One of the most significant roles of light is to act as an information carrier. This function is clearly and successfully demonstrated in guided wave optics and for the purposes of telecommunication or sensing. Impressive achievements are also demonstrated with full-field optical methods, in which spatially extended light beams, propagating in free space, are used to illuminate and create images of entire objects. Operating with light beams in free space offers the opportunity of performing many interesting types of measurement and our Nanophotonics & Metrology Laboratory devotes a reasonable part of its resources to the development of such full-field methods.
The suitability of optics for 3D investigations depends on a couple of deep roots. Solutions to the d'Alembert's equation in transparent, homogenous, linear media, together with elementary solutions to the diffraction problem, provide the key properties obeyed by optical waves. Although very well known, these basic properties are worth recalling:
- In a vacuum or in linear media, optical waves propagate without interacting between each other. When two or more waves are superimposed, the net result is simply the addition of the individual effects of each constituent wave, taken as isolated from each other. Wavefronts observe the superposition principle, first expressed by Thomas Young. The physical effects of the waves are primarily described by the electric field. Wavefront superposition is therefore described by the sum of the corresponding electric fields. In free space, as in guided optics, the visible bandwidth of the electromagnetic spectrum, centred around 7.1014 Hz, provides a huge capacity for the transmission of information.
- The 2D knowledge of the wavefront within an aperture is sufficient to obtain complete 3D knowledge of this wavefront propagating everywhere in space, as "seen" through this pupil. The finite spatial extent of this pupil only has the detrimental effect of restricting the possible 3D description of the full wave beyond a given accuracy - the resolution limit. However, coarser details are preserved. The reconstructed beams are said to be "diffraction limited". More precisely, the resolution limit is proportional to the wavelength, and inversely proportional to the angle subtended by the aperture in the observation region. This phenomenon is a direct consequence of the Huygens' principle. In the visible spectrum, and given standard aperture angles, resolution cells are typically several microns wide. Aerial wavefronts containing more than 107 independent cells per cm² are easily produced.
- Wavefronts propagate at the speed of light. In a vacuum, or in a medium like air with a refractive index quite close to 1, no signal can travel any faster.
- Due to the wave nature of light, a very large number of intrinsic and extrinsic optical quantities can be object-modulated for the purpose of carrying information or performing measurements. As intrinsic quantities enabling to probe objects at a distance we have: intensity, amplitude, phase and wavefront, frequency, wavelength, velocity, polarization, and coherence. Extrinsic quantities linked to measurement and monitoring possibilities include reflectance, transmittance, optical density, refractive index and birefringence, dispersion, absorption, diffusion.
For these reasons, optical methods dealing with extended light beams possess a high degree of parallelism: a multitude of points on an object surface can be interrogated simultaneously without any crosstalk; the information can be transmitted for detection and recording at a remote location, at the speed of light; perfect time resolution is available as long as only optics is involved. Furthermore, there are only vanishingly small interactions between the light and any object, due to the infinitesimal energy and momentum transfers to the object.
Imaging and interferometric methods offer a vast number of techniques and lend themselves to a large number of types of measurement. In the NAM context, we are principally interested in shape and deformation measurements, for which experimental mechanics is the principal field of application. Material science studies, structural engineering problems and non-destructive testing are concerned. Many disciplines, including civil and environmental engineering, biomechanics and biophysics, micro- and nano-technologies make daily use of these techniques.
In the early 60's, the invention of the laser and the birth of holographic and speckle interferometry gave an impressive impetus to the old science of interferometry. For the first time, demonstrations were given that classical interferometric measurements, only applicable by definition to optically polished surfaces, could indeed be extended to objects of arbitrary surface finish thanks to these new techniques. Furthermore, owing to the diffusing properties of rough surfaces, it appeared that there are many more possible optical arrangements in speckle than in classical interfererometry, combining imagery and interferometry, and leading to measurement types impossible to be carried out by means of the latter. As will be evoked below, the price to pay for this extraordinary extension of the domain was in the necessity to carefully analyse how the speckle phenomenon influences both image and interference fringe formation. The following examples give an idea of the appearance of different sorts of fringe patterns.
Examples of fringe patterns obtained at NAM (from left to right): a) classical Fizeau fringes of a computer hard disc; b) holographic interferometry fringes of a cylinder locally heated; c) contouring using coherent fringe projection on a cast of pathological scar; d) speckle interferometry fringes related to the in-plane deformation of a rubber sample; e) out-of-plane Bessel-type speckle interferometry fringes of a disc vibrating at a resonance frequency of 1.9 kHz.
The range and accuracy of our measurement techniques can be adapted to the problem in hand. For example, we have measured, in a single shot, the shape of specimens larger than 10 m² with a height resolution of 10-4 of the object's largest dimension and a spatial resolution of 4x4 mm². For deformation measurements, typical figures are a displacement resolution better than 10 nm for 105 independent points per single interferogram acquisition.
The clip below illustrates the high degree of spatial and temporal resolution that can be achieved using whole field methods. For about 5000 points over the object surface, it presents the time evolution, along the vertical axis, of the in-plane displacement component (u) of a rubber specimen. The sample is increasingly stressed in its own plane, along the line joining the points of maximum deformation. The maximum difference of displacement between these two points is 9.5 μm. The recording method used is in-plane speckle interferometry. The high resolution obtained in the displacement field is also worthy of note. Interferogram processing is carried out by methods developed in our group, based either on dynamic phase-shifting, Morlet wavelet or other definite transforms, or still signal dependent transforms.
Movie 1: High resolution, full-field dynamic micro-deformation measurement.
Lines of research
Our group acquired much of its competence and know-how while first working in the domain of holographic interferometry. Following on the one hand the progressive shortage of very high resolution recording media, in particular of silver halide emulsions and photothermoplastic plates or films, and on the other hand the increasing availability of photoelectric devices like CCD or CMOS cameras, the group turned its activities towards speckle interferometry techniques. Highly coherent sources illuminating unpolished surfaces invariably produce a 3D random distribution of light - the so-called speckle waves. These waves encode, in a quite intricate way, geometrical and micro-roughness characteristics of the diffusing surfaces, as well as their position. Emphasis was first laid on establishing the fundamental properties of these fields, following which we are trying to make optimal use of these properties for metrological purposes. The research lines in this direction can be summarised as follows:
Two examples of application
1. Evaluation of the reinforcement capability of composite laminates
A speckle interferometry method reveals the reinforcement effects of various types of laminate. The CFRP laminates are glued onto steel specimens to prevent the initiation and subsequent propagation of fatigue cracks.
|Fatigue machine with notched steel specimen||In situ speckle interferometry set up|
The results indicate that a dramatic increase of the useful life of structures can be achieved. The optical set-up is designed for in situ investigations, directly adapted to the fatigue testing equipment. A finite element computer model is validated on the basis of the optical measurements. This work was carried out in collaboration with the ICOM-Steel Structures Laboratory of the EPFL.
|Interferogram showing the progressive transfer of load bearing capacity from the partially cracked specimen to the CFRP laminate||Validation of the FEM simulation of the fatigue test (Abaqus & Diana)|
2. Dynamic mechanical analyses of welded and riveted plates
The video clip below shows the out-of-plane deformation motion of a rectangular plate of 1 m².
Movie 2: Movie showing the evolution of the out-of-plane component of the deformation of a vibrating plate; the colour code from red to violet and from red to green represents the formation of bumps and holes respectively.
The plate is subjected to an acoustic wave (in this case at 47 Hz) by means of a loudspeaker placed behind its centre. An out-of-plane configuration of speckle interferometry is used for recording the successive interferograms. The phase sensitivity of the arrangement is 23.62 radian/µm. The movie frames are separated in time by 1 ms. The maximum peak-valley deformation is 4.25 µm. The illumination source is a Q-switched YAG laser, emitting 5 ns light pulses at a repetition rate of 1 kHz. The imager is a 1 kHz CCD camera synchronised with the pulsed laser emission. In this particular case, we used the Fourier transform method, in association with the generation of straight carrier fringes, for obtaining the successive out-of-plane displacement maps. The work was carried out in the framework of the EUREKA VISILAS programme in collaboration with Holo3-Saint-Louis, France.
The Swiss National Science Foundation and the Technology and Innovation Commission (CTI) awarded our group a series of research grants, supporting in particular our theses works. We participated in the EUREKA "VISILAS" programme, grouping 9 European industrial partners and research centres. The programme ended with the delivery of a prototype of a speckle interferometry system operating at 1 kHz for in situ NDT applications and deformation measurements, applicable to specimens or structures greater than several square meters in size. Other projects have been conducted in close partnership with European institutions. Using holographic interferometry we analysed the thermal behaviour of the carbon-carbon structure of a space telescope intended for telecommunications, in collaboration with ESA-EADS. We evaluated the specifications of an adaptive X-ray mirror using speckle interferometry at 532 nm, with the ESRF in Grenoble. The current group interests lie in diffractive optical elements, numerical analysis of interferograms, digital speckle interferometry, and fringe projection techniques. In collaboration and with the guidance of IOA, EPFL, optical trapping and binding of particles at the mesoscopic level has recently been added to the research activities. The latter and the project of rationalising and optimising the processing of fringe patterns and interferometric signals are presently supported by Swiss National Science Foundation grants.
- Massimo Monti, CREHOS. Un scanner laser à détection holographique pour la reconnaissance en temps réel de fissures dans les revêtements routiers, (A laser scanner and a crack recognition holographic system for real time road pavement inspection), Thesis Nº 1278, EPFL, 1994.
- Xavier Colonna de Lega, Processing of non-stationary interference patterns: adapted phase-shifting algorithms and wavelet analysis. Application to dynamic deformation measurements by holographic and speckle interferometry, Thesis Nº 1666, EPFL, 1997.
- Mathias Lehmann, Statistical theory of two-wave speckle interferometry and its application to the optimization of deformation measurements, Thesis Nº 1797, EPFL, 1998.
- Michel Cherbuliez, Wavelet analysis of interference patterns and signals: development of fast and efficient processing techniques, Thesis Nº 2377, EPFL, 2001.
- Anne-Isabelle Desmangles, Extension of the fringe projection method to large objects for shape and deformation measurement, Thesis Nº 2734, EPFL, 2003.
- Sébastien Equis, Phase extraction of speckle interferometry signals, EPFL thesis, completion scheduled for the end of 2009.
At different levels, in the EPFL at Lausanne and abroad, see e.g.:
by means of specialised lectures, seminars, short courses or laboratory works, we participate in teaching full-field optical methods, mainly holographic and speckle methods, and in training both young and seasoned engineers and scientists. We regularly welcome student exchanges and academic visitors for typically 2-4 month stays in our group for developing agreed joint projects.