Fiber-based all-optical sampling for ultrafast waveform monitoring

Doktorsavhandling, 2004

Digital sampling is a technique to visualize a time-varying waveform by capturing quasi-instantaneous snapshots of a signal via a sampling gate. The gate is opened and closed by narrow pulses in a pulse train having a well defined repetitive behaviour such that all parts of the waveform are measured. Sub-picosecond temporal resolution sampling can be achieved with all-optical techniques that utilize the ultrafast response of nonlinear materials in order to implement the required gating functionality. In this thesis, the focus is on the performance of a fiber four-wavemixing (FWM) based all-optical sampling oscilloscope (OSO) in terms of a trade-off between temporal resolution, signal optical bandwidth and signal sensitivity. This work also addresses the fact that the nonlinear processes utilized to implement all-optical sampling gates are inherently strongly dependent on the relative state of polarization (SOP) between the sampling pulses, used to open the sampling gate, and the signal being monitored. A very simple technique is described that completely removes the problem with signal polarization dependence of a fiber FWM-based OSO. All digital sampling schemes require a precise time-base in order to position the acquired samples correctly in time. The time-base is usually dependent on a hardware generated trigger signal, which is normally extracted from the monitored signal. We demonstrate a novel technique to achieve the required time-base by means of a software algorithm applied on the acquired samples. As a result, we can avoid clock-recovery from the signal, and hence, the OSO becomes independent of the waveform repetition frequency. The detrimental polarization dependence of the FWM process can be utilized to implement novel functionality to an OSO. By rotating the state of polarization (SOP) of the sampling pulses during the sampling process, a time-resolved measurement of the SOP of the monitored signal was demonstrated with picosecond temporal resolution. Finally, we used the OSO as a tool to monitor the transmission quality of a 160 Gb/s RZ data signal subjected to polarization mode dispersion (PMD) and third-order dispersion (TOD). Techniques to compensate for PMD and TOD are presented and a large improvement of the transmission quality was achieved.