Abstract:
A grating for an optical device. The grating includes a working area comprising a periodic grating structure and a non-working area comprising a support structure for mating with an additional component of the optical device. The support structure supports the additional component in order to prevent strains from developing in the grating. The strains may come from physical stresses applied to the optical device during manufacturing or use or may come from changes in the temperature of the optical device. The support structure may operate by providing an element whose width to height ratio is greater than a width to height ratio of the elements in the grating structure. The support structure may also operate by being higher than the working area of the grating. Furthermore, the non-working area of the grating may include alignment marks used during the manufacturing of the optical device.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention pertains generally to manufacturing devices with nanometer scaled features and more specifically to manufacturing components for a Polarization Beam Splitter (PBS). 
     2. Description of the Related Art 
     PBSs have been created having a multilayer polarization splitting elements. The multilayer polarization splitting elements are composed of layers having a high refractive index alternating with layers having a low refractive index. These multilayer polarization splitting elements are constructed using TiO 2 . Such a PBS is described in U.S. application Ser. No. 11/122,153 entitled “POLARIZATION ELEMENT AND OPTICAL DEVICE USING POLARIZATION ELEMENT” filed May 3, 2005. which issued Dec. 26, 2006 as U.S. Pat. No. 7,155,073. 
       FIG. 1  is a structural view showing a PBS.  FIG. 1  shows a state in which a polarization splitting layer  23  composed of a plurality of periodic structures each having structural birefringence is sandwiched by two prisms. The polarization splitting layer  23  and the two prisms compose an optical element having a polarization splitting function. 
     In  FIG. 1 , the polarization splitting layer  23  is tilted at Brewster angle relative to an incident surface  25  of the prism. When an incident light beam including a P-polarized light component  18  and an S-polarized light component  20  is perpendicularly made incident on the incident surface  25 , the P-polarized light component  18  passes through the polarization splitting layer  23  to become passing light  19 , and the S-polarized light component  20  is reflected on the polarization splitting layer  23  to become reflective light  21 . As illustrated herein, the optical element is assumed to be used for visible light. 
       FIG. 2  is a conceptual view showing the polarization splitting layer  23 . The polarization splitting layer  23  has a plurality of grating structures (periodic structures) stacked therein. Periodic directions of adjacent grating structures are substantially orthogonal to each other. In this embodiment, five one-dimensional grating structures corresponding to five layers are stacked. ( FIG. 2  is the conceptual view so only three one-dimensional grating structures are shown therein.) Assume that first, second, third, fourth, and fifth one-dimensional gratings are arranged in order from a light incident side (upper side of  FIG. 2 ). A period of each of the grating structures is shorter than a wavelength of any incident light. Each of the grating structures exhibits structural birefringence. 
     As shown in  FIG. 2 , an incident surface on which the incident light beam (P-polarized light component  18  and S-polarized light component  20 ) is made incident is orthogonal to a periodic direction of the first one-dimensional grating. The periodic direction of the first one-dimensional grating is assumed to be a grating direction V. As shown in  FIG. 2 , a periodic direction of the second one-dimensional grating is orthogonal to the grating direction V and assumed to be a grating direction P. 
     When the light is made incident on the polarization splitting layer  23 , the S-polarized light component is reflected thereon and the reflective light  21  thereof exits from an exit surface  26  different from the incident surface  25  located on the light incident side of the prism. At this time, the P-polarized light component passes through the polarization splitting layer  23  and the passing light  19  thereof exits from an exit surface  27  located on the light exit side of the prism. 
     This PBS performs well as it has a performance such as wide incident angle as well as broad wavelength. But, it is difficult to make such a device. One difficulty is lies in the stresses that must be applied to the PBS during manufacturing. As the ratio of the height of the grating elements to the width of the grating elements is high, the grating elements may not tolerate lateral stresses induced when the grating structures are stacked or when additional optical elements are bonded to the stack of gratings. In addition, once the PBS is constructed, the PBS may be fragile as shocks experienced by the PBS may be transmitted to the active portions of the stacked gratings which may cause toppling of the grating elements. Finally, as traditional manufacturing processes for the stacked periodic structures leave the periodic structures tightly bound, expansion and contraction of the structures because of temperature changes may cause the shape of the periodic structures to change. 
     Therefore, a need exists for a manufacturing process that provides for reduced physical and thermal stresses being transmitted to the grating structures in a PBS. Various aspects and embodiments of the present invention meet such a need. 
     SUMMARY OF THE INVENTION 
     In one aspect of the invention, a grating for an optical device is provided. The grating includes a working area comprising a periodic grating structure and a non-working area comprising a support structure for mating with an additional component of the optical device. 
     In one aspect of the invention, a ratio of a width of an element of the support structure to a height of the element of the support structure is greater than a ratio of a width of an element in the periodic grating structure to a height of the element. 
     In another aspect of the invention, a height of an element of the support structure is greater than a height of an element in the periodic grating structure. 
     In another aspect of the invention, the additional component is another grating in the optical device. 
     In another aspect of the invention, the additional component is an optical component of the optical device. 
     In another aspect of the invention, the non-working area further comprises an alignment mark. 
     In another aspect of the invention, the non-working area is peripheral to the working area and the non-working area and support structure are contiguous and enclose the working area. The contiguous support structure may further include one or more openings into the enclosed working area. 
     In another aspect of the invention, the support structure is non-contiguous providing one or more openings in communication with the working area. 
     In another aspect of the invention, the support structure comprises a plurality of non-contiguous support elements. 
     In another aspect of the invention, the working area has an effective area within the periodic grating structure and a transitional area within the periodic grating structure between the effective area and the non-working area. 
     In another aspect of the invention, a layered periodic structure is built using periodic grating structures with each periodic grating structure having effective areas and transitional areas, thus creating effective and transitional areas in the layered periodic structure. When the layered periodic structure is used in an application, light to be polarized is transmitted into the layered periodic structure into an effective area of the working area of the layered periodic structure and without transmitting light through the transitional area. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be more readily understood from a detailed description of the preferred embodiment taken in conjunction with the following figures. 
         FIG. 1  is a schematic view showing a polarization splitting element. 
         FIG. 2  is a schematic view of a polarization splitting layer. 
         FIG. 3  is a cross-sectional schematic view of a polarization splitting element in accordance with an exemplary embodiment of the present invention. 
         FIGS. 4   a  to  4   k  are schematics illustrating a manufacturing process for a layered structure in accordance with an exemplary embodiment of the present invention. 
         FIGS. 5   a ,  5   b  and  5   c  are top views of periodic grating structures in accordance with an exemplary embodiment of the present invention. 
         FIG. 5   d  is a top view of a stack or pile of periodic grating structures in accordance with an exemplary embodiment of the present invention. 
         FIGS. 6   a ,  6   b ,  6   c , and  6   d  are schematics illustrating a series of periodic grating structures having alignment marks in accordance with an exemplary embodiment of the present invention. 
         FIG. 6   e  is a schematic illustrating an alignment mark in accordance with an exemplary embodiment of the present invention. 
         FIG. 7  is a cross-sectional schematic view of another optical device in accordance with an exemplary embodiment of the present invention. 
         FIGS. 8   a ,  8   b  and  8   c  are schematics illustrating a manufacturing process for a layered structure in accordance with an exemplary embodiment of the present invention. 
         FIGS. 9   a ,  9   b  and  9   c  are top views of periodic grating structures in accordance with an exemplary embodiment of the present invention. 
         FIG. 9   d  is a top view of a stack or pile of periodic grating structures in accordance with an exemplary embodiment of the present invention. 
         FIGS. 10   a ,  10   d  and  10   e  are top views of periodic grating structures in accordance with an exemplary embodiment of the present invention. 
         FIGS. 10   b  and  10   c  are cross-sectional views of a periodic grating structure in accordance with an exemplary embodiment of the present invention. 
         FIG. 11  is a partial cross-sectional view of a transitional area of a periodic grating structure in accordance with an exemplary embodiment of the present invention. 
         FIG. 12  is a top view of a transitional area of a layered periodic structure in accordance with an exemplary embodiment of the present invention. 
         FIG. 13  is a cross-sectional view of a transitional area and an effective area of a layered periodic structure as utilized in an optical application in accordance with an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     As previously described,  FIG. 1  is a structural view showing a polarization splitting element.  FIG. 1  shows a state in which a polarization splitting layer  23  composed of a plurality of periodic structures each having structural birefringence is sandwiched by two prisms. The polarization splitting layer  23  and the two prisms compose an optical element having a polarization splitting function. 
       FIG. 2  is a conceptual view showing the polarization splitting layer  23 . The polarization splitting layer  23  has a plurality of grating structures (periodic structures) stacked therein. Periodic directions of adjacent grating structures are substantially orthogonal to each other. In this embodiment, five one-dimensional grating structures corresponding to five layers are stacked. ( FIG. 2  is the conceptual view, so only three one-dimensional grating structures are shown therein.) Assume that first, second, third, fourth, and fifth one-dimensional gratings are arranged in order from a light incident side (upper side of  FIG. 2 ). A period of each of the grating structures is shorter than a wavelength of any incident light. 
     A complete description of the polarization splitting layer  23 , the element incorporating the polarization splitting layer and several embodiments is provided in co-pending U.S. application Ser. No. 11/122,153 entitled “POLARIZATION ELEMENT AND OPTICAL DEVICE USING POLARIZATION ELEMENT” filed May 3, 2005, the contents of which are incorporated by reference as if stated in full herein. 
     While the following description applies specifically to formation of gratings for a polarization splitting layer, such as polarization splitting layer  23 , various embodiments of the manufacturing processes disclosed herein are applicable to forming periodic or aperiodic structures for components for other optical and RF applications. For example, a stacked grating structure could be used for filters for specific RF frequencies, detectors, couplers; or for telecommunication applications such as waveguides, lasers, detectors, modulators, multiplexers or demultiplexers. Stacked structures created according to the following descriptions may also be used in optical imaging devices such as a the described PBS, polarizers, diffraction elements for antireflection coatings, and a chromatic aberration correction lens and for use in optical memories such as a DVD or CD, or an optical head. 
     Furthermore, various methods have been proposed for manufacturing a polarization splitting layer. As an example, U.S. patent application Ser. No. 11/500,321 entitled “METHOD FOR MANUFACTURING LAYERED PERIODIC STRUCTURES” filed Aug. 8, 2006, the contents of which are hereby incorporated by reference, describes several such methods. 
     Having discussed the types of devices  FIG. 3  is a cross-sectional schematic view of an optical device, such as a polarization splitting element, in accordance with an exemplary embodiment of the present invention. A polarization splitting element  100  includes a layered periodic structure  102  bonded to a first component, such as prism  104 , and a second component, such as prism  106 . The layered periodic structure includes a plurality of periodic grating structures, such as periodic grating structures  108 ,  109  and  110 . Each periodic grating structure includes at least one working area, for example working area  112  of periodic grating structure  108 , and at least one non-working area, such as non-working area  114  of periodic grating structure  108 . The non-working area includes one or more support structures that include elements, such as element  116 , that mate with support structure elements, such as elements  118  and  120 , in other non-working areas of other layered periodic structures. In this way, the support structure elements take the loads applied to the support elements&#39; respective periodic grating structures without transferring the loads to working elements, such as working element  117 , in the working areas of the support elements&#39; respective periodic grating structures. As illustrated, the non-working area is peripheral to the working area; however, the non-working area may be located otherwise than peripheral to the working area, such as adjacent to or surrounded by the working area. 
     The elements of the support structures have lower aspect ratios, that is, a ratio of a height of the element to a width of the element, than individual elements, such as element  118 , within the working areas of the periodic gratings. This allows the support structures in the non-working areas of the periodic gratings to absorb and transfer stresses induced during manufacturing and usage of the optical device without disturbing unduly the elements of the working areas of the periodic gratings. 
     Having described an optical element having periodic gratings with working and non-working area, a manufacturing process in accordance with an exemplary embodiment of the present invention will now be described with reference to  FIGS. 4   a  to  4   k .  FIG. 4   a  is a cross sectional view of a layer of substrate material  400 , such as a Si wafer. As illustrated in  FIG. 4   b , a layer of grating material  402 , such as TiO2, is formed on a surface of the substrate layer  400 . As illustrated in  FIG. 4   c , a resist layer  404  is formed on a surface of the grating material layer  402 . The resist layer  404  includes openings, such as opening  406 , extending through the resist layer  404  to expose portions of the surface of the grating material. The openings may define a periodic pattern. The pattern may be formed in the resist layer by several methods including lithography by phase shift mask, interference lithography using multiple laser beams or an imprint process. 
     In a resist layer in accordance with an exemplary embodiment of the present invention, the openings extend along one surface dimension of the resist layer  404  creating a line hole pattern of spaced apart grooves. Such a line hole pattern is useful for creating periodic gratings and the like. 
     In a resist layer in accordance with an exemplary embodiment of the present invention, the height of the resist layer is in the range of 400 nm, the openings are in the range of 110 nm wide with a spacing in the range of 140 nm leaving lands or ridges, such as ridge  410 , of around 30 nm wide. Furthermore, in one portion  414  of the pattern, one or more ridges or lands  416  are formed having a width of about 400 nm. These dimensions are representative of grating dimensions for a component for a PBS for visible wavelengths. As can be readily understood by those skilled in the art, the pitch depends on the wavelength. For example, in the case of infrared applications, the pitch might be larger than that of the above example in proportion to wavelength. 
       FIG. 4   d  illustrates forming a pattern in the grating material layer  402 . The pattern includes one or more openings, such as opening  418 , extending substantially from a surface of the resist layer  404 , through grating material layer  402  to substrate layer  400 . In a manufacturing process in accordance with one exemplary embodiment of the present invention, the pattern in the grating material layer  402  is formed by a dry etching process such as Reactive Ion Etching (RIE) through the openings  406  in the resist layer  404 . In this manufacturing process, the Si layer is prepared as an etch stopping layer to avoid any damage to the Si surface of the substrate layer  400 . Accordingly, conventional enchants may be used in Si Large-Scale Integration (LSI) processes to etch by high selectivity such as CF4+H2, C2F6, CHF3, and C3F8. 
       FIG. 4   e  illustrates a pattern formed in the grating material layer  402  by the etching process and then removing the resist layer leaving behind the patterned grating material layer  402  atop the substrate layer  400 . The resulting patterned grating material layer  402  includes one or more elongated elements  422 , extending away from the plane of  FIG. 4   e , that are part of a working area  423  of a periodic grating. Furthermore, the patterned grating material layer includes a non-working portion  425  of the periodic grating that includes one or more elements  424  that function as support structures as previously described. 
       FIG. 4   f  illustrates depositing a sacrificial layer  420  filing the spaces, such as space  426 , in the patterned grating material layer  402 . The sacrificial layer may be of a material such as SiO2 deposited by a process such Chemical Vapor Deposition (CVD).  FIG. 4   g  illustrates removing any over-deposition  428  of the sacrificial layer and flattening by a process such as Chemical Mechanical Polishing (CMP). As the grating material layer now includes one or more support structures  424 , the support structures may be used as a stopper indicating when the flattening process may be stopped. 
       FIG. 4   h  illustrates adding an additional periodic grating structure. To do so, a thin etch stopper layer  430  is deposited on the layer of patterned grating material  402 . Then, an additional layer of grating material  432  is deposited and used to construct an additional periodic grating structure as previously described. 
       FIG. 4   i  illustrates continuing to add periodic grating structures by adding additional etch stop layers, such as etch stop layer  434 , additional layers of grating material, such as additional grating material layer  436  and then constructing an additional periodic grating structure as previously described. Although only three periodic grating structures are illustrated in  FIG. 4   i , it is to be understood that any number of periodic grating structures may be stacked or piled up in this manner. 
       FIG. 4   j  illustrates bonding an additional component  438  to the stack of periodic grating structures  440  and then removing the sacrificial filler material by a chemical process.  FIG. 4   k  illustrates removing the substrate layer  400  (of  FIG. 4   j  ) and bonding an additional component  442  to the stack of periodic grating structures to create a working device. 
     In one embodiment of the present invention the each stopper layer is of a material such as Al 2 O 3 , and is deposited at a thickness of around 10 nm to 20 nm. 
     In another embodiment of the present invention, the additional periodic grating structure layer is about 65 nm in thickness. 
     In another embodiment of the present invention, the additional periodic grating structure has a different orientation than the first periodic grating structure. 
       FIGS. 5   a ,  5   b  and  5   c  are top views of a series of periodic grating structures,  500 ,  502  and  504 , each having different orientations, that are stacked on top of each other in accordance with the previously described manufacturing processes. As illustrated in  FIG. 5   a , each periodic grating structure has a working area  506  and a non-working area  508  peripheral to the working area. The non-working area includes support structures as previously described. 
       FIG. 5   d  is a top view of the periodic grating structures stacked or piled atop each other. In the stack or pile  510 , there is a working area  512  and a non-working area  514  including support structures (not shown) that mate with each other. The resulting stack  510  corresponds to the layered periodic structure  102  shown in cross-section in  FIG. 3 . 
       FIGS. 6   a ,  6   b  and  6   c  are top views of a series of periodic grating structures,  600 ,  602  and  604 , each having different orientations, that are stacked on top of each other as in  FIGS. 5   a ,  5   b  and  5   c . The periodic grating structures  600 ,  602  and  604  include a working area, such as working area  606  in periodic grating structure  600 . The periodic grating structure  600  further includes a peripheral non-working area  608  surrounding the working area. The peripheral non-working area includes one or more openings, such as opening  610 . These openings allow fluids, such as gases and liquids, to enter the periodic grating structure  600  during processing and manufacturing when the periodic grating structure  600  is stacked with periodic grating structures  602  and  604 . 
       FIG. 6   e  is a schematic illustrating use of alignment marks in accordance with an exemplary embodiment of the present invention. Each periodic grating structure in a stack  610  may include one or more alignment marks, such as alignment mark  612 . During assembly of the stack  610 , the alignment marks may be used to align the periodic grating structures. 
       FIG. 6   e  is a schematic illustrating alignment marks in accordance with an exemplary embodiment of the present invention. In  FIG. 6   e , two alignment marks are shown. Alignment mark  613  is superimposed with alignment mark  614 . The alignment marks  613  and  614  are on separate periodic grating structures (not shown) that are stacked on top of each other. 
     In one type of alignment mark in accordance with an exemplary embodiment of the present invention, alignment marks on successive layers of structures have different formats. For example, alignment mark  613  on a first layer is wider than alignment mark  614  on a successive layer. This configuration allows aligning layers of structures by placing one alignment mark within or over another. 
     In another type of alignment mark in accordance with an exemplary embodiment of the present invention, the alignment marks include Vernier scales  616  and  618 . The Vernier scales can be used to more accurately align the periodic grating structures during assembly. 
       FIG. 7  is a cross-sectional schematic view of another optical device in accordance with an exemplary embodiment of the present invention. A polarization splitting element  700  includes a layered periodic structure  702  bonded to a first component, such as prism  704 , and a second component, such as prism  706 . The layered periodic structure includes a plurality of periodic grating structures, such as periodic grating structures  708 ,  709  and  710 . Each periodic grating structure includes at least one working area, for example working area  712  of periodic grating structure  708 , and at least one non-working area, such as non-working area  714  of periodic grating structure  708 . The non-working area includes one or more support structures that include elements, such as element  716 , that take the loads applied to the periodic grating structures. The working area  712  of the periodic grating structure  708  includes elements, such as element  717  that are shorter in height than the element&#39;s corresponding support structure  716 . This ensures that any loads applied to the periodic grating structures are carried by the mating support structure elements  716 ,  718  and  720  in the non-working areas of the periodic grating structures and not the elements in the working areas of the periodic grating structures. This is because there is a gap, such as gap  720 , between the elements of the working area of the periodic grating structure  708  and the adjacent periodic gratings structure&#39;s  709  working area  724 . Furthermore, there will be a gap, such as gap  726 , between elements in a working area  728  of the periodic grating structure  710  and prism  704 . 
       FIGS. 8   a ,  8   b  and  8   c  are schematics illustrating a manufacturing process for a layered structure having support structures that have heights greater than the height of elements in a working area of the layered structures.  FIG. 8   a  is a cross sectional view of a layer of substrate material  800 , a layer of grating material  802 , formed on a surface of the substrate layer  800 , and a first resist layer  804  formed on a surface of the grating material layer  802 . The resist layer  804  includes an opening  806  extending through the resist layer  804  to expose a portion  807  of the surface of the grating material layer. The opening may be formed in the resist layer by several methods including lithography by phase shift mask, interference lithography using multiple laser beams or an imprint process. 
       FIG. 8   b  illustrates etching the grating material layer  802  in order to develop a height difference  808  between a surface  810  of the resist material protected portion  812  of the grating material layer  802  and a surface  814  of the exposed portion  807  of the grating material layer  802 . During the manufacturing of the structured layer, the exposed portion  807  will be used to create elements in a working area of a periodic grating structure while the protected portion  812  of the grating material layer  802  will be used to create an elevated support structure. 
       FIG. 8   c  illustrates forming a second resist layer pattern  816  on the lower surface  814  of the exposed portion of the grating material layer  802 . The remaining manufacturing process for a stack or pile of periodic grating structures is then similar to the manufacturing process illustrated in  FIGS. 4   d  to  4   k.    
       FIGS. 9   a ,  9   b  and  9   c  are top views of a series of periodic grating structures in accordance with an exemplary embodiment of the present invention. Each periodic grating structure,  900 ,  902  and  904 , includes a working area, such as working area  906 , and a non-working area, such as non-working area  908 . Each non-working area includes one or more elements that have a higher height relative to a height of elements in the working area as described in  FIG. 8   c.    
       FIG. 9   d  illustrates a top view of stacking or piling the periodic grating structures  900 ,  902  and  904  of  FIGS. 9   a ,  9   b  and  9   c  respectively, on top of each other. Once stacked, the non-working areas  910  of the periodic grating structures overlay one another. In addition, the working areas  912  of the periodic grating structures also overlay each other. This allows the support structures within the non-working areas to contact each other and transmit loads to each other without affecting the non-contacting working areas of the periodic grating structures. 
     Referring now to  FIGS. 10   a ,  10   b  and  10   c ,  FIG. 10   a  is a top view of a periodic grating structure in accordance with an exemplary embodiment of the present invention,  FIG. 10   b  is a cross-sectional view along line AA of  FIG. 10   a  and  FIG. 10   b  is a cross-sectional view along line BB of  FIG. 10   a . The periodic grating structure  1000  includes a non-working area  1002  having one or more support structure elements  1004  that have a height that exceeds a height of any element, such as element  1006 , in a working area  1008  of the periodic grating structure. Furthermore, the height of the support structure elements also exceeds a height of other portions  1010  of the non-working area. 
     Referring now to  FIGS. 10   a ,  10   d  and  10   e , top views of a series of periodic grating structures  1000 ,  1012  and  1014 , are shown. Each periodic grating structure  1000 ,  1012  and  1014  includes a non-working area  1002 ,  1016  and  1018 , respectively. In addition, each non-working area,  1002 ,  1016  and  1018  includes one or more support structure elements  1004 ,  1020  and  1022 , respectively. Each support structure element  1004 ,  1020  and  1022  has a height that is greater than a height of their respective working areas  1008 ,  1024  and  1026 . This allows the periodic grating structures  1000 ,  1012  and  1014  to be stacked or piled on top of each other with each periodic grating structure being supported in the stack or pile by the support structures in the non-working areas of the periodic grating structures without affecting the working areas. Furthermore, each support structure element  1004 ,  1020  and  1022  has a height that is greater than a height of a portion,  1010 ,  1028  and  1030  respectively, of each support elements&#39; respective non-working areas. This ensures that openings exist between support structures in a particular layer in a stack or pile such that fluids may reach the interior working areas of the stack or pile. 
       FIG. 11  is a partial cross-sectional view of a transitional area of a periodic grating structure during manufacturing in accordance with an exemplary embodiment of the present invention. As previously described, during the manufacture of a periodic layered structure, a layer of substrate material  1100  is overlain with a layer of grating material  110 . A resist layer  1104  is formed on a surface of the grating material layer  1102 . The resist layer  1104  includes openings, such as opening  1106 , extending through the resist layer  1104  to expose portions of the surface of the grating material. The openings may define a periodic pattern. 
     As previously described, a pattern is formed by etching portions of the grating layer  1102  exposed by the openings in the resist layer  1104 . During the etching process, the pattern is formed such that peripheral support structures in a non-working area of a grating, such as support structure  1108 , are formed around a series of grating elements, such as grating elements  1110  and  1112 . During the etching process, the grating material is etched downward ( 1113 ) towards the substrate layer  1100 . In addition, the etching process also etches horizontally ( 1114 ,  1116  and  1118 ) into the grating elements. 
     Furthermore, the etching rate is different for those grating elements closer to the support structure, such as elements  1110  and  1112 , than for those grating elements further away from the support structure, such as elements  1115 ,  1117  and  1119 , because of a process known as microloading. This results in the grating elements having progressively different sizes depending on how close a grating element is to the support structure. However, after a finite number of grating elements, the grating elements attain a relatively uniform size as the microloading effect is diminished. For example, grating elements  1117  and  1119  are shown has having a relatively uniform size as the rate of horizontal etching ( 1118 ) is approximately the same for both grating elements. Therefore, as a result of the microloading effect, a periodic structure may end up having a non-working area  1120  and a working area  1122  wherein the working area includes a transitional area  1124  and an effective working area  1126 . In the transitional area  1124 , the grating elements  1110 ,  1112  and  1115  have differing widths because of microloading effects. However, in the effective working area  1126 , the grating elements, such as grating elements  1117  and  1119 , the widths of the elements are relatively uniform. 
       FIG. 12  is a top view of a transitional area and an effective area of layered periodic structure in accordance with an exemplary embodiment of the present invention. In the top view, a stack or pile of periodic grating structures (not shown), each having their own transitional and effective areas are used to create a layered periodic structure  1200  as previously described. The resultant layered periodic structure  1200  has a peripheral non-working area  1202 . Furthermore, as each periodic grating structure includes both a transitional area and an effective area, an interior portion of the layered periodic structure also includes a transitional area, as indicated by  1206   a ,  1206   b ,  1206   c  and  1206   d  surrounding an effective area  1204 . 
       FIG. 13  is a cross-sectional view of a transitional area and an effective area of a layered periodic structure as utilized in an optical application in accordance with an exemplary embodiment of the present invention. A light source  1300  projects a beam of light  1302  through a focusing device  1304 . The focusing device creates a second beam of light  1306  that is transmitted into a layered periodic structure  1310  including a plurality of periodic grating structures (not shown) in an optical device  1308 . Each of the periodic grating structures has an effective area and a transitional area (not shown). Therefore, and as previously described, when the layered periodic structure  1310  is built using the periodic grating structures, the layered periodic structure  1310  will also have a working area  1316  including an effective area  1318  and a transitional area, as indicated by  1320   a  and  1320   b . In operation, the layered periodic structure  1310  splits the second beam of light  1306  into a first polarized beam of light  1312  and a second polarized beam of light  1314 . The light source and focusing device are configured such that they transmit the second beam of light  1306  into the effective area  1318  of the working area  1316  of the layered periodic structure  1310  without transmitting light into the transitional area, as indicated by  1320   a  and  1320   b , of the working area  1316 . 
     The present invention has been described above with respect to particular illustrative embodiments. It is understood that the present invention is not limited to the above-described embodiments and that various changes and modifications may be made by those skilled in the relevant art without departing from the spirit and scope of the invention.