Multiple-beam interferometric determination of Poisson's ratio and strain distribution profiles along the cross section of bent single-mode optical fibers

Recent Advances in Forward Brillouin Scattering: Sensor Applications

In-fiber opto-mechanics based on forward Brillouin scattering has received increasing attention because it enables sensing the surrounding of the optical fiber. Optical fiber transverse acoustic resonances are sensitive to both the inner properties of the optical fiber and the external medium. A particularly efficient pump and probe technique-assisted by a fiber grating-can be exploited for the development of point sensors of only a few centimeters in length. When measuring the acoustic resonances, this technique provides the narrowest reported linewidths and a signal-to-noise ratio better than 40 dB. The longitudinal and transverse acoustic velocities-normalized with the fiber radius-can be determined with a relative error lower than 10^-4, exploiting the derivation of accurate asymptotic expressions for the resonant frequencies. Using this technique, the Poisson's ratio of an optical fiber and its temperature dependence have been measured, reducing the relative error by a factor of 100 with respect to previously reported values. Using a single-point sensor, discriminative measurements of strain and temperature can be performed, achieving detection limits of ±25 με and ±0.2 °C. These results show the potential of this approach for the development of point sensors, which can be easily wavelength-multiplexed.

High accuracy measurement of Poisson's ratio of optical fibers and its temperature dependence using forward-stimulated Brillouin scattering

Transverse acoustic mode resonances enable a high accuracy determination of Poisson's ratio and elastic properties of optical fibers. An all-optical pump and probe technique is used for efficient excitation and accurate characterization of both, radial and torsional-radial acoustic resonances of optical fibers. Simple and precise algebraic expressions for the frequencies of high order acoustic resonances are derived, enabling a rigorous analysis of the experimental data using standard least squares fitting. Following this approach, the determination of Poisson's ratio does not require the measurement of any physical length, but only frequency measurements are required. An accuracy better than 1 ‰ is achieved. The dependence of the fiber Poisson's ratio with temperature is also determined experimentally.

Highly sensitive vibration sensor based on the dispersion turning point microfiber Mach-Zehnder interferometer

In the present work, we introduced a highly sensitive vibration sensor, which is based on the dispersion turning point (DTP) microfiber Mach-Zehnder interferometer. The axial strain and vibration sensing characteristics of the microfiber Mach-Zehnder interferometer were investigated. First, we theoretically analyzed the spectrum evolution characteristics of the microfiber Mach-Zehnder interferometer caused by axial strain. Second, the microfiber with different diameters was fabricated using the electrode discharge and fused taper method, and the axial strain experiments were conducted; the maximum sensitivity of the DTP microfiber with a diameter of ∼2.2 µm reached -45.55 pm/µɛ at ∼1550 nm. Finally, based on the axial strain principle of the microfiber, we designed a highly sensitive vibration sensor using a DTP microfiber integrated into a rectangular through-hole cantilever beam. The 30-3500 Hz vibration signal monitoring could be realized, the maximum signal-to-noise ratio (SNR) was ∼75 dB at 52 Hz, and the acceleration sensitivity reached as high as 0.764 V/g at 45Hz. These results suggested the high performance of the microfiber in axial strain and micro-vibration sensing fields.

Nonlinearity in bent optical fibers

An interferometric method to study the induced variations of nonlinear parameters in bent optical fiber such as third-order susceptibility chi((3)) and second-order refractive index n(2) is presented. Due to the expected nonlinear response of the Young's modulus of fiber material, the profiles of asymmetric variations of the two parameters with curvature are observed and calculated, revealing the high spatial and index resolution of the method. The investigation is done in single-mode optical fibers at the standard operating wavelengths of 1300 and 1550 nm and at radii of curvature from 5 to 11 mm. At the minimum radius of curvature R=5 mm, the cladding chi((3))=4.131x10(-15) esu on the tensile side, whereas on the compressed side it is 4.601x10(-15) esu for lambda=1300 nm. On the tensile side n(2)=1.09x10(-13) esu, whereas on the compressed side it is 1.216x10(-13) esu. For lambda=1550 nm, the cladding chi((3)) and n(2) on the tensile side are 3.96x10(-15) esu and 1.055x10(-13) esu, whereas in the compressed cladding side they are 4.435x10(-15) esu and 1.174x10(-13) esu, respectively. At lambda=1300 nm and R=5 mm, the core chi((3)) is given by 4.631x10(-15) esu on the tensile side and 4.649x10(-15) esu on the compressed side. The asymmetry in n(2) is given by 1.223x10(-13) esu on the tensile side and by 1.227x10(-13) esu on the compressed side. With lambda=1550 nm, the core chi((3)) asymmetry is given by 4.46x10(-15) esu on the tensile side and by 4.477x10(-15) esu on the compressed side. For n(2) its asymmetry is provided by 1.181x10(-13) esu on the tensile side and by 1.185x10(-13) esu on the compressed side.

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.

Theoretical and experimental evaluation of the modal field shift and the associated transition loss in perturbed index-profile single-mode optical fiber with application of Fizeau interferometry

Fizeau micro-interferometry is applied to evaluate some parameters of a curved single-mode optical fiber. The field shift of the fundamental mode and the associated transition loss in a perturbed index-profile fiber due to bending are determined. The preceding fiber parameters are determined as a function of the shift of multiple-beam Fizeau fringes. For a curvature range between 0.13 and 0.053 mm(-1), a range of field shift between 0.44 and 0.21 microm is determined. A fraction of the transition loss ranging between 0.0056 and 0.028 is calculated within the same curvature range. Because our method has high index resolution and spatial resolution, it shows good agreement with theory. The results and the agreement with theory indicate that the use of multiple-beam Fizeau fringes is a promising technique that is capable of determining with high accuracy some guidance parameters of the optical fibers.

Evaluation of some GRIN fiber parameters and the associated fraction mode loss due to mechanically induced optical anisotropy

Some of the optical parameters of the bent multimode graded-index (GRIN) optical fiber in terms of indices of refraction, where the bending stresses broke the radial symmetry, are evaluated by use of multiple-beam Fizeau fringes. The variation of the index difference between the cladding index and core index in both the compression and tensile fiber regions is measured. The accuracy of measuring the index is +/- 1 x 10(-4). The spatial resolution of the method is 1.39 microm. Evaluation of the acceptance angle, the numerical aperture, and the V number profiles of the bent fiber from the interference pattern at both sides of the bent fiber are presented. The fraction of the mode number lost has been evaluated. The method was used to study the influence of compression on diminishing the index difference that leads to a dissipation of energy and a considerable mode loss. It is obvious from the experimental data that the change of the index difference due to bending strongly affects the fraction of propagating mode number, especially at the small radii of curvature. Ignoring the variation of the index difference we evaluating the number of propagated modes number leads to an insufficient determination of the mode loss. It subsequently leads to an incorrect determination of the mode dispersion and the interface loss in bent GRIN fibers. The study confirms that the deviation of the guide axis from straightness with the radius of curvature of less than 1 cm could lead to a significant fraction mode loss.

Determination of bending-induced strain in optical fibers by use of quantitative phase imaging

Quantitative phase microscopy with polarized light has been used to determine the change in refractive index introduced into an optical fiber when the fiber is bent through a constant radius of curvature. By obtaining phase images for two orthogonal directions of the polarization of the incident light, one can infer the induced axial strain profile within the fiber. Radii of curvature from 1 to 8 cm were considered, and in each case excellent agreement, within lambda/100, was obtained between experimental results and theory.

Theory and measurement of Young's modulus radial profiles of bent single-mode optical fibers with the multiple-beam interference technique

Multiple-beam Fizeau fringes in transmission have been applied to the study of the nonlinear stress-strain relationship due to bending in the cladding of single-mode optical fibers. The present study yields a relation between the variation of refractive indices of bent single-mode fibers, represented by the fringe shift, and the nonlinear radial change of Young's modulus along the fiber cross section. Experimentally, the study confirms the nonlinear asymmetric stress-strain relation across the fiber cross section. This relation is due to the asymmetric distribution of the compression and tensile stresses over the fiber cross section rather than to the shift in the centroid (neutral axis).