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In this paper we present long-term stability results for a Fabry-Perot optical fibre sensor in high temperatures environments. We introduce an intrinsic Fabry-Perot type of sensor, which its sensing element is made with an undoped pure fused silica cavity. Furthermore, we will present results detailing dopant diffusion from core to cladding of standard optical fibres demonstrating their inherent unsuitability for high temperature environments. Finally, we display the manufacturing technique required to produce this sensor and we present results of 33 days long stability tests conducted at elevated temperatures of more than 900°C.

We present high temperature (~1100°C) stability tests of, Ge-doped and F-doped, optical fibre sensors. Our analysis includes the variation in their behaviours within high temperature environments and how the dopant diffusion affects their long term stability.

A smart metal component having the potential for high temperature strain sensing capability is reported. The stainless steel (SS316) structure is made by selective laser melting (SLM). A fiber Bragg grating (FBG) is embedded in to a 3D printed U-groove by high temperature brazing using a silver based alloy, achieving an axial FBG compression of 13 millistrain at room temperature. Initial results shows that the test component can be used for up to 700°C for sensing applications.

Applications of fibre optic sensors at high temperatures have gained a huge interest recently, as they appeared to be suitable for temperature recording in harsh environments. In this paper, we are demonstrating two intrinsic Fabry-Perot (F-P) fibre optic sensors for high temperature monitoring. The sensors are consisting of a 125μm diameter single mode fibre (SMF28) and a 125μm diameter PCF ESM-12B pure fused silica fibre spliced to a SMF28, respectively. The result was a low finesse optical SMF-Cr-SMF, and SMF-Cr-PCF, sensor with cavity lengths varying from 50μm to 100μm. Both types of Fabry-Perot sensors were tested in a tube furnace over a temperature range from room temperature up to 1100°C. Following a number of annealing cycles, between the above mentioned temperatures range, very good repeatability of the phase response was achieved. During the cycling process, thermal stress relief takes place which makes the sensors suitable for temperature testing at temperatures just in excess of 1000°C. After initial cycling the sensors are subjected to long term stability tests. The phase response is stable, less than 4°C, over a period of 5 days at a temperature of 1050°C for both sensors. The temperature resolution is around 3°C.

Smart metal having integrated high temperature sensing capability is reported. The SS316 structure is made by additive layer manufacturing via selective laser melting (SLM). Sensor component is embedded in to the structure during the SLM build process. The strain isolated in-fiber Fabry-Perot cavity sensor measures temperature up to 1100 °C inside the metal.

Focused Ion Beam (FIB) machining has been demonstrated to be capable of fabricating nano and micro scale elements onto optical fibres. In this paper we exploit FIB to fabricate core aligned 45° mirrors at the end of multi-core fibres (MCF). The resulting fibre is used as a component in a two dimensional optical fibre accelerometer. The mirror is produced using a two step process: first a scanning process is used to make a rough cut to define the overall mirror structure. This is followed by a polishing process to create an optical surface finish. The machined 45° mirror can be accurately aligned with optical fibre core, which avoids issues associated with the alignment of external turning mirror components. Proof-of-concept tests demonstrate the use of such a fibre as a two axis acceleration sensor that is interrogated interferometrically. The sensor operated between 0.5g and 4.5g with a cross talk of -24.3dB between axes.

Fiber Bragg Grating (FBG) sensors are embedded into Stainless Steel (SS) 316 components using bespoke Selective Laser Melting (SLM) technology. SS 316 material is added on substrates by SLM, incorporating U-shaped grooves with dimensions suitable to hold nickel coated optical fibers. Coated optical fibers containing fiber Bragg gratings for strain monitoring are placed in the groove. Melting subsequent powder layer on top of the fiber completes the embedding. Strain levels exceeding 3 me are applied to specimens and are measured by embedded fiber optic sensors. Elastic deformation of the steel component is reliably measured by the Bragg grating from within the component with high accuracy. During plastic deformation of the steel the optical fiber is slipping due to poor adhesive bonding between fused silica and metal surround.

We present the fabrication, characterization and encapsulation of a fibre optic temperature sensor based upon a micro Fabry-Perot (F-P) cavity. The F-P cavity is formed between a reflective in-fibre metallic splice and the air-fibre boundary at the end of the sensor. A change in temperature modifies the optical cavity length and this is observed as a shift in the reflected interference spectrum. The temperature resolution of the sensor is

Micro-cantilever sensors have shown great promise in a wide range of application are as including chemical and biological sensing. However, many of these devices are based upon a sensor ‘chip’ that requires careful alignment between the cantilever and the read-out system, which can be challenging. Furthermore, optical interrogation typically involves a bulky free-space system. Optical fibre addressed cantilevers have been reported previously in the literature and in this paper we propose techniques to design and fabricate polymer micro-cantilevers for attachment onto the end of standard single mode fibres using laser machining. Low-cost optical sources and a fibre coupled spectrometer are employed to monitor the cantilever deflection and therefore observe biological binding between a species of interest and an activated cantilever. Proof-of-concept experiments show that the sensor is capable of detecting pathogen concentration with down to a level of 10 5 cfu/ml.

Metal clad single mode optical fibres containing high reflectivity Bragg gratings are successfully embedded into stainless steel components using bespoke laser based additive manufacturing technology. Temperature cycling of the embedded sensor demonstrates the ability of gratings to survive the embedding process, and act as temperature or strain sensors. At extreme temperatures up to 500 °C, discontinuities in the gratings response are observed and these have been attributed to possible delamination issues at the interface between glass fiber and the surrounding metal.

Additive manufacturing or 3D printing of structural components in metals has potential to revolutionise the manufacturing industry. Embedded sensing in such structures opens a route towards SMART metals, providing added functionality, intelligence and enhanced performance in many components. Such embedded sensors would be capable of operating at extremely high temperatures by utilizing regenerated fibre Bragg gratings and in-fibre Fabry-Perot cavities.

Focussed Ion Beam (FIB) machining has been demonstrated to be capable of fabricating nano and micro-structure elements. In this paper we demonstrate techniques to design and fabricate a 45° mirror on the end of both conventional single mode and multi-core fibres (MCF) using FIB processing. The mirror is finished by a two step process: first a scanning process is used to make a rough cut followed by a polishing process to create an optical surface finish. The machined 45° mirror can be accurately aligned with optical fibre core, which avoids issues associated with the alignment of external turning mirror components. Proof-of-concept tests demonstrate that the fabricated structure is capable of measuring two axis displacements interferometrically with a maximum displacement up to 1.0mm and an rms error of ~50nm.

Metal clad single mode optical fibres are successfully embedded into stainless steel using a layer by layer based additive manufacturing technology based on selective laser sintering of powered stainless steel material. The single mode fibre has been coated with a protective Ni metal layer to facilitate thermal protection and bonding to the stainless steel matrix. A loss of ~1 dB is observed after embedding which is attributed to micro bending in the embedded section.

In this work we present the delivery of high energy Er:YAG laser pulses operating at 2.94 μm through a hollow-core negative curvature fibre (HC-NCF) and a hollow-core photonic crystal fibre (HC-PCF) and their use for the ablation of biological tissue. In HC-NCF fibres, which have been developed recently, the laser radiation is confined in a hollow core and by an anti-resonant or reflection principle (also known as ARROW). Both fibres are made of fused silica which has high mechanical and chemical durability, is bio-inert and results in a fibre with the flexibility that lends itself to easy handling and minimally invasive procedures. The HC-NCF structure consists of only one ring of capillaries around a realtively large core, followed by a protecting outer layer, hence the preform is relatively easy to build compared to traditional HC-PCF. The measured attenuation at 2.94 μm is 0.06 dB/m for the HC-NCF and 1.2 dB/m for the HC-PCF. Both fibres have a single mode output beam profile which can be advantageous for surgical applications as the beam profile is maintained during fibre movement. We demonstrate delivery of high energy pulses through both fibres, well above the thresholds needed for the ablation of biological tissue in non-contact and contact mode. Delivered energy densities reached > 750 J/cm after 10 m of HC-NCF and > 3400 J/cm through a 44 cm HC-PCF.

Micro-cantilevers are one of the most popular Micro-Electronics-Mechanical-Sensor (MEMS).They've demonstrated in a number of application areas such as chemical and bio-sensing. However, these devices usually need the alignment of the cantilever with the read-out system, which can be challenging. Furthermore, it involves a bulky free-space optical detection system. In this paper, we propose techniques to design and fabricate micro-cantilevers onto the end of standard single mode fibres using a picoseconds (ps) laser machining technique. In this way the cantilever can be aligned with the core of the fibre therefore offering stable and accurate means of optically addressing the cantilever. Low-cost optical sources and fibre coupled spectrometers are employed to interrogate the final cavity with a resolution around 15nm. Experiment show this optical fibre cantilever can be used as a displacement sensor with a dynamic range up to 7μm. Proof-of-concept experiments demonstrate that the cantilever could also be used as a temperature sensor in the range of 24-320°C with a temperature sensitivity of 0.5°C. © (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

We investigate the phase matching conditions and sensitivities of higher order metal jacketed long period gratings (LPGs). These higher order modes have been previously demonstrated to have flatter, and therefore more sensitive, phase matching conditions leading up to the phase matching turning point. We demonstrate this increased sensitivity as applied to a Pd jacketed LPG hydrogen sensor illustrating an improvement in both the refractive index and temperature sensitivity (of the 17th order mode) of an order of magnitude over the lower order (1st-9th) modes. © (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Presently, there is no truly flexible delivery system for light from Er:Yag medical lasers (λ = 2.94 μm) which allows surgeons to work unrestricted. Instead, either a relatively inflexible articulated arm or multi-mode fibre, limited to large bend radii, must be used. One proposed solution is the use of novel types of hollow core - band gap optical fibre rather than more traditional large area solid core fibres. In these silica based fibres, material absorption and damage limitations are overcome by using a photonic band gap structure. This confines radiation to lower order modes, that are guided in a small diameter air core. The overall fibre diameter is also smaller, which allows a smaller mechanical bend radius. Together with the guidance in air, this improves the laser power damage threshold. However, there are many practical hurdles that must be overcome to achieve a robust system for use in surgery. One of the main problems is that the fibre structure is hollow and ingress of dust, vapour, fluids and other contaminants need to be prevented to ensure safe in-vivo usage. Additionally, any infibre contamination will degrade the laser damage resistance of the fibre leading to potential catastrophic failure. The development of a robust and hermetically sealed end cap for the fibre, without adversely affecting beam quality or damage threshold is an essential prerequisite for the safe and efficient use of such fibres in surgery. In this paper we report on the progress on implementing end caps and describe novel methods of sealing off these hollow fibres in particular for surgical applications. This work will demonstrate that the use of these superior fibres with low loss guidance at 2.94 μm in surgery is feasible.

We demonstrate for the first time true distributed sensing of scale and location of transverse loading along an extended length of polarisation maintaining [PM] fibre with high spatial resolution using Optical Frequency Domain Reflectometry [OFDR]. Transverse loading of PM fibres induces local and differential changes in refractive index in slow and fast axis, depending on scale and direction of load relative to the axes. The change in refractive index is detected by monitoring the change in local periodicity of the statically distributed Rayleigh scatter for each PM fibre axis simultaneously using OFDR.

We demonstrate a new and highly flexible technique for high speed dynamic interrogation of fiber Bragg gratings [FBG]. The technique is based on a ratiometric measurement employing a pulsed broad band light source, a single detector and 2 filter FBGs. The filter FBGs are mounted on a temperature controlled cantilever where the strain of the filter FBGs can be adjusted to requirement. The technique can be used on multiplexed FBGs and has been demonstrated here at a frequency of 20kHz although much higher frequencies are possible with a light source of higher power. Sub micro-strain resolution of 0.6 μe at 23 kHz is demonstrated and the noise for strain e(=ΔL/L) is 5•10-9e√Hz-1.

Conventionally optical parametric oscillators (OPOs) have been used in high-resolution absorption-spectroscopy as narrow-band tuneable sources where the measurement resolution is determined by the OPO output linewidth, rather than the wavelength resolution of the detector. In contrast, the absorption spectroscopy of gases and other media has for many years been carried out using instruments such as Fourier-transform infrared (FTIR) spectrometers or high-resolution diffraction-grating-based tuneable monochromators. These techniques commonly utilise broadband thermal sources with highly-divergent illumination beams limiting their use in remote sensing or fibre delivery applications. The work presented here reports a new approach to FTIR spectroscopy based around a novel Ti:sapphire pumped, signal-resonant OPO that uses a 10mm crystal of aperiodically-poled lithium niobate (APPLN) as the gain medium producing an idler output covering a 3.2-3.85μm tuning range with a typical full-width-half-maximum bandwidth of 85nm. Methane was used to demonstrate the technique since the OPO tuning range almost completely covers the strongest mid-infrared absorption lines of methane from 3.0-3.7μm (limited only by the available resonator optics). A double-beam Michelson interferometer was built around the OPO idler beam using a helium-neon laser as the second beam to self-calibrate each trace. Course tuning of the OPO resulted in the measurement of absorption data across the 3.2-3.85μm tuning range using methane held at pressures ranging from 2000mbar down to 25mbar. A maximum resolution of around 1cm -1 was achieved using a simple rapidly scanning mirror assembly indicating that with further development this approach could yield very high-resolution measurements.

Palladium and its alloys show a high and selective affinity for hydrogen, resulting in a volume expansion. Fibre Bragg gratings attached to palladium elements are used to monitor the resultant strain from the hydrogen uptake. The technique is aimed at low concentration hydrogen monitoring to a few hundred ppm, where H2 is the result of ageing of polymer materials. We demonstrate a sensor exhibiting 10 ppm sensitivity and report on observed cross sensitivity to environmental conditions.

In this paper we describe absolute distance measurement through an optically dense transparent polymer, using a fibrebased, broadband interrogated, Fabry-Perot interferometer. The dual fibre sensor demonstrates a system that provides 'on-line' compensation for temperature and stress effects through recovery of refractive index information. Over a typical sensing range of 1mm distance measurements with sub-micron accuracy are reported.

We describe an optical system to monitor small long-term changes in the shape of a surface by using a network of optical fibre Bragg grating strain gauges, for applications in which space does not permit the use of techniques such as photogrammetry or structured light methods. Gratings are bonded to copper beryllium strips held under tension in contact with the test surface. The copper beryllium strips enable sufficient force to be transferred to the optical fibre from the compliant surface. Shape changes are revealed as strain changes in the sensor strips, inferred from wavelength shifts in the Bragg peaks. The optical signals are obtained in reflection by illuminating the sensor fibres with a broadband source and using a scanning Fabry-Perot filter to generate the spectrum with a wavelength resolution of 0.3 pm over the range 1530 to 1570 nm. Laboratory tests show that a strain resolution of 8 microstrain can be achieved with temperature compensation over the range 20 to 50 C, with a multiplexing capability of between 11 and 16 temperature - strain sensor pairs, depending on temperature gradients on the test surface. We present experimental measurements on a cylindrical test object subject to diametral loading, and show a comparison with a finite element model.