Marc L. Dufour

Deformable and durable phantoms with controlled density of scatterers.

We have developed deformable and durable optical tissue phantoms with a simple and well-defined microstructure including a novel combination of scatterers and a matrix material. These were developed for speckle and elastography investigations in optical coherence tomography, but should prove useful in many other fields. We present in detail the fabrication process which involves embedding silica microspheres in a silicone matrix. We also characterize the resulting phantoms with scanning electron microscopy and optical measurements. To our knowledge, no such phantoms were proposed in the literature before. Our technique has a wide range of applicability and could also be adapted to fabricate phantoms with various optical and mechanical properties.

Artifact removal in Fourier-domain optical coherence tomography with a piezoelectric fiber stretcher

We describe an artifact removal setup swept-source optical coherence tomography (OCT) system that enables high-speed full-range imaging. We implement a piezoelectric fiber stretcher to generate a periodic phase shift between successive A-scans, thus introducing a transverse modulation. The depth ambiguity is then resolved by performing a Fourier filtering in the transverse direction before processing the data in the axial direction. The dc artifact is also removed. The key factor is that the piezoelectric fiber stretcher can be used to generate discrete phase shifts with a high repetition rate. The proposed experimental setup is a much improved version of the previously reported B-M mode scanning for spectral-domain OCT in that it does not generate additional artifacts. It is a simple and low-cost solution for artifact removal that can easily be applied.

Optical delay line using rotating rhombic prisms

We propose a robust and efficient delay line using an ensemble of rotating rhombic prisms. Delay lines relying on rotating elements provide fast and stable operation. Optical systems using rhombic prisms are quite easy to align since these prisms are efficient even when slightly misaligned. Optical delay lines with a single rotating element usually have a poor duty cycle and show large nonlinearity in the variation of the optical path lengh with the angular position. Our delay line improves over existing technology by using off-centroid rotation and reinjection. Off-centroid rotation allows the use of multiple prisms and, by optimizing the conditions of operation, the duty cycle is increased and the nonlinearity is decreased. The duty cycle and repetition rate are further increased by reinjecting the incoming ray towards the delay line when it is not first intercepted by the prism ensemble. We have designed and built such a delay line using five prisms. The experimental device was tested at 2000 delay scans per second and provided a duty cycle larger than 80% with about 5% nonlinearity. Higher delay scan rates are easily achievable with this technology. The delay line was introduced in a time-domain optical coherence tomography system and example of imaging of biological tissue is provided.

Artery phantoms for intravascular optical coherence tomography: healthy arteries.

We present a method to make phantoms of coronary arteries for intravascular optical coherence tomography (IV-OCT). The phantoms provide a calibrated OCT response similar to the layered structure of arteries. The optical properties of each layer are achieved with specific concentrations of alumina and carbon black in a silicone matrix. This composition insures high durability and also approximates the elastic properties of arteries. The phantoms are fabricated in a tubular shape by the successive deposition and curing of liquid silicone mixtures on a lathe setup.

Characterization of wear damage in coatings by optical profilometry

The accurate determination of the volume loss of plasma- sprayed coatings submitted to abrasive and erosive wear and the visualization of wear track or crater profiles are of major concern when ranking coatings, developing wear- resistant coatings, or identifying the mechanism responsible for failure. The determination of the volume loss by liquid displacement measurements is impractical when the size of coated pieces is large and the volume loss is small. For evaluating coating damage and directly measuring the volume loss, a three- dimensional surface mapping method is proposed. The three- dimensional image of the worn surface is obtained by a laser triangulation method. The experimental setup is basically composed of an illuminating source and a detecting device. The light source is focused on the sample surface, and the reflected light is then collected on a network of charge couple detectors linked to a computer. Because the spot location on the network is a direct function of the measured height, a three- dimensional image can be obtained after scanning the entire damaged surface so that the volume loss can be calculated easily. Intensity- coded depth images of the worn surface and computerized cross sections of the damaged area can also be obtained. Inspection of coatings damaged by abrasion wear or slurry erosion by optical profilometry reveals that the volume loss measurements by this technique are very accurate as opposed to the volume measured by liquid displacement methods or calculated from weight loss measurements. Moreover, intensity- coded depth images of worn surfaces and computerized cross sections of damaged areas provide relevant information about the coating performance or defects resulting from the deposition process.

Intravascular optical coherence tomography on a beating heart model

The advantages and limitations of using a beating heart model in the development of intravascular optical coherence tomography are discussed. The model fills the gap between bench experiments, performed on phantoms and excised arteries, and whole animal in-vivo preparations. The beating heart model is stable for many hours, allowing for extended measurement times and multiple imaging sessions under in-vivo conditions without the complications of maintaining whole-animal preparation. The perfusate supplying the heart with nutrients can be switched between light scattering blood to a nonscattering perfusate to allow the optical system to be optimized without the need of an efficient blood displacement strategy. Direct access to the coronary vessels means that there is no need for x-ray fluoroscopic guidance of the catheter to the heart, as is the case in whole animal preparation. The model proves to be a valuable asset in the development of our intravascular optical coherence tomography technology.

Surface inspection of hard to reach industrial parts using low-coherence interferometry

Optical inspection tools based on low-coherence interferometry and specialized for hard to reach industrial parts are presented. A common path configuration using optical fiber components is described. Small diameter probes originally developed for biomedical applications have been specialized for industrial inspection. Probes that can be used with a Cartesian surface scanning system or a cylindrical scanning system are presented. The probes include a reference that makes absolute accuracy measurements easier. Characterization of the internal surface of a worn plasma torch electrode has been realized using that technique. Surface profiling of the barrel of a gun was also performed.

Durable coronary artery phantoms for optical coherence tomography

We developed coronary artery phantoms that should be of great use for intravascular optical coherence tomography. Our phantoms mimic the OCT signal profile of coronary arteries, show mechanical properties approaching those of real tissue, and are durable.

Particle size measurement in glass powder beds using optical coherence tomography

Optical coherence tomography was used to collect cross- sectional images of glass powder beds consisting of microspheres with diameters ranging from 8 to 175 m. Images were formed by a collection of individual interferogram envelopes that give the backscattered light amplitude as a function of the optical path in the glass powder bed. The diameter distribution, for microspheres located near the surface of the beds, is obtained by appropriate peak distance measurements on threshold-selected envelopes after having performed the surface profilo- metry. The measured distributions are in good agreement with those obtained by laser diffraction. When considering the whole powder vol- ume, the evaluation of the mean light penetration depth inside the pow- der beds proves to be a useful approach to evaluate the mean particle diameter, although no information is obtained on the actual particle size distribution in this case. Two simplified models are introduced to under- stand the linear relationship observed between the penetration depth and the mean particle size.

Laser-ultrasonic inspection of steel slabs using Saft processing

The detection of inclusions or small defects located below the surface of cast slabs is addressed in this paper. The technique combines laser-ultrasonics and synthetic aperture data processing for inspection on descaled slabs. An improved synthetic aperture data processing (F-SAFT) performed in the Fourier domain which includes the control of the aperture as well as spatial interpolation is used. A further improvement in F-SAFT reconstruction is made by taking into account the inspected surface profile, previously or simultaneously measured by an optical surface profiler. Samples with wavy inspected surface and flat-bottom holes at different depths are tested to validate the correction method for surface profile. Industrials steel slab samples are then tested to confirm the reliability of the proposed laser-ultrasonic approach. The inspection time duration and the resolution limit are also discussed.