FR2731086A1 - Recording of colour holograms on photosensitive material using illumination in stages by several different coherent wavelengths along common path to record interference fringes - Google Patents

Abstract

A recording of the object (17) may be made by successively increasing wavelengths or simultaneously at the different wavelength sun a single layer photosensitive support (18), where three wavelengths are used, 488nm, 532nm and 610 nm may be selected. A reflector (9) is placed in the bram of each of the sources (5-7) in turn to send the beams along the common path (16). Alternatively, one reflector and two partial reflectors and transmitters enable three simultaneously beams. Fibre optic cables may be used with a pair of two-input couplers two produce a single beam in an optical fibre directed towards the object.

Description

 The present invention relates to a method for recording color holograxses by wavelength multiplexing and to a device for implementing this method.

 For many applications such as, for example, holographic microscopy or endoscopic holography, the resolution of holographic images -that is to say the ability to be able to clearly distinguish on the vines restored by the holographic recording two points distinct slightly distant from each other- is of essential importance.

 The best resolutions currently known are of the order of about three microns and have been obtained using monochromatic holographic recordings. These resolution values, although already high, prove however still insufficient for certain applications such as endoscopic holography applied to medicine, or resolutions of the order of a micron, corresponding to the size of the nucleus of a cell. individual human tissue, are commonly sought after.

 The monochromatic aspect of the holographic recordings described above also constitutes an important limit to the development of holographic applications. It is indeed difficult for a doctor, for example in the case of a holographic cytological analysis, to establish a reliable diagnosis on the basis of the examination of a monochromatic image on which the tissues, cells and blood are differentiated only by faint nuances of a colored nene.

 The object of the present invention is to remedy these difficulties by proposing a method for recording holograms in color by multiplexing the light waves used to record the hologram as well as a device for implementing this method.

 The general principles underlying the production of a hologram are well known: a sensitive surface or recording surface receives the light wave coming from the object to be reproduced in just one other wave, consistent with the first, called reference wave. These two waves create, in the thickness of the recording material, a system of interference fringes constituting the hologram proper. To read it, it is then necessary to illuminate the recording by means of a new light wave, called reading wave, vis-à-vis which the system of interference fringes behaves like a diffraction grating. diffracted by the system of fringes, one is identical to that emitted by the object and the eye that receives it does not make the difference, thus making it possible to create in the observer the impression of three-dimensionality.

 The recording of interference fringe systems and the conditions for the formation of images from these networks are closely linked to the degree of coherence of the light sources available.

The appearance and continuous improvement of laser radiation sources have made it possible to make significant progress in the field of holography.

 Several factors, however, still considerably limit the quality of holographic images, and in particular their resolution. Among these factors, one of the most important is said to be "phenomenon of coherent granularity" and can be explained succinctly in the following manner. Each irregularity in the hologram behaves like a light source emitting a spherical wave. These waves are coherent and interfere in the space beyond the hologram. As the phase shift between these waves is random, so is their state of interference. Finally, the background noise of granularity (also called "speckle" by those skilled in the art) is formed by a plurality of bright spots standing out against a more obscure background.

 According to the Rayleigh resolution criterion, this is assessed as a difference in brightness between two neighboring points. It is therefore easily understood that the background noise of granularity negatively affects the resolution of the holographic image, and it is usually estimated that the details of a holographic image must be three to four times greater than the size of granularity in order to be able to be distinguished.

As described above, the granularity can be represented as a regular network of light points whose mean statistical diameter 2R is given by Rayleigh's formula
2R = 1.22 k / nsina (1) where. R is the average radius of a light point, XX is the wavelength of light in the
empty, nn is the refractive index of the middle of
propagation of the wave between the object and the support
registration, ns usina is the digital opening of the system
optical.

 The nsina digital aperture depends exclusively on the geometry of the optical system. As for the refractive index n (X), its variation An as a function of the wavelength is very small, of the order of 10-5.

 Leith and Yang then showed that a first way to limit the phenomenon of granularity and therefore to improve the resolution of holographic images, was to increase the size of the light source, and therefore to decrease the spatial coherence of it. this.

 According to its essential characteristic, the inventive method proposes to improve the resolution of holographic color images by multiplexing at least two different recording wavelengths, that is to say by superimposing the granularity networks linked to each of the wavelengths used to illuminate the object for which the hologram is to be recorded.

 This arrangement proves to be quite remarkable since it makes it possible both to improve the resolution of holographic images by a factor of two while offering the user color recordings.

 The device for implementing the method according to the invention will preferably comprise three coherent lighting lines of different wavelengths, for example three laser lighting lines, the additive combination of the granularity networks of which is obtained by interposing on the path of each of the beams a dichroSque mirror or a fiber optic multiplexer directing the beams in the same direction of propagation towards the object to be illuminated.

Other characteristics and advantages of the present invention will emerge more clearly from the description which follows of a particular and nonlimiting embodiment of the device for implementing the method according to the invention with reference to the accompanying drawing in which FIG. 1 is a schematic view of the principle of a first embodiment of the device according to the invention; FIG. 2 is a schematic view of the principle of the Lippmann interference photography device; Figure 3 is a schematic view of the principle of a second embodiment of the device according to the invention;
Figure 4 is a schematic principle view of a third embodiment of the device according to the invention.

 The invention proceeds from the general inventive idea of multiplexing the wavelengths used to record the hologram of a given object. Thanks to this arrangement, the granularity networks of each wavelength are averaged by superimposing them, making it possible to reach resolution values for the hologram and its restored image never before achieved, even in monochromatic holography.

 The optical device or system according to the present invention, represented in FIG. 1 and designated as a whole by the general reference 1, preferably comprises, but is not limited to, three lighting lines 2, 3 and 4 each comprising a coherent lighting source , for example three sources of laser radiation 5, 6 and 7.

 According to a first alternative embodiment, the recording device 1 comprises a dichroic mirror 8 of which a reflecting surface 9 forms an angle ss, for example of 450, with the directions of propagation 10, 11 and 12 of the incident laser beams 13, 14 and 15 of respective wavelengths X1, X2 and X3 produced by the laser sources 5, 6 and 7.

 According to an essential characteristic of the inventive method, the laser beams 13, 14 and 15 are reflected by the mirror 8 in the same direction of propagation 16 and come to light an object 17 whose hologram is to be recorded through a recording medium 18 .

 The recording of the hologram being carried out sequentially, by successive illuminations of the object 17 with the three light waves X1, X2 and X3, the mirror 8 is mounted mobile on an optical bench 19 between two extreme positions 20 and 21 determined by the geometry of the optical system 1 according to the invention.

 When recording the wavelength X1, the mirror 8 is brought into position 20 so as to reflect the incident laser beam 13 in the common propagation direction 16 towards the object 17 to be illuminated. The same operation is repeated for the wavelengths X2 and X3.

 According to the essential advantage of the invention, the refection of the beams 13, 14 and 15 in the same propagation direction 16 makes it possible to superpose the granularity networks linked to each of the wavelengths X1, X2 and A3, and therefore to significantly improve the resolution of the holographic recording by averaging the granularity networks and therefore reducing the background noise that they generate.

 The slightest difference in the relative positioning of the mirror 8 and of each of the laser beams 13, 14 and 15 having irreversible consequences on the final quality of the holographic recording, the movement of the latter along the optical bench 19 will preferably be automated. , for example by means of a step-by-step motor controlled by a computer (not shown in the figures).

 Furthermore, on the propagation direction 16 of the laser beams 13, 14 and 15, there is a shutter 22, the opening and closing of which control the passage and then the extinction of the recording light waves, as well as a spatial filter 23 intended to ensure a uniform spatial distribution of these waves over the entire surface of the recording medium 18.

 For reasons of clarity of the figure, the optical bench 19 is shown parallel to and distinct from the direction of propagation 16 of the laser beams 13, 14 and 15, and the optical accessories such as the mirror 8, the shutter 22 and the spatial filter 23 dissociated therefrom.

 As is clear from FIG. 1, and according to another of its essential characteristics, the inventive method proposes to apply to the recording of color holograms the principles established by Lippmann for interference photography which are briefly recalled. below.

 The Lippmann device represented in FIG. 2 and generally designated by the general reference 24 essentially comprises a recording medium 25 consisting of a layer of photosensitive emulsion 26 deposited on a rigid substrate 27, for example a glass plate , and brought into contact, on its rear face 28, with a surface of mercury 29 with high reflecting power. This then reflects light into the emulsion layer 26 where it interferes with light waves such as 30 coming from an object 31 to be photographed, forming a network of interference fringes throughout the thickness of the emulsion 26 with a periodic spacing of X / 2n, where n is the refractive index of the emulsion and X the wavelength of light in a vacuum.

 By analogy with the Lippmann device 24, and according to another essential characteristic of the invention, the object 17 whose hologram is to be recorded is placed behind the recording medium 18 with respect to the direction of propagation of the laser beams 13 , 14 and 15 along the common propagation direction 16. The light reflected by the object 17 is returned to the recording medium 18 where it interferes with the incident light waves X1, X2 and k3. Finally, the light waves X1, X2 and X3 simultaneously play the role of reference waves and illumination waves of the object 17. The holographic recordings thus obtained are said to be by reflection, the light waves coming from the object. 17 and containing the information wimagew reflecting first on this before sensitizing the recording medium 18.

 According to a variant, the individual laser sources 5, 6 and 7 can be replaced by a single source of emission wavelength tunable over the range of the wavelength spectrum sought for recording the hologram.

 The recording medium 18 is preferably a support of the monolayer type consisting of a photosensitive recording layer 32, for example a silver halide emulsion with very high resolving power or any other known recording material such as , for example, dichromatic gelatin, deposited on a rigid substrate 33, for example a glass plate.

 The hologram is recorded sequentially, by successive illuminations of the object 17 with the light waves Xi selected where i = 1, ..., N is the number of exposures.

As is well known, the diffraction efficiency of a holographic recording varies inversely proportional to the square of the number
N of successive exposures of the recording medium 18. For this reason, the number of wavelengths Xj is preferably limited to three, which makes it possible to achieve a satisfactory compromise between the rendering of the colors of the holographic image and the diffraction power of the hologram itself.

 The choice of recording wavelengths Xi is also of great importance and largely determines the color rendering of the holographic image. One classically chooses three wavelengths Xi corresponding to the primary colors, namely blue, green and red, for which different triplets of values have been proposed in the literature. Hubel and Solymar have for example proposed 464, 572 and 606 nm respectively in the case of a holographic recording medium of the sandwich type. More generally, taking into account the characteristics of the laser radiation sources currently available on the market, they propose 458, 529 and 633 nm as an optimal combination of the wavelength values Xi for recording holograms in color.

According to the preferred embodiment of the invention, the wavelengths chosen for recording the hologram are X1 = 488 nm for blue, X2 = 532 nm for green and X3 = 610 nm for red . The light wave X1 = 488 nm is produced by an argon laser, the light wave X2 = 532 nm by a laser
Nd: YAG doubled in frequency and the light wave X3 = 610 nm by a HeNe laser. The value of 488 nm used for the blue color makes it possible to obtain a network of granularity in blue which is brighter and better balanced compared to the other two colors.

 We start classically by recording the shortest wavelength, namely X1 = 488 nm corresponding to the blue color, this being the most difficult color to register because the most sensitive to the diffraction phenomena occurring in the corresponding part of the spectrum of the photosensitive emulsion. The colors green X2 = 532 nm and red X3 = 610 nm are recorded in succession, with longer exposure times than the color blue, in agreement with the work of Johnson.

 The diffraction efficiency of the holographic recording also depends on the time interval between two successive exposures Xj, Xj j + j. A first solution conventionally consists in reducing the duration of the time interval separating two successive recordings Xi, Xj to a value 7i, j where the phenomenon remains insensitive. It is experimentally observed that a value of ri-j of less than 2.5 s allows this condition to be verified.

 According to a new and entirely surprising arrangement for those skilled in the art, the inventive method proposes to simultaneously expose the recording medium 18 to all of the recording waves Xi.

 As shown in FIG. 3, the movable mirror 8 is replaced for this purpose by a more complex optical assembly comprising a first fixed dichroic mirror 34 arranged on the path of the incident laser beam 13 and two additional fixed mirrors 35 and 36 arranged on the path beams 14 and 15 making it possible to superimpose successively and simultaneously beams 13 and 14 then 15 in the direction of common propagation 16.

 According to another variant implementation of the inventive method, the output of the laser sources 5, 6 and 7 is provided with optical fiber couplers 37, 38 and 39 respectively for the injection of the incident laser beams 13, 14 and 15 into fiber optic segments 40, 41 and 42, the additive and simultaneous combination of the beams 13 and 14 then 15 being produced by means of couplers with two inputs and an output 43 and 44. Finally, the output of the coupler 44 is connected to a segment fiber 45 for directing the laser beam resulting from the superposition of the three incident beams 13, 14 and 15 towards the object 17 to be reproduced.

Claims (10)

REVBDIQTIOs  1. A method for recording color holograms comprising the steps of illuminating the object (17) to be reproduced by means of a plurality of recording light waves ki produced by sources of coherent light and of recording a system of interference fringes in the thickness of a photosensitive recording medium (18), characterized in that the recording light waves Xi are multiplexed, that is to say directed according to a same direction of propagation (16) towards the object (17) to be reproduced.  2. Method according to claim 1, characterized in that the object (17) to be reproduced is illuminated by the recording light waves Xj through the photosensitive recording medium (18).  3. Method according to one of claims 1 or 2, characterized in that the holographic recording is carried out sequentially, by successive exposures of the object (17) to be reproduced at the different wavelengths of recording #i .  4. Method according to claim 3, characterized in that the holographic recording is carried out in the increasing order of the recording wavelengths Xi.  5. Method according to one of claims 1 or 2, characterized in that all the light waves Ai are recorded simultaneously.  6. Method according to any one of claims 1 to 5, characterized in that three different recording wavelengths Xi are used.  7. Method according to claim 6, characterized in that the recording wavelengths are X1 = 488 nm, X2 = 532 nm and B3 = 610 nm.  8. Method according to any one of claims 1 to 7, characterized in that the recording medium (18) is of the monolayer type.  9. Device for implementing the method according to any one of claims 1 to 8 comprising a plurality of sources of coherent optical radiation emitting recording light waves Xi for recording, in the thickness of a support d photosensitive recording (18) of a system of interference fringes constituting the holographic recording of an object (17) to be reproduced, characterized in that it comprises a reflecting device interposed on the path of each of the lengths d recording wave xi incident and reflecting these in the same direction of propagation (16) towards said object (17) to be reproduced.  10. Device according to claim 9, characterized in that the output of each of the sources of coherent optical radiation is provided with a fiber optic coupler for the injection of the incident light recording wave Xj into a fiber section individual optics connected at its other end to one of the inputs of a coupler with two inputs and a single output for the additive and simultaneous combination of all the light waves Xj, the resulting cumulative optical signal being directed along a single optical fiber (45) towards the object (17) to be reproduced.