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The differential modal gain (DMG) in a few-mode erbium-doped fibre amplifier (FM-EDFA) is successfully improved by employing a ring-core FM erbium-doped fibre (RC-EDF). The DMGs between LP01 and LP11 mode signals in the FM-EDFA with the RC-EDF are 1.8 and 1.6 dB for LP01 and LP11 mode pumping, respectively, whereas those in a FM-EDFA with a conventional step-index FM EDF are 9.2 and 6.1 dB, indicating 7.4 and 4.5 dB DMG improvement for LP01 and LP11 mode pumping, respectively.

A novel mode control device is described, comprising a point-load mode scrambler in tandem with a mode filter. The device may be tuned to ensure compliance of fibre test equipment with new link performance standards. Improvement in the reproducibility of insertion loss measurements on concatenated fibres is demonstrated.

A mode scrambler based on an annealed step-index fibre is shown to have reproducible performance and source-launch-alignment insensitivity. Devitrification produces scattering centres in the fibre that distribute light uniformly over both angle and radius. An all-fibre mode scrambler with a 10 cm length and an insertion loss < 3.5 dB is demonstrated.

A new design of a mode scrambler for optical fibre measurements is presented. A rotation-splice technique has been used to mix the incoming mode power distribution in the fibre. The device is tested under different launch conditions to check its efficiency to produce a definite mode power distribution. Experimental results are presented.

For measuring the frequency response of an optical fibre, a mode scrambler utilising a sinusoidal bend and a new set-up based on the attenuation-comparison method have been proposed. Measurements for multimode step-index fibres have revealed that the long-fibre response can be closely predicted from the short-fibre response, using an appropriate choice for the scrambler parameter.

Mode coupling can change the noise properties of optical-fibre links. Fluctuations of the input field distribution of laser diodes with filamentary emission patterns can be smoothed out by statistical mode mixing along the fibre line.

An approach to single- (and low-) mode waveguides is described that enables the important mode characteristics to be derived from much reduced profile data. Uniformity with generalised parameters for multimode guides is demonstrated, and experimental determination discussed.

An experimental investigation about the reliability of an attractive method recently proposed for modal power distribution measurements in multimode fibres is performed. The method is shown to work also in some common cases when the hypotheses of validity are not rigorously fulfilled. Finally, an illustrative application to the testing of mode scramblers is presented.

We report a simple derivation of the electric field and modal properties of the fundamental mode of single-mode fibres only from the results of the variable-aperture launch method performed on the fibre.

The effect of a mode filter using fibre bend has been clearly shown experimentally. The mode filter effectively removes the LP11-mode, and has a negligibly small effect on the attenuation of the LP01-mode. The single-mode propagation was attained at the V-value of 2.8 by using the filter.

A method is presented for calculating the effects of random mode coupling in a multimode optical fibre. The calculated results are in good agreement with the experiment, and the relative error in the steady-state power loss is about 1%.

We report a simple and accurate approximation of modal dispersion in single-mode optical fibres deduced from spectral behaviour of mode spot size; neither knowledge of index profile nor of maximum refractive index difference are required in this approximation.

The dynamic optical spectrum and optical transmission characteristics of a 1.55 μm conventional distributed feedback (DFB) laser with a side mode were analysed. The laser used in the experiment came to operate in two modes although initially it had operated in single-mode. When this laser was directly modulated by setting the bias current around the threshold, the mode switching between a main mode and a side mode was observed to be similar to mode partitioning in Fabry-Perot (FP) lasers. The mode switching affects considerably the bit error rate performance of 113 km single-mode fibre transmission at 140 Mbit/s.

We report a simple calculation allowing the prediction of the modal dispersion in single-mode optical fibres from the spectral behaviour of the mode spot size and the profile index.