Introduction to Wavelength-division Multiplexing

Wavelength-division multiplexing (WDM) is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using differentwavelengths (i.e., colors) of laser light. This technique enables bidirectional communications over one strand of fiber, as well as multiplication of capacity. A WDM system uses a multiplexer at the transmitter to join the signals together, and a demultiplexer at the receiver to split them apart. With the right type of fiber it is possible to have a device that does both simultaneously, and can function as an optical add-drop multiplexer. WDM systems are popular with telecommunications companies because they allow them to expand the capacity of the network without laying more fiber.

WDM systems are divided into different wavelength patterns: Coarse Wavelength Division Multiplexing (CWDM) and Dense Wavelength Division Multiplexing (DWDM).

CWDM is the technology of choice for cost efficiently short-haul transmission in telecoms or enterprise networks. CWDM typically has the capability to transport up to 16 channels (wavelengths) in the spectrum grid from 1270 nm to 1610 nm with a 20 nm channel spacing. Passive CWDM is an implementation of CWDM that uses no electrical power. It separates the wavelengths using passive optical components such as bandpass filters and prisms.

While Dense Wavelength Division Multiplexing (DWDM) is designed for long-haul transmission where wavelengths are packed tightly together, providing a high-capacity solution in telecom networks. It’s effective for wavelengths between approximately 1525-1565 nm (C band), or 1570-1610 nm (L band). A basic DWDM system contains several main components: a DWDM terminal multiplexer, an intermediate line repeater, an intermediate optical terminal, an optical add-drop multiplexer, and a DWDM terminal demultiplexer.

As an additional optical transport layer, DWDM technology plays an important role in fiber optic communication field. Wavelength-converting transponders serve originally to translate the transmit wavelength of a client-layer signal into one of the DWDM system’s internal wavelengths in the 1,550nm band. In the mid-1990s, signal regeneration in transponders quickly evolved through 1R to 2R to 3R and into overhead-monitoring multi-bitrate 3R regenerators. We need wavelength converters, which is exactly what a transponder is. A transponder can be made up of two transceivers placed after each other: the first transceiver converting the 1550 nm optical signal to/from an electrical signal, and the second transceiver converting the electrical signal to/from an optical signal at the required wavelength. Transponders that don’t use an intermediate electrical signal are in development.

Since communication over a single wavelength is one-way (simplex communication), and most practical communication systems require two-way (duplex communication) communication, so two wavelengths will be required. As a result, at each end both a transmitter and a receiver will be required. A combination of a transmitter and a receiver is called a transceiver. It converts an electrical signal to and from an optical signal. There are usually transceiver types based on WDM technology: Coarse WDM (CWDM) Transceivers (as the picture below) and Dense WDM (DWDM) Transceivers.

From the above introduction, we can know better of what the WDM is, the main types of it and some main concepts related to WDM. WDM technology is now widely used in fiber optic communications.