Helen D. Ford

Fibre imaging bundles for full-field optical coherence tomography

An imaging fibre bundle is incorporated into a full-field imaging optical coherence tomography 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 endoscopic 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 ?m. 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 discussed.

Slope measurement by two-wavelength electronic shearography

Abstract A technique is presented for the slope measurement of objects using an electronic shearography system. To detect the gradients of an object shape, a laser diode is modulated to produce two wavelengths on successive image frames. These two frames are then subtracted to generate correlation fringes which depict slope variations of the object surface. The theory of this technique is described and measurements of conical, cylindrical, and spherical test objects are presented. Com parisons of experimental results with theoretical data are demonstrated to be in good agreement. The implementation of fringe analysis using phase stepping techniques is also discussed.

Development of extended field Doppler velocimetry for turbomachinery applications

The development of a portable, single component Doppler global velocimetry (DGV) head, based around a wavelength-stabilised argonion laser and a fast digital image-processing system, is described. The normalised two-dimensional DGV image, in which intensities are linearly related to velocities, can be displayed and updated at the 25 Hz camera frame rate, greatly easing the problem of system alignment. The effect of each individual system component upon the velocity resolution achieved for the system as a whole is discussed, and correction factors are calculated to account for the finite aperture and field of view of real systems and for divergence of the illuminating light sheet. Axial velocities of up to 100 m/s in a straight duct flow have been measured, demonstrating an rms velocity resolution of 2.5 m/s. The potential of the technique for gas turbine applications has been demonstrated by measuring the position of a shock in a transonic flow. At a Mach number of 2.3 and mass flow rate of 0.79 kg/s the velocity change across the shock was measured to be approximately 130 m/s.

Two-frequency planar Doppler velocimetry (2ν-PDV)

A planar Doppler velocimetry (PDV) system has been designed which is able to generate two beams from a single source separated in frequency by 690 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 and a set of acousto-optic modulators used to affect the frequency shifts. One illumination frequency lies on an absorption line of gaseous iodine, and the other in a region of zero absorption. 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 reference beam that lies in a zero absorption region is unaffected by passage through the cell, and provides a reference image. The signal 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 illumi...

Characterization of optical fiber imaging bundles for swept-source optical coherence tomography

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.

Low-coherence optical fibre speckle interferometry

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.

Full-field optical coherence tomography using a fibre imaging bundle

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.

Comparative signal-to-noise analysis of fibre-optic based optical coherence tomography systems

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.

Phase-stepped speckle shearing interferometer by source wavelength modulation

Phase stepping by wavelength modulation of the optical source is demonstrated in a path-length unbalanced shearing interferometer. A magnification difference, which accompanies the path-length imbalance in the interferometer, introduces an unwanted radial shear, which is shown to be compensated by inclusion of a block of high-refractive-index material in the longer interferometer arm. The block also increases the phase shift obtained for a given wavelength change. The phase-stepping technique is demonstrated with a three-step algorithm to measure out-of-plane strain on a f lat metal plate.

Heterodyning of speckle shearing interferometers by laser diode wavelength modulation

Laser diode emission wavelength modulation is demonstrated as a technique for measuring the phase and gradient of displacement amplitude of a vibrating object using continuous wave illumination in a path length imbalanced speckle shearing interferometer. The unwanted mismatch in the magnification of the two images in the shearing interferometer and sensitivity to radial shear introduced by a path length imbalance is overcome by the use of a transparent block of high refractive index in one path in the interferometer.