Multipoint side illuminated absorption based optical fiber sensor for relative humidity

Abstract

A side illuminated optical fiber sensor with three sensing points and an absorption-based indicator in the cladding was demonstrated for the first time. This device is easy to manufacture, uses leaky modes as the signal carrier and can measure RH in air, soil, concrete and other environments. So far, only side illuminated fluorescence sensors have been reported. They were thought, erroneously, to have their entire signal generated by evanescent wave coupling when, in fact, leaky modes also play an important role. This, coupled to the prevailing misconception that leaky modes propagate for very short lengths of fiber, prevented the earlier discovery of this absorption-based configuration. A 25 cm long fiber, with a cladding doped with an absorption dye sensitive to Relative Humidity (RH), was used in this demonstration. The fiber was side illuminated by a broadband LED, a fraction of this light was absorbed by the cladding and the remaining light guided to the fiber tip as low loss leaky modes. A total of three sensors, two with three sensing points and one with two, were calibrated using a low cost photometer. The signal was linear, stable, increased with RH and had resolutions between 0.11% and 0.25% in RH. With 5 mm diameter LEDs, devices with at least two sensing points per centimeter of fiber can be easily fabricated resulting in sensors with a very high density of sensing points. Compared to the prevailing axial illumination approach, the side illuminated sensor was found to be far simpler and inexpensive. Keywords:optical fiber sensor; side illuminated optical fiber; side excited optical fiber; side pumped optical fiber; absorption based sensor; relative humidity; multi point sensor; high density of sensing points; evanescent wave coupling; high spatial resolution

Fiber-Optic Chemical Sensors and Biosensors (2013–2015)

“…The sensor is capable of measuring RH in the range of 0–100%. A side-illuminated optical fiber sensor for RH was designed by replacing the cladding of a fiber by poly­(vinyl acetate) doped with the polarity-sensitive indicator probe called Reichardt’s dye or ET30 . The device uses leaky modes as the signal carrier, and the signal increases linearly with increasing RH.…”

An Intra-Oral Optical Sensor for the Real-Time Identification and Assessment of Wine Intake

“…On one hand, lateral application of the incident radiation through the polished fiber surface eliminates the limiting effects of sensing area length and polishing depth, which is the case with evanescent-wave optical sensors [38]. On the other hand, lateral illumination of the optical fiber accounts for increased flexibility and is more convenient for applying light into the fiber core, while relaxing the specifications for expensive coupling optics, which is the case for axial illumination [42,43,44]. In addition, the proposed sensor topology can be straightforwardly applied to extend the sensing area.…”

Distributed fluorescent optical fiber proximity sensor: Towards a proof of concept

“…A similar principle enables the implementation of a variety of sensors for environmental monitoring. For exemplification, C. O. Egalon developed a relative humidity sensor in [4]. This solution employs a fluorescent fiber which produces fluorescence within the cladding material which is then transmitted along the fiber.…”

“…This paper considers fluorescent fibers which emit light as a response to side illumination in the visible spectrum. One of the main benefits of fluorescent optical fibers is the increased flexibility of side illumination vs. traditional axial illumination [4]. Whereas axial illumination requires complicated and expensive coupling optics for confining the light radiation into the fiber core, side illumination is simply performed by illuminating the fiber cladding from an LED.…”