Investigation Of Ultrafast Dynamics In Semiconductor Optical Amplifiers

The demands for key devices with high performance for optical signal processing increase rapidly with the increase of the speed in one wavelength channel in optical networks. The dynamic characteristics of semiconductor optical amplifiers (SOAs) which are extensively used in nodes of optical networks as nonlinear devices determine the performance of the entire optical communication systems directly. The recovery time of the carrier density in SOAs, reflecting the gain recovery speed is a critical parameter to the performance of the entire system, especially for ultra-high speed wavelength conversion, multiplexing/demultiplexing and signal regeneration.With the development of both theory and fabrication technology of SOA, gain polarization independence, high gain as well as differential gain coefficient and ultrafast gain recovery speed are realized in low-dimensional SOAs. On the one hand, the designs of low-dimensional structures are essential to reduce the carrier lifetime in SOAs, so that the improvement of ultrafast dynamics in SOAs can be achieved. On the other hand, optimization for acceleration of gain recovery for a given SOA in its related applications can improve the performance of the device in high speed optical communication system. The activities and contributions of this thesis can be summarized as follows:(1) Several theoretical models for describing dynamic characteristics in SOAs at different physical levels are analyzed and compared. For incidence of optical pulses with pulse width at different time scale level, some kinds of ultrafast physical phenomenon and mechanisms in SOAs are theoretically investigated. Furthermore, for interaction with optical pulses with pulse width at different time scale, the applicabilities of models have been analyzed within the error tolerance. Starting from this principle, the rules for choosing a proper model should be based on the studied objective and satisfy with both the efficiency and accuracy.(2) A type of asymmetry multiple-quantum well (MQW) SOA with carrier reservoir is proposed. A carrier transport model for MQW SOAs is used to numerically simulate the ultrafast dynamic characteristics of this asymmetry MQW SOA. The simulated results show that this structural design can effectively accelerate gain recovery. (3) An analytic form solution is derived by using multisection model to demonstrate small signal frequency response (SSFR) of wavelength conversion based on cross-gain modulation (XGM) in SOAs. The influence of parameters including length of the active region, current injection and cavity loss on the characteristics of SSFR is theoretically analyzed. Also, the results manifest that gain dispersion affects the SSFR more evidently in long SOAs than short ones as a result of deeper saturation in the formers. Finally, the measured SSFR is compared with the simulated results.(4) The analytic form solution for spectral characteristics of amplified spontaneous emission (ASE) noise in consideration of facet reflection of active cavity with gain is derived for the first time. Utilizing multisection model and slicing frequency spectrum, the longitudinal dependence of the optical power distribution of ASE propagating in the cavity and the ASE spectral characteristics emitting from the front and rear facets of the active cavity are numerically simulated. The Newton method and parallel computational algorithm are used to control the convergence speed based on this wideband model. And high efficiency is achieved in the numerical simulation. The acceleration of gain recovery in SOA by injection of assist light near transparency and design of certain reflectivities on two facets are theoretically analyzed using this model. Key parameters related to both static and dynamic characteristics of SOAs are theoretically estimated for the two schemes. Simulated results show that gain recovery can be effectively accelerated by both two schemes. Also the quality of the output signals in wavelength conversion based on cross-gain modulation (XGM) and cross-phase modulation (XPM) can be greatly improved. A half-reflection structure is suggested based on the design of facet reflection. Theoretical investigation manifests that improvement in both wavelength down-conversion based on XGM and wavelength up-conversion based on XPM can be achieved by this design.(5) Nonlinear patterning effect (NLP) of the output waveform in wavelength conversion based on transient XPM by using a single SOA and a detuned optical bandpass filter (OBF) is estimated by using a non-adiabatic model while the pulse width is comparable to the characteristics time of the temperature relaxation of carriers in SOAs. A scheme of incidence of an assist light near the gain peak to reduce the NLP of the output waveform in wavelength conversion and improve the quality of the output signal is proposed. The ultrafast dynamics of the gain of the probe light with different wavelengths and the carrier temperature in the SOA are estimated while the pump light emitted near the gain peak composed of ultrashort pulses is incident into the SOA. Also the ultrafast phase dynamics of the probe light is numerically calculated by Kramers-Kronig integration relation. The NLP of the output probe waveforms in the schemes of without assist light injection, with assist light co-propagating and with assist light counter-propagating is estimated and compared, respectively. Results show that NLP can be effectively reduced in the proposed scheme by using assist light injection. Furthermore, the improvement of NLP of output waveform is most evident in wavelength up-conversion and assist light present in counter-propagating scheme.(6) A wavelength converter based on a cascaded SOA and electroabsorption modulator (EAM) is proposed to mitigate patterning effect in wavelength conversion. An adiabatic model in consideration of the gain compression is used to analyze this semiconductor waveguide. Simulated results show that the optimization of the quality of the output signal can be always achieved for different length of the EAM by changing the reverse voltage applied on the EAM and regulating the time-delay line. It is further theoretically proved that by simulating the receiver electronics, the optimized power penalty estimated at the output attributed to a concatenated SOA-EAM compared to a single SOA can be achieved by changing the sweep-out time of the EAM practically realized by adjusting the reveres bias applied on the EAM. By simulating the wavelength conversion system at different bitrate, it's demonstrated that for the ideal situation, the quality of the eye-diagram of the output probe signal in even high speed transmission system is only limited by the ultrafast response in SOAs, the time of which is approximate 1～2ps. This scheme is further experimentally proved by incidence of ultrashort optical pulses emitted from a mode locked laser with a repetition of 10GHz. Furthermore, photonic generation of ultrawideband (UWB) monocycle and doublet pulses is experimentally demonstrated by exploiting of XGM in SOA and XAM in EAM by incidence of return-to-zero pump signal into this wavelength converter. The polarities and shapes of UWB monocycle and doublet pulses can be simply controlled by regulating the time-delay line and the reverse voltage applied on the EAM. Also the output signal is carried on one single wavelength which is supposed to be tunable within the whole C band. The corresponding measured radio-frequency (RF) spectra meet the UWB criteria defined by federal communications commission (FCC).