W.A. Gambling

Propagation in radially-inhomogeneous single-mode fibre

The propagation of electromagnetic waves along a radially-inhomogeneous single-mode optical fibre with small index difference between core and cladding is studied. As in the case of the step-index fibre the propagation modes are weakly guided and their transverse fields are essentially polarized in one direction. The simple field descriptions and characteristic equation obtained here enable mode spot size and bending loss to be determined and are useful for practical design work of radially-inhomogeneous single-mode optical fibres.

Wave propagation in a single-mode fibre with dip in the refractive index

A theoretical study has been carried out on propagation in single-mode fibres having a dip in the refractive index at the centre of the core. The energy of the mode gradually spreads from the centre of the core with increase of the degree of the dip. Nevertheless the near and far-field distributions of such single-mode fibres are very nearly Gaussian in shape. A simple method of estimating the limit of the single-mode region for any kind of index profile is presented. For a particular class of profiles, dip widths up to 30% have a negligible effect on the propagation characteristics.

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.

Gigahertz bandwidths in multimode, liquid-core, optical fibre waveguide

An attenuation of 7 dB/km has been achieved over kilometre lengths of liquid-core, multi-mode optical fibre waveguide. The measured pulse dispersion depends on the mode distribution launched and on mode conversion which is a function of bend radius. Using a laser source a dispersion corresponding to a bandwidth ~ 1 GHz has been achieved and does not appear to be limited by fibre imperfections.

Pulse dispersion in a lens-like medium

An analysis is given of dispersion in an optical fibre waveguide having a continuous radial variation of refractive index using the scalar wave approximation. Solutions are presented for the particular case of Selfoc fibre taking into account mode dispersion, material dispersion and group delay. It is shown that for a correctly matched input Gaussian beam the pulse dispersion is small although in practice it is likely to be ∼ 1 ns over a length of 1 km.

A photometer to measure light scattering in optical glass

A photometer is described for the measurement of light-scattering in high-quality, optical bulk glass. The photometer operates continuously over the wavelength range 0.5 to 1μm and can measure scattering coefficients down to a level of 5×10−7 cm−1. A photon counting technique is used for light measurement and calibration is by reflection of the incident beam from an ideal diffusing screen. Measurements with high-purity benzene are in good agreement with computed and previous experimental values. Results of scattering from two samples of optical glass are given.

Measurement of attenuation in low-loss optical glass

An improved spectrophotometer for measuring the bulk attenuation of optical quality bulk glass over the wavelength range 500 to 1000 nm is described. It is capable of measuring loss coefficients equivalent to an attenuation of a few dB/km. Results obtained with lead flint and barium crown glasses are described.

The preparation of multimode glass- and liquid-core optical fibres

A preform technique for drawing cladded-glass and hollow fibres suitable for application to optical communications is described. The parameters which need to be controlled are discussed and the preparation of the preforms is described. The resulting fibres have a high geometric uniformity and a probe beam remains largely at the same angle to the axis after more than 106 reflections at the core/cladding interface. Fibre attenuations of 150, 60 and 5.8 dB km−1 have been obtained with commercial glasses, preforms made from a special melt at Sheffield University [5], and a commercial liquid, respectively. The fibre drawing process does not appear to introduce any additional impurities and heat treatment has produced a significant reduction of transmission loss in glass fibres.

Angular distribution of light scattering in bulk glass and fibre waveguides

Measurements of the angular distribution of light scattering in samples of optical quality glass and in cladded glass fibres have been made and the total scattering loss over the wavelength range 0.5-1.0µm has been obtained. The scattering shows a Rayleigh distribution indicating that (a) in similar samples the scattering loss can be inferred from a single measurement of the Rayleigh ratio at an angle of 90° to the incident beam and (b) no large inhomogeneities are introduced during the fibre-drawing process.

Transmission measurements in optical glass with an improved twin-beam spectrophotometer

Improvements are described to a twin-beam spectrophotometer for measuring the bulk attenuation of optical quality glass over the wavelength range 0.4 to 1.1 μm. Results are presented for a number of glasses and a comparison is shown between the attenuation of a multimode fibre with a core of F7 glass and various samples of F7 rods.