Study On Nonlinearities In Quantum Well Semiconductor Optical Amplifiers

As the increasing of telecommunication networks due to the advancement of opticaltransmission technologies, optoelectronic devices play more and more important role inthe nodes of the optical networks. Semiconductor optical amplifiers (SOAs) have beenwidely used in the optical networks thanks to the high nonlinearities, the low-powerconsumption and the potential for integration. However, Different applications needdifferent types of SOAs. On the other hand, the material, the width and the number ofQWs in the active region of QW-SOA can be adjusted based on the energy bandengineering to obtain the optimum QW-SOA. In this dissertation, we focus on thestructural optimization of QW-SOA to meet the demands of different applications. Theresearch achievements and contributions are summarized as follows:(1) We have built a comprehensive numerical model based on the band theory insemiconductors. The Schr dinger equations in the conduction band and valence band arenumerically solved. On the other hand, we have also built the dynamic model of QW-SOAand the interactions between photon and electron are considered in the simulated model.Therefore, the nonlinearities of QW-SOA such as cross phase modulation (XPM), crossgain modulation (XGM) can be analyzed in detail.(2) Based on the simulated model, we proposed a novel QW-SOA, the intra-carrierrecovery time of which can be shortened significantly. In the active region of the novelQW-SOA, the bandgap of the first subband and the second subband in the conductionband are nearly36meV, which is the energy of a longitudinal optical (LO) phonon. Theinteraction between electron and phonon are enhanced in the active region. The intrabandcarrier recovery speed is accelerated significantly.(3) Based on the simulated model, we proposed a novel asymmetric QW (AQW)SOA to enhance the gain recovery. In the active region, a shallow QW and a deep QW areact as a continuum and the quantum barrier between the deep QW and the shallow QW areas thin as3nm. Thus the tunneling effect will be very strong. On the other hand, theelectrons with high energy mainly locate in the shallow QW while both the electrons with low energy in the conduction band and the holes with low energy in the valence bandmainly locate in the deep QW. Therefore, the deep QW is the ‘real’ active region. Therecombination between the electron and hole is almost in the deep QW. The shallow QWis a perfect carrier reservoir. Gain recovery time (from10%to90%) can be as short as15ps.(4) Based on the simulated model, we have analyzed the phase modulation, gainmodulation as well as the third susceptibility in the active region of the QW-SOAs. Wediscussed the phase dynamics as well as gain dynamics with different operatingconditions such as different optical power, injected current and operational wavelength.Secondly, we have theoretically analyzed the linewith enhancement factors (αNand αT) inthe active region of QW-SOA. The dependence of the material system, the design of theactive region and the operating conditions on the linewith enhancement factors arediscussed in detail. Lastly, the third susceptibility (χ(3)) in the active region is analyzed.We found that the χ(3)in asymmetric QW-SOA can be enhanced due to the interactionbetween the shallow QW and the adjacent deep QW.(5) Based on the simulated model, we have theoretically demonstrated the SOA withstrong wavelength dependence of gain and phase could be used for all-optical inverted andnon-inverted wavelength conversion over a wide range with the assistance of opticalfiltering. Through simulation, it is found that the quality of converted signal had littledependence on the operational wavelength. Both the inverted and the non-inverted WC areachieved in a large wavelength range.(6) Based on the simulated model, we have presented an optimized QW-SOA whichis capable of regenerating phase-modulated signals, such as return-to-zerodifferential-phase-shift keying (RZ-DPSK) signal. Based on the optimized QW-SOA, theamplitude fluctuations are suppressed while the phase information is preserved. Theessential mechanism of the optimized QW-SOA is the low linewidth enhancement factor(α-factor). Furthermore, the optimized QW-SOA can be used for dual-and four-channelRZ-DPSK signal regeneration.(7) We have experimentally demonstrated that, based on the operational conditionoptimization, a common QW-SOA has the ability of regeneration for single-and dual- RZ-DPSK signal. On the other hand, we also experimentally demonstrate single andmulticasting inverted wavelength conversion at80Gb/s by using the XGM and XPM in asingle SOA. In all cases, the converted signals with high extinction ratio (ER) and largeeye opening are obtained.