Bending-induced Birefringence Research Articles

Polarizing Grating-Based Ultrasensitive All-Fiber Angular Displacement Sensor

We have proposed and experimentally demonstrated an ultrasensitive all-fiber angular displacement sensor based on in-fiber polarizing gratings and half waveplate (IFHWP), in which the polarizing gratings are based on 45°-tilted fiber gratings (45°-TFGs) and the IFHWP is constructed by bending-induced birefringence in single mode fiber (SMF). Compared with the existing angular displacement sensors, the structure of our proposed sensor is more desirable to bring the prominent advantages of all-fiber structure, low transmission loss and high sensitivity. The experimental results show that the proposed angular displacement sensor has a sensitivity and a resolution up to 20.477 dB/rad and $2.8\times 10^{-3\circ }$ , respectively, within a large measurement range of about 25°. In addition, such sensor also has very low temperature sensitivity around $2.1\times 10^{-4}$ dB/°C.

Measurement of bending-induced birefringence in a hollow-core photonic crystal fiber.

We report the measurement of bending-induced birefringence in the presence of large intrinsic birefringence in a hollow-core photonic crystal fiber (HC-PCF). The fast axis of bending-induced birefringence was found to be normal to the bending plane, in contrast to the conventional fiber case. The dependence of the induced birefringence on the bending radius was also different from the typical inverse square law. Possibilities and design criteria for polarization controllers using bending-induced birefringence in HC-PCFs are presented.

Analysis of Bending-Induced Degradation of Orbital Angular Momentum Modes in Optical Fibers

In this work, bending-induced deterioration of orbital angular momentum (OAM) modes in ring core fiber (RCF), photonic crystal fiber (PCF), and vortex fiber (VF) was theoretically investigated: Bending losses, coupling losses, and intermodal crosstalk at the interface between straight and bent optical fibers were investigated from the modal analysis of those three types of OAM mode fibers. In addition, the degradation of a topological charge number of an OAM mode due to the bending-induced birefringence and horizontal mode asymmetry was also investigated. Our investigation revealed that, in all aspects, the PCF is most robust to bending among the three types of optical fibers, and the most serious bending-induced problem in the VF and the RCF is the degradation of the topological charge number. The allowed minimum bending radii of VF and RCF appeared to be ~15 and ~45 mm, respectively, for the specific structures considered in this work. We expect that the methodology and results of our quantitative analysis on bending-induced degradation of OAM modes will be of great use in the design of OAM mode fibers for practical use.

Combined Bending and Torsion Sensing by Induced Birefringence in Distributed Bragg Reflector Laser

Measurement of torque or twist angle and mechanical bending is important in several fields such as structural health monitoring, movement analysis, robotics instrumentation, and mechanical systems in general. Several optical fiber sensors have been already proposed to measure these parameters separately. However, their high-cost interrogation systems are the main drawback for their large commercial application. Distributed Bragg reflector (DBR) allows to overcome this issue since its inherent heterodyne behavior leads to interrogation schemes in the frequency domain instead optical domain reducing significantly the interrogation systems cost. In this context, this paper presents a DBR to measure twist angle up to 180°. The enhancement of twist sensitivity by bending-induced birefringence is analytically and experimentally demonstrated achieving a value of $-$ 2.95 MHz/rad. With a constant bending angle of 20°, a twist sensitivity of $-$ 32.73 MHz/rad is achieved, which represents a enhancing sensitivity by a factor of $\sim$ 7, when compared to some DBR sensor configurations reported in the literature.

High-Sensitivity Temperature Sensor Based on Polarization Maintaining Fiber Sagnac Loop

A high-sensitivity all-fiber temperature sensor based on a Sagnac interferometer is demonstrated by splicing a section of polarization maintaining fiber (PMF) between two sections of standard single mode fibers (SMFs). In this sensor, the SMF-PMF-SMF structure in the Sagnac loop is bent into a circle to enhance the sensitivity. The length and curvature of the PMF in the loop are investigated and can be optimized to further increase the temperature sensitivity of the sensor. Results show that the radius of the circle has an important effect upon temperature sensitivity due to the bend-induced birefringence variation of the PMF. The SMF-PMF-SMF structure bent into a circle with a radius of 30 mm exhibits a high-sensitivity temperature of 1.73 nm/°C. The sensor is provided with the advantages of easy fabrication, low-insertion loss, and high sensitivity, which may find potential applications in the field of high precision temperature measurement.

Distributed polarization analysis with binary polarization rotators for the accurate measurement of distance-resolved birefringence along a single-mode fiber.

We describe a first distributed polarization analysis (DPA) system using binary polarization rotators in an optical frequency domain reflectormeter (OFDR) capable of measuring the variations of polarization states along a single-mode fiber (SMF). We demonstrate using such a DPA system to accurately measure the distance-resolved birefringence with 12 fiber loops of different radii with different birefringence values along a length of SMF and obtain a bending-induced birefringence coefficient (BBC) of 6.601 × 10-10 m2, agreeing well with the theoretically estimated value of 5.334 × 10-10 m2. To further verify the measurement accuracy, we obtain the birefringence values of the 12 fiber loops of different radii one at a time using a non-distributed polarization analysis system with an accuracy traceable to a birefringence standard made with a quartz crystal, and obtain a BBC value of 6.490 × 10-10 m2, agreeing well with our distributed measurement with a relative error of only 1.68%. In addition, we measure the residual birefringence of the SMF with both distributed and non-distributed polarization analysis systems and obtain similar results with a relative error of only 0.59%. Our experiments not only validate the performances of our DPA system, but also the first to use DPA to experimentally obtain the accurate birefringence values along the SMF and verify the theory of bending-induced birefringence. Our work further proves that such an OFDR-based DPA system is a practical tool for optical component characterization, nondestructive optical material inspection, and distributed fiber optic transversal stress sensing.

Bending effect characterization of individual higher-order modes in few-mode fibers.

We propose an approach to directly measure the bending effect on individual modes in few-mode fibers using a wavelength-scanning spatially and spectrally resolved imaging technique. By collecting a fiber output beam profile with a scanning wavelength at different bending diameters and analyzing the peaks of its Fourier transformation, we have distinguished higher-order modes (HOMs) of the fiber and investigated their group delay, beam profile, bend loss, and mode splitting individually. We have experimentally verified with a multilayer core fiber at 1-8cm bending diameters that its HOMs experience more loss compared to lower-order modes, but delay at approximately the same speed as the bending diameter decreases. Mode splitting of the LP11 mode at a small bending diameter due to bending-induced birefringence is also observed and discussed. This method provides an efficient tool to study the bending effect on individual HOMs in fibers and could be extended to studying fiber stretching and twisting.

Novel configuration for an enhanced and compact all-fiber Faraday rotator with matched birefringence.

We propose a novel configuration for an improved and compact all fiber Faraday rotator based on phase matching between the Faraday rotation and bend-induced birefringence. The device utilizes a coiled fiber within two electro-magnetic toroids, such that the fiber length required for getting the beat length is quite long and several rounds of fiber are needed. Analysis of the capabilities of the proposed device and its sensitivity to different parameters is presented. Faraday rotation of 13° was experimentally measured in six meters of single mode silica fiber, with a magnetic field of about 0.06T at a wavelength of 1064nm. We show that phase matching between the two phenomena significantly improves the polarization rotation by a factor of 4-10. In addition, we demonstrate the ability to achieve higher rotation by using Fabry Perot resonator in low terbium doped glass.

Helical Fiber Interferometer Using Flame-Heated Treatment for Torsion Sensing Application

An interferometer using a single-mode fiber helix (SFH) was fabricated by the flame-heated treatment for torsion sensing application. For the first time to the best of our knowledge, this kind of interferometer was designed and experimentally demonstrated. By launching a linear polarized beam into the SFH, the interference was formed at the output of the structure because of the fiber bending-induced birefringence. Theoretical analysis shows that the geometrical change will bring a resonant dip wavelength shift when torsion is applied to the sensor part. Additionally, by adjusting the helix radius and pitch, the torsion sensitivity can be further optimized. From the experiment data, the torsion sensitivity reaches 1.691 nm $\cdot \text{m}$ /rad from −9.39 to 13.42 rad/m for the SFH with a radius of 0.89 mm and a pitch of 2.04 mm.

Nonplanar fiber-optic sensing head for the compensation of bending-induced birefringence in Faraday current sensors.

We demonstrate the compensation of bending-induced linear birefringence in single-mode fibers coiled in a nonplanar path by alternating orthogonal bending planes. This effect can be applied for the construction of birefringence-free fiber coils in Faraday sensor heads (e.g., in current sensors) to improve their sensitivity. Validation experiments are presented.

Polarization-resolved imaging of an ensemble of waveguide modes

We demonstrate polarization-sensitive measurement of the modal content of waveguides by generalizing the classic rotating wave-plate-based polarimeter to wide-field optical low-coherence interferometry. The spatial phases of the modes are retrieved with principal component analysis. By applying this polarization-sensitive cross-correlation (C2) imaging technique to the characterization of a few-mode fiber, we reveal that different modes experience distinct bend-induced birefringence in optical fibers. This polarization-resolved C2 imaging is well suited for analyzing the impact of polarization on wave propagation in high-power fiber lasers as well as in mode-division-multiplexed communications systems.

One Degree of Freedom Fiber Ring Depolarizer

A single fiber ring depolarizer with one degree of freedom of adjustment is proposed by setting the bend-induced birefringence in the fiber ring as a half-wave plate and choosing proper coupling coefficient of the coupler. Depolarization can be realized by just rotating the whole set of the fiber ring. A 12-m fiber ring depolarizer is demonstrated to depolarize a Fabry-Pe¿rot laser with 10-m coherent length, and a degree of polarization of 1.5% is obtained.

Bending-induced birefringence of optical fiber cladding modes

The bend-induced lateral stresses that generate birefringence in an optical fiber are analytically determined throughout the fiber cross section. This stress profile is used to determine the birefringence of a cladding mode in a single-mode fiber. The result demonstrates that the birefringence of cladding modes may be much smaller than that of the core mode and knowledge of the mode profile and stress distribution is necessary for an accurate prediction.

Cancellation of bending-induced birefringence in single-mode fibers: application to Faraday sensors

We demonstrate that one can cancel the bending-induced linear birefringence in single-mode fibers by inducing a controlled anisotropy in a direction orthogonal to the bending plane. In particular, the controlled anisotropy can be generated by application of a lateral compressive stress on the fiber. This effect can be applied for the construction of birefringence-free fiber coils in Faraday sensor heads (e.g., in current sensors) to improve their sensitivity.

A standard fiber-based loop mirror as a gain-flattening filter for erbium-doped fiber amplifiers

We report flattening of the amplified spontaneous emission (ASE) spectrum from an erbium-doped fiber (EDF) using a standard fiber-based loop mirror (FLM), realized with an "over-coupled" fiber coupler. Through simulation, a bend-induced birefringent FLM as a gain-flattening filter is designed. Subsequently, an over-coupled coupler with a free spectral range of 140 nm is fabricated to realize the FLM, which is integrated to an EDF that is pumped by a 980-nm laser diode. By introducing an appropriate amount of bend-induced birefringence in the loop, the ASE spectrum of the EDF could be flattened within /spl plusmn/0.5 dB over a wavelength range of 32 nm in the C-band.

Determination of bend mode characteristics in dielectric waveguides

A new and effective method for the electromagnetic analysis of generic bent dielectric waveguides is presented. The method is based on the expansion of the bend mode in modes of the straight waveguide and permits determination of the shape and the phase constant of the fundamental mode of the bend waveguide with great accuracy at a very low computational cost. Simple analytical expressions of the phase constant, coupling losses, and bending-induced birefringence are derived under very general conditions. The proposed method is useful for the design and optimization of each bent section of integrated optics components.

Highly Sensitive and Widely Dynamic Fiber-Coiled Deformation Sensor Immune from External Disturbances

This paper describes a double fiber-coiled interferometric deformation sensor fabricated using paired polarization maintaining single-mode fibers. A change in bending-induced birefringence plays a fundamental role in the sensor performance as the fiber-coil is being deformed across its cross-section. Differential optical heterodyne detection processes allow us to implement two-mode operations; one is widely dynamic operation and the other highly sensitive operation around a target region. The deformation sensitivity for one-turn fiber-coil is 2.5 deg/mm for the widely dynamic operation, whereas 9.5X10 deg/mm for the highly sensitive operation. The double fiber-coiled deformation sensor can eliminate external, undesirable temperature disturbances.

Suppression of coupling between orthogonally polarized modes in weakly anisotropic light guides when winding fiber

The goal of this paper is to demonstrate the possibilities of decreasing the coupling between orthogonally polarized modes in a weakly anisotropic single-mode light guide (SLG) when winding fiber. The coupling parameter h can, in principle, be decreased at the expense of increasing the bending-induced birefringence. Multimode light guides based on silicon rubber are also being developed. They allow stronger (compared to quartz fibers) bending without noticeable radiation losses. By creating a silicon rubber-based SLG, one can markedly reduce h if the fiber i wound on a moderate-diameter coil

Birefringence examination in a practical fiber-current-sensing system

The output of a fiber-current sensor with a nonideal input (either misaligned linear or elliptical polarization) and moderate output misalignment in the setup (the fiber end versus the Wollaston prism) has been considered. Based on this, a novel method that uses a statistical approach, including a set of scanning elliptical polarization inputs and the corresponding curve-fitting outputs for a given current, is used to determine the birefringence of the fiber-current-sensing system. The experimentally measured bend-induced birefringence agrees with the estimated value of the bend birefringence.

Dual-core fiber as a tunable directional coupler

The bending-induced birefringence in a fiber loop is used to restore locally perfect coupling conditions between the two slightly detuned cores of a dual-core fiber. It is also shown that varying the angle between the birefringence and the core's axis allows one to obtain perfect control (0-100%) of the coupling ratio.