The Polarization Mode Dispersion Monitor And Compensation In Optical Subcarrier Multiplexing System

Optical subcarrier multiplexing (SCM) has been shown to be a useful technique for reliably transmitting both analog and digital signals over fiber. It applies to analog cable television, and whose future applications may include local-area network (LAN) traffic, and header control for optical packet-switched networks. But the transmission of analog and digital SCM signals over fiber can be severely affected by PMD. PMD blocks the improvement of bit rate and relays station in long optical fiber communication. Consequently, one of the most important problems is to solve the PMD problem so as to achieve high speed SCM optical fiber communication.This paper describes the concept of PMD firstly, which is the theory for latter chapters. Then the PMD is monitored in SCM system and different monitor methods are compared such as RF, eye-diagram, DOP, etc. Meanwhile the character of SCM system is also considered. Finally, DOP is selected as the monitor signal to PMD. By analyzing in SCM system, while the power of optical carrier is much greater than that of the sideband, and the phase differences is not distinct, it is not easy to be monitored. In order to solve the problem, a narrow-band optical filter is applied in the design to equalize the power of optical carrier and the sideband, that is to filtrate one sideband from double sidebands, increase the power of the sideband, thereby it keeps the same level as the power of optical carrier. By doing so, the sensitive degree of DOP to DGD is obviously increasing. Finally, the paper discussed the PMD compensation in SCM system, which can also be done in electrical domain and optical domain. By analyzing all kinds of compensation methods, the simplest method is fixed-delay line method if DOP is applied as feedback signal, and the compensation effects is also good without higher-order PMD. If RF is used as feedback signal, variable-delay line will be a better compensation method. If higher-rank PMD influence is taken into account, coherent monitor with DFB local oscillator can be used in the compensation.