Patent Publication Number: US-6665110-B2

Title: Diagonal to rectangular pixel mapping for spatial light modulator

Description:
This application claims priority under 35 USC §119(e)(1) of provisional application No. 60/344,854 filed Dec. 31, 2001. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates to spatial light modulators, and more particularly to a method of mapping between diagonal pixel control patterns and rectangular pixel control patterns. 
     BACKGROUND OF THE INVENTION 
     A Digital Micromirror Device™ (DMD™) is a type of microelectromechanical systems (MEMS) device. Invented in 1987 at Texas Instruments Incorporated, the DMD is a fast, reflective digital spatial light modulator. It can be combined with image processing, memory, a light source, and optics to form a digital light processing system capable of projecting large, bright, high-contrast color images. It may also be used for other applications, such as for optical switching and in optical equalization systems. 
     The DMD is fabricated using CMOS-like processes over a CMOS memory. It has an array of individually addressable mirror elements, each having an aluminum mirror that can reflect light in one of two directions depending on the state of an underlying memory cell. With the memory cell in a first state, the mirror rotates to +10 degrees. With the memory cell in a second state, the mirror rotates to −10 degrees. When the mirror surfaces are illuminated with a light source, the mirrors in the array can be set to one state or the other, such that “on” mirrors reflect light to one location and “off” mirrors reflect light to another location. 
     This on or off capability of the mirror elements has led to numerous applications for DMDs. For imaging applications, the “on” mirror elements reflect light to an image plane. For optical switching applications, the mirror positions can be used to direct a beam of light to a given output path. Another application is for optical equalization, where input wavelengths are directed to different portions of the DMD array, and each wavelength is selectively attenuated so that the outputs are equalized. 
     The DMD architecture is such that the rotation of each mirror element is along the diagonal of the mirror. Because of this, the mirror surfaces are illuminated along their diagonals. 
     Thus, when the DMD is used for optical equalization, the input signal illuminates the DMD mirror surfaces along the mirror diagonals. As a result, pixel control patterns are along diagonal paths. This requires the control values to be mapped to the rectangular control architecture of the DMD pixel array. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention is a method of using a spatial light modulator (SLM) to control the gain of an input optical signal. The gain control achieved by determining a two-dimensional grid of values, each value representing an on or off position of pixel elements of the SLM. A is u,v space is defined for the diagonal rows and columns, such that the u axis and v axis run through diagonals of each grid element. Each grid element is assigned a u,v coordinate pair. For each grid element in the u,v space, an x,y coordinate pair is calculated by performing the following operation:          [         x           y         ]     =     int                   (         [         0.5       1             -   0.5         1         ]     ·     [         u           v         ]       +     [         0.5           0.5         ]       )                       
     Each non-integer value is then rounded to an adjacent pixel value. For example, the next smallest integer value, int(8.5)=8 and int (−9.5)=−10. 
     An advantage of the invention is that it provides a means to describe pixel control values in diagonal pixel space, then translate the results onto the rectangular space of the SLM. The conventional alternative is to work in rectangular space, then use complex mathematical operations to rotate pixel control values. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a DMD-based optical equalization system having a DMD that operates in accordance with the invention. 
     FIG. 2 illustrates the mirror and hinge elements of a single mirror element of the DMD of FIG.  1 . 
     FIG. 3 illustrates diagonal pixel paths of the DMD mapped to a u,v space. 
     FIG. 4 illustrates the diagonal pixel paths of FIG. 3 mapped to rectangular paths of an x,y space. 
     FIG. 5 illustrates an example of equivalent pixel coordinates in the u,v and x,y spaces. 
     FIGS. 6-8 illustrate three examples of u,v coordinates and associated pixel control values. 
     FIG. 9 illustrates the pixel control values of FIGS. 6-8 mapped to an x,y space. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a DMD-based optical equalizer system, having a DMD  11  that operates in accordance with the invention. As illustrated, an input fiber  12  directs an optical signal to a first grating  13 . Grating  13  spreads the signal diagonally across the DMD  11 , with different wavelengths reaching different portions of DMD  11 . The DMD  11  equalizes the wavelength components of the signal. As explained below, the equalization is accomplished when the DMD portion associated with each wavelength reflects only a predetermined amount of that wavelength as determined by the number of mirror elements that are “on”. The optical signal is reflected from DMD  11  to a second grating  14 , which realigns the components and directs the signal to an output fiber  15 . 
     Although various equalization schemes are possible, in general, the DMD operates by attenuating each wavelength component a desired amount, so that stronger signals are attenuated more and weaker signals are attenuated less, with the result being that the output signals have substantially equal intensity. A specific portion of the DMD array is associated with each input wavelength component. If all mirror elements of that portion are “on” the signal is not attenuated. If some portion of the mirror elements of that portion are “off”, the signal is attenuated. 
     A DMD control unit  16  implements the various operational functions for DMD  11 , such as setting the mirror elements to their on or off positions. A diagonal to rectangular mapping unit  16   a  implements the method described herein. Apart from the mapping functions of mapping unit  16   a,  the general aspects of DMD operation are described in various publications and patents. 
     FIG. 2 illustrates the mirror  21  and hinge  22  of one mirror element  20  of DMD  11 . Only those upper portions of a single DMD mirror element are shown; each mirror element has other components not shown, such as addressing electrodes and bias circuitry, underlying the mirror  11 . For purposes of this description, the mirror elements are also referred to as “pixels”. 
     Each mirror  21  is mounted over a torsion hinge  22 , typically by means of a hinge support post  23 . Hinge  22  permits the mirror  21  to tilt. Each mirror  21  is further individually addressable by means of applied electrostatic forces, and tilts in response to address signals. As illustrated, each mirror  21  tilts with an axis of rotation along its diagonal. 
     Because of the diagonal tilt of mirror  11 , the mirror  11  is illuminated diagonally. Thus, as stated above, the grating  13  spreads the wavelength components along the diagonal of the surface of the DMD  11 . 
     Because of the diagonal line paths of the optical input, the mirrors  21  are controlled along diagonal line paths. This permits the gain of equalizer  10  to be controlled for each wavelength. More specifically, gain control is accomplished by using a diagonal masking function, which is a two-dimensional grid of values. Each value in the grid is associated with a mirror element, and ultimately determines whether that mirror element will be on or off. 
     FIG. 3 illustrates a two-dimensional space, (u,v), having diagonal pixel coordinates in accordance with the invention. As explained below, the method described herein defines a new u,v space with diagonal pixel coordinates that maps to a conventional x,y space with rectangular pixel coordinates. The mapping of pixel coordinates permits a two-dimensional grid of pixel control values to be mapped from diagonal paths to rectangular rows and columns. 
     The method is directed to DMDs and their diagonal illumination. However, the method could also be applied to other spatial light modulators (SLMs) that are controlled along diagonal paths of pixel elements. 
     In the example of this description, each value describes a thresholding weight of a mirror element. Depending on whether the threshold is exceeded, the mirror is set to either an on or off position. Various other equalization schemes are possible. For example, the pixel patterns could be simply a pattern of 1&#39;s and 0&#39;s representing on or off mirror positions. 
     The most convenient way to describe and store the masking function is as a rectangular grid. This presents the problem of translating the masking function, which represents diagonal mirrors onto the rectangular space of the DMD. 
     In FIG. 3, the u,v space is shown with diagonal paths of mirror elements (pixels) of DMD  11  rotated and mapped onto the space. As illustrated, the horizontal and vertical tick lines of the u,v space run through the diagonals of the pixels and the u axis counts along a zig zag row of pixels. Thus, one column and one row of pixels are shown mapped onto the u,v space. 
     FIG. 4 illustrates the same pixels as shown in FIG. 3, but mapped onto the (x,y) plane of DMD  11 . The zig zag row of pixels from FIG. 3 is considered as being at v=0. Along the u axis, each pixel of the zig zag row is one unit. 
     The mapping from u,v space to x,y space is accomplished using the following equation:            [         x           y         ]     =     int                   (         [         0.5       1             -   0.5         1         ]     ·     [         u           v         ]       +     [         0.5           0.5         ]       )         ,                   
     where int(x) represents a rounding down to the next smallest integer. 
     Mapping from x,y space to u,v space can be accomplished using a similar equation:            [         u           v         ]     =     int                   (         [         1         -   1             0.5       0.5         ]     ·     [         x           y         ]       +     [         0.5           0.5         ]       )         ,                   
     where int(x) represents a rounding down to the next smallest integer. 
     Referring again to FIG. 1, the above-described mapping functions may be easily implemented using logic circuitry  16   a,  including hardware or processor-run software. The operations for performing the mapping functions include shifts, adds, and bit masks. 
     FIG. 5 is a table that sets out equivalent coordinates in x,y and u,v space. Using the mapping functions, a rectangular grid of numbers described in diagonal (u,v) space can be mapped onto rectangular (x,y) space. 
     FIGS. 6-8 illustrate three examples of u,v coordinates. Each pair of u,v coordinates has a corresponding pixel control value. FIG. 9 illustrates the results of the mapping. 
     The mapping method of the invention permits computations and masks to be stored in rectangular formats, then transformed to the rectangular space of the DMD array. Each entry in the mask maps one-to-one to a location in the DMD space, thereby ensuring that no pixels are missed or uncontrollable. 
     Other Embodiments 
     Although the present invention has been described in detail, it should be understood that various changes, substitutions, and alterations can be made hereto without departing from the spirit and scope of the invention as defined by the appended claims.