| WHAT IS CLAIMED IS: 1. A photonic textile product comprising a textile material including photonic fibers structured to reflect light of a first given color in response to ambient illumination and to irradiate light which is guided within the photonic fibers, wherein the irradiated light has a second given color different from the first given color. 2. A photonic textile product according to claim 1 , wherein the photonic fibers are structured to reflect light of the first given color when the photonic fibers are exposed to ambient white light. 3. A photonic textile product according to claim 1 , wherein the photonic fibers are structured to guide and irradiate light of the second given color when white light is launched within the photonic fibers. 4. A photonic textile product according to claim 1 , wherein the textile material is a woven material and the photonic fibers are woven in the textile material. 5. A photonic textile product according to claim 1 , wherein the photonic fibers are selected from the group consisting of hollow core PCF optical fibers and solid core PCF optical fibers. 6. A photonic textile product according to claim 1 , wherein the photonic fibers are selected from the group consisting of hollow core PBG optical fibers and solid core PBG optical fibers. 7. A photonic textile product according to claim 1 , wherein the photonic fibers comprise PCF or PBG optical fibers including a core and a reflector surrounding the core, wherein the reflector contains imperfections to enhance irradiation of guided light of the second given color. 8. A photonic textile product according to claim 1 , wherein the photonic fibers comprise PCF or PBG optical fibers including a core and a reflector surrounding the core, wherein the reflector contains powder to enhance irradiation of light of the second given color. 9. A photonic textile product according to claim 1 , wherein the photonic fibers comprise PCF or PBG optical fibers including a core and a reflector surrounding the core, wherein the reflector contains bubbles to enhance irradiation of light of the second given color. 10. A photonic textile product according to claim 1 , wherein the photonic fibers comprise PCF or PBG optical fibers including a core and a multilayer reflector surrounding the core, wherein the multilayer reflector contains structural imperfections including roughness at interfaces between layers of the multilayer reflector. 11. A photonic textile product according to claim 6, wherein the PBG optical fibers comprise a core and a periodic or quasi-periodic reflector including a sequence of low and high refractive index regions surrounding the core. 12. A photonic textile product according to claim 6, wherein the PBG optical fibers comprise a core and a periodic reflector including a sequence of low and high refractive index layers surrounding the core. 13. A photonic textile product according to claim 10, wherein the core has a refractive index lower or equal to the low refractive index of the layers. 14. A photonic textile product as defined in claim 1 , wherein the photonic fibers are transparent to light having colors other than the first given color. 15. A photonic textile product as defined in claim 1 , wherein the photonic fibers have a mirror-like colored appearance. 16. A photonic textile product as defined in claim 1 , comprising a source of white light coupled to at least a portion of the photonic fibers. 17. A photonic textile product as defined in claim 16, comprising a control of an intensity of white light from the source launched in said at least a portion of the photonic fibers to control an intensity of the light of the second given color guided in said at least a portion of the photonic fibers to thereby control the color of said at least a portion of the photonic fibers and the corresponding portion of the textile material. 18. A photonic textile product as defined in claim 1 , comprising a control of an intensity of the light of the second given color guided through at least a portion of the photonic fibers for adjusting the color of the light irradiated from said at least a portion of the photonic fibers. 19. A photonic textile product as defined in claim 1 , wherein the photonic fibers define bundles of photonic fibers guiding light of different colors. 20. A photonic textile product as defined in claim 19, comprising a white light source for launching white light in the photonic fibers of the respective bundles, the photonic fibers in each bundle being responsive to the white light from the source for guiding light of respective, different colors. 21. A photonic textile product as defined in claim 20, comprising a control of the intensities of white light launched in the respective photonic fibers of each bundle to control the intensities of light of the different colors guided within the respective photonic fibers of each bundle and therefore the color of the textile material. 22. A method of adjusting an overall color of a photonic fiber structured to reflect light of a first given color in response to ambient illumination and to irradiate light which is guided within the photonic fibers, wherein the irradiated light has a second given color different from the first given color, the method comprising: controlling relative intensities of the reflected light of the first given color and the irradiated light of the second given color; and mixing the reflected light of the first given color and the irradiated light of the second given color to determine the overall color of the photonic fiber. 23. A method of adjusting the overall color of a photonic fiber according to claim 22, wherein, under a given ambient illumination, controlling the relative intensities comprises controlling the intensity of the light guided within the photonic fiber. 24. A method of adjusting the overall color of a photonic fiber according to claim 22, comprising launching white light in at least a portion of the photonic fibers to enable the photonic fibers of said at least a portion to guide and irradiate light of the second given color. 25. A method of adjusting the overall color of a photonic fiber according to claim 22, wherein the photonic fibers are selected from the group consisting of hollow core PCF or PBG optical fibers and solid core PCF or PBG optical fibers. 26. A method of producing a photonic textile product and adjusting a color of the photonic textile product, comprising: incorporating in a textile material photonic fibers structured to reflect light of a first given color in response to ambient illumination and to irradiate light which is guided within the photonic fibers, wherein the irradiated light has a second given color different from the first given color; controlling relative intensities of the reflected light of the first given color and the irradiated light of the second given color; and mixing the reflected light of the first given color and the irradiated light of the second given color to determine the overall color of the photonic fibers and textile material. 27. A method as defined in claim 26, wherein controlling the relative intensities comprises propagating light within a first portion of the photonic fibers and propagating no light within a second portion of the photonic fibers, whereby first and second sections of the textile material corresponding to the first and second portions of the photonic fibers, respectively, present different colors. 28. A method as defined in claim 27, comprising changing an intensity of the light propagating within the first portion of the photonic fibers to change the color of the first section of the textile material obtained by mixing the light reflected and the light irradiated by the photonic fibers of the first portion. 29. A method as defined in claim 26, wherein controlling the relative intensities comprises: propagating light within a first portion of the photonic fibers and propagating no light within a second portion of the photonic fibers, whereby first and second sections of the textile material corresponding to the first and second portions of the photonic fibers, respectively, present different colors; changing the ambient illumination to change the color of both the first and second portions of the photonic fibers and therefore of the first and second sections of the textile material that still present different colors. 30. A method as defined in claim 26, comprising propagating light of one color within a first portion of the photonic fibers and propagating light of another, different color within a second portion of the photonic fibers, whereby first and second sections of the textile material corresponding to the first and second portions of the photonic fibers, respectively, present different colors. 31. A method as defined in claim 26, wherein the textile material is a woven material, said method comprising weaving the photonic fibers in the textile material. 32. A method as defined in claim 26, wherein the textile material is a non-woven material in which the photonic fibers are introduced. 33. A method as defined in claim 26, wherein incorporating the photonic fibers in the textile material comprises one of the following operations: - introducing the photonic fibers in the textile material by knitting; and - introducing the photonic fibers in the textile material by braiding. 34. A method as defined in claim 26, wherein, under a given ambient illumination, controlling the relative intensities comprises controlling an intensity of the light of the second given color guided within the photonic fibers. 35. A method as defined in claim 26, comprising launching white light in at least a portion of the photonic fibers to enable the photonic fibers of said at least a portion to guide and irradiate light of the second given color. 36. A method as defined in claim 26, wherein the photonic fibers are selected from the group consisting of hollow core PCF or PBG optical fibers and solid core PCF or PBG optical fibers. 37. A method as defined in claim 26, comprising defining bundles of photonic fibers guiding light of different colors, respectively. 38. A method as defined in claim 37, comprising launching white light in the photonic fibers of the respective bundles, the photonic fibers in each bundle being responsive to the white light for guiding light of the respective, different colors. 39. A method as defined in claim 38, comprising controlling the intensities of white light launched in the respective photonic fibers of each bundle to control the intensities of light of the different colors guided within the respective photonic fibers of each bundle and therefore the color of the textile material. 40. A photonic bandgap fiber, comprising: a light-propagating core; and a periodic reflector surrounding the light-propagating core and defining at least one bandgap; - wherein, in response to white light, only light at frequency located within the at least one bandgap is guided by the photonic bandgap fiber. 41. A photonic bandgap fiber as defined in claim 40, further comprising a cladding surrounding the periodic reflector. 42. A photonic bandgap fiber as defined in claim 40, wherein the light- propagating core is a light-propagating hollow core. 43. A photonic bandgap fiber as defined in claim 40, wherein the light- propagating core is a solid core. 44. A photonic bandgap fiber as defined in claim 43, wherein the solid, light- propagating core has a low refractive index. 45. A photonic bandgap fiber as defined in claim 40, wherein the periodic reflector comprises a sequence of high and low refractive index layers. 46. A photonic bandgap fiber as defined in claim 45, wherein the sequence of high and low refractive index layers comprises a finite number of high and low refractive index layers, which causes irradiation of light guided by the photonic bandgap fiber across the periodic reflector. 47. A photonic bandgap fiber as defined in claim 45, wherein the sequence comprises a number of high and low refractive index layers, wherein the high and low refractive index layers of the sequence have respective thicknesses, and wherein the number of high and low refractive index layers and the thicknesses of the high and low refractive index layers control a color of the light guided by the photonic bandgap fiber and a level of irradiation of light guided by the photonic bandgap fiber through the periodic reflector. 48. A photonic bandgap fiber as defined in claim 40, wherein the bandgap is a spectral region in which the periodic reflector efficiency reflects light due to interference effects inside the periodic reflector. 49. A photonic bandgap fiber as defined in claim 40, wherein the light at frequency located within the at least one bandgap is confined in the light propagating core through reflection from the surrounding periodic reflector 50. A photonic bandgap fiber as defined in claim 40, wherein the light guided by the photonic bandgap fiber has a given color, light having other colors being irradiated out of the fiber after a certain length of propagation. 51. A photonic bandgap fiber as defined in claim 41 , comprising imperfections at an air/cladding interface that cause irradiation of the guided light through the periodic reflector. 52. A photonic bandgap fiber as defined in claim 46, comprising imperfections at interfaces between the high and low refractive index layers of the sequence that cause irradiation of light guided by the photonic bandgap fiber through the periodic reflector. 53. A photonic bandgap fiber as defined in claim 40, wherein the periodic reflector contains nanopowder to cause and control irradiation of light guided by the photonic bandgap fiber through the periodic reflector. 54. A photonic bandgap fiber as defined in claim 40, wherein the periodic reflector presents properties of reflection of ambient light at normal incidence. 55. A photonic bandgap fiber as defined in claim 54, wherein the reflected and guided light have different colors. 56. A method of producing a photonic textile product, comprising: incorporating in a textile material photonic bandgap fibers as defined in any one of claims 40 to 55. 57. A method of adjusting a color of light guided by a photonic fiber comprising a light-propagating core and a periodic reflector surrounding the light-propagating core and defining at least one bandgap whereby, in response to white light, only light at frequency located within the at least one bandgap is guided by the photonic fiber, said method comprising stretching the photonic fiber to shift the bandgap and therefore change the color of the light guided by the photonic fiber. 58. An anti-counterfeit label comprising a plurality of sets of photonic fibers comprising a light-propagating core and a periodic reflector surrounding the light- propagating core and defining at least one bandgap whereby, in response to white light, only light at frequency located within the at least one bandgap is guided by the photonic fiber, wherein: - the photonic fibers of at least two sets have respective, different bandgaps to guide light of different colors in response to white light; and - the photonic fibers of said at least two sets have a same or respective, distinct colors under reflection of ambient light. 59. An anti-counterfeit label comprising a plurality of sets of photonic fibers comprising a light-propagating core and a periodic reflector surrounding the light- propagating core and defining at least one bandgap whereby, in response to white light, only light at frequency located within the at least one bandgap is guided by the photonic fiber, wherein: - the photonic fibers of at least two sets have respective, different colors under reflection of ambient light; and - the photonic fibers of said at least two sets have a same bandgap or respective different bandgaps to guide light of a same color or respective, different colors in response to white light. |
Photonic Fiber and Color-Changing and Color-Tunable Photonic Textile Product Using this Photonic Fiber
FIELD
[0001] The present invention generally relates to a photonic fiber and a color-changing and color-tunable photonic textile product using this photonic fiber.
BACKGROUND
[0002] Driven by the consumer demand of unique appearance, increased performance and multi-functionality of woven items, smart textiles became an active area of current research. Various applications of smart textiles include: interactive clothing for sports; hazardous occupations; military; industrial textiles with signage; and fashion accessories and apparel with unique and variable appearance. Major advances in the textile capabilities could be achieved through further development of its fundamental element - the fiber.
BRIEF DESCRIPTION OF THE DRAWINGS
[0003] In the appended drawings:
[0004] Figure 1a is a schematic diagram of a standard total internal reflection (TIR) optical fiber or a photonic bandgap (PBG) optical fiber bent in a textile product; Figure 1b is a schematic diagram of a TIR or PBG optical fiber showing leakage of guided light due to scattering on artificially created or inherent imperfections of the optical fiber; Figure 1c is a schematic diagram of a straight and otherwise unperturbed hollow core PBG optical fiber showing leakage of guided light of a given color due to finite size of a reflector of the PBG optical fiber; and Figure 1d is a schematic diagram of a solid core PBG optical fiber showing enhanced leakage of guided light of a given color due to scattering on imperfections of the reflector of the PBG optical fiber;
[0005] Figures 2a, 2b, 2c and 2d are photographs of sections of solid core PBG optical fibers approximately 20 cm long and fabricated from a same preform, but drawn to different outside diameters;
[0006] Figure 3a is a photograph of one end of a typical solid core fiber preform; Figure 3b is a photograph showing an enlarged portion of the solid core fiber preform of Figure 3a; Figure 3c is a photograph in perspective of a resulting solid core PBG optical fiber comprising a central solid core surrounded by a multilayer periodic Bragg reflector itself surrounded by a cladding; and Figure 3d is a graph showing regions of phase space where no delocalized states exist inside of the periodic reflector (bandgaps);
[0007] Figure 4a is a schematic diagram of a PBG optical fiber showing color synthesis produced by the mixing of (a) light of a first color reflected from the PBG optical fiber under white light ambient illumination and (b) guided light of a second color irradiated from the PBG optical fiber; Figure 4b is a photograph taken under ambient daylight illumination with additional lighting from the top, and no light propagating through the solid core PBG fibers, resulting in a green color of the PBG optical fibers; Figure 4c is a photograph taken under ambient daylight illumination with light guided through the solid core PBG fibers, resulting in a red color of the solid core PBG fibers; Figure 4d is a photograph taken both under ambient daylight illumination and guided light using a microscope focused on the solid core PBG fibers, wherein the resultant appearance of the PBG optical fibers in the near field is a set of green and red stripes; and Figure 4e is also a photograph taken both under ambient daylight illumination and guided light using a microscope defocused from the solid core PBG fibers to get an image of the PBG optical fibers in the far field, which revealed a yellow fiber bundle; [0008] Figure 5a shows schematic images illustrating a first application, under ambient daylight, of the photonic textile product to a variable color uniform for enhanced visibility during daylight illumination; Figure 5b shows schematic images still illustrating the application of the photonic textile product to a variable color uniform, at nighttime and under variable exposure to the headlights of a car; Figure 5c shows images illustrating an application of the color changing photonic textile product to an anti-counterfeit label; and Figure 5d illustrates a further application of the color changing photonic textile product to discreet and intelligent jewelry;
[0009] Figure 6a is a photograph showing use of yarns or bundles of three (3) PBG fibers respectively propagating and irradiating red (R), green (G) and blue (B) light to adjust the color of a photonic textile product by controlling the light intensities guided through the respective red (R), green (G) and blue (B) light irradiating PBG fibers of the yarns or bundles; and Figure 6b is a schematic diagram showing a photonic textile product comprising laterally adjacent bundles such of three (3) PBG fibers respectively propagating and irradiating red (R), green (G) and blue (B) light; in Figure 6b, a single white light source and a 1X3 variable intensity coupler is used to control the color of the PBG fiber-based photonic textile product.
[0010] Figure 7a is a photograph of a sample of photonic textile product comprising woven in PBG optical fibers irradiating guided light, under no ambient illumination; Figure 7b is a photograph of a sample of photonic textile product comprising woven in PBG optical fibers under ambient illumination, with no light guided by the PBG optical fibers; Figure 7c is a photograph of a sample of photonic textile product comprising woven in PBG optical fibers under ambient light illumination, with no light guided by the PBG optical fibers; Figure 7d is a photograph of the sample of photonic textile product of Figure 7c with the woven in PBG optical fibers irradiating guided light under ambient illumination, wherein reflected light and irradiated guided light mix to synthesize textile color; Figure 7e are photographs of portions of the sample of photonic textile product of Figure 7c showing change of the color via variation of the intensity of the irradiated guided light; and
[0011] Figure 8a is a photograph of a setup in which a 20 cm long piece of Bragg fiber is placed inside fiber chucks and then stretched; and Figure 8b is a graph showing spectra of fiber light transmission taken before and after stretching, and illustrating a shift by about 1% (7 nm) between the two (2) spectra.
DETAILED DESCRIPTION
[0012] According to a first non-restrictive, illustrative embodiment of the present invention, there is provided a photonic bandgap fiber, comprising: a light- propagating core; and a periodic reflector surrounding the light-propagating core and defining at least one bandgap; wherein, in response to white light, only light at frequency located within the at least one bandgap is guided by the photonic bandgap fiber.
[0013] According to a second non-restrictive, illustrative embodiment of the present invention, there is provided a method of producing a photonic textile product, comprising incorporating in a textile material photonic bandgap fibers as defined hereinabove.
[0014] According to a third non-restrictive, illustrative embodiment of the present invention, there is provided a photonic textile product comprising a textile material including photonic fibers structured to reflect light of a first given color in response to ambient illumination and to irradiate light which is guided within the photonic fibers, wherein the irradiated light has a second given color different from the first given color.
[0015] According to a fourth non-restrictive, illustrative embodiment of the present invention, there is provided a method of adjusting an overall color of a photonic fiber structured to reflect light of a first given color in response to ambient illumination and to irradiate light which is guided within the photonic fibers, wherein the irradiated light has a second given color different from the first given color, the method comprising: controlling relative intensities of the reflected light of the first given color and the irradiated light of the second given color; and mixing the reflected light of the first given color and the irradiated light of the second given color to determine the overall color of the photonic fiber.
[0016] According to a fifth non-restrictive, illustrative embodiment of the present invention, there is provided a method of producing a photonic textile product and adjusting a color of the photonic textile product, comprising: incorporating in a textile material photonic fibers structured to reflect light of a first given color in response to ambient illumination and to irradiate light which is guided within the photonic fibers, wherein the irradiated light has a second given color different from the first given color; controlling relative intensities of the reflected light of the first given color and the irradiated light of the second given color; and mixing the reflected light of the first given color and the irradiated light of the second given color to determine the overall color of the photonic fibers and textile material.
[0017] According to a further non-restrictive, illustrative embodiment of the present invention, there is provided a method of adjusting a color of light guided by a photonic fiber comprising a light-propagating core and a periodic reflector surrounding the light-propagating core and defining at least one bandgap whereby, in response to white light, only light at frequency located within the at least one bandgap is guided by the photonic fiber, the method comprising stretching the photonic fiber to shift the bandgap and therefore change the color of the light guided by the photonic fiber.
[0018] According to another further non-restrictive, illustrative embodiment of the present invention, there is provided an anti-counterfeit label comprising a plurality of sets of photonic fibers comprising a light-propagating core and a periodic reflector surrounding the light-propagating core and defining at least one bandgap whereby, in response to white light, only light at frequency located within the at least one bandgap is guided by the photonic fiber, wherein: the photonic fibers of at least two sets have respective, different bandgaps to guide light of different colors in response to white light; and the photonic fibers of said at least two sets have a same or respective, distinct colors under reflection of ambient light.
[0019] According to a still further non-restrictive, illustrative embodiment of the present invention, there is provided an anti-counterfeit label comprising a plurality of sets of photonic fibers comprising a light-propagating core and a periodic reflector surrounding the light-propagating core and defining at least one bandgap whereby, in response to white light, only light at frequency located within the at least one bandgap is guided by the photonic fiber, wherein: the photonic fibers of at least two sets have respective, different colors under reflection of ambient light; and the photonic fibers of said at least two sets have a same bandgap or respective different bandgaps to guide light of a same color or respective, different colors in response to white light.
[0020] The foregoing and other objects, advantages and features of the present invention will become more apparent upon reading of the following non- restrictive detailed description of the illustrative embodiments thereof, given by way of example only with reference to the accompanying drawings.
[0021] The non-restrictive illustrative embodiments described in the following disclosure relates to photonic crystal fibers (PCF optical fiber(s)) usable in textile products (hereinafter photonic textile product(s)) and to the resulting photonic textile product(s). In particular, but not exclusively, real-time color- changing capability of PCF-based textile products with potential applications in dynamic signage and environmentally adaptive coloration will be described. [0022] Photonic textile products integrate light-emitting or light- processing elements into the mechanically flexible matrix of a woven material (weave of fibers), whereby appearance of these photonic textile products can be controlled. Practical implementation of photonic textile products is realized through integration of PCF optical fibers during the weaving process of textile fabrication. This approach is quite natural since PCF optical fibers, being long threads of sub- millimeter diameter, are geometrically and mechanically similar to regular textile fibers and, therefore, suitable for a similar weaving process. Applications of photonic textile products include, amongst others, large area illumination, clothes with unique esthetic appearance, flexible and wearable displays, etc.
[0023] The present disclosure will describe the following features:
- Since PCF optical fibers includes total internal reflection (TIR) fibers and photonic bandgap (PBG) fibers, the principles of operation behind the TIR fibers and the PBG fibers for application to photonic textile products;
- The advantages of PBG fibers, compared to TIR fibers, for light extraction from the core of the optical fiber;
- Advantages of PBG fibers in implementing static multicolor displays including see-through displays, in particular tri-fiber PBG yarns where the color of the irradiated light can be varied dynamically; and
- The mechanism of changing the fiber color by mixing two colors respectively resulting from irradiation of guided light and reflection of ambient light, including implementations of both woven and non-woven color changing photonic textile products that operate using this concept of color mixing.
[0024] Generally stated, a standard optical fiber efficiently guides light from an optical source to a detector. A TIR fiber, by its nature, confines light very efficiently in a core of the fiber. For example, silica glass-based telecommunication grade optical fibers are easily available on the market at low cost, but are not suitable for use in photonic textile products since such optical fibers are designed for ultra-low loss transmission with virtually undetectable side leakage. A problem related to the application of silica glass-based telecommunication grade optical fibers in photonic textile manufacturing thus becomes the irradiation of guided light from the optical fibers. In TIR fibers, light is guided via consecutive reflections at the fiber/air interface. Only light rays within the cone defined by the fiber numerical aperture are guided along the optical fiber, while the light rays with steeper angles of propagation leak out within a short distance (typically several centimeters) of propagation from the optical source.
[0025] Extraction of guided light from the core of a TIR fiber can be accomplished by introducing perturbations at the fiber/air interface.
[0026] Figure 1a illustrates a first technique to realize such perturbations. The technique of Figure 1a comprises macro-bending of the optical fibers such as 101 by the threads such as 102 in a textile product 103. A disadvantage of the macro-bending technique is the high sensitivity of the intensity of the scattered light such as 104 to the value of the radius of bending. As the threads 102 of the textile product 103 are typically quite elastic, insuring that the optical fiber 101 is sufficiently bent with a constant radius throughout the whole textile product becomes challenging. If uniformity of the fiber bending radii over the whole surface of the textile product is not well controlled, only portions of the textile product featuring tightly bend fibers will be lit up. This problem becomes especially severe in the case of a wearable photonic textile product in which local textile structure is prone to changes due to variable force loads during wear, thus resulting in 'patchy' looking non-uniformly luminescent textile products. Moreover, the optical and mechanical properties of the silica glass-based telecommunication grade optical fibers degrade irreversibly when the fibers are bent into tight bends (for example bending radii in the range of millimeters) which are necessary for efficient light extraction, thus resulting in somewhat fragile textile products. [0027] Figure 1b illustrates a second technique to realize the perturbations. The technique of Figure 1b uses scratching of the outer surface 112 of the optical fiber 113 to create defects such as 111 that produce scattered light such as 114. A disadvantage of the scratching technique is that mechanical or chemical processes used to roughen the fiber surface 112 also generally tend to introduce mechanical defaults into the fiber structure, thus resulting in weaker optical fibers prone to breakage. Also, due to the random nature of, for example, mechanical scratching or chemical etching, these techniques tend to also introduce a number of randomly located strong optical defaults which result in almost complete leakage of light at a few singular points, making the appearance of the photonic textile product unappealing.
[0028] Finally, photonic textile products incorporating macro-bent or scratched optical fibers are non-transparent due to strong scattering within the optical fibers, thus precluding the possibility of see-through photonic textiles-based displays.
[0029] The use of PBG fibers instead of TIR fibers eliminates technological problems associated with light extraction from the optical fibers, while also allowing additional functionalities. For example, amongst others, PBG fibers enable the following additional functionalities: static multi-color flexible displays requiring only white light sources, semi-transparent flexible displays, and passive photonic textile products that change color as a function of the intensity of ambient light. Although the following description holds for any PBG fiber, hollow and solid core PBG Bragg fibers will be used as non-limitative examples.
[0030] Figure 1c is a schematic diagram of a hollow core PBG fiber 121 comprising an air-filled light-propagating hollow core 122 surrounded by a periodic sequence of high and low refractive index layers 123 forming a so-called periodic Bragg reflector 124. A quasi-periodic Bragg reflector could also be used. A distinguishing feature of such a periodic reflector 124 is the presence of at least one bandgap, i.e. of a spectral region in which the periodic reflector 124 is highly efficient in reflecting light; this phenomenon is caused by interference effects inside the periodic multilayer reflector 124. In a PBG optical fiber, light with frequency inside of a bandgap of the reflector 124 can be effectively confined in the hollow core 122 through reflections from the surrounding periodic Bragg reflector 124. The effective refractive index of a core guided mode is typically somewhat smaller than that of air filling the hollow core 122. In practice, due to the finite number of layers 123 of the reflector 124, there are always tunneling and leakage, that is irradiation of light guided by the PBG optical fiber 121 sideways across the periodic reflector 124. By changing the number of layers 123 of the reflector 124, it is possible to control the rate of light leakage or irradiation. Thus, for energy transmission applications, the number of reflector layers 123 is increased to suppress radiation loss while, for illumination applications, a relatively small number of reflector layers 123 is chosen to allow sizable sideway irradiation of guided light. Another characteristic feature of bandgap guidance is wavelength filtering. More particularly, when launching white light 125 from a white light source 126 into the hollow core PBG fiber 121 , only a particular light color 127 determined by the bandgap will be guided, while all the other light colors 129 will be irradiated out of the PBG optical fiber 121 after a few centimeters of propagation. An advantage of the hollow core PBG Bragg fiber technology for photonic textile products is that such fiber can irradiate guided light sideways without the need of any mechanical deformations. Moreover, the rate of light leakage or irradiation and the color of irradiated light 128 can be controlled by varying the number of layers 123 of the reflector 124, and the thicknesses of the reflector layers 123, respectively.
[0031] Figure 1d is a schematic diagram of a solid core PBG fiber 131 which constitutes another type of optical fiber that can be used in the fabrication of photonic textile products. Similar to the hollow core fiber, geometry of a solid core PBG fiber features a Bragg periodic reflector 132 made of a sequence of high and low refractive index layers 133. The reflector 132 surrounds a low refractive index core 134, which is typically made of the same material as the low refractive index layers 133 of the periodic reflector 132. In response to white light 140 launched in the solid core PBG optical fiber 131 from a light source 141 , light 135 of a given color determined by the at least one bandgap of the surrounding periodic reflector 132 is guided in the low refractive index, light-propagating core 134 of the solid core PBG fiber 131 , while light 139 of non-guided colors leaks through the reflector 132 and into the fiber cladding 137 (layer surrounding the reflector 132) within the first few centimeters of propagation. The effective refractive index of a core guided mode in a solid core PBG fiber is somewhat smaller than that of a core refractive index, although larger than the refractive index of air. For that reason, guided light that leaks from the fiber solid core 134 and into the cladding 137 will be contained in the fiber cladding material. In this respect, a solid core PBG fiber exhibits an overall TIR light guidance and no sideways radiation of light is expected. In practice, due to scattering on imperfections or defaults such as 138 in the geometry and materials of a solid core PBG fiber, light 136 is always partially irradiated outside of the fiber. Examples of imperfections or defaults can be micro-scratches on the fiber surface or dust particles integrated into the multilayer structure during, for example, co-rolling of the layers. Typical sizes of such imperfections of defaults are from sub-micron to several tens of microns. Moreover, irradiation of light 136 can be further enhanced by introducing scattering centers in the form of nano- powders or bubbles (not shown) that can be easily integrated into the structure, for example the layers of the reflector 132 of the solid core PBG fiber 131 , without significantly compromising the fiber mechanical properties. Examples of nano- powders can be almost any dielectric (ceramic) or metallic powders of a typical size from tens of nanometers to tens of microns. A non-restrictive example of the nano- powders can be zirconia or alumina powders. Bubbles sizes are typically from the tens of nanometers to the tens of microns and can be introduced intentionally during preform fabrication procedure by, for example, adding agents (including solvents) that release gas during drawing at elevated temperatures. Alternatively, scattering centers can be in the form of interface roughness between the individual layers 133 of the periodic Bragg reflector 132. Such roughness can be controllably introduced into the structure of the multilayer periodic Bragg reflector 132 by changing fabrication conditions. As a non-limitative example, co-rolling of two individual low and high refractive index films around a core rod during fabrication of the preform will result in a higher interfacial roughness than co-rolling of a single multilayer film produced by co-extrusion.
[0032] Figures 2a and 2b are photographs of sections of solid core PBG fibers 202 approximately 20 cm long and fabricated from a same preform, while drawn to different outside diameters. Upon propagating white light 201 , the fibers 202 are glowing uniformly along their lengths with respective, distinct colors determined by the bandgaps of their corresponding periodic Bragg reflectors (not shown). Figures 2c and 2d are photographs showing ambient illumination of the fibers 202. As shown in Figures 2c and 2d, even in the absence of guided light, the solid core PBG fibers 202 appear colored when externally illuminated and, at the same time, remain semi-transparent due to transparency of plastics used in the fabrication of the fibers 202.
Understanding colors of PBG optical fibers
[0033] Solid and hollow core PBG optical fibers can be fabricated using layer-by-layer deposition of polymer films, as well as co-rolling of commercial or home-extruded polymer films around a solid core or a core mandrel in the case of a hollow core optical fiber.
[0034] Optical fiber fabrication typically proceeds by first making a fiber preform, which is a macroscopic elongated member having a cross section similar to that of a desired optical fiber. A typical preform can be a cylinder several centimeters to several meters long having a diameter of several centimeters to several tens of centimeters. By placing a preform into a vertical furnace and lowering it into the hot zone, the lower extremity of the preform softens. Finally, pulling the soft section of the preform reduces dramatically its transversal dimension to produce the optical fiber. This process is called fiber drawing and results in an optical fiber of typically sub-mm size and a cross section similar in structure to that of the original preform. Figure 3a is a photograph of one end of a typical solid core fiber preform 302. Figure 3b is a photograph showing an enlarged portion 303 of the solid core fiber preform 302 of Figure 3a, and Figure 3c is a photograph in perspective of the resulting solid core PBG optical fiber 301. The solid core PBG optical fiber 301 of Figure 3c comprises a central solid core 305 surrounded by a multilayer periodic Bragg reflector 304 itself surrounded by a cladding 306. During fabrication of PBG optical fibers two material combinations, for example polystyrene (PS)/poly(methyl methylacrylate) (PMMA) and polycarbonate (PC)/poly(vinylene difloride) (PVDF) featuring refractive index contrasts of 1.6/1.48 and 1.58/1.4 respectively, are typically used to make the core 305 and the alternate layers of different refractive indices of the periodic reflector 304. Description of guided states in such PBG optical fibers typically starts with finding the bandgap or bandgaps of the periodic Bragg reflector 304 (Figure 3c). Thus, Figure 3d is a graph illustrating a typical band diagram (frequency ω versus propagation constant β) of the guided modes of an infinite periodic Bragg reflector
304 fabricated from PC/PVDF and having layers of equal thicknesses.
[0035] In the graph of Figure 3d, gray regions on the band diagram describe states delocalized over the whole periodic Bragg reflector 304. Such guiding states are efficiently irradiated out of the solid core PBG optical fiber 301 by the imperfections at the air/cladding 306 interface of the fiber, as well as by the imperfections at the numerous layer interfaces inside of the multilayer periodic Bragg reflector 304. Thus, when launching white light from a white light source into the solid core PBG optical fiber, guiding states delocalized over the whole fiber cross section are typically irradiated in the first few centimeters of propagation.
[0036] Figure 3d also shows regions of phase space where no delocalized states exist inside of the periodic Bragg reflector 304; these are the reflector bandgaps (white areas 307). The periodic Bragg reflector 304, therefore, can confine light in the solid core of the PBG Bragg fiber 301 if such guided light falls into the reflector bandgaps. As the size of the core of a PBG optical fiber is large (compared to the radiation wavelength), light propagation inside of the core
305 can be seen as a sequence of consecutive bounces of light rays traveling at shallow angles with respect to the core 305/reflector 304 interface. The effective refractive index of such rays will be close, while somewhat smaller than that of the material of the core 305. Thus, on the band diagram of Figure 3d, a core guided mode (shown as a solid curve 308) appears as a state inside of the Bragg reflector bandgaps (white areas 307), which is positioned somewhat above the light line 309 of the material forming the core 305; the light line 309 can be described by a simple Equation ωn CO re=β, where n cor e is the refractive index of the core material. The color of the light guided by the fiber core 305 is then defined by the spectral position of a bandgap that supports the core guide mode. Spectral position of such a bandgap can be varied at will by changing the thicknesses of the reflector layers, with thicker layers shifting the bandgap to longer wavelengths. For example, the thicknesses of the layers forming the periodic Bragg reflector 304 can be changed by drawing the same preform to fibers of different diameters. In this case, larger-diameter fibers will have thicker reflector layers and, as a consequence, will have different color guiding and reflection properties.
[0037] An interesting property is that, even with no light guided and traveling inside a PBG optical fiber, while only under the ambient (external) illumination, the PBG optical fiber still appears colored (see Figures 2c and 2d and corresponding description). Typically, the color of the PBG optical fiber is determined by properties of the periodic Bragg reflector to reflect ambient light at close to normal incidence (/3=0). Figure 3d shows that, in the case of a low refractive index-contrast all-polymer PBG optical fiber, the bandgap of the Bragg reflector at normal incidence is located at a spectral position different from a reflector bandgap that supports core guided mode (see white areas 307). Therefore, the color of the PBG optical fiber under ambient illumination is generally different from the color of the PBG optical fiber due to irradiation of core guided light. This property opens an interesting opportunity of adjusting the overall color of the PBG optical fiber by controlling the relative intensities of the ambient and guided light. Although it appears from Figure 3d that color of the guided light (for example green) always has a frequency higher than the frequency of the color of the PBG optical fiber due to ambient illumination (for example red), this is not a general rule. For instance, under normal incidence and above the fundamental bandgap of the Bragg reflector of the PBG fiber, a number of higher order bandgaps are typically found. Therefore, if the guided light is red, the fundamental bandgap will be in the near-IR (invisible to the human eye) and, then, the color of the PBG fiber under ambient illumination is determined by the spectral position of the higher order bandgaps of the Bragg reflector; this can result in a color of higher frequency (say green) than the frequency of the color of the guided light.
Color-changing textiles under variable ambient illumination
[0038] As can be appreciated from the foregoing description, an interesting property of PBG fibers photonic textile products is therefore their ability to change color by mixing the ambient light reflected from the PBG optical fibers with guided light irradiated from these PBG optical fibers.
[0039] Figure 4a is a schematic diagram of a hollow core PBG fiber
401. As described herein above, launching white light 402 from a light source 403 into such a hollow core PBG fiber 401 causes only the color of the light 405 guided by the bandgap of the periodic Bragg reflector 404 to propagate along the hollow core PBG fiber 401 (a solid core PBG fiber could also be used). Light 406 of all the other colors is irradiated out of the fiber 401 in the first few centimeters of propagation. Due to the finite number of layers such as 407 of the periodic Bragg reflector 404, and due to presence of imperfections at the interfaces between the layers 407 of the Bragg reflector 404, the color of the guided light 405 slowly leaks out (see 411) of the hollow core 408 of the PBG optical fiber 401 , thus resulting in coloration of the PBG fiber 401 with the color of the guided light 405. On the other hand, under daylight or other ambient white light illumination 409, and in the absence of guided light 405, the hollow core PBG fiber 401 is still colored (Figures 2c and 2d). As described in the foregoing description, this phenomenon is caused by reflection of light 410 of a given color by the reflection bandgap of the Bragg reflector 404 at close to normal angles of incidence of the ambient light 409 as shown in Figure 4a. As it was demonstrated in the above description with reference to Figure 3d, color of the reflected ambient light 410 is generally different from the color of the irradiated core-guided light 411. Therefore, when both ambient illumination 409 and guided light 405 are present, the overall light color of the hollow core PBG optical fiber 401 will be determined by mixing 412 of the two colors in the radiation far field. This opens an interesting possibility of actively controlling the resulting overall color of the hollow core PBG optical fiber 401 under a given ambient illumination by changing the intensity of the guided light 405 and therefore the intensity of the guided light 411 irradiated from the PBG optical fiber 401.
[0040] Figures 4b to 4d are photographs of a practical demonstration of the above described color-mixing concept. For the purpose of this demonstration, four (4) solid core PBG optical fibers such as that of Figure 4a were suspended in the air parallel to each other.
[0041] Figure 4c is a photograph that was taken under ambient daylight illumination with light guided through the solid core PBG optical fibers, resulting in a red color of the solid core PBG fibers.
[0042] Figure 4b is a photograph that was taken under ambient daylight illumination with additional lighting from the top, and no light guided through the solid core PBG optical fibers, resulting in a green color of the solid core PBG fibers.
[0043] Figure 4d is a photograph that was taken both under ambient illumination and guided light using a microscope focused on the solid core PBG optical fibers. The resultant appearance of the solid core PBG fibers in the near field is a collection of green and red stripes.
[0044] Figure 4e is also a photograph that was taken both under ambient illumination and guided light using a microscope defocused from the solid core PBG fibers to get an image of the solid core PBG fibers in the far field; this revealed a yellow fiber bundle. [0045] Although all the photographs of Figures 4b-4e were captured under ambient daylight illumination, the backgrounds appear black. This is due to the fact that to snap the pictures of fibers a 5x microscope was used. As the solid core PBG fibers were suspended in the air, there was no reflective background in the field of view of the microscope, thus resulting in a black background.
Potential applications of the photonic textile products
[0046] A first application of the photonic textile product is a variable color uniform for enhanced visibility during daylight. The principle of operation of such uniforms is presented in Figure 5a. In this particular example, the photonic textile product 501 comprises green PBG optical fibers. Due to high reflectivity of the green light from the PBG optical fibers under ambient illumination (daylight) 503, such uniforms will be well discernible (green color) on a bright day. Additionally, for example, the green PBG optical fibers 505 of a portion of the photonic textile product 501 in the form of a particular word 502 such as "STOP" can be connected to a source of while light 504. Under ambient daylight illumination, the color of the word 502 will be determined by the combination of the green color (from the reflected ambient light) and, for example, the red color (irradiated guided light from the green PBG optical fibers). Thus, by injecting no light into the green PBG fibers 505, the word 502 remains green and not visible on the green background (left image of Figure 5a). However, by launching white light from the source 504 and changing the intensity of the white light launched from the source 504 into the green PBG fibers 505, the word 502 change color from yellow (central image of Figure 5a (medium intensity of white light)) to orange (right image of Figure 5a (high intensity of white light)). As the human eye is especially sensitive to detection of changes, such uniforms are expected to be highly noticeable during daytime.
[0047] Referring to Figure 5b, high intensity white light is launched from the source 504 into the green PBG optical fibers 505 to make the word 502 (in, for example, red color) highly distinguishable at nighttime. In the absence of external illumination (left image of Figure 5a), the word 502 is red and the rest of the uniform is invisible due to the absence of ambient light. Let's consider now a car approaching from a distance with the headlights on. As the unlit green PBG fibers (corresponding to the areas of the photonic textile product 501 outside the word 502) are highly reflective, when the car approaches, the uniform starts glowing green (see central and right images of Figure 5a), while the color of the word 502 (made of the same green PBG optical fibers) starts changing. Thus, when the car is relatively far, the intensity of the ambient light 506 is low, the intensity of the green light reflected from the green PBG optical fibers is correspondingly low, and mixing of the green light reflected from the green PBG optical fibers 505 with the strong red color guided light irradiated from the green PBG optical fibers 505 causes the word 502 to turn orange (central image of Figure 5b). When the car gets closer, the intensity of the ambient light 507 increases, the intensity of green light reflected from the green PBG optical fibers also increases, and mixing of the green light reflected from the green PBG optical fibers 505 with the strong red color guided light irradiated from the green PBG optical fibers 505 causes the word 502 to turn yellow (right image of Figure 5b).
[0048] Figure 5c illustrates an application of the color changing photonic textile product to an anti-counterfeit label 508. A possible implementation of such an anti-counterfeit label 508 comprises two planar sets of laterally adjacent, parallel PBG optical fibers 509 and 510, both having the same color (for example green) or respective, different colors under the reflection of ambient light 511 (top view of Figure 5c). The two sets of PBG optical fibers 509 and 510, however, are chosen to guide light of different colors, say red and blue. Another possible implementation of such an anti-counterfeit label 508 comprises the two planar sets of laterally adjacent, parallel PBG optical fibers 509 and 510, having respective, different colors under the reflection of ambient light 511. The two sets of PBG optical fibers 509 and 510, however, are chosen to guide light of the same color or respective different colors, say red and blue. To avoid complicated setups for launching light inside the PBG fibers, it is possible to make them sufficiently short so that when looking through the PBG optical fibers at any white light source 517, the distinct guided colors, for example red and blue, are seen at the edge of the anti-counterfeit label 508 (side view of Figure 5c). In principle, any color code can be implemented while the appearance of the label 508 can be also of any kind. While being relatively inexpensive, fabrication of labels such as 508 require a considerable know-how, thus creating a barrier for counterfeiting.
[0049] Figure 5d illustrates a further application of the color changing photonic textile product to discreet and intelligent jewelry 512. For example, unlit green PBG optical fibers 513 cover the surface of a broach or a button. However, white light from a source 514 is launched in the green PBG optical fibers 513 of a section 515 in the form of a symbol (for example a heart) to radiate, for example, red light. Outside and in daylight, or in a brightly illuminated office, that is under ambient illumination 516, the symbol will be discreet and almost indistinguishable from the background (bottom image of Figure 5d). On the contrary, in the dark, the symbol will show up in red to indicate, for example, the onset of the wearer's private time (top image of Figure 5d).
[0050] As the PBG fibers are colored under ambient light, mixing red
(R), green (G) and blue (B) PBG optical fibers in a proper proportion will produce highly reflective yarns and photonic textile products of any desired color. Such photonic textile products could find applications in various fields including, for example, uniforms, signage and machine vision. As no colorants are used in the fabrication of the individual plastic PBG fibers, the PBG fiber-based photonic textile products could also prove to be "greener" than their traditional counterparts for which a dyeing process is used. Moreover, the color of such photonic textile products under ambient illumination is very stable as it is determined by the fiber structure alone. No fading of color is expected as no dyes are used in the textile fabrication process.
[0051] Figure 6a is a photograph showing that it is possible to use yarns or bundles of three (3) PBG optical fibers respectively propagating and irradiating red (R), green (G) and blue (B) light to adjust at will the color of a photonic textile product by controlling the light intensities guided through the respective red (R), green (G) and blue (B) light irradiating PBG optical fibers of every yarn or bundle. Figure 6b is a schematic diagram showing a photonic textile product 601 comprising laterally adjacent bundles such as 602 of three (3) PBG optical fibers respectively propagating and irradiating red (R), green (G) and blue (B) light. The bundles 602 of three (3) PBG fibers respectively propagating and irradiating red (R), green (G) and blue (B) light are supplied with white light 603 from a white light source 604 though a 1x3 optical intensity coupler 605. Therefore, the use of PBG fibers requires only one white light source instead of three (3) different sources of red (R), green (G) and blue (B) light to adjust the overall color of the photonic textile products. This is a major advantage of the proposed PBG fiber-based textile products over the existing photonic textiles utilizing TIR optical fibers. In particular, color stability of TIR fiber-based textiles relies on constant monitoring of the relative intensities of the (R), (G), and (B) light sources. Due to intensity fluctuation of the individual source intensities over time a complex monitoring system have to be implemented to ensure stability of the textile color. In the case of a failure of one of the light sources, the textiles will completely change its color. In the case of PBG fiber-based textile products all these issues do not exist since only a single white light source is used.
[0052] In operation, the 1x3 optical intensity coupler 605 forms a control of the intensities 605, 606 and 607 of white light 603 supplied to the PBG optical fibers respectively propagating and irradiating red (R), green (G) and blue (B) light. The particular color of the photonic textile product 601 is then determined by the relative intensities 605, 606 and 607 of white light 603 coupled into the PBG optical fibers, which controls the intensities of the red (R), green (G) and blue (B) light guided within and irradiated from the PBG optical fibers of each bundle 602. In principle, by mixing the reflected color of the photonic textile product with the irradiated color, it is possible to vary at will the textile appearance both in daylight and nighttime conditions.
Fabrication of photonic textile products [0053] Fabrication of photonic textile products comprises integrating photonic crystal fibers, for example PBG optical fibers within a textile matrix.
[0054] For example, Figure 7a is a photograph of a sample of photonic textile product 701 comprising woven in PBG optical fibers such as 702 irradiating guided light, under no ambient illumination. The PBG optical fibers 702 are positioned parallel to each other with ends 703 leaving the photonic textile product 701 for optical coupling to a white light source 704.
[0055] More specifically, in Figure 7a, semi-automatic Jacquard loom
701 was used to integrate solid core PBG optical fibers 702 into black and white cotton. White cotton was used as a warp and black and white cotton was used as a weft together with the PBG optical fibers 702. Of course, the PBG optical fibers 702 could be also used as a warp. At the entrance edge (salvage edge) 705 of the Jacquard loom 701 , care was taken to make the textile loose enough so that no significant micro-bending of the PBG optical fibers was observed. The color of the light guided and irradiated from the PBG optical fibers 702 was spanning blue to yellow colors. To realize an easily discernable leaf pattern in the photonic textile product, a weaving technique in which the illuminated PBG optical fibers are either hidden behind the threads of the textile product or exposed on the surface of the textile product was used. By hiding partially or completely the PBG optical fibers behind the white or black or both threads of a fabric it was possible to control the contrast of the illuminated pattern (leafs) on the textile surface (see 706).
[0056] Figure 7b is a photograph of a sample of photonic textile product
711 comprising woven in PBG optical fibers 712 under ambient illumination, with no light guided by the PBG optical fibers 712.
[0057] Therefore, in Figure 7b, a PBG fiber-based photonic textile product 711 is shown under ambient illumination only. The photonic textile product 711 has demonstrated to be highly reflective and showed distinct metallic-like colors.
[0058] Figure 7c is a photograph of a sample of photonic textile product
721 comprising woven in PBG optical fibers 722 under ambient light illumination, with no light guided by the PBG optical fibers. Figure 7d is a photograph of the sample of photonic textile product 721 comprising woven in PBG optical fibers 722 irradiating guided light under ambient illumination, wherein reflected light and irradiated guided light mix to synthesize textile color. Figure 7e are photographs of the sample of photonic textile 721 showing change of the color via variation of the intensity of the irradiated guided light. Figures 7c-7e demonstrate the color changing functionality of the photonic textile product 721. For example, the same sample of photonic textile product 721 is shown under ambient light illumination only in Figure 7c and under both ambient light illumination and irradiated guided light in Figure 7d. A clear distinction in textile appearance is visible in Figures 7c and 7d
[0059] Finally, Figure 7e illustrates portions of the photonic textile product 721 to demonstrate clearly how the perceived textile color changes due to color-mixing principle. The left column of photographs of Figure 7e shows the sample of photonic textile product 721 with the woven in PBG optical fibers 722 irradiating guided light under ambient illumination, while the right column of photographs show the sample of photonic textile product 721 with the woven in PBG optical fibers 722 under ambient light illumination, with no light guided by the PBG optical fibers. The left and right columns of photographs of Figure 7e clearly show the resulting contrast.
[0060] The PBG optical fibers could also be integrated to a non-woven textile material, by introducing the PBG optical fibers in the textile materias by knitting or braiding. [0061] Stretching fabrics
[0062] Figure 8a is a photograph of a setup in which a 20 cm long piece of PBG optical fiber 801 is placed inside fiber chucks 802 and 803 and then stretched. With this setup, the fiber 801 could be stretched up to 1% in length before fiber slippage occurred. Figure 8b is a graph showing spectra of fiber transmission taken before (804) and after (805) stretching, and bandgap shift by about 1% (7 nm) can be easily observed. Shift in the transmission spectrum of the PBG optical fiber 801 is equivalent to a change in the color of the light guided and irradiated by the PBG optical fiber 801.
[0063] PBG optical fibers such as 801 can therefore be integrated into a photonic textile product with the color of the irradiated, guided light changing upon stretching of the photonic textile product. Such a photonic textile product can therefore be applied to the area of dynamic apparels that change their appearance in response to stretching. In principle, it is possible to realize PBG optical fibers such as 801 from very soft materials, thus allowing considerable elongations of the PBG optical fibers, which would result potentially in dramatic changes of color of such PBG optical fibers under relatively small stress.
[0064] It is to be understood that the invention is not limited in its application to the details of construction and parts illustrated in the accompanying drawings and described hereinabove. The invention is capable of other embodiments and of being practiced in various ways. It is also to be understood that the phraseology or terminology used herein is for the purpose of description and not limitation. Hence, although the present invention has been described hereinabove by way of non-restrictive, illustrative embodiments thereof, these embodiments can be modified at will within the scope of the appended claims, without departing from the spirit and nature of the subject invention.
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