1.8 Optical Interfaces

Optical fibres are now playing a larger part in everyday data communications, and the cost : performance ratio of such fibre interconnects makes them a reasonable proposition when a large amount of data is to be carried over long distances. The key features of optical fibres are a large bandwidth, very low losses and immunity to interference, coupled with small size and flexibility. Also, when fibres are used as interconnects, devices are electrically isolated thus avoiding problems such as ground loops , shorts and crosstalk. Data is transferred over fibres by modulating a light source with the data, the light being carried within the fibre to an optical detector at the receiving end. The light source may be an LED or a laser diode, with the LED capable of operation up to a maximum of a few hundred megahertz at low power, whilst the laser is preferable in applications at higher frequencies or over longer distances.

1.8.1 Fibre Principles

The typical construction of an optical fibre is shown in Figure 1.19, and light travels in the fibre as in a typical ' waveguide ', by reflection at the boundaries. Total internal reflection occurs at the boundaries between the fibre core and its cladding, provided that the angle at which the light wave hits the boundary is shallower than a certain value called the critical angle , and thus the method of coupling the light source to the fibre is important in ensuring that light is propagated in the right way. Unless the fibre is exceptionally narrow, with a diameter approaching the wavelength of the light travelling along it (say between 4 and 10 m), the light will travel along a number of paths known as 'multimodes' and thus the time taken for a source pulse to arrive at the receiver may vary slightly depending on the length of each path . This results in smearing of pulses in the time domain, and is known as modal dispersion , or, looked at in the frequency domain, it represents a reduction in the bandwidth of the link with increasing length. Up to distances of around 1.5km the bandwidth decreases roughly linearly with distance (quoted in MHz per km), and after this in proportion to the square root of the length. Optical signals will also be attenuated with distance due to scattering by metal ions within the fibre, and by absorption due to water present within the structure. Losses are usually quoted in dB per km at a specified wavelength of light, and can be as low as 1dB/km with high quality silica, graded index multimode fibres, or higher than 100dB/km with plastic or ordinary glass cores. Clearly the high loss cables would be cheaper, and perhaps adequate for consumer applications in which the distances to be covered might be quite small.

Figure 1.19: Cross-section through a typical optical fibre, and mode of transmission.

Single mode fibres with very fine cores achieve very wide bandwidths with very low losses, and thus are suitable for use over long distances. Attenuations of around 0.5 dB/km are not uncommon with such fibres, which have only recently become feasible due to the development of suitable sources and connectors.

1.8.2 Light Sources and Connectors

LED (Light-Emitting Diode) light sources are made of gallium arsenide (GaAs) and can be doped to produce light with a wavelength between 800 and 1300nm. The bandwidth of the radiated light is fairly wide, having a range of wavelengths of around 40nm, which is another factor leading to greater losses over distance as the light of different wavelengths propagates over different modal paths within the fibre. The light from an LED source is incoherent (i.e. the phase and plane of the wavelets is random), and the angle over which it is radiated is quite wide. Since light radiated into the fibre at angles greater than a certain 'acceptance angle' will not be internally reflected it will effectively be lost in the cladding; thus the effectiveness of the coupling of light from an LED into the fibre is not good and only a few hundred microwatts of power can be transmitted.

An ILD (Injection Laser Diode) on the other hand produces coherent light of a similar wavelength to the LED, but over a narrower angle and with a narrower bandwidth (between around 1 and 3 nm in wavelength), thus providing better coupling of the light power to the fibre and resulting in less dispersion. Because of this, ILD drivers can be used for links which work in the gigahertz region whilst maintaining low losses.

Optical detectors are forms of photodiode which are very efficient at converting received light power into electrical current. Rise times of around 10 ns or less are achievable. The main problem with detectors is the distortion and noise they introduce, and different types of photodiode differ enormously in their S/N ratios. The so-called avalanche photodiode has a noise floor considerably lower than its counterpart , the PIN diode , and thus is preferable in critical applications. The integrated detector/preamplifier (IPD) provides amplification and detection of the light source in an integrated device, providing a higher output level than the other two and an improved S/N ratio.

The important features of connectors are their insertion loss and their return loss, these being respectively the amount of power lost due to the insertion of a connector into an otherwise unbroken link, and the amount of power reflected back in the direction of the source due to the presence of the connector. Typically insertion loss should be low (less than 1dB), and return loss should be high (greater than 40dB), for a reliable installation.

There are seven principal types of fibre optic connector, and it is not intended to cover each of them in detail here. For a comprehensive survey the reader is referred to Ajemian ⁸ . The so-called FDDI (Fibre Distributed Digital Interface) MIC connector is gaining widespread use in optical networks since it is a duplex connector (allows separate communication in two directions) with a typical insertion loss of 0.6 dB, whilst the SC connector is a popular 'snap-on' device developed by the Japanese NTT Corporation, available in both simplex and duplex forms, offering low insertion loss of around 0.25 dB and small size. The ST series of connectors, developed by AT&T, is also used widely.