Single-mode engineering in semiconductor lasers using parity-time-symmetry and coupled-cavity structures
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High power single spatial mode semiconductor lasers are of interest for various applications, including optical communication, material processing, and pumping single-mode optical fibers. The output power of a typical index guided ridge waveguide single-mode laser is limited by its narrow waveguide width required to cut off higher-order optical modes. To overcome the output power limitation, conventional techniques rely on structures increasing the mode size without introducing new modes. These methods are based on lateral mode discrimination in a single waveguide to enforce single-mode operation. In contrast to the conventional methods, our work utilizes the concept of parity-time-symmetry (PTS) and coupled-cavity (CC) structures. By exploiting these two approaches, we employ multi-mode waveguides to achieve single-mode lasing in edge-emitting laser diodes. The PTS laser is based on coupling two identical waveguides. By electrically tuning the gain and loss in each waveguide, the optical modes are manipulated to realize a single-mode operation. On the other hand, the CC approach is based on the resonant coupling of waveguides with different widths to realize single-mode operation. In contrast to the PTS method, CC lasers have an unpumped waveguide to introduce loss instead of tuning the loss with an electrical pump. Towards this goal, the design parameters are numerically explored by detailed optical simulations, and their sensitivities are investigated for PTS and CC methods. I fabricated and experimentally demonstrated the control of the optical mode profiles promising single-mode operation for PTS and CC structures. The results are encouraging for future research and industrial applications.
High power semiconductor lasers