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.     

Resonance modes in optical fibres

The weakly nonlinear boundary value problem of wave propagation in an optical fibere(for the transverse electric mode,for example)is formulated and a modified linear solution is obained.It is shown that a self-consistent theory of fibre optics should be weakly nonlinear,The mode of critical refraction that does not exist in the linear theory is obtained,showing that it is a mode consisting of resonance modes,It is shown that the signal carriers in a long fibre are of resonance modes,not normal modes,Some experimental data are given for comparison with the theoretical predictions and the agreement seems satisfactory.     

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.     

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.     

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.     

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.     

Modes in weakly guiding elliptical optical fibres

Approximate and much simplified dispersion relations are obtained for the problem of optical wave propagation within weakly guiding elliptical fibres. The refractive index difference between the core and its cladding of weakly guiding optical fibres that are contenders for use as practical optical communication lines is very small; i.e., (n ₁/n₀–1) 1 wheren ₁ is the core index andn ₀ is the cladding index. These greatly simplified dispersion relations are then used to calculate the propagation constants for several higher order modes on an elliptical optical fibre.Supported partly by NELC, San Diego; this paper was presented at 1974 URSI Electromagnetic Waves Conference, London.     

Use of optical fibres in Raman spectroscopy

A detailed study of the parameters governing the design of an optical fibre system for use in Raman spectroscopy is described. The results have led to the design and construction of a flexible fibre optic remote sampling system which is easily adaptable for use on most commercial instruments. A variety of samples were used to evaluate the performance of the system and have been classified in terms of the efficiency of the new technique relative to more conventional methods of excitation and collection.     

Light polarization modulation in single-mode optical fibres

A theoretical and experimental study of light polarization modulation in single-mode anisotropic optical fibres is presented. The problem of increasing polarization modulation efficiency in slightly and strongly anisotropic fibres is considered. The requirements to be met by the optical configuration for efficient polarization modulation with both slightly and strongly anisotropic fibres are discussed. The polarization modulation signal is found to be immune to external effects on the fibre under compensated birefringence. An experimental polarization modulation-based fibredyne video transmission data highway is presented.     

Theory of scattering in multimode optical fibres

The problem of scattering in multimode optical fibres is treated in a rigorous way by means of both a ray and a statistical approach. These two methods are interlaced and harmonized. Applications to microbending, ellipticity, core radius and maximum numerical aperture variations, and fluctuations of index profile shape are performed. Useful results about power distribution and fibre attenuation are derived in each case.     

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.     

Medium loss optical fibres and cables

Many of the uses to which fibres will be put in the next few years will require at most 100 MHz bandwidth and relatively short fibre lengths. Research has therefore been concentrated on producing a high numerical aperture fibre using normal lead silicate glasses. Details are given for commercially available fibres having attenuation less than 100 dB km^-1 in the range 800–900 nm. These are available in both bundle and single fibres, the former sheathed in pvc and the latter in polypropylene.     

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.     

Curvature and microbending losses in single-mode optical fibres

Curvature of a single-mode optical fibre gives rise to two principal forms of additional transmission loss, namely transition loss and pure bend loss. The transition loss and the associated ray radiation, which have been observed at the beginning of a bend, can be satisfactorily explained by a modified coupled-mode theory. The radiation modes are represented by a quasi-guided mode having an average propagation constant _e. The introduction of a gradual change of curvature reduces the transition loss much more than the pure bend loss. Analysis of the microbending loss shows that the transition component is a maximum at a given correlation length which can be simply expressed in terms of _e. The contributions of both transition and bend components to the total microbend loss have been derived for the case of a randomly-curved fibre for several autocorrelation and density functions.     

Spectral and temporal Bloch oscillations in optical fibres

We report a detailed analysis of experimental parameters and fundamental mechanisms contributing to the strong nonlinear scattering of femtosecond and short picosecond laser pulses (50–6000 fs) at focusing in ambient air and protecting gases. The experimental conditions under consideration are typical for a variety of applications: micromachining of metals and dielectrics, high power laser exposure of targets. Such scattering, being the most noticeable manifestation of the complex phenomenon of conical emission in the focusing geometry, puts fundamental limitations to the incident power of precision laser targeting, restricting in this way potential productivity of micromachining in this pulse width range. The phenomena analyzed here are: ionization, the optical Kerr effect, the distortion of spectra via self-phase modulation, and the refractive ability of microplasma, namely the scattering. Characteristics of the last were investigated with respect to the incident wavelength and pulse width, for focusing of Gaussian and flat-top beams of different diameters. The obtained extensive data allowed us to trace the clue trends and relationships of this phenomenon and to find several ways to reduce or even eliminate the scattering in a broad range of the incident energies.     

Partially coherent optical waves in random gradient fibres

The excitation characteristics and spatial coherence of partially coherent optical waves in gradient fibres are demonstrated for incident light waves radiated from semiconductor lasers and light emitting diodes. Lower modes are efficiently excited in the case of coherent laser beams, while incident waves of low coherence such as lightwaves of LEDs excite higher modes. Pulse propagation of partially coherent optical waves in dispersive gradient fibres is also discussed for random index fluctuations. Mode coupled equations for temporal correlation functions of the electric field that are generalizations of coupled power equations are found. Mode filtering and pulse improvement with a lossy inhomogeneous cladding are described.     

Theory of dispersion in lossless multimode optical fibres

Using the ray propagation model generalized expressions for the impulse response of multimode fibres have been derived. The analysis has been applied to lossless fibres for pulses having both gaussian and lambertian spatial distributions as well as impulse and gaussian temporal distributions. Detailed results are given for output pulse shapes and fibre dispersions for various configurations.     

A ray approach to scattering in optical fibres

The problem of scattering in optical fibres is approached by means of a ray theory and by considering both the parameters that characterize each ray congruence. Starting from a general transport equation a two-variable diffusion equation is derived and its field of validity is discussed. Preliminary results (steadystate power distribution among ray congruences; steady-state attenuation and steady-state length) are given in some simple but common cases. These results permit interesting comparisons with other theories.