In recent years, distributed fiber Raman amplifier (DFRA) has become a key technology in long-span transmission because of its excellent noise characteristics. However, for ultra long-distance transmission system, sometimes only relying on the Raman amplifier can not solve the problem, and remote pumping provides a good solution. ROPA (remote optically pumped amplifier) is mainly used in non relay system to improve system power budget and extend transmission span. With the rapid development of economy and the rise of urban agglomerations, there are urban agglomerations in the Yangtze River Delta, the Pearl River Delta and the Bohai Bay. The communication between adjacent cities in the urban agglomerations may become the next application hotspot of non relay transmission in the future. In developed regions such as Europe, North America, and neighboring regions such as Southeast Asia, the communication between adjacent cities in the urban agglomerations may become the next application hotspot, The cost of laying terrestrial optical cable is high or there are obstacles from the third party, so it is difficult to lay submarine non relay system between adjacent coastal cities, which becomes another application of non relay transmission. The construction and maintenance cost of relay station in no man’s land such as swamp, desert and forest is high, which is another focus of no relay system application. In the domestic power network system, with the maturity of EHV power technology, especially with the implementation of the policy of West to East power transmission, the requirement of single span 300 km transmission system in the power communication network is also very urgent. According to statistics, deserts alone account for about a quarter of the global land area. In addition to swamps, forests, mountains and other no man’s land, there is a huge potential market application of non relay transmission in passing through no man’s land. At present, the mature solutions of single span long-distance DWDM are mainly realized through Raman + FEC, Therefore, it is very meaningful to study the remote pump amplifier for the transmission system with longer distance.
The principle of remote pumping
As the name suggests, the remote pump amplifier is a fiber amplifier which can realize optical amplification by providing pump light remotely. As an optical amplifier, the pump light and gain medium are indispensable. Therefore, the remote pump amplifier is mainly composed of two parts, one is the pump unit which provides the pump source; The other part is the gain unit. Because of the remote pumping, the two units are located at different places hundreds of kilometers apart. Because the pump light must pass through hundreds of kilometers of transmission, the output power of the pump light is required to be relatively high. At present, there are two ways to achieve it: one is through high-power fiber laser; the other is through high-power fiber laser; The other is to combine the frequency stabilized FP semiconductor laser with IPBC, and then combine it with WDM to get high power output. The principle is the same as that of Raman pump module. Because the gain medium as gain unit is composed of erbium-doped fiber, 980nm laser is the most commonly used pump light for ordinary EDFA, but 980nm Pump light can not be used in remote pump amplifier. Because 980nm Pump light is multimode in transmission fiber, the transmission loss is very large, so the pump light can only select the light near 1480nm. The schematic diagram is shown in Figure 1.
Figure 1. Basic principle of remote pumping
Classification of remote pump optical amplifiers
According to the different pumping methods, the remote pumping optical amplifier can be divided into two types: the same fiber pumping and the different fiber pumping. The so-called same fiber pumping is that the pump light and signal light are transmitted in the same fiber, while the different fiber pumping is that the pump light and signal light are transmitted in different fibers. A special fiber is needed to transmit the pump light to a gain unit of hundreds of KM. The same fiber pump not only provides the pump source for hundreds of KM gain units, but also generates Raman gain. Because the pump wavelength is concentrated around 1480nm, the flatness of the Raman gain is very poor, so it is not suitable for DWDM system. Therefore, in DWDM system, different fiber pumping mode is preferred, while in SDH system, same fiber pumping mode is preferred. The schematic diagram of the same fiber pump is shown in Figure 2
Figure 2. Basic principle of remote pumping
The schematic diagram of different fiber pump is shown in Figure 3
3. Application of remote pump optical amplifier in SDH system
Through its own development, Accelink has mastered a set of technology from device manufacturing to system application, and realized 2.5gb/s single span 505km no relay transmission in the laboratory by using remote pump technology, with a total transmission loss of 86db. The structure of the experiment is shown in Figure 5
Fig. 5. The experimental setup of 505km (86db loss) using remote pump
Accelink has developed the remote pump amplifier and its application technology, and realized the single span 505km (86db loss) relay free transmission in the laboratory. Remote pump technology is another key technology to solve the problem of ultra long distance transmission.
Editor in charge: GT