This paper looks to evaluate the performance of combined low-coherence and confocal refractometry under conditions of reduced refractive index contrast. The instrument measures the refractive index and thickness of transparent objects using a fibre-based low-coherence interferometer with a line-scan spectrometer. A sample was designed that mimics the on-axis structure of the eye, consisting of a lens tube holding a pair of windows surrounded on either side by fluid chambers and sealed at the far end by a diffuse black metallic plug. Sucrose solutions with a range of concentrations were injected into the fluid chambers, providing a linear variation in refractive index of 1.3330 to 1.4416. The instrument was used to simultaneously measure both the phase and group refractive indices np and ng, as well as the physical thickness t of the windows and the fluid in the chambers. Although the measurement accuracy is shown to decrease with reduced refractive index contrast, it remains better than 0.6% over all measured components for sucrose concentrations of up to 60.3%, which is close to the saturation limit.

Optical fibre strain and shape measurement sensors were deployed on two bearingless main rotor systems, Airbus Helicopters H135 and Airbus Helicopters H145 (or BK117 D-3) during ground runs with controlled pilot inputs and during a whirl tower test. The sensing capabilities of two optical fibre-based strain sensing techniques, optical fibre Bragg grating (FBG) and fibre segment interferometry (FSI), and direct fibre optic shape sensing (DFOSS), a shape measurement based on the FSI approach, were benchmarked against conventional strain gauge measurements. Signal-to-noise ratios and modal properties were determined from the collected strain and displacement signatures using an improved operational modal analysis suitable for the removal of amplitude-modulated rotor harmonics and large rotor speed variations. It was shown not only that all fibre-optic based sensing techniques provide detailed understanding into the dynamic properties of the blade, but the measurements also offer insights into couplings from the airframe to the rotor and couplings from the drive train to the rotor. Results and discussions of the analysis of the measurements from the DFOSS system highlight its benefits over strain gauges or of the FBG strain sensing approach.

Ever-increasing demands to improve fuel burn efficiency of aero gas turbines lead to rises in fuel system pressures and temperatures, posing challenges for the structural integrity of the pump housing and creating internal deflections that can adversely affect volumetric efficiency. Non-invasive strain and vibration measurements could allow transient effects to be quantified and considered during the design process, leading to more robust fuel pumps. Fuel pumps used on a high bypass turbofan engine were instrumented with optical fibre Bragg grating (FBG) sensors, strain gauges and thermocouples. A hydraulic hand pump was used to facilitate measurements under static conditions, while dynamic measurements were performed on a dedicated fuel pump test rig. The experimental data were compared with the outputs from a finite element (FE) model and, in general, good agreement was observed. Where differences were observed, it was concluded that they arose from the sensitivity of the model to the selection of nodes that best matched the sensor location. Strain and vibration measurements were performed over the frequency range of 0 to 2.5 kHz and demonstrated the ability of surface-mounted FBGs to characterise vibrations originating within the internal sub-components of the pump, offering potential for condition monitoring.

The development and experimental performance of instrumentation to measure ageing-related spectroscopic changes in bitumen is described. Oxidation of bitumen at the surface increases the number of carbonyl (C=O) bonds, and this can be measured in the 6 μm region (1700 cm-1) of the mid-infrared. Standoff measurements of surface reflectivity were performed using 4 discrete wavelengths, 3 for the carbonyl absorption and the fourth as a spectral reference. The standoff height of 20 cm caused problems resulting from the presence of numerous strong absorption lines of atmospheric water in the optical path, which was solved by use of wavelengths centred within available "water windows" and a pathlength-matched reference channel. The instrument was tested using bitumen samples aged artificially using UV exposure. Results illustrating the instrument's response to bitumen age, along with tolerance to changes in height and tilt, are shown. Measurements made during preliminary field trials on outdoor asphalt are also demonstrated. Part 1 of this paper describes the scientific challenges involved in designing this instrument.

A novel approach to distributed humidity sensing using polyimide-coated optical fibres and fibre segment interferometry (FSI) is demonstrated. FSI interrogates the cumulative strain along fibre segments formed between pairs of ultrashort FBG reflectors, with measurement sensitivity scaling with segment length. Polyimide, a hygroscopic coating material, induces strain on the optical fibre in response to humidity changes. Experiments were conducted using a serial array of fibre segment interferometers, comprising three 10 mm long fibre segments, with polyimide coatings on two segments and the third uncoated for control. The polyimide-coated segments exhibit strong and consistent phase responses to changes in relative humidity (RH), with responsivities of 0.049 +/- 0.05 rad/mm/%RH and 0.043 +/- 0.05 rad/mm/%RH, respectively over the 30% RH range tested. The uncoated segment displayed no measurable response to humidity.

Laser additive manufacturing uses a high power laser to melt a metal feedstock into a desired shape, which is then allowed to solidify. Layers of deposition are built up a few millimetres at a time to manufacture the desired part. Monitoring of the deposited layers allows for adjustment to the deposition parameters in future layers which is critical for completing the build of an acceptable, quality-controlled part of near net-shape. The height of the deposited layer is one of the most important physical parameters. Developed at Cranfield University, Range resolved interferometry (RRI) is an approach that applies wavelength modulation of a low-cost diode light source, and interferometry between the metal surface of the newly built part and the optical fibre tip to measure layer height, whilst also rejecting the incoherent light scattered from the high power laser. Using RRI the distance between the metal surface and the fibre tip can be determined across a range of tens-of-centimetres. Compared to Optical Coherence Tomography systems this operating range is much greater and the instrument cost is substantially lower. Here initial results are presented using an RRI system integrated into a commercial laser processing head applied to laser wire directed energy deposition laser additive-manufacture wall-building.

Accurate temperature measurement is a key parameter that determines the quality of additive manufactured components in directed energy deposition processes. Optical pyrometers which are used to provide in-process temperature data require accurate emissivity data of the metal surface. Process-specific emissivity data for metals used in these processes is not readily available. This paper provides the emissivity of a variety of metals used in wire-arc directed energy deposition processes. For the first time, the test samples were fabricated using typical deposition processes and systems. The metals evaluated were titanium alloy (Ti-6Al-4V), Inconel 718, mild steel, aluminum alloy 2319, and nickel aluminum bronze. At ambient temperature, the measured normal emissivity was 0.26–0.28 for Ti-6Al-4V; for Inconel 718, it was 0.45–0.54; for mild steel, it was 0.4–0.72; for aluminum 2319, it was 0.14; and for nickel aluminum bronze, it was 0.35. The approximate emissivity values are also given over the temperature range 20–1400 °C. The effect of residual oxygen in the shield gas on emissivity is explored for the first time. The spectrophotometric technique was used to measure the metal thermo-optical properties.

A strategy is described to make in-situ measurements of a spectroscopic marker of ageing in bitumen binders used on asphalt-paved roads. Oxidation of bitumen at the surface increases the number of carbonyl (C=O) bonds, and this can be measured in the 6 μm region (1700 cm −1 ) of the mid-infrared. A measurement strategy is proposed to make standoff measurements of surface reflectivity in this region, despite the challenge presented by numerous strong absorption lines from atmospheric water vapour within the optical path. An instrument design is described to make measurements at 4 discrete laser wavelengths, namely 1593.0, 1641.4 and 1731.3 cm −1 (around 6 µm) and at 2633.6 cm −1 (3.8 µm), the first 3 responding to carbonyl absorption and the fourth acting as a spectral reference that is substantially unaffected by ageing. Part 2 of this paper describes the implementation of such an instrument and its experimental performance.

Measurements from optical fibre interferometers incorporated within a superconducting magnet coil, recorded during energization, are presented. The interferometers were formed between low reflectivity (0.01%), broad bandwidth (10 nm) in-fibre reflectors and were interrogated using the principles of range resolved interferometry, where the retuned signals from the interferometers addressed by a sinusoidally modulated DFB laser diode were demodulated using a quadrature processing technique. The demonstrated low noise (0.3 n epsilon/root Hz), high resolution measurement capability indicates the significant potential for fibre segment interferometry for multiplexed strain measurement in cryogenic systems.

A spectrometer built using an external cavity pulsed quantum cascade laser is described. The spectrometer has a tuning range from 10 – 13 µm (1,000 – 769 cm −1 ) and is designed to target volatile organic compounds (VOCs) which often exhibit water-free molecular absorption within the region. The spectrometer utilizes a hollow silica waveguide gas cell which has an internal volume of a few millilitres, a fast response time (∼1 s), and is advantageous when only low sample volumes, similar to the cell volume, are available. Propane is used as a test gas because it is easy to handle, and its spectral profile is comparable to VOCs of interest. Its absorption in the region is primarily within the ν 21 band which spans from 10.55 – 11.16 µm (948 – 896 cm −1 ). Spectral measurements at a range of concentrations show good linearity and an Allan deviation of absorbance values recorded over a 100-minute period indicates a minimum detectable absorbance of 3.5×10 −5 at an integration time of 75 s.

We present a two dimensional angle sensor with greater than 0.01 degrees resolution, using dual-wavelength laser speckle. The technique uses speckle correlation of a double speckle pattern, determining absolute surface tilt from a single frame. A constructed sensor was used to measure a grid of 2D angles between +/- 0.25 degrees rotating around both x and y axes. Sensor performance was limited by laser mode instability effects, however an analysis of measurements demonstrated that a stable mode output could achieve bias errors of less than 0.010 degrees, and standard deviations of 0.003 degrees around x and 0.021 degrees around y.

Additive manufacturing (AM) or three-dimensional (3D) printing stands out for its remarkable ability to manufacture custom-designed preforms for specialty silica optical fibres (SOFs) featuring sophisticated structures and diverse material compositions. Here, a novel scheme for manufacturing preforms for highly birefringent multi-core silica optical fibres (Hi-Bi MC SOFs) is proposed and tested using specially formulated dual-curing resins with AM technologies. These resins, incorporating nano composites (NCs), are ultraviolet (UV) and thermally cured to form fibre preforms in respective AM processes. Sample silica fibre preforms are successfully fabricated with holey cladding (Ti-doped, UV cured), multiple cores (Ge-Ti co-doped, thermally cured) and stress applying parts (SAPs, B-Al co-doped, thermally cured). As confirmed by X-ray diffraction (XRD) tests, these preforms can be consolidated into clear amorphous silica with the required structure and strength, demonstrating the potential of the proposed scheme in developing preforms for specialty fibres with custom-designed structures and materials required for sensing applications.

This paper presents a method utilising the speckle pattern formed by dual-wavelength illumination for the measurement of the two out-of-plane surface angles with respect to the sensor frame. Theoretical expressions are derived relating the observed speckle shift between patterns formed by two wavelengths for tilted surfaces with both on-axis and off-axis detector positions. These expressions are verified experimentally, showing RMS errors of between 0.5-1.0 mu m. Finally, an on-axis implementation of the concept is presented using dual-wavelength illumination generated from two modes of a standard FP diode laser. Simplified expressions for the calculation of surface angles from measured speckle shift using this arrangement are presented, given in terms of three sensor constants; the responsivity or sensitivity of the sensor, C, and the zero surface tilt speckle shifts, A(x0) and A(y0). Experimental results using this sensor for a range of surface tilts between 0.0 degrees and 7.5 degrees showed an RMS error of 0.10 degrees in theta(x) and 0.19 degrees in theta(y).

Part quality monitoring and control in wire-based directed energy deposition additive manufacturing (w-DEDAM) processes has been garnering continuous interest from both the academic and industrial sectors. However, maintaining a consistent layer height and ensuring that the wall height aligns closely with the design, as depicted in computer-aided design (CAD) models, pose significant challenges. These challenges arise due to the uncertainties associated with the manufacturing process and the working environment, particularly with extended processing times. To achieve these goals in an industrial scenario, the deposition geometry must be measured with precision and efficiency throughout the part-building process. Moreover, it is essential to comprehend the changes in the interlayer deposition height based on various process parameters. This paper first examines the behaviour of interlayer deposition height when process parameters change within different wall regions, with a particular focus on the transition areas. In addition, this paper explores the potential of geometry monitoring information in implementing interlayer wall height compensation during w-DEDAM part-building. The in-process layer height was monitored using a coherent range-resolved interferometry (RRI) sensor, and the accuracy and efficiency of this measurement were carefully studied. Leveraging this information and understanding of deposition geometry, the control points of the process parameters were identified. Subsequently, appropriate and varied process parameters were applied to each wall region to gradually compensate for wall height. The wall height discrepancies were generally compensated for in two to three layers.

Range resolved interferometry (RRI) applied to the interrogation of an extrinsic Fabry-Perot based pressure sensor in laboratory and wind tunnel environments is presented. A simple, compact sensor head design was fabricated and subsequently characterised using RRI, which was shown to have a sensitivity of 1.627 x10(-3) rad Pa-1 with a noise standard deviation of 9 Pa over a data rate of 1.5 kHz. When installed in a high-lift wing for surface pressure evaluation during wind tunnel testing, the approach outlined here was able to perform as well as a conventionally employed commercial device for relative static pressure measurements.

The strain sensing performance of an optical fibre Bragg grating (FBG), demodulated using a state-of-the-art commercial interrogator, is compared directly with that of a fibre segment interferometer (FSI) of near equivalent gauge length. It is shown that the FSI segment displayed a 5 fold improvement in noise, 1n ε /Hz for a bandwidth of 1 kHz, compared to 5n ε /Hz at 1 kHz to that of the FBG. Furthermore, by exploiting a longer gauge length, it was also demonstrated that this could further reduce noise levels.

We present a sensor for gas detection utilising an integrating sphere and interband cascade laser for tunable diode laser absorption spectroscopy (TDLAS). Measurements were made of the methane absorption line at 3313 nm using an integrating sphere with an effective pathlength of 70.1 cm. Gas concentrations were estimated from normalised absorption spectra with line fitting to 7 absorption lines within the scan. Testing showed that measurements were linear for methane in synthetic air for concentrations in the range 0-50 ppm. With an averaging time of 20 s, the noise-equivalent methane concentration was 180 ppb (1 & sigma;). The system requires minimal alignment, with preliminary measurements indicating the system is insensitive to vibration and misalignment of the laser input. The lack of alignment optics also allows it to be compact (190x170x120mm) and robust.

This paper describes the steps taken to improve the measurement speed of a combined low-coherence and confocal refractive index measurement system. The instrument measures the refractive index and thickness of transparent plates using a fibre-based low-coherence interferometer with a line-scan spectrometer. The spectrometer allows on-line dispersion measurement which is necessary to derive the sample thickness t as well as both the phase and group refractive indices n p, n g. The measurements were performed on a sample with six surfaces consisting of three glass windows mounted in a lens tube. Experimental results show that a measurement time of 4.4 s for the multi-layered object, which has a total thickness of approximately 10.5 mm, can be achieved whilst maintaining an accuracy of better than 0.1% for n p, n g, and t. This represents an approximately hundredfold improvement over previously published measurements.

In this paper, we present a novel technique for stabilisation of widely wavelength modulated lasers (>100pm) over long time scales, where modulation depths exceed the spectral width of standard reference features, such as gas absorption lines, by over an order of magnitude. The technique operates by controlling the temporal separation between successive appearances of a gas absorption line on the up and down sweeps of a sinusoidal laser wavelength modulation waveform. The influence of the signal distortions introduced by the laser intensity modulation that are associated with laser diode injection current modulation are also addressed. The technique is applied to a range-resolved interferometric system interrogating a Mach-Zehnder interferometer operating in thermally stable conditions, using an absorption feature from a fibre-coupled gas cell as a reference. Proof-of-principle measurement results achieved using this technique are presented, demonstrating a notional fractional stability of 3.9×10−7 without further correction.

In this paper we report the application of a laser speckle odometer to a mobile industrial robot in a typical factory floor environment. The suitability of typical floor surfaces and features is assessed in terms of the ability to form speckle patterns with sufficient signal to noise for correlation-based processing. All tested surfaces including concrete, rubber tile, dried paint and oil stains, and hazard tapes were found to be suitable. A comparison of the velocimetry sensor output to the industrial robot’s internal SLAM and wheel encoder data is presented with good agreement of < 0.3mm/s at tested speeds of up to 250mm/s. Finally, a comparison of speckle odometry to the robot’s internal SLAM based navigation will be presented using a laser tracker to provide ground-truth measurement data. Both techniques were found to perform similarly, with errors of up to 80mm when traversing a 16m square path of 4m sides. The laser speckle odometry was however found to perform significantly better over the initial sides of the path with a maximum error of < 10mm in comparison to < 47mm for the robot’s internal odometry.