Abstract

Injection locking has been used to improve static and dynamic lasing characteristics of semiconductor lasers for various applications, including both long-haul, highspeed digital fiber transmission and coherent communication systems. However, a strong phase-amplitude coupling in semiconductor lasers imposes stringent operational requirements for stable locking and limits the practical benefits of the injection locking technique. It is therefore important to examine whether these limitations can be somewhat ameliorated by choosing a target mode different from the free-running dominant mode. In our investigation of side-mode injection locking characteristics, we have developed a model based on multimode rate equations, including a phase equation for the injected mode. Consideration of multimode effects is especially important when the target mode is different from the free-running dominant mode. The locking range for side-mode injection differs from the conventional result by an extra frequency offset term that is caused by an increased threshold gain deficit. We investigate the dynamic stability of injection-locked states by considering small fluctuations around the stationary solutions. As in peak-mode injection, only a part of the possible locking range is dynamically stable. However, the stable locking bandwidth can be increased significantly by injection into a side mode. This implies that better locking and eased operating tolerances can be expected with side-mode injection. Analysis of the variation of carrier concentration as a function of the injection level reveals a well-defined threshold effect for side-mode injection. This new effect can be used for fast, all-optical channel switching, and we are now exploring it experimentally. In addition, selecting a target mode at the shorter-wave-length side of the gain peak results in an increased relaxation-oscillation frequency, thus enhancing the direct modulation bandwidth.

© 1990 Optical Society of America