Improved high birefringence photonic crystal fibres with dispersion flattened and single mode operation

A kind of improved high birefringence photonic crystal fibre (PCF) is proposed in this paper. The characteristics of birefringence, dispersion and leakage loss are studied by the multipole method. Numerical results show that the improved PCF possesses the properties of a flat dispersion and single mode operation. Moreover, with the operating wavelength λ = 1.55μm, the modal birefringence increases greatly in comparison with that of the original PCF, and the leakage loss is about 10⁴ times smaller than that of the original PCF because the modification gives rise to the strong confinement of guided modes. It is expected that the improved PCF can be used as high birefringence and dispersion flattened fibres.     

Optical fibre strength/fatigue experiments

New techniques for measuring and analysing the strength characteristics of optical waveguide fibres have evolved as the strength of the optical fibres improved. A recent life-fatigue experiment is described and the results analysed in terms of the most commonly used theoretical model which results in the power law V = AKⁿ. Ten m long fibre samples were tensilely stressed in a high humidity environment by both fixed (static fatigue) and constant rate (dynamic fatigue) loads. The respective test values for the power law exponent, the fatigue constant ‘n’, were 38 and 17. The results of the tests indicate the more conservative value, 17, for practical engineering design, and the need for incorporating an ageing term in the model.     

Detecting and avoiding the necking deformation along polypropylene fibre axis using the fringe pattern analysis of multiple-beam microinterferometry

A fibre-drawing device attached with the system for producing multiple-beam Fizeau fringes in transmission is used to optimize the optical properties during the cold drawing of polypropylene (PP) fibres. This system is automated for interference pattern analysis. Two drawing processes for the PP fibres are applied and investigated. The first one is fast drawing in which the necking deformation is predicted and the other is the slow (step) drawing in which the necking can be avoided. The refractive index profiles (n and n) of PP fibres are determined at different positions along the fibre axis during the fast and slow drawing processes. The fibre interference patterns are automatically digitized and stored in a computer storage media. The slow drawing technique for PP fibres is recommended to overcome the deformation difficulties along the fibre axis due to necking during the drawing processes. Microinterferograms in case of light vibrating parallel and perpendicular to the fibre axis are given for illustration.     

Optical fibre excitation by partially coherent sources

Sources for optical fibre excitation have previously been assumed to be either totally coherent or incoherent. This paper formulates the modal excitation problem for partially coherent sources. The modal excitation coefficients are given in terms of the source complex degree of coherence and numerical results and simple analytical expressions appropriate to multimode step index fibres are presented. The assumption of equal modal power when excitation is by a very incoherent source is examined and the following simple criterion developed: for highly incoherent sources, modes with eigenvaluesU < the reciprocal of the coherence length, measured in units of fibre radius, are approximately equally excited, while the remaining modes carry little power.     

Ag微米管阵列修饰的聚合物光子晶体光纤制备工艺研究

以聚合物光子晶体光纤空气孔阵列为模板,制备了Ag微米管阵列修饰的聚合物光子晶体光纤.讨论了银氨溶液和葡萄糖溶液浓度以及反应时间和温度对银微米管形貌的影响.在Ag(NH3)+2与C6H6O浓度比为2,反应时间为20 min,温度为65℃条件下光纤孔洞表面获得了规则的Ag微米管状结构,并对Ag的生长机理进行了讨论.     

Origin of the variability of the mechanical properties of silk fibres: 1 ‐ The relationship between disorder,hydration and stress/strain behaviour

The causes of the variability in mechanical behaviour of various silks obtained from silkworms (Bombyx mori, Antheraea/Tussah) and spiders (Nephila madagascarensis) have been studied by tensile uniaxial tests, differential scanning calorimetry and Raman microspectrometry. Analyses of tensile stress–strain curves recorded for different silkworm and spider baves and single fibres, undergoing different histories (age, degumming, thermal/chemical treatments), allow the behaviour to be classified into five groups and correlated with the fibroin/spidroin structural status: (1) Type I exhibits linear elastic behaviour and then a quasi plateau, (2) Type II has a two‐step linear elastic behaviour with kinks then a quasi plateau, (3) Type III exhibits a smooth transition from a linear behaviour to a plateau, (4) Type IV has a behaviour rather similar to that of Type I but above ~8–12% hardening occurs, and (5) Type V results in the breakage of fibres during the first elastic stage. Unambiguously, Type IV is more frequent for degummed, very dried fibres and Type III for water‐saturated fibres. The most striking Raman signature differences are observed in the Raman intensity of the amorphous/‘ordered’ νN–H and H₂O band components and correlated to the water content and disorder degree according to the calorimetry study. Types I and II are frequently observed for fresh (largely amorphous) spun fibres/baves. Type V is characteristic of degraded fibres. Type IV represents the most ordered state. The similar stress–strain types for dried silkworm and spider fibres show that the general difference in amino acid contents is not the most pertinent parameter for the mechanical behaviour. Copyright © 2011 John Wiley & Sons, Ltd.     

Improvement of coupling efficiency between a 0.82 μm wavelength laser diode and a VAD single-mode fibre by conical microlenses

Conical microlenses have been fabricated directly on the end faces of single-mode fibres which demonstrate −1.8 dB coupling loss to a 0.82 μm laser diode. Experimental results on coupling loss, lateral and axial offsets are also reported.     

Experimental investigation of fibre-optical confocal scanning microscopy: Including a comparison with pinhole detection

Reported in this paper are the experimental results on fibre-optical confocal scanning microscopy (FOCSM). We have performed various measurements in the FOCSM involving axial responses, edge responses, signal levels and interference axial responses in order to demonstrate the differences in confocal scanning imaging between using optical fibres and using pinholes. Experimentally measured results are compared with our theoretical predictions, revealing good agreements. It is shown convincingly that the FOCSM is a purely coherent imaging system as predicted in our previous theory.     

Light propagation in multi‐step index optical fibres

This paper reviews the theoretical analysis of light propagation we have carried out on multimode multi‐step index (MSI) optical fibres. Starting from the Eikonal equation, we derive the analytical expressions that allow calculating the ray trajectories inside these fibres. We also analyse the effects of leaky rays on the transmission properties of MSI fibres. For this purpose, a single analytical expression for the evaluation of the ray power transmission coefficient is calculated. Afterwards, we investigate the effects of extrinsic and intrinsic coupling losses on the performance of MSI fibres, providing analytical expressions to calculate the coupling loss and, also, determining the most critical parameters. Finally, we carry out a comprehensive numerical analysis of the fibre bandwidth under different source configurations.     

Transmission properties of all-silica fibres for high-power Nd:YAG lasers

The transmission properties of two different types of all-silica fibres, step index and graded-index fibres, are investigated using a multimode high-power cw Nd:YAG rod laser. The reflections from the fibre surface back into the laser cavity affect the laser power and the beam parameters, waist diameter and divergence. A set-up for simultaneously measuring these values in front of and behind a fibre is presented. The maximum laser power transmitted by fibres is dependent upon the beam parameters, the fibre design and the surface conditions. An upper limit for the maximum divergence which can be coupled into a fibre with a certain numerical aperture without loss is given. The beam parameters behind the fibre, related to the input parameters, are presented.     

Simultaneous determination of fibre parameters by an improved four-photon mixing technique

We demonstrate the possibility of determining simultaneously the parameters of optical fibres using a stimulated four-wave mixing method without the requirement of considering the particular refractive index profile. The most suitable modal combinations that minimize the error in neglecting the refractive index profile are found and their dependence on the value of the V parameter. Experimental results that verify the accuracy of the proposed method are also given.     

Time domain measurement of dispersion in optical fibres

A time domain dispersion measurement apparatus was successfully built and tested at the Indian Institute of Technology, Madras. This instrument can measure directly the pulse broadening in a multimode optical fibre. The basic dispersion mechanisms in optical fibres and the design details of the system are discussed.     

Origin of the variability of the mechanical properties of silk fibres: 3. Order and macromolecule orientation in Bombyx mori bave,hand‐stretched strings and Nephila madagascarensis spider fibres

The comparison of the low wavenumber of polarized Raman spectra (50–300 cm^–1) from Bombyx mori (fresh cocoons fibres, hand‐stretched ‘Crins de Florence’ strings from the gland content, dried gland, regenerated silk films) and Nephila madagascarensis silks reveals the high polarisation of fibre modes and the absence of polarisation for dried gland and regenerated silk films. This is consistent with X‐ray diffraction measurements. The orientation of the fibroin/spidroin chains is due to the stretching during production, as for advanced synthetic fibres. The bandwidth of the ‘ordered chains’ signature is almost the same for the different fibres. However, the degree of polarisation seems to be higher in the case of spider fibre. The huge bandwidth of low wavenumber components of regenerated films indicates high disorder. Measurements along the fibre point out conformation changes with a periodicity (~20 mm) related to the silkworm head motion during the fabrication of the cocoon. Copyright © 2012 John Wiley & Sons, Ltd.     

New aspects in bandwidth measurements considering the effects of DMD

This paper focuses on the effects of differential mode delay (DMD) on the bandwidth of multimode optical fibres. First an analytical solution for the computation of the differential mode time delay is presented. The electrical field of each mode is calculated by the numerical solution of the Helmholtz equation. Based on this solution the modal power distribution as well as the fibre's impulse response under different launching conditions can be obtained.Next, the refractive-index profile of two fibres is modelled on the basis of DMD measurements. It is shown that these measurements provide enough information to predict the fibre's propagation characteristics under different launch conditions (excitation conditions).     

光导在折射率引导光纤、多孔光纤、光子带隙光纤和纳米线中的简要定性解释

本文是一篇教学论文,旨在通过对具体实例中不同物理过程的分析来定性解释光导拓宽固体材料折射率引导光纤光谱、多孔光纤光谱、光子带隙光纤光谱和纳米线光谱的原理。     

Detection of necking deformation along polypropylene fibres axis at low draw ratios using multiple-beam microinterferometry

A modified drawing device attached with an automated system for producing multiple-beam Fizeau fringes in transmission is used to study the mechanical deformation along polypropylene (PP) fibres at low draw ratios. Two drawing processes are used for drawing PP fibres. In the first process (fast drawing), the necking deformations are formed at draw ratios from 1.2 to 1.9 along the fibre axis. While in the other process (slow/step drawing), these deformations disappeared. The refractive indices calculated at different positions along the fibre axis during the fast- and slow-drawing processes. To overcome the formation of necking deformation along the PP fibres during fast-drawing process, the slow-drawing technique is recommended. Microinterferograms are given for illustration.     

Determination of the refractive index of fibres using the modified area method: I. Homogeneous fibres

The mathematical formulae for determining the mean refractive indices and birefringence of homogeneous fibres are modified. This modification depends on calculating the area under the fringe shift and cross-sectional area of homogeneous fibres having nearly circular cross-sectional area taking the refraction of the incident light beam by the fibre into consideration. The mathematical analysis for two-beam and multiple-beam interferometric techniques is dealt with. These formulae are applied to polypropylene fibres with draw ratios of 3.5 and 4 using the two techniques. A comparison between the results when using the new formulae and when neglecting the refraction has been studied. The diffraction of a He–Ne laser beam is used for determining the shape of the cross-sectional area of these fibres. Microinterferograms are given for illustration.     

Non-reciprocal transmission via phase conjugation in multimode optical fibres

We demonstrate phase conjugate correction of modal distortion of a single mode beam on double-passing a passive highly multimode fibre. Phase conjugation is achieved in a self-pumped photorefractive BaTiO₃ crystal which conjugates only the e-polarised input component. We show that only a small fraction of the multimode fibre output is required to faithfully conjugate the single mode input beam. This technique enables the demonstration of a novel non-reciprocal transmission device, which for example has immediate application in gain-grating holographic resonator designs with multimode fibres as the active gain medium.     

Origin of the variability of the mechanical properties of silk fibres: 2 The nanomechanics of single silkworm and spider fibres

The variability in mechanical stress–strain behaviour of various silks obtained from Bombyx mori silkworm and Nephila madagascarensis spider fibres has been studied by high resolution Raman analysis using the Raman shift induced by application of a controlled strain on the νN–H mode as a probe. Silk fibres exhibiting typical 1, 2, 3 and 4 Types have been selected from their characteristic tensile stress–strain behaviour. A perfect relationship between the nanomechanic (at the scale of the chemical bond) and macroscopic (silk single fibre) behaviour is observed as in the case of other polyamide fibres (natural keratin and synthetic PA66). Copyright © 2012 John Wiley & Sons, Ltd.