Abstract:
A modulator for modulating an incident beam of light. The modulator includes a plurality of elements, each element including a first end, a second end, a first non-linear side, a second non-linear side, and a light reflective planar surface. The light reflective planar surfaces of the plurality of elements lie in one or more parallel planes. The elements are preferably arranged parallel to each other. The modulator also includes a support structure to maintain a position of each element relative to each other and to enable movement of selective ones of the plurality of elements in a direction normal to the one or more parallel planes of the plurality of elements. The plurality of elements are preferably moved between a first modulator configuration wherein the plurality of elements act to reflect the incident beam of light as a plane mirror, and a second modulator configuration wherein the plurality of elements act to diffract the incident beam of light. The non-linear sides substantially reduce the polarization dependent losses for diffraction from the plurality of elements.

Description:
FIELD OF THE INVENTION 
   The present invention relates to an apparatus for mitigating the effects of Polarization Dependent Losses (PDL). More particularly, this invention relates to a structure for PDL mitigation for diffractive MEMS and gratings. 
   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 (KHz frame rates), 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. Nos. 5,311,360 and 5,841,579 which are hereby incorporated by reference. 
   According to the teachings of the &#39;360 and &#39;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, an electrical bias causes alternate ones of the elongated elements  12  to move toward the substrate  20 . The electrical bias is applied between the reflecting and conducting surfaces  22  of the alternate ones of the elongated elements  12  and the conductor  18 . The electrical bias results in a height difference between the alternate ones of the elongated elements  12  and non-biased 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 . In the diffraction mode, the diffractive modulator forms an optical structure similar to a square well grating. 
     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. 
   Unfortunately, when arbitrarily polarized light impinges on the diffractive light modulator depicted in  FIGS. 2 and 3 , different polarization states interact with the diffractive light modulator differently. As depicted in  FIG. 4 , at a given instant in time, any arbitrarily polarized light incident upon the diffractive modulator, or grating, can be decomposed into two components; one where the electric field of the light is parallel to the ribbons, or grating grooves, henceforth referred to as P, and another where the electric field of the light is perpendicular to the ribbons, or grating grooves, henceforth referred to as S. Two polarization states are deemed to be different if the ratio of the P and S components for the two states is different. The electric field of the P component initiates an oscillation in the electrons or dipole of a reflector along the ribbon length, and the S component initiates an oscillation in the electrons or dipoles of the reflector in a direction perpendicular to the ribbons. When the oscillating electrons or dipole returns to its original state, it emits, or scatters, light back. The extent of light scattered back depends on the extent of the induced oscillation, which in turn depends on the wavelength of light, the proximity of the oscillating electrons or dipole to the physical boundary of the material, and the fact that the electric field has to be continuous at the material boundaries. For the predominant effect, the oscillations induced by the electric field of the P component interact differently with the ribbon edge than the oscillations induced by the electric field of the S component. This results in a different extent of back scattering for P and S components. Therefore, different polarization states with different ratios of P and S components exhibit different amounts of loss for the light reflected or diffracted back. This leads to Polarization Dependent Losses (PDL) in which one polarization state is attenuated more than the other. 
   For telecommunications applications where the polarization state at the input of a device is not guaranteed and changes with time, PDL causes the intensity of light at the output of the device to vary with time. This results in the degradation of the quality of the transmission and therefore PDL in the device must be minimized. What is needed is a diffractive light modulator with an output response that is as independent of the polarization state as possible. 
   SUMMARY OF THE INVENTION 
   Embodiments of the present invention include a diffractive MEMS for modulating or switching an incident beam of light. The diffractive MEMS includes a plurality of elements, each element including a first end, a second end, a first non-linear side, a second non-linear side, and a light reflective planar surface. The light reflective planar surfaces of the plurality of elements lie in one or more parallel planes. The elements are preferably arranged parallel to each other. The modulator also includes a support structure to maintain a position of each element relative to each other and to enable movement of selective ones of the plurality of elements in a direction normal to the one or more parallel planes of the plurality of elements. The plurality of elements are preferably moved between a first modulator configuration wherein the plurality of elements act to reflect the incident beam of light as a plane mirror, and a second modulator configuration wherein the plurality of elements act to diffract the incident beam of light. 
   The first non-linear side and the second non-linear side can each include one or more projections. The shape of the projection can be an arc, sinusoid, triangle, square or any polygon. Each projection on the first non-linear side can be repeated according to a constant period, and each projection on the second non-linear side can be repeated according to a constant period. The period of the first non-linear side and the period of the second non-linear side can be the same. The shape of each projection can be the same. The projections on the first non-linear side can be symmetric in relation to the projections on the second non-linear side. 
   The first non-linear side can form a recurring pattern and the second non-linear side can form a recurring pattern. The recurring pattern on each side can be the same. The recurring pattern on the first non-linear side can be symmetric in relation to the recurring pattern on the second non-linear side. The recurring pattern can be formed by alternating halves of a circle. The recurring pattern can be formed by alternating a sector of a circle and the mirror image of the sector. The recurring pattern can be a sinusoid, triangle, square or any polygon. 
   The non-linear sides of adjacent elements are preferably separated by a constant gap width. The modulator can be a diffractive MEMS device. The selective ones of the elements are preferably alternating elements and can be moved by applying an electrostatic force. 

   
     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 an arbitrarily polarized light impinging a diffractive light modulator. 
       FIG. 5  illustrates a first embodiment of a plurality of elements included within a diffractive light modulator. 
       FIG. 6  illustrates a second embodiment of a plurality of elements included within a diffractive light modulator. 
       FIG. 7  illustrates a third and preferred embodiment of a plurality of elements included within an active area of a diffractive light modulator. 
       FIG. 8  illustrates a fourth embodiment of a plurality of elements included within an active area of a diffractive light modulator. 
       FIG. 9  illustrates examples of projection shapes that can be included on the non-linear sides of the elements. 
       FIG. 10  illustrates examples of recurring patterns that can be included on the non-linear sides of the elements. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   The present invention overcomes deficiencies of conventional approaches by performing non-linear ribbon cuts at the edges of the ribbons within a diffractive light modulator, thereby providing ribbons with non-linear sides. The non-linear sides substantially reduce the PDL for reflection and diffraction from such ribbons in response to random input polarization states. Preferably, the non-linear sides form a recurring pattern. More preferably, the recurring pattern is formed by alternating halves of a circle. Alternatively, the recurring pattern is formed by alternating a sector of a circle and the mirror image of the sector, by alternating halves of a polygon, for example a square, rectangle or triangle, by a sinusoid, or by a zigzag. Alternatively, the non-linear sides include one or more projections. The projections can form a recurring pattern or the projections can be random. Preferably, each projection or recurring pattern is consistent through the thickness of the ribbon. In this manner, the projection or recurring pattern is symmetric in the third dimension, or 3-D symmetric. The aforementioned non-linear sides act to normalize the profile seen by any incident light including a random polarization state. In the case of the preferred embodiment, the recurring pattern formed by alternating halves of a circle appears as substantially the same profile to both the polarization states TM and TE at the boundary of the ribbon. This evokes substantially the same diffraction response which manifests itself as low PDL. 
     FIG. 5  illustrates a first embodiment of a plurality of elements included within a diffractive light modulator. Preferably, the diffractive light modulator comprises a grating light valve™ light modulator  100 . Preferably, each element within the grating light valve™ light modulator  100  is a ribbon  102 . Each ribbon  102  includes a first end supported by a post  104 , a second end supported by another post  104 , and a reflective layer as its topmost layer. Preferably, the reflective layer is also conducting. The posts  104  support the ribbons  102  to maintain the position of each ribbon  102  relative to each other. The posts  104  are preferably coupled to a substrate. The support posts  104  also enable the movement of selected ones of the ribbons  102  in a direction normal to a substrate of the grating light valve™ light modulator  100 , discussed in greater detail below. Although each ribbon  102  is preferably supported at its ends by a post, it is understood that any means for supporting the ribbons to maintain the position of each ribbon  102  relative to each other and to enable movement in a direction normal to the substrate of the grating light valve™ light modulator  100  can be used. 
   It will be readily apparent to one skilled in the art that the conducting and reflecting layer can be replaced by a multilayer dielectric reflector in which case a conducting element would also be included in each of the elongated elements. Further, it will be readily apparent to one skilled in the art that the conducting and reflecting layer can be coated with a transparent layer such as an anti-reflective layer. 
   Preferably, each of the plurality of ribbons are arranged in parallel. Each ribbon  102  is separated from an adjacent ribbon  102  by a gap g. Preferably, the gap g is constant. 
   Each ribbon  102  includes a first non-linear side and a second non-linear side, where each non-linear side includes one or more projections. As illustrated in  FIG. 5 , the projections in the first embodiment are squares. Specifically, the non-linear sides of each ribbon  102  include square projections  110 . Preferably, each side of the square projection  110  is of length L. The square projections  110  preferably repeat every period P. In this manner, the square projections  110  form a recurring pattern with period P. As can be seen in  FIG. 5 , the recurring pattern on the first non-linear side is out-of-phase with the recurring pattern on the second non-linear side. Specifically, the recurring patterns on both non-linear sides are out-of-phase by 180 degrees. To alternatively describe this square projection recurring pattern, each non-linear edge can be defined as a function. The function in this case is alternating halves of a square. An axis of this function is a line parallel to and a distance L/2 outside of a linear edge defined by a ribbon width W of the ribbon  102 . 
   Each ribbon  102  includes the ribbon width W. For any given ribbon, an effective width is defined as the width of the ribbon at the point where incident light impinges the ribbon. The effective width can be defined as the ribbon width W plus the projection width at that point. The effective width can also be defined as the ribbon width in an active area of the ribbon  102 . The active area is a portion of the ribbon  102  in which the incident light impinges. In the first embodiment, a projection width L′ is equal to the projection length L, since the projection is a square. Therefore, a first effective width W 1  equals W+L. As can be seen in  FIG. 5 , the effective width of each ribbon  102  is the same. Specifically, the effective width W 1  is equal to a second effective width W 2  of an adjacent ribbon  102 . To achieve good contrast, the effective width of adjacent ribbons should be equal, as is the case in the first embodiment. More specifically, it is the reflectivity effective width of adjacent ribbons that should be equal in order to achieve good contrast. However, in the case where reflectivity is constant over the width of the ribbon, it is the effective width that should be equal. 
   Each ribbon  102  can also include one or more slits  106 . The slits can be placed in a non-active area of the ribbon  102  to relieve stress. The non-active area is a portion of the ribbon  102  in which the incident light does not impinge. Although only a single slot  106  is illustrated in  FIG. 5 , it is understood that more slits can be used as necessary. 
   The grating light valve™ light modulator  100  is operated in a similar manner as a conventional grating light valve™ light modulator. In a reflection mode, the reflecting layers of the ribbons  102  form a plane so that incident light I reflects from the ribbons  102  to produce reflected light R. The reflected light R is reflected in the zero order. 
   In a diffraction mode, an electrical bias causes alternate ones of the ribbons  102  to move toward the substrate of the grating light valve™ light modulator  100 . The electrical bias is applied between the reflecting and conducting layers of the alternate ones of the ribbons  102  and a conductor on the substrate. The electrical bias results in a height difference between the alternate ones of the ribbons  102  and non-biased ones of the ribbons  102 . 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 . For a deflection of the alternate ones of the ribbons  102  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 ribbons  102  less the quarter wavelength λ/4, the diffractive light modulator  100  produces a variable reflectivity. 
     FIG. 6  illustrates a second embodiment of a plurality of elements included within a diffractive light modulator. The second embodiment differs from the first embodiment in that the recurring pattern on the first non-linear side is symmetrical with the recurring pattern on the second non-linear side. In other words, the recurring patterns on both non-linear sides are in-phase. Due to this symmetry, less lateral stress is applied to each ribbon  202  within the second embodiment than to each ribbon  102  within the first embodiment. As a result, the second embodiment is more mechanically stable than the first embodiment. However, the second embodiment does not provide as high contrast as the first embodiment. As discussed above, higher contrast is achieved when the effective width of adjacent ribbons is equal. This is the case in the first embodiment. However, as can be seen in  FIG. 6 , the effective width W 1  and W 2  of two adjacent ribbons  202  are not equal. Since W 1  is not equal to W 2 , the contrast provided by the second embodiment is not as high as the contrast provided by the first embodiment. 
     FIG. 7  illustrates a third and preferred embodiment of a plurality of elements included within an active area of a diffractive light modulator. The third embodiment is similar to the first embodiment except that the recurring pattern on each non-linear side is different. Specifically, the non-linear sides of each ribbon  302  in the third embodiment include half-circle projections  310 . Each half-circle projection  310  includes a radius R. The half-circle projection  310  and a mirror image of the half-circle projection  310  preferably repeats every period P. In this manner, the half-circle projections  310  form a recurring pattern with period P. As can be seen in  FIG. 7 , the recurring pattern on the first non-linear side is out-of-phase with the recurring pattern on the second non-linear side. Specifically, the recurring patterns on both non-linear sides are out-of-phase by 180 degrees. To alternatively describe this half-circle projection recurring pattern, each non-linear edge can be defined as a function. The function in this case is alternating halves of a circle. An axis of this function is a line parallel to and a distance R outside of a straight line defined by a ribbon width W of the ribbon  302 .  FIG. 7  illustrates the active area of the grating light valve™ light modulator  100 . As such, posts on either end of the elements  302  are not shown in FIG.  7 . 
   Similarly to the first embodiment, the effective widths W 1  and W 2  of adjacent ribbons  302  are equal. As such, the third embodiment of the grating light valve™ light modulator  100  provides high contrast operation. 
     FIG. 8  illustrates a fourth embodiment of a plurality of elements included within an active area of a diffractive light modulator. The fourth embodiment differs from the third embodiment in that the recurring pattern on the first non-linear side is symmetrical with the recurring pattern on the second non-linear side. In other words, the recurring patterns on both non-linear sides are in-phase. Due to this symmetry, less lateral stress is applied to each ribbon  402  within the fourth embodiment than to each ribbon  302  within the third embodiment. As a result, the fourth embodiment is more mechanically stable than the third embodiment. However, the fourth embodiment does not provide as high contrast as the third embodiment. As discussed above, higher contrast is achieved when the effective width of adjacent ribbons is equal. This is the case in the third embodiment. However, as can be seen in  FIG. 8 , the effective widths W 1  and W 2  of two adjacent ribbons  402  are not equal. Since W 1  is not equal to W 2 , the contrast provided by the fourth embodiment is not as high as the contrast provided by the third embodiment. 
   PDL produced by the half-circle projections is less than the PDL produced by the square projections. This is because of the smoothly varying nature of the edge of the circle. The square is not smoothly varying at the corners, where there are abrupt changes in direction. Such abrupt changes lead to increases in PDL when compared to PDL produced at the smoothly varying edge of the circle. However, the more smoothly varying the non-linear side of the ribbon, the less distinct is the diffraction of the incident light. As such, for a smoothly varying non-linear side it is more difficult to de-couple the diffracted light from the reflected light in the zero order. Since all of the diffracted light can not be decoupled in this case, the contrast is reduced. This concept can be better understood by looking at the operation of the grating light valve™ light modulator. 
   In operation, the zero order light is collected. When in the reflection mode, virtually all of the incident light is reflected back as zero order light. In the diffraction mode, alternating ones of the ribbons are deflected to a maximum diffraction distance of a quarter wavelength λ/4 of the incident light. In an optimal case, virtually all of the incident light will diffract into the first order while in the diffraction mode. In practice, a portion of the incident light scatters. In other words, a portion of the incident light diffracts at an angle different than that of the first order. Some of the light that is scattered can be close enough to the zero order such that this scattered light is collected in addition to any zero order light. Non zero-order light may be collected because the portion of the scattered light close enough to the zero order can not be decoupled from the zero order light. As more of the scattered light is collected, contrast is reduced. 
   As the ribbons are deflected towards λ/4, the incident light reflected in the zero order is reduced and the diffracted light increases. For a non-linear side with a recurring pattern of square projections, the highest contrast is achieved at the maximum diffraction λ/4. This is because the square projections diffract a significant percentage of the incident light into the first order and less of the incident light is scattered elsewhere. However, half-circle projections diffract a lower percentage of the incident light into the first order and comparatively more light is scattered at an angle that can not be decoupled from the zero order. Therefore, as the ribbons are deflected and the zero order light is reduced, the diffracted light increases including the scattered light that can not be decoupled from the zero order light. There reaches a deflection point short of λ/4 where the amount of light that is collected can no longer be reduced. Even though the incident light reflected back as zero order light continues to decrease as the deflection of the ribbons increase, the amount of scattered light that is collected is simultaneously increasing. Therefore, the maximum contrast achieved by the half-circle projections is not as high as the maximum contrast achieved by the square projections. Recalling that PDL produced by the half-circle projections is less than the PDL produced by the square projections, there exists a trade-off between PDL and contrast. 
   PDL also varies with the distance that the ribbons are deflected. The more the ribbons are deflected, the more light is diffracted and the more PDL is introduced. When the ribbons are deflected, each polarization state TM and TE see an edge and a height. Preferably, any projection in the non-linear sides is constant through the entire thickness of the ribbon. In the case of the recurring patterns such as alternating half circles, squares, triangles and sinusoids, each polarization state TM and TE sees the same topology. Regardless of the orientation, each polarization state TM and TE sees the same edge and step, otherwise called 3-D topology. In the case of the recurring patterns such as alternating half circles, squares, triangles and sinusoids, the ribbon configuration is 3-D symmetric. This results in good PDL performance as a function of attenuation. 
   It should be clear to those skilled in the art that the half-circle projections illustrated in  FIGS. 7 and 8  can include a radius either smaller or larger than that shown. For applications using diffraction, the radius of the half-circles must preferably be greater than or equal to the wavelength of the incident light. 
   It is understood that the embodiments illustrated in  FIGS. 5-8  are intended to aid in understanding and should not be used to limit the scope of the present invention. The gap width g, the ribbon width W, the dimensions of the projections and the frequency of the projections illustrated in  FIGS. 5-8  can be increased or decreased as appropriate. Furthermore, the shapes of the projections are not limited to squares and half-circles.  FIG. 9  illustrates examples of projection shapes that can be included on the non-linear sides of the ribbons. Projection shapes can include, but are not limited to, a square, a half-circle, an arc, a polygon including a symmetrical polygon and a triangle, or any randomly shaped projection. 
   Similarly, the recurring patterns formed by the non-linear sides of the ribbons are not limited to alternating halves of a circle or alternating halves of a square.  FIG. 10  illustrates examples of recurring patterns that can be included on the non-linear sides of the ribbons. Recurring patterns can include, but are not limited to, alternating half-circles, alternating arcs, a sinusoid, alternating triangles, alternating squares, or any other recurring function. Arcs can also be considered as sectors of a circle, where a recurring pattern that alternates between a sector of a circle and a mirror image of the sector has a decreased amplitude as compared to a recurring pattern that alternates half-circles of the circle. 
   It will be readily apparent to one skilled in the art that other various modifications may be made to the embodiments without departing from the spirit and scope of the invention as defined by the appended claims.