Patent Publication Number: US-2009238514-A1

Title: Optical waveguide having grating and method of forming the same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority of U.S. Provisional Application No. 61/038,483, filed in the U.S. Patent and Trademark Office on Mar. 21, 2008, the entire contents of which is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Apparatuses, devices, and methods consistent with the present invention relate to optical waveguides and, more particularly, to an optical waveguide sensor having a long period grating. 
     2. Description of the Related Art 
     In a related art optical waveguide, the characteristic of light transmission depends on the interface between a core and a cladding material and, more specifically, on the difference between the refractive index of the core and the refractive index of the cladding material. Recently, long period gratings (LPGs) have been added to manipulate the light resonance between the core and the cladding material. The LPG couples the core guided mode with the cladding modes, propagating in the same direction. When a coupling between the guided mode and the cladding mode occurs, a relationship between those modes is given by 
       λ 0 =( N   0   −N   m )Λ  (1) 
     where λ 0  is a resonance wavelength, at which the guided mode and m-th cladding mode are coupling, N 0  is the effective refractive index of the guided mode, N m  is the effective refractive index of the cladding mode, and Λ is the grating period. The excitation of the cladding mode attenuates the light intensity of the guided mode after the LPG, which results in a resonant loss in the transmission spectrum. 
     The LPG is fabricated as either a phase grating, which periodically manipulates the material refractive index of the waveguide core by means of inscription, or by corrugation grating, which periodically creates geometrical features by means of material removal. 
     This fabrication process has a few disadvantages. For example, many different operations are involved, such as using laser inscription, laser cutting, thermal inscription, reactive ion beam (RIB) etching, and the like. Also, many different materials are involved, with each operation requiring a different equipment setup. Accordingly, the fabrication cycle time of the related art—optical waveguide is long and costly, and there are many constraints on the geometry of the optical waveguide. 
     SUMMARY OF THE PRESENT INVENTION 
     Exemplary embodiments of the present invention address the above disadvantages and other disadvantages not described above. However, the present invention is not required to overcome the disadvantages described above, and thus, an exemplary embodiment of the present invention may not overcome any of the disadvantages described above. 
     According to an exemplary embodiment of the present invention, there is provided an optical waveguide comprising a first cladding layer; a first waveguide core formed on the first cladding layer, the first waveguide core comprising a first long period grating formed in at least one sidewall of the first waveguide core; and a second cladding layer formed over the first waveguide core. 
     According to another exemplary embodiment of the present invention, there is provided a process for forming an optical waveguide, the process comprising forming a first waveguide core on a surface of a first cladding layer; patterning the first waveguide core with a long period grating that is perpendicular to a surface of the first cladding layer; and forming a second cladding layer on the first cladding layer so as to cover the first waveguide core. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects of the present invention will become more apparent and more readily appreciated from the following description of exemplary embodiments of the present invention taken in conjunction with the attached drawings, in which: 
         FIGS. 1A and 1B  show a perspective view and a front view, respectively, of an optical waveguide according to a first exemplary embodiment of the present invention; 
         FIGS. 2A and 2B  show a perspective view and a front view, respectively, of an optical core array waveguide according to a second exemplary embodiment of the present invention; 
         FIG. 3  shows a front view of another example of an optical core array waveguide; 
         FIGS. 4A and 4B  show a perspective view and a front view, respectively, of a stacked optical core array waveguide according to a fourth exemplary embodiment of the present invention; 
         FIG. 5  shows a front view of another example of a stacked optical core array waveguide; and 
         FIGS. 6A to 6I  show perspective views illustrating a process for making an optical waveguide according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE PRESENT INVENTION 
     Exemplary embodiments of the present invention will now be described with reference to the accompanying drawings. In the following description, like reference numerals refer to like elements throughout. 
     Turning now to  FIGS. 1A and 1B , an optical waveguide  10  according to a first exemplary embodiment of the present invention is shown. The optical waveguide includes a first cladding layer  15  as an undercladding layer. A waveguide core  20  is disposed on top of the first cladding layer  15 . The waveguide core  20  is generally rectangular in shape, and has two sidewalls  24 , a bottom surface  26  and a top surface  25 , extending in the length direction. The waveguide core  20  includes a long period grating  30 , which is formed in a portion of at least one of the two sidewalls  24 . Alternatively, the long period grating  30  may be formed in portions of both sidewalls  24  of the waveguide core  20 . A second cladding layer  35  is formed on top of the first cladding layer  15  and covers the waveguide core  20 . 
       FIG. 1B  shows a front view of the optical waveguide  10 . In  FIG. 1A , the height and width of the waveguide core  20  are shown as roughly square. However, as shown in  FIG. 1B , the height and width of the waveguide core  20  may alternatively be rectangular in the height and width direction. 
     The first cladding layer  15  may be of any thickness as long as the cladding mode is confined to the second cladding layer  35 . The waveguide core  20  shown in  FIG. 1A  has dimensions of about 5 μm by about 5 μm in the height and width direction respectively. The second cladding layer  35  has a thickness of about 10 μm. 
     The first cladding layer  15 , the second cladding layer  35 , and the waveguide core  20  are each made of a polymer material, which is sensitive to ultraviolet light. The polymer materials are selected based on the refractive indices of the materials. A relationship among the refractive indices of the waveguide core  20 , the first cladding layer  15  and the second cladding layer  35  is n (core) &gt;n (clad 2) &gt;n (clad 1) , where n (core)  denotes the refractive index of the waveguide core  20 , n (clad 1)  denotes the refractive index of the first cladding layer  15 , and n (clad 2)  denotes the refractive index of the second cladding layer  35 . Under this relationship for the refractive indices of the waveguide core  20 , the first cladding layer  15  and the second cladding layer  35 , the cladding mode propagates in the second cladding layer. Alternatively, the relationship of the refractive indicies may be n (core) &gt;n (clad 1) &gt;n (clad 2) , in which case the cladding mode propagates in the first cladding layer. 
     Turning to  FIGS. 2A and 2B , an optical waveguide  50  according to a second exemplary embodiment of the invention is shown. The first cladding layer  15  and the second cladding layer  35  are the same as in the first exemplary embodiment described above. In the second exemplary embodiment, two waveguide cores are provided including a first waveguide core  40  and a second waveguide core  45 . The first waveguide core  40  and the second waveguide core  45  are separated from each other and run substantially parallel to each other in the length direction. Alternatively, the first waveguide core  40  may be deviated by a given angle from the second waveguide core  45 . 
     The first waveguide core  40  includes a first long period grating  43 , and the second waveguide core  45  includes a second long period grating  47 . In contrast to the first exemplary embodiment described above, the first long period grating  43  is provided in both sidewalls of the first waveguide core  40 , and the second long period grating  47  is provided in both sidewalls of the second waveguide core  45 . Alternatively, the first long period grating  43  and the second long period grating  47  may be provided in only one sidewall of the first waveguide core  40  and the second waveguide core  45 , respectively. The period of the first long period grating  43  and the second long period grating  47  are substantially the same. However, alternatively, the periods may be different. Additionally, the depth of the first long period grating  43  and the second long period grating  47  are substantially the same. However, alternatively, the depths may be different. 
     The refractive indices of the first waveguide core  40  and the second waveguide core  45  are substantially the same. As in the first exemplary embodiment described above, the materials are polymer materials selected to satisfy the relationship n (core) &gt;n (clad 2) &gt;n (clad 1) . Alternatively, the polymer materials may be selected to satisfy the relationship n (core) &gt;n (clad 1) &gt;n (clad 2) . 
     As shown in  FIG. 3 , a waveguide core of an optical waveguide may also be formed as a core array. This configuration may be used, for example, to form an optical waveguide sensor. As shown in  FIG. 3 , an optical waveguide  100  includes a first cladding layer  15  and a second cladding layer  35 , both of which are the same as in the first exemplary embodiment. The waveguide core of the optical waveguide  100  includes a plurality of waveguide cores  20 . Each of the plurality of waveguide cores  20  is the same as the optical waveguide core  20  of the first exemplary embodiment, and includes a long period grating. The long period gratings of the individual waveguide cores  20  may have a same period or different periods, and may be of the same depth or different depths. The polymer materials are the same as described above with respect to the first exemplary embodiment. 
       FIGS. 4A and 4B  show an optical waveguide according to a third exemplary embodiment of the present invention. The optical waveguide  200  according to the third exemplary embodiment includes a plurality of waveguide cores arranged in a stacked configuration. This configuration also may be used, for example, to form an optical waveguide sensor. The optical waveguide  200  includes a first cladding layer  15 , a second cladding layer  35 , a first waveguide core  40 , and a second waveguide core  45 , each of which is the same as in the second exemplary embodiment described above and hence a repeated description will be omitted. The first waveguide core  40  and the second waveguide core  45  respectively include the first long period grating  43  and the second long period grating  45 , which are also the same as in the second exemplary embodiment. 
     The optical waveguide  200  further includes a third waveguide core  60  and a fourth waveguide core  70 . The third waveguide core  60  and the fourth waveguide core  70  are each substantially the same as the optical waveguide core  20  of the first exemplary embodiment. The third waveguide core  60  and the fourth waveguide core  70  are formed on top of the second cladding layer  35 . A third cladding layer  80  is formed over the second cladding layer  35 , and covers the third waveguide core  60  and the fourth waveguide core  70 . Thus, the third waveguide core  60 , the fourth waveguide core  70 , and the third cladding layer  80  form a second optical layer, that is stacked on top of a first optical layer, which includes the first cladding layer  15 , the first and second waveguide cores  40 ,  45 , and the second cladding layer  35 . 
     As shown in  FIGS. 4A and 4B , the third waveguide core  60  and the fourth waveguide core  70  of the second optical layer are formed without any long period gratings. As an alternative, the third waveguide core  60  and the fourth waveguide core  70  may include respective long period gratings having the same or different periods, and the same or different depths. 
     The third waveguide core  60  and the fourth waveguide core  70  are formed in parallel over the first waveguide core  40  and the second waveguide core  45 , respectively, such that each of the first waveguide core  40 , the second waveguide core  45 , the third waveguide core  60  and the fourth waveguide core  70  run in parallel to one another in the length direction. However, the second optical layer may alternatively be deviated by a certain angle from the first optical layer such that the third and fourth waveguide cores  60 ,  70  are deviated from the first and second waveguide cores  40 ,  45 . 
     The polymer materials of the first cladding layer  15 , the second cladding layer  35 , the third cladding layer  80 , and the first, second, third, and fourth waveguide cores  40 ,  45 ,  60 ,  70  are selected based on their respective refractive indexes. The materials for the first, second, third, and fourth waveguide cores  40 ,  45 ,  60 ,  70  are selected such that the refractive index of the first, second, third, and fourth waveguide cores  40 ,  45 ,  60 ,  70  are the same. The materials are selected to satisfy the following relationship: n (core) &gt;n (clad 2) &gt;n (clad 1) ≧n (clad 3)  or n (core) &gt;n (clad 2) &gt;n (clad 3) ≧n (clad 1) , where n (core)  denotes the refractive index of the waveguide cores, n (clad 1)  denotes the refractive index of the first cladding layer  15 , and n (clad 2)  denotes the refractive index of the second cladding layer  35 , and n (clad 3)  denotes the refractive index of the third cladding layer  80 . Under this relationship of the refractive indexes, the cladding-mode propagates in the second cladding layer. Alternatively, the polymer materials may be selected according to the following relationship in which the cladding-mode propagates in the third cladding layer: n (core) &gt;n (clad 3) &gt;n (clad 2) ≧n (clad 1)  or n (core) &gt;n (clad 3) &gt;n (clad 1) ≧n (clad 2) . 
     While the third exemplary embodiment is shown with two waveguide cores in each of the first optical layer and the second optical layer, the number of waveguide cores in each layer may be more than two. Thus, as shown in  FIG. 5 , an optical waveguide  300  may also be provided with a first core array including a plurality of waveguide cores  20  arranged in the first optical layer  310 , and a second core array including a plurality of waveguide cores  20  arranged in the second optical layer  320 . As in the preceding exemplary embodiments, each of the waveguide cores in each of the layers is provided with a respective long period grating, and the respective long period grating may be provided in one or both sidewalls of the waveguide core. Alternatively, some layers may be provided with respective long period gratings and other layers may be provided without long period gratings, as long as at least one layer is provided with long period gratings. 
     As in the preceding exemplary embodiments, the polymer materials of the first cladding layer  15 , the second cladding layer  35 , the third cladding layer  80 , and the waveguide cores  20  are selected based on their respective refractive indexes. The materials are selected to satisfy the following relationship: n (core) &gt;n (clad 2) &gt;n (clad 1) ≧n (clad 3)  or n (core) &gt;n (clad 2) &gt;n (clad 3) ≧n (clad 1) , where n (core)  denotes the refractive index of the waveguide cores, n (clad 1)  denotes the refractive index of the first cladding layer  15 , and n (clad 2)  denotes the refractive index of the second cladding layer  35 , and n (clad 3)  denotes the refractive index of the third cladding layer  80 . Under this relationship of the refractive indexes, the cladding-mode propagates in the second cladding layer. Alternatively, the polymer materials may be selected according to the following relationship in which the cladding-mode propagates in the third cladding layer: n (core) &gt;n (clad 3) &gt;n (clad 2) ≧n (clad 1)  or n (core) &gt;n (clad 3) &gt;n (clad 1) ≧n (clad 2) . 
     Turning now to  FIGS. 6A to 6I , a process for manufacturing an optical waveguide according to an exemplary embodiment of the present invention is shown and will now be described. 
     As shown in  FIG. 6A , a first cladding layer  601  is formed from a polymer material. The polymer material is sensitive to ultraviolet light. In  FIG. 6B , the first cladding layer  601  is exposed to ultraviolet light in order to set the refractive index of the first cladding layer  601 . 
     In  FIG. 6C , a core layer  602  is formed on top of the first cladding layer  601 , and covers the first cladding layer  601 . The material used for the core layer  602  is a polymer material having a certain refractive index. In  FIG. 6D , a photolithography mask  605  is aligned on top of the core layer  602 . The photolithography mask  605  includes a channel  605  and a long period grating  604  formed in one side of the channel  605 . In this exemplary embodiment, the long period grating  604  is formed in only one side of the channel  605 . However, alternatively, the long period grating  604  may be formed in both sides of the channel  605 . The photolithography mask  605  is then exposed to ultraviolet light, as shown in  FIG. 6E . 
     The photolithography mask  605  is then removed, as shown in  FIG. 6F , and the core layer  602  is developed in order to form a waveguide core  606 , as shown in  FIG. 6G . The waveguide core  606  includes a long period grating  607 , which is in an inverse relationship to the long period grating  604  of the channel  605 . Accordingly, the long period grating  607  is in one side of the waveguide core  606 . However, as described above, the long period grating  607  may be provided in both sides of the waveguide core  606 . 
     In  FIG. 6H , a second cladding layer  608  is formed on top of the first cladding layer  601 , and covers the waveguide core  606 . The second cladding layer  608  is then exposed to ultraviolet light in order to set the refractive index. 
     Although the process has been described with respect to forming only one waveguide core  606 , the process may be applied to produce a core array of a plurality of waveguides, each having a long period grating. In such a case, the photolithography mask is formed to correspond to the core array of a plurality of waveguides, and the core array is formed at one time using the mask. Each successive optical waveguide layer may then be formed by iterative application of the process. 
     While the present invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.