Optical Fiber Humidity Sensors: Measurement Techniques

Humidity Measurement Techniques in Optical Fiber Humidity Sensors

Optical fiber humidity sensors have gained increasing attention in recent years due to their potential to provide high-performance, long-distance sensing capabilities. These sensors are composed of an optical fiber structure interacting with a humidity-sensitive material, such as polyvinyl alcohol (PVA), polyimide, chitosan, or agarose. Among the various optical fiber structures, optical fiber interferometers have shown relatively high sensitivity and simple structures.

Optical Fiber Interferometers

Optical fiber interferometers are a common type of sensor used for humidity measurement. They work by exploiting the Fabry-Perot interferometer (FPI) principle, in which light is reflected between two mirrors, creating an interference pattern. In the case of humidity sensing, a humidity-sensitive material is used to alter the cavity length of the FPI, which in turn changes the wavelength of the reflected light. By monitoring this wavelength shift, the relative humidity (RH) can be determined.

Fiber Bragg Grating (FBG)

To enhance the performance of optical fiber interferometers, researchers often incorporate fiber Bragg gratings (FBGs) into the sensing probe. FBGs are periodic structures inscribed into the core of an optical fiber, which reflect specific wavelengths of light. By cascading an FBG with an FPI sensing probe, it becomes possible to monitor both humidity and temperature changes simultaneously. The FBG can be used to monitor temperature by reflecting light at the wavelength corresponding to the Bragg condition, while the FPI sensing probe measures humidity.

Challenges and Future Developments

Despite the advances in optical fiber humidity sensors, achieving all desired performances, including high sensitivity, large measurement ranges, fast response speeds, and good repeatability remains challenging. To overcome these challenges, researchers continue to explore new sensor designs by combining new humidity-sensitive materials with various optical fiber structures and fabrication techniques.

In summary, optical fiber humidity sensors play a crucial role in monitoring and controlling humidity in various fields such as food, medicine, storage, and environmental monitoring. The use of optical fiber interferometers, FBGs, and novel humidity-sensitive materials has enabled the development of high-performance sensors that can provide accurate, real-time measurements in a range of applications.