Abstract:
A modulator for and a method of modulating an incident beam of light including means for supporting a plurality of active elements and a plurality of bias elements, each active and bias element including a light reflective planar surface with the light reflective planar surfaces of the plurality of active elements lying in a first parallel plane and the plurality of bias elements lying in a second parallel plane wherein the plurality of active and bias elements are parallel to each other and further wherein the plurality of bias elements are mechanically or electrically deflected with respect to the plurality of active elements. Each of the plurality of bias elements is deflected an odd multiple of the wavelength of an incident light wave divided by four and the plurality of light reflective planar surfaces of the plurality of active elements move between the first parallel plane to the second parallel plane. The deflection of bias elements is optimized to minimize optical attenuation error due to voltage error.

Description:
FIELD OF THE INVENTION 
     The present invention relates to mitigating the effects of voltage error on light diffraction. More particularly, this invention relates to a grating light valve array structure for light diffraction error mitigation due to voltage error. 
     BACKGROUND OF THE INVENTION 
     Designers and inventors have sought to develop a light modulator which can operate alone or together with other modulators. Such modulators should provide high operating speeds, a high contrast ratio or modulation depth, have optical flatness, be compatible with VLSI processing techniques, be easy to handle and be relatively low in cost. Two such related systems are found in U.S. Pat. No. 5,311,360 and 5,841,579 which are hereby incorporated by reference. 
     According to the teachings of the ‘360 and ‘579 patents, a diffractive light modulator is formed of a multiple mirrored-ribbon structure. An example of such a diffractive light modulator  10  is shown in FIG.  1 . The diffractive light modulator  10  comprises elongated elements  12  suspended by first and second posts,  14  and  16 , above a substrate  20 . The substrate  20  comprises a conductor  18 . In operation, the diffractive light modulator  10  operates to produce modulated light selected from a reflection mode and a diffraction mode. 
       FIGS. 2 and 3  illustrate a cross-section of the diffractive light modulator  10  in a reflection mode and a diffraction mode, respectively. The elongated elements  12  comprise a conducting and reflecting surface  22  and a resilient material  24 . The substrate  20  comprises the conductor  18 . 
       FIG. 2  depicts the diffractive light modulator  10  in the reflection mode. In the reflection mode, the conducting and reflecting surfaces  22  of the elongated elements  12  form a plane so that incident light I reflects from the elongated elements  12  to produce reflected light R. 
       FIG. 3  depicts the diffractive light modulator  10  in the diffraction mode. In the diffraction mode, voltage applied to alternate ones of the elongated elements  12  causes those alternating elongated elements  12  to move toward the substrate  20 . The charged alternating elongated elements are referred to as the “active” elements. The voltage is applied between the reflecting and conducting surfaces  22  of the alternate active ones of the elongated elements  12  and the conductor  18 . The voltage results in a height difference between the alternate active ones of the elongated elements  12  and noncharged or “bias” ones of the elongated elements  12 . A height difference of a quarter wavelength λ/4 of the incident light I produces maximum diffracted light including plus one and minus one diffraction orders, D +1 , and D −1 . 
       FIGS. 2 and 3  depict the diffractive light modulator  10  in the reflection and diffraction modes, respectively. For a deflection of the alternate ones of the elongated elements  12  of less than a quarter wavelength λ/4, the incident light I both reflects and diffracts producing the reflected light R and the diffracted light including the plus one and minus one diffraction orders, D +1 , and D −1 . In other words, by deflecting the alternate ones of the elongated elements  12  less the quarter wavelength λ/4, the diffractive light modulator  10  produces a variable reflectivity. 
       FIG. 4  depicts a graphical representation of diffraction of 0 th  order light of the diffractive light modulator  10  at various active ribbon voltages with respect to intensity and attenuation. The active ribbon voltage  30  is graphically represented along the horizontal axis in Volts(V) and a normalized intensity  32  scale and attenuation  34  scale in decibels(dB) are shown along each vertical axis. Both the intensity graph  36  and the attenuation graph  38  have large negative slopes that decrease drastically as the active ribbon voltage  30  exceeds 10V.  FIG. 4  is a representation of how voltage error, as will be discussed below, affects the performance of the diffractive light modulator  10 . 
     Unfortunately, diffractive light modulators  10  are sensitive to voltage errors that may occur in normal operation. Specifically, there is a large dependence of attenuation with active ribbon voltage. This is particularly a problem at larger attenuations (−15 dB) where the dependence exceeds 10 dB per volt. Such error makes it extremely difficult to diffract the proper amount of light from the diffractive light modulator  10  in that the electrical bias applied to every other one of the elongated elements  12  will have an additional voltage component. This additional voltage will separate the active and bias elongated elements  12  more or less than the desired amount, thereby causing light diffraction inconsistent with the desired operation of the diffractive light modulator  10 . 
     The non-linearity in the voltage versus attenuation behavior places severe design constraints on the voltage source. A stable, high bit-depth voltage supply or precision non-linear supply is required for control of the attenuation level. For attenuation applications, a key metric of performance is the slope of the attenuation versus voltage response, namely decibels per volt (dBN). A low and constant dBN is desired.  FIG. 4  illustrates a less desirable performance—varying slope, and high slope at larger attenuation levels. 
     Lastly, current diffractive light modulators  10  are sensitive to voltage errors caused by power supply noise. Such error is often referred to as “ripple.” What is needed is a diffractive light modulator  10  that is less sensitive to ripple in order to diffract a correct amount of light 
     SUMMARY OF THE INVENTION 
     According to the embodiments of the present invention, a modulator and method for modulating an incident beam of light including a plurality of active elements, each active element including a first end, a second end, and a light reflective planar surface with the light reflective planar surfaces of the plurality of active elements lying in a first parallel plane wherein the plurality of active elements are parallel to each other. The modulator also includes a plurality of bias elements, each bias element including a first end and a second end, and a light reflective planar surface with the light reflective planar surfaces of the plurality of bias elements lying in a second parallel plane wherein the plurality of bias elements are parallel to each other and further wherein the plurality of bias elements are mechanically or electrically deflected with respect to the plurality of active elements. 
     Lastly, the modulator includes a support structure coupled to each end of the plurality of active elements and to each end of the plurality of bias elements to maintain a position of each active and bias element relative to each other and to enable movement of selective ones of the plurality of active elements and the plurality of bias elements in a direction normal to the one or more parallel planes of the plurality of active and bias elements, and between a first modulator configuration wherein the plurality of active elements and the plurality of bias elements act to reflect the incident beam of light as a plane mirror, and a second modulator configuration wherein the plurality of active elements and the plurality of bias elements act to diffract the incident beam of light. 
     Each of the plurality of bias elements is deflected an odd multiple of the wavelength of an incident light wave divided by four and the plurality of light reflective planar surfaces of the plurality of active elements move from the first parallel plane to the second parallel plane. 
     The bias elements can be deflected by applying a voltage difference over each of the plurality of bias elements, or covering the light reflective surface of each of the plurality of bias elements with a transparent over layer, or affixing each of the plurality of bias elements such that the light reflective planar surfaces of the plurality of bias elements are lying in the second parallel plane wherein the plurality of bias elements are parallel to each other or securing the light reflective planar surfaces of each of the plurality of bias elements to a substrate such substrate defining the second parallel plane. 
     According to the embodiments of the present invention, a modulator for modulating an incident beam of light includes means for supporting a plurality of active elements and a plurality of bias elements, each active and bias element including a first end and a second end, the supporting means being coupled to each end of the plurality of active and bias elements to maintain a position of each active and bias element relative to each other, and a light reflective planar surface with the light reflective planar surfaces of the plurality of active elements lying in a first parallel plane and the plurality of bias elements lying in a second parallel plane wherein the plurality of active and bias elements are parallel to each other and further wherein the plurality of bias elements are deflected with respect to the plurality of active elements. 
     The modulator also includes means for moving selective ones of the plurality of active elements and the plurality of bias elements in a direction normal to the one or more parallel planes of the plurality of active and bias elements, and between a first modulator configuration wherein the plurality of active elements and the plurality of bias elements act to reflect the incident beam of light as a plane mirror, and a second modulator configuration wherein the plurality of active elements and the plurality of bias elements act to diffract the incident beam of light. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary diffractive light modulator. 
         FIG. 2  illustrates a cross-section of the exemplary diffractive light modulator in a reflection mode. 
         FIG. 3  illustrates a cross-section of the exemplary diffractive light modulator in a diffraction mode. 
         FIG. 4  illustrates a graphical representation of the operation of the exemplary diffractive light modulator 
         FIG. 5  illustrates a cross-section of the light modulator of the preferred embodiment of the present invention. 
         FIG. 6  illustrates a graphical representation of the operation of the light modulator of the preferred embodiment of the present invention. 
         FIG. 7  illustrates a graphical representation of the operation of the light modulator of the preferred embodiment of the present invention. 
         FIG. 8  illustrates a cross-section of the light modulator of an alternative embodiment of the present invention. 
         FIG. 9  illustrates a cross-section of the light modulator of an alternative embodiment of the present invention. 
         FIG. 10  illustrates a cross-section of the light modulator of an alternative embodiment of the present invention. 
         FIG. 11   a  illustrates a side view of a ribbon constructed according to an embodiment of the present invention. 
         FIG. 11   b  illustrates a side view of a ribbon constructed according to an embodiment of the present invention. 
         FIG. 11   c  illustrates a side view of a ribbon constructed according to an embodiment of the present invention. 
         FIG. 12  illustrates a top view of a plurality of ribbons constructed according to an embodiment of the present invention. 
         FIG. 13  illustrates a top view of a plurality of ribbons constructed according to an embodiment of the present invention. 
         FIG. 14   a  illustrates a top view of a plurality of ribbons constructed according to an embodiment of the present invention. 
         FIG. 14   b  illustrates a side view of a ribbon constructed according to an embodiment of the present invention. 
         FIG. 14   c  illustrates a side view of a ribbon constructed according to an embodiment of the present invention. 
         FIG. 14   d  illustrates a side view of a ribbon constructed according to an embodiment of the present invention. 
         FIG. 15   a  illustrates a side view of a ribbon constructed according to an embodiment of the present invention. 
         FIG. 15   b  illustrates a side view of a ribbon constructed according to an embodiment of the present invention. 
         FIG. 15   c  illustrates a top view of a plurality of ribbons constructed according to an embodiment of the present invention 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The present invention overcomes deficiencies of conventional diffractive light modulators  10  by deflecting bias elements, commonly referred to as ribbons, at an amount substantially equal to λ/4 or an odd multiple thereof in order to reverse the characteristics of the 0 th  order light, thereby reducing the sensitivity of the active ribbons to various forms of voltage errors. 
       FIG. 5  depicts a cross-section of the light modulator  40  of the preferred embodiment of the present invention. The bias ribbons  42  are deflected such that the top surface of each Bias Ribbon is substantially equal to any odd multiple of λ/4 below the top surface of the active ribbons  44 . The active ribbons  44  operate nominally between position  44  and position  46 . Voltage errors will cause deflection errors in the active ribbons  44 , which in turn, gives an attenuation error. The position or deflection of the bias ribbons  42  can be adjusted to minimize that optical attenuation error. The type of voltage error is not limited to the types discussed above. 
     The preferred embodiment as depicted in  FIG. 5  reverses the ON/OFF state of a device operating in 0 th  order, and implements the deflected bias ribbons  42  in a stable, predictable manner, for the total operation lifetime of the light modulator  40 . Optimum bias deflection has been found to range from approximate 95% of λ/4 to 105% λ/4, and any odd multiples thereof. Deflections in this range show a smooth, near linear response of attenuation dB versus voltage as is depicted in FIG.  6 . The large dependence of deflection with voltage (for the active ribbons  44 ) is moved to the low attenuation (high intensity) state, where it can be better tolerated. 
       FIG. 6  depicts a graphical representation of diffraction of 0 th  order light of the light modulator  40  of the preferred embodiment of the present invention at various active ribbon voltages with respect to intensity and attenuation. The active ribbon voltage  50  is graphically represented along the horizontal axis in Volts(V) and a normalized intensity  52  scale and attenuation  54  scale in decibels(dB) are shown along each vertical axis. Both the intensity graph  56  and the attenuation graph  58  have slopes with less variation that do not show drastic increase or decrease as the active ribbon voltage  50  increases. 
       FIG. 7  is a graphical comparison of the attenuation slopes associated with the operation of the diffractive light modulator  10  having no deflection of the bias ribbons and the light modulator  40  of the preferred embodiment of the present invention having a bias ribbon  42  deflection of λ/4 in order to display the desired slope of dB per Volt to minimize the sensitivity of the light modulator  40 . Here, the attenuation  60  scale is graphically represented along the horizontal axis in decibels(dB) and the slope  62  scale in dB per Volt is shown along each vertical axis. The non-deflected bias slope  64  shows an ever decreasing slope as the attenuation  60  value decreases. However, the light modulator  40  of the preferred embodiment of the present invention has a deflected bias slope  66  having desirable characteristics in decreasing sensitivity to the light modulator  40 . More specifically, the deflected bias slope  66  is at or near a constant level. Such a slope indicates that the light modulator  40  is operating with decreased sensitivity to voltage error. 
     Referring back to  FIG. 5 , a number of methods and configurations can be used to deflect the bias ribbons  42  either mechanically or electrically. In other words, the bias ribbons  42  may be deflected to an amount substantially equal to an odd multiple of the 95%-105% λ/4 range by actually applying a separate voltage of approximately 20V to the bias ribbons  42 , thus holding the bias ribbons  42  in a deflected position electrically. This method is advantageous because electrical positioning of the bias ribbons  42  is highly accurate, and can be implemented on most light modulating devices. Furthermore, electrically deflecting the bias ribbons  42  provides an optimum response with respect to the deflected bias slope  66  as graphically demonstrated in FIG.  7 . 
     Also, the bias ribbons  42  may be deflected mechanically by producing a light modulator  40  having bias ribbons  42  that are deflected by a predetermined distance or amount, thereby deflecting the bias ribbons  42  mechanically. The following description will disclose known methods to deflect the bias ribbons  42 . 
       FIG. 11   a  depicts a blind etch process of an embodiment of the present invention, wherein a side view of a sacrificial layer  110  is deposited and is etched to an etch depth  116  that reflects the desired deflection level. The posts  114  are then etched and a ribbon layer  112  is deposited. Once the ribbon layer  112  is in place, the sacrificial layer  110  is removed, the ribbon layer  112  is cut and released, creating a ribbon layer having a pre-biased ribbon height  118 . 
       FIGS. 11   b  and  11   c  depict a more detailed side view showing the characteristic effect that this etching has on the ribbon layer  112 . In  FIG. 11   b,  the posts  114  are shown supporting a ribbon layer  112  after the sacrificial layer  110  of  FIG. 11   a  has been removed, but before the tension in the ribbon layer  112  has been released. Here, the original etch depth  116  is clearly shown in the ribbon layer  112 .  FIG. 11   c  depicts the same ribbon layer  112  after the tension has been released. Naturally, the ribbon layer  112  relaxes to a pre-bias ribbon height  118 . The active ribbon height  118 , which will be discussed below, depicts the level of the ribbon layer  112  prior to the etching process. 
       FIG. 12  depicts a method of mechanically deflecting the bias ribbons  122  of the preferred embodiment of the present invention. In this figure, a top view of a number of ribbons (bias  122  and active  120 ) depict how the etching process described above is utilized, by etching the sacrificial layer of every bias ribbon  122 . Here, the posts  126  support both the active ribbons  120  and the bias ribbons  122  in a single row. In order to deflect every bias ribbon  122  so that every other ribbon is deflected, the sacrificial layer of each of the bias ribbons  122  is etched as depicted by the etching area  124  as shaded in FIG.  12 . The bias ribbons  126  are then deflected as described in the description of  FIGS. 11   a-c,  to the desired etch depth  116  ( FIG. 11   b ).  FIG. 12  only depicts one end of the bias ribbons  122  and active ribbons  120  for exemplary purposes only. It will be clear to one skilled in the art that there may be more than the four ribbons depicted, and that the etching process will also occur on the other end of the bias ribbons  122  as described above. 
       FIG. 13  depicts a method of mechanically deflecting the bias ribbons  122  of an additional embodiment of the present invention. Again, the bias ribbons  132  are etched using the method described above. In this embodiment the entire length of the sacrificial layer of each bias ribbon  132  as shown by the etching area  134 . Again, the active ribbons  130  are not etched, while the posts  136  remain in two parallel rows at either end of the active and bias ribbons  130 ,  132 . This embodiment allows for deflected bias ribbons  132 , while making the etching process easier by expanding the etching area to the entire length of the bias ribbons  132  rather than to a precise etching area  124 , as was depicted in FIG.  12 . 
       FIGS. 14   a-d  depict a method of mechanically deflecting the bias ribbons  144  of an additional embodiment of the present invention. Referring first to the top view of  FIG. 14   a , the posts  146  are staggered such that the posts  146  of the bias ribbons  144  and the posts of the active ribbons  142  each create their own rows. Further, an LM 1  layer  140  is inserted under the active and bias ribbons  142 ,  144  as shown in  FIG. 14   a such that the posts  146  of the active ribbons  142  rest on the LM 1  layer  140 . The LM 1  layer is then covered with sacrificial layer and the active and bias ribbons  142 ,  144  are etched as depicted in  FIGS. 14   b-d.    
     Referring now to the side views depicted in  FIGS. 14   b-d,  the LM 1  layer  140  is shown supporting the posts  146  of the active ribbons  142 , while the etch depth  148  is shown in the bias ribbons  144 . Referring to  FIG. 14   d , as the active and bias ribbons  142 ,  144  are cut and the pressure is released, the resulting configuration is a deflected bias ribbon  144  as the posts  146  supporting the active ribbons  142  are elevated on the LM 1  layer  140 . 
       FIGS. 15   a-c  depict a method of mechanically deflecting the bias ribbons  158  of an additional embodiment of the present invention. Referring first to the side view of  FIG. 15   a,  the posts  154  of the active ribbons  156  are placed on a pedestal  150 , such that the pedestal thickness  160  is defined by the difference between the height of the post  154  and the active ribbon  156 . The sacrificial layer  152  is removed, and now referring to  FIG. 15   b , as the tension is released the active ribbon  156  rises to the level of the elevated post  154 . Thus, the bias ribbons  158  are effectively deflected by mechanically raising the level of the active ribbons  156 . 
     Referring now to  FIG. 15   c,  a top view is of this embodiment is depicted such that the active ribbons  156  have a post  154  that is supported by a pedestal  150 , while the bias ribbons  158  are supported by a non-elevated post  154 . As described above, this embodiment thereby elevates each active ribbon  156 , thus effectively deflecting each bias ribbon  158  by mechanical means. Deflected ribbon operation and further embodiments of the present invention will now be discussed below. 
       FIG. 8  depicts a cross-section of a light modulator  70  according to an alternative embodiment of the present invention. Some light modulators  70  do not include bias ribbons  42  as described in the preferred embodiment, rather only active ribbons  72 . A description of this type of light modulator  70  is available in U.S. Pat. No. 5,311,360, which has been incorporated by reference. In this embodiment, a substrate  74  provides a reference reflective surface and the gaps  76  between the active ribbons  72  are the same width as the active ribbons  72 . This configuration can be modified mechanically to operate to eliminate light refraction error due to voltage drift, ripple and other voltage error. This can be accomplished by fastening bias ribbons  78  with widths identical to the gaps  76  into the substrate  74 . Such a mechanically deflected configuration operates as the bias ribbons  78  in the preferred embodiment of the present invention by creating an Optical Path Difference  80 , thereby when the active ribbons  72  receive a voltage error, they deflect to cause phase shift with respect to the substrate  74  and the bias ribbons  78 . 
       FIG. 9  depicts a cross-section of a light modulator  90  according to an alternative embodiment of the present invention. In this embodiment, the bias ribbons  92  are effectively deflected with respect to the active ribbons  94  by covering the bias ribbons  92  with an Overlayer  96 . The Overlayer  96  is any of a number of transparent materials having a index of refraction coefficient which will cause a phase shift to incoming light with respect to the active ribbons  94 . Ideally, the bias ribbons  92  are covered with an Overlayer  96  which will increase the path length of incident light by a distance substantially equal to the 95%-105% λ/4 range, or any odd multiple thereof. 
       FIG. 10  depicts a further cross-section of a light modulator  100  according to an alternative embodiment of the present invention. In this embodiment, the bias ribbons  102  are deflected using a combination of mechanical and electrical deflection. The bias ribbons  102  are deflected a Mechanical Deflection Fraction  104  of the Total Deflection  106  with respect to the active ribbons  108  first through mechanical methods as described above. In other words the Mechanical Deflection Fraction  104  is produced by constructing the light modulator  100  having bias ribbons  102  deflected a Mechanical Deflection Fraction  104  using one of the mechanical disclosures outlined previously in this disclosure. Then a relatively smaller voltage can be applied to the bias ribbons  102  to create the Electrical Deflection Fraction  110 . The sum of the Mechanical Deflection Fraction  104  and the Electrical Deflection Fraction  110  achieves a Total Deflection  106  substantially equal to the 95%-105% λ/4 range, or any odd multiple thereof. It is not a requirement of this embodiment that the Mechanical Deflection Fraction  104  be smaller than the Electrical Deflection Fraction  110  as represented in FIG.  10 . In fact, the percentage of each fraction is variable to the extent that their sum is substantially equal to the 95%-105% λ/4 range, or any odd multiple thereof. 
     It will be readily apparent to one skilled in the art that other various modifications may be made to the preferred embodiment without departing from the spirit and scope of the invention as defined by the appended claims.