Abstract:
An optical system provides high-contrast operation by collecting zero order light. The optical system comprises a light modulator and a collector. The light modulator is preferably a grating light valve™ light modulator including a plurality of elements selectively operable in a first mode and a second mode, wherein a gap between adjacent elements is equal to or less than a wavelength of an incident light beam. The plurality of elements in the first mode reflect light along a return path, where the plurality of elements in the second mode direct light away from the return path. The collector is coupled to the light modulator to collect zero order light along the return path while the plurality of elements are in the first mode and to collect zero order light along the return path while the plurality of elements are in the second mode.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method of and an apparatus for modulating a beam of light. More particularly, this invention relates to a light modulator structure for producing high-contrast operation using zero-order light. 
     BACKGROUND OF THE INVENTION 
     Light modulators are used in many telecommunications applications including optical switching. Light modulators, and in particular grating light valve™ light modulators, utilize first order light to obtain higher-contrast operation. A conventional grating light valve™ light modulator includes a series of elongated, reflective ribbons arranged adjacently and in parallel. When the ribbons lie in an un-deflected, or flat, state, an incident beam of light reflects off the grating light valve™ light modulator as a mirror. If alternating ribbons are pulled down, or deflected, then the incident light diffracts. In operation, a through-state is considered to be when the alternating ribbons are deflected by a predetermined distance, thereby obtaining maximum diffraction of the incident light, and the diffracted first order light is collected. Inefficiencies arise since the first order light is not the only diffracted light in the deflected state. Higher orders of light are also produced including second order, third order, etc. These higher orders are not collected and are therefore wasted. This reduces efficiency. 
     Problems arise in association with the collection process of the first order light When the incident light is diffracted in the through-state, different wavelengths diffact at different angles. Larger wavelengths have larger diffraction angles. As such, any wavelength combiner used for collecting the first order light must be sufficiently large to account for the varying wavelength diffraction angles. A wavelength combiner is also called a wavelength multiplexer, examples of which include a dichroic filter, a diffraction grating, and an array waveguide. Unfortunately, the larger the combiner, the less efficient is the collection process and the lower the contrast ratio provided. Further, since the first order light includes a plus and minus component, two such filters are necessary to collect each the plus and minus first order light. 
     Design of the optical system must not only account for collecting of the plus and minus first order light, but also must isolate the first order light from the higher order light. Since the different wavelengths diffract at different angles, the optical system must ensure sufficient discrimination of zero and first order diffraction of all wavelengths. Considering the isolation and collection constraints of such a system, the optical design considerations using a conventional grating light valve™ light modulator are substantial. 
     Additionally, high-contrast operation is essential for performing optical switching where 30-40 dB of contrast is necessary. However, conventional zero and first order operation often does not produce high enough contrast needed for optical switching. 
     What is needed is a spatial light modulator and associated optical system that is easier to design and implement. What is also needed is a spatial light modulator and associated optical system with increased contrast and improved efficiency. 
     SUMMARY OF THE INVENTION 
     An optical system of the present invention provides high-contrast operation by collecting zero order light. The optical system comprises a light modulator and a collector. The light modulator is preferably a grating light valve™ light modulator including a plurality of elements selectively operable in a first mode and a second mode, wherein a gap between adjacent elements is equal to or less than a wavelength of an incident light beam. The plurality of elements in the first mode reflect light along a return path, where the plurality of elements in the second mode direct light away from the return path. The collector is coupled to the light modulator to collect zero order light along the return path while the plurality of elements are in the first mode and in the second mode. 
     The collector comprises Schlieren optics and an optical train. The Schlieren optics pass the zero order light. The optical train directs the zero order light to an optical fiber, a display device, a thermal printer or any other appropriate application. 
     The first mode comprises a reflection mode in which the plurality of elements reflect the incident light beam as a plane mirror. The second mode comprises a diffraction mode in which the plurality of elements comprise a first group of elongated elements interdigitated with a second group of elongated elements. A height difference between the first elongated elements and the second elongated elements causes the incident light beam to diffract away from the return path. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a cross-section of an exemplary grating light valve™ light modulator in a reflection mode. 
     FIG. 2 illustrates a cross-section of an exemplary grating light valve™ light modulator in a diffraction mode. 
     FIG. 3 illustrates a cross-section of a grating light valve™ light modulator in a reflection mode according to the preferred embodiment of the present invention. 
     FIG. 4 illustrates a cross-section of a grating light valve™ light modulator in a diffraction mode according to the preferred embodiment of the present invention. 
     FIG. 5 illustrates an top-down view of an exemplary optical system using first order operation. 
     FIG. 6 illustrates a top-down view of an optical system using zero order operation according to the preferred embodiment of the present invention. 
     FIG. 7 illustrates collected zero order light where the grating light valve™ light modulator of the present invention is in the on-state. 
     FIG. 8 illustrates collected zero order light where the grating light valve™ light modulator of the present invention is in the off-state. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The present invention provides high-contrast operation in the order of 30 dB and higher while improving efficiency and substantially reducing the optical design constraints of a conventional grating light valve™ light modulator-based modulator. Such results are achieved by paying proper attention to the grating light valve™ light modulator structure and collecting the zero order light. Zero order light is the light that is specularly reflected off the grating light valve™ light modulator. This is in contrast to conventional operation in which it was believed to be essential to collect the diffracted first order light in order to achieve high-contrast optical switching. Prior designs of the grating light valve™ light modulator emphasize the width of the grating light valve™ light modulator ribbons and the pixel pitch as the most relevant design parameters. The design of the present invention demonstrates that reduction of the size of the gap between the grating light valve™ light modulator ribbons below a certain fraction of the wavelength light interacting with the grating light valve™ light modulator permits very high-contrast ratio performance in zero order operation. To further illustrate this point, consider the FIGS. 1-4. 
     A cross-section of an exemplary grating light valve™ light modulator  10  is illustrated in FIG.  1 . The grating light valve™ light modulator  10  comprises elongated elements  12  suspended above a substrate  14 . The elongated elements  12  can be suspended by posts (not shown) or some other support structure that maintains the positions of each elongated element  12  relative to each other. The elongated elements  12  comprise a conducting and reflective surface  16  and a resilient material  20 . The substrate  14  comprises a conductor  18 . Each elongated element  12  is separated from an adjacent elongated element by a gap g. In FIG. 1, the gap g is greater than a wavelength λ of incident light I. FIG. 1 depicts the grating light valve™ light modulator  10  in the reflection mode. In the reflection mode, the conducting and reflecting surfaces  16  of the elongated elements  12  form a plane so that incident light I reflects from the elongated elements  12  to produce reflected light R. However, a portion of the incident light I impinges the grating light valve™ light modulator  10  at the gaps g instead of the elongated elements  12 . Due to the gap g being greater than the wavelength λ, a portion of this incident light I will pass between adjacent elongated elements and reflect off the conductor  18 . 
     It will be readily apparent to one skilled in the art that the conducting and reflecting surface  16  can be replaced by a multilayer dielectric reflector in which case a conducting element would also be included in each of the elongated elements  12 . Further, it will be readily apparent to one skilled in the art that the conducting and reflecting surface  16  can be coated with apparent layer such as an anti-reflective layer. 
     FIG. 2 depicts the grating light valve™ light modulator  10  in the diffraction mode. In the diffraction mode, an electrical bias causes alternate ones of the elongated elements  12  to move toward the substrate  14 . The electrical bias is applied between the reflecting and conducting surfaces  16  of the alternate ones of the elongated elements  12  and the conductor  18 . The electrical bias results in a height difference of a quarter wavelength λ/4 of the incident light I between the alternate ones of the elongated elements  12  and non-biased ones of the elongated elements  12 . The height difference of the quarter wavelength λ/4 produces diffracted light including plus one and minus one diffraction orders, D +1  and D −1 , D +1  and D −1  are considered together as first order light. 
     FIGS. 1 and 2 depict the grating light valve™ 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 off the elongated elements  12  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 less the quarter wavelength λ/4, the grating light valve™ light modulator  10  produces a variable reflectivity. 
     It can be seen from FIG. 2 that since the gap g is sufficiently large, g&gt;λ, there remains the portion of the incident light I that is reflected as zero order light and diffracted as even order light, e.g. plus and minus second order, fourth order, etc., regardless of the elongated elements  12  being pulled down or not. This portion of the incident light I is reflected regardless of the state of the elongated elements  12 . Such a condition reduces contrast. If contrast is defined as the difference between the lightest and darkest measured conditions, then the optimal contrast can be defined as all light versus no light. Optimal contrast can also be defined as all incident light I being reflected versus all incident light I being diffracted. If a portion of the incident light I is never diffracted, then the contrast is reduced from optimal. An essential aspect of achieving good contrast is to make sure that the most light possible is being diffracted when the elongated elements are pulled down. Conventional GLVs of the type shown in FIGS. 1 and 2 attempt to circumvent the impact of the un-diffracted light by collecting the first order light instead of the zero order light In this first order operation, the GLV is in the through-state when the elongated elements are in the diffracted mode and the resulting first order light is collected. However, with first order operation comes the previously discussed characteristics of reduced light intensity and decreased efficiency due to the unused higher order light, and increased optical complexity related to collection of the first order light. 
     FIG. 3 illustrates a grating light valve™ light modulator  30  according to the preferred embodiment of the present invention. The grating light valve™ light modulator  30  comprises elongated elements  32  suspended above a substrate  34 . The elongated elements  32  can be suspended by posts (not shown) or some other support structure that maintains the positions of each elongated element  32  relative to each other. The elongated elements  32  comprise a conducting and reflective surface  36  and a resilient material  40 . The substrate  34  comprises a conductor  38 . Each elongated element  32  is separated from an adjacent elongated element by a gap g. In contrast to FIG. 1 in which the gap g is greater than the wavelength λ of incident light I, the gap g in FIG. 3 is less than or equal to the wavelength λ. With g≦λ, high-contrast operation is possible by collecting the zero-order light. The gap g effective to enable this zero order operation is preferably less than or equal to the wavelength λ of the incident light I, g≦λ, and more preferably g≦λ/2. 
     FIG. 3 depicts the grating light valve™ light modulator  30  in the reflection mode. In the reflection mode, the conducting and reflecting surfaces  36  of the elongated elements  32  form a plane so that incident light I reflects from the elongated elements  32  to produce reflected light R. Since the gap g is sufficiently small, none of the incident light I passes through the gap g unaffected. Any portion of the incident light I that passes through the gap g is diffracted, as shown in FIG.  3 . This is in contrast to the grating light valve™ light modulator  10  in FIG. 1 where, due to the gap g being greater than the wavelength λ, a portion of this incident light I will pass un-effected between adjacent elongated elements and reflect off the conductor  8 . 
     FIG. 4 depicts the grating light valve™ light modulator  30  in the diffraction mode. In the diffraction mode, an electrical bias causes alternate ones of the elongated elements  32  to move toward the substrate  34 . The electrical bias is applied between the reflecting and conducting surfaces  36  of the alternate ones of the elongated elements  32  and the conductor  38 . The electrical bias results in a height difference of a quarter wavelength λ/4 of the incident light I between the alternate ones of the elongated elements  32  and non-biased ones of the elongated elements  32 . The height difference of the quarter wavelength λ/4 produces diffracted light including plus one and minus one diffraction orders, D +1  and D −1 . D +1  and D −1  are considered together as first order light. 
     FIGS. 3 and 4 depict the grating light valve™ light modulator  30  in the reflection and diffraction modes, respectively. For a deflection of the alternate ones of the elongated elements  32  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 less the quarter wavelength λ/4, the grating light valve™ light modulator  30  produces a variable reflectivity. 
     In operation, the grating light valve™ light modulator  30  collects the reflected zero order light resulting from the incident light I impinging the grating light valve™ light modulator  30 . In this zero order operation, the grating light valve™ light modulator  30  is in a through-state when the elongated elements are in a flat state and the reflected light R is collected, as illustrated in FIG.  3 . The through-state is also referred to as an on-state. The reflected light R is also the zero order light. When electrical bias is applied to alternating elongated elements  32 , the grating light valve™ light modulator  30  is in a deflected state, as in FIG. 4, and the incident light I is diffracted. In this deflected state, the grating light valve™ light modulator  30  essentially extinguishes any of the incident light I from being reflected. This essentially eliminates any zero order light from being collected while in the deflected state. While in this deflected state, the grating light valve™ light modulator is also referred to as being in an off-state. Any portion of the incident light I that impinges the grating light valve™ light modulator  30  while in the off-state and is not diffracted will be collected as zero order light, thereby reducing contrast. To achieve optimal contrast, the incident light I is either completely specularly reflected (in the on-state), or completely diffracted (in the off-state). Any variation from this will reduce contrast. 
     The present invention designs and engineers the gaps g to achieve contrast ratios of 30-40 dB and higher. Once the gaps g reach the scale of the wavelength λ and below, g≦λ, the incident light I will inevitably be affected. For telecommunication applications, the wavelength λ is preferably about 1.55 μm and the gap g is preferably about 0.5 μm. Incident light I that impinges a gap g larger than the wavelength I can remain unaffected. When working with parts per thousand, as in high-contrast operation, this unaffected light negatively impacts the contrast. 
     The modulator of the present invention can be applied to any optical switching applications, as well as to displays. Optics for this type of modulator are easier and cheaper to produce than modulators using conventional grating light valve™ light modulators and collecting first order light. Also, the efficiency of the present invention is improved, and polarization dependent loss (PDL) effects are typically lower in zero order operation. 
     By collecting the zero order light, efficiency of the modulator of the present invention is improved relative to collecting the first order light. In an optimal case where gap g=0, a maximum efficiency associated with first order operation is 81% and a maximum efficiency associated with zero order operation is 100%. While collecting light of a diffracted state, as in first order light, there is inevitably light in the higher orders. The diffracted light may not be only in the first order. Therefore, when collecting first order light, there may also be uncollected light at the higher orders. This higher order light may be wasted thereby limiting the first order operation to maximum efficiency of 81%. In contrast, zero order operation collects light in a mirror state so ideally there is no diffraction. With no diffraction, and therefore no higher orders of light, all of the light is collected as zero order light. So as the gap g approaches zero, the zero order operation efficiency approaches 100%. 
     The size of the gap g also impacts the efficiency of the modulator. As the size of the gap g is reduced, efficiency improves which also increases the contrast. This is a result of the fill factor, which represents the percentage of the grating light valve™ light modulator that effects impinging light. The fill factor is represented as: 
     
       
         fill factor= w /( w+g ) 
       
     
     where w is the width of the elongated element. In scalar theory, efficiency of the zero order operation is proportional to (fill factor) 2 . Ideally, the gap g=0 and the fill factor is 1.00, or 100%. With a fill factor of 1.00, all of the impinging light is affected. The impact of the fill factor relates to the previous discussions regarding the portion of the incident light I that passes through the gap g. Whether the incident light I is affected when passing through the gap g, as in FIGS. 1 and 2, or unaffected, as in FIGS. 3 and 4, the portion of the incident light I that passes through the gap g is not collected while the grating light valve™ light modulator is in the through-state. By losing this portion of the incident light I, efficiency is reduced and contrast is lowered. As the gap g is reduced, the fill factor increases and efficiency is improved. Since the present invention reduces the gap g to g≦λ, the efficiency of the of the modulator of the present invention is improved over conventional modulators. 
     There is also strong motivation for using zero order operation in an optical train because it is much easier optically. It is well known in the art of optics that is easier working on-axis, or parallel to the optical axis, as in zero order light, versus working at an angle to the optical axis, as in first order light. Conventionally, first order light is collected using Schlieren optics, which is discussed in greater detail below. In zero order operation, the Schlieren optics are substantially easier to design and more efficient in their results. 
     Contrast can be set in great part by two factors. The first factor is the distance that the alternating elongated elements are deflected. Maximum contrast as influenced by this first factor is obtained when the elongated elements are deflected λ/4. In the off-state of the present invention, the distance λ/4 causes the reflected light to be essentially extinguished. 
     The second factor is the architecture of the filter used to collect the light. To better understand the architectural benefits of the present invention, a closer look at the first order operation using a conventional grating light valve™ light modulator is warranted. FIG. 5 illustrates an exemplary top-down view of an optical system using first order operation. This first order optical system includes the grating light valve™ light modulator  10 , relay lenses  22  and  24 , and a Schlieren aperture  26 . The relay lenses  22  and  24  and the Schhieren aperture  26  together are known as Schlieren optics. In operation, an incident light I impinges the grating light valve™ light modulator  10 . While in an off-state, the incident light I is reflected off the grating light valve™ light modulator  10  as reflected light R. The reflected light R also includes the zero order light. The reflected light R is relayed by relay lens  22  to the Schlieren aperture  26 . The relay lens  22  is configured such that all reflected light R is directed to the center of the Schhleren aperture  26 . The Schlieren aperture  26  is configured to pass all first order light and to stop all non-first order light. To accomplish this, the Schlieren aperture  26  includes a stop in its center for stopping zero order light, an opening on either side of the center stop for passing the plus and minus first order light, and a lateral stop on the far side of each opening for stopping second order and higher light. While in an on-state, the incident light I is diffracted off the grating light valve™ light modulator  10  as first order light, D +1  and D −1 , and higher order light, which is not shown in FIG.  5 . The first order light passes through the Schlieren aperture  26  to the relay lens  24 . The higher order light is stopped by the lateral stops of the Schlieren aperture  26 . The relay lens  24  relays the first order light as appropriate, depending on the application. 
     As previously discussed, the different wavelengths have different diffraction angles. To account for these varying diffraction angles, the openings in the Schlieren aperture  26  must be large enough to pass the entire wavelength band of interest In addition, there must be two openings to account for the plus and minus first order light. These larger openings reduce efficiency and, therefore, lower contrast. Also, overlapping of the higher order light with the first order light must be prevented. Specifically, due to the varying diffraction angles of the different wavelengths, the optical system illustrated in FIG. 5 must have sufficient discrimination among zero and first order diffraction at all wavelengths. This increases the complexity of such an optical system. 
     FIG. 6 illustrates a top-down view of an optical system using zero order operation according to the preferred embodiment of the present invention. This zero order optical system includes the grating light valve™ light modulator  30 , relay lenses  42  and  44 , and a Schlieren aperture  46 . In operation, an incident light I impinges the grating light valve™ light modulator  30 . While in an on-state, the incident light I is reflected off the grating light valve™ light modulator  30  as reflected light R. The reflected light R is also the zero order light. The reflected light R is relayed by relay lens  42  to the Schlieren aperture  46 . The relay lens  42  is configured such that all reflected light R is directed to the center of the Schlieren aperture  46 . The Schlieren aperture  46  is configured to pass all zero order light and to stop all non-zero order light. To accomplish this, the Schlieren aperture  46  includes a center opening for passing the zero order light and a lateral stop on either side of the center opening for stopping all non-zero order light. While in an off-state, the incident light I is diffracted off the grating light valve™ light modulator  30  as first order light, D +1  and D −1 , and higher order light (not shown). The non-zero order light is stopped by the lateral stops of the Schlieren aperture  46 . 
     The zero order light passes through the Schhieren aperture  46  to the relay lens  44 . The relay lens  44  relays the zero order light as appropriate, depending on the application. Although FIG. 6 only shows the zero order light reaching the relay lens  44 , the zero order light can be further directed to a display, an optical fiber, a thermal printer, or any other appropriate application using any combination of lenses, waveguides or other optical means for directing light. 
     Unlike first order light, different wavelengths in zero order light do not have varying diffraction angles. Since the zero order light is reflected in a specular fashion and there is no diffraction, all wavelengths are perfectly superimposed. This greatly reduces the size of the center opening of the Schlieren aperture  46 . Using a smaller combiner with a single opening is desirable for higher contrast, and it is easier to design optically. Therefore, the combiner used in zero order operation significantly improves efficiency over first order operation, which results in greater contrast. 
     FIGS. 7 and 8 illustrate the collected zero order light for a telecommunications application where the grating light valve™ light modulator of the present invention is in the on-state and the off-state, respectively. The incident light for the measurements indicated in FIGS. 7 and 8 is 1547.72 nm and the grating light structure has 8 um wide elongated elements with 0.625 um wide gaps between the elongated elements. Thus the gaps are only 0.4 wavelengths wide and tend to only diffract into second order or otherwise scatter, producing a negligible level of specular reflection. Narrowing the gaps and increasing the widths of the elongated elements has the additional benefits of reducing the fraction of light incident to the grating light valve™ light modulator that interacts directly with the gaps. In other words, the fill factor is improved. This also tends to enhance contrast, so long as an adequate first order diffraction angle is maintained. The curves illustrated in FIGS. 7 and 8 clearly demonstrate that 30 dB of on-off contrast can be attained when operating a grating light valve™ light modulator-based switch in zero order operation with a gap between elongated elements equal to 0.4 wavelengths of the incident light. By reducing the gap size to 0.4 um, 35-40 dB of on-off contrast has been achieved. Further refinements on the gap, the sacrificial layer, the cover glass (and its coating) of the grating light valve™ light modulator should permit single-pass contrasts of greater than 40 dB. 
     The 1547.72 nm wavelength of the incident light is consistent with telecommunications applications. The use of the present invention also has an application in display devices. However, the gap must be reduced by a factor of three since the wavelength of visible light, approximately 0.5 um, is a factor of three smaller than the wavelength used in telecommunications. Irrespective of the particular application, the same principles apply. 
     The present invention has been described in terms of specific embodiments incorporating details to facilitate the understanding of the principles of construction and operation of the invention. Such reference herein to specific embodiments and details thereof is not intended to limit the scope of the claims appended hereto.