Generation and Applications of Picosecond Pulses in High-Capacity Fiber Optic Systems

Doktorsavhandling, 2004

This thesis deals with short optical pulse generation from actively mode-locked Erbium-doped fiber ring lasers (ML-EFRL) and their applications in all-optical signal processing (optical sampling and switching) and transmission experiments for high-capacity fiber optic systems. In future ultra-high bit-rate optical time division multiplexed (OTDM) systems, one of the key requirements is to develop reliable optical pulse sources that can generate high quality optical pulses. ML-EFRLs are very attractive candidates in this respect. Such a laser system with a non-polarization-maintaining cavity has been built and characterized. This laser can generate 10 GHz transform-limited 0.7-1.3 ps, sech2-shaped pulses in the 1536-1569 nm wavelength region, with less than 0.1 ps RMS timing jitter and over 80 dB super-mode noise suppression. Despite the non-PM nature of the laser cavity, long-term stability can still be maintained by a fiber-stretcher-based cavity length maintaining feedback circuit. By using this ring laser as the sampling pulse source, two optical sampling schemes based on 50 m highly nonlinear fiber (HNLF) have been demonstrated. One scheme uses fiber parametric amplification, which has resulted in up to 300 Gb/s real-time eye-diagram measurements with 1.6 ps temporal resolution. A sensitivity of 10 mW and more than 30 nm operational optical bandwidth were also demonstrated. The other scheme is based on cross-phase modulation (XPM)-induced wavelength shifting and optical filtering, which has led up to 500 Gb/s eye-diagram measurements with a temporal resolution of 0.7 ps. Signal operational wavelength range covering the whole EDFA gain range (1535-1569 nm) was also demonstrated. Furthermore, by exploiting XPM-induced wavelength shifting in HNLF, all-optical demultiplexing of 160 Gb/s OTDM data and simultaneous time domain add-drop multiplexing at 80 Gb/s have been experimentally demonstrated. Further numerical simulations show that with proper signal and control pulse widths, simultaneous time domain add-drop multiplexing at 160 Gb/s or beyond is possible. Finally, nonlinear pulse transmissions that demonstrate soliton robustness to polarization-mode dispersion (PMD) and intra-channel four-wave mixing (FWM) suppression have been performed. The results show that at least for individual signal pulses, both conventional and dispersion managed (DM) solitons are indeed robust to PMD. It was also demonstrated that proper phase modulation is an effective method to suppress intra-channel FWM for strongly DM nonlinear transmission systems.