Optical Waveguide Sensor Research Articles

Optical Micro/Nanofiber Enabled Multiaxial Force Sensor for Tactile Visualization and Human-Machine Interface.

Tactile sensors with capability of multiaxial force perception play a vital role in robotics and human-machine interfaces. Flexible optical waveguide sensors have been an emerging paradigm in tactile sensing due to their high sensitivity, fast response, and antielectromagnetic interference. Herein, a flexible multiaxial force sensor enabled by U-shaped optical micro/nanofibers (MNFs) is reported. The MNF is embedded within an elastomer film topped with a dome-shaped protrusion. When the protrusion is subjected to vector forces, the embedded MNF undergoes anisotropic deformations, yielding time-resolved variations in light transmission. Detection of both normal and shear forces is achieved with sensitivities reaching 50.7dB N-1 (14% kPa-1) and 82.2dB N-1 (21% kPa-1), respectively. Notably, the structural asymmetry of the MNF induces asymmetrical optical modes, granting the sensor directional responses to four-directional shear forces. As proof-of-concept applications, tactile visualizations for texture and relief pattern recognition are realized with a spatial resolution of 160µm. Moreover, a dual U-shaped MNF configuration is demonstrated as a human-machine interface for cursor manipulation. This work represents a step towards advanced multiaxial tactile sensing.

Thin films with implemented molecular switches for the application in polymer-based optical waveguides

Complexes like iron (II)-triazoles exhibit spin crossover behavior at ambient temperature and are often considered for possible application. In previous studies, we implemented complexes of this type into polymer nanofibers and first polymer-based optical waveguide sensor systems. In our current study, we synthesized complexes of this type, implemented them into polymers and obtained composites through drop casting and doctor blading. We present that a certain combination of polymer and complex can lead to composites with high potential for optical devices. For this purpose, we used two different complexes [Fe(atrz)3](2ns)2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$[\ ext {Fe}(\ ext {atrz})_{3}](2\\, \ ext {ns})2$$\\end{document} and [Fe(atrz)3]Cl1.5(BF4)0.5\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$[\ ext {Fe}(\ ext {atrz})_{3}]\ ext {Cl}_{1.5}(\ ext {BF}_{4})_{0.5}$$\\end{document} with different polymers for each composite. We show through transmission measurements and UV/VIS spectroscopy that the optical properties of these composite materials can reversibly change due to the spin crossover effect.

A brief review on optical properties of polymer Composites: Insights into Light-Matter interaction from classical to quantum transport point of view

In recent years, the optical capabilities of polymer composites have dominated the marketplace for applications in optoelectronic and energy devices. Supercapacitors, light-emitting displays, optical waveguide sensors, and organic photovoltaic cells are a few examples of the applications of the optical behavior of composites. Polymer composites are composed of nano/micro-sized inorganic particles (such as semiconductors, metal complexes, and synthetic nanoparticles) that are distributed within a polymer matrix. These materials have a significant role because they combine the optical properties of inorganic materials with the ease of processing of polymers, which utilizes the potential and performance of polymer-based optical systems.Optical features of polymer composites, such as indices of refraction, transparency, dispersion, and coefficients of absorption, influence their potential uses in optical systems and devices. The purpose of this research is to establish more insight into the optical characteristics of polymer composites and to understand the correlations between the structure and optoelectronic behavior of polymer composites. This review involves general concepts, scientific guidelines, and feasible mathematical sections. The unique optical characteristics of some common polymers (PEO, PMMA, PVA, and CS) and polymer composites are briefly reviewed. This review article establishes the fact that metal complexes are excellent over ceramic filler or nano-particles to improve optical absorption and decrease the optical band gap. An inspection of the fundamentals of light-matter interaction, ranging from classical (Drude-Lorentz model) to quantum methods for studying electron transition was exhibited. Furthermore, the applications of Tauc’s model and optical dielectric loss parameters to estimate the optical energy band gap of polymer composites are explained. Other fundamental optical parameters, such as absorption coefficient, optical dielectric constant, and refractive index are also explored. Finally, the Wemple-DiDomenico (WD) model is applied to investigate; the refractive index, optical dielectric constant, and optical spectra moments. The correlations between the optical dielectric function and several parameters such as ε∞, τ, N/m*, µopt, ρopt, ωp, and Eg are derived. Various models based on refractive index and absorption coefficient are discussed in detail to estimate crucial optical parameters.

A novel TiO2-modified THPP-BCP composite optical waveguide sensor for the determination of ethylenediamine at ppb level.

Composite thin films OWG sensors show higher sensitivity than single film-based OWG sensors, making them particularly useful in the detection of trace gases. In this work, we developed a new tetra hydroxyphenyl porphyrin (THPP)-bromocresol purple (BCP)/TiO2 gel composite film-based OWG (THPP-BCP/TiO2-OWG) sensor for identifying ethylenediamine (EDA) gas. The fabricated sensor was characterized using ultraviolet and infrared spectroscopy and atomic force microscopy. The test result shows that the response of the THPP-BCP/TiO2-OWG composite film sensor to analytes was 3-4 times higher than that of the single film THPP-OWG sensor. The THPP-BCP/TiO2-OWG sensor exhibited excellent selectivity and remarkable response to EDA gas at a concentration of < 1ppb at room temperature, with fast response (1s) and recovery (3s) times. The high refractive index TiO2 film contributes to OWG's sensitivity, while the THPP and BCP's sensitivity to bases further increased the sensor's response to EDA gas. The combination of THPP-BCP thin film and TiO2 gel film provides a powerful platform for OWG sensors that are highly sensitive, specific, and stable in the detection of trace gases.

Sensitivity Performance of Optical Waveguide Sensors with Different Shapes of Cladding Layer for Microplastics Detection

With microplastics pollution becoming a global concern, there comes a need for sensors to attain an optimal level of sensitivity to detect microplastics in water. This work investigated the effects of cladding layer shapes on the sensitivity performance of an optical waveguide sensor for microplastics detection in water. In this research, three different cladding shapes—C-shaped fiber, D-shaped fiber, and rectangular waveguide with circular core—were simulated by using Wave Optics Module-COMSOL Multiphysics® software. The results indicated that the C-shaped fiber exhibited significantly higher sensitivity, with a sensitivity value of 1.070x10−3 compared to the D-shaped fiber and rectangular waveguide with 3.845x10−4 and 3.842x10−4, respectively. The sensitivities of the D-shaped fiber and rectangular waveguide were relatively similar and did not exhibit any significant difference. The higher sensitivity of the C-shaped fiber is attributed to its larger exposed core area to the analyte, resulting in higher interaction of the evanescent wave with the analyte. However, fabricating the C-shaped fiber is more challenging compared to the other two shapes. This research highlights the significance of cladding shapes in optical waveguide sensor sensitivities and provides design optimization insights for microplastics detection in water.

A Ti: LiNbO3 optical waveguide sensor for measurement of intensive electric field

In this paper, a Ti: LN optical waveguide intensive electric field sensor without metal structure is designed and fabricated. By using the finite element simulation, the refractive index difference Δno and Δne of the waveguide are designed as 0.0105 and 0.0305 respectively when the prediffusion depth d is 5 μm, which results in the static operating point of the sensor is 86.4° when the length L is 45 mm and the prediffusion width w is 10 μm. The simulation results show that when the design of LN substrate is 6 mm (x) × 1 mm (y) × 45 mm (z), the ratio of the external and internal electric fields is 4.216, and the half-wave electric field is 10053 kV/m. The sensor is measured by the power-frequency electric field measurement system, and the half-wave electric field is about 10135 kV/m, and the maximum detectable electric field is about 3041 kV/m.

A Multidirectional External Perception Soft Actuator Based on Flexible Optical Waveguide for Underwater Teleoperation

As a friendly tool, the soft gripper can be used directly for grasping vulnerable objects in underwater environments. These tasks are generally performed by teleoperation based on the visual feedback, rather than the soft actuators’ sensing information. However, vision sensors’ function may be restricted in some complex underwater environments with poor visibility and narrow spaces. This will greatly reduce the efficiency of the underwater operations. Therefore, soft actuators strongly require an organism‐like perception system to sense environmental stimuli and can be applied to complex underwater environments. To address the problem, a multidirectional external perception soft actuator based on two flexible optical waveguide sensors is developed and machine learning methods are utilized to build its perceptual model herein. The experimental results indicate that the soft actuator can recognize 12 contact positions based on the sensing model, and the identifying accuracy is up to 99.82%. Additionally, according to the contact location feedback, teleoperation can be more efficiently completed in unknown underwater environments.

Composite optical waveguide sensor based on porphyrin@ZnO film for sulfide-gas detection

Composite optical waveguide sensor based on porphyrin@ZnO film for sulfide-gas detection

Silicon Waveguide Sensors for Carbon Dioxide Gas Sensing in the Mid-Infrared Region

Two optical waveguide sensors based on SOS (silicon-on-sapphire) for detecting CO2 are theoretically proposed. The operational wavelength is 4.23 μm, which is the maximum absorption line of CO2. The power confinement factor (η) value is over 40% and 50%, the propagation loss is 0.98 dB/cm and 2.99 dB/cm, respectively, in the slot waveguide and SWGS (subwavelength grating slot) waveguide. An inverted tapered structure is used for the transition from strip waveguide to slot waveguide and constitutes the sensing absorption region, with the coupling efficiency that can reach more than 90%. When the optimal absorption length of the slot waveguide and SWGS waveguide is 1.02 cm and 0.33 cm, respectively, the maximum sensitivity can reach 6.66 × 10−5 (ppm−1) and 2.60 × 10−5 (ppm−1). Furthermore, taking the slot waveguide as an example, spiral and meander structures enable the long-distance sensing path to integrate into a small area.

Gas sensing properties of a maghemite-coupled porphyrin thin film–based optical waveguide sensor

This study concerns the development of a maghemite (γ-Fe2O3)-coupled tetra sulfophenyl porphyrin (TSPP) thin film–based optical waveguide (OWG) sensor (TSPP-γ-Fe2O3-OWG) for the detection of ethylenediamine (EDA). For ease of manufacture, we fabricated sensors using a spin-coating technique, and characterized their response based on their ultraviolet-visible (UV-vis) absorption spectra and the response intensity of OWG sensor. We have observed significant changes in the location of the peaks in the sensor’s UV-vis absorption spectra after exposed to EDA gas, indicating aggregation of TSPP. Optical characterization confirmed that the combination of TSPP with maghemite (TSPP-γ-Fe2O3-OWG) had improved device response, with twice as high as those of the acid-treated TSPP-OWG sensor, and 30 times higher than that of the pure γ-Fe2O3-OWG sensor. At ambient temperature, the composite sensor exhibited a linear response to EDA gas in the concentration range of 0.1–1000 ppm with signal-to-noise ratio (S/N) 8.33, confirming that the improved stability of the composite TSPP-γ-Fe2O3 thin-film OWG enables the detection of trace amounts of gas with high sensitivity and selectivity.

Bending-Sensitive Optical Waveguide Sensor With Carbon-Fiber Layer for Monitoring Grip Strength.

With the gradual popularity of wearable devices, the demand for high-performance flexible wearable sensors is also increasing. Flexible sensors based on the optical principle have advantages e.g. anti-electromagnetic interference, antiperspirant, inherent electrical safety, and the potential for biocompatibility. In this study, an optical waveguide sensor integrating a carbon fiber layer, fully constraining stretching deformation, partly constraining pressing deformation, and allowing bending deformation, was proposed. The sensitivity of the proposed sensor is three times higher than that of the sensor without a carbon fiber layer, and good repeatability is maintained. We also attached the proposed sensor to the upper limb to monitor grip force, and the sensor signal showed a good correlation with grip force (the R-squared of the quadratic polynomial fitting was 0.9827) and showed a linear relationship when the grip force was greater than 10N (the R-squared of the linear fitting was 0.9523). The proposed sensor has the potential for applications in recognizing the intention of human movement to help the amputees control the prostheses.

A Surface-Scattering-Based Composite Optical Waveguide Sensor for Aerosol Deposition Detection

Aerosol is a suspension of fine chemical or biological particles in the air, and it is harmful, easily causing air pollution, respiratory diseases, infrastructure corrosion, and poor visibility. Therefore, the development of advanced optical sensors for real-time detection of aerosol deposition is of great significance. In this work, a prism-coupled composite optical waveguide (COWG) sensor for aerosol deposition detection based on surface scattering is proposed and demonstrated theoretically and experimentally. The COWG consists of a single-mode slab glass waveguide locally covered with a tapered thin film of high-index metal oxide. The tapered film can greatly enhance the evanescent field through the adiabatic transition of the fundamental transverse electric (TE0) mode between the uncovered and film-covered regions, thereby enabling the COWG to serve as a simple yet highly sensitive evanescent-wave scattering sensor for sensitive detection of aerosol deposition. The COWG with a tapered layer of Ta2O5 was prepared by masked sputtering, aerosol salt particle deposition on the COWG was successfully detected, and the influence of surface water droplets on the COWG sensor performance was analyzed. The experimental results indicate that the sensitivity of the COWG is 30 times higher than that of the bare glass waveguide.

Environmental Monitoring: A Comprehensive Review on Optical Waveguide and Fiber-Based Sensors.

Globally, there is active development of photonic sensors incorporating multidisciplinary research. The ultimate objective is to develop small, low-cost, sensitive, selective, quick, durable, remote-controllable sensors that are resistant to electromagnetic interference. Different photonic sensor designs and advances in photonic frameworks have shown the possibility to realize these capabilities. In this review paper, the latest developments in the field of optical waveguide and fiber-based sensors which can serve for environmental monitoring are discussed. Several important topics such as toxic gas, water quality, indoor environment, and natural disaster monitoring are reviewed.

Stretchable Optical Waveguide Sensor Capable of Two-Degree-of-Freedom Strain Sensing Mediated by a Semidivided Optical Core

With the rising demand for flexible strain sensors for soft robots, optical sensing solutions have been proven to be electrically safe, stable, and precise. This study presents a highly stretchable optical waveguide sensor that extends existing solutions to two degrees of freedom while maintaining low manufacturing complexity and part count. Through casting in a 3D-printed mold, a polymer waveguide is manufactured, featuring a semidivided core cross section. The light from a near-infrared LED light source is guided to two phototransistors connected to each chamber. The connection between both core chambers makes the power throughput amplitude and ratio at the two outputs dependent on the strain amplitude as well as its location or direction. The proposed waveguide is experimentally compared with single and dual core designs in four different strain modes, to sense elongation, local deformation amplitude and position across length, twisting angle and direction, as well as bending amplitude and direction in a soft finger. While influences of the manual manufacturing process are apparent, the results verify that the presented waveguide sensor can be effectively applied in these four strain scenarios. We also demonstrate that information about superimposed strain states can be obtained from the time signal.

A Novel Stretchable and Flexible Sensor Using Graphite-Added Optical Waveguide for Human Motion Detection

According to the Bill-Lambert law, this article reports a flexible optical waveguide sensor based on the polydimethylsiloxane (PDMS) doped with graphite. We quantitatively study the attenuation spectra induced by adding graphite with different particle sizes and amounts into the optical waveguide of the sensor. Adding the graphite’s particle size with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.5\mu $ </tex-math></inline-formula> m and its concentration of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5\times 10^{-{3}}$ </tex-math></inline-formula> wt% shows that the relationship between the optical attenuation and the tensile strain in the range of 90% is approximately linear. Our further experiment shows that the radial shrinkage of a waveguide is more than 16.8% when the axial tensile strain is larger than 90%, this increases the density of graphite particles on the optical waveguide’s cross-section, resulting in a non-negligible effect on light transmission, which makes the relationship between light attenuation and the tensile strain nonlinear. Our proposed waveguide strain sensor is applied to the motion detection of human joints and muscles. We obtained a series of stable, reliable and durable response no matter the static and dynamic conditions. The sensitivity of the proposed flexible strain sensor is improved by a factor of 3.9 and 1.2 than that of sensors made of the pure PDMS and the polymethylmethacrylate (PMMA), respectively. Meanwhile, it has a high signal-to-noise ratio (SNR) and is insensitive to installation angle errors. There will be a good application prospect in wearable devices that can adapt to the complex curved surface, due to the proposed waveguide strain sensor’s stretchable, sensitive and low-cost characteristics.

Metallic Effects on p-Hydroxyphenyl Porphyrin Thin-Film-Based Planar Optical Waveguide Gas Sensor: Experimental and Computational Studies.

Metal effects on the gas sensing behavior of metal complexes of 5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin (THPP) thin film was investigated in terms of detecting NO2 gas by the planar optical waveguide. For this purpose, several THPP and metal complexes were synthesized with different central metal ions: Co(II), Ni(II), Cu(II), and Zn(II). Planar optical gas sensors were fabricated with the metalloporphyrins deposited on K+ ion-exchanged soda-lime glass substrate with the spin coating method serving as host matrices for gas interaction. All of the THPP complex’s films were fully characterized by UV-Vis, IR and XPS spectroscopy, and the laser light source wavelength was selected at 520 and 670 nm. The results of the planar optical waveguide sensor show that the Zn–THPP complex exhibits the strongest response with the lowest detectable gas concentration of NO2 gas for both 520 nm and 670 nm. The Ni–THPP and Co–THPP complexes display good efficiency in the detection of NO2, while, on the other hand, Cu–THPP shows a very low interaction with NO2 gas, with only 50 ppm and 200 ppm detectable gas concentration for 520 nm and 670 nm, respectively. In addition, molecular dynamic simulations and quantum mechanical calculations were performed, proving to be coherent with the experimental results.

A New Strategy for Real-Time Humidity Detection: Polymer-Coated Optical Waveguide Sensor

This paper proposes a novel strategy for low humidity detection, an optical waveguide (OWG) sensor that is locally coated with polyvinylpyrrolidone (PVP) film. The humidity sensor was fabricated using a spin coating on a K+-exchanged glass optical waveguide with PVP film. Its sensing properties were investigated by injecting a humid air range of 10.6~32%RH (relative humidity) at room temperature. The surface morphology of the PVP film was characterized by an atomic force microscope (AFM). The possible humidity sensing mechanism of the proposed sensor was discussed by using absorption spectra. This study showed that the PVP-coated OWG sensor possessed high sensitivity, stability, and rapid response/recovery. Therefore, these observed results demonstrate that the low-cost OWG humidity sensor could be applied in real-time low concentration water vapor monitoring.

Design and Simulational Verification for a Stretchable Optical Waveguide Sensor Featuring a Semi-Divided Core Cross Section

Design and Simulational Verification for a Stretchable Optical Waveguide Sensor Featuring a Semi-Divided Core Cross Section

Multifunctional flexible optical waveguide sensor: on the bioinspiration for ultrasensitive sensors development

This paper presents the development of a bioinspired multifunctional flexible optical sensor (BioMFOS) as an ultrasensitive tool for force (intensity and location) and orientation sensing. The sensor structure is bioinspired in orb webs, which are multifunctional devices for prey capturing and vibration transmission. The multifunctional feature of the structure is achieved by using transparent resins that present both mechanical and optical properties for structural integrity and strain/deflection transmission as well as the optical signal transmission properties with core/cladding configuration of a waveguide. In this case, photocurable and polydimethylsiloxane (PDMS) resins are used for the core and cladding, respectively. The optical transmission, tensile tests, and dynamic mechanical analysis are performed in the resins and show the possibility of light transmission at the visible wavelength range in conjunction with high flexibility and a dynamic range up to 150 Hz, suitable for wearable applications. The BioMFOS has small dimensions (around 2 cm) and lightweight (0.8 g), making it suitable for wearable application and clothing integration. Characterization tests are performed in the structure by means of applying forces at different locations of the structure. The results show an ultra-high sensitivity and resolution, where forces in the μN range can be detected and the location of the applied force can also be detected with a sub-millimeter spatial resolution. Then, the BioMFOS is tested on the orientation detection in 3D plane, where a correlation coefficient higher than 0.9 is obtained when compared with a gold-standard inertial measurement unit (IMU). Furthermore, the device also shows its capabilities on the movement analysis and classification in two protocols: finger position detection (with the BioMFOS positioned on the top of the hand) and trunk orientation assessment (with the sensor integrated on the clothing). In both cases, the sensor is able of classifying the movement, especially when analyzed in conjunction with preprocessing and clustering techniques. As another wearable application, the respiratory rate is successfully estimated with the BioMFOS integrated into the clothing. Thus, the proposed multifunctional device opens new avenues for novel bioinspired photonic devices and can be used in many applications of biomedical, biomechanics, and micro/nanotechnology.

Optical Waveguide Sensors for Measuring Human Temperature and Humidity with Gel Polymer Electrolytes.

In this work, multimodal responsive optical waveguide sensors using a stable cross-linking gel polymer electrolyte are successfully designed and fabricated by bottom metal-printing technology. Temperature and humidity sensing characterization based on the polymer electrolyte is simulated and analyzed. The multimodal responsive properties of the photonic chip are defined based on the analysis of ion relaxation dynamics: optical phase variable to monitor temperature and optical attenuation variable to detect humidity. In the supervising temperature (36.0-38.0 °C) and relative humidity (45-65%) range, the temperature and humidity sensitivities of the device are measured as 0.5π rad/°C and 1.14 dB/% RH, respectively. The fast-response time for both temperature and humidity of the multifunctional sensor can be obtained as 4.21 ms and 1.32 s, respectively. These findings provide a feasible scheme for the design and application of temperature and humidity sensors in potential medical treatment. From gel polymer electrolytes to multimode monitoring applications, the application exploration of high stability and ultrafast response multimode waveguide sensors is gradually being carried out. This study has great significance for the comprehensive monitoring of sophisticated human physical signs by multimodal responsive waveguide sensors.