In this paper, amplified spontaneous emission (ASE) in quantum dot semiconductor optical amplifier (QDSOA) is investigated and analyzed theoretically. The presented model is based on a set of rate equations that consider all possible carriers transitions including the second excited state (ES2). This assumption is possible, if the QDSOA’s active region is grown in such a way that the presence of ES2 becomes distinguishable from upper states and wetting layer. Optical gain of QDSOA is calculated using density matrix approach. The coupled rate equations are solved numerically along with propagation equations. It is shown that in the presence of ES2, QDSOA performance can be improved and an ultra-high bit-rate signal amplification without wave distortion is possible. Furthermore, it is illustrated that the obtained ASE spectrum has three peeks which are related to ground, first excited and second excited states. Moreover, noise figure (NF) is calculated numerically using ASE power. It is shown that considering the effect of ES2 in the band diagram of quantum dots (QDs) decreases the NF of amplifier. The effect of increased input power on the reduction of NF is illustrated, too. It is shown that ASE and NF of amplifier can be ignored in the modeling QDSOA at high input powers.
