Theory of propagation in optical fibres : Ray approach

The present paper is mainly a survey on geometrical methods and results in the study of propagation in optical fibers. Anyway the exposition is based on a new formalism which permits some simplifications on calculations and more immediate physical interpretation of results. About all problems of main interest in the theory of propagation in optical fibers are treated. Starting from coupling problems the paper investigates near-and-far-field intensity, about all kinds of attenuation (including scattering) and pulse distortion. The various arguments are homogeneously treated, the due evidence is given to results and to practical use of different methods. For some subjects of particular interest, like the problem of leaky rays or scattering phenomena, detailed discussion are performed. Useful results are supplied both by formulae and plots.     

Modes of propagation in optical fibres

Light propagates along optical fibres in form of normal modes. As long as fibres are uniform in axial direction, these modes follow as solutions of the eigenvalue problem which Maxwell’s equations together with the boundary conditions of the particular fibre represent. Such modes propagate independently from each other and transmit and distort signals according to their respective delay, dispersion and attenuation. Imperfections of the fibre couple its modes with each other and change its signal attenuation and transmission characteristics. When a randomly imperfect fibre guides many modes, the random coupling leads to a new set of statistical modes which again propagate independently from each other and describe the average power in each of the perfect fibre modes. Interaction between statistical modes occurs only at points along the fibre where the statistics of imperfections change such as at connectors or splices.     

Pulse propagation in multimode optical fibres

Coupled mode theory is unnecessary for the analysis of pulse propagation in multimode optic waveguides. Instead, a very simple ray or continuum equation is derived from elementary plane-wave concepts to account for the effects of absorption loss and scattering. The formalism reduces to a diffusion equation for highly forward directed scatterers.     

Fluoride-glass-cladded optical fibres for mid-infra-red ray transmission

Cladded-type fluoride glass fibres with a transmission loss of 0.1 dB/m at 2.5 ?m and 0.3 dB/m at 3.6 ?m, respectively, were fabricated by a novel casting method. Relative refractive-index differences between core and cladding ranging from 0.17% to 0.33% were obtained using BaF2-GdF3-ZrF4-AlF3 glasses as core and cladding components.     

Influence of laser fluctuations on soliton propagation in optical fibres

Laser fluctuations influence soliton propagation in optical fibres by randomly affecting their speed. The maximum propagation distance allowed for a given superposition probability between adjacent solitons is computed, permitting the evaluation of system limitations.     

A new approach to 3-D ray tracing for propagation prediction incities

A vertical-plane-launch (VPL) technique for approximating a full three dimensional (3-D) site-specific ray trace to predict propagation effects in cities for frequencies in the 300 MHz-3-GHz band is described and its predictions are compared with measurements for Rosslyn, VA. The VPL technique employs the standard shoot and bounce method in the horizontal plane while using a deterministic approach to find the vertical displacement of the unfolded ray paths. This approximation is valid since building walls are almost always vertical. The VPL method shows significant improvement compared with the slant-plane/vertical-plane (SP/VP) method for rooftop antennas. For a base station located at street level, the VPL method gives better predictions than the two-dimensional (2-D) method in locations where propagation over buildings is significant     

General formalism for the study of nonlinear pulse propagation in optical fibres

A general formalism for the study of nonlinear pulse propagation in optical fibres is presented. It is then applied to study the time evolution of a Gaussian laser pulse in a fibre with cubic and fifth-order nonlinearity in the index of refraction. The results are presented and discussed.     

Visible-region optical absorption property for germanium-doped silica optical fibres induced by UV light propagation

Visible-region absorption and fluorescence are first observed for GeO2-doped silica optical fibres propagated by UV light in the 350?370 nm region. An absorption band at 1.97 eV and fluorescence at 1.91 eV are observed in the UV-propagated fibre. The absorption and fluorescence are considered to be caused by creation of the oxygen-hole centre.     

Flip-chip approach to endfire coupling between single-mode optical fibres and channel waveguides

We report a practical approach to the positioning of a number of single-mode fibres for endfire coupling to channel waveguides. Boivin's approach has been extended to a flip-chip configuration which permits its use with substrates other than silicon.     

Analytical approach to prediction of dispersion properties of step-index single mode optical fibres

The use of the Rudolph-Neumann approximation at later stages of the derivation yields a waveguide dispersion formula much more accurate than previously published results. Material dispersion for an arbitrary dopant content is estimated by interpolating derivatives of the Sellmeier equation. Overall dispersion is thus predicted on an analytical basis.     

Limits to power transmission in optical fibres

Measurements of the transmission characteristics of a multimode fibre have been performed to determine the power throughput limitations set by Raman scattering. The results show that, in the presence of severe pump depletion, the total transmitted power is effectively determined only by the fibre loss.     

Soliton propagation in the presence of source chirping and mutual interaction in single-mode optical fibres

The effect of source chirping on soliton interaction is studied. For small chirping, soliton interaction can be reduced by the method of solitons with unequal amplitudes. For large chirping, however, even this method fails to work. In this case two initial pulses give rise to three solitons. Thus large chirping leads to disastrous mutual interaction between solitons.     

Attenuation changes in optical fibres due to hydrogen

The attenuation in an optical fibre due to dissolved hydrogen has been studied experimentally and theoretically as a function of time as the hydrogen is allowed to escape. The agreement confirms that the hydrogen is in molecular form and that the diffusion constant is 1 5×10?11 cm2/s at room temperature.