Study On Key Devices And Papr Reduction Technologies In All-optical Ofdm Systems

Orthogonal frequency division multiplexing (OFDM) has been widely applied in wireless communications to enhance spectral efficiency and to combat intersymbol interference caused by a dispersive channel. Only recently, OFDM has been introduced to optical communications. Aided by electronic fast Fourier transform circuit and sophisticated digital signal processing technologies, optical OFDM exhibits advanced tolerances both of chromatic dispersion and polarization mode dispersion (PMD). To mitigate the limitation of processing speed set by electronics, all-optical OFDM has attracted great attention and is promising to eliminate the electronic bottleneck. The key devices in an all-optical OFDM system are the all-optical multiplexer and demultiplexer.Firstly, this dissertation investigates into OFDM fundamentals by mathematical models. Based on this, two kinds of all-optical OFDM multiplexer are studied: an all-optical OFDM multiplexer based on SiO₂ planar lightwave circuit (PLC) and an all-optical OFDM multiplexer based on LiNbO3 waveguide. The first kind of multiplexer utilizes SiO₂ PLC-based directional couplers, optical delay lines and thermo-optical phase shifters to implement time delay and phase shifting respectively, so as to implement optical inverse discrete Fourier transform. The second kind, firstly proposed by this dissertation, employs LiNbO3 phase modulators driven by electrical triangular waveforms to realize light frequency shifting. This dissertation gives the design principals, implementation methods, numerical simulation results, and performance evaluations of the two kinds of multiplexers. A 2×20Gb/s and a 2×10Gb/s SiO₂ PLC-based all-optical OFDM multiplexers have been manufactured. The optical charecteristics of 2×10Gb/s SiO₂ PLC-based all-optical OFDM multiplexers have been tested. Measured results show 4.92 dB optical loss, 13 dB extinction ratio, and obvious thermo-optic phase shifting effect.Secondly, two kinds of all-optical OFDM systems have been designed based on the two kinds of all-optical OFDM multiplexers and have been numerically analyzed. In a 4×40Gb/s SiO₂ PLC-based all-optical OFDM system, signals are successfully rebuilt after transmitting through 400 km G.655 fibers. In a 16×10Gb/s LiNbO3 waveguide-based all-optical OFDM system, PMD tolerance is 42 ps and dispersion tolerance is more than 3000 ps/nm when bit error rate is 1×10-9. Simulation results validate the feasibility and superiority of all-optical OFDM. The comparisons between the three kinds of all-optical OFDM are also given.Finally, this dissertation studies the theory of peak-to-average power ratio (PAPR) in OFDM systems and its reduction schemes. A novel all-optical PAPR reduction scheme based on phase pre-emphasis is proposed in this dissertation. Numerical simulations show that phase pre-emphasis brings about significant PAPR reduction in OFDM systems that use intensity modulation format: 3.74 dB when the number of subcarriers is 16; 14.88 dB when the number of subcarriers is 512. The implementation methods of phase pre-emphasis in all-optical OFDM systems based both on SiO₂ PLC and LiNbO3 waveguide are given. Phase pre-emphasis can be easily implemented without resulting in noticeable additional device or system complexity.