Abstract
This work addresses a new configuration that improves the sensitivity of a humidity sensor based on a long-period fiber grating coated with a SiO2-nanospheres film. An intermediate higher refractive index overlay, deposited through Electrostatic Self-Assembly, is placed between the fiber cladding and the humidity sensitive film in order to increase the total effective refractive index of the coating. With this intermediate design, a three-fold improvement in the sensitivity was obtained. Wavelength shifts up to 15 nm against 5 nm were achieved in a humidity range from 20% to 80%.
Highlights
Fiber optic humidity sensors have been widely explored due to their practical importance in several fields in which specific environments require small sensor size and electromagnetic immunity
The long-period fiber gratings (LPG) was coated with a first overlay of higher refractive index than that of the cladding (PDDA/PolyR-478) with the only purpose of increasing the total effective refractive index of the coating, and with a sensitive-to-humidity coating of lower refractive index (PAH/SM30)
If the sensor is located near the vanishing area, it would be more sensitive to any humidity change, but it would not be possible to detect any change once the peak vanished
Summary
Fiber optic humidity sensors have been widely explored due to their practical importance in several fields in which specific environments require small sensor size and electromagnetic immunity. The time response of this type of sensors as well as their sensitivity can be optimized by adding an intermediate overlay with higher refractive index, as will be further explained. In previous work [8] it was demonstrated that SiO2 nanospheres coated on a LPG could provide a fast response humidity sensor. Due to its low index of refraction, lower than the fiber itself, the sensitivity is quite low and should be improved in order to avoid expensive equipment. This can be done by increasing the total effective refractive index of the sensitive coating using an intermediate overlay with higher refractive index
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