Optical-fiber based hydrogen sensing at high temperature using ALD coated oxide sensing layers

Optical fiber-based sensors show unique advantages for high temperature and harsh environment sensing, with off-the-shelf silica fibers being relevant for application environments at temperatures approaching approximately 800oC. Through the integration of oxide-based sensing layers with the optical fiber platform in an evanescent field sensing approach, the optical response of a sensing layer which is modified in response to changing ambient conditions can be directly correlated to the sensor environment. Numerous deposition techniques have been explored in prior publications including sol-gel based wet chemistry deposition and sputtering. As an alternative, atomic layer deposition (ALD) allows for deposition of high quality, nanometer-scale thin films and is uniquely suited for coating of optical fiber-based sensors due to a lack of directionality during the deposition process and compatibility with scalable coating of optical fiber samples. In the current publication, ALD coated TiO2 sensing layers are demonstrated to show a pronounced optical absorption in the visible range which depends upon subsequent processing temperatures and chemical environments. More specifically, the ALD deposition conditions result in the formation of initially amorphous TiO2 layers which show a broad absorption band across the visible range due to the amorphous structure. A reversible temperature dependent response is observed, and above a critical temperature (~400-500°C), crystallization of the TiO2 results in an irreversible change in optical absorption with a sharpened absorption peak associated with the band edge for which the temperature dependence is consistent with prior experiments and theoretical results. Following crystallization of the initially amorphous TiO2 layer, a strong and stable H2 sensing response is also demonstrated for H2 concentrations of various levels ranging from 0-3.9% H2 in nitrogen balance, and at temperatures up to 800°C. Particularly attractive responses are shown in the telecom wavelengths (1550 nm) indicating potential application for distributed sensing with commercially available techniques.