What is WDM and how it works?
WDM (wavelength division multiplexing) is a technique where two or more optical carrier signals of different wavelengths (carrying various information) are combined at the transmitting end through a combiner (multiplexer) and connected to the same optical fiber for transmission. At the receiving end, the optical signals of various wavelengths are separated by a demultiplexer (demultiplexer), and then further processed into the original signal by the photons. The technology that allows two or more optical wavelength signals to transmit information through different optical channels in the same optical fiber at the same time is called wavelength division multiplexing (WDM).
The basic structure of a general WDM system
The basic structure of a WDM system is mainly divided into two modes: dual-fiber unidirectional transmission and single-fiber bidirectional transmission.
Dual-fiber unidirectional means that all optical paths are transmitted in the same direction on one optical fiber at the same time. Different wavelengths carry different optical signals. They are combined at the transmitting end and transmitted through one optical fiber. At the receiving end, they are demultiplexed to complete the transmission of multiple optical signals, while the reverse direction is transmitted through another optical fiber. The transmission in two directions is completed by two optical fibers respectively.
Single-fiber bidirectional means that the optical path is transmitted in two different directions on one optical fiber at the same time. The wavelengths used are separated from each other to achieve full-duplex communication between the two parties.
The general WDM system mainly consists of five parts: network management system, optical transmitter, optical relay amplifier, optical receiver and optical monitoring channel.
A simple WDM system mainly includes media converter, WDM mux & demux, patch cord, and dark fiber components.
In the entire WDM system, optical wavelength division multiplexers and demultiplexers are key components in WDM technology, and their performance plays a decisive role in the transmission quality of the system.
Advantages of WDM wavelength division multiplexing
Large capacity
An important feature of WDM is that it can make full use of the bandwidth resources of optical fibers, increase the data transmission capacity without changing the basic architecture of the existing network, and increase the transmission capacity of an optical fiber many times that of a single wavelength. For example, a DWDM system can support up to 192 wavelengths in a pair of optical fibers, and the transmission capacity of each wavelength is as high as 100Gbit/s to about 400Gbit/s and one Terabit/s.
Good compatibility
WDM has good compatibility with different signals. When transmitting different signals such as images, data and voice in the same optical fiber, each wavelength is independent of each other and does not interfere with each other, ensuring the transparency of transmission.
High network flexibility, economy and reliability
Wavelength division multiplexing technology allows new channels to be connected as needed without interrupting existing traffic services, making upgrades easier. When upgrading and expanding the network, there is no need to modify the optical cable line. New services can be opened or superimposed by adding wavelengths, saving a lot of optical fiber and 3R regenerators in large-capacity long-distance transmission, and significantly reducing transmission costs.
Wavelength routing
WDM technology is one of the key technologies to realize all-optical network. In the all-optical network that is expected to be realized in the future, by changing and adjusting the wavelength of the optical signal on the optical path, various telecommunication services can be added/dropped and cross-connected.
What are Mux and Demux?
Multiplexer MUX
The main function of the combiner MUX is to combine multiple signal wavelengths in one optical fiber for transmission. At the transmitting end, N optical transmitters work on N different wavelengths, which are separated by appropriate intervals, denoted as λ1, λ2, … λn. These N light waves are modulated as carriers and carry signals. A combiner combines these optical carrier signals of different wavelengths and couples them into a single-mode optical fiber. Since optical carrier signals of different wavelengths can be regarded as independent of each other (without considering the nonlinearity of the optical fiber), multiple optical signals can be multiplexed and transmitted in one optical fiber. Through multiplexing, communication operators can avoid maintaining multiple lines, effectively saving operating costs.
Demultiplexer DEMUX
The main function of the demultiplexer DEMUX is to separate multiple wavelength signals transmitted in an optical fiber. In the receiving part, a demultiplexer separates the optical carrier signals of different wavelengths, and the optical receiver further processes them to restore the original signal. A multiplexer (Demux) is a device that performs reverse processing on a multiplexer.
In principle, the device is reciprocal (bidirectionally reversible), that is, as long as the output and input ends of the demultiplexer are used in reverse, it is a multiplexer.
Performance indicators affecting WDM wavelength division multiplexers
1. Working band
The working band of WDM devices, such as 1550 wavelength, is divided into three bands: S band (short wavelength band 1460~1528nm), C band (conventional band 1530~1565nm), and L band (long wavelength band 1565~1625nm).
2. Number of channels and channel spacing
The number of channels refers to the number of channels that can be synthesized or separated by the wavelength division multiplexer/demultiplexer. This number can range from 4 to 160. The design is enhanced by adding more channels. Common channel numbers include 4, 8, 16, 32, 40, 48, etc. Channel spacing refers to the difference between the nominal carrier frequencies of two adjacent channels, which can be used to prevent interference between channels. According to the recommendations of ITU-T G. 692, the spacings less than 200GHz (1.6nm) include 100GHz (0.8nm), 50GHz (0.4nm) and 25GHz, etc. At present, 100GHz and 50GHz channel spacings are preferred.
3. Insertion loss
Insertion loss is the attenuation caused by the insertion of a wavelength division multiplexer (WDM) in an optical transmission system. The attenuation of the optical signal by the wavelength division multiplexer itself directly affects the transmission distance of the system. Generally, the lower the insertion loss, the less signal attenuation.
4. Isolation
Isolation refers to the degree of isolation between signals in each channel. A high isolation value can effectively prevent crosstalk between signals, which may cause distortion of the transmitted signal.
5. Polarization Dependent Loss PDL
Polarization Dependent Loss PDL is the distance between the maximum and minimum loss caused by the same polarization state at a fixed temperature, wavelength and the same band, that is, the maximum deviation of insertion loss under all input polarization states.
In addition to the above, there are of course other performance parameters that affect WDM devices, such as operating temperature, bandwidth, etc.
FAQ
What is the difference between WDM Fiber Multiplexers and Optical Splitter?
WDM is to separate multiple wavelengths of light in the line and transmit them separately, of course, it can also combine multiple wavelengths of light for transmission.
Fiber optic splitter is to divide a wavelength of light into multiple beams according to the use, and the power of each beam of light is determined by the specifications of the splitter used.
The most important difference between the two is that the former can combine and transmit optical signals of various business wavelengths, while the latter only transmits one wavelength of light to split according to a certain splitting ratio.
How to use WDM technology to expand fiber capacity?
In the traditional transmission mode, one optical fiber can only transmit an optical carrier signal carrying one type of information. If different services are required, countless different and independent optical fibers are needed for transmission. However, if the amount of business information is large, a large number of optical fibers need to be laid for transmission, which is a great challenge to the wiring space and cost. The application of a WDM system can quickly solve the above problems. The WDM system can carry “business” signals in various formats, such as ATM, IP, etc. Through multiplexing and demultiplexing technology, multiple business signals can be transmitted through one optical fiber, which greatly reduces the amount of optical fiber used. It is an ideal means of capacity expansion in network expansion and development. When introducing new broadband services, such as CATV, HDTV, B-ISDN, etc., only one additional wavelength needs to be added.
If you want to know more about the types of CWDM DWDM FWDM, please refer to the article: What are the types of optical multiplexers?
