1 Department of Photonics Engineering, Technical University of Denmark2 Networks, Department of Photonics Engineering, Technical University of Denmark3 Systems, Department of Photonics Engineering, Technical University of Denmark4 Nanophotonics, Department of Photonics Engineering, Technical University of Denmark5 Copenhagen Center for Health Technology, Center, Technical University of Denmark
This thesis analyzes semiconductor optical amplifier (SOA) based all-optical switches experimentally and through numerical simulations. These devices are candidates for optical signal processing functionalities such as wavelength conversion, regeneration, and logic processing in future transparent optical networks. The factors governing the modulation bandwidth of SOAs are determined, and schemes for reducing detrimental patterning effects are discussed. Three types of SOA-based switches are investigated numerically: so-called standardmode and differential-mode switches, and the filtering assisted switch. Differential -mode switches are shown to eliminate one contribution to the patterning effects, referred to as the linear patterning. This enables operation at bitrates far beyond the limit set by the carrier lifetime, but ultimately a saturation-induced patterning effect, nonlinear patterning, is found to limit the performance. Two implementations of differential-mode switches, the Mach Zehnder interferometer (MZI) and the delayed-interferometer signal converter (DISC), are compared at bitrates up to 160 Gb/s, and fundamental differences in terms of noise filtering are demonstrated. The DISC, consisting of an SOA and an asymmetric MZI filter, is analyzed in the small-signal regime, and the obtainable modulation bandwidth is expressed analytically. A new optical spectrum approach to small signal analysis is introduced, and is used to assess the bandwidth enhancing effect of different optical filters, as well the impact of the filter phase response. Experiments at 40 Gb/s verify the predictions of the small-signal analysis. Wavelength conversion is demonstrated experimentally at 40 Gb/s using a simple filtering-assisted scheme with an ultra-low optical switching energy, and up to 80 Gb/s employing MZIs operated in the standard mode, also assisted by bandwidth enhancing filtering. The impact of 2R regeneration (re-amplification and re shaping) is explained through simulations, and demonstrated using MZIs at 10 Gb/s. In addition, the 2R regenerative capability of a novel all-active 2x2 coupler is verified, also at 10 Gb/s. 3R regeneration (2R + re-timing), based on a cross-gain modulation wavelength converter and a MZI, is demonstrated at 40 Gb/s in a recirculating loop experiment over 4000 km. Moreover, an optical subsystem for NRZ clock recovery, based on self-phase modulation and cross-phase modulation in an SOA, and capable of generating the 40 GHz spectral component from a 40 Gb/s NRZ signal, is presented. All-optical Boolean logic gates and functionalities involving several gates are investigated experimentally and numerically. Boolean AND and XOR gates are realized experimentally with MZIs, at 20 Gb/s and 10 Gb/s, respectively, whereas combinations of Boolean functions in MZIs are used to demonstrate a 3-input XOR gate, a data segment bit comparator, and a compact parity checking scheme, all at 10 Gb/s.