Mid-infrared fiber laser research: Tasks completed and the tasks ahead

Abstract

After decades of research, there are almost half a dozen efficiently pumped rare earth laser transitions in a fiber laser format capable of Watt-level output. These systems use near-IR laser diodes for excitation and have developed into reliable sources of high beam quality light with some commercially available. This maturation of the mid-IR fiber laser is entirely based on a high quality fluoride glass fiber, which has emerged as the primary fiber gain material for emission up to 4 µm. The other major mid-IR transparent glass families, the heavy metal oxides, and chalcogenides have always been challenged by consistent hydrogen diffusion into the glass that creates strong absorption features in the high-frequency portions of the mid-IR. This problem along with challenges to sufficiently concentrate the rare earth doping level has historically stifled progress preventing fiber laser emission in the mid-IR. In recent years, great efforts in precursor purification and reducing contamination during fabrication have resulted in pioneering demonstrations of mid-IR lasing using these glasses with emission now extending beyond 5 µm. As a result, mid-IR fiber laser research has entered a new era with more breakthroughs and applications to benefit from the efficiency potential, reliability, and relatively simple architecture of the optical fiber.