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Optical fibre strain and shape measurement sensors were deployed on a rotor blade during a full-speed helicopter ground run, with real-time data wirelessly streamed from rotor hub-mounted sensor interrogators. In part 2 of a 2-part paper series, two-dimensional direct fibre-optic shape sensing (DFOSS), using fibre segment interferometry-based interrogation is investigated. The concept of blade shape change visualisation over one rotation period using rotation displacement surfaces is introduced and the usefulness of DFOSS data to gain additional insights by determining operational modal frequencies independently for both horizontal and vertical vibration directions of the blade is demonstrated.

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 n ϵ / Hz for the FSI and FBG-based sensing approaches, respectively, were observed over a strain range of 3500 µϵ.

Displacement measuring interferometers, commonly employed for traceable measurements at the nanoscale, suffer from non-linearities in the measured displacement that limit the achievable measurement uncertainty for microscopic displacements. Two closely related novel non-linearity correction methodologies are presented here that allow for the correction of non-linearities in cases where the displacement covers much less than a full optical fringe. Both corrections have been shown, under ideal conditions, to be capable of reducing all residual non-linearity harmonics to below the 10 pm level. Engineering and Physical Sciences Research Council (EPSRC) EP/R511894/1 (Project 2199198). Department for Business, Energy and Industrial Strategy; Royal Academy of Engineering Research Fellowship F\201718\174.

We describe the first use of range-resolved interferometric signal processing for measurement of spectral transmission. This was applied to gas sensing using tunable diode laser spectroscopy, allowing the simultaneous and independent measurement of methane concentrations in multiple gas cells. The system uses a single injection-current modulated diode laser and a single photodetector. For three gas cells, we show the ability of the system to measure methane at noise equivalent concentrations of less than 200 ppm for a 0.5 s measurement period and a potential noise equivalent concentration (1σ) of

Laser standoff Instrumentation has been developed to measure surface reflectivity of UV-aged bitumen at four specific mid-IR wavelengths. Preliminary measurements reveal heterogeneity across individual bitumen samples and an overall trend with bitumen age.

High sensitivity pressure measurements are reported using two intrinsic optical fibre methods: Fibre Bragg Gratings and Fibre Segment Interferometry. The optical fibres sensors are surface mounted onto a composite membrane and respective sensitivities of 79.2 µɛ/kPa and 65.8 µɛ/kPa have been achieved at resolutions of 0.79 µɛ and 0.04 µɛ.

Chemical sensors based on optical absorption require accurate knowledge of the optical pathlength of the sample cell. Integrating spheres offer increased pathlengths compared to single pass cells combined with tolerance to misalignment, making them attractive for use in challenging environments subject to vibration. However, the equivalent optical pathlength can be degraded by dirt and / or condensation on the inner surface of the sphere. We present a new scheme for in-situ calibration that uses a ratiometric two-beam approach. Results are presented for an integrating sphere used in the measurement of methane by tunable diode laser spectroscopy (TDLS) at 1651nm. Reduced sphere reflectivity was simulated by applying small areas of black tape on the inner surface. At methane concentrations of 1500ppm and 3125 ppm, for areas of contamination up to 2.3% of the sphere wall, the technique reduced the error from over 50% to within ±4%. At a concentration of 6250 ppm and the most severe fouling corresponding to 2.9% wall coverage, the technique reduced the error from 55-65% to within ±11%.

Multiplexed optical fibre sensors were embedded into a carbon-fibre-reinforced-preform during the industrial production of a full-sized, one-piece tail cone assembly for a regional jet aircraft. Optical fibre Fresnel sensors monitored both the infusion of the resin, via measurement of the refractive index-dependent attenuation in the reflected light signal, and the degree of cure of the resin, via measurement of the chemical cure reaction-dependent change in refractive index. The resin cure was also monitored by optical fibre Bragg gratings (FBGs) fabricated in high linearly birefringent optical fibre, which measured through-thickness strain development, while FBGs in standard single mode optical fibre measured longitudinal strain development. The magnitudes and profiles of the transverse and longitudinal strains developed during the curing process were consistent across different locations on the tail cone. Typical transverse and longitudinal strains, related to cure reaction-induced shrinkage, were −1500 ± 17 μepsilon and −500 ± 5 μepsilon, respectively. Post-production, the same embedded FBG sensors were used subsequently to monitor structural strains when the tail cone was subjected to vacuum pressure loading. The longitudinal strains measured using the embedded FBG sensors were generally in good agreement with the longitudinal strains measured by the surface-bonded resistance foil strain gauge (RFSG) sensors, both qualitatively and quantitatively. The in-plane transverse and circumferential strains, oriented collinearly, were measured by the embedded FBGs and appropriately oriented surface-bonded RFSG sensors, respectively, and were, qualitatively, in good agreement

We have investigated the release of gases and volatile organic compounds (VOCs) from a carbon fibre reinforced epoxy composite matrix used in aircraft structural components. Analysis was performed at several temperatures both up to and above the recommended operating temperature (121 °C) for the material, to a maximum of 250 °C. Gas chromatography-mass spectrometry (GC-MS) combined with thermal desorption (TD-GC-MS) was used to identify and quantify VOCs, and in parallel real-time gas detection with commercial off-the-shelf (COTS) gas sensors. Under hydrocarbon free air, CO, SO2, NO, NO2 and VOCs (mainly aldehydes, ketones and a carboxylic acid) were detected as the gaseous products released during the thermal exposure of the material up to 250 °C, accompanied by increased relative humidity (4%). At temperatures up to 150 °C, gas and volatile emission was limited.

Long period gratings (LPGs) as a sensing platform have the potential for multi-parameter measurement through the utilization of 2nd order coupling resonance bands. Although the current literature has produced LPGs with 2nd order resonance bands, the fabrication parameters required to generate these features have not been elucidated. Here, using UV irradiation via the point-by-point method, it is shown that by varying the duty cycle it is possible to fabricate LPGs that exhibit 2nd, 3rd, and 4th order coupling resonance bands. Fabrication of LPGs with a 25% or 75% duty cycle produces distinct 2nd order resonance bands, which are not observed when a 50% duty cycle is adopted.

This paper reports on the development of a new, non-contact, cost-effective and widely-applicable optical sensor for the measurement of the three degrees of translational freedom of a robotic end-effector. The workpiece positioning sensor (wPOS) tracks the relative position between the workpiece and end-effector in real-time, using laser speckle correlation for the measurement of in-plane position and Range-Resolved Interferometry for out-of-plane positioning. The sensing principles of the techniques and the development of the instrument are discussed along with example results for applications in robotic additive manufacturing for tool speed and layer height measurements.

We describe a combination of Tunable Diode Laser Spectroscopy and Range Resolved Interferometry that enables multipoint gas detection using a single laser and photodetector. Tunable Diode Laser Spectroscopy (TDLS) permits gas detection via the measurement of its optical absorption while a diode laser is tuned through an absorption line. Range Resolved Interferometry (RRI) employs a signal processing technique previously used for position sensing and vibrometry. The principle of operation is the sinusoidal modulation of the emission wavelength of a laser which is coupled into a network of interferometers. The detected signal is demodulated to give the signal amplitude within each interferometer. Signals from different interferometers can be distinguished based on the optical path length difference, enabling continuous interrogation of multiple measurement channels simultaneously. In this work, a number of gas cells are combined with optical fibre to create systems of nested interferometers, including a combination of gas cells and a common local oscillator. The laser wavelength was modulated with a combination of fast sinusoidal (49kHz) and slow (2Hz) ramp wave forms, the ramp sweeping the central wavelength of the laser through the methane absorption line at 1653nm. The intensity signal for each cell was extracted using the RRI technique in order to measure the methane concentration within each cell A Mach-Zehnder topology allows the measurement of methane at concentrations as low as 200ppm. We discuss the potential performance of other topologies that are more suitable for field deployment.

We have evaluated the performance of a range of retroreflective tapes for their potential to improve detection of casualties in maritime search and rescue (SAR). The potential for cockpit distraction in the visible region was also evaluated. Different material samples were placed at a representative distance from an operational UK SAR aircraft under different illumination conditions. Footage was recorded from pilot night-vision goggles (NVGs) and from the different cameras on board operating in the visible, near IR and mid IR regions. Six materials were tested. Two silver coloured, SOLAS approved materials were tested with broadband visible retroreflection, one with microsphere construction, the other with microprismatic construction. Three microprismatic materials were tested that included overlaid filters to colour them red, orange and yellow. Finally, a reflective microprismatic tape was tested that incorporated a visible-light-blocking filter that rendered it black in the visible but reflective in the near infrared. These tests confirmed that pilot perception of the microprismatic silver, orange and yellow materials was highest in both the visible and near IR (700nm - 2.5μm) regions. Cockpit reflectance tests showed that, when applied to an orange suit, colour-matched retroreflectors provide minimal visual distraction from reflection.

A dual beam optical coherence tomography (OCT) instrument has been developed for flow measurement that offers advantages over microscope derived imaging techniques. It requires only a single optical access port, allows simultaneous imaging of the microfluidic channel, does not require fluorescent seed particles, and can provide a millimetre-deep depth-section velocity profile (as opposed to horizontal-section). The dual beam instrument performs rapid re-sampling of particle positions, allowing measurement of faster flows. In this paper, we develop the methods and processes necessary to make 2D quantitative measurements of the flow-velocity using dual beam OCT and present exemplar results in a microfluidic chip. A 2D reference measurement of the Poiseuille flow in a microfluidic channel is presented over a spanwise depth range of 700 m and streamwise length of 1600 m with a spatial resolution of 10 m , at velocities up to 50 m m / s . A measurement of a more complex flow field is also demonstrated in a sloped microfluidic section.

A non-contact speckle correlation sensor for the measurement of robotic tool speed is described that is capable of measuring the in-plane relative velocities between a robot end-effector and the workplace or other surface. The sensor performance has been assessed in the laboratory with sensor accuracies of ±0.01 mm/s over a ±70 mm/s velocity range. The effect of misalignment of the sensor on the robot was assessed for variation in both working distance and angular alignment with sensor accuracy maintained to within 0.025 mm/s (

We describe a simple method for detecting formaldehyde using low resolution non-dispersive UV absorption spectroscopy. A two channel sensor was developed, making use of a strong absorption peak at 339 nm and a neighboring region of negligible absorption at 336 nm as a reference. Using a modulated UV LED as a light source and narrow laser-line filters to select the desired spectral bands, a simple detection system was constructed specifically targeted at formaldehyde. By paying particular attention to sources of noise, a minimum detectable absorbance of $5\times 10^{-5}$ absorbance units (AU) was demonstrated with a 20 s averaging period (as $\Delta I/I_{0}$ ). The system was tested with formaldehyde finding a limit of detection of 4.3 ppm for a 195 mm gas cell. As a consequence of the low gas flow rates used in our test system, a time period of over 8 min was used in further tests, which increased the minimum detectable absorbance to $2\times 10^{-4}$ AU, 17 ppm of formaldehyde. The increase was the result of thermal drift caused by unwanted temperature variation of the UV LED and the filters, resulting in a zero uncertainty estimated at −560 ppm °C−1 and 100 ppm °C−1 respectively.

The process of fabricating tapered long period gratings (TLPGs) using a CO2 laser is described. The objective of this work is to optimize the CO2 laser based fabrication of TLPGs so that their transmission spectra are comparable with those made using the arc-discharge technique, which typically exhibits transmission losses below 2 dB. The losses currently observed with CO2 laser fabricated TLPGs are typically greater than 10 dB over the spectral waveband 650-1100 nm. In this work, the reduction in transmission loss was achieved by optimizing the micro-taper geometry and the duty-cycle of the device. TLPGs with a period of 378 μm were fabricated. The 6-period TLPGs exhibited a pass-band transmission loss of 0.6 dB, resonance band extinction values of 3 dB and had a physical length of 2.27 mm. The refractive index sensitivity of a 6 period TPLG was measured and found to be 372 nm/ RI.

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.

Non-linearities in interferometric displacement measurements commonly affect both homodyne and heterodyne optical interferometers. Unwanted back reflections (ghost reflections) or polarisation leakage introduce non-linearity terms at harmonics of the illuminating wavelength that cannot be fully corrected for with standard non-linearity correction techniques. A two-wavelength interferometric approach, operating at 632.8 and 785 nm, is presented here that is capable of correcting such non-linearities. Non-linearities are separated from the difference between two displacement measurements made at differing wavelengths with a Fourier approach. Compared to a standard Heydemann ellipse fitting correction, the proposed approach reduces estimated residual non-linearities from 84 to 11 pm in the case of a linear displacement profile. In particular this approach is applicable to the correction of higher order non-linearities that are caused by multiple reflections, and that are therefore very sensitive to alignment conditions.

Cranfield University's National Flying Laboratory Centre (NFLC) has developed a Bulldog light aircraft into a flight test facility. The facility is being used to research advanced in-flight instrumentation including fibre optic pressure and strain sensors. During the development of the test bed, Computational Fluid Dynamics (CFD) has been used to assist the flight test design process, including the sensor requirements. This paper describes the development of the Bulldog flight test facility, including an overview of the design and certification process, the in-flight data taken using the installed fibre optic sensor systems and lessons learned from the development programme, including potential further applications of the sensors.

The development of an optical fiber based pressure sensor, created using a Fiber Bragg Grating (FBG) and a glass fiber reinforced polymer membrane for use in aerodynamic applications, is reported. The FBG was co-bonded on the surface of the membrane to measure the strain induced by the deflection of the diaphragm with applied pressure. The sensor exhibited high sensitivities of 56.7 pm/kPa, with a minimum resolution of 21 Pa over a 0 to 35 kPa range. The outputs from a mathematical model, Finite Element analysis and the experimental results are compared and discussed.

A novel suite of instrumentation for the characterisation of materials held inside an air-tight tube furnace operated up to 250 °C has been developed. Real-time detection of released gases (volatile organic compounds (VOCs), CO2, NO, NO2, SO2, CO and O2) was achieved combining commercial off-the-shelf (COTS) gas sensors and sorbent tubes for further qualitative and semi-quantitative analysis by gas chromatography-mass spectrometry coupled to thermal desorption (TD-GC-MS). The test system was designed to provide a controlled flow (1000 cm3 min−1) of hydrocarbon free air through the furnace. The furnace temperature ramp was set at a rate of 5 °C min−1 with 10 min dwell points at 70 °C, 150 °C, 200 °C and 250 °C to allow time for stabilisation and further headspace sampling onto sorbent tubes. Experimental design of the instrumentation is described here and an example data set upon exposure to a gas sample is presented.

The use of optical fibre Bragg gratings (FBGs) and direct fibre optic shape sensing (DFOSS) in the ground vibration testing of a full size helicopter rotor blade has been evaluated, with the performance benchmarked against measurements made using accelerometers. An array of FBGs was used to monitor the amplitudes of the mode frequencies at specific locations along the blade and DFOSS was used to measure directly the shape of the blade and to characterise its modal frequencies. While it was possible to measure modal frequencies using both approaches, DFOSS proved capable of detecting modal frequencies using a single sensor array located along the longitudinal axis of the blade.

Confocal scanning combined with low-coherence interferometry is used to provide remote refractive index and thickness measurements of transparent materials. The influence of lens aberrations in the confocal measurement is assessed through ray-trace modeling of the axial point-spread functions generated using optical configurations comprised of paired aspherics and paired achromats. Off-axis parabolic mirrors are suggested as an alternative to lenses and are shown to exhibit much more symmetric profiles provided the system numerical aperture is not too high. The modeled results compare favorably with experimental data generated using an optical instrument comprised of a broadband source and line-scan spectrometer. Refractive index and thickness measurements are made with each configuration with most mirror pairings offering better than twice the repeatability and accuracy of either lens pairing.

Many applications of precision interferometry suffer from uncertainties in knowing the values of the air refractive index or the laser wavelength and often require the use of complex setups or costly components to mitigate these effects. In this work, using range-resolved interferometric measurements, we show how differential measurements along a single optical beam can be used to perform displacement measurements with drastically reduced systematic errors. In a proof-ofprinciple experiment, a semi-transparent target mounted on a moving stage is placed centrally between a window and a mirror, forming two air paths around the target that are arranged to be of nominally equal length. The evaluation of the interferometric phase signals at multiple locations along the measurement beam then allows the simultaneous measurement of changes in the length of the two air paths on either side of the target. The difference of the air path length changes yields the desired target displacement measurement, while influences that are common to both air paths such as air refractive index changes or laser wavelength drift are strongly suppressed.

A non-contact optical technique employing dual-wavelength laser speckle is investigated for absolute angle measurement. The approach uses the separation of the speckle patterns formed by two closely spaced illumination wavelengths to determine the angle of a surface. Autocorrelation is performed on a single exposure containing both speckle patterns to find their relative displacement, which is directly related to the absolute surface angle. This absolute angle determination offers an advantage over previous techniques using laser speckle that require a reference image. The underlying theory linking the speckle pattern displacement and the surface angle is presented, along with a proof-of-concept sensor. Experimental results from the sensor confirm the validity of the theory, with measurements demonstrating a mean difference from applied angles of 0.136°.

In this work, using our range-resolved interferometry (RRI) signal processing technique, we present a novel approach to multidimensional displacement measurements using only a single optical access port and very simple optical setup. By utilising surface reflections from a stage-mounted moving beamsplitter and two orthogonal stationary reference mirrors, two interferometers for the two Cartesian measurement directions are formed. With RRI, the interferometric phase signals of both interferometers can be independently demodulated, allowing simultaneous measurements of displacement in both dimensions using a single continuous-wave laser diode source and a single photodetector. In this paper, the capabilities of this approach are demonstrated using a proof-of-concept experiment with a multidimensional Piezoelectric stage performing a variety of stage movements. Measurements of displacements over a nominal stage working range of ±50μm are presented, demonstrating independent, simultaneous displacement measurements of two dimensions. The presented measurements show nanometer-level displacement resolutions with typical noise densities of 0.02 nm/√Hz over a 21 kHz bandwidth. It is thought that this approach could offer an interesting alternative to existing interferometric techniques for multi-dimensional metrology, benefiting from both simplicity and cost-effectiveness whilst maintaining the advantages that make optical techniques attractive to scientific and industrial applications.

Low-coherence interferometry is combined with confocal scanning to provide remote refractive index and thickness measurements of transparent materials. The influence of lens aberrations in the confocal measurement is assessed through investigation of the axial point-spread functions (APSFs) generated using optical configurations comprised of paired aspherics and paired achromats. Off-axis parabolic mirrors are suggested as an alternative to lenses and are shown to exhibit much more symmetric APSFs provided the system numerical aperture is not too high. Refractive index and thickness measurements are made with each configuration with most mirror pairings offering better than twice the repeatability and accuracy of either lens pairing.

This paper investigates the application of feature tracking algorithms as an alternative data processing method for laser speckle instrumentation. The approach is capable of determining both the speckle pattern translation and rotation and can therefore be used to detect the in-plane rotation and translation of an object simultaneously. A performance assessment of widely used feature detection and matching algorithms from the computer vision field, for both translation and rotation measurements from laser speckle patterns, is presented. The accuracy of translation measurements using the feature tracking approach was found to be similar to that of correlation-based processing with accuracies of 0.025&ndash, 0.04 pixels and a typical precision of 0.02&ndash, 0.09 pixels depending upon the method and image size used. The performance for in-plane rotation measurements are also presented with rotation measurement accuracies of &lt, 0.01 found to be achievable over an angle range of &plusmn, 10 and of &lt, 0.1 over a range of &plusmn, 25 , with a typical precision between 0.02 and 0.08 depending upon method and image size. The measurement range is found to be limited by the failure to match sufficient speckles at larger rotation angles. An analysis of each stage of the process was conducted to identify the most suitable approaches for use with laser speckle images and areas requiring further improvement. A quantitative approach to assessing different feature tracking methods is described, and reference data sets of experimentally translated and rotated speckle patterns from a range of surface finishes and surface roughness are presented. As a result, three areas that lead to the failure of the matching process are identified as areas for future investigation: the inability to detect the same features in partially decorrelated images leading to unmatchable features, the variance of computed feature orientation between frames leading to different descriptors being calculated for the same feature, and the failure of the matching processes due to the inability to discriminate between different features in speckle images.

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.

We present a novel multi-dimensional positional encoder approach that uses range-resolved interferometry (RRI) to simultaneously interrogate multiple interferometers differing in optical path length, using only a single photo detector and a single diode laser. Employing only stage-mounted non-polarising beamsplitters and reflections from three orthogonal, externally mounted, planar mirrors, three Cartesian measurement directions are formed, that when combined with measurements from a stage-mounted reference reflection, allow for three-dimensional measurements of stage motion obtained via a single access port. Through a variety of movements of a three-dimensional Piezo-electric stage over its nominal stage working range of ± 50 μm, simultaneous measurements of displacements from three orthogonal dimensions are acquired with a 10 kHz quadrature bandwidth, achieving typical displacement noise densities of below 0.4 nm/ Hz , with the noise floor dropping to below 0.1 nm/ Hz above 50 Hz, as well as high linearity with cyclic errors amplitudes well below 1 nm in all three directions.

A pressure sensor specified for aerodynamic applications and based on optical fibre strain sensors mounted on a circular glass fibre reinforced polymer membrane is presented. The use of two fibre optic strain sensing technologies is explored, the novel intrinsic fibre segment interferometry (FSI) approach and fibre Bragg gratings (FBGs), with the use of FSI shown to offer a pressure resolution that is 15 times larger than that achieved using an FBG. A number of design and fabrication issues are considered, including the position of the fibres relative to the neutral axis of the membrane and the influence of the membrane support structure on the thermal and pressure sensitivities of the sensor, with particular regards to pressure and temperature discrimination.

The use of polarizing optics for both beam steering and phase measurement applications in displacement measuring interferometer designs is almost universal. Interferometer designs that employ polarizing optics in this manner are particularly sensitive to the effects of unwanted optical cavities that form within the optics due to polarization leakage and back reflections from material interfaces. Modeling techniques commonly employed in the design of such interferometers are poorly suited to the analysis of multiple passes through polarizing optics. A technique, along with an accompanying software implementation, is presented here that is capable of modeling the propagation of monochromatic plane waves through an arbitrary network of linear planar optical components.

A novel dual beam Optical Coherence Tomography (OCT) system is presented for microfluidics applications. The dual sensing-light-sheet approach presented here achieves high velocity flow measurement because the rapid re-imaging by the second sensing-light-sheet allows for particle tracking between each image-frame of the pair. We have demonstrated measurements of microchannel flow using 10 μm particles in a 700 μm deep channel with peak velocities of approximately 0.28 metres-per-second (m/s), approximately two orders of magnitude higher than previously reported OCT systems operating at a few millimeters-per-second.

When used as samples cells for optical absorbance measurements, integrating spheres offer increased pathlengths compared to single pass cells combined with tolerance to misalignment. This makes them attractive during alignment of optical instruments and in challenging environments subject to vibration. However, integrating spheres can suffer problems when used in sensitive and / or accurate absorbance measurement. We present our work to date to address these issues in high resolution laser spectroscopy. Firstly, optical interference effects include both random laser speckle and structured interference fringes created by optical feedback to the laser. Secondly, the sphere’s optical pathlength is a combination of multiple paths that take an exponential pathlength distribution. At low values of absorbance, the measured signal is linear with concentration, but at higher absorbances signals follow a nonlinear but predictable function of absorbance. Thirdly, our most recent work concerns calibration of the optical pathlength, which is a sensitive function of its internal reflectivity. In-situ calibration is needed if the sphere is to be used in dirty environments or with condensing samples. Measurements from multiple independent sources and / or detectors are combined to provide compensation from fouling of the sphere walls and windows. Results are presented for an integrating sphere used in the measurement of methane. The emission from a tunable DFB laser at 1651nm was tuned across the gas absorption line to measure its concentration. Reduced sphere reflectivity was simulated by applying small areas of black tape on the inner surface. Finally, we give an example of one application where our results are being put into practice: use of an integrating sphere with a tunable laser at 3.3μm to measure atmospheric methane, installed on a two seater light aircraft.