Controlled manipulation of laser dynamics by integrated feedback sections

As telecommunications traffic grows in response to the internet and other high bandwidth users, there will be an increasing need to operate at high data rates that are beyond the processing speeds of electronics. Semiconductor lasers which incorporate individually contacted sections with direct current injection can be applied for high speed optical signal processing because they offer bistabilities, self-pulsations (SP), and excitability on a very fast time scale. SPs are particularly interesting since applications such as clock recovery for signal regeneration and demultiplexing can be addressed. Different laser designs have been investigated to tailor fast SPs. Devices containing two DFB sections provide SP due to dispersive Q-switching (DQS) with pulsation rates in the order of the relaxation oscillations, as well as mode beating pulsations in dual-mode lasers with two highly-pumped DFB sections. In this talk we focus on experiments with lasers where the optical feedback is given by semiconductor sections without a grating. This situation was intensively studied theoretically, originally by Tager and Petermann who predicted very fast SPs. Recently, our cooperation partners at WIAS have created comprehensive bifurcation diagrams, which help the experimentalist to prepare and control the desired laser dynamics. In addition, their simulation tool LDSL is used to understand the impact of effects such as spontaneous emission noise, longitudinal spatial hole burning and nonlinear gain saturation, all of which are present in fabricated devices. For the PFL device, which is the simplest design of monolithically integrated feedback device, both Tager-Petermann and DQS self-pulsations, as well as excitability at a homoclinic bifurcation are demonstrated. However, the feedback strength in experimental devices is low due to optical losses which arise in the integrated feedback cavity. Furthermore, there is no means to vary the feedback strength, which results in restricted control of the frequency and the quality of the SP.