Fiber optic evanescent wave humidity sensor based on SiO2/TiO2 bilayer films

Graphene oxide based optical fiber humidity sensor having a linear response throughout a large dynamic range and optimum sensitivity

The main objective of the present research is to develop an optical fiber relative humidity (RH) sensor with a high sensitivity, linear response over a wide dynamic range, and optimum response/recovery times by employing the simplest optical fiber sensing configuration. An optical fiber RH sensor that exploits the intensity modulation scheme through evanescent wave absorption spectroscopy and fulfills the objective is reported here. The fiber sensor employs graphene oxide (GO) diffused silica nanostructured thin sensing film as the cladding on the centrally decladded straight and uniform multimode optical fiber. Detailed experimental investigations are carried out to analyze the response characteristics of the proposed sensor. In comparison to other GO based optical fiber RH sensors, a linear response over the widest dynamic range (15.0%-95.3%RH) and shortest (fastest) response/recovery times (0.1436 s/0.1547 s) are observed for the proposed sensor. The linear sensitivity for the developed sensor is observed to be 0.1036 dB/%RH. In addition, the proposed sensor exhibits a very good degree of reversibility, reliability, and repeatability.

PVA/Tween 20 thin-film-based fiber optic humidity sensor with enhanced sensing performance

A fiber optic humidity sensor based on polyvinyl alcohol (PVA)/Tween 20 film has been fabricated by modulating the intensity of light transmitted in optical fiber. PVA/Tween 20 film was used as the cladding and humidity-sensitive material of optical fiber. The logarithmic of output light intensity exhibited a linear increase with the increase of humidity (22%-82%RH). With the addition of Tween 20 in the formation of film, average sensitivity increased by 13-fold. Fast equilibrium on adsorption and desorption of water molecules were also achieved on the film. The response and recovery times were determined to be 11 s and 9 s, respectively. Moreover, the sensor possesses good repeatability. The sensing mechanism was probably based on the swelling of PVA after adsorbing water molecules, which affected scattering of evanescent waves in the cladding. The output light intensity varied with the decay of evanescent waves.

Review of Optical Humidity Sensors

Optical humidity sensors have evolved through decades of research and development, constantly adapting to new demands and challenges. The continuous growth is supported by the emergence of a variety of optical fibers and functional materials, in addition to the adaptation of different sensing mechanisms and optical techniques. This review attempts to cover the majority of optical humidity sensors reported to date, highlight trends in design and performance, and discuss the challenges of different applications.

Binding Analysis of Functionalized Multimode Optical-Fiber Sandwich-like Structure with Organic Polymer and Its Sensing Application for Humidity and Breath Monitoring

In recent years, the chemical modification of optical fibers (OFs) has facilitated the manufacture of sensors because OFs can identify several analytes present in aqueous solutions or gas phases. Nevertheless, it is imperative better to understand the chemical interactions in this molecular system to generate low-cost and efficient sensors. This work presents a theoretical and experimental study of organic polymeric functionalized OF structures and proposes a cost-effective alternative to monitor breathing and humidity. The device is based on silicon optical fibers functionalized with (3-Aminopropyl) triethoxysilane (APTES) and alginate. The theoretical analysis is carried out to validate the activation of the silicon dioxide fiber surface; moreover, the APTES–alginate layer is discussed. The computational simulation suggests that water can be absorbed by alginate, specifically by the calcium atom linked to the carboxylic acid group of the alginate. The analysis also demonstrates a higher electrostatic interaction between the water and the OF–APTES–alginate system; this interaction alters the optical fiber activated surface’s refractive index, resulting in transmission power variation. The humidity analysis shows a sensitivity of 3.1288 mV/RH, a time response close to 25 s, and a recovery time around 8 s. These results were achieved in the range of 50 to 95% RH. Moreover, the recovery and response time allow the human breath to be studied. The proposed mechanism or device is competitive with prior works, and the components involved made this sensor a cost-effective alternative for medical applications.