Ricardo Correia

A pressure sensor based upon the transverse loading of a sub-section of an optical fibre Bragg grating

An experimental and theoretical study of locally transverse loaded fibre Bragg gratings (FBGs) for pressure sensing purposes is presented. When a load is applied to a short section of an FBG, a spectral hole is generated in the reflection spectrum, which exhibits a redshift in wavelength as the load is increased. This effect has been modelled using Rouards method and has been characterized experimentally. Two techniques for analysing the spectrum for pressure measurements at constant temperature are considered, one based on measurement of the wavelength shift of the spectral hole, and the other on measurement of peak reflectivities either side of the hole. Normalized pressure sensitivities of 3.30 ± 7.93 × 10−2 MPa−1 and 4.97 × 10−4 ± 1.37 × 10−5 MPa−1 were obtained for the reflectivity and wavelength measurement, respectively. A pressure resolution of 1.3 kPa, over a range of 745 kPa, was achieved when using a tunable laser of 1 pm wavelength resolution to interrogate the sensor. For pressure measurements in the presence of temperature changes, a technique based on the measurement of the position of the spectral hole within the Bragg envelope is presented. This technique allows the opportunity to discriminate pressure and temperature using a single FBG sensor element.

Development and application of optical fibre strain and pressure sensors for in-flight measurements

Fibre optic based sensors are becoming increasingly viable as replacements for traditional flight test sensors. Here we present laboratory, wind tunnel and flight test results of fibre Bragg gratings (FBG) used to measure surface strain and an extrinsic fibre Fabry–Perot interferometric (EFFPI) sensor used to measure unsteady pressure. The calibrated full scale resolution and bandwidth of the FBG and EFFPI sensors were shown to be 0.29% at 2.5 kHz up to 600 μe and 0.15% at up to 10 kHz respectively up to 400 Pa. The wind tunnel tests, completed on a 30% scale model, allowed the EFFPI sensor to be developed before incorporation with the FBG system into a Bulldog aerobatic light aircraft. The aircraft was modified and certified based on Certification Standards 23 (CS-23) and flight tested with steady and dynamic manoeuvres. Aerobatic dynamic manoeuvres were performed in flight including a spin over a g-range −1g to +4g and demonstrated both the FBG and the EFFPI instruments to have sufficient resolution to analyse the wing strain and fuselage unsteady pressure characteristics. The steady manoeuvres from the EFFPI sensor matched the wind tunnel data to within experimental error while comparisons of the flight test and wind tunnel EFFPI results with a Kulite pressure sensor showed significant discrepancies between the two sets of data, greater than experimental error. This issue is discussed further in the paper.

Fibre Bragg grating based effective soil pressure sensor for geotechnical applications

An effective-soil-pressure sensor for geotechnical applications based on Fibre Bragg Gratings is presented. The sensor simultaneous measures total soil pressure and pore pressure, allowing the calculation of the effective stress of soil. Calibration of the sensor using pressurised air demonstrated a pressure sensitivity of 2.02x10-3 ± 2.84x10-5nm/kPa and 1.87x10-3 ± 6.88x10-5nm/kPa for the total and pore pressure respectively. This corresponds to a pressure resolution of 4.95x10-1kPa and 5.46x10-1kPa for total and pore pressure using a 1pm interrogation system. Measurements undertaken in two types of soil demonstrated dependence of the total pressure sensitivity on soil density/stiffness. Pore pressure measurements agreed well with the preliminary calibration.

Fiber Segment Interferometry for Dynamic Strain Measurements

Using a novel range-resolved interferometric signal processing technique based on the sinusoidal optical frequency modulation of a cost-effective laser diode, a fiber sensing approach termed fiber segment interferometry (FSI) is described. In FSI, a chain of long-gauge length fiber optic strain sensors are separated by identical in-fiber partial reflectors. Targeted at dynamic strain analysis and ultrasound detection for structural health monitoring, this approach allows integrated strain measurements along fiber segments, removing the sensing gaps and sensitivity to inhomogeneities found with localized fiber sensors. In this paper, the multiplexing of six fiber segments, each of length 12.5 cm, is demonstrated. The sensor array can be interrogated at 98 kHz data rate, achieving dynamic strain noise levels ≤ 0.14 nϵ · Hz-0.5. The reflector fabrication is discussed, an analysis of linearity and noise performance is carried out and results from an exemplar experiment to determine the speed-of-sound of a stainless steel rod are shown.

Enhanced sensitivity fibre Bragg grating (FBG) load sensor

The characterization of a load sensor based on the transverse loading of a subsection of a fibre Bragg grating (FBG) embedded within a cube of epoxy resin is presented. When the epoxy resin cube is loaded transverse to the axis of the fibre, its deformation transduces the load to a strain along the axis of the optical fibre, which changes the period of the embedded section of the FBG. This creates a spectral dropout within the bandwidth of the FBG, with an absolute wavelength that is linearly dependent on the applied load. This technique enhances the sensitivity of the FBG to transverse loading by a factor of 15, to 2.9 × 10−2 ± 0.01 nm N−1, when compared to the direct transverse loading of a bare fibre, and also protects the fibre from mechanical damage at the loading point.

Simultaneous temperature and humidity measurements in a mechanical ventilator using an optical fibre sensor

An optical fibre sensor for simultaneous temperature and humidity measurements consisting of one fibre Bragg grating (FBG) to measure temperature and a mesoporous film of bilayers of Poly(allylamine hydrochloride)(PAH) and silica (SiO2) nanoparticles deposited onto the tip of the same fibre to measure humidity is reported. The hygroscopic film was created using the layer-by-layer (LbL) method and the optical reflection spectra were measured up to a maximum of 23 bilayers. The temperature sensitivity of the FBG was 10 pm/°C while the sensitivity to humidity was (-1.4x10-12 W / %RH) using 23 bilayers. The developed sensor was tested in the mechanical ventilator and temperature and humidity of the delivered artificial air was simultaneously measured. Once calibrated, the optical fibre sensor has the potential to control the absolute humidity as an essential part of critical respiratory care.

Fabrication of fiber optic long period gratings operating at the phase matching turning point using an amplitude mask

A fast method for the fabrication of the long period gratings (LPG) optical fibres operating at or near the phase matching turning point (PMTP) with the period of 109.0, 109.5 and 110.0 μm based on an amplitude mask writing system is described. The proposed system allows fabricating 3 cm long LPG sensors operating at PMPT within 20 min that is approximately 8 times faster than point-by-point approach. The reproducibility of the fabrication process was thoroughly studied. The response of the fabricated LPGs to the external change of the refractive index was investigated using water and methanol.

In-situ Curing Strain Monitoring of a Flat Plate Residual Stress Specimen Using a Chopped Stand Mat Glass/Epoxy Composite as Test Material

The curing stresses in a newly proposed bi-axial residual stress testing configuration are studied using a chopped strand mat glass/epoxy specimen. In-situ monitoring of the curing is conducted using dielectric and fibre Bragg grating sensors. It is confirmed that a bi-axial residual stress state can be introduced in the specimens during curing and a quantification of its magnitude is presented. An alternative decomposition method used for converting the dielectric signal into a material state variable is proposed and good agreement with models found in the literature is obtained. From the cure cycles chosen it is suggested that any stress build up in the un-vitrified state is relaxed immediately and only stress build up in the vitrified state contributes to the residual stress state in the specimen.

A fibre Bragg grating-based inclinometer system for ground movement measurement

The measurement of ground movement is an essential part of many geotechnical engineering operations. For decades, inclinometer systems have traditionally been used for this purpose to provide crucial information to engineers and researchers. However, conventional inclinometer systems have their limitations, such as high cost and poor durability. In this paper a fibre optic based inclinometer system is reported, which utilizes fibre Bragg grating sensors attached to the casing of a conventional inclinometer. The characterization of the sensor revealed good agreement with theory and conventional displacement measurements. For a casing of length 2.5 m, the minimum measureable deflection of the top of the casing was found to be 0.48 mm when the FBGs were interrogated with a 1 pm wavelength resolution system.

Passive Sensors for Long Duration Internet of Things Networks

In this work, three different concepts are used to develop a fully passive sensor that is capable of measuring different types of data. The sensor was supplied by Wireless Power Transmission (WPT). Communication between the sensor and reader is established by a backscatter, and to ensure minimum energy consumption, low power techniques are used. In a simplistic way, the process starts by the transmission of two different waves by the reader to the sensor, one of which is used in power transmission and the other of which is used to communicate. Once the sensor is powered, the monitoring process starts. From the monitoring state, results from after processing are used to modulate the incoming wave, which is the information that is sent back from the reader to the tag. This new combination of technologies enables the possibility of using sensors without any cables or batteries to operate 340 cm from the reader. The developed prototype measures acceleration and temperature. However, it is scalable. This system enables a new generation of passive Internet of Things (IoT) devices.