Polarization properties of elliptical core non-hexagonal symmetry polymer photonic crystal fibre

In this paper, polarization properties and propagation characteristics of polymer photonic crystal fibres with elliptical core and non-hexagonal symmetry structure are investigated by using the full vectorial plane wave method. The results show that the birefringence of the fibre is induced by asymmetries of both the cladding and the core. Moreover, by adjusting the non-symmetrical ratio factor of cladding $\eta$ from 0.4 to 1 in step 0.1, we find the optimized design parameters of the fibre with high birefringence and limited polarization mode dispersion, operating in a single mode regime at an appropriate wavelength range. The range of wavelength approaches the visible and near-infrared which is consistent with the communication windows of polymer optical fibres.     

Mode conversion and radiation losses in a step-index optical fibre due to bending

Wave propagation in a step-index optical fibre with a curved section is treated theoretically. Mode conversion and radiation take place. The mode coupling coefficients which depend on the polarization of the incident mode are calculated. Radiation losses at the transition between a straight and a curved fibre section and total losses including the continuous radiation loss due to a uniform bend are evaluated. Comparisons are also made between singly and doubly clad fibres. It is shown that the doubly clad fibre can tolerate a sharper bend.Requests for reprints should be sent to Professor G. L. Yip.     

Measured sensitivity of arc-induced long-period grating sensors in photonic crystal fibre

Reported are experimental results from investigations of the sensing properties of long-period gratings (LPGs) recorded in two different geometries of photonic crystal fibre (PCF): a large-mode area PCF and an endlessly single mode PCF. The LPGs have been characterised for their sensitivity to temperature, bending, surrounding index and strain. The LPGs in both fibres have been found to have negligible temperature sensitivity whilst exhibiting useful strain sensitivities. Strong directional bend sensitivity is shown by one PCF whilst the other shows good non-directional bend sensitivity. The fibres exhibit differing sensitivities to surrounding refractive index.     

Fibre Bragg Gratings Inscribed in Homemade Microstructured Fibres

Fibre Bragg gratings （FBGs） are inscribed in homemade microstructured fibres by the standard phase mask method. Enhanced couplings between the forward fundamental mode and backward cladding modes are obtained. The mode coupling and spectral characteristics are investigated experimentally. The cladding mode resonances can be affected by filling active materials into the air holes, which will be useful for the implementation of tunable photonic devices in optical fibre communication and sensing systems.     

Fibre gratings and devices for sensors and lasers

Although mainstream grating writing, more often than not using single photon excitation of germanosilicate based defects with CW 244 nm light, remains the key technology for complex devices it is now being complemented by a whole host of processes which can enhance and tailor the properties of both conventional and not‐so‐conventional fibre Bragg gratings. Further, processes for writing of gratings in non‐germanosilicate fibres have also continued to develop and include multi‐photon excitation directly into the band edge of the glass. It is now possible to custom tailor a gratings property based on the application and the nature of production as well as custom tailor the grating writing process to suit the type of fibre and application. Examples and suggestions where these can benefit sensors and lasers are outlined.     

Tuneable polarization mode dispersion emulator: Fixed polarization maintaining fibre sections and rotatable polarization orientations

We design a polarization mode dispersion (PMD) emulator capable of adjusting first-order (FO-) and second-order (SO-) PMD statistics, making it capable to mimick differerent fibre links or fibre plants. The emulator can adjust PMD statistics by controlling mode coupling angles between polarization maintaining fibres (PMFs) of fixed length using its seven rotatable electro-optic polarization rotators which act as half waveplates (HWPs). The stability and repeatability of the emulator under a stable laboratory environment makes PMD statistics reproducible. The control mechanism of the emulator is in real-time.     

Polarization dependent loss of arc-induced long-period fibre gratings

We have investigated the polarization properties of long-period fibre gratings fabricated using the electric arc technique. It was found that the choice of the fabrication parameters (electric current, arc duration and pulling tension) affects the polarization dependent loss of the produced gratings. In particular, a non-monotonic dependence on the external pulling tension was obtained.     

Pulsed interferometric optical fibre sensor detecting wiretapping in long transmission lines

A modified optical fibre based Mach-Zehnder interferometer was applied as a sensor to detect wiretapping in long transmission optical fibre lines. The signal consisting of short pulses (around 1?ns) was launched to the input of the interferometer based on the polarization maintaining fibres and polarization elements. When the sensing line was undisturbed, detectors registered only a single pulse. The additional two side pulses appear, if the wiretapping attempt took place. For robust detection of any alarm situation we proposed two-criteria algorithm to minimize false alarm rate. Moreover, slow environmental fluctuations were continuously monitored and compensated by polarization controllers. We measured frequency characteristics of the sensor and performed a hundred wiretapping attempts, which proved high performance of the sensor.     

Orientation-dependent electromagnetic properties of basalt fibre/nickel core--shell heterostructures

The influence of orientation on electromagnetic properties of basalt fibre/nickel core--shell heterostructures prepared by a simple electroless plating method is investigated. For comparison, the same investigation is also performed on naked basalt fibres. For electromagnetic measurement, the directions of basalt fibre/nickel and naked basalt fibres are parallel, random and perpendicular to the direction of external electric field, termed E_\vert\vert sample, random sample and E_⊥ sample, respectively. Electromagnetic anisotropy can be clearly observed in the basalt fibre/nickel core--shell heterostructures, while electromagnetic properties of naked basalt fibres are unrelated to the orientation. The E_⊥ basalt fibre/nickel shows the highest dielectric loss but the lowest magnetic loss, and E_|| basalt fibre/nickel exhibits the highest magnetic loss but the lowest dielectric loss. The dielectric loss of E_⊥ basalt fibre/nickel is several times as large as that of E_|| basalt fibre/nickel, which could be attributed to the increase of polarization relaxation time as a consequence of the nanosize-confinement effect. The magnetic loss of E_|| basalt fibre/nickel is even one order of magnitude higher than that of E_⊥ basalt fibre/nickel, which originates mainly from the natural magnetic resonance of basalt fibre/nickel core--shell heterostructures.     

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.     

Polarization dependent loss of mechanically induced long-period fibre gratings

We present results on the measurements of polarization dependent loss (PDL) of mechanically induced long-period fibre gratings. It is shown that a proper choice of the fabrication parameters leads to a considerable reduction of PDL values.     

Vectorial vortex mode transformation for a hollow waveguide using Pancharatnam-Berry phase optical elements

Transformation and inverse transformation between a free-space linearly polarized beam and the vectorial vortex mode of a circular hollow waveguide by use of Pancharatnam-Berry phase optical elements is proposed. Demonstration was achieved by fabricating GaAs subwavelength gratings and utilizing a 300 microm diameter hollow metallic waveguide for 10.6 microm wavelength CO(2) laser radiation. The mode transformations and the excitation of a single vectorial mode inside the hollow waveguide were verified by full polarization measurements. In addition, the inverse mode transformation of the single vectorial mode excitation in the waveguide enabled us to experimentally obtain a linearly polarized bright spot with a high central lobe.     

Study of the behaviour of the fibre Bragg grating fabricated with cladding mode suppression fibre

One effective method for suppression of coupling from guided optical modes into cladding modes in an optical fibre Bragg grating is to use cladding mode suppression fibre. In this specially engineered fibre, the grating is written into the core and the inner cladding, both of which are photosensitive. A theory is presented in this paper to analyse the spectral characteristics of this kind of gratings. A fibre Bragg grating was experimentally fabricated with this kind of photosensitive fibre (PS-RMS-50 from StockerYale). It is shown both theoretically and experimentally that such gratings exhibit strong suppression of the coupling between core mode and cladding mode. The experimental result is in good agreement with the theoretical analysis.     

Fundamental mode microbending loss in graded-index and W fibres

The microbending loss in single-mode or quasi-single-mode fibres is approximately determined from a calculation of the fundamental mode field in a fibre with constant curvature. The result of the microbending loss essentially depends on the spot size of the fundamental mode, only. The approximations are verified by studying a statistical model and by comparing the approximations with exact calculations in the case of W fibres.     

Broadband spectral attenuation measurements on optical fibres: an interlaboratory comparison by members of COST 208

Comparative loss measurements on optical fibres are being performed by members of COST 208, and in this paper some preliminary results are reported. Full spectral attenuation measurements have been made, covering the range from 700 to 1800 nm, on graded-index and single-mode fibres. In the multimode fibre the measured loss depends on the energy distribution launched into the fibre. By ensuring similar energy distributions the deviations between loss spectra have been maintained to within about ± 5%. In single-mode fibre the launched power is very small so the transmitted intensity was closer to the noise limit of the detector. Differences between loss spectra amounted to about ± 8% in the low-loss ranges.     

Dielectric properties of short sisal/coir hybrid fibre reinforced natural rubber composites

The dielectric properties, such as dielectric constant, volume resistivity and dielectric loss factor, of sisal/coir hybrid fibre reinforced natural rubber composites have been studied as a function of fibre loading, fibre ratio, frequency, chemical modification of fibres and the presence of a bonding agent. The dielectric constant values have been found to be higher for fibre filled systems than pure natural rubber. This has been attributed to the polarization exerted by the incorporation of fibres into the matrix. Dielectric constant values were observed to be decreased with increase in frequency due to the decreased interfacial and orientation polarization at higher frequencies. Whereas dielectric constant increases with fibre loading because of the increment in number of polar groups after the addition of hydrophilic lignocellulosic fibres. The volume resistivity of the composites was found to be decreased with fibre loading and a percolation threshold has been obtained at 15.6% volume of fibres. Fibre treatment, such as alkali, acetylation, benzoylation, peroxide and permanganate, were carried out to improve the adhesion between fibres and matrix. The dielectric constant values were lower for systems consisting of fibres subjected to chemical treatments due to the increased hydrophobicity of fibres. The addition of a two-component dry bonding agent consisting of hexamethylene tetramine and resorcinol, used for the improvement of interfacial adhesion between the matrix and fibres, reduced the dielectric constant of the composites. When the weight percentage of sisal fibre was increased in the total fibre content of the hybrid composites, the dielectric constant was found to increase. The added fibres and different chemical treatments for them increased the dielectric dissipation factor. A dielectric relaxation has been observed at a frequency of 5 MHz.     

Compensation of polarization-dependent loss in transmission fiber gratings by use of a Sagnac loop interferometer

We analyze the transmission characteristics of a Sagnac loop interferometer containing a polarization-dependent loss element and lossless polarization-converting elements by use of Jones matrices. We show that polarization independence in the transmission mode can be achieved in such a configuration and that maximum transmittance occurs when a half-wave plate is used for the polarization-converting element. The result is verified experimentally for a fiber acousto-optic tunable filter and cascaded long-period fiber gratings with either intrinsic or process-induced polarization-dependent filtering characteristics.     

弱双折射光纤布喇格光栅反射偏振对温度响应特性的研究

理论分析了切趾弱双折射光纤布喇格光栅反射偏振相关特性与温度之间的关系.数值模拟了切趾弱双折射光纤光栅的反射谱、偏振相关损耗和差分群时延随波长变化曲线.实验测出了不同温度下反射谱、偏振相关损耗和差分群时延随波长变化曲线.根据实验结果对偏振相关损耗和差分群时延的变化情况作出了分析.反射偏振相关损耗呈现两个峰值,随温度增加两峰漂移程度相同,表明偏振相关损耗无明显差异.差分群时延最大值随温度增加成线性向长波方向漂移,证明了光纤光栅正交模损耗变化的等同性.综合理论分析与实验结果表明:切趾弱双折射光纤布喇格光栅的偏振特性随温度产生明显的变化,其正交模变化呈现等比例特性.     

Optical fibres for current measurement applications

The polarization properties of optical fibres are of interest in several different communications and measurement applications. This paper indicates the properties ideally required of a fibre for use in current measurement systems and describes an extremely low birefringence fibre which meets many of these requirements. Measurements of the polarization properties of this fibre are given and a laboratory prototype current measurement system using the fibre is described.