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A comprehensive understanding of WDM wavelength division multiplexing

  1. What is WDM wavelength division multiplexing and the working principle of WDM

The technology of simultaneously allowing two or more optical wavelength signals to transmit information through different optical channels in the same optical fiber is called wavelength division multiplexing (WDM). WDM (Wavelength Division Multiplexing) is to combine two or more optical carrier signals of different wavelengths (carrying various information) at the sending end through a multiplexer (Multiplexer) and couple them into the same optical fiber for transmission , and at the receiving end, the optical signals of various wavelengths are separated by a demultiplexer (Demultiplexer), and then further processed by the optical receiver to restore the original signal.

To put it simply, you can see the figure below. 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 required for transmission. . However, if there is a large amount of business information, a large number of optical fibers need to be laid for transmission, which poses a great challenge to wiring space and cost. The application of a WDM system can quickly solve the above problems. The WDM system can carry “service” signals in various formats, such as ATM, IP, etc., and multiple service signals can be transmitted through one optical fiber through multiplexing and demultiplexing technologies, which greatly reduces the amount of optical fiber. Ideal expansion means for expansion and development. When introducing new broadband services, such as CATV, HDTV, B-ISDN, etc., only one additional wavelength needs to be added.




  1. Basic structure of general WDM system

The basic composition of the WDM system is mainly divided into two modes: dual-fiber one-way transmission and single-fiber two-way transmission. Dual-fiber unidirectional means that all optical paths are transmitted in the same direction on one optical fiber at the same time, and different wavelengths carry different optical signals, which are combined at the sending end and transmitted through one optical fiber, and demultiplexed at the receiving end to complete multiplexing. The transmission of optical signals, while the opposite 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 simultaneously transmitted in two different directions on one main fiber, and the wavelengths used are separated from each other to achieve full-duplex communication between the two parties.



The general WDM system is mainly composed of five parts: network management system, optical transmitter, optical relay amplifier, optical receiver, and optical monitoring channel.



  1. Schematic diagram of the overall structure of the WDM system

The simple WDM system mainly includes transceivers, WDM wavelength division multiplexers, jumpers, and dark fiber components.


In the whole WDM system, the optical wavelength division multiplexer and demultiplexer are key components in WDM technology, and their performance has a decisive effect on the transmission quality of the system.

  1. Advantages of WDM wavelength division multiplexing

High 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 structure of the existing network, and increase the transmission capacity of an optical fiber many times compared with that of a single wavelength. For example, the 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 signals of different natures 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

WDM technology allows for easier upgrades by allowing new channels to be connected as needed without disrupting existing traffic service. When upgrading and expanding the network, there is no need to modify the optical cable line, and new services can be opened or superimposed by adding wavelengths, saving a lot of optical fibers and 3R regenerators during long-distance transmission with large capacity, and the transmission cost is significantly reduced.

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, the up/down and cross-connection of various telecommunication services can be realized.

  1. What is multiplexing Mux and demultiplexing Demux?

Multiplexer MUX

The main function of the multiplexer MUX is to combine multiple signal wavelengths into one optical fiber for transmission. At the sending end, N optical transmitters work on N different wavelengths respectively, and there are appropriate intervals between these N wavelengths, which are respectively denoted as λ1 and λ2. . . λn. These N light waves, as carriers, are respectively modulated by signals to carry signals. A multiplexer combines these optical carrier signals of different wavelengths and couples them into single-mode fiber. Since the optical carrier signals of different wavelengths can be regarded as independent of each other (without considering the nonlinearity of the optical fiber), the multiplexing transmission of multiple optical signals can be realized 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 one optical fiber. In the receiving part, the optical carrier signals of different wavelengths are separated by a demultiplexer, and further processed by the optical receiver to restore the original signal. A multiplexer (Demux) is a device that performs the reverse processing of a multiplexer.

In principle, the device is reciprocal (two-way reversible), that is, as long as the output and input of the demultiplexer are used in reverse, it is a multiplexer.

  1. The difference between WDM wavelength division multiplexer and optical splitter

There are many people who can’t understand the difference between WDM and Optical Splitter. To put it simply, WDM is to separate and transmit the light of multiple wavelengths in the line separately, of course, it can also be combined and transmitted together; The optical power is determined according to the specification of the splitter used. The most important difference between the two is that the former can composite and transmit optical signals of various service wavelengths, while the latter only transmits light of one wavelength to split light according to a certain splitting ratio.



  1. What are the performance indicators that affect the WDM wavelength division multiplexer?
  2. working band

The working band of WDM devices, such as 1550 wavelength, distinguishes three bands: S-band (short-wavelength band 1460-1528nm), C-band (conventional band 1530-1565nm), L-band (long-wavelength band 1565-1625nm).

  1. 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 vary from 4 to 160. The design can be enhanced by adding more channels. The common channel numbers are 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 inter-channel interference. According to ITU-TG. According to the proposal of 692, there are 100GHz (0.8nm), 50GHz (0.4nm) and 25GHz for intervals less than 200GHz (1.6nm), and the channel intervals of 100GHz and 50GHz are preferred at present.

  1. Insertion loss

Insertion loss is the attenuation caused by the insertion of a wavelength division multiplexer (WDM) in an optical transmission system. The attenuation effect of the wavelength division multiplexer itself on the optical signal directly affects the transmission distance of the system. In general, the lower the insertion loss, the less signal attenuation.

  1. isolation

Isolation refers to the degree of isolation between signals of each channel. A high isolation value can effectively prevent crosstalk between signals and cause transmission signal distortion.

  1. 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 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 and bandwidth.



  1. How to distinguish O, E, S, C, L, U band wavelengths?



What is an O-band?

O band is the original band 1260-1360 nm. O-band is the first wavelength band historically used for optical communications, with minimal signal distortion (due to dispersion).

What is an E-band?

E-band (extended wavelength band: 1360-1460 nm) is the least common of these bands. E-band is mainly used as an extension of O-band, but there are few applications, mainly because many existing optical cables show high attenuation in E-band, and the manufacturing process is very energy-intensive, so its use in optical communication is limited.

What is the S-band?

The fiber loss in the S-band (Short-wavelength Band) (short-wavelength band: 1460-1530 nm) is lower than that in the O-band, and the S-band is used as many PON (passive optical network) systems.

What is C band?

The C-band (Conventional Band) ranges from 1530 nm to 1565 nm, representing the conventional band. Optical fiber exhibits the lowest loss in the C-band and has a greater advantage in long-distance transmission systems. It is usually used in many metropolitan, long-distance, ultra-long-distance and submarine optical transmission systems combined with WDM and EDFA technology. C-band becomes more and more important as transmission distances become longer and fiber amplifiers are used instead of optical-to-electron-to-optical repeaters. The use of C-band expanded with the advent of DWDM (Dense Wavelength Division Multiplexing), which enables multiple signals to share a single fiber.

What is L-band?

L-band (Long-wavelength Band) (long-wavelength band: 1565-1625 nm) is the second lowest-loss wavelength band, and is often used when the C-band is not enough to meet the bandwidth requirements. With the widespread availability of b-doped fiber amplifiers (EDFAs), DWDM systems have been extended up to the L-band, and were initially used to extend the capacity of terrestrial DWDM optical networks. Now, it has been introduced to submarine cable operators to do the same thing – expand the total capacity of submarine cables.

Because the transmission attenuation loss of the two transmission windows of the C-band and the L-band is the smallest, the signal light in the DWDM system is usually selected in the C-band and the L-band. In addition to the O-band to the L-band, there are two other bands, namely the 850 nm band and the U-band (ultra-long band: 1625-1675 nm). The 850 nm band is the dominant wavelength for multimode fiber optic communication systems incorporating VCSELs (Vertical Cavity Surface Emitting Lasers). The U-band is mainly used for network monitoring.


WDM technology can be divided into WDM, CWDM, and DWDM according to different wavelength modes. The wavelength range specified by ITU for CWDM (ITU-TG.694.2) is 1271 to 1611 nm, but considering the relatively large attenuation of the 1270-1470nm band in the application, the 1470-1610nm band range is usually used. DWDM channels are more densely spaced, using C-band (1530nm-1565nm) and L-band (1570nm-1610nm) transmission windows. Ordinary WDM generally adopts 1310 and 1550nm wavelengths.

  1. What is CWDM, DWDM, FWDM, LWDM, MWDM?

WDM bearer schemes include coarse wavelength division multiplexing (CWDM), dense wavelength division multiplexing (DWDM), medium wavelength division multiplexing (MWDM), and fine wavelength division multiplexing (LWDM).

CWDM coarse wavelength division multiplexer

CWDM (Coarse Wavelength Division Multiplexer) is a sparse wavelength division multiplexer, also known as a coarse wavelength division multiplexer. CWDM has 18 different wavelength channels, and the different wavelengths of each channel are separated by 20nm, using wavelengths from 1270 nm to 1610 nm. CWDM supports fewer channels than DWDM because it is compact and cost-effective, thus making it an ideal solution for short-range communications. The biggest advantage of the CWDM system lies in its low cost, and the device cost is mainly reflected in filters and lasers. The wide wavelength interval of 20nm also brings to CWDM the advantages of low technical index requirements for lasers and simplified structure of optical multiplexer/demultiplexer. The structure is simplified and the yield is increased, so the cost is reduced.


DWDM dense wavelength division multiplexer

DWDM (Dense Wavelength Division Multiplexer) is a dense wavelength division multiplexer. The channel spacing of DWDM is 1.6/0.8/0.4 nm (200GHz/100 GHz/50 GHz), far smaller than CWDM. Compared with CWDM, DWDM with tighter wavelength spacing can carry 8 to 160 wavelengths on one optical fiber, which is more suitable for long-distance transmission. With the help of EDFA, DWDM system can work in the range of thousands of kilometers.


FWDM Filter Chip Wavelength Division Multiplexer

FWDM (Filter Wavelength Division Multiplexing) filter wavelength division multiplexer is based on mature thin film filter technology. Filter-type wavelength division multiplexers can combine or separate light of different wavelengths in a wide wavelength range, and are widely used in erbium-doped optical amplifiers, Raman amplifiers and WDM optical fiber networks.

MWDM Medium Wavelength Division Multiplexing

MWDM reuses the first 6 waves of CWDM, compresses the 20nm wavelength interval of CWDM to 7nm, and uses TEC (Thermal Electronic Cooler, semiconductor cooler) temperature control technology to expand 1 wave into 2 waves. In this way, the capacity can be increased while the optical fiber can be further saved. MWDM is based on the 6 waves of CWDM, with a left and right offset of 3.5nm and expanded to 12 waves (1267.5, 1274.5, 1287.5, 1294.5, 1307.5, 1314.5, 1327.5, 1334 .5, 1347.5, 1354.5, 1367.5, 1374.5nm). MWDM is mainly proposed in the context of China’s 5G fronthaul network.