On transmission impairments in optical systems: Investigation, suppression and monitoring.

摘要

To achieve long transmission distance and high bit-rates, transmission impairments in lightwave systems, such as amplifier noise, chromatic dispersion, polarization mode dispersion and fiber nonlinearities, should be carefully handled. It is desirable to predict, control, suppress and monitor these impairments to maintain high transmission performance.; Before controlling the impairments, it is necessary to investigate the system impacts they cause to acquire insights on how to optimize the transmission system. In the first part of the thesis, the cross-phase modulation (XPM) induced nonlinear impairments, such as nonlinear polarization fluctuation (NPF) in DPSK-WDM and OOK-WDM systems and nonlinear phase noise in OOK-DPSK hybrid WDM systems are experimentally investigated by using a simple pump-probe configuration. Detailed quantification of the XPM-induced penalty helps network operators identify the nonlinear impairments.; Although the fiber nonlinear effect significantly degrades the system performance, it also offers the possibility of processing the signal in an all-optical manner. Among these fiber nonlinearities, the XPM effect in nonlinear fibers plays an important role in all-optical signal processing due to its ultrafast response and low polarization dependence. In this thesis, the NPF effect, also known as cross-polarization modulation (XPolM), in highly nonlinear photonic crystal fiber is used to realize an all-optical wavelength converter, and the XPM effect in dispersion-shifted fiber is employed to implement an all-optical RZ-DPSK encoder. Meanwhile, a new simple modified-duobinary transmitter based on a Mach-Zhender delay interferometer is demonstrated by experiment and simulation to suppress the intra-channel nonlinearities.; In order to minimize or mitigate the degradation caused by impairments, performance monitoring of signal quality is required to effectively manage a network. In this thesis, we tackle three of the most challenging issues in performance monitoring: polarization mode dispersion (PMD), optical signal-to-noise ratio (OSNR), and alignment monitoring. First, a self-phase modulation (SPM)-assisted PMD monitoring scheme based on frequency-resolved polarization state rotation is proposed. The use of broadened signal spectrum induced by SPM not only relaxes the filter requirement and reduces the computational complexity, but also improves the estimation accuracy, and relaxes the pulsewidth requirement. As OSNR monitoring has become an indispensable part of optical performance monitoring (OPM) to allow in-service characterization of the signal quality, two simultaneous OSNR and PMD monitoring schemes using polarization techniques are proposed and experimentally demonstrated. The first one employs enhanced RF spectral analysis, while the second is based on degree-of-polarization (DOP) measurement. Finally, the misalignment between pulse-carver and data modulator in an RZ-DPSK transmitter is effectively monitored by simply performing off-center optical power filtering.