Manuel H. De la Torre Ibarra

Simultaneous measurement of in-plane and out-of-plane displacement fields in scattering media using phase-contrast spectral optical coherence tomography

The use of phase-contrast spectral optical coherence tomography to measure two orthogonal displacement components on a slice within a scattering medium is demonstrated. This is achieved by combining sequential oblique illumination of the object and recording two interferograms before plus two after the deformation. The proposed technique is illustrated with results from a sample undergoing simple shear. Depth-resolved out-of-plane and in-plane sensitivities of 0.14 and 4.2 microm per fringe are demonstrated up to a depth of 400 microm in a water-based polymer.

Simultaneous two-dimensional endoscopic pulsed digital holography for evaluation of dynamic displacements

An endoscope is used in pulsed digital holography to simultaneously evaluate in-plane and out- of-plane transient and harmonic displacements on a flat metallic plate. The plate is illuminated from two different directions. The optical path for each illumination direction is matched to its corresponding reference beam, but also in such a way that each object-reference beam pair optical path is mismatched such that they are incoherent and can be stored in a single CCD frame. As is typical in these types of interferometric arrangement, two digital holograms are needed to compare two different states of the plate. Each hologram is Fourier transformed and due to the incoherence introduced, two separate spectra are readily identified, each belonging to an object-reference beam pair. On comparing by subtraction the phase obtained from the two pulsed digital holograms, it is possible to gather quantitative in-plane and out-of- plane results from transient and harmonic displacements.

Displacement measurements over a square meter area using digital holographic interferometry

Abstract. Current industrial demand for optical nondestructive testing includes the displacement analysis of large object areas. This paper reports on the use of a digital holographic interferometer to measure displacements over an area of 1.14  m2. The object under study is a framed working table covered with a Formica layer fixed to a granite bench, and it is observed and illuminated employing a high speed and high resolution camera and a continuous wave high output power laser, respectively. A stabilization procedure needs to be established as long illumination distances are required in order to retrieve the entire surface optical phase during a series of continuous deformations. As a proof of principle, two different tests are presented: the first involves a slow continuous loading process and the second a vibration condition. The wrapped phase and displacement maps are both displayed.

Low level free vibration measurements using high speed digital holography

Recent advances in high speed image acquisition along with high power cw lasers open up the possibility of measuring very low level free vibrations using high speed digital holographic interferometry. In this paper we report on the minimum detected level of instantaneous deformation for an elastic material fixed to a rectangular frame, and for one segment of a spherical object. The signal to noise ratio for each acquisition frame rate is dealt with as well as the limitations due to the digital holographic technique with respect to the pixel size and maximum sampling for free vibrations. An out of plane optical arrangement is considered with an object illuminated by cw laser at 532 nm. After Fourier processing each digital hologram a series of consecutive phase maps is obtained from a set of one thousand images taken from data acquired at 5000 fps. Experiments were also implemented using a different frame rate from images of a rather small segment of a spherical object, at 2000, 4000, 6000 and 8000 fps. Th...

Detection of inhomogeneities in a metal cylinder using ESPI and 3D pulsed digital holography

ESPI and 3D pulsed Digital Holography have been applied to detect inhomogeneities inside a metal cylinder. A shaker was employed to produce a mechanical wave that propagates through the inner structure of the cylinder in such a way that it generates vibrational resonant modes on the cylinder surface. An out of plane ESPI optical sensitive configuration was used to detect vibrational resonant modes. A 3D multi-pulse digital holography system was used to obtain quantitative deformation data of the dynamically moving cylinder. The local decrease in structural stiffness inside the cylinder due to an inhomogeneity produces an asymmetry in the resonant mode shape. Results show that the inhomogeneity produces an asymmetry in its vibrational resonant modes. The method may be reliably used to study and compare data from inside homogeneous and inhomogeneous solid materials.

Qualitative characterization of collagen hydrogel membranes using digital holographic microscopy and SHG microscopy

Crosslinked collagen hydrogel membranes are widely used in engineering tissue to promote the performance of biological processes involved in the healing of wounds, mainly in the presence of chronical diseases such as diabetes. One of the standard techniques used in biology to measure mechanical properties of hydrogels and tissues is based on a methodology called rheology. Rheological studies consist on the measurement of stress, strain and the ratio of stress to strain of several biological membranes to determine their viscoelastic properties. In this research work we propose as a proof of concept the use of digital holographic microscopy (DHM) and second harmonic generation (SHG) microscopy to compare qualitatively some basic image properties of collagen hydrogel membranes. Once demonstrated that this comparison is equivalent, we study under controlled excitation the biomechanical properties of collagen membranes by means of the analysis of the optical phase that results from comparing two consecutives holograms from a reference and deformed state. At this stage of characterization, an in depth study between rheological and holographic results will be performed in the near future.

Controlled compression test applied to composite materials reinforced with particles to predict fracture formation

Composite materials are continuously being developed for all kinds of applications, making an impossible task to test each one during their development. The main purpose of these new and advanced materials is to improve the physical and mechanical properties of two or more materials by combining them. Without this combination each material by its own does not meet the specifications that the composite one will. New composite materials are subject to several types of tests trying to predict their behavior under specific chemical and mechanical conditions. In this work we deal with the mechanical characterization under a controlled compression test in circular composite probes. The use of a home-made specifically designed testing machine for non-destructive optical testing allows repeatable and controlled compression loads. Associating this testing machine with the well-known analysis capabilities of digital holographic interferometry in high speed mode, makes it possible to register and analyze the precise instant where the composite samples develop a crack by compression, its propagation and finally the fracture formation. A series of composite samples with three different concentrations of metallic particles (reinforcement) within the polymer (matrix) were manufactured. These specimens were subjected to a controlled compression and the obtained interferometric results show that it is possible to estimate an unknown particle concentration density of the composite material by identifying the load value when the crack appears. The latter is possible if a data base of similar samples are characterized before and then the crack load point is correlated to estimate this reinforcement.

Inspection of micrometric size semitransparent biological samples using a transmission digital holographic interferometer

A new configuration of a transmission digital holographic interferometer to observe micro scale samples is presented. The optical system uses high magnification to analyze the phase of semitransparent biological samples in order to measure their size. As proof of principle some previously characterized pollen grains are placed in the object’s beam path and magnified by projection on the neutral phase screen whose image is conveyed on the camera sensor and thus is used to retrieve the optical phase that is related to the size of each particle. As validation fluorescence and transmission images from a confocal microscope are used to compare the phase intensity retrieved with the interferometric system. Results show a direct relation between the pollen size and the resulting phase magnitude after geometrical processing with the advantage of having a micrometric measurement with a simple optical set up.

CMOS and sCMOS imaging performance comparison by digital holographic interferometry

Abstract. We use a digital holographic interferometric setup to assess, as a proof of concept, two state-of-the-art sensors (CMOS and sCMOS cameras) that are widely used in nondestructive testing (NDT). This interferometric study is intended to evaluate the image quality recorded by any camera used in NDT. The assessing relies on the quantification of the optical phase information recovered by the cameras used for this study. For this, we calculate the signal-to-noise ratio, correlation coefficient, and quality index (Q-index) as main figures-of-merit. As far as we know, the Q-index has not been used for evaluation of the optical phase coming from image holograms. The CMOS and sCMOS sensors used record the same deformation event under the same experimental conditions. The experiment involves the inspection of a large sample (>1  m2 of area) which implies low illumination conditions for the imaging sensors. The retrieved CMOS optical phase shows artifacts that are not observed in the sCMOS. An analysis of these two groups of interferometric images is presented and discussed. The methodology set forth here can be applied to evaluate other sensors such as CCDs and EM-CCDs.