Optical Fiber Communications

Author: the photonics expert Dr. Rüdiger Paschotta

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:

• Due to the large optical bandwidth, the transmission capacity of fibers for data transmission is huge: a single silica fiber can carry millions of telephone channels, utilizing only a small part of the theoretical capacity. In the last decades, 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 propagation losses of infrared light in fibers are amazingly small: ≈ 0.2 dB/km in the 1.5-μm spectral region for modern single-mode silica fibers, so that many tens of kilometers can be bridged without amplifying the signals.
• A large number of data 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.
• Fiber-optic cables are immune to problems that arise with electrical cables, such as ground loops or electromagnetic interference (EMI). Such issues are important, for example, for data links in industrial environments.

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.

Telecom Windows

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

• 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 propagation 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 (often through plastic optical fibers).
• 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 (erbium-doped fiber amplifiers). Also, low dispersion has been found not to be necessarily ideal for long-haul transmission, as it increases the effect of optical nonlinearities.
• The third telecom window, which is now very widely used, utilizes wavelengths around 1.5 to 1.6 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 utilization of even longer-wavelength bands has been considered, but there has not been much progress. In principle, one could achieve still substantially lower propagation losses, but only in certain non-silica fibers (e.g. fluoride fibers) with very low infrared absorption e.g. between 2 μm and 3 μm. It would be challenging to develop and commercially introduce such new fiber technologies, together with adapted transmitters, receivers and amplifiers.

The second and third telecom windows are further subdivided into the following wavelength bands:

The 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.

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. Bidirectional transmission is also possible, with a transceiver (transmitter and receiver combined) on each end. The article on fiber-optic links gives more details.

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.

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.

Multimode Fibers

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

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.

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.

Further Prospects

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.

Key Components for Optical Fiber Communications

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 propagation 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
• 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.

Fiber Cable Management

Various devices and accessories for fiber cable management are needed for managing telecom fiber cables, particularly where there are many of those. Some examples:

• Fiber patch panels can contain many adapters for fiber connectors. They may be used, for example, where fiber cables come into a building, and their signals need to be distributed.
• Fiber shuffles can provide fixed or even reconfigurable routing of signals between many fibers.
• Various types of accessories allow one to safely and orderly fix many fibers, or to protect sensitive things like fiber splices.
• Labeling solutions help one to keep track of many fibers.

More to Learn

Encyclopedia articles:

• fibers
• fiber cables
• telecom fibers
• silica fibers
• erbium-doped fiber amplifiers