Research On The Key Technologies Of High-Speed Optical Communications

Nowadays, the ever-increasing communication bandwidth requires a larger and larger transmission capacity of the backbone networks and the high-speed optical communication has become a current research focus. The high-speed optical communication systems, with complex structures, involve many key technologies, for example, high-speed optical time-division multiplexing (OTDM) systems needs high-quality pulse source, wavelength-division multiplexing (WDM) systems needs flat optical frequency comb as multi-carrier source, and the generation of high-precision optical signal needs electrical clock signal with high stability and tunability. Each technology not only can be used in transmission systems, but also has many important applications in other fields. In this paper, our work focuses on the high-speed optical communication systems, which includes high-speed OTDM systems and Nyquist pulse source, generation of optical frequency comb and the technology of generating ultra-stable and frequency-tunable microwave signal.First, in the field of high-speed optical transmission system, we demonstrate a640Gbit/s DQPSK-OTDM transmission system. Error-free transmission over410km fiber link was realized and the power penalty is4.46dB in the worst case. Then we performed an experiment of coherent detection of320Gbit/s Nyquist-OTDM signal. A comparison between Gaussian sampling and Nyquist matched sampling was made and it is verified that the latter approach could suppress the inter-symbol interference (ISI) more effectively at the same pulse width. At last, we proposed a scheme of Nyquist pulses generation based on a dual parallel Mach-Zehnder modulator (DPMZM) driven by a single RF signal. A detailed theoretical analysis was made and it is theoretically proved that Nyquist pulses with a spectrum of5flat comb lines can be generated using a single DPMZM. This scheme is further investigated experimentally.40GHz Nyquist pulses with full-width-at-half-maximum (FWHM) less than4.65ps, signal-to-noise ratio (SNR) better than29dB, and normalized root-mean-square error (NRMSE) less than2.4%are generated. The tunability of repetition rate is experimentally verified by generation of1GHz to40GHz Nyquist pulses with SNR better than28.4dB and NRMSE less than6.15%.Second, in the field of optical frequency comb generation, we proposed a novel scheme based on stimulated Brillouin scattering (SBS). In the experiment,10comb lines with frequency spacing of9.7GHz and flatness lower than1dB are generated. To overcome the problem of fixed frequency spacing, we further proposed a modified scheme and successfully obtained6comb lines with frequency spacing of49.7GHz. The measured flatness is below0.9dB and the side-comb suppression ratio (SCSR)is more than14.7dB.Finally, in the field of microwave signal generation, we proposed a dual-loop optoelectronic oscillator (OEO) employing balanced detection and Brillouin OEO based on single-sideband suppressed-carrier (SSB-SC) modulation. In the former experiment, we obtained high-quality40GHz microwave signal with timing-jitter of32.31fs. The generated electrical clock and optical clock are further used for pulse compression.40GHz short optical pulses with FWHM of2.1ps are obtained. In the latter experiment, employing a RF source lower than10GHz, fundamental-frequency and doubled-frequency microwave signal with a tuning range from10.9to20.9GHz and from21.8to41.8GHz are generated respectively.