The Research Of In-band OSNR Monitoring Techniques For Coherent Optical Orthogonal Frequency Division Multiplexing System

As a great demand for transmission capacity, traditional single-carrier optical fiber communication has been unable to meet the requirements. The multi-carrier technology based on coherent orthogonal frequency division multiplexing(OFDM) is attracting more and more attention due to its high spectrum efficiency and great tolerance for chromatic dispersion(CD) and polarization mode dispersion(PMD). Optical performance monitoring(OPM) is very important for advanced fiber communication systems because accurate OPM is useful for managing the whole systems. The optical signal to noise ratio(OSNR) is one of the most important parameter because it is correlated with the end-terminal bit error rate. Accurate OSNR monitoring can be helpful for link optimization and analysis of the system's problems. Previously, OSNR monitoring involved the use of traditional out-of-band techniques. However, out-of-band techniques are no longer accurate with the application of ultra-dense wavelength division multiplexer systems and the introduction of reconfigurable optical add/drop multiplexers. Therefore, it is necessary to perform the in-band OSNR monitoring in a cost-effective way. This paper is mainly force on OSNR monitoring for coherent OFDM system. The main work is as follows :1. We propose an OSNR monitoring technique based on coherent hybrid. The principle of proposed technique is derived and the feasibility is confirmed through simulation and experiment. The results show the OSNR monitoring error is less than 0.5d B from 10 d B to 25 d B OSNR and is insensitive to CDand PMD.2. We propose an OSNR monitoring technique based on training symbol. OSNR of the polarization division multiplexed(PMD) signals on both polarization tributaries can be simultaneously monitored. Matlab and VPI are employed to build simulation PDM coherent OFDM transmission system which is used for verifying the proposed OSNR monitoring technique. The simulation results show the OSNR monitoring error is less than 0.5d B from 12 d B to 30 d B OSNR and is insensitive to CD, PMD, polarization-dependent loss(PDL) and laser linewidth.In summary, we propose two in-band OSNR moitoring techniques for CO-OFDM systems. The OSNR monitoring technique based on coherent hybrid can achieve cost-effective OSNR monitoring. Another technique based on training symbol can monitor the OSNR of each polarization tributarie of PDM-CO-OFDM system.