Production and use of an interferometric optical strain gauge with comparison to conventional techniques

The use of optical fibres in transducers has recently attracted much interest and the adoption of optical techniques in strain detection is growing due to the reliability and accuracy they can offer in harsh environmental conditions (strong electromagnetic fields, high temperature, corrosive chemicals, etc.) where the conventional electrical strain gauges often fail.In this paper an optical strain gauge based on a true fibre Michelson interferometric scheme is developed and tested. The discrete optical strain gauge is realised by embedding the optical fibre sensing leads into a resin support which allows its easy installation and use. The related static and dynamic behaviours are investigated and compared with those of similar optical sensing systems either directly bonded to the structure or embedded in composite laminates. Comparisons are also made with theoretical predictions and measurements supplied by semiconductor electrical strain gauges.     

Optical extensometer and elimination of the effect of out-of-plane motions

A novel optical extensometer is developed to estimate the local uniform strain on planar surface accurately. The proposed system consists of a shared large format lens and two image sensors, which acquire pairs of images of two isolated small regions on the object surface simultaneously. Digital image correlation (DIC) algorithm is applied to determine the relative displacement between gauge points designated on recorded pairs of images. Then local strain can be extracted after dividing the relative displacement by the scale distance. Moreover, a special experimental setup called “correction sheet” is used to eliminate the virtual strain induced by out-of-plane motions. Uni-axial tensile experiments are performed to validate the reliability and resolution of the optical extensometer, and the measurement results demonstrate that the resolution of the optical extensometer achieves 2–3 με.     

Fibre raman amplifier properties for applications to long-distance optical communications

This paper reviews recent theoretical and experimental studies on optical amplification by stimulated Raman scattering (Raman amplification) in single-mode optical fibres, and discusses its possible applications to long-distance optical communications. The fibre Raman amplifier can be used as an in-line optical amplifier in both repeaterless and non-regenerative all-optical repeater configurations. In the former application, signal transmission of over 400 km has been predicted theoretically, although high-power pump sources are considered to be required. In the latter configuration, in which optical fibre losses are compensated for by Raman gains, it has been shown that extremely long-distance transmission over more than 1000 km is feasible in terms of signal-to-noise ratio, where the distance achievable is mainly limited by the accumulation of Raman noise. Recent experimental demonstrations including bit-error rate measurements on the Raman amplified signal, and laser-diode pumped amplification, etc., are also reviewed. The results confirm the effectiveness of fibre Raman amplifiers in future long-haul systems.     

Breakpoint detection of heating wire in wind blade moulds using infrared thermography

In order to manufacture the fibre glass wind blades, one kind of mould embedded with heating wire is used not only for making numerous ‘copies’ of the original sample, and also heating the mould to a certain temperature for curing. The heating wire is embedded in fibre glass as a sandwich structure, and it may break after a long time usage at high temperatures. In this study, a high voltage discharging (HVD) circuit is used to trigger HVD at the breakpoint, which generates heat and therefore causes temperature increase at the corresponding front surface, one infrared camera is used to monitor the temperature evolution. It successfully and quickly detects breakpoints in spar moulds.     

Flat Supercontinuum Generation at 1550 nm in a Dispersion-Flattened Microstructure Fibre Using Picosecond Pulse

The generation of a flat supercontinuum of over 80nm in the 1550nm region by injecting 1.6ps 10 GHz repetition rate optical pulses into an 80-m-long dispersion-flattened microstructure fibre is demonstrated. The fibre has small normal dispersion with a variation smaller than 1.5 （ps·nm^-1·km^-1） between 1500 and 1650nm. The generated supercontinuum ranging from 1513 to 1591 nm has the flatness of ±1.5 dB and it is not so flat in the range of several nanometres around the pump wavelength 1552nm. Numerical simulation is also used to study the effect of optical loss, fibre parameters and pumping conditions on supercontinuum generation in the dispersion-flattened microstructure fibre, and can be used for further optimization to generate flat broad spectra.     

Evaluating damage in smart composite laminates using embedded EFPI strain sensors

Extrinsic Fabry-Perot interferometric (EFPI) strain sensors (SSs) were assessed for their sensing capacity and survivability via surface mounting and interior embedment in the carbon/epoxy beams. Artificial delaminations of two different sizes were used to simulate delamination. The embedded EFPI-SSs along with surface-mounted SGs were used to evaluate the effect of the delaminations on bending stiffness induced by quasi-static loading on a comparative basis. The linear responses of EFPI-SSs up to a bending strain of 0.5% were in good agreement with the interpolated strains of surface strain measurements. The embedded EFPI-SSs were able to differentiate the effect of the tensile delaminations on bending stiffness. The interior strain data from embedded EFPI-SSs were shown to be very useful to extrapolate the strain difference on both sides of the delaminations as well as deducing local stresses.     

Measurement of polarization fluctuations in installed single-mode optical fibre cables

Measurements were made of the random polarization fluctuations of conventional single-mode fibres of five optical fibre cables installed in the city of Berlin (West). The measurements were performed with the aid of a semiconductor laser, the frequency (f) of which was 227.3 THz (wavelength 1320 nm). Such preliminary studies are very important for optically coherent transmission over cables not maintaining the polarizations. The signal intensity fluctuations as the fibre output can be represented both as a function of time and by the related spectral power density.     

Linear self-referenced complex-field characterization of fast optical signals using photonic differentiation

This paper reviews recent work on a new group of linear and self-referenced techniques for full (amplitude and phase) characterization of fast optical signals based upon the concept of photonic differentiation, generally referred to as ‘phase reconstruction using optical ultrafast differentiation’ (PROUD). These techniques are particularly well adapted for applications in the context of fiber-optics telecommunications. PROUD methods can be implemented using simple and practical optical fiber-based setups and they rely on a direct, non-iterative phase recovery numerical algorithm. They can be used over a very wide range of pulse time durations, from the sub-picosecond to the nanosecond regime, and they can provide measurements in a single shot and in real time with power sensitivities down to the microwatt level. Previously reported PROUD methods are treated here under a unified, single framework, facilitating their analysis and comparison.     

Mechanical tests of the conduit tubes of a conductor for the Toroidal winding of the International Thermonuclear Experimental Reactor (ITER)

Extremely stringent requirements, which include the impact toughness at the liquid-helium temperature, are imposed on the material of the conduit tubes for International Thermonuclear Experimental Reactor (ITER) Toroidal Field (TF) conductors. Modified 316LN-IG stainless steel is recommended as the conduit tube material. Steel 316LN-IG tube samples (both full-size samples and sub-sized samples) are subjected to mechanical tests at various stages of the process of conductor production: in the as-recieved state and after compacting, preliminary elongation by 2.5% at room temperature, and annealing at 650°C for 200 h in a pure helium gas atmosphere. The tests are carried out at room, liquid nitrogen, and liquid helium temperatures and satisfy the standards of the American Society of Mechanical Engineers (ASME and ASTM). The results of sub-size and full-size samples testing show that the last one gives more representative results to qualify the weld joints in liquid nitrogen. When the temperature decreases or the strain increases, the magnetization of the samples increases, especially in the weld area. Strain measurements with an extensometer demonstrate that the intracrystal processes occurring at the liquid-helium temperature can lead to a significant change in the local load, up to complete unloading in a deformation zone. Unusual local serrated deformation is observed with an extensometer installed in the weld area during tests in liquid helium: this deformation is the result of compressive jumps opposite to the loading direction.     

Quantum logic operations on two distant atoms trapped in two optical-fibre-connected cavities

Based on the coupling of two distant three-level atoms in two separate optical cavities connected with two optical fibres, schemes on the generation of several two-qubit logic gates are discussed under the conditions of Δ = δ - 2ν cos πk/2 >> g/2 and ν ~ g. Discussion and analysis of the fidelity, gate time and experimental setups show that our schemes are feasible with current optical cavity, atomic trap and optical fibre techniques. Moreover, the atom-cavity- fibre coupling can be used to generate an N-qubit nonlocal entanglement and transfer quantum information among N distant atoms by arranging N atom-cavity assemblages in a line and connecting each two adjacent cavities with two optical fibres.     

Power coupling between core and cladding of an optical fibre and its effect on measurements

A model describing ray propagation in both the core and cladding regions of a multimode optical fibre is developed. This model is then used to determine the influence of the fibre outer protective sheath on far-field measurements and to determine the equilibrium source conditions for the modified near-field technique.     

Effect of light-collection geometry on reconstruction errors in Abel inversions

The Abel inversion, used to reconstruct axisymmetric radial profiles from line-of-sight intensity measurements, is increasingly used to make spatially resolved combustion measurements. An Abel deconvolution is valid only when incoming rays are parallel, whereas most practical optical setups used for emission imaging consist of single-lens and multilens systems that collect light in a cone, over a nonzero solid angle. A ray-tracing simulation was performed to aid in understanding how optical collection geometry affects measured intensity signals and the resultant reconstructed emissivity profiles. Simulation results are compared with emission tomography measurements performed on an axisymmetric laminar diffusion flame.     

Optical mouse digital speckle correlation

The possibilities with which an optical mouse can be applied to obtain scientific measurements are currently limited by the maximum distance possible between the mouse and surface under interrogation. We demonstrate that a simply adapted optical mouse is able to operate as a recording instrument in the digital speckle correlation mode; wherein greater distances between optical mouse and translating surface are possible. Linear (R² > 0.98) and repeatable measurements in the x and y axis were obtained with the optical mouse and translating surface separated at 300 mm. In both cases, the measurement resolution was 0.025 mm. Digital speckle correlation with an optical mouse offers the ability to obtain measurements of displacement and deformation at a fraction of the investment in time and cost of conventional setups.     

The principles of remote interferometric optical fibre strain measurement

Measurement of mechanical strain using optical fibre techniques may be implemented in many configurations. This paper concentrates on interferometric measurements which offer high sensitivity and reasonable scale factor stability. The emphasis here is on the remote measurement of slowly varying strains in the region of a few microstrains. The system is based on a remote fibre or integrated optic interferometer, and the device is read using a frequency swept laser source. The potential resolution is of mechanical displacements of about 10 nm, and the device may be optically interrogated over ranges of several kilometers. The concept is also compatible with passive sensor multiplexing.     

Polymer optical fibre Bragg gratings based fibre laser

For the first time the fibre laser constructed from a polymer optical fibre Bragg grating is reported. The single frequency laser with the peak power of −5 dBm and signal to noise ratio greater than 45 dB has been achieved. Further examination demonstrates the excellent characteristics of the fibre laser. First, the fibre laser can be easily tuned over 35 nm by the simple axial tension method. Second, the fibre laser has the high strain sensitivity of 1.48 pm/με with the dynamic measurement range as large as 2.37%.     

基于变刚度结构的光纤布拉格光栅低温环境下的温度増敏方法研究

光纤布拉格光栅采用非电磁信号测试方式,具有显著的抗电磁干扰以及可嵌入被测结构内部等特性,已成为近些年广受关注的光学应变、温度传感与测试新技术。但在极端条件下(如超低温环境),光纤由于受到其自身材料特性的局限,传感特性不明显甚至限制了其应用。基于结构变刚度的力学増敏方法,本文提出了一种适用于低温区的光纤布拉格光栅温度传感増敏的结构设计方法。研究结果表明：该传感结构可有效感应低温下的温度变化,増敏效果明显;并获得増敏效果随结构材料与几何特性的依赖关系,经过优化可使得光栅区对温度和应变的敏感性大大提高,甚至提高1个数量级。最后,完成了这一増敏结构的设计和制备,实验验证了所提方法的有效性和可靠性。     

Tensile testing machine for optical fibres and cables

A tensile testing machine suitable for optical fibre and optical cable testing is described. This machine uses specially developed gripping arrangements to obviate damage to the sample during testing. Some preliminary measurements of the stress/strain behaviour of coated fibres and optical cables are presented.     

Broadening free spectral range of an evanescent-wave pumped Whispering-Gallery-Mode fibre laser by Vernier effect

Evanescent-wave pumping scheme has been successfully used to excite Whispering-Gallery-Mode fibre laser. The free spectral range of the lasing spectrum from a single optical fibre is found to be very narrow, to increase the free spectral range, a coupled cylinder-cavity structure has been fabricated by binding two bare optical fibres together, based on Vernier effect, the free spectral range of the coupled cylinder-cavity structure has been effectively broadened over 15 times compared with that of a single optical fibre. The measured free spectral ranges for both coupled cylinder-cavity structure and the single optical fibre are consistent with theoretical prediction.     

The rotation of the polarization in low birefringence monomode optical fibres due to geometric effects

Monomode optical fibres of very low intrinsic linear and circular birefringence have been developed for use in polarimetric optical fibre sensors, most notably the fibre optic current sensor. The polarization of light travelling along such fibres is known to be modified by external stress applied to the fibre. In this paper it is shown that the polarization state may also be rotated if the path of the fibre is bent in a nonplanar curve. A theory is developed which enables the effect to be quantified and the results are shown to agree with some measurements on a fibre bent into a helix.     

Measurements and analysis of antireflection coatings reflectivity related to external cavity lasers

Facets of semiconductor optical amplifiers (SOA) designed for external cavity lasers must be coated with an antireflection (AR) film of high quality and extremely low reflectance. Therefore measurements of facet reflectance play a crucial role in the fabrication of such AR coating. The reflectance can be estimated by studying the optical power reflected from the Fabry-Perot cavity formed for that purpose by the examined SOA facet and the end of a single-mode fibre. We have made analysis of practical suitability of such measurement method. Theoretical calculations show that, for the low reflectance coatings, losses due to light coupling into optical fibre cannot be omitted in the analysis of the experimental results. To verify this conclusion a theoretical model was tested for a low reflectance surface and we have found that the relative error of the measurements supported by the theoretical model is on the order of 8%.