Optical Fiber Communications

Optical fiber communications typically operate in a wavelength region corresponding to one of the following “telecom windows”:

• The first window at 800–900 nm was originally used. GaAs/AlGaAs-based laser diodes and light-emitting diodes (LEDs) served as transmitters, and silicon photodiodes were suitable for the receivers. However, the fiber losses are relatively high in this region, and fiber amplifiers are not well developed for this spectral region. Therefore, the first telecom window is suitable only for short-distance transmission.
• The second telecom window utilizes wavelengths around 1.3 μm, where the loss of silica fibers is much lower and the fibers' chromatic dispersion is very weak, so that dispersive broadening is minimized. This window was originally used for long-haul transmission. However, fiber amplifiers for 1.3 μm (based on, e.g. on praseodymium-doped glass) are not as good as their 1.5-μm counterparts based on erbium. Also, low dispersion is not necessarily ideal for long-haul transmission, as it can increase the effect of optical nonlinearities.
• The third telecom window, which is now very widely used, utilizes wavelengths around 1.5 μm. The losses of silica fibers are lowest in this region, and erbium-doped fiber amplifiers are available which offer very high performance. Fiber dispersion is usually anomalous but can be tailored with great flexibility (→ dispersion-shifted fibers).

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 nm

The second and third telecom windows were originally separated by a pronounced loss peak around 1.4 μm, but they can effectively be joined with advanced fibers with low OH content which do not exhibit this peak.

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. The article on fiber-optic links gives more details.

A typical channel capacity for long-haul transmission for somewhat older systems is 2.5 or 10 Gbit/s; advanced systems offer 40, 100 or 160 Gbit/s or even more, and experimental systems have been demonstrated around 1 Tbit/s in one channel. 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. 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. 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.

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; this is called 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. Even such a huge capacity does 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:

• optical transmitters, based mostly on semiconductor lasers (often VCSELs), fiber lasers, and optical modulators
• optical receivers, mostly based on photodiodes (often avalanche photodiodes)
• optical fibers with optimized properties concerning losses, guiding properties, dispersion, and nonlinearities
• dispersion-compensating modules
• semiconductor and fiber amplifiers (mostly erbium-doped fiber amplifiers, sometimes Raman amplifiers) for maintaining sufficient signal powers over long lengths of fibers, or as preamplifiers before signal detection
• optical filters (e.g. based on fiber Bragg gratings) and couplers
• optical switches and multiplexers (e.g. based on arrayed waveguide gratings); for example, optical add/drop multiplexers (OADMs) allow wavelength channels to be added or dropped in a WDM system
• electrically controlled optical switches
• devices for signal regeneration (electronic or optical regenerators), clock recovery and the like
• various kinds of electronics e.g. for signal processing and monitoring
• computers and software to control the system operation

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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[1]	N. A. Olsson, “Lightwave systems with optical amplifiers”, J. Lightwave Technol. LT-7, 1071 (1989)
[2]	D. O. Caplan, “Laser communication transmitter and receiver design”, J. Opt. Fiber Commun. Rep. 4, 225 (2007), doi:10.1007/s10297-006-0079-z
[3]	R.-J. Essiambre et al., “Capacity limits of optical fiber networks”, J. Lightwave Technol. 28 (4), 662 (2010), doi:10.1109/JLT.2009.2039464
[4]	P. J. Winzer, D. T. Neilson and A. R. Chraplyvy, “Fiber-optic transmission and networking: the previous 20 and the next 20 years”, Opt. Express 26 (18), 24190 (2018), doi:10.1364/OE.26.024190
[5]	International Telecommunication Union (ITU), http://www.itu.int/home/index.html
[6]	G. P. Agrawal, Fiber-Optic Communication Systems, John Wiley & Sons, New York (2002)
[7]	H. J. R. Dutton, Understanding Optical Communications, http://www.freeinfosociety.com/media/pdf/5475.pdf, IBM Redbooks
[8]	Illustrated fiber optic glossary, http://www.fiber-optics.info/glossary-a.htm
[9]	R. Paschotta, tutorial on "Passive Fiber Optics"

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This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics Consulting GmbH. How about a tailored training course from this distinguished expert at your location? Contact RP Photonics to find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, training) and software could become very valuable for your business!