1. What is WDM (Wavelength Division Multiplexing) ?
WDM is the abbreviation for Wavelength-Division Multiplexing and represents one of the optical communication technologies for transmitting high-capacity signals. Figure 1 shows a schematic diagram of WDM transmission. The transmitting end of WDM has a plurality of semiconductor lasers that emit light of different wavelengths. Each LD beam is modulated to create a signal light. The created signal lights are combined through the multiplexer and are transmitted over a single optical fiber. At the receiving end, the signals are separeted again into light of each wavelength by the demultiplexer, and are received by the PDs. Since the numbers of transmittable signals are propotional to the numbers of wavelength, the use of many more wavelengths makes possible to transmit many more signals.
Fig.1. Schematic diagram of WDM transmission.
2. CWDM (Coarse WDM) is a WDM with wide wavelength spacing
In the case of WDM transmission, many more signals can be sent with the use of many more wavelengths. However, a certain wavelength band must be divided with narrow spacing in order to use many wavelengths. The narrow spacing requires to use high precision components of LD, filter and etc., so and the system become expensive. On the other hand, wide wavelength spacing reduces the transmission capacity of signals but achieves economical systems. Therefore, the ITU-T (International Telecommunication Union Telecommunication Standardization Sector) defines two types of WDM wavelength spacing so that appropriate systems are available depending on the application. One of the standards is DWDM with narrow wavelength spacing suitable for large-capacity, long-distance transmission. The other is the CWDM with wide wavelength spacing suitable for medium capacity, 50 ~ 80km distance transmission
Figure 2 shows the CWDM wavelengths. There are 18 center wavelengths of 20 nm spacing from 1271 nm to 1611 nm, covering the O-, E-, S-, C- and L-band. In fact the case of using all 18 wavelengths is not a lot, but the four wavelengths of 1531nm ~ 1611nm or the 8 wavelength of 1471nm ~ 1611nm are used frequently. This is because the mass production optical components, like a CWDM filter, a Mux/DeMux unit, and an optical add-drop module, can be available in this wavelength range.
Fig.2. Center wavelengths of CWDM．
3. DWDM (Dense WDM) is a WDM with narrow wavelength spacing
Figure 3 shows the DWDM wavelengths. The wavelength spacing of DWDM is defined by light frequencies rather than the wavelengths. Because DWDM is premised on long-distance transmission using an optical amplifier, the C and L bands suitable for amplification are used, and the wavelength bands are divided finely to increase the transmission capacity. There are two standard channel spacings of 100 GHz and 200 GHz. Refer to the following link for details: ITU Grid DWDM Reference Table.
Fig.3. Center wavelength of DWDM.
4. Use of a WDM network
Figure 4 shows an image diagram of the optical communication network. Network is classified into three categories, i.e., the core network connecting the major cities, the metro network connecting the main area in the city, and the access network connecting each home and company. In the core network, DWDM suitable for a large-capacity and long-distance transmission is used. CWDM is mainly used in the metro network. CWDM is premised not to use an optical amplifier, and communication distance is about 50km ~ 80km. Though most areas in the city can be covered without an optical amplifier, the CWDM amplifiers developed by FiberLabs can serve to extend reach if necessary.