Ultrafast pulse-pair amplification in InGaAs quantum-dot amplifiers

Semiconductor optical amplifiers (SOAs) are important devices for optical communications being low-cost and having small size, high gain, and large amplitude and phase nonlinearities. However, slow dynamics of the gain nonlinearities in conventional bulk and quantum well SOAs (mainly due to carrier heating) usually limit the performance of these amplifiers for optical signal processing at high bit rates (≫10Gb/s). SOAs with InGaAs quantum dots (QDs) in the active region have recently attracted increasing attention because of their ultrafast gain recovery dynamics (dominated by spectral hole burning nonlinearities) [1] and suppression of pattern effects in amplification and cross-gain modulation [2]. It has been pointed out in several experimental and theoretical works (see e.g. [3] and reference therein) that the measured ultrafast (∼100fs) gain recovery of the QD ground-state (GS) optical transition following a single pulse excitation in QD SOAs is due to the presence of a reservoir of carriers in the QD excited states (ES) that undergo a fast relaxation into the GS via Auger-like carrier-carrier scattering as long as the ES are highly populated. However, it was also pointed out [4] that in the presence of a slower refilling of the ES levels via carrier capture from the wetting layer, ultrafast amplification of an optical pulse sequence is severely limited (already at 40GHz repetition rate in [4]). Using a pump-probe differential transmission technique in heterodyne detection with sub-picosecond time resolution we have measured the GS ultrafast gain recovery dynamics in electrically-pumped InGaAs/GaAs QD SOAs operating near 1.3µm at room temperature in the presence of a pump pulse pair. A description of the investigated samples and of the pump-probe experiment using a single pump can be found in [5]. Here, we have introduced a pre-pump at a variable delay time relative to the pump pulse, which can be independently adjusted in intensity and modulated separately. We measured the effect induced by both pump pulses on the modal gain experienced by a weak probe as a function of the pump-probe delay (Fig. 1(top)). Moreover, we isolated the effect of the pre-pump on the gain dynamics by modulating the second pump only and measuring the pump-probe dynamics for a gain and carrier distribution modified by the pre-pump (Fig.1(bottom)). If the effect of the pre-pump would be as speculated in [4], the gain recovery dynamics should be slower at short pre-pump delay times, and should recover the fast single-pump gain dynamics only at large pre-pump delay times. Surprisingly, our experimental finding indicate that the differential gain recovery dynamics is ultrafast even for short pre-pump delays and does not change much when changing this delay (see Fig1(bottom) at 30mA injection current corresponding to GS gain saturation). Even for injection currents below gain saturation (e.g. 7mA in Fig.1) the main effect of the pre-pump is a reduction of the gain experienced by the second pump at short pre-pump delay, rather than a slowing down of the recovery dynamics. These results suggest a dominant effect of an inhomogeneous distribution of recovery times, as could be expected from a microstate model, rather than the effect of ES depletion. Comparison with such model is in progress. From the application point of view, our findings indicate that a pulse sequence of 2 THz repetition rate would undergo ultrafast amplification.