Novel properties of population-inverted quantum dots (QDs), represented by QD semiconductor optical amplifiers (SOAs) depicted in Fig. 1, have been shown to be practically attractive to achieve high-level network functionalities which had not been realized before with a single semiconductor optical device without complexity of monolithic integration [1-21]. High-power (> 20 dBm) low-noise (NF < 7 dB) amplification obtained over an ultrabroad wavelength range (> 120 nm) which we have demonstrated [21] is now ready to be combined with broadband polarization insensitivity realized by controlling the QD shape with vertical stacking and coupling of Stranski-Krastanow (SK) QDs (Fig. 2). An ultrafast nonlinear gain response on which this talk focuses is another unique property of QD SOAs [1-12]. This is realized with an intraband thermally non-equilibrium carrier distribution (selective extraction of carriers at the incident wavelength due to stimulated emission) caused by longer intraband relaxation time compared to quantum-well (QW) and bulk SOAs. The intraband relaxation time of QDs, which has been estimated to be a few ps, is still much shorter than the interband ones which limits the speed of QW and bulk SOAs [1-14], and short enough to handle a 40 Gb/s non-return-to-zero signal (Fig. 3 (b)).

© 2007 Optical Society of America

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