In recent years, the main business carried by the access network is gradually changing from voice and data to video and image. This requires a high-speed, symmetrical, large-scale (20km) access network to connect the metropolitan area network / backbone network and the user network. Traditional access networks have gradually failed to meet this demand, and optical fiber is considered to be the best choice for access network media due to its nearly unlimited bandwidth. Fiber to the home (FTTH) has also become the ultimate choice to meet the skyrocketing bandwidth. In order to make full use of optical fiber and equipment resources and save operation and maintenance costs, people choose point-to-multipoint passive optical network (PON) for access, and on this basis, the integration of traditional telecommunications network, computer network and cable TV network "Three networks in one" is a feasible and effective solution to further reduce costs, and is also the general trend of network development.
Fig. 1 is a FTTH scheme based on PON structure, which is widely used at present, and uses wavelength division mulTIplexing (WDM) technology to realize single fiber transmission of upstream 1310nm band, downstream 1490nm band and broadcast CATV 1550nm band signal light, and uses TIme-division mulTIple access (TDMA) enables multi-user bandwidth sharing.
Figure 1 Schematic diagram of FTTH scheme based on PON structure, OLT-OpTIcal Line Terminal, ODN-Optical Distribution Network, ONU-Optical Network Unit
At this stage, CATV signals mostly use analog modulation, and the carrier-to-noise ratio (CNR) is an important indicator to measure system performance. When CNR "50dB, the human eye can't perceive the damage of the image. China's relevant standards stipulate that the CNR of the CATV user receiving end is not less than 43dB. According to the relevant theory of analog modulation systems, in order to obtain high CNR at the receiving end, it is necessary to reduce CNR degradation during line transmission and increase the optical power into the receiver (-6 ~ 2dBm, typical value is -1 ~ 0dBm @ 1550nm ), That is, to increase the transmission power of CATV signal light at the central office. In actual triple play applications, the CATV signal processing at the central office is often shown in Figure 2 (corresponding to the red dotted frame in Figure 1).
Figure 2 Optical signal synthesis in a triple-play system
If the optical power of the 1550nm signal bearing CATV service participating in multiplexing is considered to be 16dBm, when N >> 8 in FIGS. 1 and 2, the total output power of the optical amplifier OA should be> 27dBm (assuming that the inherent loss of Splitter and the connector is 2dB), this is very difficult for conventional EDFA, regardless of technology or cost. The high-power fiber amplifier (HFA) developed by Wuhan Guangxun Technology Co., Ltd. uses erbium and ytterbium co-doped double-clad fiber and lateral cladding pumping technology. Its maximum output power is 36dBm, and the price per unit of mW is significantly lower than ordinary EDFA. Table 1 is the performance index of OPAMS integrating HFA and 1310 + 1490 / 1550WDM.
Table 1 High-Power OPAMS Specifications
Among various indicators of the amplifier, the noise index NF is a key parameter that determines the amount of CNR degradation introduced by the amplifier. It can be found from the data in the table that the HFA NF is equivalent to the power amplifier EDFA. In addition, the fiber of HFA is not long, and the CSO and CTB of the system will not drop significantly. Our experimental data also shows that the cost of CSO / CTB introduced by HFA is not significantly different from that of conventional EDFA. Therefore, HFA and HFA-based OPAMS can meet the application requirements of CATV systems with a large number of optical nodes and triple play systems, and can effectively reduce costs to make up for the deficiencies of conventional EDFA.
In the era of information explosion, in order to meet people's ever-increasing bandwidth requirements, "light into copper" has become an inevitable trend. As the optical fiber is gradually advanced to the user (until FTTH), the power budget of the system must be higher and higher. Therefore, we believe that Guangxun's high-power fiber amplifiers will make a difference in the continuous development of optical networks.
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