Underwater optical communication in the presence of ambient daytime solar illumination requires a detector with very small bandwidth (preferably ≦ 0.01 A), combined with a wide solid angle of reception (≥ 3 steradians) and sensitivity capable of approaching the quantum limit in the 400-550 nm spectral range. Basic feasibility and principles of operation of Quantum-Limited Optical Resonance Detectors (QLORDs) are discussed, which exploit atomic resonance transitions in potassium, rubidium, and cesium vapors. QL0RD operation is experimentally demonstrated and shown to satisfy all three basic detector requirements.
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