Nonlinear applications of semiconductor optical amplifiers for all-optical networks

In the present work several optical network functionalities based on Semiconductor Optical Amplifiers (SOA) are investigated by means of numerical simulations. An optical network has the function of connecting two or more optical links through a network node. Although in the transmission paths the data flux is all-optical, in the network nodes much of the signal switching/processing has been implemented through electronic circuits whose speed is much lower than the speed of optics. One of the challenges of the present work was to propose novel all-optical network functions to substitute electronic signal processing in order to avoid a bottleneck in the network nodes. In this context, several network functionalities have been proposed and theoretically investigated in the present work. In the beginning of the work, Optical TimeDivision Multiplexing (OTDM) technology was considered to be a potential solution to future-generation optical networks. Therefore, the simulation environment developed was first envisaged to deal with OTDM signals. In this context an existing SOA time-domain model was upgraded and a SOA frequency-domain model was developed both able to process short optical pulses. The main features of both models are described in detail throughout the work. Having these SOA models as basic building blocks, the following network functionalities were proposed/investigated. First, a solution for high-speed simultaneous OTDM Add/Drop multiplexing is presented. The proposed setup enables independent optimization of drop and clear ports simultaneously, can work with high aggregate and high base data rates, can be monolithic integrable, possess low polarization dependency and has the potential of low cost and low power consumption. Theoretical results showed that OTDM ADM from 320 Gbit/s to 10 Gbit/s, 40 Gbit/s and 80 Gbit/s is possible by using this setup. A detailed investigation of a simplified setup for wavelength conversion of optical signals at bit rates varying from 40 Gbit/s to 320 Gbit/s is also carried out. The setup consists of a continuous wave (CW) pulse source, a semiconductor optical amplifier and an optical bandpass filter. The investigations for up and down-conversion took into consideration data signals with duty cycles varying from 8 % to 67 %. The optical bandpass filter has a Gauss or a second-order Gauss profile or is a delay interferometer (Mach-Zehnder interferometer with a delay in one of its arms). The Gauss and second-order Gauss filters are placed asymmetrically to the CW center wavelength in order to suppress the red-shifted (low-frequency) components of the CW spectrum. The delay interferometer is centered to the CW carrier in order to suppress it. A good wavelength conversion performance up to 160 Gbit/s is obtained when using short optical pulses. Moreover, error-free 40 Gbit/s wavelength conversion operation is demonstrated experimentally by implementing the delay interferometer by means of a birefringent fiber and a polarizer. A scheme for all-optical Exclusive-OR (XOR) is proposed and investigated for a bit rate of 160 Gbit/s. The setup consists of a semiconductor optical amplifier placed in both arms of a Mach-Zehnder interferometer (SOA-MZI) and coupled to a delay interferometer (SOA-MZI-DI). With help of the delay interferometer a completed degraded signal could be restored. Error-free operation is predicted by using a quasi-Monte-Carlo method. An interesting feature of this setup is that amplitude crossings lead to phase jumps of π that assist the restoration process. A setup for the regeneration of Differential Phase-Shift Keying (DPSK)-signals is proposed and analytically analyzed. The transfer function of the setup shows that depending on the working point either amplitude or phase can be regenerated. This behavior indicates that in order to enable the regeneration of both amplitude and phase of a degraded DPSK signal, a cascade of two consecutive such setups will be necessary. Another alternative would be to first regenerate the amplitude of the degraded DPSK signal before launching it to the proposed setup. Theoretical investigations on parametric amplification in semiconductor optical amplifiers are performed. Specially, ultra-long devices (> 2 mm) are investigated because of their improved efficiency. First results confirmed the existence of parametric amplification in SOAs which is related to the SOA gain spectrum asymmetry.