# Patent

## Title
Optical identification element having a non-waveguide substrate

## Abstract

An optical identification element including a non-waveguide optical substrate. The optical substrate has a volume and includes an inner region surrounded by an outer region. The inner region has an index of refraction that prevents the optical substrate from forming an optical waveguide. The optical substrate includes a diffraction grating within the volume, and the grating provides an output signal indicative of a code when illuminated by an incident light.

## Background
CROSS REFERENCES TO RELATED APPLICATIONS
This application is a continuation of and claims benefit to U.S. patent application Ser. No. 11/063,660, filed on Feb. 22, 2005, which claims benefit to U.S. Provisional Patent Application No. 60/546,445, filed on Feb. 19, 2004, all of which are incorporated by reference in their entirety.


BACKGROUND OF INVENTION
1. Technical Field
The present invention relates to an identification element, and more particularly to an optical identification element having a diffraction grating written therein.
2. Description of Related Art
Many industries have a need for uniquely identifiable objects or for the ability to uniquely identify objects, for sorting, tracking, and/or identification/tagging. Existing technologies, such as bar codes, electronic microchips/transponders, radio-frequency identification (RFID), and fluorescence and other optical techniques, are often inadequate. For example, existing technologies may be too large for certain applications, may not provide enough different codes, or cannot withstand harsh temperature, chemical, nuclear and/or electromagnetic environments. Therefore, it would be desirable to obtain a coding element or platform that provides the capability of providing many codes (e.g., greater than 1 million codes), that can be made very small, and/or that can withstand harsh environments.
SUMMARY OF INVENTION
In its broadest sense, the present invention provides a new and unique optical identification element (also known herein as a microbead) made from pieces of an optical fiber or substrate that includes an inner core or region being surrounded by an outer cladding region, the optical fiber or substrate having an identification code imparted therein containing coded information. The identification code may be in the form of a Bragg grating inscribed or written in either the inner core or outer cladding.
The optical identification element may be microscopic in size having a length in a range of 1-1,000 microns or smaller; or for larger applications may have a length of 1.0-1,000 millimeters or more. The outer diameter may be as small as less than 1,000 microns, as well as in a range of 1.0 to 1,000 millimeters for larger applications. Using manufacturing techniques developed in conjunction with the development of the present invention, one optical fiber or substrate can be drawn and processed to produce hundreds of thousands, as well as even a million or more of such unique microbeads.
Objects of the present invention include encoded micro-particles that are coated with a substance for multiplexed experiments, which are very small, capable of providing a large number of unique codes, and/or are resistant to harsh environments.
According to the present invention, an optical identification element having a chemical attached thereto comprises a substrate; at least a portion of said substrate being made of a substantially single material and having at least one diffraction grating embedded therein, said grating having a resultant refractive index variation within the substantially single material at a grating location; and said grating providing an output optical signal indicative of a code when illuminated by an incident light signal propagating from outside the substrate, said output optical signal being a result of passive, non-resonant scattering from said grating when illuminated by said incident light signal; and the chemical being attached to at least a portion of said substrate.
The invention is a significant improvement over chip based assay and existing bead assay technology, as discussed above.
The microbeads are inexpensive to manufacture and the identification codes are easy and inexpensive to imprint into the microbeads. The codes are digitally readable and easily adapted to optical coding techniques. Thus, the optical readout is very simple and inexpensive to implement. The code is not affected by spot imperfections, scratches, cracks or breaks. In addition, splitting or slicing an element axially produces more elements with the same code; therefore, when a bead is axially split-up, the code is not lost, but instead replicated in each piece. Unlike electronic ID elements, the elements of the present invention are not affected by nuclear or electromagnetic radiation.
The invention may be used in any assay or multiplexed experiment. The assay may be reused or disposed upon completion of the assay.
The present invention may be used with any known combinatorial chemistry or biochemistry assay process, and are especially adaptable to assays having solid phase immobilization. The invention may be used in many areas such as drug discovery, functionalized substrates, biology, proteomics, combinatorial chemistry, and any assays or multiplexed experiments. Examples of common assays are SNP (single nucleotide polymorphism) detection, DNA/genomic sequence analysis, genotyping, gene expression assays, proteomics assay, peptide assays, antigen/antibody assays (immunoassay), ligand/receptor assays, DNA analysis/tracking/sorting/tagging, as well as tagging of molecules, biological particles, cell identification and sorting, matrix support materials, receptor binding assays, scintillation proximity assays, radioactive or non-radioactive proximity assays, and other assays, high throughput drug/genome screening, and/or massively parallel assay applications. The analyte can be labeled, detected or identified with any technique capable of being used in an assay with arrays or beads, including but not limited to fluorescent, luminescent, phosphorescent, quantum dot, light scattering colloidal particles, radioactive isotopes, mass spectroscopy, NMR (nuclear magnetic resonance), EPR (electro paramagnetic resonance), ESR (electron spin resonance), IR (infrared), FTIR (Fourier transform infra red), Raman spectroscopy, or other magenetic, vibrational, electromagnetic, or optical labeling or detection techniques. The invention provides uniquely identifiable beads with reaction supports by active coatings for reaction tracking to perform multiplexed experiments. The invention may also be used in any chemical and/or biochemical purification, isolation, or filtering-type process where bead or bead-like solid supports may be used (e.g., chromatographic techniques, such as affinity column purification). In that case, the above techniques for labeling, detection or identification may be used.
Microbead Using Conventional Waveguide Technology
In one embodiment, the optical identification element may be manufactured from a conventional waveguide used in the telecommunications industry in which the refractive index of the core is higher than the cladding using, for example, the technique shown and described in patent application Ser. No. 10/661,234, which is hereby incorporated by reference, as well as other techniques described below. In this case, the Bragg grating is written in the core of the conventional waveguide.
In this embodiment, conventional waveguides are used that are known in the art and used in the telecommunications industry, which are made from optical fiber (e.g. 125 micron optical fiber). Conventional fiber Bragg gratings are primarily formed in single mode fibers and are used for coupling forward propagating modes into backward propagating modes. The coupled modes are confined to propagate in the core region of the fiber along its axis; such a constraint defines a waveguide. If only one mode is allowed to propagate then the fiber is called a single mode waveguide, if two or more modes are permitted to propagate the fiber is referred to as a multimode waveguide. The key to the function of guiding any number of modes in a fiber is the existence of both a core region and a cladding region, where the refractive index of the core is higher than the cladding. Conditions for single mode propagation are met when the V number of the fiber is less than 2.405, larger values will result in more than one mode. The V number is related to the geometry and refractive indices of the core and clad by the following relationship:



  
    
      V
      ⁢
      
        :
      
    
    =
    
      
    
    ⁢
    
      
        2
        ⁢
        
          π
          ·
          a
          ·
          NA
        
      
      
        λ
        ⁢
        
          
        
        ⁢
        c
      
    
  




  
    
      NA
      ⁢
      
        :
      
    
    ⁢
    
      
    
    =
    
      
    
    ⁢
    
      
        
          n
          ⁢
          
            
          
          ⁢
          
            1
            2
          
        
        -
        
          n
          ⁢
          
            
          
          ⁢
          
            2
            2
          
        
      
    
  



where n1 and n2 are the refractive indices of the core and clad respectively. Practical single mode fibers are restricted to NA's in the range of 0.05 to 0.3. Fibers on the high end of the NA range have extremely small core diameters, ranging from 1 to 3 microns, while fibers on the low end of the range have larger cores, but their sensitivity to bend loss increases substantially. As the NA approaches zero, the fiber behaves less and less like a waveguide, which is inconsistent with the stringent demands of the telecommunications industry.

The broad list of devices and applications of fiber Bragg gratings has thus far been restricted to operation in single or few moded fibers; these include band pass optical filters, strain sensors, dispersion compensators, and gain flattening filters. In addition to the technique shown and described in patent application Ser. No. 10/661,234, the following list includes United States patents disclosing techniques for forming Bragg gratings in a conventional telecommunications waveguide:
1. U.S. Pat. No. 5,367,588—Method of fabricating Bragg gratings using a silica glass phase grating mask and mask used by same, by Ken Hill;
2. U.S. Pat. No. 5,327,515—Method for forming a Bragg grating in an optical medium, by Dana Anderson; and
3. U.S. Pat. No. 5,351,321—Bragg grating made in optical waveguide, by Elias Snitzer, which are all hereby incorporated by reference.
Microbead Using a New and Unique Optical Substrate
However, using such a conventional telecommunications waveguide to make such optical identification elements may be expensive to manufacture because the manufacturing techniques for making conventional telecommunications waveguides involve drawing optical fiber from a preform under strict predefined optical conditions so as to produce optical fiber having strict predetermined optical characteristics. Moreover, using such a conventional telecommunications waveguide to make such optical identification elements may also be expensive to manufacture because the manufacturing techniques for making such conventional telecommunications waveguides involving writing strong gratings in the optical fiber with grating writing techniques requiring very precise and expensive lasers in order to meet the demands of the telecommunications industry. In view of this, the inventor has also developed an alternative optical identification element in which a substrate is used such as an optical substrate having the refractive index of the core less than or equal to the cladding, that has important advantages in that it is less expensive to manufacture than when using the conventional waveguide technology otherwise used in the telecommunications industry.
In this alternative embodiment, the optical identification element is manufactured using, for example, the technique shown and described in patent application Ser. No. 10/661,234, as well as other techniques described herein.
Since in a typical usage, the optical identification element is interrogated from the side in order to read the coded information contained in the Bragg grating, the Bragg grating may be written not only in the core, but also in the cladding.
The microbeads in the alternative embodiment can be manufactured using the same process for inscribing Bragg gratings as those described in some of the above patents. Moreover, due to fact that the beads are interrogated from the side, it is not necessary that the optical substrate be manufactured to perform as a conventional waveguide. It is also well known that the incorporation of Boron as a dopant enhances the photosensitivity of the optical substrate to UV radiation. Boron is also known as an index depressor when it is incorporated into silica glass. When designing a single mode waveguide the amount of Boron is usually very carefully balanced with Germanium to provide the correct index profile in conjunction with enhanced photosensitivity. Again, because the requirement for waveguiding is removed for the microbead applications, excess amounts of Boron can be incorporated into the glass to increase its photosensitivity without regard to its optical guiding properties. This has the benefit of reducing the cost of manufacturing the optical substrate when compared to manufacturing the conventional waveguide, and increasing the photosensitivity without concern for tradeoffs involving other waveguiding issues.
When an optical substrate does not have to perform as a conventional waveguide, the core index of the optical substrate may be made lower than the cladding index. Under these conditions, the above equation demonstrate that the NA is imaginary, and thus the optical substrate is not considered a waveguide.
In the alternative embodiment, the optical identification element is a non-waveguide optical substrate having the Bragg grating inscribed therein. The optical identification element structure consists of a photosensitive region in the geometric center of the element surrounded by a non-photosensitive and non-absorbing region. The photosensitive region has an index of refraction less than or equal to the surrounding region, thus preventing the element from supporting any modes. The photosensitive region is also designed to produce the appropriate Bragg envelope when interrogated from the side. Bragg envelopes in the range of 1 to 10 degrees are desirable for most microbead applications. In order to achieve such an envelope with visible light, the diameter of the photosensitive region must be no larger than 30 um, and for most applications the ideal size is approximately 12 um. Due to this small size and the practical issues that would arise from fabricating and handing such a small optical identification element, it is convenient to include an outer region, which is neither photosensitive nor absorbing, which has an outer diameter between 50-130 um. The optical identification element had a core diameter of 24 um when the outer diameter was 125 um, and a core diameter of 12 um when it was drawn to 65 um in diameter. The index of the core region was −0.003, thus ensuring the optical identification element would not guide a mode.
The New and Unique Optical Substrate Specification
The new and unique optical substrate may take the form of a photosensitive non-waveguide optical fiber consisting of two sections, an outer section and at least one inner section. The outer section (or cladding) is made entirely of fused silica, without any dopants; however, trace amounts of impurities commonly found in fused silica or natural fused quartz are acceptable. The inner section (or core) is made of Germanium and Boron doped fused silica. The exact constituents of Boron and Germanium are determined based on the desired refractive index profile (RIP). In one example, the photosensitive non-waveguide optical fiber may have an outer diameter of 28 um+/−1 um, and the inner section diameter of 8 um+/−0.5 um. The scope of the invention is intended to include using other dimensions as well.
The photosensitive non-waveguide optical fiber may be made by drawing a glass preform on a known fiber draw tower. Also, the photosensitive non-waveguide optical fiber may be drawn with an outer coating or buffer layer to protect the fiber during handling, e.g., a polymer based coating or other coating.
The RIP of the preform is used to calculate the following parameters of the photosensitive non-waveguide optical fiber as follows:

    
    
        Fiber Core Diameter=(Fiber Outer Diameter)/Ratio,
        where Ratio=(Preform Outer Diameter)/(Preform Core Diameter); the Ratio stays the same for the Preform and the Fiber.

Measurements are typically taken along the length of the preform in intervals of about 1 cm; however, other intervals may be used if desired. The Delta refractive index (or Delta Index) between the outer cladding and the inner core is typically defined as the difference: Clad index−Core index. In one example, the Delta Index may be greater than 0.001 as measured from the preform RIP (and as will also exist in the fiber). Other values for the Delta Index may be used if desired.

    
    


The elemental (or dopant) constituents (Germanium and Boron dopants) in the core may be about 20 mole % Germanium (Ge) and about 10 mole % Boron (B). Other percentages of the dopants may be used if desired. The Germanium helps increase photosensitivity and the Boron reduces the refractive index to create the depressed core shape of the fiber refractive index profile. Other values of Ge and B may be used provided the inner core region has a refractive index that is less than the outer cladding region (“depressed core”). Also, other dopant(s) now known or later developed may be used to create the depressed core substrate. Also, instead of depressing the refractive index of the core, the refractive index of the cladding may be increased to create the desired depressed core condition. In such a depressed core condition, light will not be guided along the inner core region because it does not satisfy the well known waveguide condition; thus, the substrate does not act as a waveguide. Furthermore, the substrate will not propagate light along the core that could be reflected or diffracted by a diffraction grating located in the substrate.
As discussed herein, the substrate may be photosensitive to impress the diffraction grating therein. In that case, the fiber inner region (or core) has a photosensitivity that allows for a refractive index modulation of greater than about 5×10−4, using approximately 248 nanometer (nm) light, and about 0.5 Joules/cm2. Other levels of photosensitivity may be used, provided it is sufficient to give the desired grating profile. The photosensitivity may be achieved by any technique now known or later developed that allows the region of the substrate where the diffraction grating is to be written to experience a change in the refractive index of the substrate material when exposed to incident radiation of a desired wavelength, e.g., Ultraviolet (UV) exposure or actinic radiation or other wavelength radiation. For example, the fiber may be loaded with Hydrogen (H2) or Deuterium (D2), or doped with lead, boron, germanium, flame brushing, tin-germanium or other dopants, such as is described in U.S. Pat. No. 5,287,427 issued to Atkins on Feb. 25, 1994; or U.S. Pat. No. 5,325,659 issued to Atkins et al. on Aug. 10, 1993; or U.S. Pat. No. 5,157,747 issued to Atkins et al. on Oct. 20, 1992; or U.S. Pat. No. 6,221,566issued to Kohnke et al. on Apr. 24, 2001; or U.S. Pat. No. 6,327,406 issued to Cullen et al. on Dec. 4, 2001; or U.S. Pat. No. 6,097,512 issued to Ainsle et al. on Aug. 1, 2000; or U.S. Pat. No. 6,075,625 issued to Ainsle et al. on Jun. 13, 2000; or U.S. Pat. No. 5,495,548 issued to Hill on Feb. 12, 1996; or U.S. Pat. No. 6,436,857 issued to Brubeck et al. on Aug. 20, 2002; or as described in articles by X-C Long et al., entitled “Large Photosensitivity in Lead Silicate Glasses”, FE3-1/219 to FE3-3/221; or F. Ouellette et al, Applied Physics Letters, Vol. 58(17), p 1813; or G. Meltz et al, SPIE, Vol. 1516, “Int' Workship on Photoinduced Self-Organization in Optical Fiber”, May 10-11, 1991, Quebec City, Canada, pp 1516-1518; or D. McStay, SPIE, Vol. 1314, “Fibre Optics ‘90”, pp. 223-233; or as also discussed in the book by Kashyap, entitled “Fiber Bragg Gratings”, including Chapter 2, pp 13-54, Academic Press 1999; or as described in the references of one or more of the foregoing documents referred to therein, each of which is hereby incorporated herein by reference in its entirety; or some combination of the aforementioned documents.
While the substrate has been described as being used with a cylindrical fiber geometry, it should be understood and appreciated that other geometries may be used, such as planar, D-shaped, or any other shape, such as is described for example in the aforementioned co-pending co-owned patent applications. Also, when the material is referred to as a photosensitive material it is understood to mean any material whose index of refraction can be changed by exposing the material to light of a predetermined intensity at a wavelength in a predetermined range.
The foregoing and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.



BRIEF DESCRIPTION OF THE DRAWING
The drawing includes FIGS. 1-10, which are not drawn to scale, and include the following:
FIG. 1 is a side view of an optical identification element in accordance with the present invention.
FIG. 2 is a top level optical schematic for reading a code in an optical identification element in accordance with the present invention.
FIG. 3 is a schematic of Bragg grating envelope measurement equipment that may be used to read an optical identification element according to the present invention.
FIG. 4 includes FIG. 4(a) which shows a schematic of the fiber geometry of an optical fiber or substrate according to the present invention, and FIG. 4(b) which shows the refractive index profile in relation to the radius in microns of the fiber or substrate.
FIG. 5 shows a graph of a Bragg envelope measurement for depressed core fiber with 7 collocated gratings (65 μm, 13 μm core), having the integrated peak power (arbitrary units) plotted in relation to the angle (degrees) of incidence.
FIG. 6 shows a refractive index profile for a substrate having an outer diameter (OD) of 125 μm and a core diameter of 25 μm, having the refractive index difference plotted in relation to the X or Y position.
FIG. 7 shows a refractive index profile for a substrate having an outer diameter (OD) of 65 μm and a core diameter of 14 μm, having the refractive index difference plotted in relation to the X or Y position.
FIG. 8 shows a refractive index profile for a preform having an outer diameter of 13 mm and a core having a diameter of 3 mm.
FIG. 9 shows a refractive index profile for a preform having an outer diameter of 12 mm and a core having a diameter of 2.4 mm.
FIG. 10 shows a refractive index profile for a preform, showing the delta index in relation to the preform core diameter and the preform outer diameter.



DETAILED DESCRIPTION OF THE INVENTION
FIG. 1: The Basic Invention
Referring to FIG. 1, a diffraction grating-based optical identification element 8 (or encoded element or coded element) comprises a known optical substrate 10, having an optical diffraction grating 12 disposed (or written, impressed, embedded, imprinted, etched, grown, deposited or otherwise formed) in the volume of or on a surface of a substrate 10. The optical identification element 8 described herein is the same as that described in Copending patent application Ser. No. 10/661,234, filed Sep. 12, 2003, which is incorporated herein by reference in its entirety. The grating 12 may have a periodic or aperiodic variation in the effective refractive index and/or effective optical absorption of at least a portion of the substrate 10. It is important to note that the grating shown and described herein is provided by way of example. The scope of the invention is not intended to be limited to the type or kind of grating 12 in the substrate, the type or kind of variations forming the same, or the manner or technique for disposing the grating 12 into the substrate 10. Moreover, the scope of the invention is intended to include gratings and techniques for disposing the same both now known in the art, as well as those developed in the future.
As shown, the substrate 10 has an inner region 20 where the grating 12 is located. The inner region 20 may be photosensitive to allow the writing or impressing of the grating 12. The substrate 10 has an outer region 18, which does not have the grating 12 therein.
The grating 12 is a combination of one or more individual spatial periodic sinusoidal variations (or components) in the refractive index that are collocated at substantially the same location on the substrate 10 along the length of the grating region 20, each having a spatial period (or pitch) Λ. The resultant combination of these individual pitches is the grating 12, comprising spatial periods (Λ1-Λn) each representing a bit in the code. Thus, the grating 12 represents a unique optically readable code, made up of bits, where a bit corresponds to a unique pitch Λ within the grating 12. Accordingly, for a digital binary (0-1) code, the code is determined by which spatial periods (Λ1-Λn) exist (or do not exist) in a given composite grating 12. The code or bits may also be determined by additional parameters (or additional degrees of multiplexing), and other numerical bases for the code may be used, as discussed herein and/or in the aforementioned patent application. However, it is important to note that the scope of the invention is not intended to be limited to the type or kind of code represented by the grating 12, or the manner or technique for reading or interpreting the same. Moreover, the scope of the invention is intended to include the grating represent codes, and/or or the manner or technique for reading or interpreting the same, both now known in the art, as well as those developed in the future.
The grating 12 may also be referred to herein as a composite or collocated grating. Also, the grating 12 may be referred to as a “hologram”, as the grating 12 transforms, translates, or filters an input optical signal to a predetermined desired optical output pattern or signal.
The substrate 10 has an outer diameter D1 and comprises silica glass (SiO2) having the appropriate chemical composition to allow the grating 12 to be disposed therein or thereon. By way of example, the substrate 10 may be made of any glass, e.g., silica, phosphate glass, borosilicate glass, or other glasses, or made of glass and plastic, or solely plastic; however, other materials for the optical substrate 10 may be used if desired, including materials now known or later developed in the future. For high temperature or harsh chemical applications, the optical substrate 10 made of a glass material is desirable. If a flexible substrate is needed, plastic, rubber or polymer-based based substrate may be used. In effect, the optical substrate 10 may be any material capable of having the grating 12 disposed in the grating region 20 and that allows light to pass through it to allow the code to be optically imparted therein and read within the spirit of the invention described herein.
The optical substrate 10 with the grating 12 has a length L and an outer diameter D1, and the inner region 20 diameter D. The length L can range from very small “microbeads” (or microelements, micro-particles, or encoded particles), about 1-1,000 microns or smaller, to larger “macrobeads” or “macroelements” for larger applications (about 1.0-1,000 mm or greater). In addition, the outer dimension D1 can range from small (less than 1,000 microns) to large (1.0-1,000 mm and greater). Other dimensions and lengths for the substrate 10 and the grating 12 may be used within the spirit of the invention described herein.
The grating 12 may have a length Lg of about the length L of the substrate 10. Alternatively, the length Lg of the grating 12 may be shorter than the total length L of the substrate 10. The scope of the invention is not intended to be limited to any particular length Lg of the grating, or the length Lg in relation to the length L of the substrate 10.
The outer region 18 is made of pure silica (SiO2) and has a refractive index n2 of about 1.458 (at a wavelength of about 1553 nm), and the inner grating region 20 of the substrate 10 has dopants, such as germanium and/or boron, to provide a refractive index n1 of about 1.453, which is less than that of outer region 18 by about 0.005. Other indices of refraction n1, n2 for the grating region 20 and the outer region 18, respectively, may be used, if desired, provided the grating 12 can be impressed in the desired grating region 20. For example, the grating region 20 may have an index of refraction that is larger than that of the outer region 18 or grating region 20 may have the same index of refraction as the outer region 18 if desired. In other words, the scope of the invention is not intended to be limited to any particular refractive index of the inner or outer region, or the relationship of the refractive indices in relation to one another, or the materials or dopants used to provide the same in these regions, within the spirit of the invention.
FIG. 2 shows, by way of example, a technique for reading the code in the optical identification element 8. As shown, an incident light 24 of a wavelength λ e.g., 532 nm from a known frequency doubled Nd:YAG laser or 632 nm from a known Helium-Neon laser, is incident on the grating 12 in the substrate 10. Any other input wavelength λ can be used if desired provided the wavelength λ is within the optical transmission range of the substrate (discussed more herein and/or in the aforementioned patent application(s)). A portion of the input light 24 passes straight through the grating 12, as indicated by the arrow 25. The remainder of the input light 24 is reflected by the grating 12, as indicated by the arrow 27 and provided to a detector 29. The output light 27 may be a plurality of beams, each having the same wavelength λ as the input wavelength λ and each having a different output angle indicative of the pitches (Λ1-Λn) existing in the grating 12. Alternatively, the input light 24 may be a plurality of wavelengths and the output light 27 may have a plurality of wavelengths indicative of the pitches (Λ1-Λn) existing in the grating 12. Alternatively, the output light 27 may be a combination of wavelengths and output angles. The above techniques are discussed in more detail herein and/or in the aforementioned patent application. It is important to note that the scope of the invention is not intended to be limited to any particular input wavelength used, or the number of wavelengths used, or the number of beams used, or the angle of the beams used, in the technique for reading the code in the optical identification element 8 within the spirit of the invention.
The detector 29 has the necessary optics, electronics, software and/or firmware to perform the functions described herein. In particular, the detector reads the optical signal 27 diffracted or reflected from the grating 12 and determines the code based on the pitches present or the optical pattern, as discussed more herein or in the aforementioned patent application. An output signal indicative of the code is provided on a line 31. Optical detectors like 29 are known in the art, and the scope of the invention is not intended to be limited to any particular type or kind thereof.
The Grating Writing Process
The diffraction grating(s) 12 may be written or shot, for example, in the manner shown and described in detail in the technique shown and described in the aforementioned patent application Ser. No. 10/661,234 filed Sep. 12, 2003, which is incorporated herein by reference in its entirety. The grating 12 may be impressed in an optical fiber or substrate by any technique for writing, impressed, embedded, imprinted, or otherwise forming a diffraction grating in the volume of or on a surface of a substrate now known or later developed in the future. Examples of some known techniques are disclosed in U.S. Pat. Nos. 4,725,110 and 4,807,950, entitled “Method for Impressing Gratings Within Fiber Optics”, issued to Glenn et al. and U.S. Pat. No. 5,388,173, entitled “Method and Apparatus for Forming Aperiodic Gratings in Optical Fibers”, issued to Glenn, respectively; and U.S. Pat. No. 5,367,588, entitled “Method of Fabricating Bragg Gratings Using a Silica Glass Phase Grating Mask and Mask Used by Same”, issued to Hill; and U.S. Pat. No. 3,916,182, entitled “Periodic Dielectric Waveguide Filter”, issued to Dabby et al. and U.S. Pat. No. 3,891,302, entitled “Method of Filtering Modes in Optical Waveguides”, also issued to Dabby et al., which are all hereby incorporated herein by reference to the extent necessary to understand the present invention.
Alternatively, instead of the grating 12 being impressed within the fiber material, the grating 12 may be partially or totally created by etching or otherwise altering the outer surface geometry of the substrate to create a corrugated or varying surface geometry of the substrate, such as is disclosed in U.S. Pat. No. 3,891,302, entitled “Method of Filtering Modes in Optical Waveguides”, issued to Dabby et al., which is also hereby incorporated herein by reference to the extent necessary to understand the present invention, provided the resultant optical refractive profile for the desired code is created.
Further, alternatively, the grating 12 may be made by depositing dielectric layers onto the substrate, similar to the way a known thin film filter is created, so as to create the desired resultant optical refractive profile for the desired code.
FIG. 3
In one experiment, an optical identification element was exposed to 7 collocated gratings and interrogated from the side in the manner described in U.S. patent application Ser. No. 10/661,234 filed Sep. 12, 2003 incorporated herein by reference. Resulting diffracted beams 58 were imaged and read using a CCD camera like device 60 in FIG. 3 and the images were captured and processed using a frame grabber card in a personal computer (PC) or other suitable processing device, not shown. The Bragg envelope for the two optical identification elements were also measured by capturing the images of the diffracted beams for a range of rotation angles. It was found that under normal exposure conditions, the excess Boron in the core provided enhanced photosensitivity over the standard “photosensitive” telecom fiber. FIG. 3 shows the laser 52 providing an input light beam 54 on two elements 56a, 56b, and an output beam 58 being read by the CCD camera 60.
FIG. 4
FIG. 4(a) shows a schematic of the fiber geometry of an optical fiber or substrate 100 having a core 102 with a grating 103 and also having a cladding 104. As shown, the core 102 has a length L of about 10 μm and the grating 103 has a pitch of about 0.5 μm. In response to an optical signal having a wavelength λ=632 nanometers (nm) received at an angle Θi, the grating 103 provides a reflected optical fiber at an angle Θo with respect to the normal consisting of a plurality of uniquely distinguishable signals covering an angle range of ΘR. The index of refraction of the cladding and core respectively are 1.458 and 1.455 (i.e. difference of 0.003). The reader is also referred to the graph in FIG. 5.
SCOPE OF THE INVENTION
Although the invention has been described and illustrated with respect to exemplary embodiments thereof, the foregoing and various other additions and omissions may be made therein and thereto without departing from the spirit and scope of the present invention.

## Claims

1. An optical identification element comprising a non-waveguide optical substrate having a volume and including an inner region surrounded by an outer region, the inner region having an index of refraction that prevents the optical substrate from guiding light in a propagation mode, the optical substrate including a diffraction grating within the volume, the grating providing an output signal indicative of a code when illuminated by an incident light.

2. The element in accordance with claim 1 wherein the inner region and the outer region extend along an axis, the outer region surrounding the inner region about the axis, the inner region preventing the optical substrate from guiding light in a direction along the axis.

3. The element in accordance with claim 1 wherein the inner and outer regions are made of a common material.

4. The element in accordance with claim 1 wherein the inner region has an index of refraction less than or equal to the outer region.

5. The element in accordance with claim 1 wherein the inner region includes the diffraction grating therein or thereon.

6. The element in accordance with claim 1 wherein the diffraction grating is a Bragg grating.

7. The element in accordance with claim 1 wherein the inner region is a photosensitive inner region.

8. The element in accordance with claim 1 wherein the inner region includes a geometric center of the substrate.

9. The optical identification element in accordance with claim 1 wherein the volume is a continuous volume of material.

10. The optical identification element in accordance with claim 1 wherein the optical substrate is processed from a fiber preform.

11. The optical identification element in accordance with claim 1 wherein the optical substrate has a length that is less than about 1000 microns.

12. The optical identification element in accordance with claim 1 further comprising an object attached to the optical substrate, the object being associated with the code for at least one of identifying, sorting, tracking, and tagging the object.

13. The optical identification element in accordance with claim 1 further comprising a chemical attached to the optical substrate.

14. The optical identification element in accordance with claim 13 wherein the chemical is a nucleic acid.

15. A method for reading an optical identification element comprising:
obtaining an optical identification element that includes a non-waveguide optical substrate having a volume and an inner region surrounded by an outer region, the inner region having an index of refraction that prevents the optical substrate from guiding light in a propagation mode, the optical substrate including a diffraction grating within the volume;
illuminating the grating with an incident light, the grating providing an output signal indicative of a code; and
reading the output signal.

16. The method in accordance with claim 15 wherein illuminating the grating includes illuminating the grating from outside the substrate.

17. The method in accordance with claim 15 wherein the inner region and the outer region extend along an axis, the outer region surrounding the inner region about the axis.

18. The method in accordance with claim 15 wherein the inner and outer regions are made of a common material.

19. The method in accordance with claim 15 wherein the inner region has an index of refraction less than or equal to the outer region.

20. The method in accordance with claim 15 wherein the inner region includes the diffraction grating therein or thereon.

21. The method in accordance with claim 15 wherein the diffraction grating is a Bragg grating.

22. The method in accordance with claim 15 wherein the inner region is a photosensitive inner region.