Optical Fiber Communications

Optical fibers can be used to transmit light and thus information over long distances. Fiber-based systems have largely replaced radio transmitter systems 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, and increasingly also for shorter distances. In most cases, silica fibers are used, except for very short distances, where plastic optical fibers can be advantageous.
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Compared with systems based on electrical cables, the approach of optical fiber communications (lightwave communications) has advantages, the most important of which are:

See also our useful tutorial "Passive Fiber Optics"! This explains many aspects of fiber optics using interesting simulations.

The capacity of fibers for data transmission is huge: a single silica fiber can carry hundreds of thousands of telephone channels, utilizing only a small part of the theoretical capacity. In the last 30 years, the progress concerning transmission capacities of fiber links has been significantly faster than e.g. the progress in the speed or storage capacity of computers.

The losses for light propagating in fibers are amazingly small: ≈ 0.2 dB/km for modern single-mode silica fibers, so that many tens of kilometers can be bridged without amplifying the signals.

A large number of channels can be reamplified in a single fiber amplifier, if required for very large transmission distances.

Due to the huge transmission rate achievable, the cost per transported bit can be extremely low.

Compared with electrical cables, fiber-optic cables are very lightweight, so that the cost of laying a fiber-optic cable can be lower.

Fiber-optic cables are immune to problems that arise with electrical cables, such as ground loops or electromagnetic interference (EMI).

However, fiber systems are more sophisticated to install and operate, so that they tend to be less economical if their full transmission capacity is not required. Therefore, the “last mile” (the connection to the homes and offices) and is usually still bridged with electrical cables, whereas fiber-based communications do the bulk of the long-haul transmission. Gradually, however, fiber communications are being used within metropolitan areas (metro fiber links), and even fiber to the home (FTTH) is being developed – particularly in Japan, where private Internet users can already obtain affordable Internet connections with data rates of 100 Mbit/s – well above the performance of current ADSL systems, which use electrical telephone lines.

Telecom Windows

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.

System Design

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 is nowadays 2.5 or 10 Gbit/s; 40, 100 or even 160 Gbit/s may be used in the future. More advanced systems increase the transmission capacity by simultaneously using several, dozens or even hundreds of different wavelength channels (coarse or dense wavelength division multiplexing). 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.

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.

Transmission Capacity of Optical Fibers

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. Current commercial telecom systems typically transmit 2.5 or 10 Gbit/s per data channel over distances of ten kilometers or more. Future systems may use higher data rates per channel of 40 or even 160 Gbit/s, but currently 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 several terabits per second, sufficient for transmitting many millions of telephone channels simultaneously. Even this capacity does not reach by far the physical limit of an optical fiber. In addition, note that a fiber-optic cable can contain multiple fibers.

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.