Degradation of mode discrimination in distributed feedback lasers due to spatial hole burning

Abstract

Distributed feedback (DFB) lasers have become of increasing interest in fields which require single longitudinal mode (SLM) sources. This SLM behavior occurs because the threshold gain for a longitudinal mode in a DFB laser increases as that mode’s frequency moves away from the Bragg frequency. We have derived coupled-mode equations for the above threshold behavior of homogeneously broadened DFB lasers, including the spatial hole burning grating induced by the laser modes. These equations were numerically integrated for boundary conditions appropriate to a variety of DFB structures. To study the mode discrimination of the DFB laser, we allow one longitudinal mode to exceed threshold and induce a spatial hole burning grating. We simultaneously solve for the threshold of a second longitudinal mode in the presence of both this field-induced grating and the externally induced grating. The ratio of the threshold of this second longitudinal mode to the average saturated line center gain available in the laser is defined as the mode discrimination. The higher the discrimination the more likely the laser is to be SLM. Our analysis shows that the mode discrimination of DFB lasers with nonsaturable index coupling, such as semi-conductor DFB lasers, is seriously degraded as the laser exceeds output powers a few times above threshold. The same does not hold true for gain coupled DFB lasers, indicating that these types of laser may be more suitable for high output power SLM applications.