Study On The Ultrafast Dynamics And Processes In Semiconductor Optical Amplifiers

With streaming video and other new internet services emerging, the demand for network bandwidth increases rapidly at a rate beyond Moore's Law. All-optical networks based on large-capacity and high-speed optical transmission and switching have become the trend of the backbone of the next generation communication networks. With the increase of the transmission-link speed which may be more than Tb/s, to make a breakthrough in the research of high-speed, low-cost and low-power all-optical devices becomes an important key to promote the research process of all-optical networks. In recent years, owing to their high nonlinearity, small volume, easy integration and other unique properties, semiconductor optical amplifiers (SOAs) as key functional elements in all-optical signal processing have been attracting huge attentions from the fields of optics and communications. Supported by the National 973 Program of "Research on the theory and key techniques of optical devices towards the all-optical switching networks", with the purpose of providing ultrahigh-speed all-optical signal processing devices for the all-optical networks, we deeply and theoretically study the ultrafast gain dynamics, ultrafast phase dynamics and ultrafast chirp dynamics in SOAs. The main contributions of this dissertation are listed as follows:1. Verified by the existing experimental literatures, a novel wideband model for describing the ultrafast dynamics and processes in SOAs is presented and realized. Compared with the current ultrafast dynamics models of SOAs, this model has several advantages:the integral in this model easily converging, obviating the approximations of the constant linewidth enhancement factor and the gain compression factors, and especially applicable to large-signal analysis and broadband case such as a wide interval between probe and pump wavelengths. In the frame of this theoretical model, based on object-oriented approach and the standard C++ language, a software library for numerically simulating the ultrafast dynamics in SOAs is developed. Through this software library, for the first time, the numerical simulation processes are unified for the conventional SOAs, SOAs gain-clamped by a longitudinal laser and SOAs gain-clamped by a vertical laser.2. The ultrafast dynamic properties in a SOA excited by a sub-picosecond optical pulse are systematicaly and theoretically studied in the wavelength range of 1450-1650 nm. For the first time, the wavelength dependence of the ultrafast gain and phase dynamics is discovered. The shorter the wavelength, the larger the gain change induced by carrier heating, and the smaller the phase change induced by carrier heating. The closer the wavelength to the pump wavelength, the larger the gain change induced by spectral-hole burning, and the smaller the phase change induced by spectral-hole burning. The effect of two-photon absorption is nearly independent of wavelength. Owing to the shorter gain recovery time, larger gain dynamic range and smaller phase dynamic range, in the band of longer wavelength, it is easy to realize high-speed applications based on cross-gain modulation. Owing to the shorter phase recovery time, larger phase dynamic range and smaller gain dynamic range, the band of shorter wavelength is beneficial to high-speed applications based on cross-phase modulation.3. Different from the situation in which a SOA is excited by a picosecond optical pulse, when considering the effects of spectral-hole burning and two-photon absorption, the chirp dynamic process in a SOA excited by a sub-picosecond optical pulse contains the middle part of red shift and the two side parts of blue shift. Moreover, the ultrafast chirp dynamics is obviously dependent on wavelength. It is shown that the proposed combination of transient cross-phase modulation and a detuned optical bandpass filter to effectively reduce the pattern-effect in cross-gain modulation in the situation of picosecond optical pulses is out of action in the situation of sub-picosecond optical pulses.4. An ultrahigh-speed all-optical XOR logic gate based on a Mach-Zehnder interferometer and cross-gain modulation in bulk SOAs is proposed, which uses continueous-wave light in the gain bandwidth and two long SOAs to reduce the pattern-effect inflicted by long carrier lifetime. Through numerical simulation, we evaluated this XOR logic gate operating at 250 Gb/s,500 Gb/s and 1 Tb/s, respectively. The simulation results show that the eye-diagrams of these XOR output signals are clearly open with high Q-factor.5. For the first time, two cascaded SOAs with different bandgaps are proposed and evaluated to reduce the pattern-effect in high-speed cross-gain modulation. Through appropriately arranging the two SOAs'bandgaps, injected currents and lengths, the pattern-effect in high-speed cross-gain modulation can be effectively reduced. This proposal has a simple structure and can be easily integrated into a single chip.