Abstract:
One embodiment disclosed relates to a system for modulating a plurality of micro-electromechanical (MEM) devices. The system includes a means for providing an amplitude modulation signal to each MEM device at a base frequency and a means for providing a width modulation signal at the base frequency. In addition, the system includes a clock means for providing a higher-frequency clock signal with a frequency that is a multiple of the base frequency. In this embodiment, the width modulation signal for each MEM device indicates at least one position on the higher-frequency clock signal.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field Of The Invention  
           [0002]    The present invention relates generally to optical systems. The present invention relates more particularly to micro-electromechanical systems (MEMS) for modulating optical signals.  
           [0003]    2. Description Of The Background Art  
           [0004]    A two-dimensional projection image may be formed by using one or more linear arrays of light-modulating elements. The light-modulating elements may comprise, for example, GRATING LIGHT VALVE (GLV) devices or other types of devices that modulate optical signals in a controllable manner. Publications describing GLV devices and their application to display systems include: “The Grating Light Valve: Revolutionizing Display Technology,” by D. M. Bloom, Projection Displays III Symposium, SPIE Proceedings, Volume 3013, San Jose, Calif., February 1997; “Grating Light Valve Technology: Update and Novel Applications,” by D. T. Amm and R. W. Corrigan of Silicon Light Machines in Sunnyvale, Calif., a paper presented at the Society for Information Display Symposium, May 19, 1998, Anaheim, Calif.; and “Optical Performance of the Grating Light Valve Technology,” David T. Amm and Robert W. Corrigan of Silicon Light Machines, a paper presented at Photonics West-Electronics Imaging, 1999. Each of the above-mentioned publications is hereby incorporated by reference in its entirety.  
           [0005]    Such projection systems may be configured so that a linear array of light modulating elements modulates an incident light beam to display pixels along a column (or, alternatively, a row) of the two-dimensional (2D) image being formed. A scanning system is used to move the column horizontally such that each light-modulating element is able to generate a row of the 2D image. In this way, the entire 2D image is displayed.  
         SUMMARY  
         [0006]    One embodiment of the invention relates to a method for driving a plurality of light-modulating elements. The method includes, for each element, providing an amplitude modulation signal and providing a width modulation signal. Modulation of light by the element is determined with respect to height by the amplitude modulation signal and with respect to width by the width modulation signal.  
           [0007]    Another embodiment of the invention pertains to an apparatus for driving a plurality of light-modulating elements. Circuitry is included that provides for each element an amplitude modulation signal at a base frequency. Circuitry is also included that provides for each element a width modulation signal at the base frequency.  
           [0008]    Another embodiment of the invention relates to a system for modulating a plurality of micro-electromechanical (MEM) devices. The system includes a means for providing an amplitude modulation signal to each MEM device at a base frequency and a means for providing a width modulation signal at the base frequency. In addition, the system includes a clock means for providing a higher-frequency clock signal with a frequency that is a multiple of the base frequency. In this embodiment, the width modulation signal for each MEM device indicates at least one position on the higher-frequency clock signal.  
           [0009]    These and other features of the present invention will be readily apparent to persons of ordinary skill in the art upon reading the entirety of this disclosure, which includes the accompanying drawings and claims.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a diagram depicting the reflective and diffractive operational states of a conventional GRATING LIGHT VALVE (GLV) element.  
         [0011]    [0011]FIG. 2 is an illustration depicting a conventional GLV element comprising pairs of fixed and movable ribbons.  
         [0012]    [0012]FIG. 3 is a diagram depicting deflections of reflective members for a conventional GLV element in reflective state.  
         [0013]    [0013]FIG. 4 is a graph illustrating a non-linear electro-optic response for a conventional GLV element.  
         [0014]    [0014]FIG. 5 is a top view depicting a projection display system utilizing an array of light-modulating elements.  
         [0015]    [0015]FIG. 6 is a picture showing an example light-modulating array and drivers for the array.  
         [0016]    [0016]FIG. 7 is a timing diagram that illustrates conventional driving of a GLV-type element.  
         [0017]    [0017]FIG. 8 is a timing diagram that illustrates the driving of a GLV-type element in accordance with an embodiment of the invention.  
         [0018]    [0018]FIG. 9 is a timing diagram that illustrates use of the higher-frequency clock signal to adjust a turn-off time for a pixel.  
         [0019]    [0019]FIG. 10 depicts a schematic diagram of registers for use in controlling a light-modulating element in accordance with an embodiment of the invention.  
         [0020]    [0020]FIG. 11 is a flow chart depicting a conventional method for controlling an array of light-modulating elements.  
         [0021]    [0021]FIG. 12 is a flow chart depicting a method for controlling an array of light-modulating elements in accordance with an embodiment of the invention. 
     
    
       [0022]    The use of the same reference label in different drawings indicates the same or like components. Drawings are not to scale unless otherwise noted.  
       DETAILED DESCRIPTION  
       [0023]    [0023]FIG. 1 is a diagram depicting the reflective and diffractive operational states of a GRATING LIGHT VALVE (GLV) element. The left side of the diagram depicts the reflective (dark) state, while the right side of the diagram depicts the diffractive (bright) state.  
         [0024]    In the example illustrated in FIG. 1, the substrate may comprise a silicon substrate with oxide (for example, about 5000 angstroms thick) overlaying the silicon, and tungsten (for example, about 1000 angstroms thick) overlaying the oxide. The reflective members lie above the tungsten with an air space between the members and the tungsten. For example, three pairs of reflective members (i.e. six members) are shown. The reflective members may, for example, comprise reflective ribbons comprising nitride (for example, about 1000 angstroms thick) with a reflective layer of aluminum (for example, about 500 angstroms thick) on the nitride. Incident light is beamed onto the reflective members. The incident light beam may be at a perpendicular angle to the plane of the substrate.  
         [0025]    In the reflective or dark state (left side), all the reflective members are in the same plane, and the incident light is reflected from the surfaces of the members. This reflective state may be called the dark state because it may be used to produce a dark spot (dark pixel) in a projection display system. Such a dark pixel may be produced by blocking the light that is reflected back along the same path as the incident light.  
         [0026]    In the diffractive or bright state (right side), alternate ones of the reflective members are deflected downward. This results in the diffraction of the incident light in a direction that is at an angle to the path of the incident light. This reflective state may be called the bright state because it may be used to produce a bright spot (bright pixel) in a projection display system. Such a bright pixel may be produced because the angularly reflected light is not blocked. As discussed further below, the optical response of the element depends on the amount of downward deflection of the alternate members.  
         [0027]    [0027]FIG. 2 is an illustration depicting a GLV element comprising pairs of fixed and movable ribbons. As depicted in FIG. 2, the GLV element may include pairs of reflective ribbons, each pair having one fixed and one movable ribbon.  
         [0028]    [0028]FIG. 3 is a diagram depicting deflections of reflective members for a GLV element in a diffractive state. The GLV element comprises a plurality of reflective members  302 . In the example illustrated, the GLV element includes three pairs of reflective members  302  (i.e. six of them).  
         [0029]    In the diffractive state, the reflective members are controllably arranged in an alternating configuration at two heights ( 304  and  306 ). A first height  304  may correspond to reflective members at a fixed height, while the second height  306  may correspond to reflective members pulled down by application of a voltage.  
         [0030]    The grating plane  308  is a theoretical plane that corresponds to the plane on or about which the reflective members are aligned. As illustrated in FIG. 3, the incident light beam  310  impinges upon the element at an angle perpendicular to the grating plane  308 . Diffracted light  312  travels away from the element.  
         [0031]    The difference between first and second heights may be defined as the deflection distance γ. The amount of the deflection γ may be varied by application of different voltages to control the amount of incident light reflected from the element. When γ is near zero, the element would be near a maximally reflective state. When γ is near λ/4, where λ is the wavelength of the incident light, the element would be near a maximally diffractive state.  
         [0032]    [0032]FIG. 4 is a graph illustrating a non-linear electro-optic response for a GLV element. The graph shows intensity of light (in arbitrary units) versus voltage. The higher the voltage is, the larger is the displacement γ of the element. Depending on the voltage applied to the alternate reflective members, the light intensity varies. For the most part, the higher the applied voltage, the higher the light intensity. (This relationship reverses for sufficiently high voltages where the light intensity reduces with higher voltages, and hence the downward slope of the graph at the far right.)  
         [0033]    The two states shown in FIG. 1 produce a dark state and a bright state when the optics collect the plus and/or minus first-orders of diffracted light. If the optics collect the zero-order light instead, the left side of FIG. 1 is the bright state and the right side is the dark state. In addition, the intensity versus voltage characteristic of FIG. 4 would show high intensity near zero voltage and low intensity near the rollover voltage for zero-order operation.  
         [0034]    [0034]FIG. 5 is a top view depicting a projection display system  500  utilizing an array of light-modulating elements. The system  500  includes one or more light sources  502 , one or more arrays of light-modulating elements  504 , an optical scanner  506 , and a screen  508 .  
         [0035]    The light source  502  may comprise one or more laser light sources. Three laser light sources of different colors may be utilized for a color display system. The light-modulating array  504  may comprise an array of GLV elements (also called GLV “pixels”) described above. Each light source  502  may illuminate a light-modulating array  504 . Each element of an array  504  modulates the light incident on it to control the amount of light diffracted therefrom. The diffracted light from the elements of the array  504  is then directed to the optical scanner  506 .  
         [0036]    The optical scanner  506  is used to move the column (or row) of light across the screen  508 . Various types of scanners  506  may be used. For example, galvonometer-based scanners, resonant scanners, polygon scanners, rotating prisms, or other types of scanners may be used. A drive signal is applied to the scanner to control (“drive”) the movement of the column (or row) of light. For example, to achieve a progressive scan of the column across the screen (e.g., from left to right), a sawtooth drive signal may be used.  
         [0037]    [0037]FIG. 6 is a picture showing an example light-modulating array and drivers for the array. The vertical array  602  of light-modulating elements is shown near the center of the picture. The array shown includes 1088 GLV elements or pixels. Surrounding the array  602  are shown four driver integrated circuits ( 604   a ,  604   b ,  604   c  and  604   d ). The drivers are configured for line times as short as 4 microseconds (corresponding to a pixel rate of 250 kHz per drive channel) which is adequate to support a 1,920×1,080 HDTV display at a 96 Hz refresh rate. Each driver output is programmable to 256 levels.  
         [0038]    [0038]FIG. 7 is a timing diagram  700  that illustrates conventional driving of a GLV element. The top of the diagram shows a pixel clock  702 . The bottom of the diagram shows an example pixel amplitude signal  704  for a particular pixel.  
         [0039]    The pixel clock  702  is a clock signal that is provided in common to the various GLV elements. The pixel clock  702  provides the timing for transitioning the pixel amplitude for each pixel from one column (or row) to the next. In this instance, every rising edge indicates that the pixel drivers are transitioned from the amplitude of one column (or row) to the next. For example, the first edge  703  triggers the pixel driver for column (or row) N; the second rising edge  709  triggers the same pixel driver for the next column (or row) N+1; the third rising edge  713  triggers the same pixel driver for the next column (or row) N+2.  
         [0040]    The pixel amplitude signal  704  is generated per GLV element (i.e. per pixel). In other words, the drawing shows a single pixel amplitude signal  704 , but there is actually one such signal for each pixel.  
         [0041]    At the first rising edge  703  of the pixel clock  702 , the pixels are all turned on. In particular, each pixel amplitude signal begins to rise  706  to the appropriate amplitude level  708  as determined by the corresponding amplitude modulation signal. A higher amplitude level typically indicates a higher intensity for that pixel.  
         [0042]    At the second rising edge  709  of the pixel clock  702 , the pixels are all turned on to an amplitude value that may be different from the last amplitude. In particular, each pixel amplitude signal begins to change  710  to the new level  712 . For some types of light-modulating elements, the pixel turn-off time is longer than the pixel turn-on time. This is shown by the lower slope in the falling edge  710  than in the rising edge  706  of the pixel amplitude signal  704 . This asymmetry results in the effective center  711  of the pixel being shifted to the right (i.e. towards the side with the lower slope). Such a shift may be undesirable in various applications.  
         [0043]    At the third rising edge  713  of the pixel clock  702 , the pixels are again all turned on. In particular, each pixel amplitude signal begins to change  714  to an amplitude level  716  as determined by the corresponding amplitude modulation signal. The operation continues from there with the pixel amplitudes being changed at the clock rising edges FIG. 8 is a timing diagram  800  that illustrates the driving of a GLV-type element in accordance with an embodiment of the invention. The bottom of the diagram shows the same example pixel amplitude signal  704  as in FIG. 7. However, the top of the diagram now depicts a higher-frequency clock signal  804  in accordance with an embodiment of the invention.  
         [0044]    The example higher-frequency clock signal  802  has a frequency that is eight times that of the conventional pixel clock signal  702 . More generally, the higher-frequency clock signal  802  may have a frequency that is any multiple of the frequency of the conventional pixel clock signal  702 . In a preferred embodiment, the multiple may be a power of two so that bits may be advantageously used to indicate positions within the higher-frequency clock  802 . The higher frequency clock signal may have a frequency that is equal to or greater than twice the conventional pixel clock signal and does not necessarily need to be an integer multiple of the conventional pixel clock frequency. If both the rising and falling edges of the higher frequency clock signal are utilized, then the actual clockfrequency necessary may be reduced.  
         [0045]    For the particular pixel illustrated in FIG. 8, the pixel amplitude signal  704  is the same as in FIG. 7. However, the higher-frequency clock  802  triggers transitions in the pixel amplitude signal  704  in a different manner. In the situation illustrated, the transitions are triggered at every eighth rising edge of the higher-frequency clock  802 . In particular, every rising edge labeled “0” triggers a transition. At the first “0” rising edge  804 , the pixel is turned on. At the second “0” rising edge  806 , the pixel amplitude may be changed. At the third “0” rising edge  808 , the pixel amplitude may be changed again. And so on.  
         [0046]    The higher-frequency clock signal  802  may be used advantageously in various applications. As one example of an advantageous use, the higher-frequency clock signal  802  may be used to compensate for an undesired shift in the center  711  of a pixel due to asymmetry in rise/fall times. Such a correction is described below in relation to FIG. 9.  
         [0047]    [0047]FIG. 9 is a timing diagram  900  that illustrates use of the higher-frequency clock signal  802  to adjust a turn-off time for a pixel. Here, instead of turning off the pixel at the second “0” rising edge  806 , the pixel is turned off at the preceding rising edge  902  (i.e. at the first “7” rising edge). As a result, the falling transition  904  of the pixel amplitude  704  is shifted to the left. This advantageously shifts the effective center  906  of the pixel so as to compensate for the asymmetry in rise/fall times.  
         [0048]    More generally, there may be a separate width modulation signal provided for each pixel. In accordance with a preferred embodiment, the width modulation signal comprises a plurality of bits. For example, the width modulation signal or a portion thereof may specify at which rising edge to transition the pixel driver to its next amplitude. Use of such width modulation signals provides support for independently clocking each pixel. In other words, the timing of transitions for each pixel amplitude signal  704  may be adjustable per pixel.  
         [0049]    In a preferred embodiment, width modulation bits are appended to amplitude modulation bits for each pixel. This advantageously provides an efficient digital mechanism for controlling both the width and amplitude of each pixel. The interpretation of the width modulation bits can vary with the application. Note that not all of the available high frequency clock (rising and/or falling) edges may be necessary for a possible transition to the next amplitude level.  
         [0050]    Note that, in FIGS.  7 - 9 , the pixel amplitude signal  704  differs in level (e.g.,  708  and  716 ) depending on the corresponding amplitude modulation signal. In other embodiments, the signal  704  may instead be a simple on/off signal that indicates whether or not the pixel is to be turned on or off. In such an embodiment, another signal would indicate the amplitude level to be applied in each instance.  
         [0051]    In another embodiment, the width modulation bits may be the same for all pixels in the current column (or row or other grouping of pixels) eventhough the amplitude modulation bits are not the same. In such an embodiment it is not necessary to replicate the width modulation bits for every pixel in the column (or row or other grouping of pixels).  
         [0052]    [0052]FIG. 10 depicts a schematic diagram of register system  1000  for use in controlling a light-modulating element in accordance with an embodiment of the invention. The register system  1000  includes a buffer register  1002  and an output register  1004 .  
         [0053]    The output register  1004  provides the amplitude and/or width modulation bits for use in modulating light by the element. As described above, the amplitude and/or width modulation bits may be used to determine the amplitude of the light modulation applied by the element.  
         [0054]    While such an output register  1004  by itself is sufficient in conventional systems where the pixels are clocked simultaneously for all the elements, the output register  1004  alone may be insufficient, in some instances, to avoid synchronization problems when the flexibility of width modulation is provided as described above. To circumvent such potential problems, the buffer register  1002  may be added to work in conjunction with the output register  1004 . The buffer register  1002  receives and temporarily holds the next amplitude and/or width modulation bits for the element. The next amplitude and/or width modulation bits are transferred to the output register  1004  upon being triggered. The triggering is preferrably timed such that the next amplitude and/or width modulation bits are not transferred until after the current amplitude and/or width modulation bits in the output register  1004  are no longer needed.  
         [0055]    [0055]FIG. 11 is a flow chart depicting a conventional method  1100  for controlling an array of light-modulating elements. In the first step  1102 , a pixel clock is provided. The pixel clock may be, for example, the pixel clock  702  described above in relation to FIG. 7. In the second step  1104 , amplitude modulation signals are provided for each pixel. As depicted, the second step  1104  is generally performed in parallel with the first step  1102 . The third step  1106  uses the pixel clock (from the first step  1102 ) to turn the pixels on/off simultaneously. In other words, each of the elements in the array begins a pixel at a same time and ends the pixel at a same time. When the pixels are on, each element applies the amplitude modulation (from the second step  1104 ) to modulate that pixel to the appropriate amplitude level.  
         [0056]    [0056]FIG. 12 is a flow chart depicting a method  1200  for controlling an array of light-modulating elements in accordance with an embodiment of the invention. In the first step  1202 , a higher-frequency clock is provided. The higher-frequency clock may be, for example, the higher-frequency clock  802  described above in relation to FIGS. 8 and 9. In the second step  1204 , amplitude modulation signals are provided for each pixel. In addition, width modulation signals are provided for each pixel in the third step  1206 . As depicted, the first three steps ( 1202 ,  1204  and  1206 ) are generally performed in parallel (not one after the other). The fourth step  1208  involves buffering. As described above in relation to FIG. 10, the buffering receives and temporarily stores the amplitude and/or width modulation signals. The buffering may be optional, depending on the application. The fifth step  1210  turns the pixels to the next amplitude modulation level depending on the width modulation signal for each pixel. In other words, when a pixel changes amplitude may be controlled independently for each pixel. For each pixel, when that pixel is on, the corresponding amplitude modulation (from the second step  1204 ) is applied to modulate that pixel to the appropriate amplitude level.  
         [0057]    The method  1200  of FIG. 12 contrasts with the conventional method  1100  of FIG. 11. In the prior conventional method  1100 , all the pixels were clocked simultaneously, and the pixel width was not adjustable per pixel. In the method  1200  of FIG. 12, each pixel may be independently clocked to different edges of the higher-frequency clock. In addition, the pixel width is adjustable per pixel.  
         [0058]    The present invention may be applied in a driver for a projection display application, such as in a system like that described above in relation to FIG. 5. Alternatively, the invention may be applied in a driver for a computer-to-plate application, such as in systems used in the publishing industry to print magazines and the like. The invention may also be used in other linear scan applications.  
         [0059]    In one embodiment, the amplitude and width modulation may be used together to advantageously increase the effective depth (i.e. resolution) of the modulation of a pixel. For example, two pixels may be modulated so as to have the same amplitude but different widths. The wider pixel would effectively appear to be brighter than the narrower pixel.  
         [0060]    In another embodiment, the width modulation for a pixel may be varied between image frame refreshes in a display application. For example, the variation of the width modulation may effectively move a pixel about an average pixel position. This may be used to advantageously create effective blur of the pixel for a smoothening effect.  
         [0061]    Note that while a preferred embodiment of the invention utilizes GLV or GLV-type elements. In other embodiments, other types of light-modulating elements may be used.  
         [0062]    In the present disclosure, numerous specific details are provided such as examples of apparatus, process parameters, materials, process steps, and structures to provide a thorough understanding of embodiments of the invention. Persons of ordinary skill in the art will recognize, however, that the invention can be practiced without one or more of the specific details. In other instances, well-known details are not shown or described to avoid obscuring aspects of the invention.  
         [0063]    While specific embodiments of the present invention have been provided, it is to be understood that these embodiments are for illustration purposes and not limiting. Many additional embodiments will be apparent to persons of ordinary skill in the art reading this disclosure. Thus, the present invention is limited only by the following claims.