WDM/DWDM technical features

Wavelength Division Multiplexing

It is a technology that uses multiple lasers to simultaneously send multiple laser beams of different wavelengths on a single fiber. Each signal is modulated with data (text, voice, video, etc.) and transmitted within its own color band. WDM can greatly increase the capacity of existing fiber-optic infrastructures of telephone companies and other operators. Manufacturers have introduced WDM systems, also known as DWDM (Dense Wavelength Division Multiplexing) systems.

DWDM can support the simultaneous transmission of more than 150 light waves of different wavelengths, and each light wave can reach a data transmission rate of up to 10Gb/s. Such a system can deliver data rates in excess of 1Tb/s on a fiber optic cable thinner than a human hair.

Optical communication is a method in which light is used to carry signals for transmission. In the field of optical communication, people are used to naming by wavelength rather than frequency. Therefore, the so-called wavelength division multiplexing (WDM, Wavelength Division Multiplexing) is essentially only frequency division multiplexing. WDM is a system that carries multiple wavelengths (channels) on one optical fiber, and converts one optical fiber into multiple "virtual" fibers. Of course, each virtual fiber works independently on different wavelengths, which greatly improves the transmission capacity of the optical fiber. .

Because of the economy and effectiveness of the WDM system technology, it has become the main means of expanding the capacity of the current optical fiber communication network. As a system concept, wavelength division multiplexing technology usually has three multiplexing methods, namely, wavelength division multiplexing of 1310 nm and 1550 nm wavelengths, sparse wavelength division multiplexing (CWDM, Coarse Wavelength Division Multiplexing) and dense wavelength division multiplexing. Use (DWDM, Dense Wavelength Division Multiplexing).

This multiplexing technology only used two wavelengths in the early 1970s: one wavelength in the 1310 nm window and one wavelength in the 1550 nm window, using WDM technology to achieve single-fiber double-window transmission, which is the initial use of wavelength division multiplexing. .

wavelength division multiplexing

Coarse Wavelength Division Multiplexing

Following the application in the backbone network and long-distance network, the wavelength division multiplexing technology has also begun to be used in the metropolitan area network, mainly referring to the coarse wavelength division multiplexing technology. CWDM uses a wide window of 1200-1700 nm, and is mainly used in systems with a wavelength of 1550 nm. Of course, a wavelength division multiplexer with a wavelength of 1310 nm is also under development.

The spacing between adjacent channels of the coarse wavelength division multiplexer (large wavelength interval) is generally ≥20 nm, and the number of wavelengths is generally 4 or 8 waves, with a maximum of 16 waves. When the number of multiplexed channels is 16 or less, since the DFB laser used in the CWDM system does not require cooling, the CWDM system has more advantages than the DWDM system in terms of cost, power consumption requirements and equipment size, and CWDM is more and more widely used. accepted by the industry. CWDM does not need to choose expensive dense wave decomposition multiplexer and "optical amplifier" EDFA, and only needs to use cheap multi-channel laser transceivers as relays, so the cost is greatly reduced. Today, many manufacturers have been able to provide commercial CWDM systems with 2 to 8 wavelengths, which are suitable for use in cities where the geographic scope is not particularly large and the data service development is not very fast.

DWDM

Dense Wavelength Division Multiplexing (DWDM) can carry 8 to 160 wavelengths, and with the continuous development of DWDM technology, the upper limit of the number of demultiplexed waves is still increasing, and the interval is generally ≤1.6 nm. distance transmission system. Dispersion compensation techniques (overcoming nonlinear distortion in multi-wavelength systems - the phenomenon of four-wave mixing) are required in all DWDM systems.

In the 16-wave DWDM system, the conventional dispersion compensation fiber is generally used for compensation, while in the 40-wave DWDM system, the dispersion slope compensation fiber must be used for compensation. DWDM can combine and transmit different wavelengths in the same fiber at the same time. In order to ensure effective transmission, one fiber is converted into multiple virtual fibers. Using DWDM technology, a single fiber can transmit data traffic up to 400 Gbit/s, and as manufacturers add more channels to each fiber, the transmission speed of terabits per second is just around the corner.

use

DWDM can combine and transmit different wavelengths simultaneously in the same fiber. In order to be effective, one fiber is converted into multiple virtual fibers. So, if you plan to multiplex 8 fiber carriers (OC), that is, 8 signals are transmitted in one fiber, the transmission capacity will be increased from 2.5 Gb/s to 20 Gb/s. Due to the use of DWDM technology, a single fiber can transmit data traffic up to 40Gb/s. As manufacturers add more channels to each fiber, terabits-per-second transmission speeds are just around the corner.

technology

Wavelength division multiplexing (WDM) is to combine two or more optical carrier signals of different wavelengths (carrying various information) at the transmitting end through a multiplexer (also known as a multiplexer, Multiplexer), and coupled to the optical carrier. The technology of transmitting in the same fiber of the line; at the receiving end, the optical carriers of various wavelengths are separated by a demultiplexer (also known as a demultiplexer or a demultiplexer), and then the optical receiver is used for Further processing to restore the original signal. This technique of simultaneously transmitting two or more optical signals of different wavelengths in the same fiber is called wavelength division multiplexing.

WDM is essentially a frequency division multiplexing FDM technology in the optical domain

Each wavelength channel is realized by dividing the frequency domain, and each wavelength channel occupies the bandwidth of a section of optical fiber. The wavelengths used by the WDM system are all different, that is, the specific standard wavelength. In order to distinguish it from the ordinary wavelength of the SDH system, it is sometimes called the color optical interface, and the optical interface of the ordinary optical system is called "white optical port" or "white optical port". ".

The design of the communication system is different, and the spacing width between each wavelength is also different. According to the different channel spacing, WDM can be subdivided into CWDM (sparse wavelength division multiplexing) and DWDM (dense wavelength division multiplexing). The channel spacing of CWDM is 20nm, while the channel spacing of DWDM is from 0.2nm to 1.2nm, so compared with DWDM, CWDM is called sparse wavelength division multiplexing technology.

Features

(1) Ultra-large capacity transmission

Since the multiplexed optical channel rate of the WDM system can be 2.5Gbit/s, 10Gbit/s, etc., and the number of multiplexed optical channels can be 4, 8, 16, 32, or even more, the transmission capacity of the system can reach 300 -400Gbit/s and even more.

(2) Save fiber resources

For a single wavelength system, one SDH system requires a pair of optical fibers; while for a WDM system, no matter how many SDH sub-systems there are, the entire multiplexing system only needs a pair of optical fibers. For example, for 16 2.5Gbit/s systems, a single-wavelength system requires 32 fibers, while a WDM system requires only two fibers.

(3) Transparent transmission of each channel, smooth upgrade and capacity expansion

As long as the number of multiplexed channels and equipment are increased, the transmission capacity of the system can be increased to achieve capacity expansion. The multiplexed channels of the WDM system are independent of each other, so each channel can transparently transmit different service signals, such as voice, data and Images, etc., do not interfere with each other, which brings great convenience to users.

(4)Using EDFA to realize ultra-long distance transmission

EDFA has the advantages of high gain, wide bandwidth, low noise, etc., and its optical amplification range is 1530 (1565nm), but the relatively flat part of its gain curve is 1540 (1560nm), which can almost cover the 1550nm operating wavelength range of the WDM system. So Using an EDFA with a wide bandwidth can simultaneously amplify the multiplexed optical channel signals of the WDM system to achieve ultra-long-distance transmission of the system, and avoid the situation that each optical transmission system needs an optical amplifier. The ultra-long transmission distance can reach hundreds of kilometers while saving a lot of relay equipment and reducing costs.

(5) Improve the reliability of the system

Since most WDM systems are optoelectronic devices, and optoelectronic devices are highly reliable, the reliability of the system can also be guaranteed.

(6) All-optical network can be formed

All-optical network is the future development direction of optical fiber transmission network. In an all-optical network, the up-down and cross-connection of various services are realized by scheduling optical signals on the optical path, thereby eliminating the bottleneck of electronic devices in E/O conversion. The WDM system can be mixed with OADM and OXC to form an all-optical network with high flexibility, high reliability and high survivability to meet the development needs of the bandwidth transmission network.