Fiber Optic Tutorials
Subcarrier Multiplexing (SCM)
In some LAN and MAN applications the bit rate of each channel is relatively low but the number of channels can become quite large. An example is provided by common antenna (cable) television (CATV) networks. The basic concept behind subcarrier multiplexing (SCM) is borrowed from microwave technology, which employs multiple microwave carriers for transmitting multiple CATV channels (electrical FDM) over coaxial cables or free space. The total bandwidth is limited to well below 1 GHz when coaxial cables are sued to transmit a multichannel microwave signal. However, if the microwave signal is transmitted optically by using optical fibers, the signal bandwidth can easily exceed 10 GHz for a single optical carrier. Such a scheme is referred to as SCM, since multiplexing is done by using microwave subcarriers rather than the optical carrier. It has been used commercially by the CATV industry since 1992 and can be combined with TDM or WDM. A combination of SCM and WDM can realize bandwidths in excess of 1 THz. The SCM technique essentially sends a radio or microwave signal over optical fibers and is also referred to as radio over fibers.
Optical Time-Division Multiplexing (OTDM)
TDM is commonly performed in the electrical domain to obtain digital hierarchies for telecommunication systems. In this sense, even single-channel lightwave systems carry multiple TDM channels. The electrical TDM becomes difficult to implement at bit rates above 40 Gb/s because of the limitations imposed by high-speed electronics. A solution is offered by the optical TDM (OTDM), a scheme that can increase the bit rate of a single optical carrier to values above 1 Tb/s. The OTDM technique was studied extensively during the 1990s, and further research has continued in recent years in the context of WDM systems with channel bit rates of 100 Gb/s or more. Its deployment requires new types of optical transmitters and receivers based on all-optical multiplexing and demultiplexing techniques. In this section we first discuss these new techniques and then focus on the design and performance issues related to OTDM lightwave systems.
WDM System Performance Issues
The most important issue in the design of WDM lightwave systems is the interchannel crosstalk. The system performance degrades whenever crosstalk leads to transfer of power from one channel to another. Such a transfer can occur because of the nonlinear effects in optical fibers, a phenomenon referred to as nonlinear crosstalk as it depends on the nonlinear nature of the communication channel. However, some crosstalk occurs even in a perfectly linear channel because of the imperfect nature of various WDM components such as optical filters, demultiplexers, and switches. In this section we discuss both the linear and nonlinear crosstalk mechanisms and also consider other performance issues relevant for WDM systems.
WDM Components
The implementation of WDM technology for fiber-optic communication systems requires several new optical components. Among them are multiplexers that combine the output of several transmitters and launch it into an optical fiber; demultiplexers which split the received multichannel signal into individual channels destined to different receivers; star couplers that mix the output of several transmitters and broadcast the mixed signal to multiple receivers; tunable optical filters which filter out one channel at a specific wavelength that can be changed by tuning the passband of the optical fiber; multiwavelength optical transmitters whose wavelength can be tuned over a few nanometers; add-drop multiplexers and optical routers that can distribute a WDM signal to different ports.
WDM Lightwave Systems
In principle, the capacity of an optical communication system can exceed 10 Tb/s because of a large frequency associated with the optical carrier. In practice, however, the bit rate was limited to 10 Gb/s or less until 1990 because of the limitations imposed by the dispersive and nonlinear effects and by the speed of electronic components. Since then, transmission of multiple optical channels over the same fiber has provided a simple way for extending the system capacity to beyond 1 Tb/s. Channel multiplexing can be done in the time or the frequency domain through time-division multiplexing (TDM) and frequency-division multiplexing...