A high-sensitivity optical fiber relative humidity sensor based on microsphere WGM resonator

A high-performance fiber-optic relative humidity (RH) sensor was prepared by layer-by-layer self-assembly of polycation, poly (4-vinylpyridinium chloride), and polyanion, sodium alginate, on the surface of a thin-core fiber modal interferometer. The process of layer-by-layer self-assembly was monitored by UV-vis spectroscopy and surface profilometer. The morphology of the fabricated multilayer film was characterized by atomic force microscopy and field emission scanning electron microscopy. A fast, reversible, and linear response to RH with a high resolution of 0.1 nm/1% RH has been experimentally demonstrated by the proposed fiber-optic sensor.

We report a detailed study of a miniature optical relative humidity (RH) sensor based on a polymer-infiltrated photonic crystal fiber interferometer. The sensor shows a high sensitivity to RH variations with a change in its reflected power of about 12 dB for a humidity change of 84% RH. The sensor has the advantages of a very compact length of 1 mm, and an end-type probe configuration makes it suitable for monitoring humidity in microenvironments. The repeatability, long-term stability, measurement accuracy, and temperature dependence of the sensor are studied in this paper. The observed low thermal sensitivity of the sensor suggests that temperature compensation may not be needed if it is used in normal environments. The response time of the sensor is found to be 400 ms for a change in RH of ~ 30% RH. The fast response time suggests that the sensor can potentially be used as a human breath rate monitor in a clinical situation.

A SiO2/TiO2 bilayer thin-film-based fiber optic humidity sensor was fabricated via a modified dip coating process with enhanced sensitivity. SiO2 film was coated on the surface of the fiber core, followed by deposition of the TiO2 layer on SiO2. The relative humidity (RH) is measured by modulation in intensity of the transmitted laser at room temperature. The optical fiber humidity sensor based on SiO2/TiO2 film shows two-segmented linearity in measurement with sensitivities of 5.35 and 1.94 µW/% RH at 15%-50% RH and 50%-95% RH, respectively. The response time and recovery time are 25 s and 50 s, respectively. To our knowledge, the superior response time and recovery time of the sensor in our study were achieved over those fiber optic humidity sensors reported with modulation in intensity. Furthermore, this fiber optic humidity sensor has a good reproducibility and long-term stability. The sensing mechanism is attributed to effects of moisture on the refractive index and the light absorption coefficient of SiO2 film and modulation in the transmission characteristic of evanescent waves in the optical fiber.

We propose a novel relative humidity (RH) sensor based on a hollow core-photonic bandgap fiber (HC-PBF) using a wavelength scan and a reference technique in this paper. The wavelength scan of water vapor absorption peak around 1368.59 nm is realized by injecting sawtooth modulated current to a DFB laser diode. The humidity level is determined by the voltage difference (ΔV) between the reference signal and absorption signal at the peak of water vapor absorption. The reference signal is used not only as a zero absorption baseline but also to reduce the influence of intensity noise from DFB laser source. We have experimentally implemented the wavelength scanning modulation scheme with a HC-PBF sensing head of 5 cm and a small gap between SMF and HC-PBF to allow air diffusion into air holes of HC-PBF. High humidity sensitivity of 3.02 mV/1% RH over the range from 0 to 90% RH is achieved without the use of any coating material for humidity sensing.

The utilization of printing techniques for the development of high-performance humidity sensors holds immense significance for various applications in the fields of the Internet of Things, agriculture, human healthcare, and storage environments. However, the long response time and low sensitivity of current printed humidity sensors limit their practical applications. Herein, a series of high-sensing-performance flexible resistive-type humidity sensors is fabricated by the screen-printing method, and hexagonal tungsten oxide (h-WO3 ) is employed as the humidity-sensing material due to its low cost, strong chemical adsorption ability, and excellent humidity-sensing ability. The as-prepared printed sensors exhibit high sensitivity, good repeatability, outstanding flexibility, low hysteresis, and fast response (1.5s) in a wide relative humidity (RH) range (11-95%RH). Furthermore, the sensitivity of humidity sensors can be easily adjusted by altering the manufacturing parameters of the sensing layer and interdigital electrode to meet the diverse requirements of specific applications. The printed flexible humidity sensors possess immense potential in various applications, including wearable devices, non-contact measurements, and packaging opening state monitoring.

Long-period fiber grating humidity sensor based on spider silks

Wearable humidity sensors based on bar-printed poly(ionic liquid) for real-time humidity monitoring systems

A novel concept for an intrinsic relative humidity (RH) sensor that uses polyimide-recoated fiber Bragg gratings is presented. Tests in a controlled environment indicate that the sensor has a linear, reversible, and accurate response behavior at 10-90% RH and at 13-60 degrees C. The RH and temperature sensitivities were measured as a function of coating thickness, and the thermal and hygroscopic expansion coefficients of the polyimide coating were determined.

Humidity sensors based on flexible sensitive nanomaterials are very attractive in noncontact healthcare monitoring. However, the existing humidity sensors have some shortcomings such as limited sensitivity, narrow relative humidity (RH) range, and a complex process. Herein, we show that a tin sulphide (SnS) nanoflakes-based sensor presents high humidity sensing behaviour both in rigid and flexible substrate. The sensing mechanism based on the Schottky nature of a SnS-metal contact endows the as-fabricated sensor with a high response of 2491000% towards a wide RH range from 3% RH to 99% RH. The response and recovery time of the sensor are 6 s and 4 s, respectively. Besides, the flexible SnS nanoflakes-based humidity sensor with a polyimide substrate can be well attached to the skin and exhibits stable humidity sensing performance in the natural flat state and under bending loading. Moreover, the first-principles analysis is performed to prove the high specificity of SnS to the moisture (H2O) in the air. Benefiting from its promising advantages, we explore some application of the SnS nanoflakes-based sensors in detection of breathing patterns and non-contact finger tips sensing behaviour. The sensor can monitor the respiration pattern of a human being accurately, and recognize the movement of the fingertip speedily. This novel humidity sensor shows great promising application in physiological and physical monitoring, portable diagnosis system, and noncontact interface localization.

A microfiber interferometer based on a single-mode tapered-no-core single-mode (STNCS) fiber structure coated with a thin layer of graphene oxide (GO) was proposed and demonstrated as a relative humidity (RH) sensor. The STNCS fiber structure has a strong evanescent field, which can sensitively response to changes in the surrounding refractive index. The moisture-sensitive material GO was used to form the film on the no-core fiber by a dip impregnation method. The effects of GO concentration and the tapered waist diameter on RH sensitivity were studied experimentally. The experimental results showed that a thinner waist diameter provides higher RH sensitivity. When the concentration of GO solution is 0.01 mg/mL, the STNCS fiber structure with a waist diameter of 2.9 μm performs better in RH sensing; a high sensitivity of 0.461 nm/%RH in the range of 40-98% was achieved. The sensor has good stability with wavelength fluctuations of ±0.009 nm and ±0.008 nm over 60 mins at RHs of 50% and 80%, respectively. The proposed fiber optic humidity sensor has a high sensitivity over a wide measurement range and offers good stability showing promising potential application in the field of RH sensing.

Retracted.

Developing dual-signal sensing system that enables simultaneous optical and electrical humidity detection holds significant application foreground in the realm of real-time environmental humidity monitoring. In this work, utilizing cobalt chloride (CoCl 2 ) hybridized with polyvinyl alcohol (PVA) as sensitive materials, and cellulose paper with pencil-drawn graphite electrodes as the substrate, a paper-based humidity sensor capable of dual-signal sensing with colorimetric visualization and electrical readout is proposed for the monitoring of environment humidity across multiple application scenarios. This designed dual-mode humidity sensor exhibits superior electrical sensing properties, with a three-order-of-magnitude change in impedance over a relative humidity range from 11 % to 95 %, and a desired maximum hysteresis of only 3.5 %. Additionally, the sensor reveals remarkable electrical long-term stability and reliability, with a response variation of merely 5.11 % after one month of operation. Furthermore, this designed dual-mode humidity sensor also manifests outstanding optical sensing performance with reversible naked-eye readable color change in different humidity environments, without significant degradation in colorimetric performance even after 30 days. The designed dual-signal humidity sensor is further explored as wearable devices , demonstrating its potential practicality in multi-scenario fields such as respiration monitoring, non-contact sensing, and smart diapers. The proposed dual-mode humidity sensor allows for the simultaneous visualization and precise electrical quantification of humidity changes , offering a significant contribution to the field of environmental sensing and opening new avenues for the development of multifunctional sensing technologies. • A paper-based humidity sensor capable of optical and electrical dual-signal sensing is developed. • The dual-mode humidity sensor presents superior electrical sensing performance. • The humidity sensor manifests reliable optical sensing performance with reversible naked-eye readable color change. • The designed dual-signal sensor can be exploited as wearable devices for multifunctional applications.

Water vapor adsorption is the least studied form of non-rainfall water inputs but is likely the most common one in arid and hyper-arid areas. It is determined by the magnitude of the downward gradient in water vapor pressure between the atmosphere and the soil; the surface area of the adsorbing soil; and the penetration depth of water vapor adsorption. Water vapor adsorption was measured using micro-lysimeters and profiles of relative humidity (RH) sensors in both loess and sand in the Negev desert, Israel, over the summers of 2021 and 2022. The RH sensor array allowed measurement of detailed changes in water content in the soil profile and provided an unprecedented insight into processes governing water vapor adsorption dynamics under arid conditions in-situ. The RH sensors significantly underestimated total water vapor adsorption, indicating that a finer array is needed to capture the full process. However, even with the current array, extremely small changes in water content were captured. With these measurements we explored the three main factors contributing to water vapor adsorption. The onset of a downward vapor pressure gradient coincided with the arrival of the sea breeze, indicating that the sea breeze is the primary source for water vapor adsorption in the uppermost soil layer. Water vapor adsorption was higher in loess than in sand, due to its finer texture and larger surface area. The most important finding of this research is that the dominant mechanism for water vapor flow under natural arid conditions (relative humidity in the soil (RHs)

An optical fiber waist-enlarged bitaper-based Michelson interferometric sensor is proposed and experimentally demonstrated for humidity measurement. The waist enlarged bitaper is created for light coupling between core mode and cladding modes propagating in the fiber interferometer. A chitason layer is plated onto the surface of the interferometer to act as a humidity-to-refractive index (RI) transducer and thus humidity measurement can be realized by monitoring the wavelength shifts of its interferogram induced by RI variations. The influence of the coating thickness and concentration of chitason on relative humidity (RH) measurement is experimentally studied. The coating sensor demonstrates an optimal humidity-sensing ability, with a humidity sensitivity and fast time-response of ~26 pm/%RH and ~5 s respectively, when it is 3-dip coated in chitason solutions of the concentration of 1 wt.%. The proposed humidity sensor is compact, cost-effective and of easy-operation, therefore it has potentials in many practical applications.

Novel application of attapulgite on high performance and low-cost humidity sensors