Direct Water Thermal Exchange for Laser‐Driven Lighting: Achieving Miniature Super‐Bright Green Light Source for Underwater Illumination and Broadcast Communication

Underwater laser‐driven lighting is limited by the poor heat dissipation of color converters, which causes luminance saturation and prevents miniaturization. This study presents a direct water thermal exchange method, allowing both the color converter and laser diode to dissipate heat directly into the surrounding water. A water‐contact green‐emitting Y 3 (Al,Ga) 5 O 12 :Ce 3+ (YAGG:Ce 3+ ) phosphor‐in‐glass (PiG) with a sandwich waterproof structure (SW‐PiG) was thus developed as the color converter, maintaining 89% of its luminous intensity even after 40 days of immersion in 80°C seawater. The SW‐PiG achieved an underwater illuminance of 2.5 × 10 5 lux at 15 cm under blue laser excitation, which is 14 times higher than that of a conventional heatsink‐based setup (1.8 × 10 4 lux). Notably, no luminescence saturation was observed even at 25 W laser excitation, while the conventional setup suffered thermal rupture at just 3.5 W. Finally, a miniature, super‐bright green light source was demonstrated, highlighting its potential to improve personal diving safety and enable wide‐area (60° divergence angle, >40 meters) underwater point‐to‐multipoint broadcast communication across diverse applications.