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.     

Evaluation of material dispersion in low loss phosphosilicate core optical fibres

A method is described for the measurement of material dispersion in the core of an optical fibre, over a wide wavelength range. The method is relatively insensitive to the pulse dispersion caused by group delay differences between modes in the fibre. It is found that the dispersion of phosphosilicate core material is no greater than that of fused silica and is independent of composition over the range measured.     

The effects of source configuration on bandwidth and loss measurements in optical fibres

Bandwidth and attenuation are not uniquely defined for multimode fibres. We present a theoretical examination of the effects on bandwidth and loss measurements of varying source configurations. Choice of source radius and numerical aperture can greatly influence the results, particularly when those parameters are chosen to be substantially less than the corresponding fibre values. Other factors such as source efficiency for exciting only bound modes and the effects of source misalignments are discussed.     

Twin-hollow-core optical fibres

Twin-hollow-core microstructured optical fibres have been fabricated and characterised for the first time. The fibre cladding structure results in guidance by the inhibited coupling mechanism, in which there is a low overlap between the core modes and surrounding structure. This results in minimal interaction between the modes of each core in the transmission bands of the fibre and hence minimal coupling between the cores. It is shown that light is able to couple between the cores via coupling to cladding struts in the high loss wavelength bands.     

Active fibres and optical amplifiers

The incorporation of rare-earth ions into glass fibres to form fibre lasers and amplifiers is not a recent development. In fact the first glass laser ever demonstrated [l] was flashpumped in the form of an optical fibre, a configuration that was used to overcome the difficulties of obtaining high-quality glass in bulk form. Apart from a report [2] in 1974 of laser operation in an Nd³⁺-doped silica multimode fibre, the idea of guided-wave glass lasers attracted little attention for the next 24 years. The idea resurfaced [3] in 1985 because both optical fibre and laser-diode technologies had advanced to a stage where low-loss, rare-earth-doped, single-mode fibres could be made and high-power semiconductor sources were available to pump them. In addition, low-cost fibre components (couplers, polarizers, filters) were available, which allowed construction of complex, all-fibre ring and Fabry-Perot resonators [4] to form a unique and powerful new fibrelaser technology. Even so, it was only the announcement in 1987 of a high-gain, erbiumdoped fibre amplifier (EDFA) [5] operating in the third telecommunications wave length window at 1.54 μm that sparked widespread interest in rare-earth-doped fibres in the optical telecommunications community. From that moment, frenzied worldwide activity has brought numerous new fibre amplifier developments and in 1990 resulted in several commercial products appearing, a time-lag of only three years after the first research announcement.     

Effects of mechanically induced periodic bends on the optical loss of glass fibres

In several experiments different glass fibres were periodically distorted by mechanical means. The shape of the distorted fibre was nearly sinusoidal with an amplitude x and a periodic A. These periodic bends induced in all fibres an additional loss α_B. For multimode fibres, α_B was constant for the measured optical wavelengths from 500 to 1100 nm. When the amplitude x of the distortion is increased, α_B increases according to the empirical power law α_B = b₁x^b2 with b₂ in the range of 3 to 4.For single mode fibres, the additional loss α_B depends on the optical wavelength. A maximum of α_B has been found in the wavelength range around 900 nm. At this maximum, α_B is larger than for multimode fibres.     

Asymmetries in optical fibres

Two methods for treating the effects of small asymmetries in the cross sections of roughly cylindrical optical fibres are described. These predict the presence of stable power distributions, ribbons for example, within the fibre core. Experimental results on actual fibres showing these effects are given. To be effectively asymmetric a fibre of elliptical cross section need have major and minor diameters differing by only 0.08 μm.     

介质损耗与电磁能量

根据能流密度公式,推出时谐场中存在线性有损介质情形的坡印亭定理.与能量守恒定律结合,得到极化损耗、磁化损耗和焦耳损耗功率密度公式,以及有损介质中的电磁场能量密度公式.最后,推出上述各量的平均值公式.     

Fluoride fibres for optical transmission

Optical fibres made from heavy metal fluorides have been under development for much of the past decade. There have been significant advances in understanding the fundamental characteristics of these materials. Progress towards achieving low optical losses and in optimizing the design of fluoride fibres for use in long transmission systems is reviewed.     

Bend radiation of optical fibres

Theoretical and experimental investigations of the radiation far field from a curved optical fibre have been carried out for the lowest order LP-modes. The investigations concern the characteristics of pure bend loss radiation in a direction orthogonal to the plane of the bend. Possible applications involving the bend radiation are briefly discussed.     

The similarities in loss creation in LTS and HTS cables

The coupling current losses represent an essential contribution to AC losses in most practical superconducting conductors. The origin of this loss type is well known, being caused by induced currents in different loops consisting of superconducting and non-superconducting parts. However, the ‘current pattern' in different conductor types (strands, flat or round cables, more complicated cable structures, CICC) varies appreciably. These differences are mainly due to geometrical effects (size and shape of filaments and/or strands, their spatial distribution, conductor aspect ratio, demagnetization effects). Although the general knowledge about AC losses in low temperature cable structures is by far not complete (mainly at higher frequencies, in inhomogeneous fields and for inhomogeneous cable structure), an attempt is made to summarize those results which can be adopted to high T_c conductors and some remarks are made about new features of AC losses in these conductor types.     

Efficient non-linear optical fibres and their applications

There seem to be three possible approaches to achieve efficient non-linear optical effects in fibres: the increase of optical intensity in fibres, the control of group velocity dispersion in fibres and the use of efficient non-linear optical materials for fibres. From this viewpoint, high-numerical-aperture single-mode fibres with high optical intensity, single-mode fibres with extremely small dispersion at the operating wavelength and LiNbO₃ single-crystal fibres with efficient non-linear effects, are now being investigated. This paper describes the fabrication of these optical fibres, and their non-linear optical applications.     

Cleaving of microstructured polymer optical fibres

The issue of how best to cleave PMMA microstructured polymer optical fibres (mPOF) is addressed. The impact of the following parameters on the cleaving process is considered: (i) temperature of the cutting blade, (ii) temperature of the platen holding the fibre, (iii) time allowed for thermal equilibration between fibre and platen, (iv) blade speed, and (v) blade condition. The strong influence of a temperature-dependent phase transition in the polymer on the cleaving process is established. Optically acceptable mPOF end-faces can be achieved but only over a limited range of cleavage conditions.     

Anomalous group delay in optical fibres

It is shown, by means of computation on a specific model, how pulse broadening in multimode gradedindex optical waveguides is significantly affected by the levels of excitation of the high-order modes. Pulse widths are computed as functions of the profile parameter, under conditions of equal excitation, high-order mode suppression and GaAs laser excitation.     

Non-linear applications of microstructured optical fibres

The enhancement of different non-linear processes in microstructured optical fibres can be achieved through manipulation of the dispersion characteristics of the fibre. This is demonstrated by extending the region of short wavelength operation of high power supercontinuum generation through four wave mixing in a cascaded fibre geometry where the dispersion of each fibre decreased on propagation. The technique is further refined in a demonstration utilizing long lengths of dispersion decreasing tapered microstructured fibres, where the supercontinuum extends to around 300 nm with spectral power densities in excess of 2 mW/nm in the uv. These long length tapers can also be utilized for adiabatic soliton pulse compression in new spectral regions, allowing the compression of 655 fs pulses to 45 fs.