Symmetry Considerations Regarding the Optimal Design of Non-Hermitian Complex-Coupled DFB Lasers

This work focuses on the design of non-Hermitian DFB lasers incorporating phase-shifted dielectric and metallic gratings, emphasizing the symmetry aspects of the complex refractive index profile. Conventional DFB lasers featuring dielectric Bragg gratings and metallic gratings are also investigated. The impact of the phase shift between the gratings, as well as facet boundary conditions, was analyzed in terms of threshold gain and gain contrast relative to the next-higher-order laser pole. Significant performance improvements were predicted for complex-coupled DFB lasers wherein the dielectric and metallic gratings are either in-phase or out-of-phase. In contrast, for PT-symmetric configurations, we found that reflection asymmetry, which was expected to further enhance DFB laser performance, is compromised by facet reflections. To address this issue, lasing must occur at the Bragg wavelength, and the contribution of the PT-symmetry-induced asymmetric reflection must be enhanced. The solution involves using high-reflectivity and anti-reflective facet coatings. Modeling results show a threshold gain as low as 26 dB/cm, with laser emission occurring exactly at the Bragg wavelength. The gain contrast relative to the next-higher mode reached 72 dB/cm. Such gain contrast should ensure extremely robust single-mode laser operation. These results also suggest better tolerance of optical feedback and, as a potential outcome, operation without an optical isolator.