DIFFRACTIVE ILLUMINATION DEVICE WITH INCREASED DIFFRACTION ANGLE

Abstract

The present invention relates to an optical device for generating a diffracted image on a support (S), said device comprising a diffractive optical element (5) arranged to diffract an optical beam, in order to generate a diffraction image. The device further comprises an optical power mirror (7.1; 7.2; 7.3; 7.4; 7.5), the mirror being placed with respect to the diffractive optical element so as to project the diffraction figure towards the support to obtain a magnified version of the diffracted image.

Claims

1. An optical device for obtaining a diffracted image on a support, said device comprising: a diffractive optical element arranged to diffract an optical beam, so as to generate a diffraction image, and an optical power mirror placed with respect to the diffractive optical element so as to project the diffraction image towards the support in order to obtain a magnified version of said diffracted image on the support.

2. The device according to claim 1, further comprising a light source and a converging lens arranged to focus a beam of light emitted by said light source, in an intermediate image plane, said diffractive optical element being arranged to diffract said beam of light.

3. The device according to claim 2, wherein the mirror is concave with a hollow and curved reflective surface seen from the converging lens.

4. The device according to claim 2, wherein the mirror is convex with a hollow and curved reflective surface seen from the converging lens.

5. The device according to claim 2, wherein the light source, the diffractive optical element and the converging lens are all aligned along a common optical axis.

6. The device according to claim 2, wherein said mirror is placed, with respect to the converging lens, such that said intermediate image plane is located between the mirror and the focal plane of the mirror.

7. The device according to claim 5, wherein the mirror comprises a non-reflective zone located at the intersection of the common optical axis and the mirror.

8. The device according to claim 5, wherein the diffractive optical element is movable along the common optical axis.

9. The device according to claim 5, wherein the mirror is movable along the common optical axis.

10. The device according to claim 5, wherein the mirror has an optical axis that is separate from the common optical axis and forms a non-zero angle with said common optical axis.

11. The device according to claim 10, further comprising means to orient the optical axis of the mirror, according to a degree of rotational freedom around at least one axis perpendicular to said common optical axis.

12. The device according to claim 1, wherein the mirror and the diffractive optical element are provided on one same component, the diffractive optical element and the mirror being formed on two opposite faces of said component.

13. The device according to claim 1, wherein the mirror has a curvature with a spherical shape.

14. The device according to claim 1, wherein the mirror has a curvature with an aspheric shape.

15. The device according to claim 14, wherein the curvature is parabolic in shape.

16. The device according to claim 1, wherein the device further comprises means to apply vibrations to the mirror.

17. The device according to claim 1, wherein the mirror is deformable.

18. The device according to claim 2, wherein the light source is a semiconductor laser.

19. An assembly comprising: a support, and an optical device for obtaining a diffracted image on a support, said device comprising: a diffractive optical element arranged to diffract an optical beam, so as to generate a diffraction image, and an optical power mirror placed with respect to the diffractive optical element so as to project the diffraction image towards the support in order to obtain a magnified version of said diffracted image on the support.

Description

[0064] Other features, advantages and details of the present invention will be made clearer upon reading the following description of several embodiment examples of the invention, provided by way of example and not limited thereto, said description being made with reference to the appended drawings, wherein:

[0065] FIG. 1 is a schematic view of the architecture of the device according to the invention and according to a first embodiment,

[0066] FIG. 2 is a schematic view of the architecture of the device according to the invention and according to a second embodiment,

[0067] FIG. 3 shows an alternative version of the second embodiment,

[0068] FIG. 4 is a schematic view of the architecture of the device according to the invention and according to a third embodiment, and

[0069] FIG. 5 is a schematic view of the inclination of the mirror.

[0070] As is shown in FIGS. 1 to 4, the device according to the invention comprises a light source 1, a converging lens 3 arranged at the output of the source, a diffractive optical element 5 arranged at the output of the lens and a mirror 7.1; 7.2; 7.3; 7.4 arranged at the output of the diffractive optical element 5.

[0071] In the examples described below, it is considered that the light source 1 consists of a laser module able to emit coherent and monochromatic radiation (i.e. a laser beam).

[0072] Of course, the nature and emitting properties of the light source 1 can be adapted to suit the desired application. Thus, the source 1 can be a monochromatic or polychromatic coherent light source. For example, for the projection of diffracted colour images, the light source 1 can be polychromatic comprising three laser sources adapted to emit respectively laser radiation at different wavelengths. Alternatively, the polychromatic light source can consist of a single laser module with an adjustable wavelength. Such a module can be controlled to emit sequentially laser radiation at different wavelengths.

[0073] According to a preferred feature of the invention, the light source 1 consists of a semiconductor laser module. It must be noted that there are currently highly compact semiconductor laser modules on the market, typically of around several mm.sup.3 or several cm.sup.3, advantageously making it possible to make the device according to the invention very compact. The compactness of the device is particularly sought after in the context of biometric applications intended to be implemented in mobile phones.

[0074] By way of example, the diffractive optical element 5 is obtained, conventionally, by direct writing of a laser beam in a layer of photosensitive material deposited on a substrate. The writing is achieved according to a model obtained based on the desired image that is to be projected onto the support, by application of an inverse calculation and quantification algorithm. The resulting DOE has microstructures with a critical dimension of approximately 1 m.

[0075] Generally, the light source 1 is arranged so as to illuminate the DOE 5 through the converging lens 3. The source 1, the DOE 5 and the lens 3 are aligned along a common optical axis O.

[0076] In the case of the light source 1 consisting of a semiconductor laser module, the converging lens 3 is particularly useful in terms of correcting the divergence of the laser beam coming from this module 1, because in practice, compact semiconductor laser modules emit a diverging laser beam.

[0077] Thus, the converging lens 3 located between the light source 1 and the DOE 5 forms an image of the light source through the DOE in an image plane P3 of the converging lens 3. In the following description, this image plane P3 is described as intermediate plane P3. This image, called intermediate image, corresponds to the diffraction image generated by the DOE 5. As shown in FIGS. 1, 2 and 3, this intermediate plane P3 is located between the mirror 7.1; 7.2; 7.3 and a focal plane of the mirror P7, at a distance f3 from the converging lens 3 that corresponds to the conjugated image distance of the lens 3.

[0078] In practice, this distance f3 is typically several mm or cm and greater than the focal length of the lens. For example, for compact systems, such as mobile phones of the smartphone type, this distance f3 can be of less than 1 cm. In a known manner, the conjugated image distance relates to the distance separating the lens from the light source through conventional conjugation relations commonly used in geometrical optics.

[0079] In the specific embodiments, such as shown in FIGS. 1, 2 and 3, the mirror 7.1; 7.2; 7.3 is arranged so as to be aligned with the assembly formed by the light source 1, the converging lens 3 and the diffractive optical element 5, along the common optical axis O. In these scenarios, the optical device according to the invention is optically centred on the optical axis O, that is common to each of the components thereof (i.e. light source 1, converging lens 3, DOE 5).

[0080] A first embodiment of the invention is now described with reference to FIG. 1, according to which the mirror is a convex mirror 7.1.

[0081] The convex shape of the mirror is defined such that the reflective surface thereof has a curvature turned towards the surface of the diffractive optical element 5 as seen from the converging lens 3. In other words, the reflective surface of the convex mirror 7.1 is domed in the direction of light propagation, between the source 1 and the mirror 7.1.

[0082] In this embodiment, the convex mirror 7.1 is arranged along the common optical axis O, between the converging lens 3 and the intermediate plane P3, i.e. upstream from this plane with respect to the direction of light propagation between the light source 1 and the mirror 7.1.

[0083] A virtual intermediate image generated by the diffractive optical element 5 is formed by the converging lens 3 in the intermediate plane P3 located downstream from the mirror with respect to the direction of light propagation between the light source 1 and the mirror.

[0084] This intermediate image corresponds to the image diffracted by the diffractive optical element 5. Thus, the convex mirror 7.1 transforms the intermediate image contained in the intermediate plane P3 into a magnified real image intended to be projected on a support S.

[0085] Thus, the beam reflected by the convex mirror 7.1 and projected on the support S is advantageously magnified with respect to the incident beam, as shown in FIG. 1.

[0086] For example, such a support can be the face of an individual who is to be identified by facial recognition, or the surface of a landing area of an aircraft, or a wall used as projection screen.

[0087] Advantageously, the real image projected on the support S contains no zero-order diffracted light. Indeed, the light that has not been diffracted by the diffractive optical element 5 has been reflected in the direction of the light source 1 along the common optical axis O but does not reach the support S because the laser module, being in the alignment of the common optical axis O, creates an obstacle to the propagation of this radiation in the vicinity of the common optical axis O.

[0088] Thus, the zero-order diffracted radiation is advantageously filtered by the presence of the laser module 1, without requiring additional filtering components, thereby simplifying the global architecture of the device.

[0089] A second embodiment of the invention is now described with reference to FIG. 2. This second embodiment differs from the embodiment described above with reference to FIG. 1 in that the mirror is a concave mirror 7.2 instead of a convex mirror.

[0090] In the following description, concave is used to describe a mirror of which the reflective surface has a hollow curvature seen from the converging lens 3. In other words, the reflective surface of the concave mirror 7.2 is hollow in the direction of light propagation, between the light source 1 and the mirror.

[0091] In this embodiment, the concave mirror 7.2 has the feature of being arranged along the optical axis O, downstream from the intermediate plane P3 with respect to the direction of light propagation between the source 1 and the mirror.

[0092] Because of the concave shape thereof, the intermediate plane P3 where the intermediate image is formed is located upstream from the concave mirror 7.2 with respect to the direction of light propagation, between the light source 1 and the mirror. As described above, the distance f3 separating the converging lens 3 from this intermediate plane P3 is equal to the conjugated image distance of the converging lens 3.

[0093] As for the first embodiment, the concave mirror 7.2 generates a real magnified real image on the support S, such that the device according to the invention has an actual diffraction angle that is increased with respect to the maximum diffraction angle intrinsic to the diffractive optical element 5.

[0094] As for the first embodiment, the zero-order radiation diffracted by the diffractive optical element 5 and then reflected by the mirror is physically eliminated by the presence of the laser module 1 in the vicinity of the common optical axis O.

[0095] According to an alternative embodiment of the first embodiment, the convex mirror 7.3 comprises a non-reflective zone 9, as shown in FIG. 3, to prevent zero-order diffracted light from being projected on the support S.

[0096] This zone, called central zone, is located at the level of an intersection point of the mirror surface and the common optical axis O. The central zone 9 is preferably centred on this intersection point.

[0097] The non-reflective zone prevents zero-order diffracted radiation emitted by the diffractive optical element 5 from being reflected by the mirror, and is instead absorbed by this zone or transmitted through this zone along the optical axis O.

[0098] Thus, the non-reflective zone can consist of an absorbent material adapted to absorb all or some of the zero-order diffracted radiation.

[0099] For example, this zone can consist of a material that is transparent to incident light, such that the incident beam is integrally transmitted through this zone, i.e. without optical losses. Alternatively, this zone can consist of an opening adapted to let light pass through the mirror.

[0100] For example, the opening can be filled with an optically-transparent material adapted to transmit all the light, or a part thereof.

[0101] The integral transmission of the zero-order diffracted light at the output of the mirror is particularly interesting in characterising, in real time, the emission properties of the laser module 1.

[0102] The same alternative embodiments and the example embodiment provided above also apply to the second embodiment of FIG. 2. In this case (not shown), the concave mirror 7.2 comprises the same non-reflective central zone 9, such as described previously with reference to FIG. 3. It has the same effects and advantages as those described above.

[0103] Generally, this central zone 9 can be provided on any type of optical power mirror included in the scope of the present invention.

[0104] According to a feature of the invention, the diffractive optical element 5 is movable along the common optical axis O with respect to the mirror 7.1; 7.2; 7.3. This feature applies in particular to the embodiments described with reference to FIGS. 1, 2 and 3 and more broadly to any embodiment or alternative embodiment where the diffractive optical element 5 is not formed as a single-block with the mirror, as is described below with reference to FIG. 4.

[0105] By adjusting the distance Y57 between the diffractive optical element 5 and the optical power mirror, it is possible to change the magnification factor of the real image projected by the mirror. In an equivalent manner, this movement makes it possible to change the actual diffraction angle of the device. Thus, the size of the real image projected on the screen S can be dynamically adjusted, i.e. increased or reduced, depending on the intended use.

[0106] It must be noted that the actual diffraction angle can be adjusted without changing the position of the projection support S. Thus, it is possible to achieve a variable magnification of the real image projected on the support by maintaining a fixed projection distance D between the surface of the mirror and the support S.

[0107] Thus, by moving the diffractive optical element 5 closer to or away from the mirror, in a continuous (or progressive) manner along the optical axis O, it is possible to provide an illumination device able to project a real diffracted image, with a size that can be increased or reduced in a continuous manner, with the possibility of projecting at a fixed distance of this device.

[0108] This movement can be achieved by means of a platform (not shown) slidably mounted on a rail along the optical axis and on which is fixed the diffractive optical element 5. The actuation of the platform can be achieved manually or automatically by means of a motor controlled by a control module according to the requirements of the desired application.

[0109] A third embodiment of the invention is now described with reference to FIG. 4. This embodiment differs from the second embodiment in that the real image is projected outside of the common optical axis O.

[0110] Therefore, the mirror 7.4 is oriented so that its specific optical axis M (or in an equivalent manner, its axis of symmetry) forms a non-zero angle with the common optical axis O as shown in FIG. 5. In other words, the optical axis of the mirror M is different from the common optical axis O, contrary to the embodiments described with reference to FIGS. 1 to 3.

[0111] FIG. 5B shows, more generally and in three dimensions, the orientation of the mirror according to at most two degrees of rotational freedom defined by the angles and respectively with respect to the axes X and Z of an orthonormal reference X, Y, Z, these two axes being perpendicular to the common optical axis O. The value of the angles and can be adjusted based on the desired application. For example, an angle of 90 is particularly useful to simplify the inclusion of this type of device in the thickness of a smartphone or a tablet.

[0112] According to another aspect of the invention, the device comprises means to orient the mirror according to a degree of rotational freedom , around at least one axis X; Z perpendicular to the common optical axis O.

[0113] According to a first example embodiment, these means are based on MEMS (MicroElectroMechanical Systems) enabling electrically control of the orientation of a micro plate on which the mirror is fixed.

[0114] According to a second example embodiment, these means are based on a scanning galvo mirror systems.

[0115] According to a third embodiment, the zero-order diffracted radiation is still eliminated by the laser module 1, as described for the other embodiments described above.

[0116] According to an alternative version of this third embodiment, the inclined convex mirror 7.4 is formed of a single block with the diffractive optical element 5, as shown in FIG. 4.

[0117] The mirror makes the optical device highly compact, which would not be easy with the use of lenses as a means of optical divergence.

[0118] For example, the diffractive optical element 5 and the mirror 7.4 are jointly made on the same element that can be moulded, obtained by injection or by lithography, by nano-printing. Microstructures can be etched on a first face of a component made of glass, so as to form the diffractive optical element 5. A thin metallic layer can be deposited on a second face of the component, the second face being arranged opposite the first face. The second surface can be formed by moulding. Thus, a single-unit component comprising the diffractive optical element and the mirror on two of the opposite faces thereof can be easily achieved with conventional manufacturing techniques.

[0119] In the example of FIG. 4, a convex mirror has been shown. However, other alternative versions can be considered by using any other type of optical power mirror, such as the ones that have already been described (concave, spherical, parabolic) as alternative embodiments, based on the needs of the desired application.

[0120] It must be noted that the use of convex or concave mirrors to produce the desired magnification is advantageous to provide great flexibility in the choice of component and mountings to block the zero-order diffracted light.

[0121] According to an aspect of the invention, the surface of the mirror is spherical in shape. This feature is applied regardless of the concave or convex nature of the mirror, and can generally apply to any one of the embodiments, as already described with reference to FIGS. 1 to 4.

[0122] The spherical nature efficiently amplifies the actual diffraction angle, thus projecting real images with an increased size, while using diffractive optical elements that feature intrinsically diffraction angles limited by constraints relating to design and/or manufacturing means of the micro/nanostructures of these elements.

[0123] The spherical shape of the mirror is particularly well adapted to reach actual diffraction angles with a value greater than 30 (total angle value, as opposed to the value of the half-angle), with a size of the microstructures of approximately 1 m.

[0124] Thus, the use of the spherically-shaped mirror advantageously makes it possible to project diffracted images with an increased size by using diffractive optical elements that are inexpensive and easy to design and manufacture.

[0125] In the embodiments such as those shown in FIGS. 1 to 4, the mirror 7.1; 7.2; 7.3; 7.4 was selected with a spherical form.

[0126] However, according to another aspect of the invention, the mirror can be replaced, in any one of these embodiments, and even in other non-described embodiments, with a parabolic mirror, or more generally with an aspheric mirror. In a similar manner as for a spherical mirror, the parabolic or aspheric mirror can have a convex or concave shape, as described above.

[0127] The use of a parabolic mirror is particularly well adapted to correct aberrations and/or optical distortions due to a projection of the real image outside of the optical axis O, while optimising the compactness of the device.

[0128] Generally, the adjustment of the actual diffraction angle by modification of the distance Y57 separating the diffractive optical element 5 from the mirror as described in the first embodiment with reference to FIG. 1, remains valid for each of the embodiments described above, except in the case where the diffractive optical element 5 cannot be moved when it is integrated as a single unit with the mirror on one same component.

[0129] According to the embodiments described with reference to FIGS. 1, 2, 4, by positioning correctly the light source 1, the diffractive optical element and the mirror in the alignment of the optical axis O, the zero-order diffracted radiation reflected at the surface of the mirror at the level of the optical axis O is physically eliminated by the laser module casing 1.

[0130] As described above, this advantageously eliminates zero-order luminous points from the real image projected on the support S, the power of which can be relatively high and potentially dangerous to the human eye.

[0131] Thus, the device according to the invention eliminates the zero-order diffracted components in the real image, thus reducing potential ocular risks, without requiring additional filtering components. In this manner, the architecture of the device according to the invention is greatly simplified and has a relatively reduced size, in particular with respect to lens-based solutions.

[0132] Generally speaking, the device according to any one of the embodiments described above can further comprise means to apply vibrations to the mirror (not shown).

[0133] By submitting the mirror to reduced mechanical vibrations, it is advantageously possible to limit speckle-type noise in the form of shimmers seen in the diffracted image projected by the mirror, thus improving the quality of the projected image. These mechanical vibrations can be produced, for example, by means of a MEMS device or by a galvo mirror system.

[0134] In the embodiments described above, the diffractive optical element 5 used is a Fourier diffractive optical element. In this case, the intermediate image plane P3 corresponds to the image plane of the converging lens 3 located at the conjugated image distance f3 from the converging lens.

[0135] However, the invention also applies in the case where the diffractive optical element is a Fresnel diffractive optical element, i.e. having an optical power that can be diverging or converging. In this case, the above description remains valid, with the difference that the intermediate image plane P3 such as shown in FIGS. 1 to 4 does not correspond to the image plane of the converging lens 3, but to an intermediate image plane that also depends on the optical power of the diffractive optical element. Thus, the intermediate image plane P3, wherein is formed the intermediate image generated by the Fresnel diffractive optical element, is located, with respect to the converging lens 3, at a distance greater or shorter (and not equal) than the conjugated image distance f3.

[0136] The use of a Fresnel diffractive optical element is particularly advantageous to defocus, with respect to the focal plane of the image generated by the Fresnel diffractive optical element, the zero-order diffracted light in the vicinity of the common optical axis and thus improves the optical safety of users compared with laser beams transmitted through the diffractive optical element.

[0137] Of course, the invention is not limited to the example embodiments described and provided above, from which other embodiments can be developed, without departing from the scope of the invention.