1 WDM technology
Wavelength Division Multiplexing (WDM) refers to the technology of transmitting two or more optical wavelength signals through different optical channels in the same optical fiber to convey information. WDM includes Frequency Division Multiplexing (FDM) and wavelength division multiplexing. In essence, there is not significantly difference between Optical frequency division multiplexing (FDM) technology and WDM technologies because the light waves are part of the electromagnetic spectrum, there is a one-to-one correspondence the frequency and wavelength of light. Usually it can also be understood that optical frequency division multiplexing refers to the subdivision of optical frequencies, where optical channels are densely packed. Optical wavelength division multiplexing refers to the coarse division of optical frequencies, where optical multiplexes are far apart, even in different windows of the fiber.
Optical Wavelength Division Multiplexing is generally applied to wavelength division multiplexers and demultiplexers (also known as combined/splitter) placed at the two ends of the fiber, respectively, to achieve the coupling and separation of different optical waves. The principles of these two devices are the same. The main types of optical wavelength division multiplexers include fused biconic taper type, dielectric film type, grating type and flat type. The primary performance indicators insertion loss and isolation degree. Insertion loss refers to the increase in optical link loss caused by the use of wavelength division multiplexing equipment in the optical link. When the wavelength 11 and l2 are transmitted through the same fiber, the difference in power between the input end of the for demultiplexer wavelength l2 power and the output end of the multiplexer for wavelength 11 mixed is called the isolation degree. The technical features and advantages of optical wavelength division multiplexing are as follows:
1.1 Make full use of the low-loss band of optical fiber to increase the transmission capacity of the fiber, so that the physical limit of a fiber to transmit information doubled to several times. Currently, we are only utilizing a very small part of the low-loss spectrum of optical fiber (1310nm-1550nm), WDM can make full use of the huge bandwidth of single-mode fiber about 25THz, ensuring sufficient transmission bandwidth.
1.2 The ability to transmit two or more non-synchronous signals in the same optical fiber is conducive to the compatibility of digital and analog signals, independent of data rate and modulation. It also provides flexibility to remove or add channels in the middle of the line.
1.3 for the built fiber optic system, especially those with a small number of cores of fiber optic cable laid in the early stages, as long as the original system has a power margin, can further increase the capacity to achieve multiple one-way signal or two-way signal transmission without major changes to the original system, with strong flexibility.
1.4 Due to the significant reduction in the use of optical fiber, the construction costs have been greatly reduced, and because of the small number of fibers, it is quick and easy to recover in case of failure.
1.5 The shared nature of active optical equipment reduces the cost of multiple signal transmissions or the addition of new services.
1.6 Active equipment in the system is significantly reduced, which improves the reliability of the system. At present, due to the high requirements and technical complexity of equipment such as multi-carrier optical WDM optical transmitters, optical receivers and other equipment , the actual application of WDM is not widespread. Additionally, traditional broadcast television transmission services have not faced a particular shortage of multi-core fiber optic cable usage. However, with the development of integrated cable television services, the growing demand for network bandwidth, the implementation of various types of selective services, the economic costs of network upgrades, the characteristics and advantages of WDM in the CATV transmission system are gradually emerging. This demonstrates broad prospects for its application and may even affect the development pattern of CATV networks.
2 Technology Principle
In analog carrier communication systems, frequency division multiplexing is usually used to enhance the transmission capacity of the system, making full use of the bandwidth resources of the cable. This means that signals from multiple channels are transmitted simultaneously over the same cable, and at the receiving end, the signal of each channel can be filtered out by using a bandpass filter based on the frequency difference of each carrier. Similarly, the optical fiber communication system can also be used in the optical frequency division multiplexing method to improve the system’s transmission capacity . This involves using a demultiplexer (equivalent to an optical bandpass filter) at the receiving end to separate each signal’s optical carrier. WDM technology is to make full use of the huge bandwidth resources in the low-loss area of single-mode fiber, according to the different frequencies (or wavelengths) of each channel, the low-loss window of the fiber can be divided into several channels, the light wave as the carrier of the signal. At the transmitter end, WDM (Combiner) is employed to merge the signal carriers of different wavelengths into a single fiber for transmission. At the receiving end, wavelength division multiplexers (splitter) are used to separate these optical carriers of different wavelengths carrying different signals. Since the optical carrier signals of different wavelengths can be regarded as independent of each other (when fiber nonlinearity is not taken into account), multiplexing of optical signals can be achieved in a single fiber. By transmitting signals in different wavelengths in two directions, bidirectional transmission can be achieved. Depending on the wavelength division multiplexer, the number of wavelengths that can be multiplexed varies from two to several dozen. Typically, commercial systems are available with 8 and 16 wavelengths, depending on the allowed spacing between optical carrier wavelengths.
WDM is essentially a frequency division multiplexing (FDM) technology applied to optical frequency. From the perspective of transmission technology applied in China for decades, the progression followed a path of FDM-TDM- TDM FDM. In the early stage of analog transmission, coaxial cables were used with FDM analog technology in the electric domain, where each voice signal had a bandwidth of 4KHz and occupied a portion of the transmission medium’s bandwidth(such as coaxial cable). PDH and SDH systems transmitted TDM baseband digital signal over fiber, with each voice signal having a rate of 64kb/s; WDM technology,on the other hand, is a frequency division multiplexing technology for fiber optics, the 16 (8) × 2.5Gb/s WDM system combines optical frequency FDM analog technology with electrical frequency TDM digital technology.
WDM is essentially an optical frequency division multiplexing FDM technology, where each wavelength channel is realized by dividing the frequency domain. Each wavelength channel occupies a portion of the fiber’ bandwidth, which differs from the previous FDM technology used with coaxial cable.
2.1 Transmission medium is different, WDM system performs frequency division multiplexing on optical signals, while the coaxial system performs frequency division multiplexing on electrical signals.
2.2 On each path, the coaxial cable system transmits the analog signal of 4KHz voice signal, while the WDM system currently transmits digital signals, such as SDH2.5Gb/s or higher-speed digital systems, on each wavelength channel.