Hybrid fiber pump combiner could advance mid-IR laser systems

In the last two decades, the rapid advancement in applications such as environmental monitoring, medical diagnostics, and global positioning has intensified the focus on developing novel mid-IR light sources. Fiber-based mid-IR lasers, which operate beyond 2.5 μm, have emerged as particularly promising high-brightness light sources. However, extending laser generation into the mid-IR wavelength range presents significant challenges, primarily due to the extreme absorption in silica fibers, which results from the high phonon energy of glass matrices.

To overcome this, researchers have had to turn to soft glass fibers, which require a complete re-evaluation of the fiber laser development process due to their high thermal expansion, low melting points, and fragility. Consequently, most mid-IR fiber laser setups to date have relied heavily on bulk components, with the gain medium being the only fiber section.

A recent paper published in Light: Advanced Manufacturing by a team of scientists led by Dr. Maria Chernysheva from the Leibniz Institute of Photonic Technology introduces an innovative solution to one of the critical challenges in this field. The researchers have developed a novel concept for a hybrid fiber pump combiner, which is a crucial component for all-fiber laser systems.

The authors note, “Currently, all pump lasers are equipped with silica fiber outputs. The main challenge in integrating these pump sources with mid-IR fiber laser systems lies in the incompatibility of silica fibers with soft glass fibers, such as fluoride glass. These fibers have nearly half the melting point and 30 times the thermal expansion coefficient of silica, making traditional splicing methods highly challenging. Consequently, creating fused hybrid fiber components has been nearly impossible.”

To address this issue, the research team explored an alternative approach: side coupling based on the evanescent field. By polishing the fibers over a ~1 cm length and aligning them side by side, they were able to achieve over 80% coupling efficiency without the need for direct splicing. This innovative method leverages the evanescent field to transfer light from the silica pump-delivering fiber into the fluoride signal fiber, thereby overcoming the limitations imposed by the properties of soft glass fibers.

Notably, this new design effectively distributes the heat load across the extended polished fiber area, enabling high-power, long-term stable operation with an RMS stability of 0.09%. The demonstrated excess losses are less than 0.9 dB, which is comparable to commercially available silica fiber-based wavelength division multiplexors (WDMs) operating at longer signal wavelength ranges. Furthermore, the design is versatile, not imposing limitations on the types of fibers used in the combiner—whether active or passive—and can be adapted to different glass materials or polymer-based optical fibers.

The researchers conclude, “This work opens new and exciting avenues for the development of mid-IR all-fiber lasers, offering significant advantages over existing butt-coupling techniques. The design allows for more sophisticated laser configurations with advanced functionality and generation regimes. Furthermore, it has the potential to be adapted for other components, including material saturable absorbers or sensors.”