Advancements in Degenerate Distributed Feedback Lasing

We advance the concept of degenerate distributed feedback (DDFB) lasing in a double grating photonic structure that operates near a degenerate band edge (DBE) to achieve a robust single-frequency lasing regime. The DBE is an exceptional point of degeneracy (EPD) of fourth order involving four coalescing Bloch eigenmodes. A DDFB photonic cavity operating close to the DBE frequency is shown to display a large quality factor that scales with the fifth power of the cavity length. Upon the inclusion of gain, the DDFB cavity displays a low lasing threshold with the exceptional scaling as the inverse of the fifth power of the double grating length, i.e., $α_{D,th} \propto 1/N^5$, where $N$ is the number of waveguide unit cells making the mirrorless cavity. The work proposed here shows a path to single-frequency lasing mode using a mirrorless cavity with minimal device footprint. The DDFB laser is very attractive for various applications including communications, sensing, and spectroscopy.