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Definition: optical communication links where the signal light is transported in fibers
A fiber-optic link is a part of an optical fiber communications system which provides a data connection between two points (→ point-to-point connection). It essentially consists of a data transmitter, a transmission fiber (possibly with built-in fiber amplifiers), and a receiver. These components are explained in the following, beginning with a simple single-channel system. More sophisticated approaches are discussed thereafter.
Figure 1: Schematic of a fiber-optic link, with a data transmitter, a long transmission fiber with several fiber amplifiers, and a receiver. The amplifiers can be supplemented with additional components for dispersion compensation or signal regeneration.
Transmitter
The transmitter converts the electronic input signal into a modulated light beam. The information may be encoded e.g. via the optical power (intensity), optical phase or polarization; intensity modulation is most common. The optical wavelength is typically in one of the so-called telecom windows (see the article on optical fiber communications). In most cases, the transmission is digital, making the system very versatile and relatively insensitive, e.g. to nonlinear distortions.
A typical transmitter is based on a single-mode laser diode (often a VCSEL), which may either be directly modulated via its drive current (→ DML = directly modulated laser), or with an external optical modulator (e.g. an electroabsorption or Mach-Zehnder modulator). Direct modulation is the simpler option, and can work at data rates of 10 Gbit/s or even higher. However, the varying carrier density in the laser diode then leads to a varying instantaneous frequency and thus to signal distortions in the form of a chirp. Particularly for long transmission distances, this makes the signal more sensitive to the influence of chromatic dispersion. Therefore, external modulation is usually preferred for the combination of high data rates (e.g. 10 or 40 Gbit/s) with long transmission distances (many kilometers). The laser can then operate in continuous-wave mode, and signal distortions are minimized.
For even higher single-channel data rates, time division multiplexing may be employed, where e.g. four channels with 40 Gbit/s are temporally interleaved to obtain a single channel with 160 Gbit/s.
For high data rates, the transmitter needs to meet a number of requirements. In particular, the pulse duration needs to be some fraction (e.g. 1/5) of the time slot for one bit. It is also important to achieve a high extinction ratio (low pedestal pulses), a low timing jitter, low intensity noise, and a precisely controlled pulse repetition rate. Of course, a data transmitter should operate stably and reliably with minimum operator intervention.
In simple cases, a light-emitting diode (LED) is used in the transmitter, but due to the poor spatial coherence this requires the use of multimode fibers. The transmission distance is then restricted due to intermodal dispersion; longer distances require single-mode fibers.
Transmission Fiber
The transmission fiber is usually a single-mode fiber in the case of medium or long-distance transmission, but can also be a multimode fiber for short distances. In the latter case, intermodal dispersion can limit the transmission distance or bit rate.
Long-range fiber links can contain fiber amplifiers at certain points (→ lumped amplifiers) to prevent the power level from dropping to too low a level. Alternatively, it is possible to use a distributed amplifier, realized with the transmission fiber itself, by injecting an additional powerful pump beam (typically from the receiver end) which generates Raman gain. In addition, means for dispersion compensation (counteracting the effects of chromatic dispersion of the fiber) and for signal regeneration may be employed. The latter means that not only the power level but also the signal quality (e.g. pulse width and timing) is restored. This can be achieved either with purely optical signal processing, or by detecting the signal electronically, applying some optical signal processing, and resending the signal.
Receiver
The receiver contains some type of fast photodetector, normally a photodiode, and suitable high-speed electronics for amplifying the weak signal (e.g. with a transimpedance amplifier) and extracting the digital (or sometimes analog) data. For high data rates, circuitry for electronic dispersion compensation may be included.
Avalanche photodiodes can be used for particularly high sensitivity. The sensitivity of the receiver is limited by noise, normally of electronic origin. Note, however, that the optical signal itself is accompanied by optical noise, such as amplifier noise. Such optical noise introduces limitations which can not be removed with any receiver design. Noise effects are discussed below in more detail.
Bidirectional Transmission
So-called full duplex links provide a data connection in both directions. These may simply be based on separate optical fibers, or work with a single fiber. The latter can be realized e.g. by using fiber-optic splitters (fiber couplers) at each end to connect a transmitter and a receiver. However, the need for bidirectional operation introduces various tradeoffs, which in some cases (e.g. for very high data rates) make a system with two separate fibers preferable.
Multiplexing
A typical single-channel system for long-haul transmission has a transmission capacity of e.g. 2.5 or 10 Gbit/s; higher data rates of 40 Gbit/s or even 160 Gbit/s may be used in the future. For higher data rates, several data channels can be multiplexed (combined), transmitted through the fiber, and separated again for detection.
The most common technique is wavelength division multiplexing (WDM). Here, different center wavelengths are assigned to different data channels. It is possible to combine even hundreds of channels in that way (→ DWDM = dense WDM), but coarse WDM with a moderate number of channels is often preferred in order to keep the system simpler. 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.
Limitations via Noise and Cross-talk
Ultimately, the data transmission capacity of any system is limited by noise. In amplified optical systems, quantum noise e.g. in the form of spontaneous emission in fiber amplifiers (→ amplifier noise) is not avoidable. It can impact the system performance in different forms, such as timing jitter (→ Gordon-Haus jitter, particularly in soliton systems) or intensity noise affecting the photodetection.
Note that the transmitter also has an important impact on noise issues. For example, a simple directly modulated transmitter may produce some unwanted frequency chirp, which increases the effect of chromatic dispersion in the transmission fiber and thus makes it more difficult to receive a clean signal after some propagation distance.
A related and even more sophisticated topic is cross-talk between the different channels e.g. of a WDM system. In systems with constant channel spacing, the channels can also influence each other in the form that one channel is amplified at the expense of the power in another channel (→ four-wave mixing). The impact of such effects can depend strongly on the system architecture, including the transmitter type, modulation format, fiber parameters, detection techniques, etc. The modeling of these effects and the subsequent optimization of communication systems are complex tasks.
Noise and related influences always cause some bit error rate, i.e., some portion of the transmitted bits will not be correctly detected. Provided that the bit error rate is at a sufficiently low level, occasional bit errors can be detected with certain techniques and corrected (e.g. by resending of defective data packets). For increasing transmission distances and/or data rates, the bit error rate finally sets some limits. In that context, the bandwidth-distance product is often used in a comparison of different fiber-optic links.
See also: optical fiber communications, optical data transmission, fiber-optic networks, bit error rate, bandwidth-distance product, fiber to the home, fiber amplifiers
Categories: communications, fibers and other waveguides
