Birefringence Changes Research Articles

Recent Progress in Brillouin Scattering Based Fiber Sensors

Brillouin scattering in optical fiber describes the interaction of an electro-magnetic field (photon) with a characteristic density variation of the fiber. When the electric field amplitude of an optical beam (so-called pump wave), and another wave is introduced at the downshifted Brillouin frequency (namely Stokes wave), the beating between the pump and Stokes waves creates a modified density change via the electrostriction effect, resulting in so-called the stimulated Brillouin scattering. The density variation is associated with a mechanical acoustic wave; and it may be affected by local temperature, strain, and vibration which induce changes in the fiber effective refractive index and sound velocity. Through the measurement of the static or dynamic changes in Brillouin frequency along the fiber one can realize a distributed fiber sensor for local temperature, strain and vibration over tens or hundreds of kilometers. This paper reviews the progress on improving sensing performance parameters like spatial resolution, sensing length limitation and simultaneous temperature and strain measurement. These kinds of sensors can be used in civil structural monitoring of pipelines, bridges, dams, and railroads for disaster prevention. Analogous to the static Bragg grating, one can write a moving Brillouin grating in fibers, with the lifetime of the acoustic wave. The length of the Brillouin grating can be controlled by the writing pulses at any position in fibers. Such gratings can be used to measure changes in birefringence, which is an important parameter in fiber communications. Applications for this kind of sensor can be found in aerospace, material processing and fine structures.

Strain-Induced Molecular Reorientation and Birefringence Reversion of a Robust, Anisotropic Double-Network Hydrogel

A robust, macroscopically anisotropic double-network (A-DN) hydrogel has been developed by combining the first anisotropic, physically cross-linked network of semirigid poly(2,2′-disulfonyl-4,4′-benzidine terephthalamide) (PBDT) and the second chemically cross-linked network of flexible polyacrylamide (PAAm) through double-network technology. Owing to the cooperative orientation of the self-assembled bundle structure of semirigid PBDTs, the A-DN gel shows a very sensitive and strong birefringence change upon deformation, which is always dominated by PBDT for the wide strain range investigated (ε = 0−18), regardless the fact that PAAm is in large excess. When the initial alignment of PBDT is vertical to the elongation direction, a birefringence reversion occurs at ε ∼ 0.5, indicating that the initial vertical alignment of PBDT is overwhelmed by the strain-induced parallel alignment, even at such a small strain due to the cooperative reorientation of PBDT.

Ultrafast dynamics of soft phonon modes in perovskite dielectrics observed by coherent phonon spectroscopy

Coherent phonons were observed in perovskite dielectrics LaAlO${}_{3}$ and KMnF${}_{3}$ using ultrafast polarization spectroscopy. Ultrafast dynamics and softening of phonon modes, which contribute to structural phase transitions, were studied. The temperature dependences of the phonon frequency and relaxation rate were obtained from the observed damped oscillation of coherent phonons. In LaAlO${}_{3}$, typical softening of the phonon frequency was observed toward the phase-transition temperature ${T}_{c}$ ($~750$ K). The observed relaxation for the soft-mode phonons is explained well by a population decay due to anharmonic phonon-phonon coupling. In KMnF${}_{3}$, two structural phase-transition temperatures, ${T}_{c1}$ $(=187$ K) and ${T}_{\mathit{cm}}$ ($~81$ K), were found through birefringence measurement. At temperatures below ${T}_{\mathit{cm}}$, three modes of coherent phonons were observed and the phonon modes were found to undergo softening toward ${T}_{\mathit{cm}}$. The observed dependences of the coherent phonon signal on the pumping polarization and the temperature suggest an orthorhombic structure, instead of a monoclinic structure, at temperatures below ${T}_{\mathit{cm}}$. The sudden decrease in the phonon frequency at ${T}_{\mathit{cm}}$ indicates the first-order character of the phase transition at ${T}_{\mathit{cm}}$. Notable changes in birefringence and phonon frequency were observed at around 90 K, but no distinct anomaly is found, which suggests that the structure undergoes continuous change. Above 100 K, the ${E}_{g}$ mode in the tetragonal symmetry is found to undergo softening toward ${T}_{c1}$. A coherent phonon signal of relaxation type appears at around ${T}_{c1}$ and survives even at near room temperature, which suggests the existence of structural disorder even in the cubic symmetry above ${T}_{c1}$.

Reconstruction of polarization-shaped laser pulses after a hollow-core fiber using backreflection

We present a method to reconstruct the pulse shape of polarization-shaped femtosecond laser pulses after a hollow-core photonic crystal fiber by reflecting the pulses back through the fiber. First, a procedure is introduced to receive the optical fiber properties and generate parametrically shaped pulses after propagation through the fiber. Changes of the fiber's birefringence by mechanical stress are examined to investigate the correlation between the pulse shapes after one and two passes through the fiber. Finally, we demonstrate the characterization of the pulse after one pass through the fiber by calculating the pulse shape from the measured pulse after two passes.

The effect of self-assembled nanofibrils on the morphology and microstructure of poly(l-lactic acid)

The morphologies and microstructures of neat 1,3:2,4-dibenzylidene-D-sorbitol (DBS) and DBS/poly(L-lactic acid) (PLLA) samples have been investigated by polarizing optical microscopy (POM) and scanning electron microscopy (SEM). The morphology of neat DBS samples prepared from solution had unspecific structures, and no fibrils formed. In comparison, DBS molecules self-assembled into fibrils with diameters ranging from 100 nm to 1 µm when samples were prepared from the melt. The DBS fibrils were also found in DBS/PLLA systems, but the average diameter was only around 20 nm. The DBS architectures could be well tuned by varying the DBS contents and PLLA crystallization temperatures. Micron-sized fibrillar rings or disks due to the aggregation of DBS nanofibrils were found using SEM in samples with DBS contents more than 3 wt% and crystallized above 120 °C. Meanwhile, “concentric-circled” PLLA spherulites were observed by POM. The DBS nanofibrils largely formed at the circles, but some nanofibrils formed beyond the circles and were dispersed in the PLLA spherulites. These dispersed nanofibrils affected the orientation of PLLA lamellae and caused a change in birefringence, yet the growth rate of PLLA was not significantly influenced by the formation of DBS nanofibrils. In addition, porous PLLA structures could be obtained by solvent extraction of the DBS nanofibrils.

Differential optical spectropolarimetric imaging system assisted by liquid crystal devices for skin imaging

Skin cancer diagnosis depends not only on histopathological examination but also on visual inspection before and after the excision of suspected lesion. Neoplasm is accompanied with changes in birefringence of collagen, pleomorphicity, and hyperchromatic state of epithelial nuclei. These phenomena can be measured by spectral and polarization changes of light backscattered by the examined tissue. A new differential spectropolarimetric system is proposed using liquid crystal devices, one as a tunable filter and the other as a polarization rotator, both operating at wide spectral ranges from the visible to the near-infrared. Since collagen's fibrils texture orientation depends on its location in the skin and since it is not well organized, our system scans the bipolarization states by continuously rotating the linearly polarized light incident on a skin lesion, and collecting differential contrasts between sequenced images when simultaneously averaging the statistical readout of a video camera. This noninvasive method emphasizes areas on skin where the neoplasm, or tumor, minimizes the statistical polarization change of the scattered light from the lesion. The module can be considered as an assistant tool for epiluminescence microscopy. Images of skin tumors were captured in vivo before the patients having their surgery and compared to histopathological results.

Creep Tests, Flow Birefringence Measurements, and Flow Visualization of Aqueous Solutions of CTAB and NaSal in Shear Flow between Parallel Plates

Creep tests, flow birefringence measurements, and flow visualization experiments in shear flows between parallel plates were carried out for aqueous solutions of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) with excess of NaSal. The mol concentration of CTAB was fixed to 0.030 mol/l and the concentration ratio of NaSal to CTAB was varied. The creep tests indicate that it requires long time to develop micellar network structures for low to moderate concentrations at low shear stresses. Under conditions of shear rate at which the shear-thickening property in viscosity is observed in a flow curve, periodical changes in flow birefringence and shear stress occur and these frequencies are almost the same as that of the emergency of white turbidity observed in the flow visualization. These results indicate that relatively large structures of micellar network are induced by the shear flow and repeatedly appear and disappear.

Enhanced Frank elasticity and storage modulus in a diamagnetic liquid crystalline ferrogel

Combining the magnetic properties of metal particles and the orientational anisotropy of a liquid crystal has been of interest owing to potential technological and bioengineering applications. The target is to achieve the ferromagnetic state while retaining the fluid environment of a nematic liquid crystal (NLC). Such a realization in a low molecular weight material which undergoes physical gelation is all the more advantageous since it can exploit the electrical switching, the concomitant birefringence change, etc. in the sol state as well have the structure frozen in the gel state. We report here such an attempt on a NLC doped with FePt nanoparticles and an organogelator by performing calorimetry, XRD, dielectric, elastic, rheological and magnetic characterization in the anisotropic sol and gel states. While the XRD data suggest that the orientational correlations of NLC are slightly strengthened by the presence of the particles and the gelator, the Frank bend elastic constant increases by two orders of magnitude indicating the mechanical rigidity of the gels. The magnetic measurements reveal that the superparamagnetic feature of the FePt particles is weakened in these ferrogels. This is surprising since it has recently been shown that a polymer ferronematic retains the ferromagnetic characteristics of the doped metal particle. These results suggest that the local environment of the particles has an important role in quenching the thermal fluctuations which in turn influences their magnetic interaction. Despite the bulk viscosity of the gelated system being high due to the trapping of NLC by the gelator, the immediate neighbourhood of the particles is still a fluid with enough orientational and translational freedom. This results in an ineffective quenching of the thermal fluctuations and consequently weak magnetic interactions.

Optical properties of hybrid titanium chevron sculptured thin films coated with a semiconducting polymer

Optical and structural properties of a hybrid metallic chevron sculptured thin film from titanium coated with the semiconducting polymer poly(3-dodecylthiophene) (P3DDT) are reported. The nanostructured thin film with two subsequent layers of oppositely slanted nanocolumns was fabricated by glancing angle deposition and coated with P3DDT by a spin-cast process. Spectroscopic generalized ellipsometry is employed to determine geometrical structure properties and the anisotropic optical constants of the coated and uncoated film in the spectral range from 400 to 1700nm. The nanostructured thin films before and after hybridization show highly anisotropic optical properties. The complex refractive indices along major polarizability directions of the hybridized chevrons are increased in the entire investigated spectral range with respect to the as-deposited chevrons. Changes in birefringence and dichroism upon polymer infiltration are observed.

A simple instrument for measuring the light-induced birefringence in a photorefractive crystal

We present a simple homodyne phase measurement instrument to evaluate a light-induced birefringence change in an Mg-doped congruent lithium niobate crystal by using an integrated lithium niobate Zn-indiffused phase modulator (ZIPM) for optical signal modulations.The results successfully demonstrate that the focused beam and modulated phase of the ZIPM can provide a simple pump-probe method for measuring the evolutions of light-induced birefringence changes. The detection of birefringence change at the level of 10−7 is lower than the conventional Senarmont compensation method. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:367–369, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25702

Quantification of Change in Axonal Birefringence Following Surgical Reduction in Intraocular Pressure

the purpose of this study was to examine the hypothesis that retinal nerve fiber layer (RNFL) birefringence increases following surgical reduction of intraocular pressure (IOP). twenty-six glaucomatous eyes requiring trabeculectomy or drainage implant were enrolled. Optical coherence tomography (OCT), scanning laser polarimetry (SLP), and IOP measurements were performed preoperatively and 3 months postoperatively. The OCT and SLP images were aligned using a new algorithm that aligns the vessels in an OCT image to those in the corresponding SLP reflectance image. The SLP retardance values at the location of the OCT scan circle were then extracted using the OCT scan circle position inferred by the algorithm. Sixty-four corresponding RNFL segments were extracted from SLP and OCT to calculate RNFL birefringence. A significant birefringence change was defined as 1.96 times the weighted test-retest standard deviation in four contiguous segments. preoperative IOP (19.3 ± 6.1 mm Hg) was significantly (P < .001) lower than postoperative IOP (10.4 ± 3.7 mm Hg). Average birefringence magnitude did not change (P = .19) postoperatively. Localized birefringence magnitude increased significantly in 6 (23%) eyes and decreased significantly in 7 (27%) eyes. in this cohort, variable changes in localized birefringence were observed following surgical reduction of IOP.

Monitoring Method of Stray Current in Coal Mine on Optical Fiber Sensing Technique

In this work a novel monitoring method was developed to measure the underground stray current in coal mine with fiber current sensing technique. According to the theory of optical fiber current sensor based on Faraday magneto-optic effect, a fiber sensing system was designed and the relative experiments with current generator simulating stray current were completed. The results indicate that the measurement was influenced by birefringence and temperature change. The sensing heads with high cylindrical birefringence optical fiber had a good suppressing effect on linear birefringence and the polarization state of polarized light was also more stable than that with low birefringence optical fiber. The fiber current sensing method is doable in underground stray current monitoring system in coal mine.

Rheological properties and phase transition of cornstarches with different amylose/amylopectin ratios under shear stress

AbstractThe rheological properties and phase transition of cornstarch with low moisture content under shear stress was investigated using a mixer with twin‐roller rotors by the variation of torque as functions of temperature and time. Cornstarches with different AM/AP ratios (wx: 4:96, maize: 29:71, Gelose 50: 62:38, and Gelose 80: 77:23) were used as model materials in the experimental work. The results showed that, as the AM content increased from 0 to 77%, the maximum torque increased from about 54 to 66 N/m and the final torque increased from about 27 to 47 N/m, respectively, and a longer mixing time was required to achieve a stable state (constant torque value). Microscopic and differential scanning calorimetry (DSC) studies showed that the phase transition reflected by changes of birefringence and endotherms could well be associated with the torque curve measured by the mixer. The higher gelatinization temperatures of the starches with higher AM content, and especially the existence of multi‐phase transitions at high temperatures, could explain the rheological behaviors detected by the mixer. The technique can be used to study the starch melting and destructuration under shear stress with low moisture content (&lt;45%), which represents the most of extrusion conditions for starch‐based materials.

Specific Heat and Linear Birefringence in Incommensurate Phase of NH4HSeO4 Crystal

Precise measurements of the specific heat and linear birefringence changes were performed for the NH4HSeO4 crystal. Phase transitions at about 100 K, 252 K and 260 K were confirmed. The continuous second order phase transition at 260 K to the incommensurate phase has been described by the 3D XY model. Thermal parameters (as the excess enthalpy ΔH, and the excess entropy ΔS) of the N-IC transition were estimated. The critical index ≈ 0.832 was determined below Ti from the linear birefringence increment. Hysteretic behaviour of the linear birefringence in the incommensurate phase was observed at various temperature rates.

Photoinduced effects in TiO 2 nanocrystalline films with different morphology

The complex studies of photoinduced absorption, second harmonic generation and third harmonic generation were performed for TiO 2 films of different morphology. In particular we have studied the influence of a bicolor laser beam treatment by a 300 mW green cw laser emitting at 532 nm on changes of absorption, birefringence and third harmonic generation on TiO 2 films. We have performed the corresponding measurements using as a photoinducing light a glass erbium 10 ns pulsed laser with a fundamental wavelength of about 1540 nm together with its second harmonic generation (SHG) at 770 nm using the method of bicolor laser treatment and a frequency repetition of about 10 Hz. Varying the power density ratios between the fundamental and its SHG we have established the optimal conditions to achieve maximal photoinduced changes of absorption, birefringence and third harmonic generation (THG). We have detected changes of the birefringence and of the THG immediately after the laser treatment. We study an influence of the nanoparticle's sizes on the observed changes of the optical features. Additionally we have studied the role of different chemical treatment on the structural and optical parameters.

Crystal growth and spectroscopic studies of novel Yb-doped K5Nd(MoO4)4 single crystals

K5Nd(MoO4)4 crystals with different Yb3+ concentrations were grown using Czochralski technique. Room-temperature absorption spectra were recorded and assigned on the basis of Dieke's diagram for Nd3+ ion; the standard Judd–Ofelt theory has been used to analyze the spectra. Increase of Yb3+ concentration leads to variation of the corresponding Judd–Ofelt intensity parameters. Significant contribution of the Yb↔Nd energy transfer into the formation of the Nd3+ absorption spectra causes the observed changes. After illumination of the crystals with CW Nd:YAG laser changes of the birefringence for the laser line of CW He–Ne laser at 633nm were studied. Birefringence changes show good correlation with the content of Yb ions.

Real-time quantitative analysis of lipid disordering by aurein 1.2 during membrane adsorption, destabilisation and lysis

Effective antimicrobial peptides (AMPs) distinguish between the host and microbial cells, show selective antimicrobial activity and exhibit a fast killing mechanism. Although understanding the structure–function characteristics of AMPs is important, the impact of the peptides on the architecture of membranes with different lipid compositions is also critical in understanding the molecular mechanism and specificity of membrane destabilisation. In this study, the destabilisation of supported lipid bilayers (SLBs) by the AMP aurein 1.2 was quantitatively analysed by dual polarisation interferometry. The lipid bilayers were formed on a planar silicon oxynitride chip, and composed of mixed synthetic lipids, or Escherichia coli lipid extract. The molecular events leading sequentially from peptide adsorption to membrane lysis were examined in real time by changes in bilayer birefringence (lipid molecular ordering) as a function of membrane-bound peptide mass. Aurein 1.2 bound weakly without any change in membrane ordering at low peptide concentration (5 μM), indicating a surface-associated state without significant perturbation in membrane structure. At 10 μM peptide, marked reversible changes in molecular ordering were observed for all membranes except DMPE/DMPG. However, at 20 μM aurein 1.2, removal of lipid molecules, as determined by mass loss with a concomitant decrease in birefringence during the association phase, was observed for DMPC and DMPC/DMPG SLBs, which indicates membrane lysis by aurein. The membrane destabilisation induced by aurein 1.2 showed cooperativity at a particular peptide/lipid ratio with a critical mass/molecular ordering value. Furthermore, the extent of membrane lysis for DMPC/DMPG was nearly double that for DMPC. However, no lysis was observed for DMPC/DMPG/cholesterol, DMPE/DMPG and E. coli SLBs. The extent of birefringence changes with peptide mass suggested that aurein 1.2 binds to the membrane without inserting through the bilayer and membrane lysis occurs through detergent-like micellisation above a critical P/L ratio. Real-time quantitative analysis of the structural properties of membrane organisation has allowed the membrane destabilisation process to be resolved into multiple steps and provides comprehensive information to determine the molecular mechanism of aurein 1.2 action.

Tunable and rotatable polarization controller using photonic crystal fiber filled with liquid crystal

We design and fabricate a compact tunable and rotatable polarization controller using liquid crystal photonic band gap fibers. The electrically and thermally induced phase shift in the Poincaré sphere and corresponding birefringence change are measured. The direction of the electric field is managed by connecting four electrodes in different electrode configurations, and the thermal tunability is controlled by on-chip heaters. According to the results, a quarter-wave plate and a half-wave plate working in the wavelength range of 1520–1600 nm are experimentally demonstrated.

Evolution of Statistic Moments of 2D-Distributions of Biological Liquid Crystal Net Mueller Matrix Elements in the Process of Their Birefringent Structure Changes

This research is aimed to investigate the reliability of Mueller-matrix differentiation of birefringence change of optically thick layers of biological liquid crystals at the early stages of the change in their physiological state. This is performed by measuring the set of skewness and kurtosis values of Mueller matrix image of the phase element M44 in various points of the object under investigation.

High pressure sensor based on photonic crystal fiber for downhole application

We demonstrate a polarization-maintaining (PM) photonic crystal fiber (PCF) based Sagnac interferometer for downhole high pressure sensing application. The PM PCF serves as a direct pressure sensing probe. The sensor is transducer free and thus fundamentally enhances its long-term sensing stability. In addition, the PM PCF can be coiled into a small diameter to fulfill the compact size requirement of downhole application. A theoretical study of its loss and birefringence changes with different coiling diameters has been carried out. This bend-insensitive property of the fiber provides ease for sensor design and benefits practical application. The pressure sensitivities of the proposed sensor are 4.21 and 3.24 nm/MPa at ∼1320 and ∼1550 nm, respectively. High pressure measurement up to 20 MPa was achieved with our experiment. It shows both good linearity in response to applied pressure and good repeatability within the entire measurement range. The proposed pressure sensor exhibits low temperature cross sensitivity and high temperature sustainability. It functions well without any measurable degradation effects on sensitivity or linearity at a temperature as high as 293 °C. These characteristics make it a potentially ideal candidate for downhole pressure sensing.