Transmission Engineering Network scheme of WDM technology

Wavelength Division Multiplexing Technology  

1 Wavelength Division multiplexing Technology

Wavelength Division Multiplexing (WDM) refers to the simultaneous transmission of two or more optical wavelength signals through different optical channels in the same optical fiber, known as the Optical Wave Division multiplexing technology. Wavelength Division multiplexing includes frequency division multiplexing and wavelength division multiplexing. There is no obvious difference between optical frequency Division Multiplexing (FDM) technology and Wavelength Division multiplexing (WDM) technology, because light waves are part of the electromagnetic wave, and the frequency of optical frequencies has a single correspondence with wavelengths. It is also generally understood that optical frequency division multiplexing refers to the breakdown of the frequency of light, the optical channel is very dense. Optical Wavelength Division multiplexing refers to the light frequency of the coarse, light times are far apart, and even in different windows of optical fiber.

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Wavelength Division Multiplexing (WDM) and Multiplexer (also known as wave/splitter) are applied to both ends of optical fiber, respectively, to realize the coupling and separation of different light waves. The principle of the two devices is the same. The main types of wavelength division multiplexer are fused tapered, dielectric film type, grating type and planar type four kinds. The main characteristics of the index are insertion loss and isolation degree. In general, the increase in optical link loss is called the insertion loss of wavelength division multiplexing, due to the use of wavelength division multiplexing devices in optical links. When the wavelength 11,l2 is transmitted through the same optical fiber, the difference between the power of the input terminal L2 and the power mixed in the 11 output fiber in the splitter is called the isolation degree. The technical features and advantages of wavelength division multiplexing are as follows:

1.1 Make full use of the low-loss band of optical fiber, increase the transmission capacity of optical fiber, so that the physical limit of one fiber to transmit information is increased by one to several times. At present we just take advantage of the optical fiber low loss spectrum (1310nm-1550nm) a very small part, wavelength division multiplexing can take full advantage of the large bandwidth of a single mode fiber of about 25THz, sufficient transmission bandwidth.

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1.2 has the ability to transmit 2 or several non-synchronous signals in the same fiber, facilitates the compatibility of digital signals and analog signals, is independent of data rate and modulation mode, and can be flexibly removed or added to the channel in the middle of a line.

1.3 to the built-in fiber systems, especially the early laying of a few cores of optical cable, as long as the original system has a power margin, can further increase capacity, to achieve a plurality of unidirectional or bidirectional signal transmission without the original system to make big changes, has a strong flexibility.

1.4 Due to the large number of reduction in the use of optical fiber, greatly reducing the construction costs, due to the low number of optical fiber, when the failure, recovery is also rapid and convenient.

1.5 The sharing of active optical devices, the transmission of multiple signals or the increase in new services reduces costs.   

1.6 The active devices in the system are greatly reduced, which improves the reliability of the system. At present, due to the high demand of multi-channel wavelength division multiplexing for optical transmitters, optical receivers and other equipment, the implementation of the technology is difficult, while the application of multi-fiber-optic cable is not particularly scarce in the traditional broadcasting and TV transmission service, so the actual application of WDM is not much. However, with the development of CATV integrated service, the increasing demand of network bandwidth, the implementation of various kinds of selective services, the consideration of the economic cost of network upgrading and transformation, the characteristics and advantages of WDM are appearing gradually in the CATV transmission system, showing a broad application prospect and even affecting the development pattern of CATV network.

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2 Technical Principle

In the analog carrier communication system, frequency division multiplexing is usually used to improve the transmission capacity of the system, make full use of the bandwidth resources of the cable, that is, in the same cable transmission of a number of channels at the same time, the receiver based on the different carrier frequency, the use of band-pass filter can filter out each channel signal. Similarly, the optical frequency division multiplexing method can be used in optical fiber communication systems to improve the transmission capacity of the system, and a multiplexer (equivalent optical bandpass filter) is used to separate the signal light carriers at the receiving end. Because of the difference of signal frequency in the frequency domain of the light, the frequency difference is usually defined by wavelength, and the multiplexing method is called WDM. WDM technology is to make full use of the low loss Zone of single mode optical fiber to bring huge bandwidth resources, according to each letter Daoguanpo frequency (or wavelength) different can be optical fiber low loss window divided into a number of channels, the light wave as the carrier of the signal, At the transmitting end, the wavelength division multiplexer (Wave splitter) is used to combine the signal light carriers of different specified wavelengths into one fiber for transmission. At the receiving end, a wavelength division multiplexer (splitter) is used to separate the optical carriers with different signals. Because the optical carrier signals of different wavelengths can be regarded as independent of each other (without considering the nonlinearity of optical fibers), multiplex transmission of multiple optical signals can be realized in one fiber. Two-way transmission can be realized by arranging the signals in two directions separately at different wavelengths. Depending on the wavelength division multiplexer, the number of wavelengths that can be reused varies from 2 to dozens of, and the general commercialization is 8-wavelength and 16-wavelength systems, depending on the allowable interval size of the optical carrier wave length.

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WDM is essentially a frequency division multiplexing (FDM) technique in light frequency. From China's decades of application of transmission technology, the FDM-TDM-TDM FDM is the route to go. The beginning of the wire, coaxial cable used are FDM simulation technology, that is, the frequency division multiplexing technology, each voice bandwidth of 4KHz, each voice occupy the transmission medium (such as coaxial cable) a section of bandwidth; PDH, SDH system is a TDM baseband digital signal transmitted on optical fiber, each voice rate is 64kb/s While WDM technology is the optical fiber frequency division multiplexing technology, the WDM system (8) X2.5GB/S is a combination of FDM analog technology on optical frequency and TDM digital technology on the electric frequency.

WDM is essentially a frequency-division multiplexing FDM Technology in optical frequency, and each wavelength path is realized by segmentation in frequency domain. Each wavelength path occupies a section of fiber bandwidth, unlike the previous coaxial cable FDM technology:

2. 1 The transmission medium is different, WDM system is the frequency division on the optical signal, the coaxial system is the frequency division utilization of the electric signal.

2 . 2 on each channel, the coaxial cable system transmits the analog signal 4KHz voice signal, while the WDM system is currently a digital signal sdh2.5gb/s or higher rate on each wavelength path of the digital system.

Transmission Engineering Network scheme of WDM technology