Lionel Bertrand

Advantages of Phase Analysis in Fourier Transform Infrared Photoacoustic Spectroscopy

Fourier transform infrared photoacoustic spectroscopy is greatly improved by the simultaneous measurement of the photoacoustic signal amplitude and phase. Saturation effects, sample dilation, and optical scattering, even in heterogeneous samples, can be easily corrected to give the absorption coefficient real value. Quantitative analyses without any special sample preparation or special photoacoustic cell are now possible. Moreover, the phase of the photoacoustic signal permits depth localization of the absorbing species within the thermal diffusion length. Surface analyses are thus possible that have a depth resolution at least one order of magnitude higher than that obtained by changing the thermal diffusion length. In this paper we illustrate the advantages of phase analysis in Fourier transform infrared photoacoustic spectroscopy with the detection of sorbed water in polyethylene.

Absolute optical absorption spectra in graphite epoxy by Fourier transform infrared photoacoustic spectroscopy

Optical absorption is obviously of prime interest in the efficiency of laser generation of ultrasound in graphite-epoxy laminates. However, no quantitative spectrum of optical absorption in this composite material has yet been published in the literature. Transmission techniques are inefficient, and other techniques, like attenuated total reflectance or diffusive reflectance, do not give absolute values. The Fourier transform photoacoustic spectroscopy technique seems to be a good alternative that can analyze adequately and quantitatively a graphite-epoxy laminate. We used three different methods to compute the absolute optical absorption from the photoacoustic signal. The three methods are: the saturation of the real part of the photoacoustic spectrum, the comparison of the spectra obtained with two different mirror velocities, and the calibration of the photoacoustic cell with a transmission measurement. The spectra obtained in the IR band of 2.5 to 25 μm are presented, and the problems and limitations of each method are discussed. The results permit a better understanding of the absorption process in the composite laminate, and in this way, will help us enhance the efficiency of laser generation of ultrasound in graphite epoxy.

Photoacoustic characterization of subsurface defects in plasma-sprayed coatings

Abstract The photoacoustic detection of subsurface defects in metallic plasma coatings on metallic substrates has been achieved. The investigation is performed with samples that contain well-known artificial defects (Teflon, alumina or boron nitride) whose thermal properties are sufficiently different from those of the coatings (aluminium or Ni-Cr alloy) or the metallic substrates. The experimental results are interpreted in terms of a theoretical model for the layered structure of the sample. Moreover, the photoacoustic effects of the defect may be characterized by a thermal resistance at the coating-substrate interface. This study enables us to determine different characteristics of the coatings such as thermal properties, thickness and adherence to the substrate, in spite of the effects of the surface roughness on the photoacoustic signal.

Photoacoustic and photothermal evaluation of stratified materials

Photoacoustic and photothermal nondestructive evaluation techniques are an emerging approach for inspection of several materials of industrial interest. This paper focuses on stratified materials such as aluminum-epoxy or graphite epoxy laminates as well as metallurgically coated materials which are widely used, particularly in the aerospace industry. Such materials are usually inspected for layer-to-layer delaminations after manufacture by ultrasonic or radiographic techniques. Optical techniques provide an attractive alternative to such methods because they require no contact with the inspected surface so that they can be deployed in hostile and remote-located environments, while being easy to scan over large surfaces. This paper describes some applications of photoacoustic and photothermal approaches to the inspection of layered materials. Photoacoustic techniques using both microphone detection in air and interferometric detection of the laser-generated ultrasonic wave in the material are reviewed. Photothermal techniques using either a thermoelastic displacement approach or an IR time-resolved temperature monitoring method are also described, and their applicability to different materials is discussed.

Non-destructive interferometric detection of unbonded layers

Abstract Non-destructively detecting and monitoring delaminated areas in stratified materials is an important industrial problem. The possibility of characterizing such delaminations by interferometric dilatometry is analyzed in this paper. A Cu-Be strip adhesively bonded to a massive substrate is resistively heated and its thermal expansion is monitored by a laser interferometer. Large signals are obtained from the unbonded regions, as expected from the theoretical calculations. The initial curvature of the unbonded layer is shown to have a major effect on the lifting capability of uniformly heated layers.

Ultrasound Generation in Composites via Embedded Optical Fibers

The laser based ultrasound generation is now a well known and established technique used in NDE and material characterization [1]. The use of embedded optical fiber sensors in structures made of composite materials is of great interest for smart structures allowing an integrated health monitoring. The combination of both techniques could lead to an optical fiber based ultrasonic embedded system comprising both ultrasounds sources and detectors.

Mathematical formulations for the schlieren detection method applied to the measurement of photodeformation

In a schlieren detection scheme for photodeformation measurements, the divergence of the probe beam that is induced by the axisymmetric but radially inhomogeneous periodic photothermal displacement of the surface of a sample is transformed into an intensity variation by insertion of an iris in front of the detection photodiode. We present three expressions for the intensity profile of a Gaussian laser beam that is reflected by the inhomogeneous photodeformation of a solid. The first expression proceeds from geometrical optics (or photometry), whereas the second one derives from the use of the well-known ABCD law and the third one from diffraction principles. Comparing these formulations of the schlieren signal with their behavior as a function of different geometrical parameters, we obtain the domain of validity of each expression, and we deduce the advantages of the different formalisms.

A Two-Layer Model for the Laser Generation of Ultrasound in Graphite-Epoxy Laminates

We previously reported the performances of a numerical simulation model [1] that calculates the mechanical displacements induced within a sample by the absorption of a laser pulse. This model solves the heat diffusion and acoustic wave propagation equations over an orthotropic slab of finite thickness with the help of temporal Laplace and spatial 2D Fourier transformations. The parallel and normal displacements predicted by the model were found to be in generally very good agreement with experimental data obtained on various samples in various excitation conditions. Among these experiments, one consisted in the CO2 laser excitation of a graphite-epoxy sample. We performed an optical study of the graphite-epoxy composite using FTIR photoacoustic spectroscopy [2] to determine the optical penetration depth spectrum of this material. This study revealed that a thin (≈ 30 μm thick) epoxy layer covered the top graphite fiber sheet of the composite, and that the optical penetration depth of the CO2 radiation in the epoxy was about 20 μm. Consequently, when a CO2 laser pulse impinges on the composite, all the radiation is absorbed in the epoxy layer, and it is easy to simulate this situation with the model, using the rigidity-expansion tensor [λ] of the epoxy for the generation and the rigidity tensor [C] of the composite for the propagation (see [1]).

Relationship between amplitude and phase measurements of photoacoustic signal in the infrared region and water concentration in condensed milk.

Photoacoustic signals in the near infrared region (~ 2·75 μ m) have been investigated using amplitude and phase information to determine their effectiveness in estimating the water content of milk. The results obtained showed a good correlation between the photoacoustic signal and water concentration ( R 2 = 0·98) even though photoacoustic saturation was present in the range of wavelengths studied. However, this is not recommended as a fast technique for water determination because of inherent phase fluctuations in the samples analysed.

Virtual laboratories in optics: applications to a course on lasers

Virtual laboratories are used to help teaching a course on lasers. The spectral emission of a laser, the Zeeman effect on the laser frequencies, the frequency stabilisation of a laser and the Lamb dip are the subjects chosen to virtually illustrate the properties of a laser. All the laboratories are very close to reality, including possible experimental missteps. The student is under the conditions he can find in the real laboratory and can spend as long as he wishes, at any time. The teacher can track the student via a data base that includes the eventual missteps which the student would have committed.