Multi-section semiconductor lasers have emerged as prime candidates for fast, tunable transmitters in optical communication systems. It was realized that, due to the shorter carrier lifetime under stimulated emission, an active (gain) tuning section has a clear modulation speed advantage over its passive counterpart. The superiority of an active tuning section over other schemes of direct modulation became even more apparent with the utilization of the gain-lever effect, which can enhance the modulation response and widen the tuning range. However, two-active-section schemes that were investigated so far have practical limitations, such as a multimode spectrum in the two-section Fabry-Perot (FP) structure and complicated tuning behavior with red and blue-shift regions in the case of two-section distributed feedback (DFB) structures. The authors present a novel InGaAsP/InGaAs DFB structure with an active tuning section, that overcomes these limitations. Such a device consists of a two-section structure, where only one section has a distributed feedback grating formed over the active layer, while the other is flat.
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