The fabrication of micro-tapered optical fibres for sensing applications

This thesis describes the processes used to manufacture optical fibre tapers and tapered long period gratings (TLPGs) using a CO2 laser. A semi-automated system for fabricating adiabatic and non-adiabatic tapers with repeatable physical dimensions has been developed. The tapers had waist diameters which were reproducible to within ± 0.5 μm. This system has also been used to fabricate TLPGs with periods ranging from 378 μm to 650 μm. Novel techniques to monitor the process of fabricating tapers were also explored. These techniques included; monitoring the transmission of the fibre using a spectrophotometer, using an in-line fibre Bragg grating (FBG) to measure the strain experienced by the optical fibre and the use of a near infra-red (NIR) camera to aid fibre alignment and laser power optimisation. The spectrophotometer allowed the optical properties of the tapers to be tailored for specific applications and the FBG provided strain data for process optimisation. The use of a NIR camera and an FBG as an in-line strain sensor are a novel use of these devices in a fibre tapering process. Tapers were also thin-film coated using sputtering techniques to form surface plasmon resonance sensors and their refractive index sensitivity was measured. A novel protein sensor based on gold nanoparticles deposited on a fibre taper is also reported, together with a lossy mode resonance taper sensor. The TLPGs which were fabricated, comprised of between 6 to 18 periods. The refractive index sensitivity of a 6 period TPLG was measured and was 372 nm/ RI. Their resonance bands had twice the bandwidth and exhibited a higher extinction, compared to UV-written long period gratings of a similar number of periods

Monitoring techniques for the manufacture of tapered optical fibers

The use of a range of optical techniques to monitor the process of fabricating optical fiber tapers is investigated. Thermal imaging was used to optimize the alignment of the optical system; the transmission spectrum of the fiber was monitored to confirm that the tapers had the required optical properties and the strain induced in the fiber during tapering was monitored using in-line optical fiber Bragg gratings. Tapers were fabricated with diameters down to 5 μm and with waist lengths of 20 mm using single-mode SMF-28 fiber

Ammonia sensing using lossy mode resonances in a tapered optical fibre coated with porphyrin-incorporated titanium dioxide

The development of an ammonia sensor, formed by the deposition of a functionalised titanium dioxide film onto a tapered optical fibre is presented. The titanium dioxide coating allows the coupling of light from the fundamental core mode to a lossy mode supported by the coating, thus creating lossy mode resonance (LMR) in the transmission spectrum. The porphyrin compound that was used to functionalise the coating was removed from the titanium dioxide coating upon exposure to ammonia, causing a change in the refractive index of the coating and a concomitant shift in the central wavelength of the lossy mode resonance. Concentrations of ammonia as small as 1ppm was detected with a response time of less than 1min. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only

An ammonia sensor based on Lossy Mode Resonances on a tapered optical fibre coated with porphyrin-incorporated titanium dioxide

The development of a highly sensitive ammonia sensor is described. The sensor is formed by deposition of a nanoscale coating of titanium dioxide, containing a porphyrin as a functional material, onto a tapered optical fibre. The titanium dioxide coating allows coupling of light from the fundamental core mode to a lossy mode supported by the coating, thus creating a Lossy Mode Resonance (LMR) in the transmission spectrum. A change in the refractive index of the coating caused by the interaction of the porphyrin with ammonia causes a change in the centre wavelength of the LMR, allowing concentrations of ammonia in water as low as 0.1 ppm to be detected, with a response time of less than 30 s

Wavelength-locking of a semiconductor laser using an electronic technique

This work describes a novel system to control the stability of a 1583 nm telecommunications laser diode via measurement of junction voltage. This electronic technique dispenses with the optical components used in conventional wavelength locking schemes and shifts wavelength control to system level electronic instrumentation. The approach employs real-time measurement of diode series resistance (Rs), which is used to compensate the measured forward voltage (Vf) and recover the junction voltage (Vj) of the laser. Control of Vj provides wavelength control without introducing a significant error when the package temperature varies. This was implemented by measuring Rs as the dynamic resistance, δV/δI, by modulating the injection current. Recent work has reduced the modulation amplitude and noise in the electronics. Using a frequency deviation of 1 GHz, we achieved a centre wavelength variation of ± 2 pm over a package temperature variation of 20-55 °C. This gives a wavelength/ temperature coefficient of 0.03 pm/ °C, which is an improvement on 0.34 pm/ °C, as typically achieved for optical locking systems. The system has been further developed using board-level components within a compact demonstrator unit. Work is on-going to further enhance this performance over a package temperature variation of 0-70 °C

Novel highly sensitive protein sensors based on tapered optical fibres modified with Au-based nanocoatings

Novel protein sensors based on tapered optical fibres modified with Au coatings deposited using two different procedures are proposed. Au-based coatings are deposited onto a nonadiabatic tapered optical fibre using (i) a novel facile method composed of layer-by-layer deposition consisting of polycation (poly(allylamine hydrochloride), PAH) and negatively charged SiO₂ nanoparticles (NPs) followed by the deposition of the charged Au NPs and (ii) the sputtering technique.The Au NPs and Au thin film surfaces are then modified with biotin in order to bind streptavidin (SV) molecules and detect them. The sensing principle is based on the sensitivity of the transmission spectrum of the device to changes in the refractive index of the coatings induced by the SV binding to the biotin. Both sensors showed high sensitivity to SV, with the lowest measured concentration levels below 2.5 nM. The calculated binding constant for the biotin-SV pair was 2.2×10‾¹¹ M‾¹ when a tapered fibre modified with the LbL method was used, with a limit of detection (LoD) of 271 pM. The sensor formed using sputtering had a binding constant of 1.01 × 10‾¹⁰ M‾¹ with a LoD of 806 pM. These new structures and their simple fabrication technique could be used to develop other biosensors

Fibre-optic measurement of strain and shape on a helicopter rotor blade during a ground run: 1. Measurement of strain

Optical fibre strain and shape measurement sensors were deployed on a 5-m long rotor blade during a full-speed (rotation rate 6.6 Hz) helicopter ground run, with real-time data wirelessly streamed from rotor hub-mounted sensor interrogators. In Part 1 of a 2-part paper series, the strain sensing capabilities of the two optical fibre-based sensing techniques, optical fibre Bragg grating (FBG) and fibre segment interferometry (FSI), are compared, while Part 2 (Kissinger et al 2022 Smart Mater. Struct. accepted) specifically investigates the blade shape measurement based on the FSI approach. In part 1, the rotor hub-mounted instrumentation is described, and data on the dynamics of the blade obtained from a sequence of controlled pilot inputs are analysed both in the time and spectral domains. It is shown that this can provide insights into the aeroelastic properties of the blade. Noise standard deviations of 0.2 n√ ε/ Hz and 30 nε/ Hz for the FSI and FBG-based sensing approaches, respectively, were observed over a strain range of 3500 µepsilon

Application of fibre optic sensing systems to measure rotor blade structural dynamics

This paper compares two fibre optic sensing techniques for vibration characterisation: (a) optical fibre Bragg grating (FBG) strain gauges and (b) a novel direct fibre optic shape sensing (DFOSS) approach based on differential interferometric strain measurements between multiple fibres within the same fibre arrangement. Operational mode shapes and frequency measurements of an Airbus Helicopters H135 bearingless main rotor blade (5.1 m radius) were acquired during a series of ground vibration tests undertaken in a controlled laboratory environment. Data recorded by the fibre optic instrumentation systems were validated using commercially available accelerometers and compared against a baseline finite element model. Both fibre optic sensing systems proved capable of identifying the natural frequencies of the blade in the frequency range of interest (0–100 Hz). The data from the FBG sensors exhibited a dependency on their position relative to the neutral axes of the blade, which meant that full characterisation of the flapping and lagging modes required careful consideration of sensor location in the chordwise direction. The DFOSS system was able to identify all structural dynamics, despite being located on the neutral axis in the lagging direction, due to its sensitivity to angle changes, rather than strain, and its biaxial measurement capability. The DFOSS system also allowed the operational mode shapes of the blade to be determined directly, without the requirement for strain transfer from the blade to the sensor and without the requirement for a model of the underlying structure. The accuracy of obtained natural frequencies and operational mode shapes is assessed, demonstrating the potential of the use of both fibre optic sensing systems for determining blade structural dynamics

Bladesense – a novel approach for measuring dynamic helicopter rotor blade deformation

Technologies that allow accurate measurement of rotorblade dynamics can impact almost all areas of the rotorcraft sector; ranging from maintenance all the way to blade design. The BladeSense project initiated in 2016 aims to take a step in developing and demonstrating such a capability using novel fibre optic sensors that allow direct shape measurement. In this article the authors summarise key project activities in modelling and simulation, instrumentation development and ground testing. The engineering approach and associated challenges and achievements in each of these disciplines are discussed albeit briefly. This ranges from the use of computational aerodynamics and structural modelling to predict blade dynamics to the development of direct fibre optic shape sensing that allows measurements above 1kHz over numerous positions on the blade. Moreover, the development of the prototype onboard system that overcomes the challenge of transferring data between the rotating main rotor to the fixed fuselage frames is also discussed