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.
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.
Measurement of the refractive index and thickness of transparent plates is demonstrated using combined low-coherence interferometry and confocal scanning. The low-coherence measurement provides a quantity related to the group index and the confocal scan provides a parameter related to the phase index. Calculation of both the phase and group indices also requires a measurement of the confocal parameter at multiple wavelengths. This is achieved using a broadband source and a line-scan spectrometer to interrogate different regions of the spectrum. Measurements are made on a range of transparent optical materials with the mean percentage errors of each measurand being 0.08%, 0.06%, and 0.12% for np, ng, and thickness t respectively.
Imaging fibre bundles and a conical mirror have been used to construct a passive, endoscopic OCT probe with no
scanning components at the probe tip. Circular scanning of the beam projected onto the proximal face of the imaging
bundle results in a corresponding circular scan at the distal end of the probe. A 45° base-angle conical mirror turns the
output light to produce a radially-directed beam that permits circumferential OCT scanning in quasi-cylindrical ducts.
Imaging fibre bundles are widely used in image transmission systems in the visible spectral range1. Their image
transmission capabilities and flexibility make them interesting for OCT endoscopy2. However, cross-talk occurring
between individual fibres and multimode transmission cause deterioration of the images obtained with OCT systems.
In this study, a model of light propagation in a fibre bundle is developed using the Finite Difference - Time Domain
(FDTD) method in order to quantify these effects. The simulated bundle output power density distribution is
compared to experimental images.
OCT probes for endoscopy typically use a single optical fibre to deliver light to the measurement region. 2D imaging is
achieved using mechanical scanning components at the probe tip.
We are investigating an OCT system in which fibre imaging bundles replace the single probe fibre, allowing many
thousand measurement points to be addressed without any mechanical scanning at the probe tip, over a region measuring
a few millimetres in each dimension. This allows a small-diameter, electrically passive probe to be engineered using offthe-
shelf scanning optics. Images from a single-fibre, conventionally-scanned probe and from a bundle-based probe are
presented, using swept-source processing centred at 1330 nm.
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 endoscope 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.
Three 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.
An 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.
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.
We 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.
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.
The work presented here describes a method that allows three component velocity measurements to be made, quickly and non-intrusively, across a plane in a flow defined by a laser light sheet. The method, two frequency planar Doppler Velocimetry (2ν-PDV) is a modification of the Planar Doppler Velocimetry (PDV) technique, using only a single CCD camera, and sequential illumination of the flow using two frequencies, separated by about 700MHz. 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 at a frequency off the absorption line is not affected by passage through the cell, and provides a reference image. The other beam encodes the velocity information as a variation in transmission dependent upon the Doppler shift. Use of a single camera ensures registration of the reference and signal images, which is the major problem in any spilt image system such as a two-camera imaging head, and cost efficiency is improved by the simplification of the system. A 2ν-PDV system was constructed using a continuous-wave Argon ion laser and acousto-optic modulators to produce two frequencies of illuminating laser light. This was combined with multiple imaging fibre bundles, to port three different views of the measurement plane to a CCD camera, allowing the measurement of three velocity components.
3D planar flow-field measurements are made using multiple imaging fibre bundles to port different views of the measurement plane, defined by a laser light sheet, to a CCD camera. The Doppler frequency shifts of particles entrained in the flow are transduced to intensity using an iodine absorption cell. Only a single CCD camera is used eliminating the pixel matching problem. Two optical frequencies generated from the same source are used sequentially to provide a reference and signal image.
An optical fiber, Fizeau configuration Optical Coherence Tomography (OCT) system is presented in this paper. The interferometer is formed between the distal end of the sample-arm fiber and the sample itself. This ensures 'downlead insensitivity;' polarization variation is not a problem, as it is in the standard Michelson configuration. Path-length matching is performed by a secondary, bulk-optic scanning Michelson interferometer. A standard Fizeau arrangement, based around a directional coupler, makes relatively inefficient use of the optical power and has a poor SNR compared with the Michelson configuration. In this paper, we demonstrate the use of an optical circulator to permit efficient re-routing of the light, restoring the theoretical maximum SNR to about the same value obtained for comparable Michelson systems. The use of balanced detection to further improve the SNR is also discussed.
An optical fiber, Fizeau configuration Optical Coherence Tomography (OCT) system is presented in this paper. The Interferometer is formed between the distal end of the sample-arm fiber and the sample itself. This ensures 'downlead insensitivity;' polarization 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 maximize the signal-to-noise ratio.
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 vapor 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, fiber-linked imaging head developed at Cranfield will be discussed.
The development of a planar Doppler velocimetry is described. The technique is capable of measuring the three, instantaneous components of velocity in two dimensions using a single pair of signal and reference cameras. PDV can be used to measure the instantaneous 3-D velocity of a fluid by using an absorption line filter (ALF) to determine the Doppler shifted frequency of a narrow line pulsed laser (Nd:YAG) that has been scattered off particles seeded into the flow. The velocity of the fluid is determined using the Doppler formula and is dependent on the laser direction and the viewing direction. Hence, only one velocity component of the flow is measured. This component can be measured in two spatial dimensions using an array detector such as a CCD camera. To capture the three components, three such measurement heads have been used viewing from different angles. In the technique presented here the three views are ported from the collection optics to a single imaging plane using flexible fiber imaging bundles. These are made up of a coherent array of single fibers and are combined at one end as the input plane to the measurement head. The paper discusses the issues involved in developing a full three-dimensional velocity measurement system.
This paper describes a planar Doppler velocimetry (PDV) technique that is capable of measuring the three, instantaneous components of velocity in two dimensions using a single pair of signal and reference cameras. PDV can be used to measure the instantaneous 3-D velocity of a fluid by using an absorption line filter (ALF) to determine the Doppler shifted frequency of a narrow line pulsed laser (Nd:YAG) that has been scattered off particles seeded into the flow. In the technique presented here the three views required to obtain three dimensional velocity information are ported from the collection optics to a single imaging plane using flexible fiber imaging bundles. These are made up of a coherent array of single fibers and are combined at one end as the input plane to the measurement head. A fourth leg of the imaging bundle is used to image the individual laser pulses and allow correction for pulse-to-pulse frequency variations. The results reported in the paper are from the development phase of the system and are of the velocity field of a rotating wheel.
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 acousto-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.
Direct injection-current modulation of a diode laser source is demonstrated for phase-stepping, heterodyning and
stroboscopic illumination in a shearing interferometer. The techniques are proposed as an alternative to the use of mechanical
phase-shifters such as piezoelectric transducers, offering the advantages of ease of implementation, extended measurement
bandwidth and elimination of active elements from the measurement head. The phase-stepping and heterodyning techniques
require the interferometer to be unbalanced, which introduces an unwanted magnification mismatch into the shearing
interferometer. A compensation technique using a block of high refractive-index material is described, and experimental
demonstrations of each of the three techniques are presented.
Doppler Global Velocimetry (DGV) is a full-field optical technique for the measurement of fluid flow velocities. The flow is illuminated using a light sheet, and the Doppler shift imposed on light scattered from moving particles within the sheet is imaged through a cell containing iodine vapor onto a solid-state array camera, thereby converting the Doppler frequency shifts into intensity variations in the image. In this paper, a DGV system is presented based around an argon-ion laser source and a fast digital image-processing system, which allows the DGV velocity map to be updated at camera frame rate. Interpretation of DGV images is complicated by errors which arise at positions some way out in the field of view due to the modified illumination and viewing vectors corresponding to these positions. Typical magnitudes of such errors are calculated. Significant errors can arise for points more than about 5 degree(s) out from the center of the field of view, and for divergence angles of the illumination beam exceeding about 10 degree(s) at a distance of 5 cm from the beam axis. Other considerations affecting system accuracy are also discussed.
Birefringent filters, analogous to bulk-optic Solc filters, have been constructed from linearly birefringent optical fiber. Controlled twisting of portions of the fiber is used to rotate the polarization azimuth between retarder elements, and allows the devices to be made from continuous lengths of fiber. Filters having up to four elements have been made and evaluated, and a passband width of 10 nm has been demonstrated for a four-element filter.
Wavelength filters have applications in wavelength-division multiplexed optical communications and
sensor systems. Bulk-optic birefringent filters offer sub-nanometre passband widths, but are not compatible with
optical fibre systems. However, fibre analogues of the bulk-optic filters can be constructed using birefringent
fibre.
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