Suppression of slow gain recovery in ultralong quantum-dash semiconductor optical amplifier emitting at 1.55 m

Gain dynamics in two quantum-dash semiconductor optical amplifiers of different lengths emitting in the 1.55 µm region are investigated experimentally and compared. It is shown that the slow gain recovery due to the total carrier relaxation is totally suppressed in the ultralong amplifier. Consequently, the 10%-90% gain recovery time is drastically reduced from about 40 ps (short sample) to 10 ps (long one). 1. Introduction Quantum-dot and quantum-dash semiconductor optical amplifiers (QD-SOAs) are an attractive solution for future high capacity optical networks thanks to outstanding features of high non-linearity and ultrafast response. They promise high-performance operations at bit rates as high as 320 Gbit/s [1-3]. However, ultrahigh bit rate operations have been only demonstrated by using ultralong devices (6.15 mm) [2,4]. Investigations on gain dynamics with different pumpprobe experiments report that the QD-SOA gain, after being saturated by a pump pulse, always recovers with an ultrafast recovery (~few picoseconds) followed by a slow recovery (~100 picoseconds) [5-8]. In this work, we investigate the gain dynamics of two QD-SOAs of different lengths (2 mm and 5 mm), both emitting in the 1.55 µm wavelength region. We show that strong amplified spontaneous emission (ASE) in long devices contributes significantly to gain recovery acceleration which enables high speed operations. Experimental results demonstrated that the slow gain recovery in the ultralong QD-SOA can be totally suppressed under some operating conditions.