Key Technologies In Ultra-High Speed Optical Fiber Transmission Systems

In recent years, by the development of HDTV, real-time Internet games and other technologies, the capacity of optical fiber communication network is continual increasing. Traditional transmission rate of optical network cannot meet the requirement of our society. How to improve the capacity of optical transmission systems becomes an important issue for researchers all over the world. In this essay, we developed some key techniques for the future ultra-high speed optical fiber transmission systems, including optical time-division multiplexing (OTDM), multi-level modulation format and coherent receiver, generation of optical frequency comb and its applications.We investigated the short optical pulses and demultiplexing time window in OTDM systems. We experimentally generated40GHz short pulses based on a MZM and two PMs cascaded together. The pulse width of generated pulses was1.8ps, extinction ratio (ER) was20dB, and OSNR to be43dB. By using two-stage optical delay line,40GHz pulses could be multiplexed to be160GHz. The observed optical waveform has no interference between each pulses, it can be used for high speed OTDM systems. In demultiplexing stage, we generated40GHz optical time window using cascaded MZMs. The pulse width was4.2ps, ER was18dB, and OSNR to be50dB, it can meet the requirement for OTDM demultiplexing systems.We also investigated some typical multi-level modulation formats and their demodulation. We experimentally generated40Gb/s quadrature phase shift keying (QPSK) signal based on DPMZM. Both eye diagram and constellation diagram of the generated QPSK signal were clear. Then, we generated10Gb/s16quadrature amplitude modulation (QAM) signal still using DPMZM. From the experimental results, we can find16points located symmetrically in the constellation diagram. We also investigated their demodulation using coherent receiver based on digital signal processing (DSP) technology. We experimentally demonstrated the demodulation of40Gb/s-QPSK signal using coherent receiver. When the OSNR of receiver was20dB, error free transmission could be achieved.Optical frequency comb (OFC) technology plays an important role in high speed optical communication. In this essay, we firstly developed the application of OFC in high speed optical communication systems. Then, we experimentally generated a25-line ultra-flat OFC with a tunable comb spacing using cascaded DPMZMs. From the experimental results, we can see the power fluctuation between comb lines was0.75dB, and unwanted-mode suppression ratio to be20dB. Also, the comb spacing can be tuned easily by changing the frequency of input RF signals.