  Real-time measurement of relative humidity (RH) is important to many
physical, chemical, and biological processes. However, in processes that
involve harsh conditions such as high temperatures, strong electromagnetic
interferences, and complex spatial constraints, conventional electrical sensors
often fall short due to their intrinsic limitations. Here, we developed an
optical lossy-mode-resonance (LMR) RH sensor based on the SnO2 film coated
D-shaped fiber tip. Thanks to the high-temperature endurance and
electromagnetic interference immunity, the developed optical fiber-tip sensor
is ideal for RH sensing in critical environments, such as in the microwave
drying process. Furthermore, unlike other reported in-line LMR sensors, our
sensor is located at the D-shaped fiber tip with a probe-like form factor,
allowing it to be readily implemented in a spatially confined environment. We
have developed a custom fabrication setup for the novel D-shaped LMR fiber-tip
sensor. The LMR signal from the sensor was experimentally characterized. The
lossy mode resonances are understood by the finite element analysis, the
results of which agree well with the experimental measurements. The fiber-tip
sensor had a linear RH response between 6.1% to 75.0% and a resolution better
than 4.0% RH. The fiber-tip sensor demonstrated a response time and
reversibility comparable to that of commercial electrical sensors. The novel
D-shaped fiber-tip LMR RH sensor developed in this work will benefit many
applications that require RH monitoring in a harsh environment. Furthermore,
our innovative D-shaped fiber tip design can be readily applied to LMR-based
fiber sensors in general. This will allow the design's advantages of small
footprint and agile maneuverability to benefit a wide range of LMR fiber
sensing applications beyond RH sensing.
