A Deeper Dive into Fibre-Optic Communication.

Fibre-optic communication is a method of transmitting information from one place to another by sending pulses of light through an optical fibre. The light forms an electromagnetic carrier wave that is modulated to carry information. First developed in the 1970s, fibre-optic communication systems have revolutionized the telecommunications industry and have played a major role in the advent of the Information Age. Because of its advantages over electrical transmission, optical fibres have largely replaced copper wire communications in core networks in the developed world. Optical fibre is used by many telecommunications companies to transmit telephone signals, Internet communication, and cable television signals. Researchers at Bell Labs have reached internet speeds of over 100 petabit×kilometer per second using fibre-optic communication.

The process of communicating using fibre-optics involves the following basic steps: Creating the optical signal involving the use of a transmitter, relaying the signal along the fibre, ensuring that the signal does not become too distorted or weak, receiving the optical signal, and converting it into an electrical signal.

Modern fibre-optic communication systems generally include an optical transmitter to convert an electrical signal into an optical signal to send into the optical fibre, a cable containing bundles of multiple optical fibres that is routed through underground conduits and buildings, multiple kinds of amplifiers, and an optical receiver to recover the signal as an electrical signal. The information transmitted is typically digital information generated by computers, telephone systems, and cable television companies.

Transmitter:

The most commonly used optical transmitters are semiconductor devices such as light-emitting diodes (LEDs) and laser diodes. The difference between LEDs and laser diodes is that LEDs produce incoherent light, while laser diodes produce coherent light. For use in optical communications, semiconductor optical transmitters must be designed to be compact, efficient, and reliable, while operating in an optimal wavelength range, and directly modulated at high frequencies.

In its simplest form, a LED is a forward-biased p-n junction, emitting light through spontaneous emission, a phenomenon referred to as electroluminescence. The emitted light is incoherent with a relatively wide spectral width of 30-60 nm. LED light transmission is also inefficient, with only about 1% of input power, or about 100 microwatts, eventually converted into launched power which has been coupled into the optical fibre. However, due to their relatively simple design, LEDs are very useful for low-cost applications.

Today, LEDs have been largely superseded by VCSEL (Vertical Cavity Surface Emitting Laser) devices, which offer improved speed, power and spectral properties, at a similar cost. Common VCSEL devices couple well to multi-mode fibre.

A semiconductor laser emits light through stimulated emission rather than spontaneous emission, which results in high output power (~100 mW) as well as other benefits related to the nature of coherent light. The output of a laser is relatively directional, allowing high coupling efficiency (~50 %) into single-mode fibre. The narrow spectral width also allows for high bit rates since it reduces the effect of chromatic dispersion. Furthermore, semiconductor lasers can be modulated directly at high frequencies because of short recombination time.

A GBIC module (shown here with its cover removed), is an optical and electrical transceiver. The electrical connector is at top right, and the optical connectors are at bottom left.

 Receivers:

The main component of an optical receiver is a photodetector, which converts light into electricity using the photoelectric effect. The primary photodetectors for telecommunications are made from Indium gallium arsenide the photodetector is typically a semiconductor-based photodiode. Several types of photodiodes include p-n photodiodes, p-i-n photodiodes, and avalanche photodiodes. Metal-semiconductor-metal (MSM) photodetectors are also used due to their suitability for circuit integration in regenerators and wavelength-division multiplexers.

Optical-electrical converters are typically coupled with a Trans impedance amplifier and a limiting amplifier to produce a digital signal in the electrical domain from the incoming optical signal, which may be attenuated and distorted while passing through the channel. Further signal processing such as clock recovery from data (CDR) performed by a phase-locked loop may also be applied before the data is passed on.

Fibre cable types:

An optical fibre cable consists of a core, cladding, and a buffer (a protective outer coating), in which the cladding guides the light along the core by using the method of total internal reflection. The core and the cladding (which has a lower-refractive-index) are usually made of high-quality silica glass, although they can both be made of plastic as well. Connecting two optical fibres is done by fusion splicing or mechanical splicing and requires special skills and interconnection technology due to the microscopic precision required to align the fibre cores.

Two main types of optical fibre used in optic communications include multi-mode optical fibres and single-mode optical fibres. A multi-mode optical fibre has a larger core (≥ 50 micrometres), allowing less precise, cheaper transmitters and receivers to connect to it as well as cheaper connectors. However, a multi-mode fibre introduces multi-mode distortion, which often limits the bandwidth and length of the link. Furthermore, because of its higher dopant content, multi-mode fibres are usually expensive and exhibit higher attenuation. The core of a single-mode fibre is smaller (<10 micrometres) and requires more expensive components and interconnection methods, but allows much longer, higher-performance links.

In order to package fibre into a commercially viable product, it typically is protectively coated by using ultraviolet (UV), light-cured acrylate polymers, then terminated with optical fibre connectors, and finally assembled into a cable. After that, it can be laid in the ground and then run through the walls of a building and deployed aerially in a manner similar to copper cables. These fibres require less maintenance than common twisted pair wires, once they are deployed.

Specialized cables are used for long distance sub-sea data transmission, e.g. transatlantic communications cable. 2011–2013 cables operated by commercial enterprises typically have four strands of fibre and cross the Atlantic in 60-70 ms. Cost of each such cable was about $300M in 2011.

Another common practice is to bundle many fibre optic strands within long-distance power transmission cable. This exploits power transmission rights of way effectively, ensures a power company can own and control the fibre required to monitor its own devices and lines, is effectively immune to tampering, and simplifies the deployment of smart grid technology.

Multi-mode optical fibre in an underground service pit.

Amplifier:

The transmission distance of a fibre-optic communication system has traditionally been limited by fibre attenuation and by fibre distortion. By using opto-electronic repeaters, these problems have been eliminated. These repeaters convert the signal into an electrical signal, and then use a transmitter to send the signal again at a higher intensity than was received, thus counteracting the loss incurred in the previous segment. Because of the high complexity with modern wavelength-division multiplexed signals (including the fact that they had to be installed about once every 20 km), the cost of these repeaters is very high.

An alternative approach is to use an optical amplifier, which amplifies the optical signal directly without having to convert the signal into the electrical domain. It is made by doping a length of fibre with the rare-earth mineral erbium, and pumping it with light from a laser with a shorter wavelength than the communications signal (typically 980 nm). Amplifiers have largely replaced repeaters in new installations.

Wavelength-division multiplexing:

Wavelength-division multiplexing (WDM) is the practice of multiplying the available capacity of optical fibres through use of parallel channels, each channel on a dedicated wavelength of light. This requires a wavelength division multiplexer in the transmitting equipment and a de-multiplexer (essentially a spectrometer) in the receiving equipment. Arrayed wave-guide gratings are commonly used for multiplexing and de-multiplexing in WDM. Using WDM technology now commercially available, the bandwidth of a fibre can be divided into as many as 160 channels to support a combined bit rate in the range of 1.6 Tbit/s.