Abstract:
A linear light valve array including individually addressable diffractive optical elements, the diffractive optical elements having planes of diffraction substantially perpendicular to the longitudinal axis of the array.  
     A method of light modulation, including the steps of: providing a linear light valve array including individually addressable diffractive optical elements, the elements having planes of diffraction substantially perpendicular to the longitudinal axis of the array, illuminating the light valve with a light beam, and selectively activating the optical elements.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to light valve arrays and more particularly to linear Light Valve Array (LVA), in which individual members (pixels) are of controllable (switchable) diffraction grating type.  
         BACKGROUND OF THE INVENTION  
         [0002]    Various optical applications such as projection and imaging require light modulation and/or light beam steering. In order to increase the imaging speed, individual optical modulators are usually combined, to form one or two-dimensional arrays, called Spatial Light Modulators (SLM) or Light Valve Arrays (LVA). A distinctive class LVA is operative in a diffractive mode, i.e. an activated LVA member of the array diffracts the incoming light beam at a discrete multitude of angles, those angles being a function of the light wavelength and the dimensions of the member. Such LVAs can be based either on Liquid Crystal technology (for example, as described in U.S. Pat. Nos. 3,843,231 to Borel et al.; 4,639,091 to Huignard et al.; 4,937,593 to Prats; 5,148,302 to Nagano et al.; 5,151,814 to Grinberg et al.; and 5,638,201 to Bos et al.), or on Micro Electro-Mechanical Systems (MEMS) technology (for example, as described in U.S. Pat. Nos. 5,311,360; 5,459,610. 5,677,783; 5,808,797; 5,841,579; 5,982,553 to Bloom et al.; 5,629,801 to Staker et al.; 5,661,592 to Bronstein et al.; 5,920,518 to Harrison et al.; 5,949,570 to Shiono et al., 5,999,319 to Castracane; and 6,014,257; and 6,031,652 to Furlani et al.).  
           [0003]    Optical techniques for imaging using diffractive LVAs are well known in the art. For better demonstration of the present invention, an LVA and a typical optical system employing it are illustrated in FIGS. 1 a  to  1   c.    
           [0004]    [0004]FIG. 1 a  is a partial isometric view of a controllable diffractive grating array, generally denoted by numeral  10 . Each LVA diffractive member ( 12 ,  13 , and  14 ) is constructed of four (in this example) sub-elements  12   a ,  13   a  and  14   a , respectively, which introduce, when activated, different light-path length in the incoming beam, i.e. the LVA of FIG. 1 a  is an example of a phase grating type LVA. The diffraction plane  17  is parallel to the array&#39;s long axis  16  and is common to all the LVA members  12 ,  13  and  14 .  
           [0005]    [0005]FIG. 1 b  illustrates the diffraction in “piston” type active members of an LVA. When an array member is activated (for example members  12  and  14 ), its sub-elements ( 12   a  and  14   a , respectively) introduce a difference in the phase of the reflected light. For piston type grating, the optimal phase difference is λ/2 (λ/4 in both directions for reflective grating). Due to hiss phase modulation, the light is diffracted in the directions shown by the solid-line arrows. Array member  13  is non-active and all its sub-elements  13   a  return the light without modulating its phase, i.e. the member acts merely as a plane mirror. The diffracted beams from all LVA members lie in the same common diffraction plane  17  of FIG. 1 a , i.e. the diffractive planes for all members of the array coincide. This fact causes significant shortcomings which are explained below with reference to FIG. 1 c.    
           [0006]    [0006]FIG. 1 c  schematically illustrates a typical optical system employing a diffractive LVA. Optical light source  21  is projected on the LVA  10 , after passing though condenser lens  23  and folding mirror  25 . Active ( 12 ) and non-active ( 13 ) LVA members are shown. The diffraction plane  17  is common for all the LVA members and is parallel to the array&#39;s long axis. Fourier lens  24  images the focus of the condenser lens  23 , lying in the plane F′, onto the Fourier plane F. Thus, the active members  12  produce an image  12   e  (for simplicity only the +1 st  and −1 st  diffractive orders are illustrated, higher diffractive orders are not illustrated) of the light source  21 . The non-active members  13  act as plane mirrors and merely return the light to the folding mirror  25 . Thus, in this setup, the folding mirror  25  acts also as a spatial Fourier filter, preventing the “0” order diffracted light to propagate in he direction of the image plane  27 . Consequently, the imaging lens  26  images the LVA  10  onto the image plane  27 . Thus, due to the spatial filtration, the image of the LVA  10  in the plane  27  is an array of bright ( 12   e ) and dark ( 13   e ) spots, the first being the images of the corresponding active members  12  and the second being the images of tie corresponding non-active members  13 .  
           [0007]    The optical configuration of FIG. 1 c  is well known in the art and is the basis of many spectrometers. In such configurations, high efficiency and good spatial filtration (contrast ratio) can be easily achieved for small (point-like) light sources. However, in case of elongated light sources ( 22 , dashed line), such as laser-diode bar arrays, having a typical length of 10 mm, the Fourier images which are co-axial ( 22   a , dashed line) can partially overlap, leading to increased dimensions of the spatial filter  25  and hence significant vignetting of the optical beams. In such conventional art configurations, a compromise between high efficiency and high contrast has usually to be made.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention relates to linear Light Valve Array in which individual members (pixels) are of controllable (switchable) diffraction grating type.  
           [0009]    In accordance with the present invention, a system and method are provided, in which the linear LVA is designed such that the diffractive planes of the LVA members of the array do not coincide and are substantially perpendicular to the array&#39;s long axis. The LVA members can be of piston type, cantilever mirror type, step-wise blazed type or any other diffractive element including but not limited to diffractive optical elements with controllable refractive index.  
           [0010]    According to another embodiment of the present invention, a microlens array is attached to the linear LVA, increasing the overall efficiency by preventing non-diffractive parts of the LVA to reflect (in case of reflective device) or transmit (in case of transmittive device) the incident light. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 a  is an isometric view of a conventional art LVA of diffractive type;  
         [0012]    [0012]FIG. 1 b  illustrates the performance of a conventional art diffractive LVA of “piston” type;  
         [0013]    [0013]FIG. 1 c  is a schematic illustration of a conventional art optical system employing diffractive type LVA;  
         [0014]    [0014]FIG. 2 a  is an isometric view of a diffractive type LVA having LVA members of “piston” type, constructed according to an embodiment of the present invention;  
         [0015]    [0015]FIGS. 2 b  and  2   c  are schematic illustrations of the diffraction performance of a “piston” type LVA member, in the LVA of FIG. 2 a , constructed according to an embodiment of the present invention;  
         [0016]    [0016]FIG. 3 is a schematic isometric view of an optical system employing LVA of diffractive type, constructed according to an embodiment of the present invention;  
         [0017]    [0017]FIG. 4 a  is an isometric view of an LVA of diffractive type having LVA members of “cantilever mirror” types constructed according to an embodiment of the present invention;  
         [0018]    [0018]FIGS. 4 b  and  4   c  are schematic illustrations of the diffraction performance of a “cantilever mirror” type LVA member, in the LVA of FIG. 4 a , constructed according to an embodiment of the present invention;  
         [0019]    [0019]FIGS. 5 a  and  5   b  are schematic illustrations of the diffraction performance of a “step-wise blazed” type LVA member, in an LVA constructed according to an embodiment of the present invention;  
         [0020]    [0020]FIG. 6 a  is a schematic isometric view of an LVA of diffractive type having increased aperture ratio and constructed according to an embodiment of the present invention; and  
         [0021]    [0021]FIGS. 6 b  and  6   c  are schematic illustrations of the performance of the LVA of FIG. 6 a , constructed according to an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    Reference is now made to FIG. 2 a , which is a schematic illustration of a diffractive type linear array LVA  10  of reflective type, employing individual “piston” type diffractive members  12 ,  13 ,  14 , whose diffraction planes  12   d ,  13   d ,  14   d , respectively, are perpendicular or nearly perpendicular to the array&#39;s long axis  16 . This configuration is achieved by defining the dimension L (the length of the diffractive member) to be much longer than the dimension h (the height of the diffractive member) (L&gt;&gt;h). When activated, the electrodes  12   a ,  13   a ,  14   a , cause periodical change in the mechanical and/or optical parameters of the active media (MEMS structure, LC, etc.), in a direction perpendicular to the array&#39;s long axis  16 . Because of the configuration L&gt;&gt;h, the diffraction in plane  17  is much less than the diffraction in the planes  12   d ,  13   d ,  14   d.    
         [0023]    In the illustrated example of FIG. 2 a , a light beam  32  propagating along axis  30 , which is perpendicular to the LVA&#39;s plane  31 , may be either reflected back in the same direction by a non-active member (in this example  13 ), or diffracted by an active member (in this example  14 ) in directions  33 , lying in diffraction plane  14   d , normal to the LVA&#39;s plane  31 .  
         [0024]    For completeness of the explanation, FIGS. 2 b  and  2   c  are provided, illustrating the diffractive performance of MEMS LVA member of “piston” type. Both FIGS. are cross-sectional illustrations of plane  12   d  of an LVA member  12 , which is perpendicular to the array&#39;s long axis  16 . In this example, the LVA member (or its sub-elements respectively) is electrically activated. In FIG. 2 b  the applied voltage is U=0 and all strips (sub-elements)  12   a  forming the member lie in the same plane, i.e. the member acts as planar mirror and the impinged beam  32  is reflected back in direction  33 . In FIG. 2 c  voltage of value U=U 0  is applied on strips  12   c . Due to the electrostatic force, strips  12   c  are deformed downward and the impinging light beam  32  is diffracted in directions  33 . Maximum performance of this type diffractive modulator can be achieved when the deformation of the strip  12   c  is H=λ/4, where λ is the wavelength of the impinging light beam.  
         [0025]    The advantage of employing the diffractive array illustrated in FIG. 2 a  when using elongated light sources is clearly demonstrated in FIG. 3. FIG. 3 is a schematic illustration of an optical system generally similar to the one illustrated in FIG. 1 c . An elongated light source  22  with its long dimension parallel (in this example) to the optical axis  40 , is focused by the condenser  23  on the plane F′, then folded by the mirror  25  and projected as a parallel beam by the Fourier lens  24  on the reflective diffractive LVA  10 . The conjugated planes for the Fourier lens  24  are F and F′. F is the Fourier plane of the system, where minimum two images of the light source are present, being the “+1” and “−1” diffractive orders caused by all active LVA members  12  of the array  10  (for clarity, higher orders are not illustrated).  22   b  denotes where the “0” diffractive order caused by active LVA members  12  and the mere reflection by non-active LVA members  13  would be present however, the “0” order diffracted light and the mere reflected light are blocked by the folding mirror  25 , which acts in this configuration also as a Fourier filter. Consequently, the LVA  10  is imaged by lens  26  on the image plane  27 , where an array of bright spots  12   e  (being images of active LVA members  12 ) and dark spots  13   e  (being images of non-active LVA members  13 ) are present.  
         [0026]    [0026]FIG. 3 clearly demonstrates that because of the perpendicular non-coinciding diffractive planes  12   d ,  13   d  ( FIG. 2 a ), of the corresponding LVA members  12 ,  13 , the Fourier images  22   a  and  22   b  are arranged in a vertical manner, with their long dimensions not co-axial, and do not lie in the plane defined by the source  22  and the optical axis  40 . In this way, the overlapping of diffractive orders and beam vignetting is avoided. It is appreciated that in the exemplary configuration of LVA member illustrated above, the height of the LVA member h ( FIG. 2 a ) is much smaller than its active length L, and hence the diffraction in the direction of the LVA long axis  16  is negligible.  
         [0027]    It is also appreciated that other than “piston” type individual diffractive optical elements can be arranged in array in the manner illustrated in FIG. 2 a.    
         [0028]    [0028]FIG. 4 a  is a schematic isometric view of a diffractive away with perpendicular diffraction, employing “cantilever mirror” type LVA members  12 ,  13 ,  14 ,  15 . Each member consists of a mirror  12   a ,  13   a ,  14   a ,  15   a , having dimensions L×h. The dimensions L and h are chosen such that L&gt;&gt;h. Thus, the diffraction in plane  17  is negligible compared to the diffraction in the planes  12   d ,  13   d ,  14   d . For completeness of the explanation, FIGS. 4 b  and  4   c  are provided, illustrating the diffractive performance of a MEMS LVA member of “cantilever mirror” type. FIG. 4 b  is a cross-sectional illustration of an inactive member (for example  12  or  14  of FIG. 4 a ) while FIG. 4 c  is cross section of an active (diffracting) member (for example  13  or  15  of FIG. 4 a ). Both cross-sectional illustrations are in planes perpendicular to the array&#39;s long axis  16  ( 12   d ,  13   d ,  14   d ). In this example the LVA members are electrically activated. The mirrors  12   a  and  13   a  are also electrodes and are suspended over the counter electrode  20 . In FIG. 4 b  the applied voltage is U=0. The mirror  12   a  is horizontal and the member acts as a planar mirror, i.e. the angle of incidence of the impinging beam  32  and the angle of reflection of reflected beam  33  are equal with respect to the normal to the LVA plane. In FIG. 4 c  a voltage of value U=U 0  is applied to mirror  13   c . Due to electrostatic force, the mirror is deformed downward and the impinging light beam  32  is diffracted in direction  33 .  
         [0029]    A method of simulating cantilever mirror by a number of vertically deformed reflective strips is illustrated in FIGS. 5 a  and  5   b , demonstrating the diffractive performance of a diffractive optical element of “staircase blazed” type. The LVA is arranged as shown in FIG. 2 a , but the strips are deformed downward in a manner forming a staircase, rather than switched alternatively as shown in FIGS. 2 b  and  2   c . The advantages of using this type of diffractive optical elements are discussed in Provisional U.S. patent application Ser. No. 60/218,063, assigned to the same assignee as this patent application.  
         [0030]    It is well known to the skilled in the art that the angular resolution of a diffractive optical element, strongly depends on the number of the sub-elements working in synchrony it consists of. Therefore, it is appreciated that in this explanation, diffractive member or diffractive optical element means an entity imaged as such on the imaging surface and producing on this surface bright or dark spots, depending on its diffractive state, and not the individual sub-elements it may or may not consist of. In the example of FIGS. 2 a  to  2   c  each LVA member  12 ,  13 , consists of two sub-members  12   f  (FIG. 2 b ), each one having two sub-elements  12   a . The sub-members  12   f  are grouped functionally to form the diffractive member  12 , in terms that they are always working in synchrony, thus combining their diffractive power to achieve higher angular resolution. Such a configuration can be introduced not only to diffractive LVA employing piston-type elements, but to diffractive LVAs based on any type of diffractive optical modulator, such as staircase blazed or cantilever mirror.  
         [0031]    [0031]FIG. 6 a  illustrates another embodiment of the present invention, including a microlens array  50  attached to the LVA  10 . An important parameter of an LVA of any type is the Aperture Ratio, defined as the ratio between the active aperture of the pixel and the total surface occupied by it, including the gap between adjacent pixels. As can be seen from the comparison between FIG. 1 a  and FIG. 3, it is an inherent property of LVAs with perpendicular diffraction to have lower aperture ratio, compared to LVAs with parallel diffraction. This is due to the way the member electrodes  12   a ,  13   a ,  14   a , of FIG. 1 a  can be wired by the interconnections  12   b ,  13   b ,  14   b . For the LVA of FIG. 2 a , the Aperture Ratio is T, 0110   
         [0032]    which decreases when increasing the number of the member electrodes  12   a ,  13   a ,  14   a , i.e. when increasing the resolution power of the diffractive LVA members  12 ,  13 ,  14 , respectively. In the embodiment of FIG. 6 a , this drawback is avoided by attaching to the LVA  10  a microlens array  50 . Each of the individual lenses  12   c ,  13   c ,  14   c  of the array  50  is associated with a single LVA member  12 ,  13 ,  14 . The performance of the lens array can be better understood from FIGS. 6 b  and  6   c , where cross-sections along the planes A and B (of FIG. 6 a ) are illustrated, respectively. As can be seen in FIG. 6 b , the parameters of the individual microlens  13   c  are chosen (in the context of the overall optical scheme) such that only the active part L of the LVA member is illuminated. Because of this, the aperture ratio is close to its theoretical maximum and is determined mainly by the gap between the electrodes  12   a ,  13   a ,  14   a  of the LVA members  12 ,  13 ,  14  respectively (FIG. 2 a ).  
         [0033]    In the example of FIG. 6 a , the shape of the lens is cylindrical and the radius R of the curved surface lies in the diffraction plane B. Therefore, as can be seen in FIG. 6 c , lens  13   c  does not affect the diffraction properties of the LVA members  13 . It is appreciated that other types of lenses, namely spherical or aspherical can be used and the choice of the shape and the microlens parameters will be made according the optical system in which the LVA is integrated.  
         [0034]    It is appreciated that microlensing for increasing the LVA efficiency, as described above, can be applied on the designs of FIGS. 4 and 5 as well.  
         [0035]    It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described herein above. Rather the scope of the invention is defined by the claims that follow.