Wavelength Division Multiplexing (WDM) is a fiber-optic transmission technique that enables the use of multiple optical wavelengths to send data over the same medium.

Multiplexing

Optical communication systems are classified according to signal multiplexing methods: frequency division multiplexing FDM, time division multiplexing TDM, wavelength division multiplexing WDM, space division multiplexing SDM.

WDM is similar to FDM but referencing the wavelength of light to the frequency of light. WDM is done in the IR portion of the electromagnetic spectrum instead of taking place at radio frequencies (RF). Each IR channel carries several RF signals combined with FDM or TDM. Each multiplexed infrared channel is separated or demultiplexed into the original signals at final point. Data in different formats and at different speeds can be transmitted simultaneously on a single fiber by using FDM or TDM in each IR channel in combination with WDM. It allows network capacity to be gradually and cost effectively increased.

Wavelength Division Multiplexing WDM

Utilizing the feature that one optical fiber can simultaneously transmit multiple optical carriers of different wavelengths, the possible wavelength range of the optical fiber is divided into several bands, and each band is used as an independent channel to transmit an optical signal of a predetermined wavelength. The essence of optical wavelength division multiplexing is to perform optical frequency division multiplexing on optical fibers.

Using the huge bandwidth of the low-loss area of single-mode fiber, the light of different rates (wavelengths) is mixed together for transmission. The digital signals carried by these optical signals of different wavelengths can be the same rate, the same data format, or different rates. , different data formats.

Dense Wavelength Division Multiplexing (DWDM)

With the commercialization of EDFA with 1550 windows, new WDM systems only use 1550 windows. The adjacent wavelength intervals of these WDM systems are relatively narrow (<1.6nm). In order to distinguish them from traditional WDM systems, they are called dense wavelength division multiplexing. With the system, namely the DWDM system. Now wavelength division multiplexing technology (WDM) usually refers to dense wavelength division multiplexing technology DWDM and coarse wavelength division multiplexing technology CWDM.

Considerations in WDM Networking

Dispersion Distance Limited

Power Budget

OSNR

Nonlinear Effect

Composition of DWDM System

The optical transmitter at the sending end sends out optical signals with different wavelengths and accuracy and stability that meet certain requirements. After being multiplexed by an optical wavelength multiplexer, they are sent to the erbium-doped fiber power amplifier, and then the amplified multi-channel optical signals are sent to the optical fiber. In the transmission, it can be determined whether there is an optical line amplifier or not according to the situation. After reaching the receiving end and being amplified by the optical preamplifier, it is sent to the optical wavelength demultiplexer to decompose the original optical signals.

WDM Transmission Mode

Unidirectional WDM (dual-fiber one-way): Two optical fibers are used, one optical fiber only completes the transmission of optical signals in one direction, and the transmission of reverse optical signals is completed by the other optical fiber.

Bi-directional WDM (single-fiber bi-directional): Only one fiber is used to realize simultaneous transmission of optical signals in two directions in one fiber, and the optical signals in two directions should be arranged on different wavelengths (full duplex).

Dual-fiber unidirectional transmission are mostly used in WDM systems.

DWDM Features

Multi-service access capability: Access SDH, ATM and other signals; Can provide multiple virtual fiber channels on one physical fiber.

Cost reduction: a large number of electrical regeneration equipment into a small number of optical amplification equipment.

Easy to upgrade and expand.

DWDM System Types

Open type DWDM system: Adopt wavelength conversion technology to convert the optical signal of the multiplexing terminal into a specified wavelength, and convert the optical signal of different terminal equipment into different wavelengths in line with ITU-T recommendations, and then perform multiplexing (only these interfaces are required Comply with the optical interface standard recommended by ITU-T).

Integrated type DWDM system: No wavelength conversion technology is used. It requires the wavelength of the optical signal of the multiplexing terminal to comply with the specifications of the DWDM system. Different multiplexing terminal equipment sends different Different channels can be occupied by the oscillator, so as to complete the multiplexing (requires that the user signal must conform to a specific wavelength).

In real applications, open type DWDM systems and integrated type DWDM systems can be used in combination, and most of the existing networks are open type DWDM systems.