Abstract:
A method for modulating light using a micro-electro-mechanical structure includes providing a plurality of deformable mirror elements ( 30 ) having an L-shaped cross section. Each of the deformable mirror elements are comprised of a pedestal ( 32 ) and an elongated ribbon ( 33 ). Each of the ribbons has a reflective surface ( 35 ). A beam of light is directed on the deformable mirror elements. The deformable mirror elements is flexed about an axis parallel to a long dimension of the ribbons to vary a curvature of at least one of the reflective ribbons.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    Reference is made to commonly-assigned copending U.S. patent application Ser. No. ______ (Attorney Docket No. K001423US01NAB), filed herewith, entitled ASYMMETRICAL DEFORMABLE DIFFRACTIVE GRATING MODULATOR; by Nissim Pilossof; the disclosure of which is incorporated herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates in general to micro-mechanical structures and in particular to diffractive grating modulators. 
       BACKGROUND OF THE INVENTION 
       [0003]    Micro-electro-mechanical systems (MEMS) are used in many devices which require modulation of light. For example, projectors may use a digital micromirror device (DMD), which has thousands of micromirrors. A cantilever or hinged mirror  10  of this type, shown in  FIG. 1 , rotates on an axis  11  reflect a beam of light when in an on position. A disadvantage of this type of device is the relatively slow response time, ˜10 μs, due to the low natural frequency of each single hinged mirror. 
         [0004]    In another type of modulation device, the deformable diffractive grating light modulation systems, the diffractive element is formed as a long narrow ribbon. In one design, the ribbon bends along the long axis of the ribbon thus forming a “piston” type switching diffractive element. Typical piston type diffractive elements are shown in U.S. Pat. Nos. 5,311,360; 5,459,610; and 5,677,783. 
         [0005]    In another design, shown in  FIG. 2   a , the cross section of the diffractive ribbon is T-shaped and the ribbon bends along its short axis.  FIG. 2   a  illustrates two pixel elements  21  and  22  of a spatial light modulator (SLM)  20  built on a silicon substrate  23 , with each pixel comprised of three diffractive elements  210  and  220  respectfully. The pixels are driven using electrodes  24 . Pixel  21  is shown in energized state while pixel  22  is in a non-energized state. Resulting diffraction distribution of the two pixels is shown on  FIG. 2   b . In energized (diffracting) state, a pixel produces symmetrical angular distribution of light intensity consisting of many diffraction maximums. T-shaped ribbon types of diffractive elements are shown, for example, in U.S. Pat. Nos. 6,661,561; 6,836,352; and 6,856,448. 
         [0006]    Both types of diffractive elements have some advantages, while suffering from some drawbacks. In both types, however, the width of one ribbon, the pitch, determines the grating period d, and the diffracted light is distributed within multiple diffractive orders symmetrically in both directions. See  FIG. 2   b . The diffraction efficiency of a single element is very low with typical contrast of about 50%. Therefore, in optical systems using deformable ribbons light modulators, more than one ribbon is used for forming an optical pixel, rendering the minimum optical pixel size to two times the grating period or 2d. 
       SUMMARY OF THE INVENTION 
       [0007]    Briefly, according to one aspect of the present invention a method for modulating light using a micro-electro-mechanical structure includes providing a plurality of deformable mirror elements having an L-shaped cross section. Each of the deformable mirror elements are comprised of a pedestal and an elongated ribbon. Each of the ribbons has a reflective surface. A beam of light is directed on the deformable mirror elements. The deformable mirror elements are flexed about an axis parallel to a long dimension of the ribbons to vary a curvature of at least one of the reflective ribbons. 
         [0008]    The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  shows a prior art cantilever mirror. 
           [0010]      FIGS. 2   a  and  2   b  show a prior art SLM based on T-shaped ribbons and corresponding angular distribution of light intensity. 
           [0011]      FIG. 3  shows a T-shaped ribbon according to the present invention. 
           [0012]      FIG. 4  is a cross-sectional view of a plurality of diffractive element according to the present invention. 
           [0013]      FIG. 5  shows intensity versus diffraction for L-shaped ribbons based SLM according to the present invention. 
           [0014]      FIG. 6  is a schematic view of an optical system according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    The present invention will be directed in particular to elements forming part of, or in cooperation more directly with the apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. 
         [0016]    One embodiment of the present invention is a diffractive grating consisting of mirrors shaped as ribbons with asymmetrical cross-section as shown in  FIG. 3 . The single diffractive element  30  will have approximately the same diffraction efficiency as symmetrical ribbons, but at half the pitch value, allowing for two times higher system resolution, all other optical conditions being equal. Fabrication methods for deformable mirrors is not within the scope of the present invention, but it is well known and described in details in U.S. Pat. Nos. 5,311,360 and 5,661,592 and can be adapted to fabrication of the mirrors of the present invention. 
         [0017]    Referring again to  FIG. 3 , the deformable mirror element  30  is fabricated for example on a silicon substrate  31 . A deformable mirror element  30  is comprised of a pedestal  32  which supports elongated ribbon  33 . Either the pedestal  32  or the ribbon  33  or both are made electrically insulating material. The ribbon  33  has an unsupported elongated portion laterally extending along the pedestal  32 . The surface of the ribbon  33  is coated with a highly reflective layer forming a mirror surface  35 . For clarity the mirror is shown smaller than the ribbon width, but it is appreciated that it should cover whole width of the ribbon. 
         [0018]    The choice of the coating material depends on the wavelength of light the SLM is intended to work with. For example SLM intended to work with visible light may be coated with aluminum while SLM intended to work with near infrared light may be coated with gold. In an embodiment wherein the ribbon is made of electrically insulating material like silicon nitride and the reflective coating is not electrically conductive, a special conductive coating beneath the reflective coating should be considered. Thus, in  FIG. 3  element  35  represents an electrical electrode and reflective coating in the same time. 
         [0019]    A second electrode  34  is formed on the surface of the substrate. Applying voltage between electrodes  34  and  35  energizes the ribbon and due to electrostatic forces, it bends along its shorter axis assuming position  36 . When the ribbon is bent, its surface profile in the shorter direction (X-direction in  FIG. 3 ) follows a profile function F(x). F(x) is taken into account when calculating the light intensity versus angle in diffracting state and depends on the dimensions of the ribbon, the mechanical properties of the material it builds from, and the applied voltage. Methods of calculating F(x) and the intensity angular distribution in diffracting state are well known in the art. Generally, the closer F(x) is to a straight line, the higher contrast ratio the SLM can achieve. Therefore it is desirable to have the thickness t of the post  32  as small as possible. 
         [0020]    It is appreciated that the above discussed way of selecting and energizing a ribbon by means of electrostatic forces is only exemplary. Other methods like magnetic, thermal, etc. can also be used. 
         [0021]    Reference is now made to  FIG. 4  which shows a portion of SLM  40  with L-shaped deformable ribbons. The number of ribbons in a SLM can be hundreds or even thousands. The distance d between the ribbons is called the grating constant and determines the diffracting power of the device, while the ratio d/g, wherein g is the gap between the ribbons, is called the fill factor and affects the overall efficiency of the device. The higher the fill factor, the higher the device efficiency. 
         [0022]    It is well known in the art that the diffraction efficiency depends on the number of individual elements contributing to the process. If a single pixel is formed from only one diffracting element the maximum achievable contrast is about 50%, which is not enough for most applications. Therefore individual pixels are usually formed from two or more diffracting elements. It is appreciated that the simultaneous wiring of four ribbons illustrated in  FIG. 4  is only exemplary. The SLM can be built by pre-wiring groups of ribbons to form a pixel (as illustrated in  FIG. 4 ) or by wiring each individual ribbon and later forming pixels by simultaneous addressing of adjacent ribbons by the SLM driving electronics. 
         [0023]    For simplicity,  FIG. 4  illustrates only two pixel elements  41  and  42  and each pixel element consists of four ribbons (diffracting elements)  410  and  420  respectfully. Pixel  41  is in diffracting state and pixel  42  is in non-diffracting state. The incident light beam  46  from light source  43  lies in a plane at incidence angle α 44  and strikes the SLM plane at angle Θ relative to its normal  49 . The light falling at the non-energized (inactive) pixel element  42  will experience a simple reflection  47  and will propagate in the plane of reflection β 45  at angle Θ relative to the SLM  40  plane normal  49 . Light falling on the energized (active) pixel after reflection will propagate in different directions  48  governed by the laws of diffraction. Different directions of propagations constitute different diffraction orders as only one order, called “Zero order,” will propagate in the plane of reflection  45  at angle Θ relative to the SLM  40  plane normal  49 , i.e. will obey the law of simple reflection. All other orders first, second, etc. will generally propagate in different planes and at different angles relative to the plane normal  49 . 
         [0024]    For specific profile function F(x) the angle of incidence Θ can be chosen in a way that the light propagating in Zero order direction is minimized and the diffracted light is concentrated predominantly in one of the higher diffracting orders first, second, etc.  FIG. 5  illustrates such optimized angular distribution of light for pixels in diffracting and non-diffracting states. It can be seen that the diffracted light is concentrated predominantly in one diffraction maximum at one side of the grating, i.e. the grating is “blazed.” This “blazing” property of the grating means that the diffracted light will be with approximately the same brightness as the non-diffracted. 
         [0025]    The angular modulation of the light achieved by the SLM can be converted into spatial modulation. This is explained with the help of  FIG. 6 . In an exemplary optical system  600  a SLM  60  with pixel elements built and arranged as described above and shown in  FIGS. 3-5  is illuminated with beam of light  64  using a prism  62  with two reflective surfaces  63 . It is appreciated that instead of a prism, a system of mirrors and direct illumination can be used. The illumination angle Θ is the same angle discussed above. After reflecting from the SLM the light propagates within two beams  65  and  66 . Beam  65  originates from non-energized pixels while beam  66  originates from energized pictures. 
         [0026]    Beams  65  and  66  pass through lens  67  the focal plane (the system&#39;s Fourier plane) contains a blocker  68  which stops the light from beam  66 . After the blocker  68 , only light from non-energized pixel element will propagate through the rest of the optical system, i.e. only the Zero diffraction order will be allowed. 
         [0027]    The next element downstream is lens or group of lenses  69  which together with lens  67  constitute an imaging system with object plane the SLM and image plane  70 , i.e. the SLM and plane  70  are conjugate. As all light from energized pixels will be stopped at the blocker  68  and the image  71  will contain only images of non-energized pixels. It is appreciated that it is possible to place the blocker  68  in such a way that it will stop the Zero Order and allow the higher diffractive orders. In such case the image plane will contain images of all energized pixels. 
         [0028]    The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. 
       PARTS LIST 
       [0000]    
       
           10  hinged mirror 
           11  axis 
           20  spatial light modulator (SLM) 
           21  pixel element 
           22  pixel element 
           23  substrate 
           24  electrode 
           30  deformable mirror element 
           31  substrate 
           32  pedestal 
           33  ribbon 
           34  electrode 
           35  mirror surface 
           36  axis assuming position 
           40  spatial light modulator (SLM) 
           41  pixel element 
           42  pixel element 
           43  light source 
           44  incidence angle α 
           45  reflection β 
           46  incident light beam 
           47  reflection 
           48  direction 
           49  plane normal 
           60  spatial light modulator (SLM) 
           62  prism 
           63  reflective surface 
           64  beam of light 
           65  beam 
           66  beam 
           67  lens 
           68  blocker 
           69  group of lenses 
           70  image plane 
           71  image 
           210  diffractive element 
           220  diffractive element 
           410  ribbon (diffracting element) 
           420  ribbon (diffracting element) 
           600  optical system