Signal Characterization in Optical Networks

Doctoral thesis, 2008

The thesis presents and demonstrates methods to characterize high-speed (>10 Gbaud) signals in fiber optic communication systems and networks, including acquisition of the optical signal intensity, amplitude, phase and polarization. These quantities are necessary for the analysis of system performance, both in research and development stages, as well as for monitoring and management of operational systems. This becomes increasingly important as the optical communications systems develop toward using more sophisticated modulation formats. We demonstrated real-time optical intensity waveform sampling with high sampling rate and simultaneous high bandwidth by parallelizing the sampling in four optical gates. The optical sampling gates provide a total sampling rate of 100 GSample/s with sampling bandwidth limited by the Nyquist criterion. As an example, the first real-time acquisition of a 40 GHz bandwidth signal was demonstrated. The characterization of the full optical field, i.e. both the optical signal amplitude and phase, was used to analyze the regenerative properties of a saturated parametric amplifier on a 10 Gbit/s differential phase shift keying (DPSK) signal. The optical signal was visualized in a constellation diagram which enabled separation of amplitude and phase fluctuations. By introducing the phase preserving optical sampling gate to our coherent detection scheme, we were able to increase the temporal resolution tenfold. The high bandwidth was demonstrated by studying a 40 Gbit/s DPSK signal. Furthermore, we visualized the signal transitions by implementing signal averaging. Polarization analysis was utilized to characterize system properties. We investigated the polarization-mode dispersion (PMD) properties of a recirculation loop. It was found that, in contrast to the normal Maxwellian distribution, the first-order PMD statistics approaches a uniform distribution when no loop-synchronous polarization scrambling is used. Furthermore, we demonstrated methods to utilize the signal polarization for low-cost optical performance monitoring. The methods provide information about several parameters, e.g. power, wavelength, optical signal-to-noise ratio (OSNR), first-order PMD, etc., and the practical feasibility was demonstrated in an 820 km long installed optical system.