The evolution of low loss optical fibres

The possibility of guiding light by a dielectric cylinder has been known for many years and was demonstrated, for example, by the classic experiment of Tyndall using a water jet. However the effect was no more than a scientific curiosity and was not applied with any serious intent until 1951 when Hopkins & Kapany and also van Heel attempted image transmission in short coherent fibre bundles. Nevertheless real progress was not made until the idea of surrounding the light guiding core by a protective cladding was introduced in 1958. Two types of application then emerged, namely the short coherent bundle about 1 cm long in fibre optic faceplates, for example, and the incoherent flexible fibre bundle for use simply as a light conductor over distances of about 1m

Pulse dispersion for single-mode operation of multimode cladded optical fibres

Pulse dispersions as low as 0.4 ns/km have been measured in multimode cladded fibres at a normalised frequency V = 125 and for a constant bend radius of 5.5cm. Particularly when the number of launched modes is small, the pulse dispersion, as well as the polarisation and angular width of the output beam, are strong functions of the degree of mode conversion

Dispersion in low-loss liquid-core optical fibres

Measurements of pulse dispersion in liquid-core multimode fibres have been made for different fibre curvatures and core diameters. The results indicate the presence of mode-conversion effects, and a mode-filtering mechanism is demonstrated with a probing beam of narrow angular width. By measuring transmission loss as a function of angle of incidence, the loss in the cladding material is obtained

Optical fibres and the Goos-Hanchen shift

It is shown, and confirmed experimentally, that, despite the existence of the Goos-Hanchen shift, the propagation delay of a ray in a multimode cladded fibre is given to a very good approximation by simple ray theory

Radiation from curved single-mode fibres

A study of propagation in curved single-mode fibres shows that the transmission loss increases sharply below a critical bend radius. However the radiation does not leak away uniformly with distance but in a series of discrete well-defined rays

New glasses for active fibre devices

Through a series of case studies based on current research topics in Southampton, we describe optoelectronic devices whose realization is entirely dependent on new materials. The first is a practical optical fibre amplifier for the second telecommunications window at 1.3µm. Such a device based on rare-earth-doped fibres simply does not work in a silica host, where all useful emission is dissipated as heat in the glass. The second is a planar waveguide device, the lossless splitter. In this important component for fibre to the home, fibres with lengths of several metres would normally be required. New glasses allow greater concentrations of the active rare-earth dopant to be incorporated, thereby shrinking the size of the device to dimensions of a few centimetres. Thirdly, new glasses and fibres for fibre-based acousto-optic modulators will be described. These devices have the potential to allow direct modulation of the light within the fibre. Through these three case studies, we highlight the potential role of new materials in three key waveguide devices for telecommunications; amplifiers, splitters and modulators. The paper will conclude by reviewing overall efforts in Southampton in new glasses for optoelectronics, identifying key materials, their properties and applications in a global telecommunications network

Limiting bandwidth of a glass-fibre transmission line

The effect of natural dispersion on the bandwidth of a glass-fibre transmission line is analysed. It is shown that, if typical optical glasses are used, the limiting pulse rate, in both cladded fibres and those having a parabolic radial variation of refractive index, is not likely to exceed 10 Gbit/s over a distance of 10 km. The effect of the optical breakdown strength of the glass is also considered

Prospects for new glass-based optical waveguide devices

Why not silica? Over the past three decades, optical fibres based on high-purity silica, have established themselves as perhaps the ultimate communications material. These global cobwebs of glass have revolutionized telecommunications, reaching virtually every populated region on earth and providing enormous bandwidth, the full extent of which has yet to be exploited. Passive waveguides are today being spliced together with lengths of fibre doped with the rare-earth ion erbium, providing optical fibre amplifiers which can boost a fading signal by three orders of magnitude. This combination of active and passive waveguides, has made possible all optical networks, with no electrical/optical interfaces except at the signal source and receiver, and paved the way for global optical fibre telecommunications

Pulse dispersion in glass fibres

Measurements indicate that pulses incident normally on the end of a cladded multimode fibre are broadened by less than 0.1 ns over a length of 20 m. The measured dispersions in lengths of 20 m and 35 m do not exceed 5 ps/m. However, with an angle of incidence of 17°, or with a defocused input beam, the pulses are broadened by 0.6 ns

Pulse propagation along optical fibres

Mode-locked helium-neon lasers have been used to propagate pulses of ~1 ns duration along multimode cladded glass fibres. Any pulse spreading due to dispersion in a 33 m length of fibre is less than 0.5 ns which is the limit of resolution of the measuring equipment. This result indicates that a pulse transmission rate of at least 33 MHz may be possible over a distance of 1 km