to the frequency region where the wavelength in the host medium is larger than the lattice periods. It is demonstrated that, together with the known restriction for the homogenization related to the large values of the material parameters, there is an additional restriction related to their small absolute values. On the other hand, the homogenization becomes allowed in both cases of large and small material parameters for special directions of propagation. Two unusual effects inherent to the crystals under consideration are revealed: a flat isofrequency contour that allows subwavelength imaging using the canalization regime and birefringence of the extraordinary modes which can be used for beam splitting.

High-sensitivity determination of birefringence in turbid media with enhanced polarization-sensitive optical coherence tomography

Abstract: Polarization-sensitive optical coherence tomography provides high-resolution cross-sectional characterization of birefringence in turbid media. Weakly birefringent biological tissues such as the retinal nerve fiber layer (RNFL) require advanced speckle noise reduction for high-sensitivity measurement of form birefringence. We present a novel method for high-sensitivity birefringence quantification by using enhanced polarization-sensitive optical coherence tomography (EPS-OCT) and introduce the polarimetric signal-to-noise ratio, a mathematical tool for analyzing speckle noise in polarimetry. Multiple incident polarization states and nonlinear fitting of normalized Stokes vectors allow determination of retardation with ±1° uncertainty with invariance to unknown unitary polarization transformations. Results from a weakly birefringent turbid film and in vivo primate RNFL are presented. In addition, we discuss the potential of EPS-OCT for noninvasive quantification of intracellular filamentous nanostructures, such as neurotubules in the RNFL that are lost during the progression of glaucoma.

Switchable multiwavelength erbium-doped fiber ring laser with a multisection high-birefringence fiber loop mirror

Abstract: We proposed and demonstrated a new concept of switchable multiwavelength erbium-doped fiber ring laser with a multisection high-birefringence fiber loop mirror (HiBi-FLM). The HiBi-FLM results in the laser operating on different allowed wavelengths. The wavelength switching can be achieved by modifying the reflection spectrum of HiBi-FLM through two polarization controllers. The experiment demonstrates the switchable lasing operation within the random combination of three wavelengths, as well as with a large (/spl sim/40 dB) output signal to background noise ratio.

Birefringence control using stress engineering in silicon-on-insulator (SOI) waveguides

Abstract: We demonstrate that stress engineering is an effective tool to modify or eliminate polarization dispersion in silicon-on-insulator (SOI) waveguide devices, for a wide range of waveguide cross-section shapes and dimensions. The stress-induced effects on the modal birefringence of SOI waveguides are investigated numerically and experimentally. Finite-element simulations show that while the birefringence of ridge waveguides with both slanted and vertical sidewalls can be effectively modified using cladding stress, the birefringence becomes much less sensitive to dimension fluctuations with decreasing sidewall slope. To efficiently simulate the stress-induced effects we propose a normalized plane-strain model which can achieve comparable accuracy as a fully generalized plane-strain model but requires significantly less computational resources. Excellent agreement is achieved between the calculated and measured birefringence tuning using SiO/sub 2/ cladding induced stress. Finally, both calculations and experiments confirm that cladding induced stress can be used to eliminate the birefringence in SOI waveguides of arbitrary shapes, for typical SiO/sub 2/ film stress values (/spl sigma//sub film//spl ap/-100 to -300 MPa) and cladding thicknesses of the order of 1 /spl mu/m or less.

Vibrational circular dichroism in general anisotropic thin solid films: measurement and theoretical approach.

Abstract: In this study, the measurement of the true vibrational circular dichroism (VCD) spectrum is considered from an experimental and theoretical approach for any general anisotropic thin solid sample exhibiting linear as well as circular birefringence (LB, CB) and dichroism (LD, CD) properties. For this purpose, we have made use of a simple model α-helix polypeptide, namely, the poly(γ-benzylL-glutamate) or PBLG, reference sample possessing a well-known VCD spectrum and giving rise to slightly oriented films by deposition onto a solid substrate. Also, we have used a different Fourier transform infrared modulation of polarization (PM-FTIR) optical setup with two-channel electronic processing in order to record the PM-VLD and PM-VCD spectra for various sample orientations in its film plane. All the corresponding general relations of the expected intensities in these experiments and the related properly designed calibration measurements were established using the Stokes–Mueller formalism; in addition, the residual birefringence of the optical setup and the transmittance anisotropy of the detector were estimated. From a comparative study of the results obtained in solution and in the solid state, we then propose a simple new experimental procedure to extract the true VCD spectrum of an oriented PBLG thin film: its consists of calculating the half-sum of two spectra recorded at θ and atθ ± 90° sample orientations. Moreover, the complete linear and circular birefringence and dichroism properties of the ordered PBLG thin film are estimated in the amide I and amide II vibrational regions. This allows us to establish for any sample orientation various theoretical simulations of the VCD spectra that agree nicely with the observed experimental results; this confirms that the measurement of LD and LB is in this case a prerequisite in simulating the true VCD spectrum of a partly oriented anisotropic sample. This validates our combined experimental and theoretical approach and opens the route to promising future vibrational CD studies on other macroscopic anisotropic thin film samples.

Fast birefringent mode stressed liquid crystal

Abstract: We report a stressed liquid crystal (SLC) that produce a large shift in phase retardation at submillisecond speeds. The SLC consists of uniformly aligned micro-domains of a liquid crystal dispersed in a polymer structure. Mechanical stress produces uniform alignment, essentially eliminates light scattering, and substantially improves the electro-optic performance. A 22-μm-thick SLC film switches more than 2μm of phase retardation in less than 1ms. The system has a linear voltage response with essentially no hysteresis.

Control of enhanced THz transmission through metallic hole arrays using nematic liquid crystal.

Abstract: We demonstrate frequency tuning of enhanced THz radiation transmitted through a two-dimensional metallic hole array (2D-MHA) by controlling the index of refraction of the medium filling the holes and adjacent to the 2D-MHA on one side. The medium is a nematic liquid crystal (NLC) and its index of refraction is varied using magnetically controlled birefringence of the NLC. With the NLC, the peak transmission frequency of the 2D-MHA shift to the red by 0.112 THz and can be tuned from 0.193 to 0.188 THz. The peak transmittance is as high as 70% or an enhancement of 2.42 times, considering the porosity of the 2D-MHA. As a tunable THz filter, this device exhibits a continuous tuning range of 4.7 GHz, a low insertion loss of 2.35 to 1.55 dB and a quality factor of ~4–5.

Polarisation-sensitive terahertz detection by multicontact photoconductive receivers

Abstract: We have developed a terahertz radiation detector that measures both the amplitude and polarization of the electric field as a function of time. The device is a three-contact photoconductive receiver designed so that two orthogonal electric-field components of an arbitrary polarized electromagnetic wave may be detected simultaneously. The detector was fabricated on Fe+ ion-implanted InP. Polarization-sensitive detection is demonstrated with an extinction ratio better than 100:1. This type of device will have immediate application in studies of birefringent and optically active materials in the far-infrared region of the spectrum.

Optical trapping of colloidal particles and measurement of the defect line tension and colloidal forces in a thermotropic nematic liquid crystal

Abstract: We demonstrate optical trapping and manipulation of transparent microparticles suspended in a thermotropic nematic liquid crystal with low birefringence. We employ the particle manipulation to measure line tension of a topologically stable disclination line and to determine colloidal interaction of particles with perpendicular surface anchoring of the director. The three-dimensional director fields and positions of the particles manipulated by laser tweezers are visualized by fluorescence confocal polarizing microscopy.

LiGaTe2: A new highly nonlinear chalcopyrite optical crystal for the Mid-IR

Abstract: The chalcopyrite crystal LiGaTe 2 was grown by the Bridgman-Stockbarger technique with sufficient size and optical quality that allowed the characterization of its linear (dispersion and birefringence) and nonlinear optical properties. X-ray structural analysis was performed on single crystals. The transmission was recorded in the 0.5-24 μm range, and Raman and IR-spectra were recorded in the 0-400 cm -1 and 180-400 cm -1 ranges, respectively. The clear transparency range of LiGaTe 2 extends from 2.5 to 12 μm, the band-gap at room temperature is at 2.41 eV (515 nm), residual absorption limits the transmission near the band-edge, and the upper limit for the transmission is determined by the onset two-phonon absorption. LiGaTe 2 is a positive uniaxial crystal that possesses sufficient birefringence for phase-matching. Its nonlinear coefficient d 36 was estimated by phase-matched second harmonic generation to be 43 pm/V ± 10%. It is only the third crystal belonging to the A I B III C VI 2 chalcopyrite family for which phase-matched nonlinear frequency conversion could be demonstrated, and of this group it exhibits the highest nonlinearity and figure of merit for nonlinear optical applications in the mid-infrared.

Highly birefringent hollow-core photonic bandgap fiber

Abstract: A highly birefringent hollow-core photonic bandgap fiber is fabricated and characterized. The fiber group birefringence is found to be 0.025 at 1550 nm through wavelength scanning method and direct measurement of differential group delay.

Torsion characteristics of long-period fiber gratings induced by high-frequency CO2 laser pulses

Abstract: It is demonstrated, for the first time to our knowledge, that torsion characteristics of resonant wavelength of long-period fiber gratings (LPFGs) induced by high-frequency CO2 laser pulses depend strongly on the twist directions. That is, the resonant wavelength shifts linearly toward the longer wavelength as the LPFG is twisted clockwise, whereas it shifts linearly toward the shorter wavelength as the LPFG is twisted anticlockwise. On the other hand, the loss peak amplitude decreases gradually whether the LPFG is twisted clockwise or anticlockwise. If the twisted fiber is much longer than the twisted LPFG, the resonant wavelength shifts wavelike toward the longer and shorter wavelength as the LPFG is twisted clockwise and anticlockwise, respectively, and the loss peak amplitude decreases wavelike with the twist rate applied. An asymmetric refractive-index distribution exists within the cross section of the LPFG fabricated by high-frequency CO2 laser pulses, which results in obvious linear birefringence in the LPFG. The right- and left-rotatory elliptical birefringence are induced when the LPFG with a linear birefringence is twisted clockwise and anticlockwise, respectively. So the twist-induced right- and left-rotatory elliptical birefringence in the LPFG show that the shift of resonant wavelength is dependent on the twist directions. The twist-induced circular birefringence in the fiber shows that the resonant wavelength and amplitude of the LPFG change wavelike.

Stress-induced birefringence in large-mode-area micro-structured optical fibers

Abstract: We report on detailed numerical investigation of stress-induced birefringence in micro-structured solid-core optical fibers. The stress is induced either by external forces or stress applying parts inside the fiber. Both methods lead to different stress distributions where screening as well as enhancement effects due to the air-hole micro-structuring could be observed. Furthermore, we discuss the potential of the realization of polarization-maintaining low-nonlinearity micro-structured fibers that are suitable for applications in ultrafast optics.

Spectral-domain polarization-sensitive optical coherence tomography at 850 nm

Abstract: Spectral-Domain Polarization-Sensitive Optical Coherence Tomography (SD-PS-OCT) is a technique developed to measure the thickness and birefringence of the nerve fiber layer in vivo as a tool for the early diagnosis of glaucoma. A clinical SD-PS-OCT system was developed and scans were made around the optic nerve head (ONH) using ten concentric circles of increasing diameter. One healthy volunteer was imaged. Retinal nerve fiber layer thickness and birefringence information was extracted from the data. Polarization-sensitive OCT images were acquired at video rate (29 frames per second (fps), 1000 A-lines / frame) and at 7 fps (1000 A-lines / frame). The last setting improved the signal to noise ratio by approximately 6 dB. Birefringence measurements on the healthy volunteer gave similar results as earlier reported values that were obtained with a time-domain setup. The measurement time was reduced from more than a minute to less than a second.

Magnetic alignment of self-assembled anthracene organogel fibers.

Abstract: High magnetic fields are shown to be remarkably effective to orient self-assembled 2,3-bis-n-decyloxyanthracene (DDOA) fibers during organogel preparation. Magnetic orientation of DDOA results in a highly organized material displaying a fiber-orientation order parameter of 0.85, a large linear birefringence, and fluorescence dichroism. The aligned organogel is stable after removal of the magnetic field at room temperature and consists of fibers oriented perpendicular to the magnetic field direction, as shown by scanning electron microscopy. Models for the molecular organization within the gel fibers are discussed upon quantitative analysis of the birefringence. Prospectively, magnetic alignment can be used to improve specific properties of organogel materials.