Transmission Basics  |  Components  |  Applying Fiber Optic Technology  |  System Performance  

Wavelength-division Multiplexing

The fiber optic industry first deployed single wavelength transmission links. As requirements changed, the industry responded with wavelength-division multiplexing (WDM), which sends two distinct signals per fiber, doubling transmission capacity. Similar to a simple splitter, WDMs typically have a common leg and a number of input or output legs. Unlike the splitter, however, they have very little insertion loss. They do have the same range of excess loss. Two important considerations in a WDM device are crosstalk and channel separation. Crosstalk, also called directivity, refers to separation of demultiplexed channels. Each channel should appear only at its intended port. The crosstalk specification expresses how well a coupler maintains this port-to-port separation. Channel separation describes a coupler's ability to distinguish wavelengths. In most couplers, the wavelengths must be widely separated allowing light to travel in either direction without the penalty found in splitters. WDMs allow multiple independent data streams to be sent over one fiber. The most common WDM system uses two wavelengths, although four or more-wavelength systems are available. Figure 1 illustrates two WDMs permitting two streams of data to be carried on a single fiber. The type of data does not matter. For example, one stream could be a video signal and the other could be an RS-232 data stream. Alternatively, both signals could be video signals or high speed data signals at 2.488 Gb/s. The configuration shown is unidirectional, but bidirectional configurations are also available.
Figure 1 - WDM Application

WDM Application

WDM Applications

Figure 2 illustrates bulk optics WDM. Constructed from discrete lenses and filters, a dichroic filter lies at the center of the WDM. Dichroic filters, based on interferometric techniques, reflect the light that they do not transmit. Looking at the figure, image that Fiber 1 carriers two wavelengths, 850 nm and 1310 nm. Also imagine that the dichroic filter passes wavelengths longer than 1100 nm, known as long-wave pass (LWP) filter. As the light exits Fiber 1 it first passes through the lens which focuses the light at a point. As the light hits the filter, the 1310 nm light passes through the filter and is collected by Fiber 3. The 850 nm light exiting Fiber 1 on the other hand reflects off of the filter and is collected by Fiber 2. Thus the information on the two effectively paired wavelengths can be independently decoded. The dichroic filter can offer a great deal of isolation in the transmission mode, but has poor isolation in the reflection mode. Usually these types of WDMs feature both short-wave pass (SWP) and LWP filters, and combining these filters achieves the best system performance.
Figure 2 - Bulk Optics WDM

Bulk Optics WDM