1 Department of Photonics Engineering, Technical University of Denmark2 Systems, Department of Photonics Engineering, Technical University of Denmark
Optical switching has been proposed as an effective solution to overcoming the potential electronic bottleneck in all-optical network nodes carrying IP over WDM. The solution builds on the use of optical labelling as a mean to route packets or bursts of packets through the network. In addition to the selected wavelength, a short label of fixed length can be added to the information flow in order to be processed in intermediate nodes. The main advantage of this approach is the ability to route packets or bursts independently of bit-rate, packet format and packet length, increasing network flexibility and granularity. In this thesis the systems aspects of optical labelling will be investigated for several labelling techniques, in order to identify the strengths and drawbacks of each one with respect to system performance. Orthogonal labelling has been proposed as one of the potential labelling techniques. It is based on the transmission of two types of modulation on the same optical carrier. The orthogonal nature of the signal ensures that the label and the payload would be able to be detected independently of each other. In order to asses the limits and possibilities of such schemes, several orthogonal labelling schemes will be investigated with regard to generation, transmission, wavelength conversion and label swapping. The resulting subsystems are then combined in order to emulate the functionalities of a network node within a transmission system. The two main addressed labelling schemes are the IM/DPSK and the IM/FSK schemes. In IM/DPSK the payload is modulated in the intensity of the carrier, while the label is differentially encoded on its phase. The IM/FSK labelling scheme, on the other hand, modulates the payload on the intensity, while the label is modulated on the frequency of the signal. Other labelling techniques are also presented and analyzed, including carrier suppressed sideband labelling and the DPSK/IM scheme that interchanges the roles of the modulation formats from the IM/DPSK scheme. A mention on time-serial labelling is done with regard to wavelength conversion and an experimental assessment of a packet switching node. Several methods for generating the labelled signals are studied. For the FSK transmitter, a novel method is presented, which is based on direct modulation of a DFB laser, accompanied by an intensity stabilization using the conjugated data on a EAM. The receiver performance is also analyzed with respect to several filtering schemes. A major performance limitation of orthogonal labelling comes from the crosstalk between the two modulation formats induced by the simultaneous amplitude and phase/frequency modulation on the same optical carrier. The receiver sensitivity of the IM signal improves as the IM extinction ratio is increased, while the sensitivity of the DPSK or FSK label deteriorates. Thus, the IM extinction ratio has to be optimized in order to correctly detect the information in the phase or frequency modulation. This requirement on the IM extinction ratio limits the network scalability and the system transparency to signal format. However, the need to use a poor extinction ratio can be completely eliminated by utilizing special line coding on the IM signal, such as Manchester coding or 8B/10B coding. The degradation of the orthogonal modulation formats, due to transmission over fiber spans, is compensated in single channel and WDM systems. Several compensation schemes are compared and limits to channel spacing and receiver performance are identified. The stabilization of filtering process throughout the systems are shown to be of importance. Wavelength conversion of orthogonally labelled signals is performed on passive and active devices. In HNLF both the FWM and XPM effects are exploited in order to achieve wavelength conversion of 40 Gb/s signals. Active devices as SOA, EAM and SOA-MZI are also investigated for wavelength conversion of labelled signals. Most of the employed wavelength conversion processes are only sensitive to the amplitude of the incoming signal, and can therefore effectively be used for label erasure. Label insertion can in some cases also be done in the same device, thus performing label swapping and wavelength conversion in a single device. However, this method is unable to re-use the same wavelength for the output signal, and requires a fast tunable filtering scheme at the network node. These limitations are overcome by performing wavelength conversion and label swapping in a double stage system, that makes use of an intermediate wavelength between label erasure and label insertion. The above mentioned functionalities are assembled in whole network systems experiments that validates the different labelling schemes with respect to transmission, wavelength conversion, label swapping and retransmission. Optical labelling and specially the orthogonal schemes for optical labelling, are thus shown to be an effective solution to all-optical networks.