Wavelength Division Multiplexing (WDM)
Before reading this you may find the following tutorials useful:
Optical Networks, Optical Fiber
As an optical network consists of optical fibers carrying flashes of light from a laser, you can improve the speed of information transfer by increasing the number of laser light flashes per second (increasing the bit-rate). However, a point comes at which the technology of lasers cannot meet the demands of an optical network. Just as with the lone, dirty old man that flashes at passersby in the park — he can only flash so fast. But what if this man were to invite a group of his dirty old friends? Now they could all flash at the same time and vastly increase the amount of information they are transmitting to the innocent people walking by.
In an optical network, you can increase the number of lasers and have them all sending their light down the optical fiber at the same time. However, there is a catch. If the dirty old man and his friends were all wearing the same color of trench coat, then people would not be able to distinguish the different sources of information. So each flasher would need to have his own color of trench coat, to make sure that his information is not confused with that of the others. Similarly, all the different lasers must give out different colors (different wavelengths) of light so that their information can be separated at the other end of the network. The sending of many different wavelengths down the same optical fiber is known as Wavelength Division Multiplexing (WDM).
Modern networks in which individual lasers can transmit at 10 Gigabits per second can now have several different lasers each giving out 10 Gbit/s through the same fiber at the same time. The number of wavelengths is usually a power of 2 for some reason. So WDM systems will use two different wavelengths, or 4, 16, 32, 64, 128, etc. Systems being deployed at present will usually have no more than maybe 32 wavelengths, but technology advancements will continue to make a higher number of wavelengths possible.
The act of combining several different wavelengths on the same fiber is known as multiplexing. At the receiving end, these wavelengths need to be separated again, which is known, logically enough, as demultiplexing. Each wavelength will then need its own light detector to convert it back into useful information.
The exact wavelengths of light being used are usually around the 1550 nanometer region, the wavelength region in which optical fiber performs the best (it has very “low loss” or “low attenuation” at this wavelength). Each different wavelength will be separated by a multiple of 0.8nm (sometimes referred to as “100GHz spacing,” which is the frequency separation; or as the “ITU-Grid,” named after the standards body that set the figure). So if you have four wavelengths you may have them at 1549.2nm, 1550nm, 1550.8nm, and 1551.6nm. However, you could also separate each by 1.6nm, or even 2.4nm, as long as it is some multiple of 0.8nm. Newer designs that aim to cram even more wavelengths into an even tighter space, may even have half the regular spacing (0.4nm) or a quarter (0.2nm). There can be problems with wavelengths spreading out (known as dispersion) and affecting neighboring wavelengths; so this and other more complicated issues need to be considered carefully when designing a WDM system.
- Increases capacity of optical fibers
- Different wavelength lasers each transmitting at same time down same fiber
- 'Multiplexing' is combining wavelengths; 'demultiplexing' is splitting wavelengths
- Usually in powers of 2 — 2, 4, 8, 16, 32, 64, 128, etc. wavelengths
- Wavelengths separated by multiples of 0.8nm (100GHz, ITU-Grid)
Laser Basics, Tunable Lasers, Nonlinear Effects, Optical Amplification