Abstract:
The invention relates to a multimirror device for rotating the polarization of an electromagnetic signal, especially a light signal, through 90°, the output ray having approximately the same direction as the input ray.  
     This device comprises at least one combination ( 40 ) of three mirrors ( 42, 44, 46 ) which are arranged in such a way that a ray entering one of the mirrors at an angle of incidence of 45° is reflected at the same angle off the other mirrors, the polarization vector (V) of the incident ray being presented in such a way with respect to the mirrors that a reflection off one of the three mirrors changes the direction of polarization while the reflections off the other two mirrors does not change this direction of polarization. Preferably, a plurality of multiple-mirror combinations ( 40, 40′, 40 ″) is provided on a sheet, each combination forming a pattern which is repeated regularly, the patterns all having identical shapes and sizes.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to a multimirror device for rotating the polarization of an electromagnetic wave, especially a light wave. It also relates to the application of such a device to an image-projection apparatus of the liquid-crystal type.  
         BACKGROUND OF THE INVENTION  
         [0002]    In many technologies it is necessary to rotate the polarization of an electromagnetic wave. The following description will be limited to the example of a fixed-image or moving-image projector in which the images are generated in a component comprising liquid crystals of the nematic type. It is known that such liquid crystals must be illuminated by light linearly polarized in a defined direction with respect to the axes of the liquid crystal.  
           [0003]    In order to produce this linearly polarized light, an unpolarized light source is used together with a polarizing beam splitter which delivers, in a first direction, polarized light suitable for illuminating the liquid-crystal component and, in a perpendicular direction, light having the crossed polarization, i.e. light whose polarization vector is rotated through 90° with respect to the light in the first direction. In order for all, or most, of the energy delivered by the light source to be used to illuminate the liquid-crystal component, use is often made of a polarization-rotating device which rotates the polarization of the light received through 90° into the crossed polarization.  
           [0004]    There are two known ways of rotating the polarization through 90°.  
           [0005]    The first way consists in providing a plate called a quarter-wave plate, or {fraction (λ/4)}, plate which converts the crossed linear polarization into a circular polarization in one direction and a mirror which reflects the circularly-polarized light. The reflected signal has a circular polarization in the other direction and, after it has passed through the quarter-wave plate, the linear polarization of this signal is perpendicular to the direction that it had on entering this quarter-wave plate. Thus, on leaving the quarter-wave plate, after reflection off the mirror, the light has the linear polarization of suitable direction. Such a device, in which the incoming and outgoing beams have the same direction, is relatively compact; this is why it is widely used. However, it has the drawback of operating correctly only for a single wavelength and for a defined direction.  
           [0006]    The second known way consists in providing an arrangement of two mirrors forming the faces of a total-reflection prism, the light striking these faces at an angle of incidence of 45°. The orientation and the arrangement of these mirrors are such that the polarization is in the same direction after reflection off one of the mirrors and is in the perpendicular direction after reflection off the other mirror. Such a polarization-rotating device is almost wavelength-insensitive. However, it is annoyingly bulky. In addition, given that the outgoing beam of this device is perpendicular to the incoming beam, it does not fit well into the usual applications which require the incoming beam and the outgoing beam to be in the same direction.  
         SUMMARY OF THE INVENTION  
         [0007]    The invention provides a polarization-rotating device which has the advantages of the two approaches known hitherto, but without their drawbacks. The device according to the invention is therefore wavelength-insensitive, is not very bulky, and the output beam is in the same direction as the input beam.  
           [0008]    This device, in which the output beam is in the same direction as the input beam, is characterized in that it comprises at least one combination of three mirrors, the light being reflected off them each time at an angle of 45°, and in that the incident beam and the mirrors are arranged in such a way that two mirrors preserve the direction of polarization and the third rotates this polarization through 90°.  
           [0009]    In the preferred embodiment of the invention, a plurality of three-mirror combinations is provided, each combination forming a regularly repeated pattern and the patterns all having identical shapes and sizes. It is particularly advantageous for these patterns to form reliefs on one face of a sheet of transparent material. In this case, the polarization-rotating device may be produced by moulding, for example by moulding a transparent plastic such as an acrylic material.  
           [0010]    It will be noted that since the transparent materials normally used have a refractive index generally between 1.4 and 1.7, the total reflection occurs at and beyond an angle of incidence of approximately 40°. Under these conditions, incidence at 45° corresponds to total reflection, i.e. without loss.  
           [0011]    Preferably, the patterns are contiguous and, in projection on a plane perpendicular to the incident rays, these patterns entirely fill a surface without any discontinuity (interruption). 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    Other features and advantages of the invention will emerge from the description of some of its embodiments, the description being given with reference to the drawings appended hereto, in which:  
         [0013]    [0013]FIG. 1 is a perspective diagram of part of a projector comprising a polarization-rotating sheet according to the invention;  
         [0014]    [0014]FIG. 2 is a diagram showing, in perspective, the relief on the sheet according to the invention;  
         [0015]    [0015]FIG. 3 is a top view, on a larger scale, of the pattern on the sheet shown in FIG. 2; and  
         [0016]    [0016]FIG. 4 is a perspective diagram, in a coordinate system consisting of three rectangular axes, of the pattern on the sheet according to the invention, this diagram also showing light rays and their polarization vectors. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0017]    [0017]FIG. 1 shows part of a liquid-crystal projector, sometimes called an “LCD projector”. This projector comprises a light source  10  producing an unpolarized light beam  12 . This beam  12  is directed onto a polarizing beam splitter  14  which, in the example, is in the form of two prisms  16  and  18  having a common face  20  which splits the beams into different polarizations. This face  20  is oriented at 45° with respect to the incident beam  12 .  
         [0018]    The beam  12  striking this face  20  is split, on the one hand, into a linearly polarized transmitted beam  22  with a vector P 1  which, in the example, is horizontal and corresponds to the desired polarization and, on the other hand, into a beam  24  reflected by the face  20 , and therefore perpendicular to the incident beam  12 , the polarization vector S of which is vertical, i.e. perpendicular to the vector P 1 .  
         [0019]    The beam  24 , perpendicular to the beam  22 , is directed onto the sheet  26  of transparent material forming the polarization-rotating device according to the invention. The beam  24  enters the sheet  26  perpendicular to its entrance face  28 . The beam  24  is reflected off the reliefs on the face  30  opposite the face  28  in such a way that it emerges from the face  28  as a beam  32 , which is in the same direction but has a polarization P perpendicular to the polarization S, i.e. a horizontal polarization. The beam  32  passes back through the splitter  14  and emerges therefrom, as a beam  34 , having preserved the horizontal polarization P 2 , like the polarization P 1  of the beam  22 .  
         [0020]    These beams  22  and  34  are directed, by means comprising a mirror (or mirrors) (not depicted), onto an image-forming liquid-crystal component (not shown).  
         [0021]    The sheet  26  of the invention is made of a transparent plastic. It has reliefs shown in perspective in FIG. 2. These reliefs constitute a regular grating formed by the repetition of a pattern  40 .  
         [0022]    An elementary pattern  40  is shown in top view in FIG. 3 and in perspective in FIG. 4. The patterns are derived from one another by translations along vectors parallel to the plane of the entrance face  28 .  
         [0023]    Such a pattern  40  is formed by three faces  42 ,  44 ,  46 , each constituting a mirror.  
         [0024]    The first face  42  is perpendicular to the plane of the entrance face  28 . Some of the faces  42  have been shown with cross-hatching in FIG. 2, with respect to which it should be pointed out that all the faces  42  are either in the same plane or parallel to one another.  
         [0025]    The second face  44  is inclined at 45°, on the one hand, to the face  42  and, on the other hand, to the entrance face  28  of the sheet  26 .  
         [0026]    Likewise, the third face  46  of each pattern  40  is inclined at 45°, on the one hand, to the face  42  and, on the other hand, to the face  28 . It has a common edge  48  with the face  44 . The faces  44  and  46  are symmetrical with respect to the plane defined by the common edge  48  and the direction perpendicular to the face  28 .  
         [0027]    The geometry of the pattern  40  will now be described in detail with reference to FIG. 4.  
         [0028]    In order to define the shape and dimensions of the pattern, a trirectangular trihedron has been drawn with an x-coordinate axis Ox, a y-coordinate axis Oy and a z-coordinate axis Oz. The origin O of this trihedron lies at the point common to the edge  48  and to the face  42 .  
         [0029]    The plane defined by the Ox and oy axes is parallel to the face  28 .  
         [0030]    The face  42  has the general shape of an upside-down V with two end edges A 1 -A 2  and C 1 -C 2  of equal length, with the same z-coordinates and parallel to the Oz axis.  
         [0031]    The coordinates of the vertices A 1 , O, C 1 , C 2 , B 1  and A 1  of this face  42  are as follows:  
         O                   (         0           0           0         )       ;                  A   1          (         0           a             -       a        2       2             )       ;                  A   2          (         0           a               -   a          2             )       ;                  B   1          (         0           0             -       a        2       2             )       ;               C   2          (         0             -   a                 -   a          2             )       ;                  C   1          (         0             -   a               -       a        2       2             )                             
 
         [0032]    Conventionally, in each column associated with each point, the first value represents the x-coordinate, the second value represents the y-coordinate and the third value represents the z-coordinate.  
         [0033]    a is a length which, in the example, has a value of approximately 1 mm or less.  
         [0034]    It should be pointed out that the notation for the vertices is such that the index 1 corresponds to the z-coordinate  
       -       a        2       2                           
 
         [0035]    and the index 2 to the z-coordinate −a{square root}{square root over (2)}.  
         [0036]    The face  44  has the general shape of a rhombus, the vertices of which are A 1 , O, E 1  and F 2 . The coordinates of the vertices E 1  and F 2  are as follows:  
           E   1          (         a           0             -       a        2       2             )       ;                    F   2          (         a           a               -   a          2             )       .                           
 
         [0037]    Finally, the third face  46  is identical to the face  44 , that is to say it has the shape of a rhombus O, C 1 , D 2 , E 1 . The coordinates of the vertex D 2  are:  
           D   2          (         a             -   a                 -   a          2             )       .                         
 
         [0038]    An adjacent pattern  40 ′, which is derived from the pattern  40  by a translation along the Ox axis, is joined to the pattern  40  in such a way that this pattern  40 ′ has its vertices F′ 2 , E′ 1  and D′ 2  coincident with the vertices A 2 , B 1  and C 2 , respectively.  
         [0039]    The adjacent pattern  40 ″, which is derived from the pattern  40  by a translation along the direction of the y-axis Oy, has vertices A″ 2 , A″ 1  and F″ 2  coincident with the vertices C 2 , C 1  and D 2  of the pattern  40 , respectively. Thus all the patterns are joined together without any discontinuity, i.e. without any interruption, and they all have the same height. The face  30  is in the form of a face with reliefs of total depth a{square root}{square root over (2)}.  
         [0040]    It may be seen that, in projection on the Oxy plane, parallel to the Oz axis, the patterns fill the surface entirely, with no empty spaces.  
         [0041]    The thickness of the sheet  26  is at least a{square root}{square root over (2)}. In one example, this thickness is about 2 to 3 mm.  
         [0042]    The orientation of the sheet  26  with respect to the polarizing beam splitter  14  must be chosen in such a way that an incoming ray has its polarization vector parallel to the direction E 1 C 1  or parallel to the direction A 1 E 1 , i.e. at 45° with respect to the Ox and Oy axes and, in any case, parallel to the plane defined by these Ox and Oy axes and therefore parallel to the entrance face  28 .  
         [0043]    We consider firstly an incident ray  24  in a direction parallel to the Oz axis and with a polarization V parallel to A 1 E 1 , this ray being firstly reflected by the face  44 .  
         [0044]    The reflected ray  50  has a polarization vector V′ which is also parallel to A 1 E 1 . This ray  50  is reflected off the first face  42  perpendicular to the entrance face. The ray  52  which results therefrom has a polarization vector V′ 1  which is rotated through 90°, i.e. parallel to E 1 C 1 .  
         [0045]    The ray  52  is reflected off the face  46 , likewise at an angle of 45°. The outgoing ray  32  is parallel to the incoming ray  24 , i.e. parallel to the Oz axis, and its polarization vector V′ 2  is still parallel to E 1 C 1 , i.e. perpendicular to the polarization vector V of the incident beam  24 .  
         [0046]    It may be easily understood that a ray incident on the face  46  in the Oz direction and with a polarization parallel to E 1 C 1  exits with a perpendicular polarization.  
         [0047]    An incoming ray  24  with a polarization parallel to E 1 C 1  and firstly reflected by the face  44  is returned in such a way that the reflected ray  50  has a polarization of perpendicular direction, i.e. parallel to A 1 E 1 . The subsequent reflections off the faces  42  and  46  no longer change the orientation of the polarization vector, which thus emerges with a direction parallel to A 1 E 1 .  
         [0048]    The situation is summarized as follows: the rays enter the sheet  26  in a direction perpendicular to the face  28  with a polarization vector at 45° with respect to the Ox and Oy axes; each incident ray is firstly reflected by the total-reflection mirror  44  or  46  at an angle of 45°; the mirror  42 , also a total-reflection mirror, receives the ray coming from the mirror  44  or  46  and also reflects it; the angle of incidence is also 45° and the incident and reflected rays are in a plane parallel to the xOy plane; the ray reflected by the mirror  42  undergoes a final reflection, at an angle of 45°, off the mirror  46  or  44 . The polarization vector undergoes a direction change on only one of the three mirrors.  
         [0049]    Although in the embodiment described above the polarizing beam splitter consists of a two-prism combination, it goes without saying that any other polarizing beam splitter may be suitable, for example a glass/air plate operating with an angle of incidence below the Brewster angle or a polarizing beam-splitter film such as that sold under the name DBEF (Dual Brightness Enhancement Film) by the company 3M.