Research On Equalization Techniques In High-speed Optical Fiber Communication System

Since the first use of fiber in transmission systems, both the capacities and the transmission distance have steadily increased. Nowadays, the optical transmission system is being upgraded from 10G to 40G even 100G due to the increasing demands of wide-bandwidth network. The increasing capacity will benefit the customs a lot while there are many challenges for the telecom manufacturers. The first challenge comes with the realization of the high speed optical signal:how to transmit and receive the signal? The second challenge is how to deal with the distortions caused by the fiber effects which have become so significant that they have become the main obstacle in the upgrading of the system. For example, the polarization mode dispersion in the 40G system has seriously restricted its reaching distance.Concerning the first challenge, many new modulation formats have been raised recently. Traditionally, optical transmission systems have used amplitude modulation. However, for the next generation transmission systems this is not a suitable choice. They normally require a too large channel spacing, have a high optical-signal-to-noise ratio (OSNR) requirement and generate significant nonlinear impairments. Current state-of-the-art transmission systems therefore often use differential phase shift keying (DPSK). Compared to OOK, DPSK has a 3dB penalty advantage when demodulated using balanced detection and it can generate less nonlinear impairments. It also increased the tolerance of CD and PMD. Multi-level formats like the (D)QPSK and (D)8PSK signals are adopted to further increase the spectral efficiency(SE). These two formats have SE of 2bit/s/Hz and 3bit/s/Hz separately. Moreover, polarization and frequency of light are used to carry information besides amplitude and phase. Polarization multiplexing systems and optical orthogonal frequency division multiplexing systems are proposed these years.When it comes to deal with the distortions brought by linear and nonlinear effects in the fiber, equalization techniques have to be used. There are mainly two kinds of equalization techniques:passive and active equalization techniques. Passive equalization techniques are schemes proposed to relief these distortions. For instance, the use of phase modulated signal will increase the tolerance of PMD and CD as depicted above. However, as precautions against these effects passive schemes can not fundamentally solve the challenge in the transmission system. Active equalizations are further needed. According to the field where they process the signal, active equalization techniques are mainly classified into two categories:the optical equalization and the electrical equalization. The typical optical equalizations include optical CD compensator, adaptive PMD compensator and phase regenerates etc. Likewise, the electrical equalizations have the corresponding modules to deal with different distortions. Additionally, with the rapid development of coherent techniques, most of the equalizations can be realized in the DSP at the receiver side.In this thesis I will first present the researches on the novel modulation formats as passive equalization techniques which have been widely used recently. Next I will discuss there types of active equalization techniques including phase regenerator, PMD compensator, and the coherent electrical equalizations. It mainly consists of the following parts:(1) Simulations and experiments on the novel modulation formats are carried out. First I will introduce the phase modulation formats like the DPSK, (D)QPSK signals and analysis their performances. Then, the structure of polarization multiplexing system will be given combining QPSK formats. Two schemes of polarization demultiplexing will be compared experimentally including direct demultiplexing and coherent demultiplexing. It shows that the coherent demultiplexing exceed direct demultiplexing. Furthermore, we proposed two schemes of generating 16QAM with very simple structure. At last, a 400-Gbit/s signal is generated experimentally using optical OFDM PM-QPSK.(2) Theoretical analysis of both linear and nonlinear sources of phase noise in optical transmission system is given. We proposed schemes of phase regeneration for multi-level phase modulations based on phase sensitive amplifier whose principle has been analyzed in detail.(3) The inside mechanism of a polarization mode dispersion compensator is introduced in detail. And then the novel ideas in designing the OPMDC prototype are presented. The OPMDC prototype has shown good performance in field test.(4) Algorithms for coherent detection are realized and show their effectiveness in experiments.(5) A scheme of wavelength conversion based on FWM using two co-polarization pumps is proposed for 400-Gbit/s signals.In summary, this thesis is mainly about the electrical and optical equalizations in high-speed optical transmission systems. We analyzed the principles of the effects in fiber transmission, got the mathematical model, and proposed some novel solving schemes. Since these problems are urgent in the commercial applications, we hope our schemes will make some contributions.