Low-Hydroxyl tellurite fiber for Mid-Infrared Er:YAG laser transmission

• Record-low hydroxyl content: Ultra-low OH⁻ absorption (0.004 cm⁻¹) was achieved in TZL glass via vacuum melting, corresponding to a 99.8% reduction. • Superior Laser-Induced Damage Threshold (LIDT): The purified TZL glass exhibits a laser-induced damage threshold exceeding 443 J/cm², while the fiber reaches 1061 J/cm². • High-Power Flexible Fiber Delivery: A flexible core-cladding TZL fiber (300 μm core, 600 μm OD) was fabricated, which stably delivers a record-high output power of 8.3 W at 2.94 μm from an Er:YAG laser. • Greatly Reduced Transmission Loss: The purification process and core-cladding structure resulted in a significantly lowered minimum loss of 1.05 dB/m at 2.7 μm and extended the mid-infrared transmission edge to ∼ 4.5 μm. • Excellent Thermal Stability: The fiber demonstrates excellent thermal management, with end-face temperature at only 55 °C under 8.3 W laser operation. Due to pronounced hydroxyl (–OH) absorption bands around 2.7 μm in the mid-infrared region, tellurite glasses typically exhibit limited performance for mid-infrared laser transmission, which in turn constrains their suitability for Er:YAG laser delivery. This limitation arises primarily from –OH impurities, which introduce strong absorption in the mid-infrared band and significantly degrade transmission efficiency. To enhance the mid-infrared transmission performance and laser-induced damage threshold of TeO 2 –ZnO–La 2 O 3 (TZL) glass, high-temperature vacuum melting was employed as a physical purification method to effectively remove –OH impurities from the glass matrix. The effects of melting temperature and vacuum level on purification efficiency were systematically investigated. Through optimization of raw-material refining and melting parameters, low-hydroxyl tellurite glass was successfully obtained, exhibiting an –OH absorption coefficient as low as 0.004 cm –1 —representing a 99.8% reduction compared with conventional unpurified samples. The purified glass was subsequently extruded into core–cladding preforms and drawn into flexible fibers with a core diameter of 300 μm and a cladding diameter of 600 μm. The fabricated TZL fiber achieved a maximum stable output power of 8.3 W at 2.94 μm, corresponding to an energy density of 1061 J cm –2 . To the best of our knowledge, this is the first demonstration of a tellurite fiber capable of stably delivering more than 8 W of Er:YAG laser power at 2.94 μm, highlighting its strong potential for high-power mid-infrared laser applications in surgical medicine and industrial processing.