1. What are the causes of noise in optical fiber communication systems?

There are noise caused by unqualified extinction ratio, noise caused by random variation of light intensity, noise caused by time jitter, point noise and thermal noise at the receiving end, mode noise of optical fiber, noise caused by pulse broadening caused by dispersion, and mode distribution noise of LD. , the noise generated by the frequency chirp of the semiconductor laser and the noise generated by the reflection.

2. In the construction of the transmission network, what kind of optical fiber is mainly used? What are its main features?

There are mainly three types, namely G.652 conventional single-mode fiber, G.653 dispersion-shifted single-mode fiber, and G.655 non-zero dispersion-shifted fiber.

The dispersion of G.652 single-mode fiber in the C-band 1530-1565nm and L-band 1565-1625nm is relatively large, generally 17-22psnm·km, and the system speed is above 2.5Gbit/s, which requires dispersion compensation. At 10Gbit/s, the cost of dispersion compensation is large. It is the most common fiber in transmission networks.

The dispersion of G.653 dispersion-shifted fiber in C-band and L-band is generally -1～3.5psnnm·km, zero dispersion at 1550nm, and the system speed reaches 20gbit/s and 40Gbit/s respectively, which is a single-wavelength ultra-long distance transmission Best fiber. However, since DWDM has zero dispersion characteristics, it will produce nonlinear effects during capacity expansion, resulting in signal crosstalk and four-wave mixing FWM, so it is not suitable for DWDM.

G.655 non-zero dispersion-shifted fiber: G.655 non-zero dispersion-shifted fiber has a dispersion of 1-6 psnm·km in the C-band and 6-10 psnm·km in the L-band. The dispersion is small, avoiding the zero dispersion region. It can not only suppress FWM, but also can be used for DWDM expansion and high-speed systems. The effective area of the new G.655 fiber can be expanded to 1.5 times the effective area of ordinary fibers. The effective area can reduce the power density and nonlinear effects of the fiber.

3. What is fiber nonlinearity?

When the input optical power exceeds a certain value, the refractive index of the fiber will have a nonlinear relationship with the optical power, and Raman scattering and Brillouin scattering will occur, which will change the frequency of the incident light.

4. What effect does fiber nonlinearity have on transmission?

Non-linear effects can cause some additional losses and disturbances that degrade system performance. The optical power of WDM system is large, and the transmission distance along the optical fiber is long, resulting in nonlinear distortion. There are two types of nonlinear distortions: stimulated scattering and nonlinear refraction. Stimulated scattering includes Raman scattering and Brillouin scattering. Both types of scattering reduce the energy of the incident light, causing losses. When the input power is very small, it can be ignored.

5. What is passive optical network?

Passive Optical Network (PON) refers to an Optical Distribution Network (ODN) between OLT and ONU without any active electronic equipment. It is a point-to-multipoint optical fiber transmission and access technology. The downlink adopts broadcast mode, and the uplink adopts time division multiple access mode, which can flexibly form tree, star, bus and other topological structures, and only need to install a simple optical splitter at the optical branch point, so it has the advantages of saving optical cable resources, It has the advantages of sharing bandwidth resources, saving computer room investment, fast network construction, and low cost of comprehensive network construction. Passive optical networks include ATM-PON and Ethernet-PON.

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