Study Of Advanced Modulation Format And Optical Signal Processing By Utilizing Nonlinear Effects

With the magnificent development of semiconductor laser, fiber, electro-optical mod-ulator, detector and coupler, the optical communication system is significantly flourished. The transmission capacity on the optical network is continuously booming. Currently, the single channel modulation bit-rate of the optical backbone network is transforming from 10 Gbit/s to 40 Gbit/s and then to 100 Gbit/s. Thanks to the brilliant transmission char-acter of optical fiber, the long-haul transmission currently relies on the optical network. However, the intelligent control functions in the optical network nodes are still relying on the electrical signal processing. Compared to electrical signal processing, optical sig-nal processing has unrivaled speed capabilities. Besides, the time and power consuming electrical-to-optical (E/O) or optical-to-electrical (O/E) conversion for every single bit can be eliminated in optical signal processing. Currently, the optical signal processing is still in its experimental stage. With the increasing of the single channel modulation bit-rate, the cost and complexity of the electrical signal processing nodes are continuously expending. In the next generation, optical signal processing would be an important alternative method to increase the processing capacity and eliminate the O/E/O connectors. Optical comb filter is a key element in both fiber-photonics and microwave-photonics. For example, the opti-cal comb filter is frequently used in multi-wavelength lasers. Because of the homogeneous broadening effect, the multi-wavelength erbium doped fiber laser can hardly be stable at room temperature. In this thesis, four-wave-mixing (FWM) process is adopted for dynam-ically balancing the power variation among different wavelengths in a multi-wavelength fiber laser. Meanwhile, the wavelength spacing of the multi-wavelength fiber laser can also be changed through adjusting the structure of the comb filter. Besides the multi-wavelength fiber laser, the comb filter is also utilized in a rational harmonic mode-locked fiber laser to suppress the low-order harmonic modes in this paper.Various optical nonlinear effects exist in semiconductor optical amplifier (SOA), highly nonlinear fiber (HNLF) and silicon waveguide, such as self-phase modulation (SPM) ef-fect, cross-phase modulation (XPM) effect, FWM effect, and nonlinear polarization ro-tation (NPR) effect. Based on these nonlinear effects, various intelligent functions, such as wavelength multiplexing, wavelength conversion, wavelength interchange, format con-version, format generation, signal regeneration, clock recovery, time demultiplexing and noise monitoring, are investigated in this paper. On-off-keying (OOK) signal has the sim-plest modulation/detection structure and the widest application. Because of these, the op-tical nonlinear signal processing for OOK signal is the most developed one. In this work, three control functions are proposed and discussed for OOK signal. With XPM effect, the bit-rate continuously tunable clock recovery function is performed for OOK signal. With FWM process, the wavelength multiplexing experiment is performed for OOK signal with non-return-to-zero to return-to-zero format conversion. With FWM process, the time do-main demultiplexing function is demonstrated for OOK signal in silicon waveguide.Binary phase-shift keying (BPSK) and Differential phase-shift keying (DPSK) modu-lation formats have attracted lots of attention due to their high receiver sensitivity and large tolerance to nonlinear degradation. In this thesis, various signal processing functions are presented for DPSK signal using HNLF, SOA and silicon waveguide. With XPM effect, the wide-bandwidth phase regenerative wavelength conversion and the high-sensitivity phase-noise monitoring are demonstrated for DPSK signal. With one FWM process, the pattern-free wavelength conversion is completed for DPSK signal in silicon waveguide and the wavelength exchange is achieved for DPSK signal for the first time in HNLF. With two cascaded FWM processes, the driving voltage of a commercial phase modulator is reduced to one quarter in experiment. The capability of reducing the driving voltage in high-speed phase modulation is also discussed with numerical simulation.With the booming of information capacity, various advanced modulation formats emerge. With polarization multiplexing and multi-level modulation, the spectral efficiency can be largely improved. In this paper, a 20 Gbit/s polarization multiplexed DPSK signal is generated with 10 Gbit/s equipments. A 20 Gbit/s quadrature phase-shift keying (QPSK) signal is generated with 10 Gbit/s equipments. With FWM process, a 20 Gbit/s ASK-DPSK signal is experimentally generated with 10 Gbit/s equipments and reduced driving voltages for the first time to the best of our knowledge. Besides, a 120 Gbit/s 8-ary quadrature ampli-tude modulation (8QAM) signal is generated with numerically simulated FWM processes at 40 Gbaud/s.After considering the capacity, distance, quality and cost, each communication net-work has its optimized modulation format. In this circumstance, one modulation format is needed to be transformed to another between different kinds of networks. With the SPM, power saturation and NPR effects in one SOA, a multi-functional optical node is demon-strated for simultaneously OOK to BPSK format conversion, OOK signal extinction ratio enhancement and data reversion. With the XPM, cross-gain modulation and NPR effects in one SOA, the polarization insensitive OOK to BPSK format conversion, the one-to-four OOK to BPSK format conversion, and the format multiplexing from OOK and BPSK to QPSK are successfully performed. Besides, two independent channels of OOK signal can be converted to one channel of QPSK signal using two cascaded SOAs.