Optical fibers and corresponding manufacturing processes

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of the arrangement of microstructures of optical guides used to extract virtual images injected and transported in these optical guides. STATE OF PRIOR ART

[0002] An optical guide is formed from a transparent material (plastic, glass) and is used to transport, by successive internal reflections, light signals constituting a virtual image injected from an injection zone to an extraction zone. Such optical guides are typically used today in augmented reality to project, using suitable glasses called "information glasses", to provide a user with virtual images injected into the optical guide and provided to the user superimposed on a vision of a real scene seen through said glasses. The injected images are said to be "virtual" in the sense that they do not correspond to the scene seen through the information glasses.

[0003] The virtual image to be transported is injected into the optical guide using a collimation device attached to the guide at the injection zone. The collimation device comprises a light source providing the virtual image. The light source is for example of the LCoS (Liquid Crystal on Silicon), LCD (Liquid Crystal Display), OLED (Organic Light-Emitting Diode), µLED (Micro Light Emitting Diode) or MOEMS (Micro Opto Electro Mechanical System) type. The collimation device also comprises an optical system based on lenses, and possibly mirrors, making it possible to project this virtual image in the form of a collimated beam, which is then introduced into the optical guide via the injection zone.

[0004] The extraction zone comprises a set of microstructures constituting a network of semi-reflective reflectors on the surface of the optical guide. These microstructures consist of prisms having a suitable angle, allowing the light beam to exit from the optical guide towards a user's eye.

[0005] A similar arrangement is presented in patent document WO 2009 / 074638 A1. In the same principle, the Fig. 1 schematically illustrates an arrangement, where a set of juxtaposed microstructures form an optical guide extraction zone. There is shown a portion of optical guide 100 comprising microstructures 101 forming a network of semi-reflective reflectors on the surface of the optical guide. These microstructures 101 are hollow and protruding prisms, and are made up of alternating surfaces 103 and 102 inclined relative to the opposite face of the optical guide. The surfaces 103 jointly form, thanks to a semi-reflective coating, the extraction surface of the virtual images injected into the optical guide, i.e. the aforementioned network of reflectors. We speak here of active surfaces A. The surfaces 102 jointly form the transparent surface allowing the see-through effect. We speak here of passive surfaces B.

[0006] Also known is patent document US 2016 / 0313557 A1 which discloses an optical device comprising a reflective Fresnel element. Also known is patent document US 2012 / 002294 A1 which discloses an optical device for combining a first beam with a second beam which is not parallel to the first beam, so as to form a combined beam.

[0007] Another arrangement for obtaining the see-through effect is presented in patent document WO 2012 / 136470 A1. This makes it possible to see the virtual image carried by the light beam superimposed on the scene beyond the optical guide. The seeing of the scene through is notably enabled by the semi-reflective nature of the reflector array. This arrangement comprises a first optical guide part 100 comprising on the surface microstructures 101 substantially according to the diagram of the Fig. 1 The arrangement described introduces a second part 200 of the optical guide, also called a “cover piece”, comprising on the surface microstructures 201 of a shape complementary to the microstructures 101 of said extraction section, as schematically illustrated in the Fig. 2 . The semi-reflective deposit 204 mentioned above appears there. In addition, a layer of glue 205 extends between the microstructures 101 and 201, so that any microstructure 101 of said extraction section is separated from its complementary microstructure 201 by a transparent medium of substantially constant thickness.

[0008] The microstructures of such optical guides can be manufactured by machining, but are generally manufactured using a plastic injection molding technique ("injection molding" in English), compression injection molding, hot embossing or finally by a thermal forming technique, or forming by ultraviolet radiation, of monomers. For this, a metal impression (also called an "insert") (for example, made of steel or aluminum) is manufactured with the negative shape of the structure of the optical guide, in order to serve as a mold (typically after application of a layer of Nickel Phosphorus for steel impressions). The manufacture of this type of insert must be done with very precise methods to limit surface irregularities as much as possible.Some applications require surface shape dimensional tolerances much smaller than the guided wavelength λ (e.g., less than λ / 5) and also very low roughness (e.g., less than 10 nm), in order to have good sharpness of the projected virtual image. Also, the semi-reflective surfaces of the microstructures, whose role is to extract the image towards the eye of a user from the optical guide, must be precisely made. Indeed, the virtual image guided in the optical guide by successive total internal reflections is collimated to infinity. Each pixel of the image being transported by a pencil of light rays parallel to each other, the virtual image is therefore guided in the form of parallel light rays.

[0009] Today, optical guides project the virtual image to infinity. Indeed, if the transported virtual image is not collimated to infinity, then different fields composing it pass through the optical guide with a different number of successive internal reflections. Then, the fields mix and the part of the virtual image of the corresponding pixels is blurred.

[0010] Projecting collimated virtual images at infinity meets certain applications, but does not meet all user needs. More specifically, to be able to interact correctly with the content of the projected virtual image (for example, at arm's length), it is desirable to project this virtual image at a finite distance at the output of the optical guide. Furthermore, in the case of binocular augmented reality glasses, an image projected at infinity generates a conflict known as convergence-accommodation ("vergence-accommodation" in English). To avoid this conflict, it is desirable for the right eye vision and the left eye vision to converge at a finite distance, called " distance de convergence ". This convergence distance is generally less than 2 meters. It is also desirable that the focus be made at a short distance to improve the sharpness of the virtual image superimposed on a nearby real scene. This is obviously not possible with a virtual image projected at infinity.

[0011] In the existing state of the art, to project the virtual image at a finite distance, typically a short distance of 1 to 10 meters, a negative lens 301 is added to the optical guide 100 and placed at the exit of the extraction zone, therefore on the side of the user's eye 304, to bring the virtual image closer. In addition, a positive lens 302 of the same power is added on the other side of the optical guide 100 relative to the negative lens 301, and in the axis of the latter. The positive lens 302 thus compensates for the negative lens 301 so that the vision of the real world through the optical guide is not distorted. The corresponding arrangement is schematically illustrated in the Fig. 3 . One drawback is that adding these lenses increases bulk and weight, and may cause discomfort to the user.

[0012] It is desirable to provide a solution that can fully or partially overcome the disadvantages of the state of the art mentioned above. It is also desirable to provide a solution that is simple to implement and low cost. STATEMENT OF THE INVENTION

[0013] An object of the present invention is to propose an optical guide comprising a first part in a transparent material, the first part comprising on the surface a network of extraction microstructures composed of a succession of extraction microstructures arranged to project at a finite distance D a virtual image injected into the optical guide, each extraction microstructure having a prismatic shape with two faces, one face being called the active surface and having a semi-reflective deposit to extract the virtual image from the optical guide and the other face being called the passive surface and having no semi-reflective deposit, characterized in that each active surface is spherical and has a lower inclination by an average angle θ than the possible active surface immediately preceding in the succession of microstructures according to the direction of propagation of the virtual image in the optical guide.This provides a solution directly integrated into the optical guide, in a compact manner, without the need for additional lenses. Manufacturing is also simplified (limited assembly).

[0014] The optical guide is such that: θ = arctan P / D / 2 * n and each active surface has a radius of curvature R such that: R = 2 * n * D where P is a repetition period of the microstructures of the extraction microstructure array and n is a refractive index of the transparent material.

[0015] According to a particular embodiment, the optical guide further comprises a second part, called a cover part, in the same transparent material as the first part, the second part comprising microstructures arranged to take place in spaces between the microstructures of the first part, the second part being glued to the first part so as to form an optical guide with two parallel faces.

[0016] According to a particular embodiment, the second part has on the surface microstructures of shapes complementary to those of the microstructures of the first part, with a substantially constant thickness of glue.

[0017] According to a particular embodiment, the second part has on the surface microstructures forming another network of extraction microstructures which comprises another succession of extraction microstructures having active surfaces having a semi-reflective deposit and arranged to project the virtual image at a distance D' other than the distance D.

[0018] According to a particular embodiment, the distance D' is finite and each active surface of the microstructures of this other network of extraction microstructures is spherical and has a lower inclination by an average angle θ' than the possible active surface immediately preceding in the succession of microstructures according to the direction of propagation of the virtual image in the optical guide.

[0019] Alternatively, the distance D' is infinite.

[0020] According to a particular embodiment: the semi-reflective coating of the active surfaces of the first part is sensitive to a specific polarization and the semi-reflective coating of the active surfaces of the second part is sensitive to another specific polarization; or the semi-reflective coating of the active surfaces of the first part is sensitive to a specific wavelength and the semi-reflective coating of the active surfaces of the second part is sensitive to another specific wavelength; or the semi-reflective coating of the active surfaces of the first part is sensitive to a specific spectral band and the semi-reflective coating of the active surfaces of the second part is sensitive to another specific spectral band.

[0021] According to a particular embodiment, the first part further comprises on the surface a network of two-dimensional pupil multiplication microstructures placed between an injection zone through which the virtual image to be projected is injected into the optical guide and the network of extraction microstructures, the two-dimensional pupil multiplication microstructures comprising active surfaces in the form of inclined planes having a semi-reflective deposit and the other surfaces being passive surfaces having no semi-reflective deposit, the two-dimensional pupil multiplication microstructures being placed obliquely relative to the extraction microstructures so as to reflect a light ray of the transported virtual image which strikes one or more of its active surfaces towards the network of extraction microstructures.

[0022] According to a particular embodiment, the first part has a semi-reflective deposit between the injection zone and the network of two-dimensional pupil multiplication microstructures, as well as between the network of two-dimensional pupil multiplication microstructures and the network of extraction microstructures.

[0023] There is also provided an image projection device comprising an optical guide as mentioned above in any of its embodiments and a collimation device providing a virtual image collimated at infinity, the collimation device and the optical guide being assembled so that the virtual image provided by the collimation device is injected into the optical guide and projected to the distance D by the extraction microstructure array.

[0024] An augmented reality system comprising at least one such image projection device is also proposed.

[0025] A method of manufacturing an optical guide is also proposed, comprising the following steps: manufacturing a first part in a transparent material, the first part comprising on the surface a network of extraction microstructures composed of a succession of extraction microstructures arranged to project at a finite distance D a virtual image injected into the optical guide, each extraction microstructure having a prismatic shape with two faces, one face being called an active surface for extracting the virtual image from the optical guide and the other face being called a passive surface, each active surface being spherical and having a lower inclination by an average angle θ than the possible active surface immediately preceding in the succession of microstructures according to the direction of propagation of the virtual image in the optical guide; applying a semi-reflective treatment to the active surfaces, excluding the passive surfaces.

[0026] The manufacturing process is as follows: θ = arctan P / D / 2 * n and each active surface has a radius of curvature R such that: R = 2 * n * D where P is a repetition period of the microstructures of the extraction microstructure array and n is a refractive index of the transparent material.

[0027] According to a particular embodiment, the method is such that the first part further comprises on the surface a network of two-dimensional pupil multiplication microstructures placed between an injection zone through which the virtual image to be projected is injected into the optical guide and the network of extraction microstructures, the two-dimensional pupil multiplication microstructures comprising active surfaces in the form of inclined planes and the other surfaces being passive surfaces, the two-dimensional pupil multiplication microstructures being placed obliquely relative to the extraction microstructures so as to reflect a light ray of the transported virtual image which strikes one or more of its active surfaces towards the network of extraction microstructures; and the method further comprises the following step: apply a semi-reflective treatment to the active surfaces of the two-dimensional pupil multiplication microstructures, excluding the passive surfaces.

[0028] According to a particular embodiment, the method further comprises the following steps: manufacturing a second part, called a cover part, in the same transparent material as the first part, the second part comprising microstructures arranged to fit into spaces between the microstructures of the first part; gluing the first part and the second part together so as to form an optical guide with two parallel faces. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The above-mentioned and other features of the invention will become more clearly apparent from the following description of at least one exemplary embodiment, said description being given in relation to the attached drawings, among which: [ Fig. 1 ] schematically illustrates extraction microstructures in an extraction zone of an optical guide, according to the state of the art; [ Fig. 2 ] schematically illustrates another arrangement of optical guide extraction zone, according to the state of the art; [ Fig. 3 ] schematically illustrates an arrangement for short-distance focusing of an image injected into an optical guide and extracted from it using the microstructures of the Fig. 1 ou 2 , according to the state of the art; [ Fig. 4 ] schematically illustrates, in a simplified sectional view, orientations of active surfaces of optical guide extraction microstructures, according to the invention; [ Fig. 5 ] schematically illustrates an optical guide comprising a network of extraction microstructures supplemented by a network of two-dimensional pupil multiplication microstructures; [ Fig. 6 ] schematically illustrates an optical guide adapted to informative glasses; [ Fig. 7 ] schematically illustrates a simplified cross-sectional view of the extraction microstructure array and the two-dimensional pupil multiplication microstructure array; [ Fig. 8 ] schematically illustrates an arrangement for extracting images at two different distances; and [ Fig. 9 ] schematically illustrates a method of manufacturing an optical device. DETAILED PRESENTATION OF IMPLEMENTATION METHODS

[0030] There Fig. 4 schematically illustrates, in a simplified sectional view, orientations of active surfaces of microstructures of the extraction zone of an optical guide 400, according to the invention. The microstructures of the extraction zone are referred to herein as “extraction microstructures”. The Fig. 4 symbolizes the optical guide 400 with a first part comprising on the surface the extraction microstructures. This first part is glued to a second part, also called a "cover part", comprising on the surface complementary microstructures, according to the general principle disclosed in the patent document WO 2012 / 136470 A1. For illustration purposes, only two active surfaces of microstructures are shown on the Fig. 4 .

[0031] The optical guide 400 is arranged in such a way that, when a collimation device is associated with it according to the usage recommendations, the rays of the virtual image, which is provided by the collimation device and which is injected by the injection zone, travel through the optical guide 400 in total internal reflections up to the extraction zone (from right to left, according to the propagation direction “dir” on the Fig. 4 ). The active surfaces of the extraction microstructures of said first part of the optical guide 400 are spherical semi-reflecting surfaces of the same radius of curvature R. The active surfaces of the extraction microstructures of said first part of the optical guide 400 are preferably concave surfaces, i.e. hollow surfaces on the surface of the first part of the optical guide 400. A projection at a negative distance to compensate for hyperopia is also achievable using convex surfaces.

[0032] Since the rays of the virtual image are collimated to infinity at the output of the collimation device, these rays are therefore all parallel for each field of the transported virtual image. By using a spherical semi-reflecting surface 401a of radius R inclined in the optical guide 400, a part of the image is extracted from the optical guide 400 and is projected at a distance D from the surface of the optical guide 400. By considering another spherical semi-reflecting surface 401b of radius R inclined in the optical guide 400, another part of the image is extracted from the optical guide and is projected at the distance D from the surface of the optical guide 400. And so on to extract the entire virtual image and project it at the distance D from the surface of the optical guide 400 thanks to the network of extraction microstructures.The network of extraction microstructures has a period (or "pitch") of repetition P (one microstructure every period P). The active surfaces of the extraction zone are thus spherical semi-reflecting surfaces which are arranged in such a way that the virtual image of a point, or a pixel, carried by the rays extracted from the optical guide 400 by the semi-reflecting surfaces (as symbolically illustrated in the . Fig. 4 ) focus on a single point located at the distance D from the surface of the guide. To do this, the inclination of each active surface is lower by an average angle θ than the possible active surface immediately preceding in the succession of microstructures (in the direction of propagation "dir" on the Fig. 4 ). Thus, the last microstructure in sequence (in the direction of propagation “dir” on the Fig. 4 ) has a profile which forms an average angle α with the direction of propagation of the image transported in the optical guide 400, the penultimate microstructure in sequence has a profile which forms an average angle α + θ with the direction of propagation of the transported image... In other words, considering a network of extraction microstructures having M microstructures, the i-th microstructure in sequence in the direction of propagation of the transported image (in the direction of propagation "dir" on the Fig. 4 ) has a profile that forms a mean angle α + (M - i) * θ, with i = 1,...,M, with the direction of propagation of the transported image.

[0033] Thus, to project the virtual image at a distance D from the optical guide, the microstructures are arranged in such a way that the following relationships are respected: R = 2 * n * D θ = arctan P / D / 2 * n Or nis the refractive index of the material constituting the first part and which is equal to that of the second part of the optical guide 400.

[0034] This arrangement allows the extraction of the virtual image injected into the optical guide 400 at a finite distance (represented by D). The arrangement presented acts as a negative lens diverging the beam of the extracted virtual image in order to project this virtual image at a finite distance (represented by D) when it enters the eye, in the manner of an ophthalmic lens (eyeglass lens), but advantageously offers a solution integrated into the optical guide (without adding a lens). In this arrangement, only the extracted virtual image is placed at a finite distance, the rays resulting from the vision-through effect only passing through the optical guide 400 and not being deflected by the proposed arrangement.

[0035] The angle α is fixed by the optical guiding conditions, including the expected placement and characteristics of the collimating device injecting the virtual image so that the entire virtual image is guided by total internal reflections in the optical guide to the extraction microstructure array.

[0036] As an example of embodiment, to provide an optical guide which focuses the extracted virtual image at a distance D equal to 2 meters in the air, considering that the refractive index n of the material constituting the first part of the optical guide 400 and of the material constituting the second part of the optical guide 400 is equal to 1.6, then the radius R is 6.4 meters, the angle θ is equal to 0.014 degrees, for a repetition period P equal to 1.6 millimeters.

[0037] In order to reduce the dimensions of the injection zone, also called the “entrance pupil of the optical guide 400”, and consequently to reduce the size of the collimation device, the optical guide may include another network of microstructures, called two-dimensional pupil multiplication microstructures. This network of two-dimensional pupil multiplication microstructures is present between the entrance pupil and the network of extraction microstructures, i.e. between the injection zone and the extraction zone. The network of two-dimensional pupil multiplication microstructures comprises plane and mutually parallel surfaces, arranged to widen the beam carrying the virtual image collimated to infinity before extraction at a finite distance by the extraction microstructures. The arrangement is schematically illustrated in the Fig. 5 ,where the injection zone 501 is represented where the input beam is injected into a section of this zone which represents the entrance pupil of the guide, that is to say the place where all the fields cross this zone, each with its own angle of incidence.

[0038] The layout of the Fig. 5 comprises a network of extraction microstructures 505, arranged as previously described in relation to the Fig. 4 The microstructures therefore comprise spherical surfaces whose coordinated inclinations allow extraction, at a finite distance D from the optical guide, of an image injected into an injection zone 501 of the optical guide.

[0039] The layout of the Fig. 5 further comprises an array of two-dimensional pupil multiplication microstructures 503 for multiplying the size of the image beam relative to that of the entrance pupil. The two-dimensional pupil multiplication microstructures have a prismatic shape with planar surfaces. Each two-dimensional pupil multiplication microstructure comprises an active surface in the form of an inclined plane with a semi-reflective coating, and a passive surface without this semi-reflective coating. The two-dimensional pupil multiplication microstructures are arranged obliquely relative to the extraction microstructures so that the image beam reflected by the active surfaces of the two-dimensional pupil multiplication microstructures is transmitted to the extraction microstructures. Each ray carrying the virtual image thus strikes one or more active surfaces and is transmitted to the extraction microstructures.

[0040] Between the two-dimensional pupil multiplication microstructure array 503 and the extraction microstructure array 505, the optical guide optionally comprises on the surface a deposit of semi-reflective layer 504, having for example a transmission / reflection ratio of 50%. This particular optional arrangement is advantageous when the cover piece is in place. The deposit of semi-reflective layer 504 is then located in the middle of the thickness of the optical guide and makes it possible to double the beam at the input of the extraction microstructure array 505.

[0041] Between the injection zone 501 and the network of two-dimensional pupil multiplication microstructures 503, the optical guide comprises on the surface a deposit of semi-reflective layer 502, having for example a transmission / reflection ratio of 50%. This particular optional arrangement is advantageous when the cover part is in place. The deposit of semi-reflective layer 502 is then located in the middle of the thickness of the optical guide and makes it possible to double the beam at the input of the network of two-dimensional pupil multiplication microstructures 503 and therefore to reduce the dimensions of the collimation device.

[0042] Thus, once the virtual image is injected into the optical guide, the fields that compose it (each with a given direction) pass through an area of ​​the optical guide that includes the deposit of semi-reflective layer 502. Each incident beam then generates two beams (one is transmitted through the deposit of semi-reflective layer 502 and the other is reflected by this deposit of semi-reflective layer 502). This technique is described in more detail in patent EP 2791717 B1 and thus makes it possible to fill the optical guide.

[0043] The rays carrying the virtual image propagate in the optical guide by total internal reflections and are partially reflected by the network of two-dimensional pupil multiplication microstructures 503. This makes it possible to generate a multitude of reflected beams, each by one or more active surfaces of the network of two-dimensional pupil multiplication microstructures 503, which has the effect of widening the section with respect to the entrance pupil of the injection zone 501. All the beams may thus not reach the zone of the optical guide which comprises the deposit of semi-reflecting layer 504 and are therefore lost (they do not reach the eye box).

[0044] The semi-reflective layer deposit 504 is located in the middle of the thickness of the optical guide, which once again allows for beam doubling. This second beam doubling ensures better field uniformity, by filling areas of the optical guide which would otherwise not be provided with a light beam.

[0045] The network of extraction microstructures 505 then allows an extraction towards the outside, thanks to the reflection on its active surfaces, of the transported virtual image, in order to form a corresponding image in the eye of the user.

[0046] There Fig. 6 schematically illustrates an image projection device 600 adapted to information glasses (virtual reality), comprising an optical guide 602 allowing extraction at a finite distance of an injected image, superimposed with a real scene seen through the optical guide 602. The image projection device 600 makes it possible both to project a virtual image at a finite distance (D) and to offer a see-through effect. The image projection device 600 is easily integrated into an information glasses frame, the Fig. 6 schematically illustrating an image projection device 600 adapted to take place as a right eye optic (pane) of information glasses.

[0047] The optical guide comprises a first part made of transparent material, glass or plastic, arranged as shown in the Fig. 5 The material used is for example a transparent plastic with a high refractive index. nd (For example, nd > 1.6).

[0048] In particular, the two-dimensional pupil multiplication microstructure network 503 and the extraction microstructure network 505 are present on the surface of this first part. For example, the two-dimensional pupil multiplication microstructure network 503 and the extraction microstructure network 505 are obtained by plastic injection molding or by machining, for example using a diamond tip.

[0049] On the first part, semi-reflective deposits are present on the active surfaces of the extraction microstructures, as well as between the two-dimensional pupil multiplication microstructure network 503 and the extraction microstructure network 505, and between an injection zone where a collimation device 601 is attached and the two-dimensional pupil multiplication microstructure network 503. These deposits make it possible to obtain semi-transparency for the see-through effect.

[0050] The image beam is formed by the collimation device which is attached to the optical guide at its injection zone. The image to be injected is thus collimated to infinity over a total field of 50 degrees, for example. This beam is injected into the optical guide for example through an injection prism, which is preferably part of the body of the first part of the optical guide and which forms the injection zone, with an entrance pupil for example of 4 mm.

[0051] The optical guide has a second part that serves as a cover part. The second part is made of the same transparent material as the first part (same optical index). The second part is glued to the first part, so that the thickness of the optical guide is constant (thickness of the first part, thickness of the second part and thickness of glue). After assembly of the first part and the second part, the optical guide therefore has two external surfaces (faces) that are flat and parallel to each other. A transparent glue is used, with a refractive index substantially the same as that of the transparent material used for the first part of the optical guide and the transparent material used for the second part of the optical guide.

[0052] The microstructures of the second part are arranged to fit into spaces between the extraction microstructures of the first part.

[0053] The microstructures of the second part may have shapes complementary to those of the microstructures of the first part, with a substantially constant glue thickness, as according to the general principle disclosed in patent document WO 2012 / 136470 A1.

[0054] When the first part comprises a network of two-dimensional pupil multiplication microstructures, the second part comprises on the surface microstructures which are, according to the same principle, geometrically complementary to those of the network of two-dimensional pupil multiplication microstructures of the first part. This aspect is schematically illustrated in the Fig. 7 with the AA section in simplified view.

[0055] In an alternative embodiment, the microstructures of the second part have shapes arranged to allow a second image extraction at a distance D' other than the distance D. As the second part is glued to the first part so that the thickness of the optical guide is constant, this means that the glue thickness is not constant between the extraction microstructures of the first part and those of the second part. Allowing the same image to be projected at different distances (different planes) makes augmented reality vision more immersive, closer to that of the human eye, which has the ability to focus the image at different distances by adapting its ocular focal length in real time.

[0056] A corresponding arrangement is schematically illustrated in the Fig. 8 ,where a projection to infinity is achieved by the complementary microstructures of the second part (cover part) at the extraction zone.

[0057] Thus, in a particular embodiment, the cover part 800b also has an image extraction function thanks to the microstructures which take place in the spaces between the extraction microstructures of the first part 800a. The optical guide 800 thus comprises another succession of extraction microstructures arranged to project at a distance D', other than the distance D, the image injected into the optical guide. The active surfaces 802a, 802b of these microstructures of the cover part are also spherical and are provided with a semi-reflective deposit (not shown). They are placed opposite the active surfaces 801a, 801b of the extraction microstructures of the first part 800a, but are different from the active surfaces 801a, 801b opposite which they are placed.

[0058] To project the virtual image at a distance D' from the optical guide, the microstructures of the cover piece have a spherical shape and are arranged in such a way that the following relationships are respected: R ′ = 2 * n * D ′ θ ′ = arctan P / D ′ / 2 * n where R' is the radius of curvature of the spherical active surfaces 802a, 802b, and θ' represents the difference in inclination from one active surface to the other (according to the same principle as for the extraction microstructures of the first part 501a). The active surfaces of the extraction microstructures of the cover part are preferably convex surfaces, that is to say protruding from the surface of the second part of the optical guide 400. A projection at a negative distance to compensate for hyperopia is also achievable by using concave surfaces.

[0059] Thus, a light ray 803a transported (virtual image) by the optical guide 800 to the extraction zone strikes the active surface 801a. The semi-reflective treatment on the active surface 801a causes a portion 803b of the light ray 803a to be reflected and projected out of the optical guide at the distance D. Another portion is transmitted via the glue to the active surface 802a. The semi-reflective treatment on the active surface 802a causes a portion 803c of the light ray to be projected out of the optical guide at the distance D'. Similarly, a light ray 804a (virtual image) transported by the optical guide 800 to the extraction zone strikes the active surface 801b. The semi-reflective treatment on the active surface 801b causes a portion 804b of the light ray 804a to be reflected and projected out of the optical guide at the distance D. Another portion is transmitted via the glue to the active surface 802b.The semi-reflective treatment on the active surface 802b causes a portion 803c of the light ray to be projected out of the optical guide at the distance D'.

[0060] The active surfaces of these microstructures of the cover piece can be arranged so as to project the image to an infinite distance, as illustrated in the Fig. 8 . In other words, there is no change in the angle of inclination from one microstructure to another. The difference in complementarity of the shapes of the microstructures placed opposite each other is also compensated for by the glue so that the external faces of the optical guide remain parallel to each other.

[0061] The active surfaces of these microstructures of the cover piece can be arranged so as to project the image at a finite distance.

[0062] The active surfaces 801a, 801b and the active surfaces 802a, 802b may have different semi-reflective coatings. For example, the coating applied to the active surfaces of the extraction microstructures of the first part is selective for a specific polarization (perpendicular or parallel to the plane of incidence) or for a specific wavelength or for a specific spectral band, and the coating applied to the active surfaces of the extraction microstructures of the first part is selective for another specific polarization or for another specific wavelength or for another specific spectral band (respectively). This makes it possible to easily project two separate images at two different distances, each image then having its own polarization or wavelength characteristics.

[0063] There Fig. 9 schematically illustrates process steps for manufacturing an optical guide comprising a first part and a second part, as already discussed above.

[0064] In a step 901, the first part is manufactured, for example by machining a block of plastic, glass or by plastic injection process, or by molding. The first part comprises a network of extraction microstructures, as already described in detail. The first part preferably comprises a network of two-dimensional pupil multiplication microstructures, as already also described in detail, in particular with respect to the Fig. 5 .

[0065] In a step 902, the second part is manufactured, for example by machining a block of plastic, glass or by plastic injection process, or by molding. The second part has surface microstructures which are sized and arranged to then take place in spaces between the extraction microstructures and between the two-dimensional pupil multiplication microstructures of the first part.

[0066] In a step 903, a semi-reflective treatment (deposition) is applied to the active surfaces of the extraction microstructures of the first part and possibly to those of the second part when these themselves allow image extraction at the distance D'. The passive surfaces are excluded from this semi-reflective treatment (deposition).

[0067] A semi-reflective treatment (deposition) is preferably applied between the injection zone through which the collimation device will have to inject the virtual image and the network of two-dimensional pupil multiplication microstructures, either on the first part or on the second part, so that after bonding, the treatment is in the middle of the thickness of the optical guide.

[0068] Similarly, a semi-reflective treatment (deposition) is preferentially applied between the two-dimensional pupil multiplication microstructure array and the extraction microstructure array, either on the first part or on the second part, so that after bonding, the treatment is in the middle of the thickness of the optical guide.

[0069] In a step 904, the first part manufactured in step 901 and the second part manufactured in step 902 are bonded together, the surface microstructures of the first part being bonded opposite their complementary surface microstructures of the second part, as illustrated in the Fig. 7 The refractive indices of the materials used to manufacture the first part and the second part, and of the glue that joins them, are substantially equal in order to ensure optical continuity. After assembly of the first part and the second part, the optical guide therefore has two external surfaces (faces) that are flat and parallel to each other.

[0070] Then, an optical device can be manufactured by adding a collimation device to the optical guide in a step 906. An assembly of the optical guide and the collimation device is thus carried out, so that the collimation device injects a virtual image, in the form of a beam collimated to infinity, by an injection zone of the optical guide, and so that this image is transported in the optical guide by successive total internal reflections.

[0071] The assembly of the optical guide and the collimation device can then be integrated, with a light image source, into an augmented reality device, such as information glasses.

Claims

1. Optical guide (400, 800) comprising a first piece (800a) in a transparent material, the first piece comprising on the surface an array of extraction microstructures composed of a succession of extraction microstructures arranged to project, at a finite distance D, a virtual image injected into the optical guide, each extraction microstructure having a prismatic shape with two faces, one face being called active surface (401a, 401b, 801a, 801b) and having a semi-reflective deposit for extracting the virtual image from the optical guide and the other face being called passive surface and not having any semi-reflective deposit, characterised in that each active surface is spherical and has a smaller inclination by a mean angle θ than any active surface immediately preceding in the succession of microstructures in the propagation direction of the virtual image in the optical guide, characterised in that: Θ = arctan P / D / 2 * n and each active surface has a radius of curvature R such that: R = 2 * n * D where P is a repetition period of the microstructures in the array of extraction microstructures and n is a refractive index of the transparent material.

2. Optical guide according to claim 1, further comprising a second piece (800b), referred to as a covering piece, in the same transparent material as the first piece, the second piece comprising microstructures arranged to fit in spaces between the microstructures of the first piece, the second piece being glued to the first piece so as to form an optical guide with two parallel faces.

3. Optical guide according to claim 2, wherein the second piece has on the surface microstructures with forms complementary to those of the microstructures of the first piece, with a substantially constant thickness of glue.

4. Optical guide according to claim 2, wherein the second piece has on the surface microstructures forming another array of extraction microstructures that comprises another succession of extraction microstructures having active surfaces (102a, 102b) having a semi-reflective deposit and arranged to project the virtual image at a distance D' other than the distance D.

5. Optical guide according to claim 4, wherein the distance D' is finite and each active surface of the microstructures of this other array of extraction microstructures is spherical and has a smaller inclination by a mean angle θ' than any active surface immediately preceding in the succession of microstructures in the propagation direction of the virtual image in the optical guide.

6. Optical guide according to claim 4, wherein the distance D' is infinite.

7. Optical guide according to any one of claims 4 to 6, wherein: - the semi-reflective deposit on the active surfaces of the first piece is sensitive to a specific polarisation and the semi-reflective deposit on the active surfaces of the second piece is sensitive to another specific polarisation; or - the semi-reflective deposit on the active surfaces of the first piece is sensitive to a specific wavelength and the semi-reflective deposit on the active surfaces of the second piece is sensitive to another specific wavelength; or - the semi-reflective deposit on the active surfaces of the first piece is sensitive to a specific spectral band and the semi-reflective deposit on the active surfaces of the second piece is sensitive to another specific spectral band.

8. Image-projection device comprising an optical guide according to any one of claims 1 to 7 and a collimation device supplying a virtual image collimated to infinity, the collimation device and the optical guide being assembled so that the virtual image supplied by the collimation device is injected into the optical guide and projected at the distance D by the array of extraction microstructures.

9. Augmented reality system comprising at least one image-projection device according to claim 8.

10. Method for manufacturing an optical guide, comprising the following steps: - manufacturing a first piece from a transparent material, the first piece comprising on the surface an array of extraction microstructures composed of a succession of extraction microstructures arranged to project, at a finite distance D, a virtual image injected into the optical guide, each extraction microstructure having a prismatic shape with two faces, one face being called active surface for extracting the virtual image from the optical guide and the other face being called passive surface, each active surface being spherical and having a smaller inclination by a mean angle θ than any active surface immediately preceding in the succession of microstructures in the propagation direction of the virtual image in the optical guide; - applying a semi-reflective treatment on the active surfaces, excluding the passive surfaces; characterised in that the manufacturing method is such that Θ = arctan P / D / 2 * n and each active surface has a radius of curvature R such that: R = 2 * n * D where P is a repetition period of the microstructures in the array of extraction microstructures and n is a refractive index of the transparent material.

11. Manufacturing method according to claim 10, wherein the first piece further comprises on the surface a array of two-dimensional pupil multiplication microstructures placed between an injection zone via which the virtual image to be projected is injected into the optical guide and the array of extraction microstructures, the two-dimensional pupil multiplication microstructures comprising active surfaces in the form of inclined planes and the other surfaces being passive surfaces, the two-dimensional pupil multiplication microstructures being placed obliquely with respect to the extraction microstructures so as to reflect a light ray from the transported virtual image that strikes one or more of its active surfaces towards the array of extraction microstructure; and the method further comprises the following step: - applying a semi-reflective treatment on the active surfaces of the two-dimensional pupil multiplication microstructures, excluding the passive surfaces.

12. Manufacturing method according to claim 10 or 11, further comprising the following steps: - manufacturing a second piece, referred to as a covering piece, from the same transparent material as the first piece, the second piece comprising microstructures arranged to fit in spaces between the microstructures of the first piece; - glueing the first piece and the second piece together so as to form an optical guide with two parallel faces.