Patent Publication Number: US-9423604-B2

Title: Subsurface glass reticles

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. application Ser. No. 12/707,638, entitled “Subsurface Glass Reticles” and filed on Feb. 17, 2010, the entire disclosure of which is hereby incorporated by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under Contract No. W15QKN-10-C-0002 awarded by the U.S. Army. The government has certain rights in the invention. 
    
    
     BACKGROUND OF THE DISCLOSURE 
     1. Field of the Disclosure 
     The disclosure relates generally to laser-based fabrication of subsurface objects in glass and, more particularly, to subsurface sight reticles. 
     2. Brief Description of Related Technology 
     Creation of a three-dimensional object inside a glass substrate (i.e. a subsurface object) is becoming a common fabrication procedure. Systems have been developed to produce three-dimensional objects in souvenir “crystals” or like items. A number of companies offer custom-made souvenirs by using a three-dimensional photograph as the basis for subsurface formation of an object capable of producing an image of the photograph for a person looking at the crystal. 
     The objects in these inexpensive commercial items are typically created using nanosecond laser pulses. The resulting quality of the designs and images varies greatly between vendors, but in all cases images of the objects are visible due to the presence of relatively large laser-created scattering centers. Light, diffusing from the surroundings or from a punctual source, is deflected by the scattering centers into the eyes of the person viewing the object. The image appears to be brighter than the rest of the glass block, because the unaltered glass surrounding the object does not scatter light towards the viewer. 
     Other subsurface objects have been created through the use of a mixed composition glass that changes its color when exposed to proper laser illumination. Some glasses form such “color centers” when exposed to the proper excitation. These color centers can block incoming visible light. This phenomenon has been used to create subsurface objects in a block of mixed glass. Unlike the previously described approach, images of these objects are visible because the incoming light is absorbed by the color center (rather than scattered). Unfortunately, color centers generally have a limited lifetime, especially when exposed to higher temperatures. While special glass treatments or glass compositions can reduce this short lifetime drawback, fused silica or fused quartz does not form significantly visible color centers. 
     Ultrashort laser pulses have also been used to create smaller scattering centers (relative to those obtained with nanosecond pulses) and, thus, create finer imagery. However, these smaller scattering centers are not as efficient at scattering light. As a result, ultrashort pulses are typically not used for artistic applications. Instead, ultrashort pulses are commonly used when it is desirable to generate a weak or concealed marking when, for example, discreetly marking an item can help to prevent counterfeit reproductions. 
     SUMMARY OF THE DISCLOSURE 
     In accordance with one aspect of the disclosure, an optical instrument for light incoming along a principal optical axis includes a glass block and a subsurface object within the glass block. The subsurface object includes an arrangement of object marks, each object mark including a plurality of scattering layers stacked against the principal optical axis of the incoming light. First and second scattering layers of the plurality of scattering layers have different polarization responses. 
     Each of the plurality of scattering layers may be configured such that each object mark exhibits no polarization dependence. Each scattering layer may include a set of parallel intrinsic scattering tracks, each intrinsic scattering track being oriented transverse to the principal optical axis and exhibiting a polarization dependence. 
     In some cases, the optical instrument further includes an array of intrinsic waveguides disposed within the glass block, the array being configured to diffuse any light not scattered by the plurality of scattering layers. 
     The subsurface object may be disposed along a non-flat surface within the glass block. Alternatively or additionally, the glass block may have a non-flat surface through which the light passes. 
     The optical instrument may further include a shell enclosing the glass block. The optical instrument may then further include a light source configured to introduce side-illumination within the shell and an illumination scattering layer disposed within the glass block and positioned to redirect the side-illumination from the light source along the principal optical axis. The illumination scattering layer may have a spatially defined pattern that corresponds with the arrangement of object marks. Alternatively or additionally, the illumination scattering layer includes a set of intrinsic scattering tracks, at least one of which includes discontinuities to reduce a density of the scattering layer. 
     The arrangement of object marks may form a reticle. 
     In accordance with another aspect of the disclosure, an optical instrument for light incoming along a principal optical axis includes an enclosure having an opening through which the incoming light passes to allow propagation along the principal optical axis, a glass block housed within the enclosure, and a subsurface reticle within the glass block. The subsurface reticle includes an arrangement of reticle marks, each reticle mark including an array of intrinsic waveguides. The array of intrinsic waveguides is configured to diffuse the incoming light from the principal optical axis. 
     Each intrinsic waveguide may be oriented in parallel with the principal optical axis. Alternatively or additionally, at least one of the waveguides may have a tip bent outward to direct the incoming light away from the principal optical axis. 
     Each reticle mark may further include a plurality of scattering layers, each scattering layer including an intrinsic region of modified refractive index oriented transverse to the principal optical axis. Each intrinsic region includes a void. First and second layers of the plurality of scattering layers may have different polarization responses. 
     The optical instrument may further include an optical element having a numerical aperture smaller than a numerical aperture of the array of intrinsic waveguides. 
     The subsurface reticle may be disposed along a non-flat surface within the glass block. Alternatively or additionally, the glass block may have a non-flat surface through which the light passes. 
     In accordance with yet another aspect of the disclosure, a method of fabricating a reticle for a telescope sight includes the steps of directing ultrashort laser pulses toward a glass block, and forming a subsurface object within the glass block, the subsurface object having a plurality of intrinsic scattering layers. The forming step includes selecting a polarization state of the ultrashort laser pulses to vary a polarization dependence of the subsurface object over the plurality of intrinsic scattering layers. 
     The forming step may include writing the first intrinsic scattering layer within the glass block with the ultrashort laser pulses in a first polarization state, and writing the second intrinsic scattering layer within the glass block with the ultrashort laser pulses in a second polarization state. Alternatively, the forming step may include writing a set of parallel tracks within each intrinsic scattering layer, and varying the polarization state of the ultrashort laser while writing of each track. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       For a more complete understanding of the disclosure, reference should be made to the following detailed description and accompanying drawing figures, in which like reference numerals identify like elements in the figures, and in which: 
         FIG. 1  is a schematic, perspective illustration of a subsurface object within a glass block with a viewer perceiving an image of the subsurface object through side scattering; 
         FIG. 2  is a schematic illustration of a telescope sight with a standard reticle but into which a reticle constructed in accordance with one or more aspects of the disclosure may be incorporated; 
         FIG. 3  is a schematic, perspective illustration of an arrangement of subsurface object marks formed within a glass block via an ultrafast laser fabrication process in accordance with one or more aspects of the disclosure for creation of an image embedded in glass through scattering from the object marks; 
         FIGS. 4A-4C  show the appearance of an exemplary subsurface reticle object under various illumination conditions when the reticle object is not fabricated or configured in accordance with the teachings of the disclosure; 
         FIG. 5A  is a schematic illustration of a subsurface object mark or scattering center in which multiple scattering layers are fabricated in a stacked formation, with each layer having the same polarization effect; 
         FIG. 5B  is a schematic illustration of an exemplary subsurface object mark or scattering center constructed in accordance with an embodiment in which multiple scattering layers are fabricated in a stacked formation, with respective layers having different polarization effects to scatter or divert the incoming light regardless of its polarization state; 
         FIG. 6  is a schematic illustration of another exemplary subsurface object mark constructed in accordance with an embodiment in which an array of waveguides is arranged to form at least part of a reticle pattern by diverting the incoming light; 
         FIG. 7  is a schematic illustration of yet another exemplary subsurface object mark constructed in accordance with an embodiment having both a stack of scattering layers and an array of waveguides to scatter or divert the incoming light; and, 
         FIG. 8  is a schematic illustration of an exemplary subsurface object mark or scattering center having a multilayered construction for polarization-insensitive operation and an illuminated layer configured for night operation. 
     
    
    
     While the disclosed reticles and optical instruments are susceptible of embodiments in various forms, there are illustrated in the drawing (and will hereafter be described) specific embodiments of the invention, with the understanding that the disclosure is intended to be illustrative, and is not intended to limit the invention to the specific embodiments described and illustrated herein. 
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     The invention generally relates to subsurface reticles and other objects in glass blocks for weapon sights and other optical instruments. Methods of fabricating the reticles in the glass blocks are also described. The disclosed reticles may be useful in a variety of contexts and applications, including, for instance, sights for advanced weapons and weapon systems. Notwithstanding the description of the disclosed reticles in connection with weapon sights, the disclosed reticles are not limited to weapon or weapon sight applications, and instead may be incorporated into a variety of other optical instruments, including, for instance, telescopes, photomasks, microfluidic chips, microscopes, etc. 
     Currently, weapon sight reticles are typically fabricated only on a flat glass surface via etch/fill or metal deposition techniques. When the flat surface resides in a focal plane, the surface accordingly produces backward reflections, increasing the possibility of detection of the optical instrument (or weapon) into which the reticle is incorporated. These signatures have been reduced by cementing a cover plate to the reticle substrate with index matching cement applied to the reticle substrate surface. Unfortunately, the index match is usually imperfect over the light spectrum due to a mismatch in the dispersion of the glass and the cement. 
     The disclosed reticles and fabrication methods avoid these and other complications by forming the reticle within the bulk of a glass substrate rather than etching or otherwise forming the reticle on the glass surface. In so doing, the disclosed subsurface reticles can provide reduced reflection signatures (i.e., lower possibility of instrument detection), reduced assembly requirements, reduced parallax, and improved optical system design flexibility. For example, the surface of the glass substrate no longer needs to be flat, and may instead be shaped to promote various optical effects, as desired. 
     Weapon sights present a number of unique challenges relative to the inexpensive souvenir items addressed above. With the souvenir items, the amount of light scattered and the direction of the scattering is a function of the incoming light polarization. This characteristic is usually, if not always, not a significant or severe issue because the surrounding light is generally mostly unpolarized. Moreover, a variance in visibility as a function of the incoming light polarization is not perceived as a significant problem for souvenir and other kinds of artistic applications. When applied to weapon sights, however, this polarization dependence can be especially troublesome if, for example, the weapon is used at dawn or dusk, or during other periods of polarized light. As described below, the disclosed reticles and fabrication techniques address this shortcoming to enable the weapon sight to function consistently as intended regardless of the polarization state of the incoming light. 
     Other challenges involve the circumstances in which a weapon sight reticle is used. Generally speaking, a weapon sight reticle is used in a specific, well-defined geometry. The reticle is inserted inside an optical instrument, such as a telescope or the like. The reticle is located within one of the focal planes of the optical instrument, as described and shown in connection with  FIG. 2 . The reticle is viewed within a limited numerical aperture. For daylight operations, the target is typically located in the far-field, on-axis with the weapon sight (i.e., there is a straight optical path going from the target, through the optical instrument, through the reticle, to the eye of the user (or to some type of detector (e.g., CCD, night vision amplifier, etc.). For night operations, the reticle may be illuminated from a small source associated with the optical instrument. The system may operate in the visible or at other wavelengths. These and other aspects of weapon sight reticles present challenges addressed by the subsurface glass reticles described herein. 
     As described below, the disclosed reticles may be formed in a variety of different glass materials, including fused silica (or fused quartz) and various mixed glass compositions that do not form significant amount of color centers when exposed to intense light. Color centers may be undesirable for weapon sight applications, insofar as the reticle or, more generally, the instrument may be subjected to elevated temperatures for extended periods, or may be stored for extended time (e.g., multiyear) periods. These conditions are likely to fully or partially erase reticles formed by color centers. Furthermore, some applications involve use of the weapon sight at night, and visibility of a reticle at night may be difficult to achieve with color center-based markings. 
     With reference to the drawing figures,  FIG. 1  depicts an optical device  100  in which a number of subsurface objects or structures  101  have been formed in a block of glass  102 . The glass block  102  may be a monolithic block of, for instance, fused silica. The size, shape, and material composition of the glass block  102  may vary considerably. The glass block  102  may constitute only one component of the optical instrument  100  through which incoming light  105  passes. Each subsurface object  101  is configured to act as a respective scattering center for the incoming light  105 . Collectively, the subsurface objects  101  form a visible image that appears embedded in the glass block  102 . 
     A ray of the incoming light  105  is shown to depict how the subsurface objects  101  act as scattering centers within the glass block  102 . The incoming light ray  105  illuminates at least two of the subsurface objects  101  in this case. As a result, multiple rays  106  are scattered away toward an observer  107 . In this way, some of the subsurface objects  101  cause an image of the ray of the incoming light  105  to appear to the observer  107  to be embedded in the glass block  102 . The observer  107  may be an individual using the optical device  100  or an optical detector or other device configured and positioned to collect the scattering light rays  106 . In some cases, a portion of the incoming light  105  is not scattered by one of the subsurface objects  101 , in which case a ray  108  may pass through the glass block  102  as shown without any scattering or diversion. If the scattering fails to occur, then the observer  107  does not see or detect the corresponding subsurface object  101 . Several aspects of the disclosure are directed to ensuring that the subsurface objects  101  cause such scattering or diversion to indeed occur so that an image of the incoming light (or lack thereof) is discernable by a user of a weapon sight. However, as described below, in the case of a reticle, the observer is positioned such that the scattering diverts the incoming light from reaching the observer. As a result, a darker image is created that appears to the observer as an absence of incoming light from the reticle. 
     The disclosed fabrication methods generally use ultrashort laser pulses to create the subsurface objects  101 , which are shown generically in  FIG. 1  as star-shaped objects or marks for ease in illustration. The outline or shape of each object or mark, or of any collection or arrangement of objects or marks may vary as desired. Ultrashort laser pulses are capable of creating objects or marks of any desired shape with finer details than those made possible with other types of lasers, such as nanosecond lasers. The ultrashort laser pulses can create well-defined features, with sharp edges, which may be useful for most weapon sight applications of the disclosed reticles. 
     The ultrashort pulses interact with the glass block  102  via non-linear absorption to form or embed the objects  101  within the glass block  102 . Despite the embedded or subsurface nature of the objects  101 , the glass block  102  constitutes a single, monolithic structure. Each object  101  may be written in the bulk of the glass block  102  as one or more localized regions of modified refractive index. The index modification may involve a change in the index of the glass material, or a change resulting from the discontinuity of a transition between the glass material and a void (e.g., a micro-crack or micro-bubble). The shape, content (e.g., void or glass), and other characteristics of the region(s) determine the optical functionality of the object  101 . In some cases, the regions may be formed to act as scattering centers, while in other cases the regions may be formed to act as a waveguides. Either way, the refractive index modifications may be obtained via the direct-write, non-linear absorption processing with ultrafast pulsed, or femtosecond, lasers (e.g., lasers with pulses below 1 picosecond in duration). In some cases, the index modifications underlying the formation of scattering centers and other intrinsic objects described herein may be created using lasers with pulses in the picosecond range. Further details regarding index change modifications in the context of waveguide formation are set forth in U.S. Pat. Nos. 6,768,850 (“Method of index trimming a waveguide and apparatus formed of same”), 7,294,454 (“Waveguide fabrication methods and devices”), and 7,391,947 (“Waveguide fabrication methods and devices”), the disclosures of which are hereby incorporated by reference. Lasers suitable for generating ultrashort pulses are generally available from a number of companies and other sources, including Coherent, Inc. (Santa Clara, Calif.) under the trade name “RegA” and Spectra Physics (Mountain View, Calif.) under the trade name “Tsunami.” Such ultrafast pulsed lasers can typically achieve pulse widths of less than 100 femtoseconds. Direct-writing within the glass block  102  of glass regions of modified refractive index may be achieved with pulse energies of on the order of nano-Joules to several micro-Joules depending on the pulse widths as well as focusing optics, while slightly higher energies (e.g., on the order of a few micro-Joules) are used to create void-based index change regions. 
     The intrinsic region(s) of modified refractive index that make up each object  101  may be configured to form a waveguide segment or track. That is, the shape of the region(s), either individually or collectively, may result in a guiding of the incoming light  105  to divert the light away from the observer. In other cases, a scattering center is instead created to divert the light from its incoming path. Examples of both types of intrinsic objects are described below. 
     The objects or structures formed within the glass block  102  may be referred to herein as “intrinsic” to distinguish the structures from optical elements that are formed via doping (e.g., ion implantation, diffusion), material removal (e.g., ablation to create a channel waveguide), material deposition (e.g., epitaxial layering), or other introduction or addition of extrinsic material. While these fabrication processes may also result in a monolithic structure like the glass block  102 , the monolithic nature of the block  102  differs in that the substance or material of each intrinsic waveguide has a common chemical composition with the remainder of the bulk in which the structure is formed. Indeed, the glass block  102  may have a uniform chemical composition, such that the surrounding bulk of the block  102  has the same chemical composition as the regions within the block  102  that correspond with the waveguides. That is, each region within the block  102  other than a void has a common, uniform chemical composition, such that the refractive index is the defining factor between the intrinsic objects or structures and the surrounding bulk of the block  102 . In some cases, the composition of the glass block  102  may deviate from pure glass. In such cases, however, the composition may remain uniform throughout the bulk of the block  102 , which includes compositions that are effectively uniform, in the sense that any gradient in the composition profile is either unintended, ancillary to, or not determinative or definitive of any photonic characteristics of a device or component of the block  102 . Thus, the term “glass” is used herein broadly to include SiO2 without any impurities, as well as fused silica or other glasses that may have trace or larger amounts of impurities. 
     The intrinsic, embedded disposition of the objects in the glass block  102  make the disclosed reticles and optical instruments well suited for application and operation in demanding or harsh environments and other field uses. For instance, the interior disposition avoids any complications arising from surface exposure to the environment or from manufacturing techniques (e.g., bonding) used to avoid exposure. Moreover, using fused silica or other glass as the material of both the substrate and the subsurface objects formed therein provides several advantages, including high thermal and vibration stability. 
       FIG. 2  is a schematic illustration of an exemplary telescope sight  201  with an optical axis  202 . Incoming light  205  enters through an objective assembly  206 , and is then focused with, e.g., a defined numerical aperture  207 , onto a first focal plane  211 . The focal plane  211  may contain an aperture  212 . The image of the incoming light  205  may be further relayed through intermediary optics  215  onto a second focal plane  216  that contains a reticle. In this illustration, the reticle is schematically shown as being composed of two substrates or plates  221  and  222 , as is often the case with existing reticles. The reticle pattern is located at the common interface  223  between the two plates  221 ,  222 . The light is then relayed through an ocular device or eyepiece  231  toward the observer. A non-transparent telescope shell or enclosure  241  prevents side-illumination  242  from reaching the reticle or otherwise being redirected toward the observer, but has an opening through which the incoming light  205  passes to propagate along the axis  202 . Each of the optical instruments disclosed herein may have a shell or other enclosure that houses the optical elements of the instruments and helps to control the light that reaches the observer or user. 
     Described below are a number of reticles fabricated and configured in accordance with the disclosure and well-suited for incorporation into the telescope sight  201  in substitution of the two plates  221 ,  222 . 
       FIG. 3  depicts an exemplary arrangement of reticle or other object marks  301  in a block of glass  302 . In this case, each object mark  301  acts as a scattering center. The scattering centers  301  are generally arranged in a pattern or signature to form the shape of a reticle. In this example, the reticle pattern is a cross shape. Incoming light beams  305  from a distant object  306  are prevented from reaching an observer  307  due to scattering or diversion by the scattering centers  301 . The scattering centers  301  illuminated by the incoming light  305  deflect light beams  303  away form the observer, causing the observer to see a non-illuminated (e.g., dark) image of the scattering centers  301  in the shape of the reticle. 
       FIG. 4A  depicts an exemplary reticle pattern  400  that may be formed by an arrangement of scattering centers or other object marks in accordance with the disclosure. The pattern  400  is also well suited for exhibiting how the nature of the incoming light for a sight reticle can obscure portions of the pattern  400 . In this example, the reticle pattern  400  has an outside circle  401  and pairs of vertical cross lines  402  and horizontal lines  403 . In  FIG. 4A , the reticle pattern  400  is shown if the reticle is fabricated and configured in accordance with the disclosure. More generally, the reticle pattern  400  may also appear as shown in  FIG. 4A  if the instrument is illuminated by unpolarized light. As described below, correct images of the disclosed reticles are generated regardless of the polarization state of the incoming light. 
       FIGS. 4B and 4C  depict examples of incorrect images of the reticle pattern of  FIG. 4A  as a result of polarized light illuminating a reticle not fabricated or configured in accordance with the disclosure. In  FIG. 4B , the reticle appears as an image pattern  410  in which the visibility of vertical lines  411  is dramatically reduced, if not eliminated. Portions  412  of the outside circle are also partially non-visible. In  FIG. 4C , the same reticle appears as an image pattern  420  when illuminated by a different polarized light (e.g., polarization state S versus polarization state P). Under this illumination condition, different portions of the outside circle are lost (relative to the image of  FIG. 4B ), and the visibility of horizontal lines  421  is reduced or eliminated, while vertical lines  422  are clearly visible. 
     In order to avoid such incorrect images of the reticle pattern, the disclosed reticles are configured to ensure or increase the amount of light scattered or diverted by the subsurface objects or marks created by the writing laser. Substantial or sufficient scattering or diversion occurs regardless of the extent to which the incoming light is polarized, and regardless of the polarization state of the incoming light. The scattering or diversion also occurs despite the fine nature of the objects resulting from the writing laser. 
     In accordance with one aspect of the disclosure, the fabrication process includes writing or creating multiple scattering layers stacked against the incoming light. Thus, in some cases, each subsurface mark or object includes a plurality of scattering layers, each of which is oriented transverse or perpendicular to the incoming light for successive illumination. A set or arrangement of such multi-layered marks may then be used to form a desired reticle pattern. The set or arrangement may include any number of densely packed marks or objects, each of which acts as a scattering center to divert the light from an observer. As described below, the density of each scattering center may be adjusted (e.g., increased) by forming each scattering layer of a mark from an array or group of closely spaced, side-by-side (i.e., parallel) intrinsic traces or tracks of modified refractive index. The spacing between the traces or tracks may be decreased to improve the density of each scattering layer (or center), ranging, for instance, from about 0.5 microns to about 1.5 microns. The density of each scattering center may also be adjusted by introducing discontinuities or breaks along the length of each intrinsic trace or track. 
       FIG. 5A  depicts one example of a simplified reticle  500  illustrated in schematic form to highlight the multi-layer and track-based characteristics of this aspect of the disclosure. The reticle  500  includes a number of traces or tracks  501  intrinsically formed within a glass block and arranged in one of four layers  502 . Each trace or track  501  may correspond with a region of modified refractive index, e.g., a micro-crack or other void, within the glass block that acts as a scattering center as described above in connection with  FIG. 1 . Each trace or track  501 , and thus, each layer  502 , may be oriented transverse or perpendicular to incoming light  503  to promote deflection or other scattering or diversion from the main optical path of the incoming light  503 . Each layer  502  may include a desired number of the traces or tracks  501  disposed side-by-side in parallel to obtain a desired density or depth for the layer  502 . Each layer  502  is schematically shown in  FIG. 5A  with cross-hatching, which may be indicative of an operational characteristic of the group of traces or tracks  501 . The illustration of the reticle  500  is also schematically simplified in the sense that only a single scattering center is shown (rather than the entire reticle pattern or signature). Further scattering centers may be disposed at either side of, above, or below, the scattering center shown. Moreover, while the scattering center has a depth of four scattering layers relative to the incoming light  503 , the depth of the scattering center may vary from the example shown. The spacing between adjacent traces  501  may be adjusted, and the number of layers  502  may also may vary considerably to a select a desired depth. The other dimensions of the scattering center generally correspond with the desired dimensions of the reticle or object mark. 
     Despite the depth and density of the reticle  500  arising from the multiplicity of traces  501  in each layer  502 , and the stacking of the layers  502 , the reticle  500  may not operate as intended under certain operational conditions. As a default, each track  501  has a polarization dependence that is a function of the writing laser beam polarization. If the incoming light  503  is fully or partially polarized, and each layer  502  has the same polarization response or effect on the incoming light  503 , then a portion  504  of the incoming light  503  will not be strongly scattered by the any of layers  502 . As shown, the portion  504  has a first polarization state that is allowed to travel through the scattering center to reach an observer. As a result, the reticle  500  is not visible to the observer under that condition. In contrast, the polarization response or effect of each layer  502  does not allow incoming light  505  of a different polarization to pass through, deflecting or otherwise diverting it from the observer as scattered light  506 . Thus, an image of the reticle  500  is visible to the observer if all of the incoming light has the polarization state of the light  505 , but is not necessarily visible (or only partially visible) if the incoming light is otherwise polarized. Each of the layers  502  is schematically depicted as having the same polarization response via cross-hatching angled in the same direction. 
       FIG. 5B  depicts an exemplary reticle  510  having at least two different scattering layer configurations, schematically indicated via different cross-hatching. While the reticle  510  is similar in many respects (total number of layers, track spacing, etc.) to the reticle  500  ( FIG. 5A ), the scattering layers of the reticle  510  do not all have the same polarization response or effect on the incoming light. As described below, the polarization of the writing laser is varied to eliminate the polarization dependence that would otherwise result for the scattering center or reticle. In this example, a scattering layer  511  has a different polarization effect than a scattering layer  512 . As a result, incoming polarized light  513  and incoming light  514  of a different polarization state than the light  513  are both strongly deflected or otherwise diverted into scattered light  515  by the reticle  510 . The other scattering layers of the scattering center may provide yet further polarization response differentiation. Thus, in some cases, the scattering layers may present any desired number of different scattering layer configurations. 
     In accordance with one aspect of the disclosure, the variance in polarization responses is the result of selecting or controlling the polarization of the ultrashort laser pulses while writing the traces or tracks of the scattering layer. For example, the polarization of the writing laser is set to a first polarization state (e.g., P-polarization) while the traces of the scattering layer  511  are being written, and then set to a second polarization state (e.g., S-polarization) while the traces of the scattering layer  512  are being written. Generally speaking, the polarization of the writing laser causes the region of modified refractive index to exhibit a polarization preference. The writing laser polarization may create sub-wavelength or grating-like patterns in the scattering centers, the orientation of which is a function of the writing laser polarization. In this way, the writing laser polarization is selected and varied to control the scattering polarization dependence (or effect) of each layer of the scattering center and, as a result, eliminate the polarization dependence of the scattering center(s). 
     The polarization dependence of each scattering center can be eliminated or dramatically reduced if the writing laser polarization is varied over the course of fabricating the multi-layer structure. In the example of  FIG. 5B , one trace or layer is written with one polarization, and the next trace or layer is written with an alternate polarization. While the multiple layers generally provide an opportunity to vary the writing polarization and, thus, the polarization dependence of the scattering center, the polarization need not be varied solely on a layer-by-layer, or trace-to-trace, basis. Alternatively or additionally, the polarization of the writing laser may be modified within a single trace. In each case, the polarization may be modified in a continuous manner, in discrete, regular steps, or randomly. 
     Varying the writing laser polarization may avoid or control the formation of thick gratings, which can be formed when maintaining a well-defined writing polarization. The formation of gratings may be highly undesirable, insofar as the grating will diffract different incoming wavelengths into different directions with different efficiencies. Varying the spacing between the traces may also help to avoid such ineffective scattering. However, such wavelength-sensitive scattering or diffraction may be useful in some applications. 
     Each intrinsic trace or track of the scattering centers of  FIGS. 5A and 5B  may be generally configured as an intrinsic region of modified refractive index to act as an individual scattering center to divert the incoming light from the main optical axis of the optical instrument. Alternatively, the set of side-by-side or parallel traces or tracks of each scattering layer may be considered as a single scattering center with the set of traces acting collectively. 
       FIG. 6  shows another example of an object mark that diverts the incoming light from the principal optical axis. In this case, the intrinsic regions of modified refractive index are aligned with the optical axis to encourage the incoming light to be captured and guided by a waveguide. Rather than use a scattering center to divert light, the waveguide array in this case is configured to diffuse light away from an observer to form an image of a reticle or other object pattern. In this example, incoming light  601  enters an instrument  602  with a main optical axis  603  and an objective assembly or other optical element having a small numerical aperture  604 . The incoming light  601  gives rise to an optical signal that propagates through the instrument  602  until it reaches one or more arrays  610  of intrinsic waveguides  611 , each of which has a numerical aperture  612  larger than the numerical aperture of the instrument  602 . Each waveguide array  610  may form a portion of the object pattern. Taken together, the collection of waveguide arrays  610  are arranged to present the desired reticle shape. To these ends, the input end of each waveguide is disposed at a respective position in the object pattern. The waveguide arrays  610  may then be arranged side-by-side to provide a desired object dimension (height or width). 
     Each waveguide  611  has a main propagation axis oriented in parallel with the principal optical axis of the incoming light and, thus, with the axes of the other waveguides  611 , which together generally run side-by-side from the input end to an output end. The light exits each waveguide array  610  at the output end in a broader pattern  613  representative of the numerical aperture  612  of the waveguide array  610 . Thus, the direct, on-axis intensity is reduced when the incoming light  601  passes through the waveguide array  610 . As a result, an image of the object pattern is visible to an observer. This effect can be reinforced if a tip  621  at the output end of some or all of the waveguides  611  in the arrays  610  is bent outward, or away from the main optical axis  603 , thereby guiding and further redirecting light away from the main optical axis  603 . To insure that the outgoing light diverges strongly, the waveguides  611  may be multimode, and the waveguides  611  may be configured to scramble the modes via, for instance, changing the cross-sectional shape or size of the waveguide, and controlling the symmetry of the waveguide. 
     In order to further enhance the visibility or contrast of the reticle (or other object), an optical instrument may include both the waveguide array(s) and an arrangement of multilayered, high-density, scattering centers to divert the incoming light. In this approach, the optical waveguide function is to diffuse or direct away from the viewer any light that may not have been scattered away by the multilayered, high-density, scattering centers. The optical waveguides array(s) may also be shaped in the form of the desired reticle or other object pattern. 
     Turning to  FIG. 7 , an example of the combination of scattering centers and waveguide diffusers is shown. An optical instrument  701  with an optical axis  702  includes an intrinsic multilayered scattering structure  711  and multiple layers of arrays  712  of intrinsic waveguides. Incoming light  721  is mostly scattered away by the scattering centers of the structure  711 . Any remaining light  722  still propagating along the optical axis  702  is then diffused and further weakened by the waveguide array(s)  712 . 
     With reference to  FIG. 8 , one or more of the scattering layers may be configured for sight illumination to support night operation of an optical instrument. During night operation, the surrounding light is very weak. For example, an object to be viewed through a weapon sight may be visible (likely through an enhanced night vision technique), but there may not be sufficient light to see the reticle. In this case, a small light source may be integrated within the optical instrument and used for side-illumination of the reticle. To this end, one or more layers of a reticle may be configured to deflect a desired amount of the light from the light source toward the observer. In some cases, the layer(s) may be configured to deflect only a portion of the light from the light source, insofar as too much light may be just as detrimental to the use of the optical instrument as too little light. One way in which partial deflection is achieved is via discontinuities in the scattering centers, e.g., breaks in the transverse traces or tracks of the scatter layers. The layer(s) may alternatively or additionally have a spatially defined pattern scattering the side emitted light toward the observer that corresponds with the pattern or shape of the reticle. Alternatively, the layer(s) may introduce the light more comprehensively along the principal optical axis, in which case the object marks of the reticle divert the light as described above to provide a contrasting image of the reticle pattern. More generally, the layer(s) may be integrated into a stack of scattering layers of a reticle, optimized for side illumination, and generally disposed within or near the other layers forming the reticle. 
     The exemplary instrument shown in  FIG. 8  has a reticle  801  with multiple layers  802  configured to scatter incoming light  803  in accordance with the reticle pattern and without polarization dependence as described above. A layer  811  also includes a set of intrinsic objects, each of which is configured to scatter light from a side source  812  toward an observer  813 . The layer  811  may be designed to scatter light throughout uniformly through the layer, or only at well-defined location(s). In the latter case, the set of intrinsic objects may be arranged in accordance with the reticle pattern. 
     As shown in the examples of  FIGS. 7 and 8 , the stacks of scattering layers, waveguide arrays, and other intrinsic elements within the glass block need not be contiguous. Layers may be separated from an adjacent layer to a desired extent, either by unmodified bulk glass or an intervening optical element. However, the layers and other intrinsic elements may be positioned in close proximity so as to remain within the same focal plane of the optical instrument. 
     Described above are optical instruments and fabrication methods in which a reticle or other object pattern is formed by creating small, dense, scattering sites in a block of glass material. The glass material may be fused silica, fused quartz, or any other glass showing no significant color centers formation when exposed to intense light. The reticle object may include many scattering layers to ensure that the image of the reticle is visible to an observer (e.g., dark rather than light). Because the reticle is designed to be viewed on axis, the scattering layers include sets or arrays of intrinsic objects configured to block on-axis light. The reticle may be included within an optical instrument with a small numerical aperture. Because the scattering associated with each layer may exhibit polarization dependence, the scattering resulting from a combination of multiple layers exhibits no polarization dependence and, thus, effective scattering for all operational conditions. 
     As described above, each scattering layer may include multiple parallel tracks. Even though each scattering track may have a polarization response, the overall scattering center is configured such that no polarization dependence is exhibited. Each layer of tracks is fabricated using a femtosecond laser to create intrinsic objects within the glass block. The polarization of the laser is then selected to control the scattering polarization dependence of each layer or track. 
     As an alternative or additional approach to diverting the on-axis light, the disclosed reticles may use an array of waveguides to diffuse the light away from the optical instrument principal axis. 
     As described above, one or more scattering layers may be configured to optimize side illumination for night operation. The layer(s) may be formed within or near the other layers forming the reticle. Alternatively or additionally, the layer(s) may have a spatially defined pattern scattering the side emitted light towards the user. 
     In each of the foregoing embodiments, the scattering layers and other intrinsic objects do not have to be flat. One or more layers of each optical instrument may be written in or along a non-flat surface in order to compensate for optical aberrations present in the optical instrument, or more generally to introduce one or more desired optical effects. Similarly, each waveguide array or layer need not present a flat front surface. In some cases, the input ends of the waveguides in the array may instead present a curved front surface for the incoming light. 
     With the ultrafast laser writing technique of the disclosed methods, the scattering layers and other intrinsic objects need not be written in substrates having flat outer surfaces. The scattering centers, waveguide array diffusers, and other intrinsic elements may be written in non-flat glass volumes, such as a spherical lens, a cylindrical lens, a prism, etc. 
     While the present invention has been described with reference to specific examples, which are intended to be illustrative only and not to be limiting of the invention, it will be apparent to those of ordinary skill in the art that changes, additions and/or deletions may be made to the disclosed embodiments without departing from the spirit and scope of the invention. 
     The foregoing description is given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications within the scope of the invention may be apparent to those having ordinary skill in the art.