Abstract:
The present invention is directed to a system and method for tuning an optical fiber Bragg grating by using a circular mechanism which uniformly stretches the fiber along its length while at the same time preserving the minimal size for stretching.

Description:
RELATED APPLICATIONS 
     The present application claims priority of benefit to Provisional Application No. 60,266,683, filed Feb. 5, 2001, entitled “OPTICAL FIBER BRAGG GRATING TUNING DEVICE,” the disclosure of which is hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The invention relates generally to fiber optic communication technologies, and more specifically to a system and method for tuning optical fiber Bragg grating by a mechanically induced strain along the fiber. 
     BACKGROUND 
     It is well known that the wavelength center of fiber Bragg gratings can be shifted by either temperature or strain variations, or both. These tuning properties have been widely used in telecommunications. 
     Chromatic dispersion in a single mode optical fiber is an important problem to solve when such optical fibers are used for telecommunication. This phenomenon induces an undesirable broadening in data pulses, which appears because there is a time delay between different wavelength components. In order to eliminate this kind of dispersion, the negative dispersion exhibited by linearly and non-linearly optical fiber Bragg gratings has been widely used. 
     Because chromatic dispersion depends on the characteristics of the fiber in a telecommunications system, a grating with an adjustable chirp is more convenient for its compensation. 
     Tunable devices stretching a chirped fiber grating to compensate for the chromatic dispersion produced by single mode fibers in optical pulses are prior art. All of them, however, apply strain along the fiber grating in a linear fashion. 
     For those applications where long fiber gratings are needed, the use of long stretcher mechanisms increases the dimensions of dispersion compensations systems. On the other hand, mechanical vibrations alter the spectral characteristics of a fiber grating, either if is suspended or bonded, when using long linear stretcher mechanisms. This affects the performance of the tunable dispersion compensation devices. 
     For example, in U.S. Pat. No. 5,999,671, a device is shown that is longitudinally stretched to change the central wave length of the Bragg grating, and accomplishing the same function is shown in U.S. Pat. No. 6,055,348 which uses magnetostrictive devices, the disclosures of which are hereby incorporated herein be reference. As discussed above, these patents, while perhaps accomplishing the desired end result, do so at the cost of size and complexity. Another disadvantage is the lack of uniformity along the longitudinal axis of the servo strained element. 
     A different type of arrangement for changing the Bragg filter center frequency is shown in U.S. Pat. No. 5,007,705 where a cylindrical piezoelectric device is shown wrapped with the fiber around its outer circumference, the disclosure of which is hereby incorporated herein by reference. Upon the application of an energy source, the piezoelectric element changes its diameter thereby changing its circumference so as to adding more or less strain onto the transmission cable. This changes the Bragg filter center wavelength. This patent is also hereby incorporated by reference herein. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a system and method for tuning an optical fiber Bragg grating by using a circular mechanism which uniformly stretches the fiber along its length while at the same time preserving the minimal size for stretching. This method may also provide a smaller package footprint for chromatic dispersion compensation. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
     FIG. 1 shown the present invention system, which includes a uniform cross section circular beam around a positioned fiber Bragg grating; 
     FIG. 2 shows a schematic of one system and method of applying the bending moment to a circular beam; 
     FIG. 3 shows an alternate embodiment of the present invention, which includes a fiber Bragg grating bonded along its outside circumference; 
     FIG. 4 shows another embodiment for the expansion or contraction of circular beam of FIG. 3; 
     FIG. 4A shows a remote controlled actuator turning or pushing a screw or control rod to apply force through the housing to the end of the support circumference; 
     FIG. 5 shows an alternate embodiment of the present invention, which includes a spheroidal shell; 
     FIG. 5A shows a cross section of the spheroid shell depicted in FIG. 5; 
     FIG. 6 shows an alternative circular system for the fiber grating straining application, which is formed by a flexible ring covered by two discs; 
     FIG. 6A shows the cross sections of the alternative circular system for the fiber grating straining application shown in FIG. 6; 
     FIG. 7 shows a cross-section of an alternative embodiment of the present invention, which includes a flexible tubular element titled under the relative movement between two concentric elements; 
     FIG. 8 shows an alternative embodiment of the present invention, consisting of a half circle beam affixed to a plate; 
     FIG. 9 shows control where signals from transmission tests come into the CPU where they are processed, and perhaps stored for future use; 
     FIG. 10 shows an alternative embodiment of the present invention, consisting of a circular ring; and 
     FIG. 11 shows an exemplary use of a flat substrate with the present invention. 
    
    
     DETAILED DESCRIPTION 
     In FIG. 1, there is shown system  10  which includes uniform cross section circular beam  12  around which is positioned fiber Bragg grating  11  which ideally would be bonded to outer surface of beam  13 . Also, note that fiber Bragg grating  11  could be bonded to inner surface  14  or surface  15 , if so desired. Fiber grating  11  could be bonded around the periphery or there could be groove  17  of beam  13  that would be cut into the outer surface to make a smooth finish where the fiber Bragg grating would be embedded. The longitudinal axis of fiber Bragg grating  11  is at a different position than the neutral axis of beam  13 . Fiber Bragg grating  11  may be closer or father to the approximate center of the bending moment. It should be noted that the neutral axis of beam  13  corresponds to the axis of zero strain during any bending moment. 
     Applying moments M 1  and M 2  to ends  16  and  17  produce a linear strain along the circumference of beam  12 . Thus, fiber grating expands or compresses depending the direction of rotation. 
     FIG. 2 shows a schematic of one system and method of applying the bending moment to beam  12 . In this FIGURE, end  16  of beam  12  is fixed and end  17  is connected to spoke  21 , which in turn has its inner end fastened around pivot  22  at the center of beam  12 . Accordingly, a turning moment applied along beam  21  as shown by force F, effectively changes the circumference of beam  12 , thereby changing the length of fiber grating  11  (FIG. 1) around the circumference. Applying the force in one direction would produce strain along the circumference, while applying the force in the opposite direction would produce this strain in the opposite direction, thereby either compressing or stretching, respectively, fiber grating  11 . 
     Element  12  could be one element of a longitudinal spring, which when twisted in one direction causes its circumference to contract and when twisted in the other direction causes the circumference to expand. This compression or expansion would be even along the length of the spring. 
     FIG. 3 shows alternate embodiment  30  which includes beam  33  having fiber  32  bonded along its outside circumference. Squeezing forces F are applied to outside ends  301 ,  302  of beam  33  in such a way that a uniform strain is produced along the outside circumference of the beam itself. The profile of the beam itself is also designed (ideally hyperbolic) with the cross section of the beam at ends  301 ,  302  being narrower than at apex  31  of the arc. 
     FIG. 4 shows another embodiment for the expansion or contraction of beam  33 . In this embodiment, beam  33  is fixedly attached at end  301  to pin  401  on one side of housing  41 . The other end  302  of beam  33  is forced against fine threaded screw  402  such that when torque is applied to the screw, either inward or outward, beam  33  moves in or out bending the beam, and thus changing the strain distribution along the beam length. Fiber Bragg grating  32  bonded to the outer surface of beam  33  is then tuned under the applied strain. 
     Screw  402  can be either torqued manually or electronically and can be both locally or remotely controlled. It could also be that a second screw  402  could replace pin  401  for gross tuning when you have to move a longer distance and then moving down to one screw when the resolution becomes tight. In such a situation, the screw threads could, if desired, have different pitches. 
     FIG. 4A shows remote controlled actuator  410  turning or pushing (via shaft  411 ) screw or control rod  412  so as to apply force through housing  41  to end  302  of support circumference  33 . Actuators used may be of a variety of types, including, ferroelectric actuators, ferromagnetic actuators, motorized actuators, mechanical actuators, and thermal actuators. 
     FIG. 5 shows a circular mechanism to apply strain along a fiber grating, which considers the use of a flexible ellipsoidal shell  51 . Fiber grating  52  is fixed around the equatorial circumference  53 . Applying forces F 1  and F 2  (which are equals in magnitude) to the top  54  and bottom  55  shells alter the perimeter of equatorial plane  53 , thus applying an axial strain to fiber grating  52 . 
     In order to obtain uniform axial strain along fiber grating  52 , the direction of forces F 1  and F 2  have to be perpendicular with respect to equatorial plane  53 , as shown in FIG.  5 A. The forces have to be applied symmetrically with respect to the equatorial plane. Otherwise, the strain applied to the fiber grating is non-uniform. Note that force can be applied by any number of means such as hydraulic screw torque clamping action, air pressure, etc. 
     FIG. 6 shows another circular mechanism to apply strain along a fiber grating. It is formed by flexible ring  61  covered by two discs  62 ,  63 . Discs  62 ,  63  are separated by distance h. Bonding the contact regions  65  between discs avoids relative movement. 
     Ring  61  has a curved cross section, as shown in FIG.  6 A. When force F is applied to discs  62 ,  63 , distance h changes; this alters diameter D and thus the circumference of ring  61 . Fiber grating  64  fixed along this circumference is then uniformly strained under applied force F. 
     Force F can be externally applied using different methods. However, an internally applied force reduces the dimensions of the circular mechanism. For instance, piezoelectric transducer  66 , as shown in FIG. 6A, can apply the required force, from the inside, to move discs  62 ,  63  forward and backward. 
     In order to compensate for any thermally induced wavelength shift in fiber grating  64 , bar  67  can be incorporated. Choosing the appropriate material, bar  67  increases (or decreases) h causing ring  61  to contract (or expand) by a, thus changing the applied strain to fiber grating  64 . The thermal compensation system also takes into account the expansion coefficient of discs and ring materials. 
     In an alternate embodiment, strain may be induced on fiber grating  64  by selecting an appropriate material which expands and contracts relative to the temperature. Materials with greater than average coefficients of thermal expansion may be among the materials chosen. An example of such a material is Ni—Ti. 
     FIG. 7 shows a cross-section of another circular stretcher mechanism. It is formed by three elements  71 ,  72 ,  73 , connected by flexible web shells  74 ,  75 . Elements  72  and  73  are tubular, and element  71  is a solid cylinder. 
     As element  71  moves up and down under applied force F, and as element  73  remains in a fixed position, tubular element  72  tilts, by Δr, due to the pivoting action of web shells  74  and  75 . This produces a shear strain in tubular element  72 . 
     Fiber grating  76 , bounded around the circumference of tubular element  72 , can be axially strained under applied force F. If force F moves the cylindrical element  71  up, by Δh, the strain along fiber grating  76  is positive (expansion); conversely, the strain along fiber grating  76  is negative (compression) when force F moves cylindrical element  71  down, by Δh. 
     To obtain uniform strain along fiber grating  76 , force F is symmetrically applied on the circular face of cylinder  71 . Otherwise, non-uniform strain profiles are applied along fiber grating  76 . Note that force F can be applied by using different methods. 
     To compensate any thermally induced wavelength shift in fiber grating  76 , this circular mechanism incorporates bar  77 . By doing this, any force applied to the bar is transmitted to cylindrical element  71 . By choosing an appropriate thermal expansion coefficient, bar  77  expands or compresses under temperature variations, thus producing negative or positive strain along fiber grating  76 , as required for its thermal compensation. 
     Turning now to FIG. 8, there is shown system  80  which is another alternative embodiment consisting of half circle beam  81  affixed to plate  83  by, for instance, bond  801 . Fiber grating  82  is bonded to the circumference of beam  81 . As force F 1  is applied upward or downward on beam end  802 , circumference  85  changes in accordance with force F 1 . 
     For an area of ∝, shown as  84 , along circumference  85 , the linear change is uniform with respect to force F 1 . Therefore, fiber grating  802  is advantageously placed within this zone to achieve uniform control of the strain of the fiber Bragg grating. 
     For situations where it is desired to obtain non-linear changes in the fiber grating, then fiber grating  802  can be positioned at different locations around the circumference in varying degrees. Alternately, the geometry of beam  81  may be changed by variations in the width or thickness to achieve non-linear strain on fiber grating  82 . 
     FIG. 9 shows control  90  where signals from transmission tests, such as signal to noise ratio (SNR), quality measurement, spectrum analysis, etc. come into CPU  91  via  901 , where they are processed, and perhaps stored for future use. CPU  91  then generates proper force applying signals and transmits these signals, via  902 , to one or more devices, such as device  33 , shown in FIG. 4A, for selectively adjusting the spacings of the selected fiber Bragg grating. 
     FIG. 10 shows an exemplary use of the present invention wherein fiber grating  1001  is wrapped around circular elongated ring  1002  such that the pitch varies. A varying pitch results in various portions of fiber grating  1001  that are at different angles to the stretching direction. Circular elongated ring  1002  stretches in the direction shown by the arrows, thus, a section of fiber grating  1001 , such as section  1003 , parallel to the circumference of circular ring  1002  receive maximum strain, whereas a section of fiber grating  1004  at an angle perpendicular to the circumference would receive no strain. 
     FIG. 11 portrays FIG.  10 &#39;s circular ring  1002  unfurled into flat substrate  1102 , wherein fiber grating  1001  is held at various angles with respect to the stretching direction of flat substrate  1102 . The equation:          ɛ   ς     ≈         (     1   -         (          y          x       )     2        υ       )       (     1   +       (          y          x       )     2       )            ɛ   x                              
     where ε is strain relating strain in a flat substrate to strain in an attached fiber grating, may be used to calculate a desired amount of strain in fiber grating  1001  relative to the strain placed upon flat substrate  1102 . 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.