| The research works of this dissertation are supported by the National Basic Research Program of China (973 Project, No. 2003CB314900), the Foundation for the Key Program of Ministry of Education of China (No. 104046), the National 863 High Technology Project of China (No. 2003AA311010 and No. 2007AA03Z447), and the Scholarship Program of China Scholarship Council (No. 2007U48018).In this dissertation, both theoretically and experimentally, transmission characteristics of polarization-division-multiplexing fiber communication systems, as well as microstructure fiber based devices for optical communication systems, are studied. The main contents and innovations are as follows.1,Polarization mode dispersion (PMD) and polarization dependent loss (PDL) impairments in polarization-division-multiplexing (PDM) signals with optical polarization demultiplexing and direct detection are investigated. We find that the time alignment between the bits in the two polarizations has a significant impact on the PMD impairments, and PMD impairments also depend on the bandwidth of PDM signals, whereas PDL impairments have little dependence on the relative time alignment between the two polarizations and the signal bandwidth. We show that with a proper configuration of the polarization demultiplexing, the PDL-induced crosstalk between the two polarizations can be completely eliminated. The combined effects of PMD and PDL are also studied, and we find that, in the presence of concatenated PMD and PDL, the impairment from one effect does not enhance that from the other. The conclusions are important for the design and the performance evaluation of PDM systems.2,A new automatic optical polarization demultiplexing scheme for polarization-division-multiplexing (PDM) signals, which uses the amplitude signal from a low frequency electrical power detector at the receiver side as a control signal, is proposed. This scheme is based on the intrinsic characteristics of PDM signals and does not need to add a special signal at a PDM transmitter. The effectiveness of this demultiplexing method is experimentally demonstrated in a 2×10 Gb/s on-off-keying (OOK) PDM transmission system.3,A simple and accurate method to measure the Kerr nonlinearity coefficient of optical fibres is proposed and demonstrated. The method is based on the nonlinear polarisation rotations caused by cross phase modulation between two lightwaves of different wavelengths co-propagating in fibres. The Kerr nonlinearity coefficients of standard single mode fibre and TrueWaveTM Reduced Slope fibre measured with this method are in good agreement with the published data.4,In cooperation with other researchers, the impairment of nonlinear polarization scattering on polarization- and wavelength-division-multiplexing (PDM-WDM) systems is investigate. Transmission experiments on 10-Gbaud PDM differential-quadrature-phase-shift-keying (DQPSK), differential-binary-phase-shift-keying (DBPSK) and on-off-keying (OOK) show that time interleaving return-to-zero signals in two polarizations can significantly reduce nonlinear polarization scattering in PDM-WDM systems and increase system tolerance to inter-channel nonlinearities.5,A microstructure fiber structure is proposed for broadband dispersion compensation. The fiber has a triangular lattice of air-holes along its length, and there are different three air-hole diameters. The dispersion of the standard single-mode fiber, which is 98 times of the length of the microstructure fiber, can be compensated over the entire 100nm wavelength range centered on 1550nm (to within 0.5%). The influence of the random imperfections of air-hole diameters is also studied for practical applications, and the sensitivity of the broadband compensation capability of the microstructure fiber to the irregularities induced during fabrication process is estimated.6,All optical wavelength conversion of 10-GHz clock signal is demonstrated based on cross-phase modulation of optical pluses in an 80-m long microstructure fiber, and the conversion bandwidth is over 30nm. The experimental microstructure fiber with a nonlinear coefficient of～11W-1km-1 has small normal dispersion and it is characterized by flat dispersion curve in the 1530nm to 1570 nm range. The experimental results show that a compact wavelength converter can be realized by utilizing this microstructure fiber. 7,In cooperation with other researchers, all-optical wavelength conversion of 10-Gb/s signal based on four-wave mixing is experimentally demonstrated in 30-m dispersion-flattened microstructure fibers with small positive dispersion. The conversion efficiency was around—19.5dB with the small fluctuation within±1.4dB, covering a conversion bandwidth of 20nm centered at 1550nm. The eye diagram of the converted signal shows good eye opening.8,In cooperation with other researchers, the generation of a flat supercontinuum spectrum of over 100nm in the 1.55μm region by injecting 1.6ps, 10-GHz repetition rate optical pulses into an 80m-long dispersion-flattened highly nonlinear microstructure fiber is demonstrated. The generated flat broadband supercontinuum ranging from 1503nm to 1593nm has the flatness of±2.5dB.9,A fiber ring laser based on microstructure fiber is presented. Through slightly adjusting the frequency of control signal, active mode-locked state is acquired and the laser can produce correlated optical pulses with a high repetition rate of 10GHz and a narrow width of less than 8ps. In this laser, erbium-doped fiber amplifier is used for optical gain, and a 25m long microstructure fiber is used for pulse compression, which has a large anomalous dispersion and a high nonlinear coefficient. The operating wavelength can be altered by changing the central wavelength of the band-pass filter in the fiber ring, thus this laser can be used as the tunable optical source for wavelength-division-multiplexing systems.10,In cooperation with other researchers, the birefringence in microstructure fiber with squeezed hexagonal lattice is numerical analyzed based on full-vector finite element method. The correlation between the birefringence and the structural parameters, incidence wavelength is obtained. The numerical results show that the birefringence is sensitive to wavelength, it can achieved a magnitude of 10-3 by proper design, and the sign of form birefringence in such microstructure fibers can be changed as the incidence wavelength changes. In the statistical analyses, the correlation between the variation of air hole diameters and the birefringence in the microstructure fiber is obtained; results show that that the birefringence depends on the mean diameter of air holes.
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