Abstract:
A light valve of deformable grating type and a method for light modulation using the light valves is provided. The light valve of deformable grating type, includes at least three beams, one beam of the at least three beams being of a substantially fixed-position, and at least two beams of the at least three beams being deformable by electrostatic force in a substantially staircase structure, each step of the staircase creating a predefined change in the phase of an impinging light beam, a first electrode and a second electrode, the electrodes transmitting electrostatic force to at least the deformable beams of the at least three beams. The beam of a substantially fixed-position may be deformable by electrostatic force.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional application Ser. No. 60/218,063 filed Jul. 13, 2000. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to micro-mechanical light modulators and to Spatial Light Modilators (SLMs) including arrays of such modulators. 
     BACKGROUND OF THE INVENTION 
     Various optical applications, such as projection, imaging and optical fiber communication, require light modulation and/or light beam steeped In optical applications where a plurality of optical beams should be handled simultaneously, the modulation can be achieved by using optical modulators called Spatial Light Modulators (SLMs) or Light Valves (LVs), which are arrays of individually controlled members. Distinctive class SLMs work in diffractive mode; An activated individual member of the SLM array diffracts the incoming light beam at a discrete multitude of angles, these angels being a function of the light wavelength and the dimensions of the modulator. Such modulators, based on Micro Elctro-Mechanical Systems (MEMS) technology and called Deformable Diffractive Gratings, are described, for example, in U.S. Pat. Nos. 5,311,360; 5,459,610 to The Board of Trustees of the Leland Stanford, Junior University; U.S. Pat. Nos. 5,629,801; 5,661,592 to Silicon Light Machines; U.S. Pat. No. 5,677,783 to The Board of Trams of the Leland Stanford, Junior University; U.S. Pat. Nos. 5,808,797; 5,841,579; 5,982,553 to Silicon Light Machines; U.S. Pat. No. 5,920,518 to Micron Technology, Inc.; U.S. Pat. No. 5,949,570 to Matsushita Electric Industrial Co.; U.S. Pat. No. 5,999,319 to InterScience Inc.; U.S. Pat. Nos. 6,014,257; 6,031,652 to Eastman Kodak Company. 
     In the conventional art Deformable Diffractive Gratings light modulation systems, the diffractive element is usually of “piston” type or cantilever mirror type. Both types of diffractive elements have some advantages, while suffering from some drawbacks. For example, a piston diffractive grating element is always faster than a cantilever mirror diffractive grating element, however, its efficiency is lower. Reference is made now to FIGS. 1,  2   a ,  2   b  and  2   c , which show a typical conventional art design of a piston diffractive type element and demonstrate its operation. Throughout the figures, similar elements are noted with similar numeral references. 
     FIG. 1 is a schematic isometric view of a conventional art piston type deformable grating element  10 . The element  10  consists of several beams, noted  25 , created by a photolithographic process in a frame  20 . The beams  25  define a diffractive grating  22 , supported by the etched structure  30 . The bee  25  rest on a silicon substrate base  40 . Beams  21  of the beams  25  are movable and are suspended over gaps  41 , which are etched in the silicon substrate base  40 , while other beams  23  of the beams  25  are static. The beams  25  are coated with a reflective layer  60 . This reflective layer  60  is conductive and functions as an electrode. An opposite electrode  50  is deposited on the opposite side of the silicon substrate  40 . 
     FIGS. 2 a  and  2   b  show the A—A cross-section of the conventional art modulator  10  of FIG. 1 in non-active and active states, respectively. In FIG. 2 a , no voltage is applied between the suspended beams  21  and the common electrode  50 . Accordingly, all the beams  21  and  23  are coplanar and the diffractive element works as a plane mirror, i.e. incident beam  70  and reflected beam  71  are in the exact opposite directions. When voltage is applied between the suspended beams  21  and the common electrode  50 , as shown in FIG. 2 b , the suspended beams  21  are deformed in the direction of the electrical field created by the applied voltage. Thus, the non-suspended beams  23  and the suspended beams  21  define a diffractive structure returing an incident beam  70  in directions  171 . The directions  171  and the direction  70  of the incident beam constitute an angle Φ which follows the laws of diffractive optics and is called a diffractive angle. The angle Φ is a function of the light wavelength λ and the grating period d. The diffraction efficiency is a function of the grating amplitude. For piston type grating, the optimal amplitude for achieving optimal efficiency, is λ/4, as illustrated in FIG. 2 b . In this example and the example below it is assumed that the light modulation system operates in air with refractive index n=1. 
     FIG. 2 c  shows the angular distribution of the light energy for non-active (thin line) and active (thick line) λ/4 optimize piston type deformable grating light modulating element. The calculations are made for Fraunhofer diffraction of parallel light beam while λ=830 nm and grating period d=10 μm, and while King into account the interference of two simultaneously working elements (i.e. 2d ‘UP’-‘DOWN’-‘UP’-‘DOWN’ structure). It can be seen from this figure that when the element is active, most of the energy is distributed in the +1 st  and −1 st  orders, while when it is non-active, most of the energy is distributed in the “zero” order (tinner line). 
     Commonly, there are two kinds of distinctive optical systems that utilize diffractive type light modulators: optical light systems having spatial filtering of the “zero” order, and optical light systems having spatial filtering of the ±1 st  and higher orders. When the “zero” order is filtered, the maximal theoretical energy efficiency is 70%, while when the ±1 st  and higher orders are filtered, the maximal theoretical energy efficiency can be as high as 90%. In both cases, the maximal theoretical contrast ratio (the ratio between the energies passing the spatial filter in the active and non-active states, respectively) that can be achieved is 1:12. 
     However, for most applications, such as pre-press imaging and projection displays, contrast ratio as low as 1:12 is unacceptable. An additional disadvantage of the piston type diffractive grating modulators, is that when in active state, the light energy is distributed symmetrically in the ±1 st  and higher orders, which in many cases can lead to a more complex optical system, as the light has to be cutoff from both sides of the maximum. 
     SUMMARY OF THE INVENTION 
     There is provided in accordance with an embodiment of the invention, a light valve of deformable grating type. The light valve includes at least three beams, one beam of being of a substantially fixed-position, and at least two beams being deformable by electrostatic force in a substantially staircase structure, each step of the staircase creating a predefined change in the phase of an impinging light beam, and first and second electrodes for transmitting electrostatic force to at least the deformable beams. 
     There is also provided in accordance with a further embodiment of the invention, a light valve of deformable grating type, which includes at least three beams, one beam being of a substantially fixed-position, and the three beams being deformable by electrostatic force in a substantially staircase structure, each step of the staircase creating a predefined change in the phase of an impinging light beam and a first electrode and a second electrode, the electrodes transmitting electrostatic force to the deformable beams. 
     In addition, there is also provided in accordance with an embodiment of the invention, a method for light modulation. The method includes the steps of: 
     providing a light valve of deformable grating type, the light valve includes at least three beams, at least the first beam of the at least three beams being of a substantially fixed-position, and at least two beams of the at least three beams being deformable by electrostatic force in a substantially staircase structure, each step of the staircase creating a predefined change in the phase of an impinging light beam; 
     illuminating the light valve,; and 
     applying voltage between the first electrode and the second electrode. 
     providing a light valve of deformable grating type, the light valve includes at least three beams, at least the first beam of the at least three beams being of a substantially fixed-position, and the at least three beams being deformable by electrostatic force in a substantially staircase structure, each step of the staircase creating a predefined change in the phase of an impinging light beam; 
     illuminating the light valve; and 
     applying voltage between the first electrode and the second electrode. 
     Furthermore, in accordance with an embodiment of the invention, the deformable beams form the first electrode and the second electrode is common to all the deformable beams. 
     Furthermore, in accordance with an embodiment of the invention, the deformable beams form the first electrode and the second electrode includes an array of electrodes, each electrode of the array of electrodes associated with one of the deformable beams. 
     Furthermore, in accordance with an embodiment of the invention, the first electrode includes an array of electrodes, each electrode of the array of electrodes associated with one of the deformable beams, and the second electrode is common to all the deformable beams. 
     In addition, in accordance with an embodiment of the invention, a spatial light modulator is formed as an array of light valves. 
     Furthermore, in accordance with an embodiment of the invention, the beam of a substantially fixed-position is deformable by electrostatic force. 
     Furthermore, in accordance with an embodiment of the invention, the at least three beams form the first electrode and the second electrode is common to all the deformable beams. Alternatively, the at least three beams form the first electrode and the second electrode includes an array of electrodes, each electrode of the array of electrodes associated with one of the at least three beams. 
     Furthermore, in accordance with an embodiment of the invention, the first electrode includes an array of electrodes, each electrode of the array of electrodes associated with one of the at least three beams, and the second electrode is common to all the at least three beams. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view of a conventional art diffractive modulator of piston deformable grating type; 
     FIGS. 2 a ,  2   b  and  2   c  illustrate the performance of the conventional art diffractive modulator of FIG. 1; 
     FIGS. 3 a  and  3   b  are schematic isometric views of diffractive modulators of blazed deformable grating type according to the present invention; 
     FIGS. 4 a ,  4   b  and  4   c  illustrate the performance of diffractive modulators of blazed deformable grading type according to the present invention; 
     FIGS. 5 a - 5   d  are schematic views of additional diffractive modulators of blazed deformable grating type according to the present invention; and 
     FIG. 6 is a schematic view of a diffractive SLM utilizing diffractive modulators of blazed deformable grating type according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Detailed description of the technologies employed in producing IS devices can be found in conventional art publications, such as “Design, Test, and Microfabrication of MEMS and MOEMS”, B Courtois et al, SPIE proceedings 3680, 1999, ISBN 0-8194-3154-0. 
     Reference is made now to FIGS. 3 a  and  3   b , showing a deformable grating type diffractive modulator  100 , according to the present invention Modulator  100  consists of a plurality of beams  21  and  23 , the beams  21  being suspended over a silicon structure base  40  coated with insulation layer  45 . The beams  21  can be made, for example, from low stress silicon nitride and are etched in a frame  25  by sacrificial layer method. The beams  21  that, as will be explained below, form a diffractive grating, are the active part of the modulator and are coated with a highly reflective layer  60 . Layer  60  may be chosen of a material such that high reflectivity will be achieved, in accordance wit the wavelet of the light to be modulated and can be, for example, of aluminum, silver, gold or wavelength optimized metal—dielectric mirror. In addition, the layer  60  acts as the first electrode for applying a voltage between the beams  21  and the second electrode  50 . 
     Reference is made now to FIGS. 4 a  and  4   b , which illustrate the A—A cross-section of the diffractive modulator  100  of FIG. 3 a  As illustrated in FIG. 4 a , base  40  of the modulator is shaped in a star case structure  80 , so that the beams  21  are suspended at different distances from the base  40 . Faker, the beam  23  is fixed and rests on the base  40 . Preferably, for this embodiment, the number of the steps  81  of the staircase structure  80  is n (n being the total number of beams  21  and  23  in the modulator  100 ) and each step of the steps  81  of the staircase structure  80  is of the same height h, such that h=H/3, where H is the amplitude of the grating. The way of determining the parameters of the staircase structure  80 , i.e. the pitch d 0  of the beams  21  and  23  and the amplitude H will be addressed below. 
     FIG. 4 a  shows the modulator  100  of FIG. 3 a  in a non-active state—the voltage applied between the first electrodes  60  and the second common electrode  50  is U=0.Preferably in this state, all the suspended beams  21  are in their uppermost position and are preferably coplanar with the fixed beam  23 . In this state, the modulator will act as a plane mirror, thus an optical beam  70  impinging the modulator at an angle Ψ with respect to the normal  73 , will be reflected back at an angle Φ=Ψ. 
     FIG. 4 b  shows the modulator  100  of FIG. 3 a , with voltage U=U 0  applied between the first electrodes  60  and the second electrode  50 . Due to the electrostatic forces, all the suspended beams  21  are deformed and each one preferably rests on its corresponding step  81  of the staircase structure  80 . The resulting periodic structure performs as a diffractive grating with a grating constant d=n*d 0  and a grating amplitude H (n being the total number of beams  21  and  23  and the number of the steps  81  in the staircase structure  80 . In this example n⊖4). An optical beam  70  impinging the surface of modulator  100  at an angle Ψ with respect to the normal  73 , will be diffracted in multitude directions  171 , with specific angular distribution of the energy (for clarity reasons, only one direction of the directions  171  is shown in the figure). 
     For achieving maximum contrast ratio in the light modulation system, it is required that in its active state, the energy returned in the direction Φ=Ψ is zero (or minimal), i.e. E(−Ψ)=0. According to the diffraction theory, this condition can be fulfilled when              ∑     p   =   0       n   -   1            exp        (         2      π                 i     λ        p                 Δ     )         =   0     ,                          
     where λ is the wavelength and        Δ   =         H     n   -   1            (       cos                 Φ     +     cos                 Ψ       )       +       d   0          (       sin        (   Φ   )       +     sin        (   Ψ   )         )                                
     is the phase shift achieved, for example, in a “plane wave” optical beam  70  by each step  81 . 
     These equations may be used for optimizing blazing) the diffractive f or a given wavelength. An example of such optimizing is shown in FIG. 4 c , where the calculations are made for λ=830nm, d 0 =5 μm, andΨ=0, and while assuming interference of two diffractive modulators  100  of the present invention. In the figure, the dotted line represents the energy angular distribution created by a non-active modulator (U=0)—simple reflection, while the solid line represents the energy angular distribution in an active state of the modulator ( U=U 0 )—diffraction. It can be seen that the predominant part of the energy is concentrated in a narrow range of angles forming one sharp maximum, which is the essence of the blazed gratings. It can also be seen that the energy efficiency (EE) and the contrast ratio (CR) are significantly better compared to a conventional art piston grading modulator (FIG. 2 c ). In an optical system utilizing a light modulator according to the present invention, when filtering the “zero” order EE=75% and CR=20:1. When an optical system utilizing a light modulator according to the present invention filters the 1 st  order then EE=91% and CR=25:1. These numbers clearly demonstrate an advantage of the blazed modulators of the present invention. 
     The optimization procedure described above is valid for blazed modulator with equal center-to-center distance d 0  between the beams  21 , equal widths of beams and equal depth of steps h. It is however appreciated, that other designs with unequal center-to-center distances and/or unequal widths of beams and/or unequal depths of steps are also possible, and are also considered in the scope of the present invention In such cases, the optimization condition is more complicated and usually has only numerical solutions. Such optimization calculations are discussed for example, in M. Born and E. Wolf,  Principles of Optics , Pergamon, N.Y., 1975. 
     An additional embodiment of the present invention is shown in FIG. 3 b . It differs from the arrangement shown in FIG. 3 a  in the design of the base  40  and the electrode  23 . In this embodiment, the beam  23  is also suspended rather than rested on the base  40 . Beam  23  however, does not have electrical connection with the rest of the beams  21  and therefore its position is not affected by applying an electrical field to these beams. This design has the same performance as the design of FIG. 3 a  and can be optimized using the same procedure explained above with regard to FIGS. 4 a  and  4   b , The advantage of such a design is that its process of production is more convenient, especially when a plurality of such modulators are arranged in an array. 
     Reference is made now to FIGS. 5 a  and  5   b , presenting an additional embodiment of the present invention. The diffractive light modulator  200  of the present invention has the same basic structure as the embodiments of FIGS. 3 a  and  3   b , respectively, but for the common electrode  50  (FIGS. 3 a  and  3   b ), which is replaced by an array of electrodes  51 ,  52  and  53 , each associated wit a corresponding suspended beam  21 A,  21 B and  21 C, respectively. Accordingly, while the suspended beams still present one first electrode, the second electrode is now an array of electrodes. This configuration allows for fine tuning of the non-diffractive state, by applying small different bias voltages U 1 , U 2  and U 3  to each suspended beam  21 A,  21 B and  21 C respectively, thus arranging them to be essentially coplanar with the beam  23 . Optionally, a counter electrode  50   a  can be added to beam  23  of the embodiment of FIG. 5 b  (shown with dashed line) for receiving voltage U 0  for fine-tuning. In this case, the beam  23  should be short-circuited to the suspended beams  21 A,  21 B and  21 C as schematically shown by the dashed curve  24 . 
     It is appreciated that the same effect can be achieved by applying different small bias voltages to each of the suspended beams  21 A,  21 B and  21 C, relative to a common second electrode, as illustrated schematically in FIGS. 5 c  and  5   d . Optionally, beam  23  of the design of FIG. 5 d , can also be supplied with electrical connection (shown with dashed line) for receiving voltage U 0  for fine-tuning. In this case, the common counter electrode  50  should be extended, as shown by the dashed line  50   b.    
     Although the exemplary light modulators discussed above consist of four beams—one fixed and three suspended, it is appreciated that other configurations, with different number of suspended beams, are also possible. Furthermore, a higher number of suspended beams enables tuning (blazing) the grating modulator to higher diffractive orders, while maintaining similarly high EE and CR. 
     Reference is made now to FIG. 6, which is a schematic illustration of an array of blazed modulators of deformable grating type  300  according to the present invention. The modulators are placed on one common silicon wafer base  40 , by employing standard, well known in the art technology. The figure illustrates part of the array  300 , consisting of five individual modulators  101  through  105 . Modulators  101  and  104  are in a non-active state, thus working as plane mirrors. Modulators  102 ,  103  and  105  are active and diffract the incoming beam, as indicated by arrows  171 . All the suspended beams can be short-circuited to form one common first electrode, while a dedicated second electrode  151  to  155  is assigned to each individual modulator  101  to  105 , respectively. It is appreciated that a configuration in which the suspended beams of each individual modulator form several first electrodes, while the second electrode is common for all modulators is also possible. It is also appreciated that although the array  300  of FIG. 6 is constructed out of individual modulators of FIG. 3 a , a design involving individual modulators of FIG. 3 b  and FIGS. 5 a  through  5   d , or any combination thereof is possible as well.