The higher the spectral efficiency, the greater the optical signal-to-noise ratio, and the sharp reduction in optical transmission distance. However, 400G optical transmission technology can carry out safe long-distance information transmission with higher spectral efficiency and lower cost. After fiber-based development, it is necessary to build an optimal 400G system.
At present, the 400G system mainly has three implementation schemes: a four-carrier scheme, a dual-carrier scheme, and a single-carrier scheme. The four-carrier 100G PDM-QPSK method to build a 400G system has the advantages of large-scale application of the 100G system, mature technology, low cost, and long span; the dual-carrier 200G PDM-16QAM method to build a 400G system has the advantages of improved spectral efficiency High (165%), moderate span and low power consumption, high penetration rate, suitable for long-distance transmission; single carrier 400G PDM-32QAM way to build 400G system, with high spectral efficiency ( 300%) and The advantages of high system integration, but limited by Shannon’s law, there are disadvantages of high technical difficulty and short span.
The 400G system is constructed through the VPI simulation software, and the scene of setting the span length rule is set, and the simulation structure diagram of the 400G system is shown. The span is a 22 dB attenuation model, and the attenuation coefficient is 0. 275 dB/km, the span fiber length is 80 km, set 0. 03 is the maximum tolerance of the bit error rate (before forward error correction), the transmission distance of the 4-carrier 100G PDM-QPSK scheme is about 23 spans (about 1800 km), the spectral bandwidth is about 150 GHz, and the SE value is 2. 86 bit/Hz, the solution can reuse the existing 100G system components, the transmission distance is basically maintained compared with the 100G system, and the transmission capacity is increased by about 1/3; the transmission distance of the dual-carrier 200G PDM-16QAM solution is about 5 spans section (about 400 km), the signal occupies a bandwidth of 75 GHz, and the SE value is 5. 33 bit / Hz, separate components to form transmitter and receiver modules, use ultra-low loss ultra-large effective area optical fiber and low-noise amplifier to compensate nonlinearity, double the system capacity and achieve long-distance transmission; single carrier 400G The transmission distance of the PDM-32QAM scheme is only 3 spans (about 240 km), the signal occupies about 100 GHz bandwidth, and the SE value is 4 bit/Hz. This solution can achieve 400Gbit/S signal transmission at 50 GHz intervals and is compatible with WDM systems. The system has higher requirements for laser linewidth, D/A conversion sampling rate, scale and power of digital processing algorithms, and the cost is high.
Dual Carrier 200G PDM-16QAM
The high-speed 400G system adopts new optical fiber (ultra-low loss large effective area optical fiber) and dual-carrier 200G PDM-16QAM. Simulate the transmission of a single carrier at 256 Gbit/S rates, and set the FEC threshold to 0. 02. This simulation focuses on the Q factor in the eye diagram (the ratio of the signal power to the noise power of the receiver under the optimal decision threshold, signal-to-noise ratio) and transmission distance.
In the 400G system, the transmission distance is set to 800 km, and the traditional single-mode fiber (SSMF) and the new fiber are used. The relationship between the Q factor and the input power of the fiber in the receiving end. It can be seen that for a multi-channel system, nonlinear noise is generated inside the channel and adjacent channels, and the system uses a new type of optical fiber. The best Q-factor transmit power obtained from the simulation results is 1. 5 dBm, and the best Q-factor is 16. 5; The system uses traditional optical fiber, and the best Q-factor emission power obtained from simulation results is -0. 5 dBm, and the best Q factor is 15. 5. The optimal input optical power of the traditional single-mode optical fiber link is lower than that of the new single-mode optical fiber link. Around 5 dB, the optimal Q factor of a traditional single-mode fiber link is 1 dB smaller than that of a new single-mode fiber link.
When the input optical power in the traditional single-mode fiber link and the new single-mode fiber link in the 400G system is the best state, set FEC to 0. 02. It can be seen from the test results that the transmission distance of the traditional single-mode optical fiber link is about 500 km less than that of the new single-mode optical fiber link, and the transmission distance of the new single-mode optical fiber is increased by about 30%.
It can be seen that in the 400G system, the new single-mode fiber (ultra-low loss and large effective area fiber) increases the signal input optical power (about 1.5 dB), improves the nonlinear effect in the system by increasing the fiber effective area and ultra-low loss, and further increase the signal transmission distance (increased by about 30%).
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