OSN1800V WDM is the most commonly used backbone network transmission method for fiber optic networks nowadays. WDM consists of CWDM/DWDM WDM + EDFA fiber amplifier + DCM dispersion compensation module, so WDM is a systematic. Today, we are here to share the ultra-long distance WDM transmission technology.

I. Types of ultra-long distance transmission and application environment

In recent years, WDM technology is mainly developed in the two directions of ultra-long-distance and high-capacity transmission. In view of the market demand and network trend of ultra-long-distance transmission, this paper will focus on long-distance transmission and discuss the solution of ultra-long-distance transmission.

Ultra-long-distance transmission includes two kinds of long-distance powerless relay transmission with multiple spanning (amplified) segments and single spanning (amplified) segment transmission. From the perspective of morphological characteristics, the former usually refers to point-to-point transmission without photoelectric conversion for 1000km to 3000km, while the latter is point-to-point transmission without active amplification or relaying for 100km to 300km, as shown in Fig. 1. From the technical realization point of view, multi-span long-distance unrepeatered transmission needs to solve a series of problems such as optical signal-to-noise ratio (OSNR), chromatic dispersion (CD), polarization mode dispersion (PMD), nonlinear effects (NL), and power equalization, etc., while single-span transmission needs to solve a lot fewer problems, and is generally only concerned about OSNR and nonlinear effects (NL). From the network application point of view, the former is applied in the conventional environment, which can reduce the number of electric relay contacts up and down the non-service and greatly reduce the maintenance workload; the latter is mainly applied in the area between islands, deserts, no man's land and other natural obstacles can not be set up active equipment or inconvenient to maintain the area.

Second, ultra-long-distance transmission technology

Ultra-long-distance transmission is limited by the dispersion effect, nonlinear effect and other physical barriers. In order to complete the ultra-long-distance transmission, it is necessary to deal with the terminal and the line by adopting the corresponding technology.

Physical limitations of long-distance transmission

Optical signal transmission in optical fiber, the physical barriers encountered include nonlinear effect (NL), dispersion effect (CD), polarization mode dispersion (PMD) and optical signal-to-noise ratio (OSNR) limitations. Among them, the nonlinear effects in turn include the effects of self-phase modulation (SPM), mutual phase modulation (XPM), four-wave mixing (FWM), stimulated Raman (SRS) and stimulated Brillouin (SBS).

Dispersion effects (CD) can be overcome by increasing the dispersion tolerance of the end transmitting unit, line dispersion compensation, and post-compensation at the receiving end. For polarization mode dispersion (PMD), there is no good commercial means of compensation, can only use polarization mode dispersion (PMD) index is good (less than 0.1ps/km1/2) of the optical fiber to achieve long-distance transmission. Optical signal-to-noise ratio (OSNR) limitations can be resolved by reducing the OSNR threshold of the transmitting unit.

The nonlinear effect that affects the performance of long-distance transmission is mainly the self-phase modulation (SPM), which is manifested by interacting with the fiber dispersion effect (CD), the so-called dispersion mapping problem. As shown in Fig. 3, the variation of the dispersion profile is related to the transmission distance, and with the extension of the transmission distance, the residual dispersion curve shifts in the direction of positive dispersion, and the dispersion window becomes narrower at the same time.