Fiber Optic Communication Basics

The theoretical bandwidth of optical fiber transmission in the 1550 nm window alone is on the order of terabits. Current fiber optic systems have not even begun to utilize the enormous potential bandwidth that is possible.

There are two methods that are employed to achieve an increase in bandwidth. The first is known as Time Division Multiplexing or TDM. Multiple channels are transmitted on a single carrier by increasing the modulation rate and allotting a time slot to each channel. However, more sophisticated high-speed electronics, at both the transmitting and receiving ends of the communications link, are required when increasing the bit rate of a system. And as the bit rate increases, inherent modulation limiting characteristics of optical fibers become dominant. Chromatic and polarization mode dispersion cause pulse spreading, which affects the signal quality over longer transmission distances.

An alternate method for increasing the capacity of fiber optic communications systems is known as wavelength division multiplexing, or WDM. By this method, capacity can be increased by using more than one optical carrier (wavelength) on a single fiber. Therefore, adding a second transmitter and receiver to an optical fiber can double the bandwidth of that communications system. This method of increasing the capacity of an optical system has appeal for a variety of reasons. If a system were to increase in capacity using TDM alone, the existing transmitter and receiver would be replaced with a faster and more expensive transmitter/receiver pair. Using WDM, the existing transmitter and receiver do not need to be replaced. A second transmitter/receiver pair of a different wavelength is simply added. This is done by coupling, or multiplexing the output of the two lasers into a single fiber. At the receiving end, the two wavelengths are then separated, or demultiplexed, and each optical carrier is routed to its own receiver. For transmission systems using a 1310 nm laser, a second laser at 1550 nm is usually added. The reason for choosing these wavelengths is that they lie in the “windows” or ranges of least attenuation. This allows the signal to travel a longer distance.

The ITU (International Telecommunication Union) has proposed a set of closely spaced wavelengths in the 1550 nm window. This method of WDM is known as Dense Wavelength Division Multiplexing, or DWDM. These different wavelengths or channels, are spaced 100 GHz apart, which is approximately 0.8 nm. This set of channels is commonly known as the ITU-T grid, and is specified in frequency. The reason the 1550 nm window was chosen by the ITU is twofold: it is in one of the windows that has the smallest amount of attenuation; and it also lies in the band in which erbium doped optical amplifiers operate.

The all-optical network will be the next evolution in optical communications. Current DWDM systems are point-to-point links meaning that the signals have a single distinct starting and ending point. Research is being performed to help these networks evolve into fully configurable networks, which are not limited to fixed point-to-point links.

Transparency in the optical layer opens many possibilities for the future. Digital and analog transmission can occur on the same fiber. Different bit rates using different protocols will all travel together. Current research is being performed on reconfiguring an optical network in real time. Wavelength selective switching allows wavelengths to be routed through the network individually. Some of the applications of this are for network restoration and redundancy, which may reduce or entirely eliminate the need for an entire back up system to help the network recover from failures such as equipment malfunctions or fiber breaks. A reconfigurable network may offer bandwidth on demand to configure itself to optimize for traffic bottlenecks. The future may also include wavelength translation to convert traffic on one wavelength to another wavelength in the optical domain.
All optical switching is still in the research phase; however, researchers are looking for ways to create reliable, low loss switches with fast switching speeds. Investigation into the possibility of optical packet switching and other novel technologies are currently underway. The all-optical network may be just around the corner.