Pre-deflected bias ribbons

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. 11a illustrates a side view of a ribbon constructed according to an embodiment of the present invention.

FIG. 11b illustrates a side view of a ribbon constructed according to an embodiment of the present invention.

FIG. 11c 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. 14a illustrates a top view of a plurality of ribbons constructed according to an embodiment of the present invention.

FIG. 14b illustrates a side view of a ribbon constructed according to an embodiment of the present invention.

FIG. 14c illustrates a side view of a ribbon constructed according to an embodiment of the present invention.

FIG. 14d illustrates a side view of a ribbon constructed according to an embodiment of the present invention.

FIG. 15a illustrates a side view of a ribbon constructed according to an embodiment of the present invention.

FIG. 15b illustrates a side view of a ribbon constructed according to an embodiment of the present invention.

FIG. 15c 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. 11a 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. 11b and 11c depict a more detailed side view showing the characteristic effect that this etching has on the ribbon layer 112. In FIG. 11b, the posts 114 are shown supporting a ribbon layer 112 after the sacrificial layer 110 of FIG. 11a 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. 11c 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. 11a-c, to the desired etch depth 116 (FIG. 11b). 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. 14a-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. 14a, 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 LM1 layer 140 is inserted under the active and bias ribbons 142, 144 as shown in FIG. 14asuch that the posts 146 of the active ribbons 142 rest on the LM1 layer 140. The LM1 layer is then covered with sacrificial layer and the active and bias ribbons 142, 144 are etched as depicted in FIGS. 14b-d.

Referring now to the side views depicted in FIGS. 14b-d, the LM1 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. 14d, 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 LM1 layer 140.

FIGS. 15a-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. 15a, 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. 15b, 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. 15c, 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.