The field of fibre optics communications has exploded over the past two decades. Fibre is an integral part of modern day communication infrastructure and can be found along roads, in buildings, hospitals and machinery.
The fibre itself is a strand of silica based glass, it's dimensions similar to those of a human hair, surrounded by a transparent cladding. Light can be transmitted along the fibre over great distances at very high data rates providing an ideal medium for the transport of information.
This section explains the basic fibre equipment parameters, and their most common types:
- Fibre Modes
- Common Transmission Wavelengths
- 1310nm (often known as 1300nm)
- Common Connector Types
Fibre comes in 2 basic types Multimode and Singlemode:
- Multimode: used within buildings or over short distances (e.g. on a University campus). Multimode fibre and multimode fibre equipment is cheaper but is generally of lower quality, less sensitive, and suffers from higher losses, so it can only be used over limited distances, e.g. 500m for standard multimode equipment, up to a maximum of 5km for the best multimode equipment.
- Singlemode: used in carrier transmission networks over both short and long distances, and is generally laid underground by the Telecoms operators. Singlemode fibre and singlemode equipment is more expensive but is generally of higher quality, more sensitive, and suffers from lower losses, so it can be used over higher distances, e.g. 15km for standard singlemode equipment, up to 100km or even further for the best singlemode equipment.
Fibre equipment uses various basic wavelengths (colours of light):
- 850nm (actually 800nm-900nm): used in historic equipment and also some modern Gigabit Ethernet equipment. 850nm equipment is generally the cheapest, can only be used over multimode fibre, and will only operate over short distances, e.g. up to 500m.
- 1310nm (actually 1260nm-1360nm): used in most quality transmission equipment. 1310nm equipment is of medium price, can be used over both multimode and singlemode fibre, and will operate over medium distances, e.g. up to 5km for multimode and up to 70km for singlemode.
- 1550nm (actually 1430nm-1580nm): used in high end transmission systems. 1550nm equipment is generally expensive, is only used over singlemode fibre, and will operate over long distances, e.g. in excess of 100km.
- Bi-Di: where 1550nm is used as the transmission wavelength in one direction and 1310nm is used as the transmission wavelength in the other direction, allowing a single fibre to be used for a bi-directional link. Also known as WDM.
- CWDM (Up to 20 off 20nm spaced narrow bands within the 1270nm-1610nm band): used for multiplexing multiple applications over a single pair of singlemode fibre. CWDM will operate over short or long distances.
- DWDM (Up to 160 off 100GHz spaced very narrow bands with the 1270nm-1610nm band): used for Telecoms carrier backbones or where very high densities of applications share limited fibre resources. DWDM will operate over short or long distances.
Fibre equipment has a variety of connectors, with the common connector types described below:
- FC: screw on metal fibre connector, which is used on historic singlemode equipment
- ST: bayonet metal or plastic fibre connector, which is used on historic multimode equipment
- SC: push fibre plastic connector, which is used on a variety of modern equipment
- LC: push fibre plastic connector, similar but half the size of SC, which is used on modern Ethernet equipment
This section will provide explanation's for some of the terms associated with the field of fibre optic engineering for telecommunications.
- Fibre Basics
- Light in a fibre
- Transmission Characteristics of Fibre
- Jargon Buster
The diagram shows the typical structure of a singlemode fibre used for communication links. It has an inner glass core with an outer cladding. This is covered with a protective buffer and outer jacket. This design of fibre is light and has a very low loss , making it ideal for the transmission of information over long distances.
Multimode fibre is similar but has a much larger core which has different characteristics. This fibre is considerably cheaper but the light degrades quicker with distance and therefore multimode fibre be used over long distances.
Light in a fibre
The light propagates along the fibre by the process of total internal reflection. The light is contained within the glass core and cladding by careful design of their refractive indices.
The loss along the fibre is low and the signal is not subject to electromagnetic interference which plagues other methods of signal transmission, such as radio or copper wire links.
The signal is, however, degraded by other means particular to the fibre such as dispersion (described below) and non linear effects (caused by a high power density in the fibre core)
Transmission Characteristics of Fibre
The loss, or attenuation of fibre depends on the wavelength of the light propagating within it.
There are three main bandwidth 'windows' of interest in the attenuation spectrum of fibre. The 1st window is at 800-900nm, where there is a good source of cheap silicon based sources & detectors. The 2nd window is at 1260-1360nm, here there is low fibre attenuation coupled with zero material dispersion (see dispersion below) . The 3rd window of interest is at 1430-1580nm where fibre has it's attenuation minimum.
Typically the telecommunications industry use wavelengths in the 3rd window for their backbone infrastructure since this wavelength coincides with the gain bandwidth of Fibre Amplifiers (see EDFA below). In the future the search for greater bandwidth is likely to open up other windows for fibre transmission.
Cable length, fibre type, fibre quality, connectors and patch panels all contribute to attenuation in fibre optic links. To operate correctly fibre optic links needs to be compatible in terms of wavelength, fibre type (multimode or singlemode) and power level. A key parameter in determining the compatibility of fibre equipment power levels is the fibre optic loss budget which is measured in dB.
Ball park attenuation values are as follows:
- Multimode fibre: 0.5dB to 1dB per km
- Singlemode fibre: 0.2dB to 0.5dB per km
- Connectors: 0.5dB to 1dB
So by adding up the attenuation of the various components of a fibre optic link it is possible to calculate whether there is sufficient fibre optic loss budget for the link, and therefore whether the link will work properly.
Light from a typical optical source will contain a finite spectrum. The different wavelength components in this spectrum will propagate at different speeds along the fibre eventually causing the pulse to spread. When the pulses spread to the degree where they 'collide' it causes detection problems at the receiver resulting in errors in transmission. This is called Intersymbol Interference (ISI). Dispersion (sometimes called chromatic dispersion) is a limiting factor in fibre bandwidth, since the shorter the pulses the more susceptible they are to ISI.
EDFA - Erbium Doped Fibre Amplifier
Otherwise known as a fibre or optical amplifier, the EDFA is an important component in long distance fibre links. Fibre and component attenuation in modern telecommunications links degrade the transmitted signal. When the signal power becomes too low errors will occur at the optical receiver as it struggles to recognise the transmitted signal from received noise.
Before the introduction of EFDAs, in order to transmit signals over long distances the signal would be detected and retransmitted at regular intervals, this process was called regeneration. EDFAs provide the telecommunications engineer with the means to optically amplify the signal en-route without converting the signal from the optical back to the electrical domain. The component works by the principle of stimulated emission. A piece of fibre doped with Erbium irons is pumped by a laser at high powers. The excited erbium irons release their energy when the data signal is passed through the fibre. The process is such, that the energy they release matches the signal exactly, thus amplifying the signal.
TDM - Time Division Multiplexing
The diagram below illustrates that is a method of incorporating many signals into one. Many slower speed signals are sampled onto one high speed signal.
DWDM - Dense Wavelength Division Multiplex
Dense Wavelength Division Multiplexing is a method of expanding the bandwidth of fibre. Many high speed signals are multiplexed together using different wavelengths (or colours) for transmission. In this way up to 160 different applications can be multiplexed down the same singlemode fibre pair. The diagram below illustrates the concept.
Three high speed signals transmitted over the same fibre using different wavelengths.
CWDM - Coarse Wavelength Division Multiplex
Coarse Wavelength Division Multiplexing is a simpler alternative to DWDM whereby wider transmission bands spaced 20nm apart are used. The wider transmission bands means that higher tolerances are available which makes CWDM equipment cheaper than DWDM equipment. With CWDM up to 20 applications can be made to share the same fibre pair.
WDM - Wavelength Division Multiplex
Often known as bi-directional (or Bi-Di) transmission, Wavelength Division Multiplexing is a method of simultaneously putting both directions of transmission down a single fibre. This is done by using different wavelengths (1550nm and 1310nm) for each transmission direction. WDM equipment therefore needs to be used in pairs with each end transmitting and receiving on compatible wavelengths of light with its partner equipment.
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