What is WDM
WDM (Wavelength Division Multiplexing) is one of the optical communication technologies for transmitting large-capacity signals called wavelength division multiplexing. Figure 1 shows a schematic diagram of WDM transmission. On the transmission side of WDM, multiple semiconductor lasers (LD) that emit light of different wavelengths are prepared, and each LD is modulated to create signal light. These signal lights are transmitted into one optical fiber by using a multiplexer (also called Mux). On the receiving side, a wavelength demultiplexer (also called DeMultiplexer: DeMux) is used to divide the light into each wavelength, and the photodetector (PD) receives the signal. Compared to sending a signal with one wavelength, sending with two wavelengths is twice as many, and with three wavelengths, it is possible to send more signals by using more wavelengths.
Figure 1. Schematic diagram of WDM transmission
CWDM (Coarse WDM) is a WDM with wide wavelength spacing.
In WDM transmission, more signals can be sent as more wavelengths are used, but in order to use many wavelengths, it is necessary to divide a certain wavelength band into narrow wavelength intervals. If the wavelength spacing is narrow, the price will increase because it is necessary to make the LD wavelength and the parts such as filters that separate each wavelength highly accurate. On the other hand, if the wavelength spacing is wide, the amount of signals that can be sent will be small, but the system will be cheap. Therefore, the ITU-T (Telecommunication Standardization Division of the International Telecommunication Union) has defined two WDM wavelength intervals so that a system suitable for each application can be used. One is a standard with a narrow wavelength interval called DWDM, which is suitable for large capacity long distance transmission. The other is a standard with a wide wavelength interval called CWDM, which is suitable for transmission of 50 to 80 km where the capacity is not so large.
Figure 2 shows the CWDM wavelength. The center wavelength is 18 wavelengths divided from 1271 nm to 1611 nm including the O band, E band, S band, C band, and L band at 20 nm intervals. Actually, there are few cases where all 18 wavelengths are used, and 8 wavelengths of 1471 nm to 1611 nm or 4 wavelengths out of 1531 nm to 1611 nm are often used. This is because many general mass-produced optical components can be used in this wavelength band. For example, a CWDM filter used for wavelength division / multiplexing, a unitized Mux / DeMux unit, and an optical add / drop module (OADM) that extracts and adds specific wavelengths can be used.
In recent years, 100GbE-LR4 high-speed network optical communication technology using four wavelengths in the 1300nm band (O band) has been established, and CWDM in this wavelength band, which is different from CWDM in the wavelength band that has been the main Utilization is attracting attention.
Figure 2. CWDM wavelength
DWDM (Dense WDM) is a WDM with a narrow wavelength interval.
Fig. 3 shows the DWDM wavelength. In the case of DWDM, the spacing is determined by the frequency of light, not the wavelength. Since DWDM is premised on long-distance transmission using an optical amplifier, only the C band and L band that are easy to amplify are used, and the wavelength is very finely divided to increase the transmission capacity. In addition to 100 GHz spacing, there is also a 200 GHz spacing standard. For more information, see: ITU Grid DWDM Reference Table. You can use mass-produced optical components such as DWDM filters, Mux / DeMux units, AWG modules, and optical add / drop modules (OADMs) for wavelength division.
Figure 3. DWDM frequency (wavelength)
Network where WDM is used
Figure 4 shows an image of the optical communication network. The network is divided into a core network that connects major cities, a metro network that connects major areas within the prefecture, and an access network that connects homes and businesses. DWDM is used for core networks. It uses a large number of wavelengths for large capacity, and it uses an optical amplifier for long-distance transmission. CWDM is mainly used in metro networks. CWDM is based on the premise that no optical amplifier is used, and the communication distance is about 50km to 80km. Most areas in prefectures can be covered without an optical amplifier, but if the fiber loss is larger than expected or you want to extend the distance a little further, you can use the CWDM amplifier developed by us.
Figure 4. Image of optical communication network