Abstract:
An improved waveguide is disclosed. The waveguide utilizes a luminescent material disposed within or around its perimeter to introduce additional light into the waveguide. For example, the waveguide may include a plurality of planar layers having different refractive indexes. A luminescent material may be disposed along the outer edge of these layers. When light from within the waveguide strikes the luminescent material, it emits light, thereby adding to the light in the waveguide. Not only does the luminescent material introduce more light into the waveguide, it also introduces more light sources, thereby making it more difficult to introduce a probe without blocking at least a portion of the light destined for the image sensor. The luminescent material may be a phosphor.

Description:
[0001]    This application claims priority of U.S. Provisional Patent Application Ser. No. 62/130,208, filed Mar. 9, 2015, the disclosure of which is herein incorporated by reference in its entirety. 
     
    
       [0002]    This invention was made with Government support under Contract No. FA8721-05-C-002 awarded by the U.S. Air Force. The Government has certain rights in the invention. 
     
    
     FIELD 
       [0003]    This disclosure relates to waveguides used for physically unclonable functions applicable on fully functional printed circuit boards. 
       BACKGROUND 
       [0004]    Security is becoming increasingly important as the internet and electronic devices become more pervasive. For example, computers and even mobile telephones are equipped with biometrics to prevent access by unauthorized users. 
         [0005]    Encryption is also used to prevent unauthorized access to devices and information. For example, data can be encrypted before being transmitted on the internet. Other techniques, such as security tokens, are also employed to limit access to devices. 
         [0006]    In addition, many electronic systems require a unique digital identifier for authentication, key derivation and other purposes. These electronic systems are often manufactured using traditional manufacturing processes. Creating a unique digital identifier in this environment is often difficult and time consuming. Furthermore, to be effective, the unique digital identifier should be extremely different or nearly impossible to determine and copy. 
         [0007]    One method of creating this unique digital identifier is through the use of waveguides.  FIG. 1  shows a cross section of a printed circuit board  10  with a conventional planar waveguide  20 . The printed circuit board  10  includes one or more light sources  11 . These light sources  11  emit light that enters the waveguide  20  by means of angle mirror  26  cut into the waveguide  20 . The light initially appears in both the inner core  21  and the outer cladding  22 , but an absorptive layer of material  25  absorbs the light in the outer cladding  22 . The printed circuit board  10  also includes an image sensor  12 , such as a CCD image sensor. Light in the inner core  21  is not coupled to the image sensor  12 , but inhomogeneities  27  in the inner core  21  scatter light into the outer cladding  22  where some fraction of this light is received by the image sensor  12 . Thus, some portion of the light emitted from the light sources  11  reaches the image sensor  12 . The light pattern created on the image sensor  12  is then converted to a digital value. Slight differences in the structure of the waveguide  20  affect the resulting light pattern, causing unique patterns to be reflected onto the image sensor  12 . Thus, the light pattern represents the unique identifier. 
         [0008]    As mentioned above, these waveguides  20  are traditionally constructed using an inner core  21  surrounded by an outer cladding  22 . The outer cladding  22  is then covered by a reflective silver layer  24 . The inner core  21  may have a higher refractive index (n) than the outer cladding  22 . For example, the inner core  21  may have a refractive index of 1.59, while the outer cladding has a refractive index of 1.49. Light is reflected at the boundary between the inner core  21  and the outer cladding  22  or at the boundary between the outer cladding  22  and the silver layer  24 . 
         [0009]    As shown in  FIG. 1 , the incident angle of the light determines at which boundary the light is reflected. Higher incident angle light is reflected at the boundary between the inner core  21  and the outer cladding  22 , while lower incident angle light is reflected at the silver layer  24 . For example, using the refractive indices described above, light with an incident angle of 70° to 90° will remain trapped in the inner core  21 . Light with a lower incident angle, such as 60° to 70°, are contained within both the inner core  21  and the outer cladding  22 . Further, at incident angles less than roughly 60°, the light will exit the outer cladding  22  and may be reflected by the silver layer  24 . 
         [0010]      FIG. 2  shows a top view of the waveguide  20  of  FIG. 1 . Disposed under the waveguide  20  are a light source  11  and an image sensor  12 . Light is emitted from the light source  11  and traverses the waveguide  10  to the image sensor  12 .  FIG. 2  also shows an intrusive probe  13  that has been inserted into the waveguide  10 . If the probe  13  is not inserted into the direct path between the light source  11  and the image sensor  12 , its affect on the reflected light pattern received by the image sensor  12  may be minimal. For example, there may be some small amount of light  15  reflected off the probe  13  that may affect the reflected pattern; however, most of the light in the waveguide  20  that is destined for the image sensor  12  is unaffected by the probe  13 . If the probe  13  is not inserted in the direct light path, the shadow  14  cast by the probe  13  may have no affect on the reflected light pattern received by the image sensor  12 . 
         [0011]    However, ideally, the light pattern should be significantly affected by the insertion of an intrusive probe  13 , regardless of the location of that insertion. Therefore, it would be beneficial if there were a waveguide where the reflected light pattern is more significantly affected by the insertion of a probe. Furthermore, it would be advantageous if this significant change in the reflected light pattern occurred regardless of the location of the insertion. 
       SUMMARY 
       [0012]    An improved waveguide is disclosed. The waveguide utilizes a luminescent material disposed within or around its perimeter to introduce additional light into the waveguide. For example, the waveguide may include a plurality of planar layers having different refractive indexes. A luminescent material may be disposed along the outer edge of these layers. When light from within the waveguide strikes the luminescent material, it emits light, thereby adding to the light in the waveguide. Not only does the luminescent material introduce more light into the waveguide, it also introduces more light sources, thereby making it more difficult to introduce a probe without blocking at least a portion of the light destined for the image sensor. The luminescent material may be a phosphor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    For a better understanding of the present disclosure, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: 
           [0014]      FIG. 1  shows a printed circuit board with a waveguide according to the prior art; 
           [0015]      FIG. 2  shows a top view of a waveguide of the prior art with an intrusive probe inserted into the waveguide; 
           [0016]      FIG. 3A  shows a cross-sectional view of the waveguide according to one embodiment and  FIG. 3B  shows a top view of the waveguide; 
           [0017]      FIG. 4  shows the waveguide of  FIG. 3  with an intrusive probe; and 
           [0018]      FIG. 5A  shows a cross-section view of the waveguide of  FIG. 3A  and  FIG. 5B  shows a top view of a printed circuit board using the waveguide of  FIG. 3A . 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    The present disclosure describes a waveguide that may be used with fully fabricated printed circuit boards to create a physically unclonable function. As described above, the waveguide utilizes a luminescent material disposed within or around its perimeter to introduce additional light into the waveguide. When light from within the waveguide strikes the luminescent material, that luminescent material also emits light, thereby adding to the light in the waveguide. 
         [0020]      FIG. 3A  shows a cross-sectional view of the waveguide  100  according to one embodiment.  FIG. 3B  shows a top view of the waveguide  100 . As shown in  FIG. 3A , the waveguide  100  may include an inner core  110 , which is sandwiched on both sides by an outer cladding  120 . In other words, there are two layers of outer cladding  120 , where one layer is disposed on each side of the inner core  110 . In another embodiment, the outer cladding  120  may be applied to only one surface of the inner core  110 . In yet another embodiment, the outer cladding  120  may cover only a portion of the inner core  110  on one or both sides. 
         [0021]    In certain embodiments, the inner core  110  and the outer cladding  120  may both be polymers. In certain embodiments, the inner core  110  may be a transparent material. 
         [0022]    The two materials used in the waveguide  100  each have different refractive indices, with the inner core  110  having a higher index than the outer cladding  120 . The inner core  110  and the outer cladding  120  meet at an inner interface  115 . 
         [0023]    Each of the layers of the waveguide  100  may be planar, where each layer is a thin rectangular prism. Further, the layers are stacked on top of each other to form an assembly  125 , where the assembly  125  is also a thin rectangular prism. 
         [0024]    Light with a high incident angle stays within the inner core  110 , while light with a lower incident angle is contained within the outer cladding  120  and the inner core  110 . 
         [0025]    In one embodiment, a luminescent material  130  is disposed at the edges of the waveguide  100 . In this disclosure, the term “edges” refers to the dimension perpendicular to the longer dimensions of the assembly  125 . For example, in  FIG. 3A , the layers are shown as being horizontal planes, while the luminescent material  130  is disposed vertically at the outer edges of the assembly  125 . Typically, the “edge” corresponds to the short dimension of the assembly  125 . All of the edges form the perimeter of the waveguide  100 . Thus, the term “perimeter” refers to all of the edges that comprise the assembly  125  used to create the waveguide  100 . 
         [0026]    The luminescent material  130  may be any material that emits light. For example, in certain embodiments, the luminescent material may be a phosphor. In one particular embodiment, the phosphor emits red light when excited by blue light. Phosphors include micrometer powders of zinc sulfide, ZnS, or cadmium selenide, CdSe. These powders are usually mixed with a polymer of the desired index and applied at the edges of the waveguide  100  either in the inner core  110 , the outer cladding  120  or in both layers as the waveguide  100  is being manufactured. In some embodiments, the luminescent material  130  may be disposed within the perimeter of the waveguide  100  to secure a more sensitive area on the printed circuit board. 
         [0027]      FIG. 3B  shows a top view of the waveguide  100 , where the luminescent material  130  is disposed around the entire perimeter of the waveguide  100 . In other words, the luminescent material  130  may be disposed on all edges of the waveguide  100 . In other embodiments, the luminescent material  130  may be disposed on a subset of the edges of the waveguide  100 . In all embodiments, the luminescent material  130  is disposed on at least a portion of one edge of the waveguide  100 . In certain embodiments, the luminescent material  130  may be disposed on at least a portion of several edges of the waveguide  100 . The thickness of the luminescent material  130  may vary, and may be between 10 micrometers and 2 millimeters. 
         [0028]    In another embodiment, also shown in  FIG. 3B , luminescent material  131  may be disposed within the inner core  110 , the outer cladding  120  or both layers. In other words, the luminescent material  131  is disposed within the perimeter of the waveguide  100 . 
         [0029]    Further, in certain embodiments, the luminescent material  130  may be disposed along at least part of the perimeter and luminescent material  131  is also disposed within the perimeter of the waveguide  100 . 
         [0030]      FIG. 4  shows the waveguide  100  disposed on a printed circuit board. The printed circuit board includes a blue light source  410 , which may be one or more blue LEDs. The printed circuit board also includes an image sensor  420 . Blue light  411  enters the waveguide  100  from the blue light source  410  and traverses the waveguide  100 . When the blue light  411  reaches the edges of the waveguide  100 , the blue light  411  strikes the luminescent material  130 , which, in this embodiment, is disposed around the perimeter of the waveguide  100 . The luminescent material  130  then emits red light  412 . Both the blue light  411  and the red light  412  reach the image sensor  420  and combine to create the reflected light pattern. In certain embodiments, an optical filter  421  may be disposed between the waveguide  100  and the image sensor  420 . The optical filter  421  may attenuate the blue light  411  to more closely match the intensities of the blue light  411  and the red light  412 . In certain embodiments, the image sensor  420  may be a color image sensor, such as a CCD image sensor. 
         [0031]    In the event that a probe  430  penetrates the waveguide  100 , it reflects some of the blue light  411  emitted from the blue light source  410  toward the image sensor  420  as reflected light  432 . However, in addition, it also casts a shadow  431  downstream from the blue light source  410 . Wherever the shadow  431  extends to the perimeter, the luminescent material  130  will not be excited, and therefore will not emit any red light  412 . Thus, the amount of red light  412  that is produced is affected by the intrusion of the probe  430 . Therefore, two different phenomenons are employed to increase the sensitivity of the reflected light pattern to intrusion. First, the path of the originally emitted blue light  411  may be reflected, deflected or blocked by the probe  430 . Additionally, the production of red light  412  may be altered by the creation of shadows by the probe  430 . These two mechanisms create a much greater change in the reflected light pattern captured by the image sensor  420  than is achieved in the prior art. 
         [0032]    Further, in certain embodiments, multiple blue light sources  410 , disposed at different locations, may be used to further increase the effect that an inserted probe may have in the reflected image sensor. 
         [0033]    As explained above, in certain embodiments, the luminescent material may be disposed within the perimeter of the waveguide  100 , so as to create additional light sources within the waveguide  100 . This may be in addition to, or instead of, the luminescent material disposed along the perimeter. 
         [0034]    While the above disclosure describes the use of blue light sources  410  with luminescent material  130  that generates red light, other embodiments are also possible. For example, in certain embodiments, the light sources  410  emit a light having a first wavelength. The luminescent material  130  absorbs the light having the first wavelength, and emits a light having a second wavelength. In certain embodiments, the second wavelength is greater than the first wavelength. 
         [0035]      FIG. 5A  shows a cross section of a printed circuit board having the waveguide  100  of  FIG. 3A .  FIG. 5B  shows a top view of the printed circuit board. As shown in  FIG. 5A , the waveguide  100  is disposed on top of the printed circuit board  500 . A blue light source  410  is used to inject light into the waveguide  100 . The reflected light is received by an image sensor  420 , disposed on the printed circuit board  500 , separate from the blue light source  410 .  FIG. 5B  shows a top view of the printed circuit board  500 . In certain embodiments, the waveguide  100  (shown in dashed lines) is used to cover several components disposed on the printed circuit board  500 . Disposed on the printed circuit board is a memory element  513  that contains the code executed by the processing unit  514 . In operation, the code in the memory element  513  may be encrypted, where the key needed to decrypt the code is defined by the light pattern at the image sensor  420 . In some embodiments, a decryption circuit  515  is also disposed on the printed circuit board  500 . The decryption circuit  515  uses the light pattern from the image sensor  420  as the key to decrypt the encrypted code, and then passes this decrypted code to the processing unit  514 . To protect the security and confidentiality of the code, certain components on the printed circuit board  500  are covered by the waveguide  100 . For example, the processing unit  514 , which receives the decrypted code, may be covered by the waveguide  100 . In addition, the decryption circuit  515 , which outputs decrypted code, may also be covered by the waveguide  100 . The memory element  513  may optionally also be covered by the waveguide  100 . In other words, decrypted code and the light pattern output from the image sensor  420  remain hidden under the waveguide  100 . Additionally, the blue light source  410  and the image sensor  420  are located beneath the waveguide  100 . 
         [0036]    In this way, if one were to attempt to interrogate the printed circuit board  500  to gain access to the decrypted code, one would necessarily have to pierce or remove the waveguide  100 . However, any manipulation of the waveguide  100  will affect the way that light is reflected within the waveguide  100 , thereby affecting the light pattern received at the image sensor  420 , as described above. This change in the light pattern modifies the key, and renders the circuit unusable. Thus, the waveguide of  FIGS. 3A-3B  may be used to create a physically unclonable function (PUF) on a printed circuit board. 
         [0037]    The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Furthermore, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.