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. Fibre-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 fibres, which results from the high phonon energy of glass matrices. To overcome this, researchers have had to turn to soft glass fibres, which require a complete re-evaluation of the fibre laser development process due to their high thermal expansion, low melting points, and fragility. Consequently, most Mid-IR fibre laser setups to date have relied heavily on bulk components, with the gain medium being the only fibre 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 fibre pump combiner, which is a crucial component for all-fibre laser systems. The authors note, “Currently, all pump lasers are equipped with silica fibre outputs. The main challenge in integrating these pump sources with Mid-IR fibre laser systems lies in the incompatibility of silica fibres with soft glass fibres, such as fluoride glass. These fibres 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 fibre 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 fibres 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 fibre into the fluoride signal fibre, thereby overcoming the limitations imposed by the properties of soft glass fibres.
Notably, this new design effectively distributes the heat load across the extended polished fibre 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 fibre-based wavelength division multiplexors (WDMs) operating at longer signal wavelength ranges. Furthermore, the design is versatile, not imposing limitations on the types of fibres used in the combiner—whether active or passive—and can be adapted to different glass materials or polymer-based optical fibres.
The researchers conclude, “This work opens new and exciting avenues for the development of Mid-IR all-fibre 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.”
Light: Advanced Manufacturing
Side-polished Silica-Fluoride Multimode Fibre Pump Combiner for Mid-IR Fibre Lasers and Amplifiers