Optical fiber–waveguide hybrid architecture for mid-infrared ring lasers

We report a integrated fiber-waveguide mid-infrared ring laser enabled by fiber-to-waveguide hybrid integration utilizing fiber-pigtailed optical chips. Directional couplers were inscribed into a tailored 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 μm. Directional couplers with various coupling gaps and interaction lengths were fabri-cated and experimentally characterized, demonstrating control-lable power splitting ratios spanning from complete transmission (100:0) to balanced coupling (50:50). A directional coupler with a 7 μm 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 monolithic ring laser cavity. The resulting device produced stable lasing at 2716.7 nm with an output power of 22 mW. This work represents the first realization of a fully fiber-chip integrated mid-infrared laser source and opens up new avenues for 3D photonic integration in fluoride glasses, enabling design flexibility previously unattainable in traditional all-fiber architectures at mid-infrared wavelengths.