Abstract
A reflective fiber optic sensor based on multimode interference for the measurement of relative humidity (RH) is proposed and experimentally demonstrated. The proposed probe is fabricated by fusion-splicing, approximately 30 mm long coreless fiber section to a single mode fiber. A hydrophilic agarose gel is coated on the coreless fiber, using the dip coating technique. When the incident light comes from the SMF to the CSF, the high-order modes are excited and propagate within the CSF. These excited modes interfere with one another as they propagate along whole CSF length, giving rise to a multimode interference (MMI). Since the effective refractive index of the agarose gel changes with the ambient relative humidity, as the environmental refractive index changes, the propagation constants for each guided mode within the CSF will change too, which leads to shifts in the output spectra. The proposed sensor has a great potential in real time RH monitoring, exhibiting a large range of operation with good stability. For RH variations in the range between 60 %RH and 98.5 %RH, the sensor presents a maximum sensitivity of 44.2 pm/%RH, and taking in consideration the interrogation system, a resolution of 1.1% RH is acquired. This sensor can be of interest for applications where a control of high levels of relative humidity is required.
Highlights
Fiber optic sensing of relative humidity (RH) has been extensively studied and there have been a great number of proposed methods
This paper reports a fiber optic humidity sensor which is based on hydrogel coated optical fiber
When the light guided though the single mode fiber (SMF) enters the coreless silica fiber (CSF), the high-order modes are excited and propagate within the CSF. These excited modes interfere with one another as they propagate along whole CSF length, giving rise to a multimode interference (MMI)
Summary
Fiber optic sensing of relative humidity (RH) has been extensively studied and there have been a great number of proposed methods. The other type of RH fiber optic sensors involves using humidity sensitive coatings or gels (hydrophilic materials) on the surface or end face of the optical fiber, such as polyvinyl alcohol [7], polyethylene glycol [8], chitosan [9], polyethylene oxide [10] and agar [11]. This kind of polymer coatings has advantages of good performance [12], reproducibility, and long-term stability [10]. The sensor is easy to produce, presents good resolution, for environments with values of RH higher than 60.0 %RH
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