Thomas O. H. Charrett

Optical fibre laser velocimetry: a review

The applications of optical fibre technology to laser velocimetry are diverse and often critical to their successful implementation, particularly in harsh environments. Applications range from the use of optical fibres for beam delivery and scattered light collection, aiding the miniaturization of instrument probes, to the use of imaging fibre bundles for imaging the flow field in planar velocimetry systems. Optical fibre techniques have also been used in signal processing, for example fibre frequency shifters, and optical fibre devices such as amplifiers and lasers have been exploited. This paper will review the use of optical fibres in point-wise laser velocimetry techniques such as laser Doppler velocimetry and laser transit anemometry, as well as in planar measurement techniques such as particle imaging velocimetry and planar Doppler velocimetry.

Single camera three component planar velocity measurements using two-frequency planar Doppler velocimetry (2ν-PDV)

Two-frequency planar Doppler velocimetry (2ν-PDV) is used in conjunction with imaging fibre bundles to make time-averaged three component velocity measurements using a single CCD camera and iodine cell. In this scheme the flow is sequentially illuminated with two different frequencies of laser light, which are separated by approximately 700 MHz to 1 GHz. Images are captured for each illumination frequency on a CCD camera viewing through an iodine absorption filter. These images are then used to produce a normalized transmission image that can be processed as in normal PDV. Three component measurements are made possible by porting separate views to a single CCD camera using the imaging fibre bundles. Results are presented here for a 2ν-PDV system demonstrated making time-averaged velocity measurements on a rotating disc and a seeded air jet.

Investigation into the selection of viewing configurations for three-component planar Doppler velocimetry measurements

A method for the calculation of three orthogonal velocity components in planar Doppler velocimetry (PDV) using four or more measured velocity components (to the three typically used) is presented. The advantages and disadvantages are assessed by use of a Monte Carlo simulation and experimental measurements of the velocity field of a rotating disk. The addition of a fourth velocity component has been shown to lead to reductions in the final errors of up to 25%. The selection of viewing configurations for experiments is discussed by simulation of the level of errors in measured velocity components and investigation of the final level of errors in the orthogonal velocity components. Experimental measurements of the velocity field of a rotating disk are presented, demonstrating the effect of the viewing configuration on the final level of error.

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...

Range-resolved interferometric signal processing using sinusoidal optical frequency modulation

A novel signal processing technique using sinusoidal optical frequency modulation of an inexpensive continuous-wave laser diode source is proposed that allows highly linear interferometric phase measurements in a simple, self-referencing setup. Here, the use of a smooth window function is key to suppress unwanted signal components in the demodulation process. Signals from several interferometers with unequal optical path differences can be multiplexed, and, in contrast to prior work, the optical path differences are continuously variable, greatly increasing the practicality of the scheme. In this paper, the theory of the technique is presented, an experimental implementation using three multiplexed interferometers is demonstrated, and detailed investigations quantifying issues such as linearity and robustness against instrument drift are performed.

Objective speckle velocimetry for autonomous vehicle odometry

Speckle velocimetry is investigated as a means of determining odometry data with potential for application on autonomous robotic vehicles. The technique described here relies on the integration of translation measurements made by normalized cross-correlation of speckle patterns to determine the change in position over time. The use of objective (non-imaged) speckle offers a number of advantages over subjective (imaged) speckle, such as a reduction in the number of optical components, reduced modulation of speckles at the edges of the image, and improved light efficiency. The influence of the source/detector configuration on the speckle translation to vehicle translation scaling factor for objective speckle is investigated using a computer model and verified experimentally. Experimental measurements are presented at velocities up to 80  mm s(-1) which show accuracy better than 0.4%.

Mach-Zehnder interferometric filter based planar Doppler velocimetry (MZI-PDV)

A planar Doppler velocimetry system to measure flow velocity fields is described. The technique uses a Mach–Zehnder interferometric filter to convert Doppler frequency shifts into intensity variations. 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. An experimental arrangement is described that incorporates a phase-locking system designed to stabilize the interferometric filter. Two techniques to process the interference fringe images are presented. The first uses the shift of the fringe pattern to determine the Doppler shift along profiles. The second provides a full-field measurement by normalizing the received light intensity at each pixel in the image. Results are presented here for measurements of a rotating disc with maximum velocities of ~ ± 70 m s−1 at the edge. Standard deviations in the measured velocities were found to be about 2.9 and 2 m s−1 for the two methods. Measurements on a seeded air jet with a nozzle diameter of 20 mm and an exit velocity of ~85 m s−1 are also presented.

Speckle velocimetry for high accuracy odometry for a Mars exploration rover

The results of a preliminary study into speckle velocimetry for potential application on a Mars exploration rover are presented. Speckle velocimetry is a non-contact optical velocimetry technique intended to replace or supplement the current odometry information provided by wheel encoders. This initial study investigated the feasibility of forming speckle images from Mars soil analogues and investigated potential algorithms using a laboratory test system. The normalized cross-correlation approach is identified as the most promising processing method, with preliminary results suggesting an odometry error of 0.2% at speeds of up to 1 mm s−1 and that the technique should scale well to the higher velocities (~85 mm s−1) typical of exploration rovers.

Three-component planar velocity measurements using Mach?Zehnder interferometric filter-based planar Doppler velocimetry (MZI-PDV)

Interferometric filter-based planar Doppler velocimetry is used in conjunction with imaging fibre bundles to make time-averaged three-component velocity measurements using a single imaging head. The Doppler frequency shifts of light scattered by particles entrained into the flow to be measured are transduced to intensity variations using a Mach–Zehnder interferometer. 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. Three-component measurements are made possible by porting different views of the measurement plane to a single imaging head using the imaging fibre bundles. A comparison of three different image-processing techniques is presented and analysed with the aid of modelled images. Results are presented here for time-averaged measurements of a rotating disc with maximum velocities of ~ ±34 m s−1 in the field of view with the computed measurement error in the orthogonal velocity components being (0.89, 0.68, 1.42) m s−1 for the measurement geometry used. Three-component velocity measurements were also made on a seeded air jet with a nozzle diameter of 20 mm and an exit velocity of ~85 m s−1.

Objective speckle displacement: an extended theory for the small deformation of shaped objects

This paper describes an extended and improved theory of the displacement of the objective speckle pattern resulting from displacement and/or deformation of a coherently illuminated diffuse object. Using the theory developed by Yamaguchi [Opt. Acta 28, 1359 (1981)], extended expressions are derived that include the influence of surface shape/gradients via the first order approximation of the shape as linear surface gradients. Both the original Yamaguchi expressions and the extended form derived here are shown experimentally to break down as the detector position moves away from the z-axis. As such, improved forms of the expressions are then presented, which remove some of the approximations used by Yamaguchi and can be used to predict the objective speckle displacement over a wide range of detector positions and surface slopes. Finally, these expressions are then verified experimentally for the speckle shifts resulting from object translations.