Optical Fiber Humidity Sensor Research Articles

SiO2-capped ZnO quantum dot based highly sensitive optical fiber humidity sensor with potential applications in human breath monitoring and voice print recognition

This article describes the development and characterization of an optical fiber humidity sensor employing intensity modulation via the evanescent wave (EW) absorption technique. For the development of the sensor, SiO2-capped ZnO quantum dot (QDs) thin film is synthesized over the decladded portion of plastic cladding silica (PCS) fiber via the sol-gel method. A thorough experimental investigation was conducted by varying the thickness of the sensing film to optimize the sensor’s response. The sensing probe with optimized film thickness of 891 nm demonstrates a linear response over 30.5%–92.5%RH with an enhanced sensitivity of 46.2 mV/%RH (0.0138RH−1). Very fast response and recovery times of 2 s and 2.5 s are observed during humidification and dehumidification for the optimized sensing probe. The maximum resolution recorded during the short stability test is ±0.12%RH. Additionally, the proposed sensor demonstrates a very high degree of repeatability, reversibility, and stability. The proposed sensor has also been tested for human breath monitoring and voice print recognition. The result shows the sensor is able to detect minute humidity fluctuations in exhaled air during breathing and speaking.

Fast detection of humidity sensor with a weakly coupled fiber coated with carbomer

Humidity, as one of the basic environmental physical quantities, is essential in medical monitoring, food production and pharmaceutical industries.Fast and highly sensitive humidity sensing technology has become an urgent need in areas such as medical monitoring. We proposed and realized a novel coupled fiber-optic humidity sensor based on double tapered fiber twisted weakly coupled structure coated with carbomer film. The sensor adopts 2 × 2 coupler structure with waist diameter of 10 μm. The Carbomer film is coated in the weakly coupled area to improve the sensitivity of the sensor, and the thickness of this coating is about 79.2 nm. The sensor provides linear sensing over a relative humidity range of 40%–65% RH, with a dynamic response time of 208 ms and a recovery time of 464 ms. The sensitivity of the sensor is up to −0.28 dB/%RH and the linearity of the sensor is as high as 99.89%. Moreover, the sensor has good stability, reversibility, and low temperature crosstalk. Based on these characteristics, it can be expected that the sensor will bring a great breakthrough in applications where sensitive monitoring of humidity is required.

MZI-based high intensity responsive sandwiched multi-layer fiber optic humidity sensor

This paper presents a multi-layer optical fiber humidity sensor based on dual-beam Mach-Zehnder interference (MZI) for optical intensity sensitivity. The sensor is fabricated using a large-scale axial offset fusion technique between a multimode fiber (MMF) and an offset hole two-core fiber (OHTC), making it suitable for agricultural and health monitoring applications. The 27 μm axial offset enables tilted light incidence, enhancing environmental sensitivity. With a single-cycle Graphene oxide (GO) coating of approximately 22 nm, the MMF-OHTC structure achieves high humidity sensitivity (0.39 dB/%RH) over the 25 %-80 % RH range with excellent linearity (R2≈0.99). The sensor exhibits stable, rapid response (2.52 s/0.46 s) and low temperature cross-sensitivity (0.06 %RH/°C). When combined with a hollow-core fiber Fabry-Perot interferometer (HCF-FPI), the system allows zero cross-talk simultaneous humidity and temperature measurements, expanding the potential applications of multi-layered structures in multifunctional detection systems.

Miniature fiber optic SPR high sensitivity humidity sensor based on coated polyvinyl alcohol film

For the measurement of relative humidity, this study employed Surface Plasmon Resonance (SPR) technology, a sensor with a micro-conical fiber structure (sensing length is only 375 μm) was combined with Polyvinyl Alcohol (PVA) to create a humidity sensor that is highly sensitive, compact, dense, and exhibits good linearity. The humidity sensor is based on a gold film (thickness of about 40 nm) on the surface of the miniature fiber optic structure, and by applying PVA coating, the SPR humidity sensor shows resonance peaks in the transmission spectrum in ambient air, and as the number of layers of PVA coating (thickness) is added, the resonance peaks’ positions are red-shifted. The experimental findings demonstrated that this SPR humidity sensor’s sensitivity was 0.162 nm/%RH at an initial resonance wavelength of about 550 nm (one layer) and about 1.542 nm/%RH at an initial resonance wavelength of 656 nm (five layers) in the range of 46 %RH to 93 %RH. The wavelength and the relative humidity had a good linear connection. Temperature tests revealed the sensor has a −0.152 nm/°C temperature sensitivity between 20 °C and 50 °C and has a high linearity (0.999). Therefore, the sensor has potential applications in humidity measurement.

Fiber optic relative humidity and temperature sensor with the cascaded Vernier effect based on the C-shaped cavity structure

The water-absorbent sensing film, coated on the surface of traditional optical fiber humidity sensors, often suffers from detachment issues. In this paper, we present what we believe to be a new fiber-optic cascaded Fabry-Perot interferometer sensor for detecting relative humidity (RH) and temperature, without the need for sophisticated instrumentation. The sensing structure comprises two sections of single-mode optical fibers and a C-shaped cavity between them. The C-shaped cavity is created by grinding the side of a hollow-core fiber with fiber optic abrasive paper. The Vernier effect arises from the cascaded interaction between the C-shaped cavity filled with ultraviolet optical glue (NOA61) and the subsequent single-mode fiber pigtail. The sensor exhibits a high RH sensitivity of 0.248 nm/%RH (35-95%RH) and an RH resolution of up to 0.08%RH. It also has high-temperature sensitivities of -1.091 nm/°C (25 - 65°C). Furthermore, simultaneous measurement of RH and temperature is achieved by establishing a dual parameter matrix, and the sensor's response time and recovery time for RH and temperature are within 300s. Therefore, this work provides a simple and cost-effective manufacturing process and the proposed RH and temperature sensor features a compact size, strong environmental adaptability, and significant potential for practical applications.

Humidity sensing by tailoring light absorption of SiO2/bromophenol blue (BPB) thin film on optical fiber

The fabrication of an evanescent wave fiber optic humidity sensor based on bromophenol blue (BPB) doped SiO2 thin film was demonstrated, modulating in light intensity. The sensing film was coated on a fiber core via a single-step dip coating method, followed by sol-gel processing of the precursor. A good exponential relationship was established between output light intensity and relative humidity. The sensor exhibited a high sensitivity and fast response and recovery, as well as low hysteresis, good stability and repeatability. Adsorption of ambient water triggered a ring-opening reaction of BPB, which enhanced light absorption of the sensing film significantly and affected the transmission of the evanescent wave.

Improvement of fiber optic sensor measurement methods for temperature and humidity measurement in microelectronic circuits

A wide range of applications such as healthcare, human comfort, agriculture, food processing and storage, and electronics manufacturing also require fast and accurate measurement of humidity and temperature. Optical fiber-based sensors have several advantages over electronic sensors, and much research has been conducted in this area in recent years. This paper describes the current trends in fiber optic temperature and humidity sensors. The evolution of optical structures aimed at humidity detection is presented, as well as a new design of an optical sensor used for this purpose. The main methods of humidity determination using fiber-optic laser reflection based on Optical fiber humidity sensor (FPI) were analyzed and experimental results were obtained. Based on temperature-sensitive strain variation, a method for temperature determination based on the specific spectral back-reflection effect of fiber Bragg gratings (FBGs) is considered. Experimental analyses were conducted on the light reflection of humidity-sensitive agarose using optical fibers based on the Fabry-Perot Interferometer (FPI). It exhibits a good linear response to relative humidity, ranging from 25 % to 95 %. During temperature measurement, the deformation changes of the Fiber Bragg Grating fibers showed excellent performance, ranging from –5 °C to 70 °C. New structures, such as resonators, are being explored to improve the resolution of fiber optic temperature and humidity sensors. In addition, recent studies on polymer optical fibers show that the sensitivity of this type of sensor has not yet been achieved. Thus, materials sensitive to humidity and temperature still need to be investigated to improve sensitivity and resolution

High sensitivity Mach-Zehnder interferometric fiber-optic humidity sensor based on multimode interference enhancement

Variations in humidity can significantly disrupt the proper functioning of precision instruments and meters across various industrial, medical, and scientific applications, thereby necessitating real-time humidity monitoring. Here, a high sensitivity fiber-optic relative humidity (RH) sensor based on Mach-Zender interferometer (MZI) is proposed and demonstrated. The humidity sensor consists of single-mode fiber (SMF), peanut structure (PS), and multi-mode fiber (MMF). A multi-mode interference (MMI) enhanced fiber-optic MZI was constructed using a SMF-PS-SMF-MMF-SMF-PS-SMF cascade structure to detect small changes in the refractive index (RI) and expansion force of gelatin film caused by humidity variation. PS, as a beam splitter and combiner of MZI, provides the possibility for simple, economical, and efficient manufacturing of the fiber-based part of humidity sensors. The addition of MMF will excite higher-order core and cladding modes, laying the foundation for achieving high-sensitivity fiber-optic humidity sensors. The fiber-optic RH sensor prototype demonstrate a high sensitivity of about 0.633 nm/% RH in the RH range of 50 % RH to 80 % RH, and a low temperature cross-sensitivity of 0.165 % RH/°C. This sensor has the advantages of safe and easy manufacturing process, low manufacturing cost, and high sensitivity, providing continuous high accuracy humidity measurement results for humidity sensing in different fields.

Humidity sensing characteristics of compact coupled optical fiber structure by microsphere and tapered capillary tube

In this paper, an optical fiber humidity sensor has been proposed based on the coupling structure using BaTiO3 microsphere and conical cavity of capillary pigtail. The sensitive material polyvinyl alcohol (PVA) was coated near the coupling region of microsphere and conical cavity to improve its sensing performance. The humidity sensitivity has been demonstrated to be 0.09 dBm/%RH based on intensity demodulation technique. The morphological parameters of the coupling structure are further optimized to obtain the maximum sensitivity of −0.297 dBm/%RH. Compared with other microsphere based optical fiber sensors, the sensor has the advantages of compact structure, simple manufacture, low production cost, high sensitivity and good stability.

High-Sensitivity Optical Fiber Humidity Sensor Based on Iridescent Chiral-Oriented Cellulose Nanocrystals/Polyethylene Vinyl Oxide Composite Film

High-Sensitivity Optical Fiber Humidity Sensor Based on Iridescent Chiral-Oriented Cellulose Nanocrystals/Polyethylene Vinyl Oxide Composite Film

Optical Fiber Humidity Sensor with Enhanced Sensitivity by Double-Sided Contact Film

Optical Fiber Humidity Sensor with Enhanced Sensitivity by Double-Sided Contact Film

Highly sensitive optical fiber humidity sensor employing SiO2 coated ZnO nanoparticle doped PVA

Highly sensitive optical fiber humidity sensor employing SiO₂ coated ZnO nanoparticle doped PVA

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 (0.1436s/0.1547s) are observed for the proposed sensor. The linear sensitivity for the developed sensor is observed to be 0.1036dB/%RH. In addition, the proposed sensor exhibits a very good degree of reversibility, reliability, and repeatability.

Soil total suction sensing using fiber-optic technology

Measurement of the soil water suction is important for investigating geotechnical problems and mitigating associated risks; however, conventional high-range suction measurement techniques have some limitations for accurate, long-term, and quasi-distributed suction measurement in the field. Fiber-optic humidity sensors provide a viable solution due to their unique properties. Here, we present a novel microfabricated fiber-optic suction sensor for measuring the suction of water in unsaturated soils. We evaluated the performance of the sensor on four bentonite–sand mixtures by comparing it with two laboratory suction measurement methods (chill-mirror hygrometer, salt solution vapor equilibrium) and a capacitive relative humidity sensor. We determined that this sensor has a measurement range of 5–300 MPa with a root mean square error of less than 3.5 MPa, and a response time of 2–9 min over the tested suction variation range (6.5–80.5 MPa). The results of the evaporation model test showed that three fiber-optic sensors installed at different depths of bentonite could effectively capture changes in temperature, relative humidity in soil pores, and total suction during evaporation. Together, these results demonstrate the great potential of the fiber-optic suction sensor for in-situ, long-term, and quasi-distributed suction monitoring.

FP Interferometric Optic Fiber Humidity Sensor Based on Acrylate AB Adhesive Film

In this paper, an optical fiber humidity sensor based on acrylate AB adhesive film is studied, and two methods—a bubbling method and a dual pressure assisted method—for preparing thin films are proposed. The forms will make the thin film lighter and make the sensor more sensitive. Using a glass tube to connect the acrylate AB adhesive film to the single mode optical fiber, the humidity sensor is obtained. Through humidity response experiments, the sensor’s sensitivity reaches 167.5 pm/% RH, and the response time reaches 4.8 s/% RH. At the same time, the experiments show that the sensor has good repeatability and stability. Finally, the influence of temperature on the working process is analyzed, and we provide a method for improvement through FBG.

PVAc based SCMS optical fiber humidity sensor and its application in transformer oil

This paper presents a novel design of an optical fiber sensor (OFS) utilizing a single-mode-coreless-multimode-single-mode (SCMS) fiber coated with Poly-Vinyl Acetate (PVAc) for humidity and moisture detection. PVAc, a highly sensitive synthetic polymer, exhibits changes in refractive index (RI) based on relative humidity (RH) variations in the surrounding medium, thereby affecting the output power of SCMS. Numerical analysis using COMSOL Multiphysics determined the optimal coating thickness. The fabricated PVAc-coated SCMS sensor enables RH measurement in air and moisture measurement in oil. The sensor demonstrated a sensitivity of 0.19 dBm/%RH and 0.16 dBm/%RH for increasing and decreasing humidity rates, respectively, within an RH range of 60–85% in air media. Additionally, the sensor achieved a sensitivity of 0.15 dBm/ppm for moisture measurement in oil media.

Experimental fiber optic humidity sensor with applicability to civil structures health monitoring

Experimental fiber optic humidity sensor with applicability to civil structures health monitoring

SnO2/polyvinyl alcohol nanofibers wrapped tilted fiber grating for high-sensitive humidity sensing and fast human breath monitoring

A fiber-optic humidity sensor based on excessively tilted fiber grating (Ex-TFG) wrapped with tin dioxide (SnO2)/polyvinyl alcohol (PVA) hybrid nanofibers is presented. Relative humidity (RH) rise causes the changes both in refractive index (RI) and geometry size of the film. The transmitted spectrum of the Ex-TFG changes as a result, owing to the strong interaction of the excited forward propagating cladding modes to the surface coating. Experimental results demonstrate the maximum intensity sensitivity of 0.43 dB/%RH in RH range of 35%−75%. Compared with pure PVA film coated ones, the RH sensitivity of the SnO2/PVA wrapped grating is vastly enhanced by one order of magnitude. The reticular morphology of electrospun SnO2/PVA film around the Ex-TFG promises the unimpeded permeation of water molecules, and then the sensor exhibits an ultrafast response of ∼67 ms and recovery times of ∼83 ms, by monitoring the human breathing with different rhythms. Lastly, the negligible spectral deformation to temperature perturbations verifies the temperature de-correlation of the designed RH sensor. This work indicates that the proposed SnO2/PVA nanofibers wrapped Ex-TFG can provide a simple, robust and high-sensitive optic device for healthcare monitoring, environmental and biochemical sensing applications.

Performance-enhanced fiber optic humidity sensors based on SiO2/porous PMMA coatings.

Employing functional and structured thin films on fiber optic sensors has tremendously improved capabilities in humidity sensing applications. In this paper, we demonstrate fabrication of a fiber optic evanescent wave humidity sensor based on S i O 2/porous polymethyl methacrylate (PMMA) thin films. With the exposure to moisture, S i O 2/porous PMMA thin films absorb water molecules. The refractive index and absorption coefficient of thin films change with ambient humidity, resulting in modulation of the light intensity transmitted in fiber. A good linearity is determined between the logarithm of output light intensity and relative humidity (RH). An optimal average sensitivity of 188.3 lux/%RH is achieved with an increase of 11.7 fold in the RH range of 5% to 95%. The response and recovery times are 8s and 23s, respectively. Furthermore, the sensor exhibits low hysteresis, and excellent stability and repeatability.

Effect of dipping number of polyvinyl alcohol (PVA) -coated on optical Fiber for Humidity Sensor

Optical Fiber Humidity Sensors (OFHSs) have attracted significant research interest due to their several advantages including small size, high durability and can withstand the extreme environment regardless of high temperature, high pressure and high electromagnetic radiation. In this study, the proposed sensors were built from standard single-mode silica fibers working at telecommunication wavelengths and exhibited very good humidity sensing characteristics. Single-mode optical fibers were coated with different dipping numbers (2, 4, 6 8, and 10 times) of polyvinyl alcohol (PVA) solution using a dip coating method. The sensing performance of uncoated and PVA-coated optical fiber sensors in terms of its sensitivity for various relative humidity (RH) percentages ranging from 40 to 90 % RH was monitored using Ocean Optics spectrometer. The intensity peaks of PVA-coated optical fibers were increased linearly with dipping numbers. The optimum sensing performance was observed for 10 times dipping number of PVA solution with the highest intensity of 4100 counts at 60 % RH and exhibits good repeatability with precision error values in the range of 0 to 2 %. Furthermore, the gap difference between intensity counts due to % RH changes could be clearly observed for 10 times dipping numbers. These reported results are important to enhance the performance of humidity sensor for sensing and monitoring purposes.