Optical Fiber-Waveguide Hybrid Architecture for Mid-Infrared Ring Lasers
We report a mid-infrared ring laser via fiber-to-waveguide hybrid integration enabled by directional couplers fabricated in fluoride glasses. Directional couplers were inscribed into fluoride glass using femtosecond laser direct writing, forming low-loss waveguides with a high positive refractive index change. The waveguides were optimized for efficient guidance at a wavelength of 2.7 {\textmu}m. Directional couplers with systematically varied coupling gap and interaction lengths were fabricated and experimentally characterized, demonstrating controllable power splitting ratios spanning from complete transmission (100:0) to balanced coupling (50:50). A directional coupler with a 7 {\textmu}m coupling gap and a 1.5 mm interaction length, providing an 80:20 power splitting ratio, was integrated with a cladding-pumped Er:ZBLAN fiber to realize a ring laser cavity. The resulting device produced stable lasing at 2716.7 nm with an output power of 14.2 mW. This work represents the first realization of a fully fiber-chip integrated mid-infrared laser source and opens new avenues for 3D photonic integration in fluoride glasses, enabling design flexibility previously unattainable in traditional fiber-optics architectures.