The optical Vernier effect scheme has proven to be a powerful tool for significantly enhancing the sensitivity of the interferometric sensors by combining two interferometers, provided that the free spectral range (FSR) of each interferometer closely matches that of the other. However, the fabrication of a reference cell that precisely aligns with the sensing interferometer (SI) to meet the Vernier condition can be challenging. In this article, we propose a novel Vernier effect scheme consisting of only one interferometer. The single interferometer functions as both the SI and reference interferometer (RI), where the reference spectrum can be achieved by inverting the sensing spectrum along its wavelength axis. By combining the sensing spectrum and reference spectrum, the resulting Vernier envelope features a unique wavelength (UW), breaking the periodicity seen in the conventional envelope. The sensitivity enhancement principle of the proposed Vernier effect has been theoretically investigated and its effectiveness has been validated experimentally in a temperature measurement. This study offers the advantages of simplicity and cost-effectiveness, with promising potential to improve the performance of Vernier effect-based sensors. It paves the way for achieving higher sensitivity with a more compact sensor configuration, which is highly beneficial for practical sensing applications.

We report on the characterisation of relative humidity sensors based on fibre Bragg gratings coated with two hygroscopic materials: polyimide (PI) and graphene oxide (GO). In both cases, sensitivity and response time could be tuned by varying the coating thickness, but graphene oxide provided significantly higher sensitivity and a faster response time than polyimide: for PI-coated sensors, a 1 pm/% increase in sensitivity resulted in a 94 s increase in response time, whereas for GO-coated sensors, the same increase in sensitivity only cost an additional 1 s increase in response time. These results show that graphene oxide is a promising material for faster-responding humidity sensors that can maintain high sensitivity.

A refractive index (RI) and temperature dual-parameter sensor, based on a side-polished hole-assisted dual-core fiber (HADCF), has been proposed. In this sensor, SnO2 film has been deposited on the suspended core of HADCF by RF sputtering to produce loss mode resonance (LMR). The energy of the center core has been shown to be able to be coupled into the suspended core, taking advantage of the small distance involved and phase matching between two cores. The 10 mm long HADCF can be used to form a directional coupler. LMR and the directional coupler are integrated in parallel on the same section of HADCF, which realizes the high integration of sensing measurement. The sensor achieves simultaneous measurement of both RI and temperature through measuring both the LMR dip and coupling dip. The RI and temperature sensitivities achieved were 3812.5 nm/RIU and 521.4 pm/°C, respectively. The sensor discussed shows important advantages of high sensitivity, simple fabrication, high integration, and the ability to make measurements of two key parameters.

A comparative analysis of surface plasmon resonance and fluorescence detection methods employing molecularly imprinted polymers revealed their distinct capabilities in codeine detection, offering valuable insights for diverse applications requiring sensitive and specific drug analysis.

Efficient monitoring of sewage systems networks and their pump stations is vital to avoid environmental spills and enhance customer safety. This paper proposes the integration of optical fibre sensors (OFSs) into an Internet of Things (IoT) platform for better monitoring of such sewage networks. The work addresses key challenges such as scalability and interoperability of an IoT platform that supports seamless OFS-to-Cloud connectivity. The proposed platform leverages compression algorithms and edge devices to optimize data communication. This platform is based on 28 different OFSs that have been set up in a laboratory environment to simulate a similar set up in a sewage network. This work achieved 50% compression ratio for the data collected from the OFSs and Amazon Web Services (AWS) S3 cloud system is used for data storage. This work is conducted in collaboration with Sydney Water.

Even though many different designs for currently available, fluorescence-based fiber optic sensors for measuring oxygen concentration (O2) are well known (and indeed some are commercially available), they often are limited by their response time and long-term stability. This will cause problems in the important industrial applications of fiber optic sensors of this type that are developing, with limitations that are evident, for example, in physiology and other fields where rapid sensor responses are required. Research by a number of groups has discussed various new designs of fiber optical sensors, which have been developed in recent years where the key features of such probes to achieve the performance required are, for example, optimization of design features such as tip shape and coating layer thickness. The research reported in this paper represents an evaluation of such key factors to allow the design of better fiber optic-based sensors for oxygen measurement, where the optimized performance of a new, specially tapered tip O2 sensor designed has been compared with the output of conventional and commercially available probe designs. The performance of a group of such sensors has been analyzed and cross-compared, examining the key features of such a probe including sensor accuracy, response time and overall long-term stability, as well as cross-sensitivity to any temperature changes which may occur in ‘real’ measurement situations.

A compact fiber Bragg grating (FBG)-based strain sensor has been developed by embedding an FBG inside a 3D-printed structure, allowing the comparison of FBG responses across different filaments such as polylactic acid (PLA), thermoplastic polyurethane (TPU), polycarbonate (PC), acrylonitrile butadiene styrene (ABS), and nylon. Results have shown that FBG embedded in TPU can be effective in the measurements of mechanical strain, giving a responsivity value of 17.70 pm/cm with outstanding linearity of 98 %. Furthermore, small-scale field testing conducted in below-ground environments has shown that strain sensors based on FBG embedded in TPU are the most effective. They offer a responsivity of 13.9 pm/kg with a small standard deviation and high linearity. Additionally, they have the highest temperature sensitivity value of 15.4 pm/degrees C compared to the other embedded FBGs. Therefore, for most industrial applications, the FBG embedded in TPU can be considered as an alternative to existing embedment methods for strain sensing applications.

This paper reports the outcome of the research jointly developed and implemented by Sydney Water and City, University of London (City) to create innovative sensing solutions to long term monitoring of wastewater pump stations (PS) which form part of a network owned and operated by Sydney Water (SW). This has been achieved through recognising the potential of photonic devices for use in this environment and thus designing, fabricating and installing innovative optical fibre sensors, (achieving a strain resolution as low as ~1 microstrain), in two representative pump stations chosen by SW (and designated PS072 and PS001). This innovative sensor system has been complemented by conventional cover meter inspection for reinforcement identification. Being sensitive to the structural differences between the two sites chosen, the scientific rationale behind the sensor design, including the sensor positioning, the number of sensors installed, and the types of sensors required has been justified in detail in this paper. This contextualises the sensor data analysis and the key information extracted from the sensor systems designed and installed, thereby allowing better diagnosis of their respective structural operational conditions. Through this type of synergy of sensor data and structural integrity assessment, the key achievements of the work have been to create not just a continuous data-stream but allow accurate assessment of strain and vibration conditions of the structure in both time and position, through real time in situ monitoring and thus to warn of the potential for structural failure and collapse – the prime objective of the work.

This article proposes a novel highly sensitive humidity sensor with temperature compensation, mainly consisting of a special modal interferometer bonded with thick polyimide (PI) solid films. The modal interferometer is constructed by splicing a special dual-mode fiber (DMF) with two sections of regular single mode fiber (SMF), forming an SDS (SMF-DMF-SMF) structure. Due to the particular design of the refractive index of the DMF, only LP01 and LP02 modes are transmitted, which causes a unique wavelength (UWL) to appear in the spectrum. The UWL breaks the periodicity of the conventional interferometric spectrum. More importantly, both sides of UWL respond differently to various measurands, which enables the interferometer to measure dual parameters simultaneously. To achieve a higher humidity sensitivity, the thick PI solid films are used for the first time to manufacture the interferometric humidity sensors. Their thickness is at least one order of magnitude greater than that of the PI coating used in the traditional humidity sensors, which enhances the sensor's response to humidity variation significantly. The performance of the proposed sensor is further improved with the aid of the varied period Vernier effect by paralleling a bare dual modal interferometer based on the SDS structure as a reference. Similar to the sensing modal interferometric spectrum, the output spectrum of the varied period Vernier effect scheme owns the UWL as well, which makes it easy to distinguish the wavelength shifts of the specific peaks to the measurands. The validity of the proposed scheme is verified by an experimental case, where the humidity sensitivity and the temperature sensitivity are achieved as 1044.26 pm/%RH and 1467.99 pm/degrees C, respectively.

Previous research has identified the capabilities of artificial whisker sensor arrays as an innovative method of tracking and evaluating flow events and disturbances. In this work, a new approach is put forward, focusing on achieving enhanced sensitivity and improved performance of fibre-optic whisker sensors, based on the principle of fibre Bragg Grating (FBG) based optical stress sensing. Its performance is evaluated against more simplistic approaches, and previously demonstrated methods of optically tracking the tips of whisker sensors. The study has found the performance and sensitivity of these FBG-based sensors to be very satisfactory, with sufficient sensitivity to bending stresses to enable using Cross-Correlation (CC) and multilateration techniques (as evaluated in prior studies) to produce reliable Direction of Arrival (DoA) and velocity estimations, at a typical SNR of around 2 dB. The system has shown the capability for correction of potentially disruptive variations in temperature, allowing for effective measurement of the key hydrodynamic disturbances under study, irrespective of the local environmental conditions.

This research introduces a novel label-free graphene oxide-coated dual-peak long-period grating sensor for measuring protein concentrations. It achieves high sensitivity (22 nm/(g/mL)) and a low limit of detection (0.9 µg/mL), demonstrating extremely high potential for fiber optic biosensor development.

Swimming microrobots guided in the circulation system offer considerable promise in precision medicine but currently suffer from problems such as limited adhesion to blood vessels, intensive blood flow, and immune system clearance—all reducing the targeted interaction. A swimming microrobot design with clawed geometry, a red blood cell (RBC) membrane–camouflaged surface, and magnetically actuated retention is discussed, allowing better navigation and inspired by the tardigrade’s mechanical claw engagement, coupled to an RBC membrane coating, to minimize blood flow impact. Using clinical intravascular optical coherence tomography in vivo, the microrobots’ activity and dynamics in a rabbit jugular vein was monitored, illustrating very effective magnetic propulsion, even against a flow of ~2.1 cm/s, comparable with rabbit blood flow characteristics. The equivalent friction coefficient with magnetically actuated retention is elevated ~24-fold, compared to magnetic microspheres, achieving active retention at 3.2 cm/s, for >36 hours, showing considerable promise across biomedical applications.

Self-running piezoelectric robots have the advantages of being low cost, high load ratio and fast speed of operation, as well as showing few limitations in confined spaces or for underwater applications. Controlling the speed of motion of such robots by adjusting the standing wave ratio (SWR) along the vibration plate of the robot is important. Compared to the conventional dual-mode excitation method which is based on the adjustment of the excitation frequency, f , a novel SWR-based control method, using the adjustment of the temporal phase shift, θ , has been first derived by the authors. It has been found that the traveling wave component could be maximized using both methods, either by setting the value of f to the root mean square of the two adjacent modal frequencies, or by programming θ to have a sum of π when added to the spatial phase difference. It can be seen that using the “ θ -based” traveling wave control method, smoother motion and higher resolution of the motion speed is achieved. In this article, by using the novel θ -based method to drive the robot, its motion characteristics, such as voltage-speed, load capacity and ability to move on different surface materials, have been tested through a series of experiments carried out and reported.

An optical-based pressure sensor for a 150 x 150 mm surface was designed and fabricated. The sensor utilizes a fiber Bragg grating (FBG) attached to a 30 x 30 x 30 mm actuator as the pressure sensing mechanism. The middle section of the actuator, which is circular, can bend into an elliptical form and, in the process, pull the FBGP via both ends when force or pressure is applied, thus converting the pressure applied to its surface into a wavelength shift. In laboratory testing, a sensitivity of 0.152 nm / kPa was obtained. Subsequently, the pressure sensor was tested in the field by burying it 20 cm underground to measure soil pressure, while another FBG was spliced in series to the FBGP to compensate for temperature variations. Testing shows that the proposed design can realize a compact optical-based pressure sensor with enhanced soil monitoring applications such as dynamic soil pressure caused by soil movement.

Shape sensing is of importance for the manipulation of flexible needles. In this work, a 0.6 mm diameter stylet with five Fiber Bragg Gratings (FBGs) installed as triplets was designed and implemented and a novel model of local curvature was established. A gradient-based optimization method has been integrated into the complete algorithm for shape sensing and this was used to reduce the difference between the midpoint curvature and the mean curvature. The experimental results obtained show that the mean tip errors are 0.35 mm, 0.30 mm, 0.38 mm in the single-bending, double-bending and space-bending experiments, respectively. In a torsion test experiment which was performed, when the rotation angle of the tip was less than 25 degrees, the error seen was less than 0.5 mm. Furthermore, when the needle designed in this work was used to puncture a 60 mm thickness, ex vivo biological tissue, the mean error of the measurement of the needle tip was 0.39 mm.

A fiber Bragg grating (FBG)-based inclinometer probe with enhanced sensitivity has been developed for slope or ground movement monitoring. The inclinometer probe utilized six FBGs for the tilt measurement and a strain -free FBG that provided the temperature compensation factor. The inclinometer probe was fabricated entirely using a 3D printer and can fit into the standard inclinometer casing, which can be placed into the boreholes. The dimension of the probe is similar to the conventional inclinometer probe, with a total length of 70 cm. Additionally, this design was equipped with three highly compact tilt sensors within the same probe length, providing a better resolution of the inclination profile. Each tilt sensor possesses a flexible middle shaft fabricated using thermoplastic polyurethane (TPU) and was equipped with two FBGs for bi-directional tilt angle measurement (+x and-x). Initially, the tilt sensor was calibrated in the laboratory, which yielded a sensitivity value of 0.0215 nm/degrees. This value is higher than most previous designs by a factor of two because of the middle shaft's elasticity, which can induce a more significant strain on the FBG. The horizontal displacement of a conventional inclinometer casing could be observed during the field test, which proves the device's functionality. The results have indicated that the inclinometer can be applied in several geotechnical applications, particularly ground movement monitoring.

A surface-mounted tilt sensor was designed and fabricated to measure the inclination angle of engineered structures or slopes in two directions. The device utilizes two strain-sensitive fiber Bragg gratings (FBGs) for tilt angle measurement bidirectionally and one strain-free FBG to provide temperature compensation. In this work, a tilt sensor prototype was fabricated using a 3-D printer, with a robust enclosure and a miniature actuator with dimensions of $115\times 65\times30$ mm and $45\times 20\times $ 3 mm, respectively. The device was first calibrated in the laboratory for tilt and temperature parameters. For tilt calibration, the device yields a sensitivity value of 0.0135 and 0.0123 nm/° for + x- and– ${x}$ -directions. On the other hand, the device delivers a sensitivity value of 0.0105 nm/°C as the response to temperature changes. The tilt sensor was also tested for suitability in a real-field deployment where it was installed on a retaining wall and was left for four weeks. The field test data indicate no vertical displacement of the wall since the device exhibits zero inclination changes during the test period. This compact, robust, and easy-to-install tilt sensor has excellent potential for various geotechnical applications, mainly in landslide detections, ground movement, and engineered slope monitoring.

Polarizers are an essential optical element for tailoring the polarization state of electromagnetic waves in a wide range of optical devices. Such polarizers, which exhibit a wide operating bandwidth and high performance, are attracting increasing attention, due to their extensive prospects for use in applications ranging from polarization imaging, to optical communications and detection, among others. However, achieving both broadband performance and ultra-high extinction ratio (ER), and that simultaneously, is still challenging in the design of effective polarizers. To tackle that demand, in this work, an Au-on-silica grating structure has been proposed as the basis of the design of a miniaturized high-efficiency polarizer that practically can cover the entire visible and near-infrared spectral ranges. The single-layer polarizer thus designed can show an ER of 60 dB in this spectral domain, and it has been shown that the geometrical parameters selected have a significant effect on the performance characteristics of the polarizer. Furthermore, an ER of ∼150 dB could be achieved merely by regulating the thickness of the grating to achieve the optimum performance. By integrating the high-performance polarizer proposed in this work with an optical fiber “meta-tip,” a refractive polarizer with a value of the ER of >45 dB, and that over the entire spectral domain considered, has been demonstrated. Such an approach offers an alternative route to achieving a broadband, powerful, and flexible processing polarizer design.

We present a novel FBG-based pressure sensor for detection of pressure waves associated with pipe leaks. It includes the sensor design concept, sensor calibration in a pilot plant scale rig that mimics a water distribution network and the simulated pipeline leakage tests.