Birefringent Phase Research Articles

High-sensitive measurements of refractive-index and magnetic-field based on liquid-infiltrated photonic crystal fiber with an elliptically-shaped core

A high-sensitive refractive index (RI) measuring method is proposed through the combined use of elliptical core photonic crystal fiber (PCF) and Sagnac interferometer. The sensing performance of the designed structure are studied theoretically using the finite element method. We analyze the phase birefringence and group birefringence to obtain the dependence of RI sensitivity on the operable wavelength λ. The RI sensitivity can be enhanced 10–20× by introducing a liquid-filled elliptical hole into the fiber core. Additionally, the single-core structure can convert to twin-core case just through decreasing the RI of filling liquid, and the spectral response is adjustable by changing main structural parameters. The simulation results show that the average sensitivity is about 1,14,967 nm/RIU within the RI range of 1.463–1.467, which corresponds to a resolution of 1.74 × 10−6 RIU. Finally, the sensing method is applied to magnetic-field measurement as an example, and the magnetic-field sensitivity can reach up to 3.45 $${\text{nm/Oe}}$$ within the range of 89.9–271.0 $${\text{Oe}}$$ . The measurement principle also applicable to other sensing fields, and the proposed structure is insensitive to fiber bending without fabrication complexity.

Osmotic-Pressure-Induced Nematic Ordering in Suspensions of Laponite and Carboxy Methyl Cellulose.

Laponite is a synthetic clay that is known to form gels in aqueous suspensions at low concentrations (0.01 g/cm3). Although it is expected to form lyotropic liquid crystals, such phases usually do not form, as a consequence of laponite's tendency to form gels at concentrations below the threshold for liquid crystal formation. Here we show that macroscopic, birefringent phases of laponite can be prepared through osmotic compression of a laponite solution by an aqueous solution of carboxy methyl cellulose (CMC). We present polarization imaging studies showing how the initially dilute, isotropic laponite phase shrinks while developing typical birefringence colors between crossed polarizers. Using the Michel-Lévy interference charts, we were able to extract the refractive index and orientation of the laponite nanodisks in the compressed region. Our observations allow us to propose a tentative state diagram, indicating the concentration regions for which we obtain optically anisotropic gels.

Birefringent large-mode-area anti-resonant hollow core fiber in the 1.9 µm wavelength window.

We report on the development and characterization of a birefringent large-mode-area anti-resonant silica fiber. The fiber structure is composed of six non-touching capillaries. The birefringence results from the breaking of the circular symmetry of an air core with increasing of the diameters of two capillaries located across the fiber diameter. We depart from earlier designs of polarizing hollow core fibers, in which coupling of the guided modes was intentionally facilitated with the cladding layout. Instead, with the help of numerical simulations, we enhance birefringence in our design by varying the capillary wall thickness between the larger- and smaller-diameter capillary sections of the cladding. The fiber has a large, elliptical core with semi-axes of ∼55 and 41 µm in diameter, an effective area of the fundamental mode of 1200µm2, and a total outer diameter of 127 µm. The cladding is composed of two pairs of smaller capillaries, which are 18 µm in diameter with 1.66 µm thick walls, and two larger capillaries with a 24 µm diameter and 1.14 µm thick walls, located across the diagonal of the fiber. Measured group birefringence over 1820-1920 nm wavelengths is monotonically increasing from 0.4×10-4 to 2.0×10-4, while its phase birefringence is from 5×10-6 to 1.1×10-5. Despite this, the fiber holds polarization with a 12 dB polarization extinction ratio at 1900 nm over a 1.5 m long sample.

Liquid crystal lens with doping of rutile titanium dioxide nanoparticles.

A 4 mm-aperture hole-patterned liquid crystal (LC) lens has been fabricated using a LC mixture, which consisted of rutile titanium dioxide (TiO2) nanoparticles (NPs) and nematic LC E7, for the first time. The TiO2 NP dopant improves the addressing and operation voltages of the LC lens significantly because it strengthens the electric field surrounding the TiO2 NP and increases the capacitance of lens cell. Unlike the doping of common colloidal NPs, that of rutile TiO2 NPs increases the phase transition temperature and birefringence of the LC mixture, thereby helping enhance the lens power of LC lens. In comparison with a pure LC lens, the TiO2 NP-doped one has approximately 50% lower operation voltage because of the strengthened electric field around the NPs and has roughly 2.8 times faster response time because of the decreased rotational viscosity of the LC mixture and the increased interaction between the LC molecules by the NP dopants. Notably, the doping of rutile TiO2 NPs improves the operation voltage, tunable focusing capability, and response time of LC lens simultaneously. Meanwhile, this method does not degrade the focusing and lens qualities. The imaging performances of TiO2 NP-doped LC lens at various voltages are demonstrated practically by tunable focusing on three objectives at different positions. These results introduce NP in the application of LC lenses.

Dual‐Wavelength Focusing through Fresnel Zone Plate Fabricated in Lithium Niobate Crystal by Femtosecond Laser Micromachining

Herein, a simple method is proposed that realizes the dual‐wavelength focusing of second‐harmonic (SH) and fundamental frequency (FF) through amplitude‐type nonlinear Fresnel zone plate (FZP). The FZP is fabricated by femtosecond laser micromachining with 0 or +1 binary modulation of quadratic susceptibility in lithium niobate crystal. The evolution of the SH beam focusing intensity patterns with 2 cm focal length at 532 nm wavelength is theoretically and experimentally studied. Based on the birefringence phase matching, the measured normalized conversion efficiency for peak power of 1.9 × 10−5% W−1 cm−2 is achieved in the SH focal spot. The amplitude‐type nonlinear FZP realizes the focusing of the FF beam and SH beam simultaneously, which opens wide possibilities in the integration of laser beam shaping, nonlinear frequency conversion, and laser focusing for ultra‐compact on‐chip device requirement.

Second harmonic generation in femtosecond laser written lithium niobate waveguides based on birefringent phase matching

We report on second harmonic generation (SHG) through birefringent phase matching in femtosecond (fs) laser fabricated lithium niobate (LiNbO3) waveguides. Based on end-face coupling system, the guiding properties (mode profiles and propagation losses) of these waveguides are studied at 1064 nm. The effects of guiding properties on SHG of 1064 nm are investigated in detail. The maximum conversion efficiency of 0.87% is acquired in a 10-mm-long depressed-cladding waveguide at room temperature. Our results pave a way for constructing novel frequency converters based on fs-laser written LiNbO3 waveguides.

Birefringent Anti-Resonant Hollow-Core Fiber

Hollow-core fibers have demonstrated record performance in applications such as high-power pulse delivery, quantum computing, and sensing. However, their routine use is yet to become reality. A major obstacle is the ability to maintain the polarization state of light over a broad range of wavelengths, while also ensuring low attenuation and single-mode guidance. Here we simulated, fabricated and characterized a single-mode polarization-maintaining anti-resonant hollow-core fiber. The birefringence was achieved by introducing resonators of different thicknesses, thereby creating reduced symmetry in the structure. The measured group birefringence of 4.4x10-5 at 1550 nm is in good agreement with the calculated group birefringence from the simulations. This corresponds to a phase birefringence of 2.5x10-5 at 1550 nm. The measured loss of the fiber was 0.46 dB/m at 1550 nm. With its simple structure, low loss, and broadband operation this polarization-maintaining anti-resonant hollow-core fiber is a serious contender for applications in gas-based nonlinear optics and communications.

Highly Birefringent, Low-Loss, and Near-Zero Flat Dispersion ENZ Based THz Photonic Crystal Fibers

In this paper, we present and compare two experimentally feasible photonic crystal fiber (PCF) designs ( $\textit{Type-I}$ and $\textit{Type-II}$ ) which ensure near-zero flattened dispersion with ultra-high phase and group birefringence at THz frequencies. Both structures are based on a subset of a triangular array of circular air-holes, which define the cladding of the PCF and a central elliptical air-hole which breaks the symmetry of the structure, thus introducing high levels of birefringence. Additionally, we investigate the possibility of further enhancing the birefringence properties of $\textit{Type-II}$ structure by selectively filling the air-holes with Potassium Chloride (KCl) as strong Epsilon-Near-Zero (ENZ) material. Our investigation reveals that significant enhancement of birefringence can be achieved than its original counterpart with birefringence to be as high as 0.0627 at 6.2 THz and near-zero flat dispersion of $-$ 0.54 $\pm$ 0.04 ps/THz/cm over the frequency range of 6.2 $-$ 6.3 THz.

Polarization-maintaining optical fiber with an anisotropic core compatible with the SMF-28 standard

We present the theoretical study of an all-solid highly birefringent fiber with an anisotropic core fully compatible with SMF-28 fiber and without use of external stress zones or air holes. We propose a fiber with a core composed of interleaved subwavelength layers of silica and germanium-oxide-doped silica. The optimized nanostructured fiber has a phase birefringence of 1.42 × 10 − 4 , and an effective mode area and numerical aperture similar to SMF-28 fiber. We predict coupling losses of 0.1 dB for bidirectional coupling with the SMF-28 fibers.

Polarization holographic microscope slide for birefringence imaging of anisotropic samples in microfluidics.

Birefringence is an important optical property of anisotropic materials arising from anisotropies of tissue microstructures. Birefringence parameters have been found to be important to understand optical anisotropic architecture of many materials and polarization imaging has been applied in many researches in the field of biology and medicine. Here, we propose a scheme to miniaturize a double-channel polarization holographic interferometer optics to create a polarization holographic microscope slide (P-HMS) suitable for integrating with microfluidic lab-on-a-chip (LoC) systems. Based on the P-HMS combined with a simple reconstruction algorithm described in the paper, we can not only simultaneously realize holographic imaging of two orthogonal polarization components of dynamic samples in a microfluidic channel but also quantitative measurement of 2D birefringence information, both including the birefringence phase retardation and optic-axis orientation. This chip interferometer allows for off-axis double-channel polarization digital holographic recording using only a single illumination beam without need of any beam splitter or mirror. Its quasi-common path configuration and self-aligned design also make it tolerant to vibrations and misalignment. This work about the P-HMS could play a positive role in promoting the application of birefringence imaging in microfluidic LoC technology.

Polarization contrast optical diffraction tomography.

We demonstrate large scale polarization contrast optical diffraction tomography (ODT). In cross-polarized sample arm detection configuration we determine, from the amplitude of the optical wavefield, a relative measure of the birefringence projection. In parallel-polarized sample arm detection configuration we image the conventional phase projection. For off-axis sample placement we observe for polarization contrast ODT, similar as for phase contrast ODT, a strongly reduced noise contribution. In the limit of small birefringence phase shift δ we demonstrate tomographic reconstruction of polarization contrast images into a full 3D image of an optically cleared zebrafish. The polarization contrast ODT reconstruction shows muscular zebrafish tissue, which cannot be visualized in conventional phase contrast ODT. Polarization contrast ODT images of the zebrafish show a much higher signal to noise ratio (SNR) than the corresponding phase contrast images, SNR=73 and SNR=15, respectively.

Diamond-blade diced trapezoidal ridge waveguides in YCOB crystal for second harmonic generation

Trapezoidal ridge waveguides have been fabricated in YCOB nonlinear optical crystals by carbon ion irradiation and precise diamond-blade dicing. The diced ridges with smooth side-walls allow for near-infrared (1064 nm) light guiding with propagation losses around 1 dB/cm. Refractive index profile of a waveguide has been reconstructed in a reasonable manner. Green second harmonic light have been generated at room temperature via type I birefringent phase matching. Under the pump of continuous and pulsed lasers, conversion efficiencies for guided-wave frequency doubling can be up to ~1.10% W−1 and ~6.22%, respectively.

A Method for Estimating the Crustal Azimuthal Anisotropy and Moho Orientation Simultaneously Using Receiver Functions

AbstractThe P‐to‐S converted phase at the Moho (the Pms phase) contains important information regarding the crustal structure. Here, we employ a grid search approach to simultaneously extract the bulk crustal azimuthal anisotropy and Moho orientation using receiver functions. The solution of this approach is the weighted average of two individual subfunctions that separately analyze the birefringence and arrival time of the Pms phase. Since a tilted Moho deflects the polarized components of the Pms phase, the Pms birefringence subfunction minimizes the energy on all the transverse polarization components (i.e., the transverse polarization direction before Pms splitting). The implementation of this subfunction is similar to that of the transverse component minimization method, but the Moho orientation is considered. The Pms arrival time subfunction takes into account both the crustal anisotropy and the Moho orientation to predict and fit Pms arrivals. According to extensive synthetic experiments, this approach is remarkably robust to the presence of strong anisotropy, a steep Moho, and slightly tilted fast axes (less than 30°). This approach is also highly tolerant of random noise and data with uneven azimuthal coverage. We apply this approach to three permanent stations on the Tibetan Plateau. The resulting measurements boast a high quality and are consistent with the crustal anisotropy and Moho orientation revealed in previous studies.

Fast and direct measurement of the linear birefringence profile in standard single-mode optical fibers.

Phase birefringence in optical fibers typically fluctuates over their length due to geometrical imperfections induced from the drawing process or during installation. Currently commercially available fibers exhibit remarkably low birefringence, prompting a high standard for characterization methods. In this work, we detail a method that uses chirped-pulse phase-sensitive optical time-domain reflectometry to directly measure position-resolved linear birefringence of single-mode optical fibers. The technique is suitable for fiber characterization over lengths of tens of kilometers, relying on a fast measurement ($ {\sim} 1\,\, {\rm s} $∼1s) with single-ended access to the fiber. The proposed method is experimentally validated with three different commercial single-mode optical fibers.

Lipid-based lyotropic liquid crystalline phase transitions as a novel assay platform using birefringence as the visual signal output.

Amphiphilic lipids can often form various lipid-based lyotropic liquid crystalline phases when they are exposed to aqueous environments, which includes the inverse hexagonal (H2), inverse cubic (V2), inverse discontinuous cubic (I2) and lamellar (Lα) phases. Their different interactions with crossed-polarised light make some phases appear bright and some dark, which offers great potential in developing a novel universal assay platform using birefringence as the visual signal output. Here, we have developed a novel strategy for constructing an assay platform using lyotropic liquid crystalline phases as the signal transducer and using changes in their birefringence upon exposure to lipase as the signal output. The hydrolysis of the ester group of glyceryl monooleate (GMO) by lipase induced a phase transition from cubic phase to hexagonal phase and consequently "turned on" the birefringence as the signal output. The change in the intensity of the birefringence depended on both the exposure time and concentration of lipase. The streptavidin-biotin affinity was utilised to demonstrate the potential of the birefringence assay platform, using free biotin as the model analyte, biotinylated lipase and streptavidin-coated magnetic beads in the competitive format. A semi-quantitative assay with a detection limit of 5 μg mL-1 to free biotin as the model analyte was achieved in the visual birefringence mode. This study demonstrated a proof-of-concept lyotropic assay platform with birefringence as the visual signal output that could be deployed as an electronics- and colour-free diagnostic device for a wide range of applications, for example to indicate the presence of toxins in water.

Birefringence and photoluminescence properties of diphenylacetylene-based liquid crystal dimers

We herein report phase transitions, mesomorphism, birefringence behavior and photoluminescence properties of symmetric liquid crystal (LC) dimers based on diphenylacetylene or tolane.

Electro-optical, dielectric and optical properties of graphene oxide dispersed nematic liquid crystal composites

ABSTRACT Graphene oxide (GO) in various concentrations (0.04 wt.%, 0.1 wt.% and 0.3 wt.%) was dispersed into nematic liquid crystal (LC) 5CB and studied for electro-optical, dielectric and optical responses. The textural and phase transition studies were investigated by optical polarising microscopy in the nematic phase. The effect of GO concentration on the micro-texture, phase transition temperatures, birefringence, threshold voltage, contrast ratio, dielectric permittivity, dielectric loss and conductivity was also investigated. The nematic–isotropic transition temperature slight increases (~3%) in the 0.04 wt.% and 0.1 wt.% GO dispersed 5CB composites as compared to 5CB. Threshold voltage decreases in the GO–5CB composites with the increase of GO concentration. Additionally, the effect of GO concentration on the optical absorbance of UV–visible light and energy bandgap is also considered.

A new methane sensor based on compound film-coated photonic crystal fiber and Sagnac interferometer with higher sensitivity

A modified D-shaped photonic crystal fiber (PCF) based on Sagnac interferometer is proposed for methane concentration sensing. Two ultra-large air holes with methane-sensitive film are distributed horizontally in the PCF to shorten response time. Besides, a defect hole is introduced into the fiber core and theoretical analysis is carried out to enhance the sensing sensitivity. The dependence of gas sensitivity on the operable wavelength λ is obtained by analyzing the phase birefringence B(λ,C) and group birefringence Bg(λ,C). The average gas sensitivity is as high as 36.64 nm/\\% in the sensing range of 0–3.5%. The modified D-Shaped and highly sensitive PCF sensor exhibits immunity to cross-sensitivity, and provides a new design idea for methane gas monitoring.

Birefringence of nonlinearity in all-normal dispersion photonic crystal fibers

We investigate the feasibility of controlling nonlinearity of an optical fiber by changing the polarization of in-coupled light. The proposed fiber structure has a nanostructured core composed of a stack of sub-wavelength stripes made from thermally matched soft glasses with different refractive indices. Numerical simulation results show both strong phase birefringence (of around 10–3) and small effective mode area, which is around 7.5 μm2 at 2000 nm and is weakly dependent on wavelength. Designed structure further shows different nonlinear coefficients for each of the orthogonally polarized guided modes. The fiber has engineered, flat normal dispersion characteristic for both principal polarization axes and its fabrication would concern use of standard, robust and commercially available soft glasses and combination of two technologically proven structured fiber drawing approaches.

Sensing characteristics of a Sagnac interferometer based on ethanol-filled photonic crystal fiber with two ultra-large holes for temperature measurement

We propose a Sagnac interferometer (SI) based on ethanol-filled photonic crystal fiber (PCF) with two ultra-large holes for temperature measurement. The phase birefringence of the fully-filled PCF varies significantly with the temperature which is realized by infiltrating ethanol into air-holes. Two ultra-large holes in y direction near the core result in a high birefringence of PCF. By selecting a fiber length of 2.5 cm, the average sensitivity of temperature can be up to -34 nm/°C with a resolution of 0.00029 °C in the wavelength range of 780 nm–2170 nm within temperature variation of 40 °C. The proposed PCF with the simple structure is easy to prepare in the experiment. Furthermore, due to the ultrahigh-sensitivity, the proposed sensor provides a new implementation for accurately temperature measurement.