The Analysis Of Optical Modulation, Transmission And Demodulation In The DD-OFDM System

OFDM is a multi-carrier transmission technique where a data stream is carried with many lower-rate sub-carrier tones. It has taken place of serial modulation formats for broadband wireless communication systems because of its simple solution to signal dispersion and intersymbol interference resulted in by the data rate increases. The efficient algorithm of Fast Fourier Transform (FFT)/Inverse Fast Fourier Transform (IFFT) is employed to the signal processing in the OFDM transceiver which means superior scalability for the data rate. In recent years, OFDM has been introduced into optical fiber communication applications. Optical OFDM has high optical spectral efficiency because the spectra of OFDM sub-carriers are partially overlapped. It can effectively combat the chromatic dispersion and polarization mode dispersion (PMD) of the transmission fiber.Coherent and incoherent (also called direct-detection) are two kinds of optical OFDM systems. The coherent optical OFDM (CO-OFDM) is employed in long-haul links and high-speed transmission systems because of its better sensitivity. The structure of CO-OFDM is complex and the cost is high, because of the coherent detection is employed. Therefore, very narrow line-width lasers are required and sophisticated digital signal processing (DSP) is required to compensate the propagation impairments [2]. On the contrary, the incoherent optical OFDM system is simple and can reduce the complexity because only one photodiode (PD) is required by employing the direct-detection. Due to the complexity and the cost, direct-detection OFDM (DD-OFDM) system is advantageous in the short distance propagation, such as OFDM-PON systems.In this paper, the direct-detect Single Sideband Optical Orthogonal Frequency Division Multiplexing (SSB-OOFDM) system which uses only one dual-drive Mach-Zehnder modulator (DD-MZM) to generate SSB-OOFDM signal is widely used and theoretical analyzed. Every parts of the system including the optical modulation, the fiber transmission and the optical demodulation are investigated. And a simulation platform is setted up with MATLAB to help the theoretical study. It is found that in this system the signals are interfered by the nonlinear of the fiber. It is also certified that there are phase shift of the signal demodulated and the first sub-carrier is interfered by the carrier. The compensation for the phase shift and the first sub-carrier zero padded are proposed in the DSP module.