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This paper presents a theory and simulation of quantum-dot semiconductor optical amplifiers (SOAs) for high-bit-rate optical signal processing. The theory includes spatial isolation of quantum dots, carrier relaxation and excitation among the discrete energy states and the wetting layer, grouping of dots by their optical resonant frequency under the inhomogeneous broadening, and the homogeneous broadening of the single-dot gain, which are all essential to the amplifier performance. We show that high-speed gain saturation occurs due to spectral hole burning under the optical pulse trains up to at least 160 Gb s-1 with negligible pattern effect, and that the self-assembled InGaAs/GaAs quantum-dot SOAs have about two to three orders faster response speed than bulk InGaAsP SOAs, with one order larger gain saturation for the 160 Gb s-1 signals. We also show that switching functions can be realized by the cross gain modulation between the two wavelength channels when the channel separation is within the homogeneous broadening. These results indicate great potential of quantum-dot SOAs for all-optical high-speed switches. As one of their possible applications, we propose a new signal-processing scheme of a `quantum-dot 3R regenerator'.

We studied the injection current dependence of room-temperature lasing spectra of a 1.3-μm self-assembled InAs∕GaAs quantum-dot laser both experimentally and theoretically. Starting from the ground-state lasing with a few longitudinal modes, the spectra showed splitting, broadening, excited-state lasing, and quenching of the ground-state lasing as the current increased. We could explain this unique current dependence by numerical simulation based on our quantum-dot laser theory, taking into account the inhomogeneous and homogeneous broadening of the optical gain as well as the carrier relaxation processes in the spatially isolated quantum dots. Through the simulation, we found that the homogeneous broadening of the ground state is kept between 5 and 10 meV under the ground-state lasing, while it increases up to 20 meV under the excited-state lasing.

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We demonstrate temperature-insensitive eye-opening under 10-Gb/s direct modulation of 1.3-µm p-doped quantum-dot lasers without using any current adjustments. The lasers show a 6.5-dB extinction ratio between 20°C and 70°C. An active region consisting of ten quantum-dot layers with p-type doping enabled this highly temperature-stable dynamic performance, which was much superior to conventional 1.3-µm quantum-well lasers. These results make it possible to use uncooled 1.3-µm quantum-dot lasers without any current adjustments.