Light-Emitting Diodes. (Monographs in Electrical and Electronic Engineering.) By A. A. Bergh and P. J. Dean. Pp. viii+591. (Clarendon: Oxford; Oxford University: London, 1976.) £22.

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Light-emitting diodes

The authors give a brief historic perspective of the coupled mode theory. The development and applications of the theory in microwaves in early years and in optoelectronics and fiber optics in recent years are described. They then consider lossless coupling of two modes in time. Two coupled resonance circuits, or two coupled microwave or optical resonators, are the physical examples. The start-up of a parametric oscillator is another example. Then they look at the formal derivation of coupled mode theory and consider the more general case when the modes are not energy-orthogonal and the energies are not necessarily positive. A more detailed account of the nonorthogonal coupled mode theory developed in the last five years for optical waveguides is given. >

Coupled-mode theory

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Statistical Theory of Communication

technology. This has been necessitated by the need to access hostile or difficult environments in situ and in vivo. strategies to date have included the use of specialist fibres and miniaturized scanning systems accompanied by ingenious microfabricated lenses. Here we present a novel approach for this field by utilizing disordered light within a standard multimode optical fibre for lensless microscopy and optical mode conversion. We demonstrate the modalities of bright- and dark-field imaging and scanning fluorescence microscopy at acquisition rates that allow observation of dynamic processes such as Brownian motion of mesoscopic particles. Furthermore, we show how such control can realize a new form of mode converter and generate various types of advanced light fields such as propagation-invariant beams and optical vortices. These may be useful for future fibre-based implementations of super-resolution or light-sheet microscopy.

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Exploiting multimode waveguides for pure fibre-based imaging

The prediction of light propagation up to hundreds of millimetres within straight or even deformed segments of multimode fibres is demonstrated. The concept is applied in an endoscope and exceptional resolution and footprint are obtained.

Seeing through chaos in multimode fibres

A simple method has been devised for the experimental determination of mode conversion coefficients in multimode fibers and involves only the observation of the far-field output as the angle of incidence of a collimated input beam is changed. The normalized mode coupling coefficient in a liquid-core fiber is D = 3 x 10-6 rad2 / m and increases by as much as a factor of 10 when transverse pressure is applied. Values some 2 orders of magnitude larger are found in glass-core fibers. There is good agreement between the theory presented and experiment.

Mode conversion coefficients in optical fibers.

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 constantβ
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 ofβ
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.

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Curvature and microbending losses in single-mode optical fibres

The limit for single-mode operation in a graded-index fibre has been obtained by calculating the normalised cut-off frequency of the TE01 mode, The affect of diffusion at the core-cladding boundary has been estimated.

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Cut-off frequency in radially inhomogeneous single-mode fibre

Simple formulae are given for the field displacement caused by bending, the change in mode volume and for the mode coupling loss which arises at transitions between straight and curved sections of single-mode optical fibre. These formulae are in good agreement with other, more exact, theories.

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Field deformation in a curved single-mode fibre

Experimental and theoretical studies on the radiation loss from curved single-mode fibres show that it includes contributions due to transition loss at discontinuities between straight and curved sections of fibre, as well as the bend loss due to uniform curvature. These two contributions have been measured separately, and there is good agreement with the theoretical predictions. The transition loss at large bend radii can be considerably greater than the uniform bend loss. It is found that the transition loss incurred by a change of fibre curvature from R to -R is about 8 times bigger than that from R to infinity. The loss due to fibre undulation has also been measured and in this case the results can be completely explained using only the pure bending loss theory.

W.A. Gambling

Papers

Mode conversion coefficients in optical fibers.

Curvature and microbending losses in single-mode optical fibres

Cut-off frequency in radially inhomogeneous single-mode fibre

Field deformation in a curved single-mode fibre

Measurement of radiation loss in curved single-mode fibres