Optical cross-connects: Architecture and system perspective.

Abstract

This thesis focuses on the architecture and system perspective of optical cross-connect systems using semiconductor optical amplifiers. The work includes architecture study, system performance analysis, simulation, and experimental investigations. Optical cross-connects are promising for future large capacity switching systems and cross-connect systems in high-speed transport networks. To construct large capacity cross-connect systems using presently available technologies, multi-dimensional optical cross-connects are explored. A novel architecture using wavelength-division and space-division multiplexing techniques is proposed, where the wavelengths of WDM are effectively reused and the size of space switches is dramatically reduced. Conceptual designs are presented to show the feasibility of its implementation. As for the system performance, homo-wavelength crosstalk from space switches is identified as a main degradation factor. The performance is further investigated with Gaussian approximation of signal-crosstalk beat noise for a reasonably large size (64 x 64) cross-connect as a high-speed switching core. It is shown that a 64 x 64 cross-connect can be constructed with 4-wavelengths WDM and 16 x 16 space switches using presently available technology. Also, a 4 x 4 optical cross-connect based on the proposed architecture was experimentally demonstrated. Performance degradation due to homo-wavelength crosstalk in multi-wavelength optical networks using dynamic wavelength routing is addressed. Enhanced crosstalk degradation due to ASE noise of optical amplifiers is identified for the first time. Mechanisms and classification of homo-wavelength crosstalk are further investigated comprehensively. To analyze homo-wavelength crosstalk implications on system performance, a theoretical model based on moment generating functions (MGF) is presented. Further, the saddlepoint approximation is employed to evaluate the power penalty with the MGF of the received signal. The evaluation results justify the estimation of the required crosstalk ratio calculated with the Gaussian approximation for a large number of crosstalk channels $(N>4).$ Several experiments were carried out in order to verify the theoretical analysis and study the effect of wavelength alignment between the signal and the crosstalk. A good agreement is achieved between the analysis and experiments. The wavelength range for generating homo-wavelength beat noise is found around 0.05 nm for a 1 Gb/s directly-modulated signal. As primary optical switching components in the proposed cross-connect system, semiconductor optical amplifiers (SOAs) are investigated in various aspects. The available SOAs are tested and characterized for switching application. They perform very well in terms of polarization insensitivity and contrast ratio. As for system applications of SOAs, an input dynamic range of SOAs is studied experimentally and further simulated numerically. An input dynamic range can be over 20 dB for a single-stage SOA system and over 10 dB for a three-stage SOA system at 1 Gb/s. The simulation shows a signal bit rate can be up to 20 Gb/s with a reasonable input dynamic range. By using a wide optical bandwidth of SOAs, we propose to construct bi-directional cross-connects using semiconductor optical amplifiers. The feasibility of bi-directional cross-connection using SOAs is confirmed experimentally at a bit rate of 1 Gb/s. The maximum input power, however, is limited due to the cross-gain modulation, particularly for multi-stage SOA systems.