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Optical fibers can be used to transmit light and thus information over long distances. Fiber-based systems have largely replaced radio transmitter systems (ground to ground or via satellite) for long-haul optical data transmission. They are widely used for telephony, but also for Internet traffic, long high-speed local area networks (LANs), cable TV (CATV), and increasingly also for shorter distances within buildings. In most cases, silica fibers are used, except for very short distances, where plastic optical fibers can be advantageous.
Compared with systems based on electrical cables, the approach of optical fiber communications (lightwave communications) has advantages, the most important of which are:
Mostly due to their very high data transmission capacity, fiber-optic transmission systems can achieve a much lower cost than systems based on coaxial copper cables, if high data rates are needed. For low data rates, where their full transmission capacity cannot be utilized, fiber-optic systems may have less of an economic advantage, or may even be more expensive (not due to the fibers, but the additional transceivers). The primary reason, however, for the still widespread use of copper cables for the “last mile” (the connection to the homes and offices) is simply that copper cables are already laid out, whereas new digging operations would be required to lay down additional fiber cables.
Fiber communications are already extensively used within metropolitan areas (metro fiber links), and even fiber to the home (FTTH) spreads more and more – particularly in Japan, where private Internet users can already obtain affordable Internet connections with data rates of hundreds of Mbit/s – well above the performance of current ADSL systems, which use electrical telephone lines. In other countries, one often tries to further extend the transmission capacities of existing copper cables, e.g. with the technique of vectoring, in order to avoid the cost of laying down fiber cables to the premises. This, however, is more and more seen only as a temporary solution, which cannot satisfy further growth of bandwidth demand.
It is also possible to transmit analog signals through fibers; that technology is called radio and microwave over fiber.
Optical fiber communications typically operate in a wavelength region corresponding to one of the following “telecom windows” (or communication bands):
The second and third telecom windows are further subdivided into the following wavelength bands:
Band Description Wavelength range O band original 1260–1360 nm E band extended 1360–1460 nm S band short wavelengths 1460–1530 nm C band conventional (“erbium window”) 1530–1565 nm L band long wavelengths 1565–1625 nm U band ultralong wavelengths 1625–1675 nmThe second and third telecom windows were originally separated by a pronounced loss peak around 1.4 μm resulting from OH (hydroxyl) absorption, but they can effectively be joined with advanced fibers with low OH content which do not exhibit that peak.
The use of the spectral bands is still limited by the availability of suitable fiber amplifiers. Erbium-doped fiber amplifiers can be optimized for the C band and/or the L band. Various other variants have been considered for shorter and longer wavelengths – for example, thulium-doped fiber amplifiers for the S band, praseodymium-doped amplifier (with fluoride glass) for the O band, or amplifiers with bismuth-doped silica for the O, E and S band. However, these are still not used to a substantial extent.
The simplest type of fiber-optic communication system is a fiber-optic link providing a point-to-point connection with a single data channel. Such a link essentially contains a transmitter for sending the information optically, a transmission fiber for transmitting the light over some distance, and a receiver. The transmission fiber may be equipped with additional components such as fiber amplifiers for regenerating the optical power or dispersion compensators for counteracting the effects of chromatic dispersion. Bidirectional transmission is also possible, with a transceiver (transmitter and receiver combined) on each end. The article on fiber-optic links gives more details.
Wavelength division multiplexing allows for enormous channel counts and overall transmission capacities.A typical channel capacity for long-haul transmission for older systems is 2.5 or 10 Gbit/s; more advanced systems offer 40, 100 or 160 Gbit/s or even more. The transmission capacity can be further multiplied by simultaneously using several, dozens or even hundreds of different wavelength channels (coarse or dense wavelength division multiplexing). Overall transmission capacities of many dozens of Tbit/s can be reached that way. Another approach is time division multiplexing, where several input channels are combined by nesting in the time domain, and solitons are often used to ensure that the sent ultrashort pulses stay cleanly separated even at small pulse-to-pulse spacings. Finally, one can employ space division multiplexing where different spatial channels are used – either with multi-core fibers or with multimode (few-mode) fibers. Hundreds of Tbit/s over thousands of kilometers are possible with such techniques.
Another important development is that of systems which link many different stations with a sophisticated fiber-optic network. This approach can be very flexible and powerful, but also raises a number of non-trivial technical issues, such as the need for adding or dropping wavelength channels, ideally in a fully reconfigurable manner, or to constantly readjust the connection topology so as to obtain optimum performance, or to properly handle faults so as to minimize their impact on the overall system performance. As many different concepts (e.g. concerning topologies, modulation formats, dispersion management, nonlinear management, and software) and new types of devices (senders, receivers, fibers, fiber components, electronic circuits) are constantly being developed, it is not clear so far which kind of system will dominate the future of optical fiber communications.
For a discussion of aspects such as bit error rates and power penalties, see the article on optical data transmission.
Within the last 30 years, the transmission capacity of optical fibers has been increased enormously. The rise in available transmission bandwidth per fiber is even significantly faster than e.g. the increase in storage capacity of electronic memory chips, or in the increase in computation power of microprocessors.
The transmission capacity of a fiber depends on the fiber length. The longer a fiber is, the more detrimental certain effects such intermodal or chromatic dispersion are, and the lower is the achievable transmission rate.
For short distances of a few hundred meters or less (e.g. within storage area networks), it is often more convenient to utilize multimode fibers, as these are cheaper to install (for example, due to their large core areas, they are easier to splice). Depending on the transmitter technology and fiber length, they achieve data rates between a few hundred Mbit/s and ≈ 10 Gbit/s.
Single-mode fibers are typically used for longer distances of a few kilometers or more. Currently used commercial telecom systems typically transmit between 10 Gbit/s and 160 Gbit/s per data channel over distances of ten kilometers or more. The required total capacity is usually obtained by transmitting many channels with slightly different wavelengths through fibers (wavelength division multiplexing, WDM). Total data rates can be many dozens of Tbit/s or even >100 Tbit/s, sufficient for transmitting many millions of telephone channels simultaneously. The main challenges are to suppress channel cross-talk via nonlinearities, to balance the channel powers (e.g. with gain-flattened fiber amplifiers), and to simplify the systems.
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Even such huge capacities (over 100 Tbit/s) do by far not reach the physical limit of an optical fiber. In addition, note that a fiber-optic cable can contain multiple fibers; it is also possible to utilize multi-core fibers, where multiple fiber cores are contained in a single fiber. Alternatively, space division multiplexing can also be realized with multimode fibers, using multiple-input multiple-output receiver technology.
In conclusion, there should be no concern that technical limitations to fiber-optic data transmission could become severe in the foreseeable future. On the contrary, the fact that data transmission capacities can evolve faster than e.g. data storage and computational power, has inspired some people to predict that any transmission limitations will soon become obsolete, and large computation and storage facilities within high-capacity data networks will be extensively used, in a similar way as it has become common to use electrical power from many power stations within a large power grid. Such developments may be more severely limited by software and security issues than by the limitations of data transmission.
Optical fiber communication systems rely on a number of key components:
In many cases, optical and electronic components for fiber communications are combined on photonic integrated circuits. Further progress in this technological area will help optical fiber communications to be extended to private households (→ fiber to the home) and small offices.
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Further research with optical fibers found that the fiber’s absorption and scattering effects which cause fiber’s attenuation were lower as wavelength increased. Another spectrum located around 1300 nm would have attenuation losses reduced to 1.5 dB/km using multimode fibers which resulted in immediate cost savings due to the elimination of costly regenerators/repeaters. The development of new high performance photo detectors and edge emitting LEDs along with the development of new solid-state laser diodes in the late 1970s and early 1980s provided the essential optical components required. It was at this time that the term "second window" was first used implying that 850 nm was the first window.
The second "window" of 1300 nm was used to define a spectral region past and was defined as 1300 nm +/- 50 nanometers (1250 nm – 1350 nm). With the high cost of amplifiers in the late 1980’s which would be required for single-mode oceanic spans starting with TAT-8. By using laser transmitters with a center wavelength of 1308.1 nm the expensive costs and numbers of amplifiers could be reduced. Rounding this number up to 1310 nm was a result that even today we use to call out single-mode fiber systems at 1310 nm vs 1300 nm. The term 1300 nm would be used by those using multimode fibers. Yet, both 1300/1310 nm are both in the spectral range of the second window.
The third window announced by NTT in 1977 would operate with a center wavelength of 1550 nm and provide lower attenuation (> .5 dB/km). Combined with the development of the Distributed Feedback (DFB) Laser, and erbium doped fiber amplifier this allowed for lower optical dispersion and the development of high speed and Dense Wavelength Division Multiplexing (DWDM) systems.
The fourth window of 1625 nm had higher optical attenuation but expanded the usable optical spectrum available for FTTx and WDM systems. Today, this window is also specified for maintenance of live and dark fiber systems per the International Telecommunications Union (ITU).
In our next article, I’ll address how the ITU defined the term "Bands" to identify specific wavelengths and how they are used in current and future fiber optic transmission systems.
Key point: Rounding up 1308.1 nm up to 1310 nm defines single-mode transmission to this day.
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