Wavelength Division Multiplexing (WDM) Tutorial

Wavelength Division Multiplexing (WDM) Definition

Wavelength Division Multiplexing (WDM) is a method of using the huge bandwidth of a low-loss area of a single-mode optical fiber to transmit different wavelengths. The wavelengths of these optical signals carry Digital signals can be the same rate, the same data format, or it can be different rates, different data formats. By adding new wavelength characteristics, according to the user’s request to determine the network capacity. For WDM at rates below 2.5Gb / s, current technologies can completely overcome the limitations imposed by fiber dispersion and fiber nonlinear effects to meet the various requirements for transmission capacity and transmission distance. The disadvantage of WDM expansion solution is the need for more fiber optic devices, and increase the probability of failure and errors.

WDM System Structure

The overall structure of n-channel wavelength multiplexing (WDM) system is mainly composed of the terminal (OMT) unit and the Optical In-Line Amplifier (ILA) unit, which is divided into three parts: Optical Wavelength Conversion Unit (OTU) The optical wavelength conversion unit (OTU) converts non-standard wavelengths to standard wavelengths specified by the ITU-T. Optical/electrical/optical (O / E / O) conversion is applied to the system by first using a photodiode PIN or APD. The received optical signal is converted into an electrical signal, which is then modulated on a standard wavelength laser to obtain a new desired optical wavelength signal. Wavelength Division Multiplexer and Demultiplexer Unit (ODU / OMU) Wavelength Division multiplexer can be divided into wavelength division multiplexer at the transmitter end and  Wavelength Division Multiplexer at the receiving end. An optical multiplexer is used for the transmitting end of a transmission system and is a device having a plurality of input ports and an output port. Each of its input ports inputs a light signal of a preselected wavelength. The inputted light waves of different wavelengths are output from the same output port. An optical splitter is used at the receiving end of a transmission system, just opposite the optical combiner, which has an input port and a plurality of output ports that classify a plurality of different wavelength signals. Optical Amplifiers (BA / LA / PA) The optical amplifier not only amplifies the optical signal directly, but also has the all-optical amplifier in real-time, high gain, broadband, online, low noise and low loss, and is an indispensable key device in the new generation of the optical fiber communication system. Among the currently available optical fiber amplifiers, there are mainly erbium-doped fiber amplifiers (EDFA), semiconductor optical amplifiers (SOA) and fiber Raman amplifiers (FRA). Among these, erbium-doped fiber amplifiers are widely used in long- High-capacity, high-speed optical fiber communication system, as a preamplifier, line amplifiers, power amplifiers. Optical Monitor Channel Optical supervisory channels are established for the monitoring of optical transmission systems in WDM. ITU-T recommends using a preferred wavelength of 1510nm, a capacity of 2Mbit / s. Rely on high receiver sensitivity at low rates (better than-50dBm) can still work properly. However, it is necessary to go down the road before the EDFA and to glaze after the EDFA.

WDM Advantages

The capacity of optical fiber is extremely large, and the traditional optical fiber communication system is in a fiber optic transmission of optical signals, this method is actually only a small part of the fiber-rich bandwidth. In order to make full use of the huge bandwidth resources of the optical fiber and increase the transmission capacity of optical fiber, a new generation of optical fiber communication technology based on dense WDM (DWDM) technology has been produced. WDM technology has the following characteristics: Large capacity The transmission bandwidth of the ordinary optical fiber used at present is very wide, but its utilization rate is still very low. Use DWDM technology can make a fiber transmission capacity than the single wavelength transmission capacity increased several times, tens of times or even hundreds of times. Now commercial maximum capacity optical transmission system is 1.6Tbit / s system, Lucent, and Nortel Networks two companies to provide such products are used 160x10Gbit / s program structure. The capacity of 3.2Tbit / s practical system has been developed. “Transparent” transmission of data Because DWDM systems are multiplexed and demultiplexed according to the wavelength of light, they are “transparent” to the data regardless of the signal’s rate and electrical modulation mode. A WDM system can carry multiple formats of “traffic” signals, such as ATM, IP, or signals that may occur in the future. The WDM system performs transparent transmission. For the “business” layer signals, the optical wavelength channels in the WDM system are like “virtual” optical fibers. Maximize protection of existing investments when System upgrades In the expansion and development of the network, optical fiber transmission lines and optical receivers can be replaced without the need to rebuild optical fiber lines, which is an ideal means for capacity expansion and is also convenient for introducing broadband services such as CATV, HDTV and B-ISDN Means, and use the addition of a wavelength to introduce any new business or new capacity you want. High Networking Flexibility, Economy, and Reliability The new communication network formed by using WDM technology is greatly simplified than the network structure formed by the traditional electrical time division multiplexing technology, and the network is structured. All kinds of services can be scheduled only by adjusting the wavelength of corresponding optical signals. As the network structure is simplified, structured and business scheduling convenience, the resulting network flexibility, economy, and reliability is obvious. Compatible with All-Optical Switching It is foreseeable that in the all-optical network that is expected to be realized in the future, up / down and cross-connection of various telecommunication services are realized by changing and adjusting the wavelength of the optical signal on the optical. Therefore, the WDM technology will be one of the key technologies for realizing the all-optical network. Moreover, the WDM system is compatible with the future all-optical network and may in the future realize the transparent and highly survivable system based on the already-built WDM system Optical Network.