Patent Publication Number: US-6990274-B2

Title: Apparatus and method for adjusting the spectral response of an optical waveguide grating

Description:
FIELD 
   This disclosure generally relates to optical waveguide gratings. More particularly, this disclosure relates to an apparatus and method for adjusting the spectral response of an optical waveguide grating. 
   BACKGROUND 
   Optical fibers are thin strands of glass capable of transmitting information containing optical signals over long distances with low loss of signal strength. In essence, an optical fiber is a small diameter waveguide consisting of a fiber core having a first index of refraction surrounded by a fiber cladding layer having a second lower index of refraction. So long as the refractive index of the core is sufficiently higher than the refractive index of the cladding, a light beam propagating along the core exhibits total internal reflection and is guided along the length of the core. Typical optical fibers are made of high purity silica, with various concentrations of dopants added to control the index of refraction of the core and the cladding. 
   Many optical materials exhibit different responses to optical waves of different wavelengths. One such phenomenon is chromatic dispersion (or simply “dispersion”) in which the speed of light through an optical medium is dependent upon the wavelength of the optical wave passing through the medium. Since the index of refraction for a material is the ratio of the speed of light in a vacuum (a constant) to the speed of light in the material, it therefore follows that the index of refraction also typically varies as a function of wavelength in these materials. 
   Optical fibers are frequently used in data networks (such as telecommunication networks) requiring high data transmission rates. In many networks, data is also transmitted over very long distances. As the distances over which data is transmitted increase, or the rates at which data is transmitted increase, chromatic dispersion presents obstacles to achieving error free performance. Specifically, in long distance transmission of optical signals such as from a laser, the laser bandwidth and the modulation used to encode data onto the laser beam results in a range of wavelengths being used to transmit the information. The combination of this range of wavelengths and the chromatic dispersion of the fiber accumulated over a distance results in pulse broadening or spreading. For example, at high data rates two adjacent optical pulses or wave fronts may eventually overlap each other due to chromatic dispersion. Such overlapping can cause errors in data transmission. The accumulation of chromatic dispersion increases as the distance the optical signals travel increases. For low speed signals, dispersion is not typically a problem as they may use a smaller range of wavelengths, resulting in a smaller range of delays in the transmission path, and they also have a longer time period in which to determine the state of each bit. 
   Attempts to compensate for chromatic dispersion include the use of dispersion compensating fibers, dispersion compensating optical waveguide gratings (e.g., fiber Bragg gratings), and a combination of both. Dispersion compensating fibers and dispersion compensating optical waveguide gratings introduce a negative chromatic dispersion with an equal and opposite sign to the accumulated dispersion in a fiber link. 
   Optical gratings suitable for dispersion compensation may include Bragg gratings, and long period gratings. These gratings typically comprise a body of material with a plurality of spaced apart optical grating elements disposed in the material. For example, a conventional Bragg grating comprises an optical fiber in which the index of refraction undergoes periodic perturbations along its length. The perturbations may be equally spaced, as in the case of an unchirped grating, or may be unequally spaced as in the case of a chirped grating. That is, a chirp is a longitudinal variation in the grating period along the length of the grating. The wavelength reflected in a Bragg grating is directly related to the period of the perturbation. Thus, in a chirped fiber grating, the reflected wavelength of the grating changes with the position along the fiber grating. As the grating period increases or decreases along a direction in the fiber grating, the reflected wavelength increases or decreases accordingly. Therefore, different spectral components in an optical signal are reflected back at different locations along the grating and accordingly have different delays. Such wavelength dependent delays may be used to negate the accumulated dispersion of an optical signal. Chirped gratings may be linearly chirped (having perturbations that vary in a linear fashion), non-linearly chirped, or randomly chirped. 
   Fiber Bragg gratings reflect light over a given waveband centered around a wavelength equal to twice the spacing between successive perturbations. The reflected wavelength is given by the Bragg Equation λ=2 dn, where n is the effective index of the grating, λ is the reflected wavelength, and d is the distance between successive perturbations. The remaining wavelengths pass essentially unimpeded. The ability to pass some wavelengths in an unimpeded manner is desirable in optical filtering applications. In such applications, the frequency of the grating can be selected to reflect (i.e., filter) undesired wavelengths, while allowing desired wavelengths to pass. 
   Fiber gratings may be extrinsically chirped or intrinsically chirped. An extrinsic chirp refers to a chirp in the grating that is obtained by applying an external perturbation generating field to the fiber. For example, to create an extrinsically chirped grating, an external gradient, typically comprised of strain gradients or temperature gradients, is applied along the length of a non-chirped fiber grating, resulting in non-uniform changes in properties of the fiber grating, thus creating a chirp. An extrinsic chirp is valuable in that it may be applied to adjust the parameters of the grating, and it may be used to control the chromatic dispersion of a fiber Bragg grating. 
   There are disadvantages, however, in forming chirped gratings with an external gradient. The maximum range of chirping that can be achieved is limited in that relatively large gradients or strains are required to obtain a range of chirping. Such externally applied strains may have a negative impact on the reliability of the fiber, such as by causing the fiber to fracture. Thus, the maximum chirp rate that can be imposed on the grating is limited by the material properties of the fiber. 
   An intrinsic chirp refers to a chirp in the grating that has been incorporated into the fiber during the fabrication process. For example, an intrinsic chirp may be achieved by using a phase mask in which the period of the phase mask varies in some manner. When radiation is applied to the fiber through the phase mask (thus altering the index of refraction), the resulting fiber will be intrinsically chirped. Using this technique, broadband gratings may be produced which can compensate for chromatic dispersion in multi-channel system. However, intrinsic chirp corrects a fixed amount of dispersion in a specified wavelength spectrum. While intrinsically chirped gratings are useful in communication systems where a specific amount of dispersion compensation is required, the dispersion and amplitude response of the grating is essentially fixed. Thus, intrinsically chirped gratings by themselves are not well suited to situations in which dynamically adjustable devices are required. 
   A variety of approaches exist for adjusting the spectral response (i.e., “tuning”) of gratings in optical fibers. The application of strain to the grating is one method. For example, simply stretching the grating by gripping the fiber on either end of the grating and then putting the fiber in tension. The act of imparting a tensile strain on the grating results in a proportional increase in the wavelength of the spectral response, as per the following equation 
         Δ   ⁢           ⁢   λ     =         λ   max     ⁡     (     1   -     p   e       )       ⁢       Δ   ⁢           ⁢   L     L           
 
where Δλ is the wavelength shift resulting from the imparted tensile strain, ΔL/L; λ max  is the maximum wavelength; and p e  is the strain optic coefficient of the grating (an intrinsic property of the fiber in which the grating is written). Other approaches use bending of a simple structural member to put a strain on an attached grating. Each of these approaches is limited in the applications it can address.
 
   In particular, a problem with current devices and methods requiring the application of tensile strain to the grating is the potential for tensile failure of the fiber. This problem severely limits the amount of strain that can be reliably imposed. Methods wherein the fiber is attached to a support member and the support member is then put into tension are likewise limited. 
   Short gratings (less than approximately 100 mm in length) have also been tuned using axial compression. For example, a grating may be attached to the inside of a ferrule and the ferrule placed in compression. When using a uniform ferrule, the wavelengths reflected will shift uniformly downward. When using a non-uniform ferrule, both the dispersion characteristics of the grating and the center wavelength change with changes in the force applied to the ferrule. Since only compression is used, tensile failure of the grating ceases to be an issue. However, the length of grating that can be economically tuned using this method is very limited. The length is also limited by column buckling of the ferrule, which requires that for a given strain, the diameter of a ferrule increases as a function of its length. Furthermore, the force required to generate a given strain increases with the square of the diameter. These factors limit the use of the described compression tuning methods for long gratings (greater than approximately 100 mm in length). 
   Another problem current devices and methods for tuning share is that the wavelengths shift in all sections of the grating at the same time, although not necessarily by the same amount. Designs have been attempted that independently tune small elements within a grating. These designs have generally not been commercially successful due to their great complexity, large number of parts and high cost. For example, U.S. Pat. No. 5,694,501 discloses an apparatus and method of controlling strain in a Bragg grating that requires a segmented piezoelectric stack with quasi-distributed voltage control. The segmented piezoelectric stack and control system are complex, and a constant supply of power is required to maintain a selected strain profile. 
   Clearly, previous attempts to provide dispersion compensating devices and methods have not produced adequate results. In particular, previous attempts have not satisfactorily addressed the unique issues associated with adjusting the spectral response of long gratings or sections of long gratings. If methods representing the current technology were used to adjust the spectral response of long gratings, the result would be severely limited ranges of adjustability, or unacceptably high levels of tensile strain (&gt;1%) on the fiber for high reliability. Currently available devices and methods also do not provide the capability to tune the dispersion of a single channel (wavelength) of a multi-channel grating at any wavelength within a broad operating range. A need exists for an apparatus and method that addresses the above described shortcomings. 
   SUMMARY 
   The embodiments according to the invention described herein provide apparatuses and methods for adjusting the spectral response of an optical waveguide grating in a flexible manner. 
   Some embodiments according to the invention provide settable or tunable dispersion compensators. Other embodiments according to the invention may be used in tunable add/drop multiplexers or a variety of other devices that would benefit from the use of an adjustable filter. Still other embodiments according to the invention utilize mechanical adjustment of the spectral response such that once set the apparatus has no power requirement. Still other embodiments according to the invention provide compressive preload on the optical waveguide grating to reduce or eliminate reliability concerns related to tensile failure. Still other embodiments according to the invention allow selection and adjustment of any particular channel within the operating band of a multi-channel grating. Still other embodiments according to the invention allow the wavelength to be set at each of two points in an optical wavelength grating, with the result being a defined channel width and dispersion. 
   One embodiment according to the invention provides an apparatus for adjusting the spectral response of an optical waveguide grating. The apparatus includes a support member having a centroidal axis. The support member is configured for longitudinal bending and rotating about its centroidal axis. A longitudinal optical waveguide grating can be attached to the support member. The optical waveguide grating may be parallel to the centroidal axis, non-parallel to the centroidal axis, or wound about the centroidal axis. The spectral response of the optical waveguide grating is adjusted by bending and rotating the support member about its centroidal axis. 
   Another embodiment according to the invention provides a method for adjusting the spectral response of an optical waveguide grating. The method includes attaching a longitudinal optical waveguide grating to a longitudinal support member having a centroidal axis. The support member is configured for longitudinal bending and rotating about the centroidal axis. The support member is longitudinally bent and rotated about the centroidal axis and thereby alters the spectral response of the optical waveguide grating. 
   As used herein, the neutral axis is the line or plane in a support member under transverse pressure, at which the support member is neither stretched nor compressed (i.e., where the longitudinal stress is zero). 
   As used herein, the centroidal axis connects the centroids of an envelope  538  encompassing the cross sections of the support member. The support member is said to be straight or curved in accordance with the shape of its centroidal axis. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates an exemplary optical fiber Bragg grating. 
       FIGS. 2A–2C  illustrate one method of forming an apparatus for adjusting the spectral response of an optical waveguide grating according to the invention. 
       FIG. 3  illustrates the performance of an apparatus for adjusting the spectral response of an optical waveguide grating like that illustrated in  FIGS. 2A–2C . 
       FIG. 4  illustrates another embodiment of an apparatus for adjusting the spectral response of an optical waveguide grating according to the invention. 
       FIG. 5  illustrates yet another embodiment of an apparatus for adjusting the spectral response of an optical waveguide grating according to the invention. 
       FIG. 6  illustrates an alternate configuration for the apparatus of  FIG. 5 . 
       FIG. 7  illustrates yet another embodiment of an apparatus for adjusting the spectral response of an optical waveguide grating according to the invention. 
       FIG. 8  illustrates the adjustment of the spectral response of a Bragg grating using the apparatus  FIG. 7 . 
       FIG. 9  illustrates a support member that is asymmetric about its neutral axis. 
       FIG. 10  illustrates one method of tuning the spectral response of an optical waveguide grating or section thereof according to the invention. 
       FIG. 11  illustrates uniformly shifting the spectral response of an optical waveguide grating. 
       FIG. 12  illustrates altering the chromatic dispersion of an optical waveguide grating. 
       FIG. 13  illustrates altering the center wavelength and the chromatic dispersion of an optical waveguide grating. 
       FIG. 14  illustrates an embodiment of an apparatus for adjusting the spectral response of an optical waveguide grating that is adjusted by bending and rotating the support member according to the invention. 
       FIG. 15  is a cross sectional view along line  15 — 15  of the apparatus of  FIG. 14 . 
       FIG. 16  illustrates yet another embodiment of an apparatus for adjusting the spectral response of an optical waveguide grating that is adjusted by bending and rotating the support member according to the invention. 
       FIG. 17  illustrates yet another embodiment of an apparatus for adjusting the spectral response of an optical waveguide grating that is adjusted by bending and rotating the support member according to the invention. 
       FIGS. 18A and 18B  illustrate additional embodiments of an apparatus for adjusting the spectral response of an optical waveguide grating that are adjusted by bending and rotating the support member according to the invention. 
       FIGS. 19A–19E  illustrate the delay slope and dispersion curves of the apparatuses of  FIGS. 18A and 18B . 
       FIGS. 20A and 20B  schematically illustrate an embodiment of a mechanism for bending and rotating the support member of the apparatuses of  FIGS. 14–18B  according to the invention. 
       FIG. 21  schematically illustrates another embodiment of a mechanism for bending and rotating the support members of the apparatuses of  FIGS. 14–18B  according to the invention. 
   

   DETAILED DESCRIPTION 
   In the following detailed description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. 
   For reasons of clarity, the various embodiments according to the invention are described herein with respect to optical waveguides configured as optical fibers. However, the teachings of the invention are equally applicable to other types of optical waveguides. Similarly, the various embodiments according to the invention are for the most part described herein with respect to devices and methods that adjust the spectral response of portions of long gratings. However, the embodiments according to the invention are applicable to adjusting the spectral response of a portion of any length grating, including the full length of a grating. Long gratings are generally considered to be gratings having a length of 100 mm or greater. Broadband gratings in the range of 1–3 meters long (or even longer) may be used in long distance optical communication systems. 
   Referring now to the drawings and in particular to  FIG. 1 , an optical waveguide  10  is shown to be configured as an optical fiber of which only a relatively short longitudinal length is depicted and which includes a fiber core  12  and a fiber cladding  14  surrounding the fiber core  12 . The fiber core incorporates a grating  16  that includes a plurality of grating elements  18 , each grating element  18  extending substantially normal to the longitudinal axis of the core  12 . The spectral response (i.e., wavelength and chromatic dispersion) of the grating  16  can be adjusted (or “tuned”) by altering the average index of refraction of the grating structure, changing the spacing between grating elements  18 , or by using a combination of the two. 
   Axial Pre-Compression 
   One aspect of the present invention provides an apparatus for adjusting the spectral response of an optical waveguide grating having high reliability (i.e., reduced susceptibility to fiber failure due to static fatigue) by applying a compressive axial preload to the grating prior to packaging the grating. This compressive preload reduces or eliminates potentially damaging tensile strain in the grating when the spectral response of the grating is subsequently adjusted using a controlled application of tensile strain. 
   One embodiment of an apparatus for adjusting the spectral response of an optical waveguide grating having a compressive preload and a method of forming such an apparatus is illustrated in  FIGS. 2A–2C . The apparatus has an optical waveguide  10 , including an optical grating  16 , attached to a support member  30 . The optical grating  16  is subjected to axial compression by support member  30  so that optical grating  16  is under a compressive axial strain prior to any adjustment (i.e., “tuning”) of the spectral response of the grating. The apparatus is tuned by altering the strain in the optical grating  16 . In one embodiment, the strain of optical grating  16  is altered by the application of mechanical bending forces to the support member  30 . In another embodiment, the strain of optical grating  16  is altered by controlled heating or cooling of the support member  30 . 
   One method of forming an apparatus for adjusting the spectral response of an optical waveguide grating having a compressive preload is as follows. First, the range of wavelengths (operating band) of a single channel or channels to be adjusted is determined, together with the range of dispersion values (dispersion range) over which the grating will be adjusted. Next, a preliminary optical grating design that allows the optical grating to be adjusted over the desired dispersion range and operating band is determined based on the desired use. Next, the maximum wavelength increase and associated maximum tensile strain required to tune the preliminary optical grating across the desired operating range is determined. Next, a final optical grating is selected, wherein at each location in the final optical grating the reflected wavelength is longer than the wavelength in the preliminary optical grating design by the previously determined maximum wavelength increase. 
   After the final optical grating is selected using the above steps, a support member  30 , such as a straight beam having a uniform cross-section, is provided. While still separated from the optical waveguide  10  having the selected final optical grating  16 , a support surface  32  of support member  30  is elongated in a controlled manner ( FIG. 2A ) as indicated by arrows  38 . Support surface  32  may be elongated in a controlled manner by imparting a controlled tensile strain to support member  30 . A controlled tensile strain may be imparted by the application of mechanical force to support member  30 , such as by bending the support member  30  in a controlled arc having a constant bend radius by the application of a uniform bending moment across the length of the support member  30 . Alternately, a controlled tensile strain may be imparted by stretching the support member  30 . Alternately, support surface  32  may be elongated in a controlled manner by applying a controlled temperature change to the support member  30 , so long as support member  30  has a non-zero coefficient of thermal expansion. 
   After support surface  32  of support member  30  is elongated by a predetermined distance (either by the application of controlled strain or the application of controlled heating/cooling), the substantially unstrained optical waveguide  10  and its associated grating  16  is securely attached along the entire length of the grating to the elongated support surface  32  of support member  30 , such that no relative movement between the support surface  32  and the grating  16  is permitted ( FIG. 2B ). The optical waveguide grating  16  is attached to the elongated support surface  32  of the support member  30  such that the longitudinal axis of the grating  16  is in alignment with the direction of the controlled elongation in the support surface  32 . The optical waveguide grating  16  may be attached to the support member  30  in a variety of acceptable manners, for example, with adhesive, solder or mechanical clamping devices. 
   After the optical waveguide  10  and associated grating  16  are securely attached to the surface  32  of support member  30 , the mechanical or thermal conditions causing elongation of the support surface  32  are removed. As support surface  32  returns to its shorter, pre-elongated condition, a compressive axial strain (the compressive preload) indicated by arrows  40  is applied to the attached optical waveguide  10  and associated grating  16  ( FIG. 2C ). 
   The spectral response (wavelength and/or chromatic dispersion) of the axially compressed grating  16  may subsequently be adjusted (i.e., “tuned”) by application of a strain and/or temperature distribution on the support member  30  along the length of the grating  16 . The application of a strain and/or temperature distribution on the support member  30  along the length of the grating  16  imparts a corresponding tensile strain in the grating  16 , which alters both the index of refraction of the grating  16  and the spacing of grating elements  18 . The controlled application of a strain and/or temperature distribution thus allows the wavelength and/or the chromatic dispersion characteristics of the grating  16  to be adjusted to achieve a desired spectral response. 
   In one embodiment according to the invention, the controlled extension of support surface  32  is sufficient to cause the magnitude of the compressive axial strain on the grating  16  to be equal to or greater than a previously determined maximum tensile strain required to achieve the maximum wavelength change in the grating  16 . 
   In an alternate embodiment according to the invention, the support member  30  is made from a material having a coefficient of thermal expansion (CTE) different from the CTE of the optical waveguide  10  and associated grating  16 . Instead of applying a controlled strain to elongate surface  32  of the support member  30  (such as by bending or stretching the support member  30  as described above), a thermal load is applied to the support member. The support member  30  is heated (or cooled) by the thermal load to a temperature at which the differential expansion of the support member  30  and the grating  16  results in a compressive axial strain on the grating  16  when the thermal load is removed. In a preferred embodiment, the magnitude of the compressive axial strain is equal to or greater than the maximum tensile stress that might occur over the operating temperature range of the apparatus in order to achieve the maximum wavelength change. 
   Thus, in a preferred method of adjusting the spectral response of the grating  16 , the compressive axial strain (the compressive preload) applied to the grating  16  prior to tuning has a magnitude equal to or greater than the magnitude of a maximum tensile strain imparted to the grating  16  during subsequent tuning operations. In this manner, during subsequent tuning, the optical waveguide  10  and grating  16  do not experience axial tension, and therefore are not subject to tensile failure. 
   An exemplary application of the method for adjusting the spectral response of a grating is as follows. First, assume a glass grating length of 100 mm and a round trip delay in the glass grating of 10 picosec/mm (delay=2×speed of light×effective refractive index). Thus, the maximum delay (from the near end of the grating to the far end of the grating and back to the near end) is 1000 ps. Next, as illustrated in the graph of  FIG. 3 , assume that the grating will be tuned around a constant center wavelength of 1554 nm, the maximum desired dispersion is 500 ps/nm, and the minimum desired dispersion is 125 ps/nm. Then, the maximum tensile strain required to tune the grating across the desired operating range is determined by the equation: 
         required   ⁢           ⁢   strain     =       Δ   ⁢           ⁢   λ         λ   max     ⁡     (     1   -     p   e       )             
 
Thus, the Required Strain=1.5/[1554*(1−0.24)]=0.13%. Therefore, a 0.13% tensile strain is applied to the support beam prior to attachment of the grating, either by simple tension or bending of the support member. After attaching the grating, the tensile strain is removed from the support member, with the result being an axially compressive force and the elimination of tension in the waveguide. This example assumes the mechanical strength of the grating is negligible relative to the strength of the support beam.
 
Bending Moments
 
   Another aspect of the present invention provides an apparatus and method for adjusting the spectral response of an optical grating through the application of a pair of bending moments. The pair of bending moments co-operate to create a controlled bending moment gradient over a section of a support member to which the optical grating is attached. Apparatus and methods that embody this aspect of the invention are useful for mechanically tuning optical waveguide gratings of any length, or a selected portion of any length of optical waveguide grating. Because the apparatus and method can tune a selected portion of an optical waveguide grating, they are beneficially used for tuning a selected channel or channels in a multi-channel grating. Specifically, they are beneficially used in a broadband wavelength division multiplexed (WDM) optical system having a plurality of optical wavelength channels. 
   One embodiment of an apparatus  100  for adjusting the spectral response of an optical waveguide grating is illustrated in  FIG. 4 . The apparatus has attached to it a broadband optical grating  116  having an operating bandwidth encompassing a plurality of channels (wavelengths). The optical grating  116  may be chirped, and may be of any suitable type of grating. For example, the grating may be a fiber Bragg grating, or a long period grating. The grating  116  is attached to a support member  130  (which is a flat beam in this illustration). A bending moment applicator  140  is configured to apply two bending moments to the support member  130 . The grating  116  and support member  130  are moveable with respect to bending moment applicator  140 . The bending moment applicator  140  includes a base  142  having two rotatable discs  144 ,  146  attached thereto. Each disc  144 ,  146  has two pairs of pins  150  (for a total of four pins  150  on each disc  144 ,  146 ) projecting therefrom, in a direction normal to the plane of rotation of the discs  144 ,  146 . The support member  130  and attached grating  116  are positioned between each pair of pins  150 , such that rotation of the discs  144 ,  146  causes one pin  150  of each pair of pins to apply a force to the support member  130 . A thumbwheel  160  and worm gear  162  are associated with each disc  144 ,  146 . As either of the thumbwheels  160  are rotated, the worm gear  162  causes the associated disc  144 ,  146  to rotate. As the discs  144 ,  146  rotates, forces are applied to the support member  130  via pins  150 , thereby creating a bending moment gradient in support member  130  between the discs  144 ,  146 . In the embodiment illustrated in  FIG. 4 , both the right and left discs  144 ,  146 , respectively, have been rotated clockwise. In this case, the grating  116  sees a linear strain gradient between the discs  144 ,  146 . The maximum axial compression on the grating  116  is adjacent to the downward pushing right pin of the left disc  144  and the maximum tension is adjacent to the upward pushing left pin on the right disc  146 . Although eight pins  150  are shown on the two discs, it should be noted that at any point in time only four of the pins  150  are applying force to the beam  130 . 
   In one embodiment according to the invention, the bending moment applicator  140  is configured to apply the two bending moments to the support member  130  separated by a distance less than the length of the optical grating  116 . The distance separating the application points of the bending moments may be selected so that only a single channel of the plurality of channels, or a subset of the plurality of channels, in the optical grating  116  are tuned. In this embodiment, the bending moment applicator  140  is preferably moveable with respect to the support member  130  and optical grating  116 , such that the application points (of the bending moments can be located over any desired channel(s) of the broadband optical grating  116 . 
   In another embodiment according to the invention, the bending moment applicator  140  is configured to apply the two bending moments to the support member separated by a distance equal to or greater than the length of the optical grating  116 . In this configuration, the bending moments can be adjusted to tune the spectral response of the entire grating  116 . 
   Embodiments of the invention may employ axial pre-compression of the optical grating in addition to the application of a pair of bending moments to the support member. As described above, the optical grating  116  may be attached to the support member  130  such that the support member  130  axially compresses the optical grating  116 . Preferably, the compressive axial strain (the compressive preload) applied to the optical grating  116  has a magnitude equal to or greater than the magnitude of a maximum tensile strain imparted to the grating during subsequent tuning operations using the application of a pair of bending moments. 
     FIG. 5  illustrates another embodiment of the present invention, an apparatus  200  for adjusting the spectral response of an optical waveguide grating. In the apparatus  200  of  FIG. 5 , the relationship between the two bending moments is fixed so that the dispersion of an optical waveguide grating  216  is changed, but not the center wavelength. The apparatus of  FIG. 5  includes the grating  216  attached to a flat beam support member  230 . The optical grating  216  and support member  230  are moveable with respect a bending moment applicator  240 . The bending moment applicator  240  includes a housing  242  having guide members  244  adjacent both sides of the housing  242 . The support member  230  and attached grating  216  extend across the housing  242  and are positioned between the guide members  244 . To apply a moment to the grating  216 , a thumbwheel  250  turns a lead screw  252 , which in turn moves a cam  260  to the left or right of its normal centered location in the housing  242 . The cam&#39;s angled cam surfaces  262  contact a movable central part  264 , which rotates about a pivot point  266 . Pins  270  mounted symmetrically about the pivot point  266  provide bending forces to the flat support beam  230 . In one embodiment, a portion of the cam  260  is exposed through the top surface  276  of the housing  242 , so that the position of the cam  260  relative to the housing  242  can be used to determine the current tuning amount. 
     FIG. 6  illustrates apparatus  200  of  FIG. 5  slidably mounted on rails  302  of an assembly  300 . Assembly  300  includes a long grating  316  on a support beam  330 . In use, the apparatus  200  is moved to a position centered over a selected point in the grating that reflects a known wavelength that becomes the center wavelength of the tuned region. The apparatus  200  is then secured in position on rails  302  of assembly  300  and the chromatic dispersion of the tuned region immediately around the selected center wavelength is adjusted using the thumbwheel  250 . 
     FIG. 7  illustrates a test jig  400  for applying bending moments to a supporting beam  430 . Test jig  400  includes guide pins  450  mounted on stationary base  451 , and a rotating disk  452  mounted on base  451 . Application of a force, F, to the end of the tuning rod  440  causes disk  452  to rotate. Rotating disk  452  includes pins  453 ,  454 ,  455 ,  456  to apply bending moments to beam  430  and the grating  416  thereon. The bending moments created by the test jig  400  create strain profiles identical to the apparatus  200  of  FIG. 5 . 
     FIG. 8  illustrates the chromatic dispersion tuning of a Bragg grating written at 430 ps/nm and adjusted using the test jig  400  shown in  FIG. 7 . The six lines represent six different magnitudes of force applied to the end of the tuning rod  440 . As can be seen, the dispersion (slope of the delay curves) around the central wavelength is effectively adjusted using this method. In this example, the chromatic dispersion was adjusted down to a minimum of 130 ps/nm, and up to a maximum of 1040 ps/nm. In the variously described embodiments according to the invention, the support member  130 ,  230 ,  330  may optionally be formed of a material having a coefficient of thermal expansion (CTE) below approximately 5 ppm/C°. A low CTE minimizes changes in the spectral response of a grating due to thermal conditions, and is desirable where the device is not intended to be thermally responsive. For example, the support member  130  may be formed of graphite, graphite composite materials, metal alloy materials, especially nickel alloy materials, and the like. Suitable nickel alloy materials include those available under the trade names INVAR (36% nickel, about 63% iron, and less than 1% total of manganese, silicon, and carbon) and KOVAR (29% nickel, 17% cobalt, about 53.5% iron, and about 0.5% total of manganese, silicon, and carbon. 
   Although the support members  130 ,  230 ,  330  described so far have symmetric cross-sections, the support member may alternately be asymmetric about its neutral axis. For example, a support member  530  illustrated in  FIG. 9  is asymmetric about its neutral axis  532 . The optical grating  536  is attached to the asymmetric support member  530  at a region most distant from the neutral axis  532 . As used herein, the neutral axis is the line or plane in a support member under transverse pressure, at which the support member is neither stretched nor compressed (i.e., where the longitudinal stress is zero). Depending upon the shape (cross-section) of the support member, the neutral axis may be, but is not necessarily, colinear with the centroidal axis  552 . As used herein, the centroidal axis  552  connects the centroids of an envelope  538  encompassing the cross sections of the support member. The support member is said to be straight or curved in accordance with the shape of its centroidal axis. An asymmetric support member shape as shown in  FIG. 9  has the grating  536  placed at a relatively large distance from the neutral axis  532 , as compared to the size of grating  536 . When a vertical bending moment is applied, this large distance provides a larger ratio of strain per unit bending moment than symmetric support member designs. The asymmetric support member shape also results in a lower average strain on the support member material than a symmetric support member shape. 
   A method of tuning the spectral response of an optical grating or section thereof according to the invention is illustrated in  FIG. 10 . First, the optical grating is attached to support member (step  500 ). The grating may optionally be attached to the support member such that the support member axially compresses the grating, as described above. The grating is attached to the support member along its length, typically parallel to and spaced from a centroidal axis  552  of the support member. The support member preferably has a uniform cross-section along the length of the optical grating. 
   Next, a portion of the grating that will have its spectral response adjusted or “tuned” is selected (step  502 ). Typically, the selected portion of the grating will be centered about a single channel or wavelength to be adjusted. However, the selected portion may encompass a plurality of channels, and may include the entire length of the grating. 
   The support member is next deformed by applying a pair of bending moments (operating on the support member) at some distance from each other (step  504 ). The location of the bending moments is selected such that one bending moment is applied adjacent to one end of the adjusted region and the other bending moment is applied adjacent the other end of the adjusted region. Application of the bending moments on the support member creates an axial strain profile along the length of grating, thereby altering the spectral response of the grating within the adjusted region. 
   As the bending moments are applied, the spectral response of the grating can be monitored. The magnitude and location of one or more of the bending moments can then be adjusted independently and dynamically to tune the spectral response of the optical grating until a desired spectral response is achieved (step  506 ). 
   For example, assume the objective is to tune a uniformly chirped grating to provide −1300 ps/nm dispersion in the 1 nm band from 1550 nm to 1551 nm. The two bending moments could be spaced apart by a distance of approximately 130 mm. The moment at one end of the band would then be adjusted to result in the grating located at that position reflecting the 1551 nm wavelength and the moment at the other end of the band would then be adjusted so that the grating would reflect the 1550 nm wavelength at that position. That is, the wavelength is set at each of two points in the grating, with the result being a defined channel width and dispersion. In actual practice, the tuned band would typically include a safety margin and be somewhat larger and than the band of the optical signal. 
   Various combinations of bending moments and the resulting change in the spectral response of the grating are illustrated in  FIGS. 11–13 . In each of  FIGS. 11–13 , the “original” spectral response is shown as solid line “A”, while the “adjusted” spectral response is shown as dashed line “B”. Depending upon how the bending moments are applied, altering the spectral response of the optical grating may include uniformly shifting spectral response (i.e., center wavelength and chromatic dispersion) of the grating ( FIG. 11 ), altering the chromatic dispersion of the grating ( FIG. 12 ), or shifting the center wavelength and also altering the chromatic dispersion of the optical grating ( FIG. 13 ). 
   In each of the illustrated examples, a chirped grating is attached to the top surface of a support beam, the short wavelength end of the grating is to the left side of the diagram and light is launched into the grating from the right side of the diagram. In each case the delay vs. wavelength chart represents only the length of grating between the innermost force application points. 
     FIG. 11  illustrates uniformly shifting the spectral response of the grating. Specifically, the center wavelength and dispersion of the grating has shifted as a result of four equal forces creating equal and opposite moments near the ends the support beam. 
     FIG. 12  illustrates altering or tuning the chromatic dispersion of the grating. The four forces are configured so that the moment created by the inside forces is equal and opposite to the moment created by the outside forces. This results in a uniform moment gradient between the inner two force application points and results in a change in the chromatic dispersion of the grating, while the center wavelength stays the same. 
     FIG. 13  illustrates a combination of wavelength shift and chromatic dispersion tuning achieved by superimposing the examples of  FIGS. 10 and 11 . In this superposition of forces, the clockwise and counterclockwise moments at the right end of the beam tend to cancel resulting in only a small upward force. The magnitude and direction of the bending moment at each location along the grating are proportional to the wavelength shift at that location. 
   As noted above, the “two-moment” apparatus and method disclosed herein can be used to tune the spectral response of an optical waveguide grating over any portion of the grating, including the full length of a grating. However, for a long grating, the change in spectral response per unit strain would be small. Achieving a meaningful or useful change in the spectral response would require a large strain on the fiber. In many applications, it is beneficial to tune a single channel within a multi-channel grating, or independently tune two or more different channels within a long multi-channel grating. This objective can be easily achieved using the apparatus and method disclosed herein, because adjusting the spectral response of only a portion of a grating requires a lower average/peak strain than needed to tune the full length of the grating. 
   Bend and Rotate 
   Another aspect of the present invention provides an apparatus and method for adjusting the spectral response of an optical grating by attaching the grating to a longitudinal support member, applying a bending moment to the support member to create a bend or curve in the support member, and then rotating the support member about its centroidal axis. As the support member is rotated, depending upon its initial position on the support member, the grating moves from an area of tension on the outside of the support member bend to an area of compression on the inside of the support member bend (thus reflecting a shorter wavelength), or alternately move from the inside of the support member bend outward (and thus reflecting longer wavelengths). 
   One exemplary embodiment of an apparatus for adjusting the spectral response of an optical waveguide grating according to the invention is illustrated in  FIG. 14 . A strain-imparting support member  550  (in this exemplary embodiment a longitudinal rod having a circular cross-section) is configured for simultaneous longitudinal bending and rotating about its centroidal axis  552 . A longitudinal optical waveguide  554  containing a grating  556  is attached along its entire length to the strain-imparting support member  550 . In one embodiment according to the invention, the grating  556  is a long optical grating, and has a length of at least 100 mm. In another embodiment according to the invention, the long optical grating has a length of at least 1 m. 
   The support member  550  is operatively coupled to a mechanism (not shown) for bending and rotating the support member. The mechanism applies a bending moment  560  to the support member  550  for longitudinally bending the support member. The support member  550  is then rotated (as indicated by arrow  562 ) about its now curved or bent centroidal axis  552  to adjust the center wavelength of the grating  556 . 
   In one embodiment according to the invention, the support member  550  bends and/or rotates as a function of temperature, such that the apparatus is suitable for use as a thermal compensation package. The thermally responsive bending and/or rotating may be accomplished by selecting the material of the support member  550  to be responsive to temperature changes, or by configuring the mechanism for bending and rotating to be responsive to temperature changes, or a combination of both. 
   The grating  556  and associated waveguide  554  may be attached to the support member  550  in several different orientations. In one embodiment according to the invention, as illustrated in  FIG. 14 , the longitudinal axis of the grating  556  is parallel to the centroidal axis  552  of the support member  550 . In this orientation, the mechanism for bending and rotating the support member  550  about its centroidal axis  552  tunes the wavelength of the optical grating  556  by controlling the longitudinal bending and rotating of the strain-imparting support member in accordance with the equation:
 
λ=λ o [1+(1 −P ) r  cos(θ)/ R ]
 
Where:
         λ o =unstrained wavelength of the grating;   P=the strain optic coefficient of the grating;   R=radius of the bend in the support member;   r=radial distance from the centroidal axis to the core of the grating; and   θ=angle of rotation about the centroidal axis (as shown in  FIG. 15 ).       

   In another embodiment according to the invention, as illustrated in  FIG. 16 , the support member  550  has a cavity  570  extending longitudinally therethrough. The grating  556  is positioned within the cavity  570  in the support member. In the embodiment of  FIG. 16 , the cavity  570  extending through the support member  550  is parallel to the centroidal axis  552  of the support member. In that instance, the mechanism for longitudinally bending and rotating the tubular support member  550  about its centroidal axis  552  tunes the wavelength of the optical grating  556  by controlling the longitudinal bending and rotating of the support member in accordance with the equation provided above. 
   In another embodiment according to the invention, as illustrated in  FIG. 17 , the support member  550  has a cavity  570 ′ extending longitudinally therethrough that intersects the centroidal axis  552  of the support member and/or is not parallel to the centroidal axis  552  of the support member. Support member  550  is shown in an unbent condition, so that the relationship between cavity  570 ′ and centroidal axis  552  may be clearly seen. The longitudinal axis of the grating  556  is positioned within the longitudinal cavity  570 ′ in the support member. Because the longitudinal cavity  570 ′ of the support member  550  is no longer parallel to the centroidal axis  552  of the support member, the result of bending and rotating support member  550  is not a simple wavelength shift in the spectral response of the grating. The mechanism for longitudinally bending and rotating the support member about its centroidal axis  552  tunes the wavelength of the optical grating  556  by controlling the longitudinal bending and rotating of the support member in accordance with the equation provided above. However, the value of r is equal to the distance between the centroidal axis  552  of the support member  550  and the grating  556 , and changes as a function of location along the grating  556 . In the case shown where the grating crosses the centroidal axis of the support member, subsequent bending and rotation does not change the wavelength at the crossing point, but changes the dispersion over the length of the grating. 
   In another embodiment according to the invention, the longitudinal axis of the grating  556  is wound about the centroidal axis  552  of the support member  550 . In the exemplary embodiment of  FIG. 18A , an optical waveguide  554  having an optical grating  556  is helically wound about the centroidal axis  552  of a longitudinal support member  550  having a circular cross-section. In this orientation between grating  556  and centroidal axis  552 , the result of bending and rotating support member  550  is not a simple wavelength shift. Instead, when the support member  550  is bent, some sections of the grating  556  are placed in tension (on the “outside” of the bend) and other areas of the grating  556  are placed in compression (on the “inside” of the bend). The spectral response is thus modulated in a manner that correlates to the helix angle (α) of the grating  556  as it winds around support member  550 . The mechanism for bending and rotating the support member  550  about its centroidal axis  552  tunes the wavelength of the optical grating  556  by controlling the longitudinal bending and rotating of the strain-imparting support member in accordance with the equation provided above. However, the angle of rotation (θ) becomes a function of location on the grating  556  and will therefore vary for each point along the grating. 
   As an example, assume that the unstrained grating  556  of  FIG. 18A  has a uniform dispersion (i.e., the delay slope is constant) before the support member  550  is bent. Then, it can be seen in the graph of  FIG. 19A  that bending the support member  550  results in a modulation around the initial (unstrained) delay line. Since dispersion is defined as the slope of the delay line, it is clear that the dispersion changes as a function of the position in the grating  556 . The graph of  FIG. 19B  shows the dispersion of the grating  556  as a function of wavelength and relates to the strain distribution used to generate the graph of  FIG. 19A . By rotating the support member  550  without changing the radius of the bend, the dispersion at any wavelength will vary between the extremes shown in  FIG. 19B . Thus a single wavelength or narrow band within the bandwidth of the grating  556  may readily have its dispersion adjusted. This embodiment thus provides the ability to tune only a selected section of a grating  556 . 
   To avoid the creation of etalons in an optical waveguide grating (as is typically preferred in a grating-based dispersion compensation filter), the intrinsic chirp of the grating should be greater than the strain induced chirp caused by bending of the support member. This can be achieved by assuring that the following relationship is maintained:
 
Intrinsic Chirp&gt;λ o *(1 −P )*sin(α)/ R 
 
Where:
         λ o =unstrained wavelength of the grating;   P=the strain optic coefficient of the grating;   α=the helix angle (the angle between the fiber grating and the axis of the support member);   R=radius of the bend in the support member.       

   For example, assume a grating written with a dispersion of 500 ps/nm has an intrinsic chirp of about 20 nm of wavelength shift per meter of grating length (intrinsic chirp=20 nm/m). Further assume a maximum wavelength of approximately 1600 nm (λ o =1600 nm). The strain optic coefficient of a silica fiber is approximately 0.24 (P=0.24). If one desires to bend the support member in an arc having a radius of 2 meters (R=2 m), then using the relationship given above, the helix angle α should not exceed about 1.8 degrees. 
   If the bend in the support member has a larger radius, a larger helix angle may be used. For example, using the same grating as in the example above, and a bend radius of 5 meters, the helix angle should not exceed about 4.6 degrees. 
   The effect of the bend radius R on delay and dispersion is illustrated in  FIGS. 19C and 19D  for a grating helically wrapped about a support member. The plots of  FIGS. 19C and 19D  model an optical waveguide grating having a dispersion of 500 ps/nm and an intrinsic chirp of about 20 nm of wavelength shift per meter of grating length (intrinsic chirp=20 nm/m) helically wrapped around a support rod having a radius of 7 mm. The grating is wrapped with a helix angle α of 2 degrees. The strain optic coefficient of the grating is approximately 0.24 (P=0.24). The support member is variously bent in an arc having a radius R of 5 meters (curve S 1 ), 10 meters (curve S 2 ), and 20 meters (curve S 3 ). 
   In the exemplary embodiment of  FIG. 18A , a single optical waveguide grating  556  is wound about the centroidal axis  552  of the support member  550 . The slope of the dispersion curve of the grating  516  (as illustrated in  FIG. 19B ) can be seen to vary continuously between the extremes, such that there is no region where the slope of the dispersion curve is relatively constant. As a result, for a given rotation of the support member about its centroidal axis, at some points in the grating (i.e., where the slope of the dispersion curve is small) only a small change in dispersion will result, while at other points in the grating (i.e., where the slope of the dispersion curve is large) a large change in dispersion will result. In some applications, it is desirable that the slope of the dispersion curve be relatively constant, so that for a given rotation of the support member about its centroidal axis, the change in dispersion is approximately the same for all points along the grating. 
   In an embodiment illustrated in  FIG. 18B , two identical optical waveguide gratings  556 ,  556 ′ are wound about the support member  550 . Gratings  556 ,  556 ′ are helically wrapped in a similar manner, with the gratings  556 ,  556 ′ mounted on the support rod such that the two gratings are located on opposite sides of the support member (i.e. 180 degrees apart) at any point along their length. The optical signal for grating  556  is launched into the short wavelength end (right) and the optical signal is launched into grating  556 ′ using the long wavelength end (left), such that their dispersion curves are of opposite signs and 180° apart. As illustrated in  FIG. 19E , summing the dispersion curves of individual gratings  556 ,  556 ′ provides a dispersion curve having a relatively constant slope. Advantageously for some applications, the dispersion curve resulting from the summed individual dispersion curves crosses the zero axis, such that the apparatus may be used to provide either positive or negative dispersion, as needed. 
   As discussed above and as shown in  FIGS. 14–18B , the grating  556  may be attached to the support member  550  such that the longitudinal axis of the grating  556  has several different possible orientations with respect to the centroidal axis  552  of the support member. Tuning of the grating  556  is controlled in accordance with the equations provided above. 
   In another method for adjusting the spectral response of an optical grating  556  according to the invention, a generally unstrained grating  556  is attached to an already strained (i.e., bent or flexed) support member  550 . The grating  556  is attached to the section of the support member having the greatest tension (e.g., the outside of the bend). As the support member  550  is rotated about its centroidal axis  552 , the grating  556  moves from its original unstrained position on the outside of the support member  550  bend to a position where it sees a compressive strain as it moves toward the inside of the support member bend. The grating  556  is thus tuned using axial strain only, and no tensile strain is place on the grating  556 . 
   The support member  550  has so far been described herein as having a circular cross-section. However, other cross-sectional profiles of support member  550  may also be implemented. For example, support member  550  may have an elliptical cross-section or any other non-circular cross-section. Additionally, the cross-section of support member  550  may vary along the length of support member  550 . 
   The mechanism for longitudinally bending and rotating the support member and attached grating  556  about the centroidal axis  552  may comprise any suitable device that provides controlled bending and rotating of the support member. One such mechanism  600  is illustrated in  FIGS. 20A and 20B , in which a positioning plate  602  includes at least one set of positioning grooves  604 . Grooves  604  are sized to receive support member  550 , and are oriented to maintain a predetermined bend radius in support member  550 . Multiple sets of grooves (for example, grooves  604 ′,  604 ″) corresponding to different bend radii may additionally be provided so that support member  550  can be bent across a range of predetermined bend radii. Support member  550  may be rotated about its centroidal axis  552  by, for example, rotating support member by hand, or by operably coupling support member  550  to a drive motor  608 . Drive motor  608  may be manually controlled, or alternately controlled by controller  610  to automatically rotate support member  550  and thereby adjust the spectral response of grating  556 . 
   Another mechanism  620  for longitudinally bending and rotating the support member  550  and attached grating  556  about the centroidal axis  552  is illustrated in  FIG. 21 . Support member  550  is held at its ends in ferrules  622 . Ferrules  622  are pivotable with respect to base member  624  (as indicated by arrows  626 ), and are configured such that the distance between ferrules  622  may be altered (as indicated by arrows  627 ), thereby changing the bend radius of support member  550 . For example, ferrules  622  may be moved between fixed, incremental positions in base member  624 . Alternately, the distance between ferrules  622  may be continuously adjusted, such as by attaching one or both of ferrules  622  to a drive mechanism  628 , such as a screw drive, that moves ferrules  622  closer together or further apart as necessary to change the bend radius of support member  550 . Drive mechanism  628  may be operated and controlled manually, or alternately controlled by a drive motor  630  and controller  632 . Support member  550  may be rotated about its centroidal axis  552  by, for example, rotating support member within ferrules  622  by hand, or by operably coupling support member  550  to a drive motor  634 . Drive motor  634  may be manually controlled, or alternately controlled by controller  632  to automatically rotate support member  550  and thereby adjust the spectral response of grating  556 . 
   The above-described embodiments according to the invention provide simple and cost effective ways to adjust (i.e., “tune”) the wavelength and/or chromatic dispersion characteristics (i.e., the “spectral response”) of an optical waveguide grating, such as a fiber Bragg grating or long period grating. The various embodiments require a small number of relatively simple parts, as compared to the devices and methods previously available. The various embodiments according to the invention provide apparatuses and methods for achieving a dispersion compensation apparatus having high reliability (i.e., reduced susceptibility to fiber failure due to static fatigue) by using a combination of beam bending and/or compressive preload on the optical grating to reduce tensile strain in the grating. Reduced strain in the grating permits a wider bandwidth and tuning range. 
   Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a wide variety of alternate and/or equivalent implementations and embodiments that achieve the same purposes may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. Those with skill in the optical, mechanical, and opto-mechanical arts will readily appreciate that the present invention may be implemented in a very wide variety of embodiments. This application is intended to cover any adaptations or variations of the embodiments discussed herein.