FR2728361A1 - Holographic interferometer e.g. for refractive index measurement - Google Patents

Abstract

The interferometer includes a first study beam (ER,EB) which is normally red and blue and has a common axis in the measurement volume (E) and the reference beams (RR, RB) have axes which with the respective study beams, define planes that are approximately orthogonal. A slightly angled prismatic plate allows a variable shift between the reference and study beam of each laser and synchronised electronic shutters at each laser allow illumination by each laser at an identical moment. A similarly constructed third beam may be added using a third laser of distinct wavelength. The study and reference beams are orientated relative to each other to avoid the superimposition of parasitics on the useful images on reconstitution of the hologram.

Description

 The present invention relates to optical interferometry. It applies to the visualization and measurement of refractive index inhomogeneities in gaseous, liquid or solid transparent media, but also to the visualization and measurement of deformations of opaque objects subjected to stresses.

 Holographic interferometry is a known technique in which a coherent light source (laser) is used for the production of monochromatic interferogranes.

 These interferograms are generally obtained using two successive recordings on the same holographic plate placed in a recording position. A reference hologram of 1 object constitutes the first recording. It is followed by a hologram for measuring the object under the conditions of the experiment, constituting the second recording.

 After development, this holographic plate is placed again in the recording position and an adapted lighting of this plate allows the visualization, by interference fringes, displacements or modifications of low amplitude of the object studied compared to its position or shape in the reference conditions.

 This holographic interferometry technique is, for example, described in the article in Engineering techniques entitled "Visualization and optical methods of visualization in aerodynamics" by A. Boutier, n. Philbert, J. Surget and C. Veret, Edition 4-1978, chapter R 2160. This monochrome technique does not allow an identification of the order of interference of the fringes in the interference images, which can be, in some case, a major drawback in the field of aeronautics for example.

 There is also another known technique, color holography.

 If the light source and the observer are located on the same side of the plane containing the holographic plate, the holograms are called of the "Bragg-Lippmanw or" Denisyuk "type and the technique allowing their development is based on the phenomena of multiple reflections and d Optical interference. The image recorded in the hologram can be reproduced in white light, for example by means of a simple electric bulb with reduced filament. Recordings according to this technique can practically only be made in an optical laboratory and the photo processing of holographic plates must be very careful if one wishes a faithful chromatic restitution.

 If the light source (s) and the observer are located on either side of the plane containing the holographic plate, the holograms are called "by transmission" and the light sources must be lasers emitting at different wavelengths. the photographic technique is simpler to implement than the previous one, but there is a risk of superposition, on the useful image, of undesirable secondary images. These secondary images are due to the use of distinct light sources emitting at several wavelengths.

On this subject, one will quote in particular the work of
Graham Saxby, entitled "Practical holography", Prentice
Hall International (UK) Ltd (1988 - pages 61 to 67 and 260 to 272).

 A first object of the invention is to propose a device and a method allowing the production of holographic interferograms in which it is possible to identify by means of the color the interference fringes and to recognize them in relation to each other. .

 In fact, it is known in the study of interference that the use of a monochromatic light source generates the production of fringes with identical sinusoidal intensity profiles, the fringe of order 0 for example cannot be distinguished from the fringe of order 1, 2, etc ...

 On the contrary, white light interferometry gives rise to systems of fringes with variously colored sinusoidal intensity profiles, of different steps, superimposed on each other, making it possible to easily recognize the position of a given fringe and, in particular, to distinguish the central fringe, of order 0, from the other fringes.

 The first object of the invention is therefore to offer, in holographic interferometry, the advantages of interferometry in white light. This makes it possible, among other things, to recognize a given fringe on either side of a discontinuity such as that generated on the optical path by a shock wave in a gas flow, or on either side of a crack. in a mechanical room.

 The second object of the invention is to allow, using conventional interferometry in white light, to carry out a so-called "sensitive shade" adjustment which, in the absence of phase differences between the two states compared by interferometry, produces a shade uniform of purple color.

 A small phase variation then causes a modification of this uniform hue which turns blue or red, depending on the sign of the phase shift, and allows very high sensitivity to be obtained, unlike the flat hue obtained in monochromatic interferometry, in which the same phase change will only produce a change in brightness that is difficult to detect.

 This "sensitive shade" adjustment and its use are obviously particularly useful when observing and measuring very small phase variations.

 The invention can be implemented for the display of phase shifts, whether they are produced by transparent phase objects or by opaque objects, by reflection.

 The invention provides a method and a device for making interferograms comprising several colors.

 To this end, the invention relates to a holographic interferometer comprising a first laser emitting a first beam at a first wavelength 11, a first beam splitter producing a first reference beam and a first study beam and optical means study forming a measurement volume on the first study beam and directing the first study beam towards a hologram recording plane and first optical reference means directing the first reference beam towards the plane of hologram recording.

 According to the invention, this holographic interferometer comprises a second laser emitting a second beam at a second wavelength 2 a second beam splitter producing a second reference beam and a second study beam, the second study beam being coaxial of the first study beam, in the measurement volume at least, and the optical study means forming a measurement volume on the second study beam and second reference optical means directing the second reference beam towards the hologram registration plan.

 Preferably and in order to remove, during the restitution of the hologram, the spurious images due to the presence on the holographic plate of the holograms recorded at the two wavelengths A 1 and A2, each beam having an axis, the axes study beams being partially common and defining, with the axis of the first reference beam, a first plane and, with the axis of the second reference beam, a second plane, the first plane and the second plane are not confused.

 These first and second planes are advantageously approximately orthogonal.

 In order to facilitate the use and implementation of the holographic interferometer of the invention, it advantageously includes optical means making it possible to introduce a variable phase shift between each study beam and the corresponding reference beam.

 Furthermore, the interferometer advantageously comprises an electronic shutter at the output of each of the lasers, these shutters being synchronizable and allowing, for example, the printing of the holographic plate at an identical instant of illumination of the study volume by each of the two lasers.

 It may be interesting, for certain experiments, to have a wider chromatic palette. Still according to the same device, in this case, a third beam at a third wavelength λ 3, a third beam splitter producing a third reference beam and a third study beam is added to the interferometer. The third study beam is coaxial with the first and second study beams, in at least the measurement volume, and the optical study means form a measurement volume on the third study beam and third optical means The reference beam directs the third reference beam towards the hologram recording plane.

 We indicated above that the respective orientation of the planes formed by the axes of the beams can be used to minimize the effect of spurious images.

In order to further minimize the effect of these spurious images, the study beams and the reference beams can be oriented relative to one another, so as to avoid the superimposition of spurious images on the images useful during the restitution of the hologram.

 The invention also relates to a method of viewing variations in optical path introduced by an object by holographic interferometry comprising at least one exposure step and a restitution step.

 According to the invention, during each of the exposures, the object is illuminated by a first pair of coaxial, monochromatic study light beams, of different wavelengths and these two study beams interfere, in a plane of recording, each with one of the two associated reference beams, these last two beams constituting a second pair of beams, the three axes of these four beams being non-coplanar.

 According to an advantageous embodiment of the method, phase shifts of opposite signs are introduced after a first exposure step on the two beams of the same pair, before the second exposure step. It is advisable to give the phase shifts the same absolute value equal to n / 2 radians, in order to obtain maximum sensitivity.

The invention will be described in more detail with reference to the accompanying drawings, in which
- Figure 1 is a schematic elevational representation of a device according to the invention
- Figure 2 is a schematic plan view of the device of Figure 1
- Figure 3 is a schematic perspective representation of the general architecture of the assembly of the invention
- Figure 4 is a representation of an interferogram of a weakly prismatic plate obtained according to the invention
- Figure 5 is a schematic representation of an object used to highlight the quality of the results obtained with the invention
- Figure 6 is a schematic representation of the interferograms obtained by studying the object of Figure 5 according to the invention
- Figure 7 is a schematic representation of interferograms obtained by studying a plane-parallel glass slide having thickness and refractive index defects of very low importance;
- Figure 8 is a schematic representation of the different distributions of useful and parasitic interference images reproduced in the recording plane.

 The particular embodiment represented and described here in detail corresponds to a device allowing the implementation of the method of holographic multichrome interferometry proposed by combination of two distinct coherent radiations adjustable in amplitude and in phase.

 The embodiment represented here is intended for the analysis of inhomogeneities of transparent media, the importance of which is recognized for the visualization of the phenomena occurring in aerodynamic wind tunnels.

These devices can be used for the observation of a circular field with a diameter of 80, but fields ten times larger can be reached without difficulty.

 The first LR laser (Red Laser) is a 15 mW neon helium laser emitting at a wavelength of 632.8 nm and the second LB laser (Blue Laser) is an ionized argon laser with a power of 2 W (cumulative over all the lines) set to the wavelength of 457.9 nm. In general, better results are obtained when the wavelengths A, and 2 are very different. They are advantageously each located at one end of the visible spectrum.

The beam emitted by each of the lasers, respectively
FR, FB is divided by a semi-transparent plate S1, S2, so that each of them produces a reference beam RR and RB, and a study beam ER, EB.

The conjunction of the mirror M1 interposed on the first study beam and the semi-transparent plate S3 placed at the intersection of the first and second study beams
ER, EB, (as well as their adjustment) allow the superimposition of the two study beams. These beams are thus coaxial.

 The high-power lens L1 is intended to open up the study beams, so as to fill the pupil of the collimator objective 01. The beam of parallel rays emerging from 01 is then collected by a second collimator objective 02 whose focus F is located beyond the hologram registration plane (P).

Thus, objectives 01 and 02, not represented on the
Figure 3, placed on the path common to the first and second study beam, determine a volume E of measurements common to each of the beams, ER, EB. In this volume, the ER and EB beams are coaxial.

 A photographic plate H is placed in the hologram recording plane P on the common path of the study beams downstream of the volume E of measurements, and upstream of the focus F of 02.

 Each of the reference beams RR, RB is directed by first and second optical reference means constituted by the mirrors M2, M3, M4, NS and by the optical systems L2 and L3, so that they each interfere with the beam corresponding study plan hologram recording plan.

Two very slightly prismatic LP1 glass slides,
LP2 each placed on a beam and secured to a mechanical device allowing translations to be applied to them, are intended to control the phase shift between each study beam (ER, EB) and the reference beam (RR and RB) which associated with it.

The planes VL (X, Z) and HL (X, Y), respectively foreground and second plane, formed by the axis of each of the reference beams RR, RB with its study beam ER,
Associated EB, are approximately orthogonal.

 Different examples of implementation of the invention highlighting its advantages will now be described with reference to Figures 4, 6 and 7 showing the interferograms obtained during these different experiments.

 The representations involving colors are difficult here. The fringes were symbolized by continuous lines placed at the maximum intensity of the blue fringes and discontinuous lines placed at the maximum intensity of the red fringes.

 The first two experiments aim to show that the invention allows the identification of the different fringes, even in the event of discontinuities in the differences in path, that is to say a break in the array of fringes.

 The interferograms (Figures 4 and 6) were obtained by applying the holographic double exposure method. A first exposure of the holographic plate H is carried out before the introduction of the object into the field of the instrument and a second exposure of H is then carried out in the presence of it.

 The object studied during the first experiment is a transparent rectangular glass slide 120 n long by 40 mm wide with a thickness of approximately 1.3 mm.

 This strip has flatness defects but especially of parallelism of its faces, which form differences in walking of small amplitude (Figure 5A).

The recording is carried out by simultaneous lighting of the photographic plate by the four RB beams,
EB, RR and ER and the lighting of the hologram, after its development and its replacement, by the two reference beams RR and RB.

 We thus obtain with this object two sets of fringes, respectively blue 1 to 5 and red 6 to 9 (Figure 4), approximately parallel, of different pitches.

During the recording, each of the pairs of beams at a wavelength ER and RR on the one hand, EB and
RB on the other hand, generates a network of microfringes recorded on the holographic plate H.

 The use of several monochromatic light sources generates a risk of superimposing unwanted images on the useful image, when reading the holographic plate. The invention makes it possible to avoid such a drawback.

We now refer to Figures 8. The arrangement of the various elements represented is that observed from the plane of the holographic plate H. The reference light sources are represented by two points
LR and LB, respectively for the red laser and the blue laser. The analysis field where the useful image is formed is centered on a cross materializing the common axis of the two analysis beams.

 In the case of a single laser source, red for example, the recorded hologram renders on reading (FIG. 8A) a direct red IRR image in the field, but also a conjugate red CRR image, out of field.

 When a second source at another wavelength, blue for example, is added, the red and blue holograms are recorded on the same plate.

 In the ideal case where each source would only illuminate the hologram that it made it possible to obtain, we would obtain (Figure 8B) the useful restored image, of purple color, coming from the superposition of the two direct images IRR + IBB centered in the field and two CRR and CBB conjugate images located on either side of the field.

But in fact, each source illuminates the two holograms giving rise to undesirable secondary images generated by the illumination of the red hologram by the blue laser and the blue hologram by the red laser. Four additional images appear
IRB, IBR, CRB, CBR (Figure 8C), some of which may partially overlap the useful image in the field.

This is the case, in our example, of two conjugate images, one CBR recorded with the blue source and restored by the red laser, and the other CRB recorded with the red source and restored by the blue laser, which generate a parasitic CBR + CRB purple image in the field.

 This undesirable superposition results from the fact that the angle a, formed by the two planes defined one by the reference beam RB and the study beam E, and the other by the reference beam RR and the beam of study E, has not been defined.

 In the case of the invention, this angle has been chosen to be close to 90 ". The set of eight images is shown in Figure 8D.

 The useful IRR + IBB purple image is well isolated from the other images and, in particular, from the most troublesome purple parasitic image CBR + CRB.

 The second experiment (Figure 6) was carried out with the phase object, shown in Figure 5B, which was obtained by cutting longitudinally the strip used in the first experiment and by translating one of the two halves a, b thus obtained in relation to each other. The points denoted A, B make it possible to locate the identical zones of these two blades a, b.

 The process used is identical to that of the first experiment described above.

 The object studied therefore presents here discontinuities which are quite comparable to those commonly encountered in practice, when viewing the phenomena occurring in aerodynamic wind tunnels.

 In Figure 6, we have shown the interferograne obtained, composed of two networks of fringes, blue 10 to 14 and red 15 to 18 respectively. One of these networks would not allow the same fringe to be recognized on either side of the object 20 breaking. On the contrary, the presence of two distinct color networks makes it possible to follow a set of two fringes, for example 10, 15 on either side of the break 20, without it being possible to be confused with another set of two fringes , for example 11 and 16 or 12 and 17, due to the spacing of the two fringes 10 and 15 which remains approximately constant on either side of the break 20.

 The recognition of a single fringe in real experimentation by the particular spread of color that it presents and, here by the spacing between the blue fringe (solid line) and the red fringe (broken line), makes it possible to have an interference state reference common to the different parts of the field separated by the discontinuities. Thus, from the location of this original interference state in each of the parts of the field, the counting can be carried out from fringe to fringe until the determination of the interference state in the whole of the picture.

 Such situations are found more particularly when examining aerodynamic and supersonic flows, for example shock waves and vortex regions.

 Likewise, in non-destructive testing of mechanical structures and other solids, discontinuities in fringe arrays are frequently encountered. They are, for example, due to cracks and mechanical play of the constituent parts.

 Compared to monochromatic interferometry, the third experiment is intended to highlight the gain in sensitivity provided by holographic color interferometry, when its setting is adjusted to the sensitive shade.

 The sensitive hue of the restored study beam, purple, is obtained by modifying in a controlled manner, between the two recordings (without then with the phenomenon to be analyzed), the phases, either of the two blue and red reference beams, or two blue and red analysis beams. These phase modifications, obtained using phase shifting means, denoted LP1 and LP2 in Figures 1 and 2, must be of opposite signs depending on the color and have a value of o / 2 radians to obtain the optimal sensitivity.

 The search for such sensitivity is justified, for example, when studying gas flows with very low density in which only path differences occur much less than the wavelength of the light of the beams. of study. There is then no appearance of a network of fringes, but only slight local changes in luminosity.

 Such a phenomenon can be simulated using the object of Figure 5B. The object is kept in the analysis field during the two recordings. The two parts a and b are only slightly translated, in opposite directions, for the second recording, which leads to modifications of optical paths of opposite signs for the parts of the beams passing through part a or part b. In our example, these modifications are of the order of 32 manometers, which corresponds approximately to 1 / 20th of the wavelength for the red radiation and 1 / 14th of the wavelength for the blue radiation.

 Calculations and experience show that this difference in optical path leads to a slight decrease in the light intensity of each radiation compared to the purple background, if the adjustment to the sensitive shade has not been made between the two recordings.

 On the other hand, if it was carried out by applying the optimal phase shifts of - n / 2 and + iT / 2 radian as indicated above, the chromatic contrast is considerably improved, as illustrated in Figure 7.

 The background 23 appears purple, but the two zones corresponding to the object to be measured 21, 22 have turned one to blue, the other to red. The retina being very sensitive to the chromatic contrast between these two colors, the eye easily differentiates these areas 21, 22, even with a very low light contrast.

 Thus, the invention allows the monitoring of a fringe in the event of a break in the gait difference in the object studied and the visualization of very small gait differences.

 Although the invention has been presented in the case of the examination of transparent media, it can be easily transposed for the study of deformations of opaque objects.

 It was proposed above to use movable prismatic glass slides to introduce the necessary phase shifts LP1, LP2. Of course, a similar result can be obtained by using movable mirrors, for example by means of piezoelectric translators or transparent media with adjustable refractive index.

 Furthermore, the method described in the examples is the holographic interferometry method by double exposure. Those skilled in the art are able to transpose the invention to the real-time interferometry technique.

 The reference signs inserted after the technical characteristics mentioned in the claims, have the sole purpose of facilitating the understanding of the latter, and in no way limit their scope.

Claims (10)

 1. Holographic interferometer comprising a first laser (LR) emitting a first beam (FR) at a first wavelength A1, a first beam splitter (S1) producing a first reference beam (RR) and a first beam (ER ) study and study optical means forming a measurement volume (E) on the first study beam (ER) and directing the first study beam (ER) to a recording plane (P) d hologram and first optical reference means directing the first reference beam towards the hologram recording plane (P),  characterized in that it comprises a second laser (LB) emitting a second beam (FB) at a second wavelength A2, a second beam splitter (S2) producing a second reference beam (RB) and a second beam (EB) for study, the second beam (EB) for study being coaxial with the first beam (ER) for study, in the measurement volume at least, and the optical study means forming a measurement volume on the second study beam and second optical reference means directing the second reference beam (RB) towards the plane (P) of recording the hologram.  2. Holographic interferometer according to claim 1, characterized in that each beam having an axis, the axes of the study beams being common, in the measurement volume at least, and defining, with the axis of the first reference beam (RR), a first plane and, with the axis of the second reference beam (RB), a second plane, the first plane and the second plane are not confused.  3. Holographic interferometer according to claim 2, characterized in that the first plane and the second plane are approximately orthogonal.  4. Holographic interferometer according to any one of claims 1 to 3, characterized in that it comprises optical means making it possible to introduce a variable phase shift between each study beam and the corresponding reference beam.  5. Holographic interferometer according to any one of claims 1 to 4, characterized in that it comprises an electronic shutter at the output of each of the lasers, these shutters being synchronizable.  6. Holographic interferometer according to any one of claims 1 to 5, characterized in that it comprises a third beam at a third wavelength λ 3, a third beam splitter producing a third reference beam and a third beam d study, the third study beam being coaxial with the first and second study beams, in the measurement volume at least and the optical study means forming a measurement volume on the third study beam and the third optical reference means directing the third reference beam towards the hologram recording plane (P).  7. Holographic interferometer according to any one of claims 1 to 6, characterized in that the study beams (ER, EB) and the reference beams (RR, RB) are oriented relative to each other, so to avoid the superimposition of spurious images on the useful images during the restitution of the hologram.  8. Method for viewing variations of the optical path introduced by an object, by holographic interferometry comprising at least one exposure step and one restitution step,  characterized in that, during each of the exposures, the object is illuminated by a first pair of coaxial, monochromatic study light beams, of different wavelengths, and that these two study beams interfere, in a plane (P) recording, each with one of the two associated reference beams, these last two beams constituting a second pair of beams, the three axes of these four beams being non-coplanar.  9. Method according to claim 8, characterized in that after a first exposure step, phase shifts of opposite signs are introduced, on the two beams of the same pair, before the second exposure step.  10. Method according to claim 9, characterized in that the phase shifts have the same absolute value equal to n / 2 radians.