Material of the Month: Fibre optics

Materials World magazine
1 Jul 2014
network cable

This month, Anna Ploszajski uncovers the history of fibre optics.

In this digital age, our hunger for information is insatiable. We have the internet’s enormity at our fingertips, yet are dissatisfied to the point of abandonment if accessing it takes more than a few seconds. Fibre optics allow us to harness the power of the fastest data carrier in the universe – light – and keep pace with the ever accelerating present.

An optical fibre is a cylindrical waveguide that transmits light along its axis. Light is guided down the fibre by total internal reflection. Discovered by Johannes Kepler in 1611, and demonstrated in a curved jet of water by Daniel Collandon in 1841, this phenomenon occurs when light, travelling in an optically dense medium, meets a boundary at an angle greater than its critical angle and is completely reflected back into the material. The critical angle of a boundary is dependent on the refractive indices of the materials present. Refractive index is a measure of the speed of light through a material compared to the speed of light in a vacuum (300,000km/s). The larger the refractive index, the slower light travels.

Optical fibres consist of a core, surrounded by a cladding layer, and an outer buffer layer that acts as a protective coating. Because the refractive index of the core is greater than that of the cladding (typically 1.62 and 1.52 respectively), when light travels down the core it bounces off the cladding layer. Step-index fibres have an abrupt boundary between core and cladding, whereas graded-index fibres have a gradual boundary. This means that the light follows a sinusoidal path along the fibre length rather than a zig-zag, which minimises the effect of modal dispersion, whereby the signal becomes spread over time because its velocity is different for different modes.

For communications, the two main types of fibres are multi-mode and single-mode optical fibres. Multimode fibres have a larger diameter core (>50μm, half the width of a human hair), which allows the use of cheaper transmitters, receivers and connections than for thinner single-mode fibres (<10μm), but exhibit a lower performance than single-mode fibres.

Optical fibres are produced by drawing largediameter preforms into thin fibres at temperatures up to 2,200°C. The preforms are made by chemical vapour deposition and have the desired refractive index profile of the product fibre. A single preform can produce kilometres of optical fibre. The strength and attenuation of optical fibres, as well as their resistance to signal losses caused by microbending (tiny deviations along the fibre axis), are all dependent on environmental conditions, so fibres are coated with UV-cured urethane acrylate during the drawing process for protection. An additional metallic layer can be added for extra protection.

This is the modern world

Modern communications systems transmit telephone, internet and television signals by creating an optical signal, using LEDs in a transmitter. The signal is relayed along the fibres, and amplified several times before being received in a photodetector that converts light into electricity by the photoelectric effect. In 2011, the bandwidth record for an optical fibre was set at more than 100 terabits (100x1012 bits) per second. Wavelength division multiplexing divides the spectrum into signals of different wavelength, meaning each fibre is able to simultaneously transmit up to 80 independent signals in both directions.

As light travels through optical fibres, its intensity is reduced by transmission losses, known as attenuation, which are measured in decibels per kilometre. Attenuation occurs because of scattering and absorption of the optical signal. Rough and irregular internal interfaces in the fibre, even at the molecular level, cause a wide range of reflection angles and scatter the signal. In glass fibres, compositional fluctuations of short-range order and microstructural defects are large enough to act as scattering centres.

Attenuation also occurs because of the selective absorption of light of specific wavelengths in the UV, visible and IR parts of the spectrum. Incident UV and visible photons are absorbed by atoms in the fibre material, causing the excitation of outer electrons to higher energy states. This means the output signal is devoid of these portions of the input signal spectrum and this is the same phenomenon responsible for giving matter the colours we observe. Absorption of IR light occurs when the frequency of the incident photon matches that at which the particles of the material vibrate at their natural frequency. Different atoms and molecules have different natural frequencies of vibration. Therefore, different materials will selectively absorb different frequencies of IR light. Both mechanisms of UV-visible and IR absorption are routinely used in laboratory spectroscopy techniques, but are detrimental to the performance of optical fibres.

To compensate for attenuation, optical amplifiers are spaced along the fibre. Here, the input signal is mixed with a high-powered beam of light with a significantly different wavelength to the signal, using a wavelength selective coupler. The mixed beam is directed into a section of fibre doped with erbium ions. The high-powered light excites the erbium ions into a higher energy state, which transfer some of their energy to the signal by producing additional photons in their return to the lower energy state. These additional photons are of the exact same phase and direction as the original signal, thus amplifying the signal along the fibre axis.

The latest generation of optical fibres, photonic crystals, became commercially available in 2000. These fibres are made of silica and contain a triangular pattern of tiny air holes, a few micrometers across, which run along its length. They can carry higher power than conventional fibres, and their performance can be optimised by exploiting their wavelength-dependent properties.

Silica is the material of choice for most optical fibres, because it boasts good transmission over a wide range of wavelengths. Ultra-pure silica with a low concentration of hydroxyl groups achieves particularly high transparency in the near-IR region. Silica fibres have high mechanical strength, are chemically inert, do not absorb water, and the end surfaces have good optical quality, so can be joined effectively. Silica can be doped to tailor its refractive index – germanium dioxide or aluminium oxide increases the refractive index, whereas fluorine or boron trioxide lowers it. Doping fibres with rare earth metals makes laser-active fibres for amplifiers, but aluminosilicate-based or phosphate glass fibres are more suitable than pure silica due to their higher solubility for rare earth ions. Fluoride glass is used for specialist imaging applications and plastic optical fibres made from PMMA or polystyrene cores with silicone resin cladding are used for short distance (<100m) transmission. They have the advantage of high mechanical strength and low cost, since the associated optical connectors and installation are a lot cheaper than those of silica fibres, although the high attenuation (1dB/m) limits their working length.

Better than the rest
Optical fibre communication offers many advantages over copper wire systems using electrical signals. Firstly, optical fibres are able to transmit broadband signals, such that audio, video, microwave, text and computer data can all be transmitted over a single fibre. Secondly, optical fibres are immune to electromagnetic interference, since light is unaffected by other electromagnetic radiation and glass fibres are electrically non-conductive. As an electrical insulator, there are no ground loops or leakage of current. Attenuation is low in particular optical windows of the electromagnetic spectrum, therefore long-distance communication is possible. Furthermore, copper theft has become commonplace due to its inherent value on the scrap metal market, a problem not shared with optical fibres. Finally, data is more secure in fibre optic cables since wiretapping, or fibre-tapping in this case, is much more difficult.

Other applications of fibre optics are in remote sensors of strain, temperature and pressure, which can provide distributed sensing over distances up to one metre. These parameters modify the intensity, phase, polarisation, wavelength or transit time of light in the fibre. Such sensors can measure the temperature inside jet engines, whereby a fibre transmits radiation to a pyrometer (high-temperature thermometer). In medicine, endoscopes made of fibre optics allow doctors to view or operate on patients with minimal invasion and a similar set-up is employed to inspect the interior of jet engines. Fibre optics can also be incorporated in buildings to brighten internal rooms with natural sunlight, and optical fibre lamps make for decorative and artistic lighting.

When NASA sent a man to the moon, the optical fibres in their television cameras were so high-tech and confidential that only those with sufficient security clearance were permitted to handle them. The introduction of commonplace light speed information transmission by fibre optics has truly revolutionised the way we live, learn and relate to each other, and there’s no slowing down now.