Fast Dynamic Characteritics Of Semiconductor Optical Amplifiers And Their Application In Optical Signal Processing

With the rapid progress of internet services, the demand for bandwidth isdramatically increasing, and optical communication networks are developing towardsall-optical networks to achieve all-optical transparency at the network notes, inparticular, for the backbone networks. One key factor in the all-optical networks is todevelop the optical signal processing integrated chips, which have the advantages ofultra-fast operating speed, low power consumption, and large scale of integration. Thesemiconductor optical amplifiers (SOAs) are promising devices in the all-optical signalprocessing due to the small size, low power consumption, high nonlinearity and beeasily integrated with other semiconductor devices. The main drawback of the SOA isthe slow carrier density recovery speed, which limits its operating speed. In order tomeet the requirements of high speed optical signal processing, it is desirable that theSOAs can operate in high speed. Based on this consideration, the SOA ultra-fastdynamical characteristics and the associated physical phenomenon are studied. Themain contributions of this dissertation are summarized as following:1. The basic propagation equation of electromagnetic field and the carrier rateequation for the SOAs are introduced, and the ultrafast intraband carrier dynamics areanalyzed. To meet the requirements of investigating the SOA ultrafast dynamics andultra high-speed optical signal processing, an extended SOA model, which includes theultrafast nonlinear effects, is built up.2. The contributions of the intraband effects for the gain dynamics and phasedynamics of SOA under different pulse width injection are investigated, and theultrafast phase recovery in the phase dynamics is demonstrated. It is found that thecarrier heating (CH) becomes stronger due to the relative long relaxation time of CHwhen the ultrashort pump pulse is injected, resulting in a ultrafast phase recovery inphase dynamics of SOA. The reason why the delay occurs between SOA gain dynamicsand phase dynamics is explained by analyzing the timescales of the intraband effectsand interband effect. The delay variations with the pulse width and peak pulse power arealso investigated. The impact of the SOA operating conditions on carrier lifetime is investigated，which are useful to increase the SOA operating speed.3. The ultrafast phase recovery in phase dynamics of SOA is demonstrated by thepump-probe experiment based on an integrated SOA-MZI. The results show that theultrafast phase recovery contributes about20%phase recovery (0.15rad) and theduration of the ultrafast process is about2ps when the pump pulse width is2ps. Basedon the measured gain dynamics and phase dynamics, the variation of time-resolvedlinewidth enhancement factor (α-factor) is investigated. The results show that theα-factor drastically varies when the short pump pulse injection due to different phaseresponses of intraband effects and interband effects, and the α-factor related to thecarriery density increases with the carrier density recovery from the minimum of4.5.4. The chirp variation characteristics of SOA are analyzed under the injection ofclock pulses and random pulses. It is found that the gain saturation induced by pumppulses can increase recovery speed of carrier density, which results in larger blue-shiftchirp. The chirp variation characteristics induced by bit pattern are analyzed in detailand the physical explanation is given, the further comprehensively explanation theoperating principle of using blue-shift band pass filter to reduce the bit pattern effect isalso given. The640Gbit/s all-optical wavelength conversion is numericallydemonstrated by using a blue-shift Gaussian band pass filter.5. The SOA ultrafast dynamical characteristics related to the gain dynamics,phase dynamics and chirp dynamics are investigated in the counter-propagating setup. Itis found that the SOA has smaller ultrafast recovery in the gain and phase dynamics inthe counter-propagating setup than in co-propagating setup. The smaller ultrafastrecovery also results in the smaller blue-shift in the chirp. It is also found that the gainsaturation time is a key factor of limiting the SOA operating speed in thecounter-propagating setup. The gain saturation time becomes large with the increase ofthe SOA active region length. And it can be decreased with the increase of work currentand pulse energy. To solve the the SOA slow operating speed in the counter-propagatingsetup, an effective way is to reduce the SOA active region length to increase the SOAultrafast dynamics. The ultrafast operating speed can be also achieved with theassistance of detuning filter or delay interferometer.6. The blue-shift detuning filter technology is induced in the counter-propagatingsetup and its feasibility to achieve high speed all-optical wavelength conversion is demonstrated. A high-speed optical wavelength converter is presented, which is capableof converting the data to the same wavelength using a single semiconductor opticalamplifier in counter-propagating setup. Error-free wavelength conversion to a differentwavelength or the same wavelength is demonstrated at a bit-rate of40Gbit/s.