Browsing by Author "Ford, Helen D."
Now showing 1 - 20 of 30
Results Per Page
Sort Options
Item Open Access 3D planar velocity measurements, using Mach-Zehnder interferometric-filter-based planar Doppler velocimetry (MZI-PDV) and imaging fibre bundles(2008-12-31T00:00:00Z) Lu, Z. H.; Charrett, Thomas O. H.; Ford, Helen D.; Tatam, Ralph P.; David, Sampson; Stephen, Collins; Kyunghwan, Oh; Ryozo, YamauchiThree component planar flow-field measurements are made using imaging fibre bundles to port different views of the measurement plane, defined by a laser light sheet, to a single imaging head. The Doppler frequency shifts of light scattered by particles entrained in the flow are transduced to intensity variations using a Mach-Zehnder interferometric filter. The free spectral range of the filter can be selected by adjusting the optical path difference of the interferometer. This allows the velocity measurement range, sensitivity and resolution to be varied. Any laser wavelength may be used. A phase locking system has been designed to stabilise the interferometric filter.Item Open Access Acousto-optic frequency switching for single-camera planar Doppler velocimetry(International Society for Optical Engineering; 1999, 2001-12-31T00:00:00Z) Ford, Helen D.; Nobes, David S.; Tatam, Ralph P.; Mercer, Carolyn R.; Cha, Soyoung S.; Shen, Gongxin X.A single-camera Planar Doppler velocimetry system has been demonstrated, in which illumination beams at two closely-spaced optical frequencies are derived from a single Argon-ion laser. The frequency of one beam lies on an absorption line of iodine vapour, and the other just off the absorption line. The beams sequentially illuminate a plane within a seeded flow and the Doppler-shifted scattered light passes through an iodine cell onto a single solid-state camera. Light scattered from the measurement plane from the beam with its wavelength set to be off the absorption line is not affected by its passage through the cell, and provides a reference image, while that from the beam on the absorption line encodes the velocity information as a variation in transmission dependent upon the Doppler shift. The complex imaging system required for conventional PDV is eliminated, since superposition of the reference and signal images is automatic. The two beams are generated by an optical arrangement incorporating an acoustio- optic modulator, and are transmitted to the region of interest by a single mode optical fibre, which ensures both a smooth Gaussian illumination profile and co- linearity of the beams. The system is demonstrated using a spinning disc and a velocity resolution of +/-1 ms-1 is achieved.Item Open Access Characterization of optical fiber imaging bundles for swept-source optical coherence tomography(Osa Optical Society of America, 2011-02-03T00:00:00Z) Ford, Helen D.; Tatam, Ralph P.Fiber imaging bundles have been investigated for use in endoscopic optical coherence tomography (OCT) systems, to obviate the requirement for scanning components within the endoscope probe section. Images have been acquired using several optical configurations, two of which are common path in design. Configurations have been selected as having potential for miniaturization and inclusion in endoscopic-type systems, since the advantages of employing imaging bundles are most clearly seen in this type of system. The various types of bundle available are described, and the properties of the leached bundles used here are discussed in detail, with reference to their effect upon the performance of OCT systems. Images are displayed from measurements made on a range of samples.Item Open Access Coherent fibre bundles in full-field swept-source OCT(2009-02-20T00:00:00Z) Ford, Helen D.; Tatam, Ralph P.; Fujimoto, J. G.; Izatt, J. A.; Tuchin, V. V.Endoscopic OCT probes deliver light to the measurement region via a single optical fibre mounted in a probe head. The output beam is focused onto the sample, providing a single point measurement. The beam is translated, using mechanical scanning at the probe tip, to address a line or area of sampling points and produce an image. We are investigating a swept-source OCT system incorporating coherent fibre bundles, to allow many measurement points to be addressed, within an area of the sample, without the need for mechanical motion within the endoscope probe. Scanning components are still present at the input of our system, but are no longer required within the flexible ndoscope section. This allows a small-diameter, electrically passive probe to be engineered using off-the-shelf scanning components. A common-path probe design is proposed, in which the bundle is external to the OCT interferometer. This eliminates contrast variations caused by non-controllable differences in the state of polarisation between fibres. Imaging bundle fibres are typically few-moded, which can lead to ghost features and reduced SNR in OCT images, but the common-path configuration also removes cross-mode interference problems, and reduces dispersion artefacts. OCT images of a microscope cover-slip and a sample of spring onion, acquired using the swept-source, bundle-based OCT system are presented. Features peculiar to the inclusion of the fibre bundle are discussed, and directions for future development of the system are outlined.Item Open Access Comparative signal-to-noise analysis of fibre-optic based optical coherence tomography systems(Taylor & Francis, 2005-09-20T00:00:00Z) Ford, Helen D.; Beddows, R.; Casaubieilh, P.; Tatam, Ralph P.Several optical coherence tomography ( OCT) systems are proposed using optical. fibre components and based around Fizeau sensing interferometers. The theoretical signal- to- noise ratio ( SNR) is calculated for each of the proposed configurations, using a constant set of assumed values for illumination and detection parameters. The SNR values obtained are compared with values calculated for typical existing configurations based around Michelson interferometers. Fizeau- based systems incorporating a secondary processing interferometer offer the advantage over current interferometer configurations of down- lead insensitivity, which prevents signal fading and reduces thermal fringe drift. The most basic form of the Fizeau system makes inefficient use of optical power, and has a low SNR compared with the widely used Michelson configuration. However, the results of the analysis described in this paper show that the SNR for more sophisticated Fizeau configurations, incorporating optical circulators and balanced detection systems, can be as high as the value for the most sensitive existing fibre- based OCT systems. Fizeau configurations therefore offer the combined advantages of optimized SNR and down- lead insensitivity, indicating their suitability for use in relatively poorly controlled environments such as in- vivo measurements.Item Open Access Dual beam swept source optical coherence tomography for microfluidic velocity measurements(2018-11) Rigas, Evangelos; Ford, Helen D.; Tatam, Ralph P.Microfluidic flows are an increasing area of interest used for “lab-on-a-chip” bioanalytical techniques, drug discovery, and chemical processing. This requires optical, non-invasive flow-visualization techniques for characterising microfluidic flows. Optical Coherence Tomography (OCT) systems can provide three-dimensional imaging through reasonably-opaque materials with micrometre resolution, coupled to a single optical axis point using optical fibre cables. Developed for imaging the human eye, OCT has been used for the detection of skin cancers and endoscopically in the human body. Industrial applications are growing in popularity including for the monitoring of bond-curing in aerospace, for production-line non-destructive-testing, and for medical device manufacturing and drug encapsulation monitoring. A dual beam Optical Coherence Tomography system has been developed capable of simultaneously imaging microfluidic channel structures, and tracking particles seeded into the flow to measure high velocity flows, using only a single optical access point. This is achieved via a dual optical fibre bundle for light delivery to the sample and a custom high-speed dual channel OCT instrument using an akinetic sweep wavelength laser. The system has 10 μm resolution in air and a sweeping rate of 96 kHz. This OCT system was used to monitor microfluidic flows in 800 μm deep test chips and Poiseuille flows were observed.Item Open Access Dual Illumination Planar Doppler Velocimetry using a Single Camera(International Society for Optical Engineering; 1999, 2003-12-31T00:00:00Z) Charrett, Thomas O. H.; Ford, Helen D.; Nobes, David S.; Tatam, Ralph P.A Planar Doppler Velocimetry (PDV) illumination system has been designed which is able to generate two beams, separated in frequency by about 600 MHz. This allows a common-path imaging head to be constructed, using a single imaging camera instead of the usual camera pair. Both illumination beams can be derived from a single laser, using acousto-optic modulators to effect the frequency shifts. One illumination frequency lies on an absorption line of gaseous iodine, and the other just off the absorption line. The beams sequentially illuminate a plane within a seeded flow and Doppler-shifted scattered light passes through an iodine vapour cell onto the camera. The beam that lies at an optical frequency away from the absorption line is not affected by passage through the cell, and provides a reference image. The other beam, the frequency of which coincides with an absorption line, encodes the velocity information as a variation in transmission dependent upon the Doppler shift. Images of the flow under both illumination frequencies are formed on the same camera, ensuring registration of the reference and signal images. This removes a major problem of a two-camera imaging head, and cost efficiency is also improved by the simplification of the system. The dual illumination technique has been shown to operate successfully with a spinning disc as a test object. The benefits of combining the dual illumination system with a three-component, fibre-linked imaging head developed at Cranfield will be discussed.Item Open Access Dual-channel OCT for velocity measurement in microfluidic channels.(OSA, 2018-09-28) Rigas, Evangelos; Hallam, Jonathan M.; Ford, Helen D.; Charrett, Thomas O. H.; Tatam, Ralph P.A dual-beam Optical Coherence Tomography system has been developed, using a bespoke dual optical fibre, to simultaneously image microfluidic channel structures and measure high velocity flows (presently 250μm/s) from a single optical access point.Item Open Access Fibre imaging bundles for full-field optical coherence tomography(Iop Publishing Ltd, 2007-01-01T00:00:00Z) Ford, Helen D.; Tatam, Ralph P.An imaging fibre bundle is incorporated into a full-field imaging OCT system, with the aim of eliminating the mechanical scanning currently required at the probe tip in endoscopic systems. Each fibre within the imaging bundle addresses a Fizeau interferometer formed between the bundle end and the sample, a configuration which ensures down lead insensitivity of the probe fibres, preventing variations in sensitivity due to polarization changes in the many thousand constituent fibres. The technique allows acquisition of information across a planar region with single-shot measurement, in the form of a 2D image detected using a digital CCD camera. Depth scanning components are now confined within a processing interferometer external to the completely passive endoscope probe. The technique has been evaluated in our laboratory for test samples, and images acquired using the bundle-based system are presented. Data are displayed either as en-face scans, parallel to the sample surface, or as slices through the depth of the sample, with a spatial resolution of about 30 ï Âm. The minimum detectable reflectivity at present is estimated to be about 10-3, which is satisfactory for many inorganic samples. Methods of improving the signal-to- noise ratio for imaging of lower reflectivity samples are discusseItem Open Access Full-field optical coherence tomography(International Society for Optical Engineering; 1999, 2005-12-31T00:00:00Z) Ford, Helen D.; Tatam, Ralph P.; Heidi, Ottevaere; Peter, DeWolf; Wiersma, Diederik, S.To eliminate mechanical scanning in the probe head of an endoscopic OCT system, we propose the use of an imaging fibre bundle for probe beam delivery. Each fibre in the bundle addresses a Fizeau interferometer formed between the bundle end and the sample, allowing acquisition of information across a plane with a single measurement. Depth scanning components are now contained within a processing interferometer external to a completely passive endoscope probe. The technique has been evaluated in our laboratory for non-biological samples, including glass/air and mirrored/air interfaces. Images resulting from these experiments are presented. The potential of the system is assessed, with reference to SNR performance and acquisition speed.Item Open Access Full-field optical coherence tomography using a fibre imaging bundle(2006-12-31T00:00:00Z) Ford, Helen D.; Tatam, Ralph P.; Tuchin, V. V.; Izatt, J. A.; Fujimoto, J. G.An imaging fibre bundle is demonstrated for spatially-multiplexed probe beam delivery in OCT, with the aim of eliminating the mechanical scanning currently required at the probe tip in endoscopic systems. Each fibre in the bundle addresses a Fizeau interferometer formed between the bundle end and the sample, allowing acquisition of information across a plane with a single measurement. Depth scanning components are now contained within a processing interferometer external to the completely passive endoscope probe. The technique has been evaluated in our laboratory for non-biological samples. Images acquired using the bundle-based system are presented. The potential of the system is assessed, with reference to SNR performance and acquisition speed.Item Open Access Imaging fibre bundles for Fizeau-based optical coherence tomography(2008-03-05T00:00:00Z) Sarantavgas, G. F.; Ford, Helen D.; Tatam, Ralph P.; Izatt, Joseph A, Fujimoto, James G, Tuchin, Valery VAn OCT system incorporating a coherent fibre imaging bundle is described. Fibres are accessed sequentially by a beam focused onto the input face of the bundle, allowing 2D or 3D images to be acquired using point detection. A Fizeau interferometer configuration is used, in which light from the distal end of a fibre in the bundle (forming the reference arm) mixes with light reflected by the sample itself (forming the sample arm). The use of coherent imaging bundles for OCT beam delivery allows mechanical scanning parts to be removed from the sample arm, resulting in a passive probe. Such a configuration can form a compact, robust and "downlead insensitive" OCT system. In the common-path configuration used, an inherent path-length difference is present in the Fizeau sample interferometer, so an additional processing interferometer is required to ensure path-length matching. The depth scanning mechanism is confined within the processing interferometer, external to the sample probe.Item Open Access Influence of aberrations on confocal-based remote refractive index measurements(Optical Society of America, 2019-08-13) Ford, Helen D.; Francis, Daniel; Hallam, Jonathan M.; Tatam, Ralph P.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.Item Open Access Instantaneous, three-component planar Doppler velocimetry using imaging fibre bundles(Springer Science Business Media, 2004-01-01T00:00:00Z) Nobes, David S.; Ford, Helen D.; Tatam, Ralph P.Abstract This paper describes a planar Doppler velocimetry (PDV) technique that is capable of measuring the three, instantaneous and time average components of velocity over two spatial dimensions using a single pair of signal and reference cameras. The three views required to obtain three-component velocity information are guided from the collection optics to a single imaging plane using flexible fibre imaging bundles. These are made up of a coherent array of single fibres and are combined at one end as the input plane to the measurement head. Measurements of the velocity field of a rotating disk are used in the development of the technique and initial results of the instantaneous velocity field of a jet are presented.Item Open Access Low-coherence optical fibre speckle interferometry(Iop Publishing Ltd, 2006-02-10T00:00:00Z) Balboa, Itziar; Ford, Helen D.; Tatam, Ralph P.This paper describes the development of optical fibre low-coherence speckle interferometers capable of three-dimensional surface profiling with a resolution of 10–20 µm and a depth range of typically tens of centimetres. The technique is absolute, enabling the measurement of steps and through holes. The inclusion of optical fibres enables compact, flexible configurations to be realized, alleviating the experimental difficulties encountered with bulk interferometers, particularly when using long path lengths for measurements on large test objects. Sources including light-emitting and superluminescent diodes and multimode laser diodes are compared, and the use of a multimode laser diode source in pulsed mode is shown to improve depth resolution. Fibre-based systems using both single mode and polarization-maintaining fibre are described, and the results of experimental measurements on a stepped test object, a tilted plate and a coin are presented. A depth resolution of ±20 µm is obtained for the standard fibre system and ±14 µm for the system based on polarization-maintaining fibre.Item Open Access Mach-Zehnder interferometric filter based planar Doppler velocimetry (MZI-PDV)(Springer, 2008) Lu, Zenghai H.; Charrett, Thomas O. H.; Ford, Helen D.; Tatam, Ralph P.This paper describes the development of a Mach-Zehnder interferometric filter based planar Doppler velocimetry (MZI-PDV) flow measurement technique. The technique uses an unbalanced Mach-Zehnder interferometer (MZI) to convert Doppler frequency shifts into intensity variations. The free spectral range of the interferometric filter can be selected by adjusting the optical path difference of the MZI. This allows the velocity measurement range and resolution to be varied. In contrast to molecular filter based PDV any laser source with single-frequency operation and a narrow linewidth can be used as the requirement for a suitable absorption line is no longer necessary. The processing methods used to extract the velocity information are described and discussed. The construction of a MZI-PDV system that incorporates a phase-locking system designed to stabilize the filter is described and example measurements made on the velocity field of a rotating disc and an axis-symmetric air jet are presented.Item Open Access Optical coherence tomography with a Fizeau interferometer configuration(International Society for Optical Engineering; 1999, 2005-12-31T00:00:00Z) Casaubieilh, P.; Ford, Helen D.; James, Stephen W.; Tatam, Ralph P.; Heidi, Ottevaere; Peter, DeWolf; Diederik, S. WiersmaWe report the investigation of a Fizeau interferometer-based OCT system. A secondary processing interferometer is necessary in this configuration, to compensate the optical path difference formed in the Fizeau interferometer between the end of the fibre and the sample. The Fizeau configuration has the advantage of 'downlead insensitivity', which eliminates polarisation fading. An optical circulator is used in our system to route light efficiently from the source to the sample, and backscattered light from the sample and the fibre end through to the Mach-Zehnder processing interferometer. The choice of a Mach- Zehnder processing interferometer, from which both antiphase outputs are available, facilitates the incorporation of balanced detection, which often results in a large improvement in the Signal-to-Noise ratio (SNR) compared with the use of a single detector. Balanced detection comprises subtraction of the two antiphase interferometer outputs, implying that the signal amplitude is doubled and the noise is well reduced. It has been discerned that the SNR drops when the refractive index variation at a boundary is small. Several OCT images of samples (resin, resin + crystals, fibre composite) are presented.Item Open Access Optical fibre Fizeau-based OCT(SPIE - International Society for Optics and Photonics, 2004-06-09) Casaubieilh, P.; Ford, Helen D.; Tatam, Ralph P.An optical fibre, Fizeau configuration Optical Coherence Tomography (OCT) system is presented in this paper. The interferometer is formed between the distal end of the sample-arm fibre and the sample itself. This ensures 'downlead insensitivity' ; polarisation variation is not a problem, as it is in the standard Michelson configuration. Path-length matching is performed by a secondary, bulk-optic scanning Mach-Zehnder interferometer. In this paper, we demonstrate the use of an optical circulator and balanced detection to permit optimum use of the light and maximise the signal-tonoise ratio.Item Open Access Optical fibre wavelength filters and multiplexers(Cranfield University, 1994-03) Ford, Helen D.; Tatam, Ralph P.Polarisation-based wavelength-division multi-/demulti-plexers (WDMs) and filters have been designed, constructed and evaluated. These are compact devices made from single-mode optical fibre possessing high linear birefringence. Ihe WDMs accept two input wavelengths in a common state of polarisation, and produce orthogonally polarised outputs which are spatially separated using a polarisation-selective element. The maximum separation demonstrated is 1.5 nm, and the minimum 0.04 nm, which is two orders of magnitude better than that achieved b most other in-line WDMs. A few metres of fibre only are required. Instantaneous cross- talk values below -15 dB have been measured experimentally, and theoretical analysis of the devices predicts that values below -28 dB should be attainable. Temperature sensitivity currently limits long-term cross-talk, but also offers the possibility of tuning the centre wavelength. Methods of reducing environmental sensitivity include judicious choice of fibre type and appropriate packaging of the WDMs. These techniques are investigated and discussed. Incorporation of the WDMs in optical fibre sensing networks is demonstrated, as is separation of frequency-shifted stimulated Brillouin scattered light, generated in a fibre, from the pump frequency. Filters are constructed using concatenated sections of birefringent fibre, positioned between two polarisers. Rotation of the polarisation azimuth between sections is achieved by twistíng portions of the fibre to induce circular birefringence. Thus devices are made from a single strand of linearly birefringent fibre, less than one metre in length. Passband widths and separations are selected b changing the number of twisted sections in the filter, and altering lengths and twist angles of sections. A minimum passband width ofI0 nm has been demonstrated, at a passband separation of60 nm. Environmental stability is suíciently good for the devices to be operated as passive components.Item Open Access Passive OCT probe head for 3D duct inspection(Iop Publishing Ltd, 2013-07-24T00:00:00Z) Ford, Helen D.; Tatam, Ralph P.A passive, endoscopic optical coherence tomography (OCT) probe has been demonstrated, incorporating an imaging fibre bundle and 45° conical mirror, and with no electromechanical components at the probe tip. Circular scanning, of the beam projected onto the proximal face of the imaging bundle, produces a corresponding circular scan at the distal end of the bundle. The beam is turned through 90° by the conical mirror and converted into a radially-scanned sample beam, permitting circumferential OCT scanning in quasi-cylindrical ducts. OCT images, displayed as polar plots and as 3D reconstructions, are presented, showing the internal profile of a metallic test sample containing a 660µm step in the internal wall. Results have been acquired using two methods: one that makes use of multiple beam-circle diameters, and a mechanical 'pull-back' technique. The effects of the convex surface of the conical mirror on spatial resolution are discussed, with suggested working distances given for different application regimes.