Harsh‐Environment‐Resistant OH‐Vibrations‐Sensitive Mid‐Infrared Water‐Ice Photonic Sensor

State-of-the-art ultrahigh-sensitivity photonic sensing schemes rely on exposing the evanescent field of tightly confined light to the environment. Yet, this renders an inherent fragility to the device, and since adding a protective layer disables light exposure, there exists a technology gap for highly sensitive harsh-environment-resistant surface photonic sensors. Here, a novel type of mid-infrared waveguide sensors is reported which exploit vibrational resonance-driven directional coupling effects besides absorption, with optical sensing elements that can be buried (≈1–10 µm) and resist systematic exposure to industrial environments without failure. A harsh-environment-resistant, fiber-coupled, surface sensor for monitoring the structural phase of water (liquid-supercooled-solid), as well as the type of ice microstructure (clear rime), is shown. It is demonstrated how this type of sensor can be designed to detect ice layers with nanometric (≈100 nm) to microscopic (≈30 µm or higher) thicknesses, and the first experimental tests both in optical laboratory and in icing wind tunnel inflight aircraft simulation tests are reported.