Abstract:
A fiber Bragg grating may be written at an arbitrary wavelength without extensive recalibration or reconfiguration of the writing equipment in some embodiments. A pair of writing beams may be used to expose the fiber. The crossing angles of the writing beams may be adjusted.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on provisional patent application Ser. No. 60/289,930 filed May 9, 2001. 
    
    
     BACKGROUND 
     This invention relates to optical networks and, particularly, to waveguides including Bragg gratings. 
     An add/drop multiplexer is an important component in most optical networks. The multiplexer pulls down the desired channels from a network branch and replaces those channels with different contents. At the same time, the multiplexer lets the other channels pass through without significant insertion loss. 
     Generally, the add/drop multiplexer uses a fiber Bragg grating. The fiber Bragg grating drops input light at the Bragg wavelength. The characteristics of a fiber Bragg grating and, particularly, its Bragg wavelength, are fixed. In other words, a particular fiber Bragg grating is written, for example, using an ultraviolet light, to have a predetermined Bragg wavelength. As a result, a given grating may only be able to reject or drop one wavelength. 
     At different times, however, it may be desirable to write a grating that rejects different wavelengths. Enabling an interferometer manufacturing device to convert to writing a different Bragg wavelength may be complicated and time consuming. Substantial realignment and/or recalibration may be required in some cases. 
     Thus, there is a need for the ability to write a waveguide with an arbitrary Bragg wavelength. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic depiction of one embodiment of the present invention; 
         FIG. 2  is a depiction of an apparatus for implementing the embodiment shown in  FIG. 1 , in accordance with one embodiment of the present invention; 
         FIG. 3  is a schematic depiction of another embodiment of the present invention; and 
         FIG. 4  is a schematic depiction of another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     An interferometric fabrication apparatus  100 , shown in  FIG. 1 , enables waveguide fabrication that, in some embodiments, is tolerant to substantial errors in alignment, laser beam pointing, laser wavelength, laser spectral and temporal coherence, and other potential error sources. Furthermore, the apparatus  100  permits control of grating period and grating phase in the manufacture of fiber Bragg gratings (FBGs). 
     The apparatus  100  includes a laser  101  (or other light source) that produces a laser beam  102  that is incident to a first diffraction grating  104  having a period Λ pm . Diffraction orders  106 ,  107  (the +1 and −1 diffraction orders, respectively) are produced and directed to respective gratings  108 ,  109  that have grating periods of about Λ pm /2. The gratings  108 ,  109  may have grating periods ranging from about 80%-120% of Λ pm /2 in some embodiments. An undiffracted component  111  of the beam  102  is blocked by a baffle  110 . 
     A diffraction order  114  (−1 order) produced by the grating  108  and a diffraction order  115  (+1 order) produced by the grating  109  are directed to a fiber  112  and produce an interference pattern  116  of period Λ/2. The period (or spatial frequency) of the interference pattern  116  at the fiber  112  can be controlled by, for example, rotating the gratings  108 ,  109 . In a representative example, the gratings  108 ,  109  are rotated by substantially equal amounts but in opposite directions as indicated by angles a o  and a r  of FIG.  1 . 
     The apparatus of  FIG. 1  is well suited to the manufacture of FBGs, exhibiting several advantageous properties. For example, the apparatus  100  is achromatic. The first derivative of Λ with respect to laser wavelength λ is dΛ/dλ=0, i.e., the period of the interference pattern is independent of the wavelength λ. Thus, the interference pattern is relatively insensitive to changes in laser wavelength or laser spectral bandwidth. Alternatively, because Λ is insensitive to changes in laser wavelength λ, a broadband light source or a laser having a relatively broad emission wavelength range can be used. 
     If a o =a r =0, then the phase of the interference pattern used to form a Bragg grating is independent of the angle of incidence of the writing beam. Because the phase of the interference pattern does not depend on the angle of incidence of the laser beam, laser pointing stability need not be precisely controlled. Accordingly, suitable light sources for the apparatus  100  include both single transverse mode and multiple transverse mode lasers, and other light sources. The angles a o  and a r  are adjusted to control the phase of the FBG during fabrication, but even as these angles are changed to control the FBG phase, the dependence of the phase of the interference pattern on the angle at which the laser beam is incident to the gratings  104 ,  108 ,  109  remains small because small values of a o  and a r  can be used for tuning the period of the interference pattern  116 . 
     The interference pattern  116  produced by the apparatus  100  is tunable over a large range of periods or spatial frequencies by changing a o  and a r . Such tuning does not significantly increase interference pattern sensitivity to positioning errors or light source imperfections such as those described above. In addition, diffraction orders such as the zeroth order or higher orders can be readily blocked as needed. In order to deliver an appreciable portion of the power of the laser beam  102  to the interference pattern  116  to reduce FBG exposure times and increase manufacturing throughput, blazed gratings can be used to deliver power to a selected diffraction order or orders in some embodiments. 
     The apparatus  100  of  FIG. 1  can be arranged in several ways for FBG fabrication. According to one embodiment, shown in  FIG. 2 , a fiber  152  (in which a grating is to be written) and a primary grating G 1  ( 154 ) are mounted on a translation stage  156  and secondary gratings G 2 , G 3  (the gratings  158 ,  159 , respectively) are fixed with respect to the translation stage  156 . Beams  171 ,  172  produced by the gratings  158 ,  159 , respectively, produce an interference pattern in the fiber  152 . The fiber  152  is situated to be illuminated with the interference pattern such as the interference pattern  116  of  FIG. 1 , or other patterned illumination or radiation. The primary grating G 1  is additionally mounted on a high precision stage  162  so that the grating G 1  can be translated by a selected fraction of a period of the interference pattern with respect to the fiber  152 . 
     A fiber Bragg grating is conveniently written in the fiber  152  section by section or by continuous scanning. For a selected section, the phase of the interference pattern is determined by translating the high precision stage  162  to within a selected fraction of a period of the pattern, and the period of the interference pattern is selected by rotation of the secondary gratings G 2  and G 3 . After the phase and period of the interference pattern are determined, exposure of the fiber  152  to form a fiber Bragg grating begins by, for example, turning on the laser and/or opening a laser shutter. The amplitude of a grating written in a particular section of fiber (e.g., the amplitude of a periodic refractive index change produced by exposure to the interference pattern) can be adjusted by either dithering G 1  along an axis  174  (partially washing out fringes of the interference pattern) or by lowering the exposure intensity. In additional embodiments, one or more of the gratings  154 ,  158 ,  159  are dithered along one or more axes to, for example, produce apodized FBGs. The dithering or other motion of the gratings can also be rotational. Such dithering can be uniform (the same in both directions with respect to an axis and about a central position) or asymmetric. Grating rotations can be configured to select a period of the interference pattern. 
     Typically, control of average refractive index in an FBG is advantageous. As a particular example, maintaining the average refractive index at or near a constant value while still allowing for variation of grating period, grating phase, and refractive index modulation permits control of FBG properties. Control of average refractive index can be accomplished by, for example, providing a constant, uniform total radiation exposure to the grating along the length of the grating. With such control, the phase, period, and amplitude of the index modulation can be selected. According to some methods, gratings can be written and erased with a series of one or more exposures (a “recording schedule”) in which a phase and/or other characteristic of the interference pattern are selected for one or more of the exposures. In a representative embodiment, the phase of the interference pattern is selected for each exposure of the recording schedule. The recording schedule is generally selected based on the amplitude and phase of the refractive index modulation to be obtained in the fiber or other waveguide. For example, a sinusoidal dithering of the phase of the interference pattern provides a refractive index modulation amplitude that is proportional to a Bessel function of the dither amplitude. In other embodiments, a ratio of the intensities of the two interfering beams (i.e., the beams  171 ,  172 ) at the fiber  152  is varied while keeping the total exposure intensity substantially constant. According to additional embodiments, a low coherence writing source can be used, and the refractive index modulation amplitude can be controlled by varying a path length difference between the two beams interfering at the fiber. Such embodiments produce interference patterns having fringe visibilities that are selected based on source coherence. Low coherence sources include incoherent light sources such as arc lamps as well as lasers having limited coherence. According to additional embodiments, control of the polarization of the two interfering beams by for example half-wave plates placed in the path of the beams (i.e. beams  106  and  107 , or  114  and  115 ) can also provide control of the fringe contrast while maintaining a constant illumination and hence average refractive index. 
     After exposure of a selected fiber segment is complete, the translation stage  156  is adjusted to position another fiber segment for exposure to the interference pattern, and the exposure process is repeated. This method generally uses a primary grating G 1  that is as long as the total length of the fiber Bragg grating to be produced. In additional embodiments, the fiber and primary and secondary gratings are stationary while the laser is moved. The laser beam can be moved by, for example, translating the laser or controlling the beam with one or more mirrors, prisms, or other reflective or refractive optical elements. 
     In other methods, diffraction orders other than the first order can be used, and reflection gratings can be used instead of the transmission gratings illustrated in  FIGS. 1 and 2 . In addition, secondary gratings having periods that are not equal to Λ pm /2 can be used. A single grating can be used for both secondary gratings, but with limited tunability. The gratings can be amplitude gratings or phase gratings, or a combination thereof, and, as noted above, can be blazed to provide efficient transfer of power from an incident laser beam (or other optical beam) into a predetermined diffraction order. For fabrication of FBGs, radiation at wavelengths of between about 150 nm and about 450 nm is typically selected, but other wavelengths can be used depending on the photosensitivity of the fiber. In addition, exposure times for FBG fabrication can be reduced using one or more spherical or cylindrical lenses or mirrors, or other focusing elements. 
     Exposure methods using step-wise exposure of fibers are described above. In additional embodiments, a recording schedule can include one or more exposures of at least some portions of the fiber. For example, one section of fiber can receive multiple exposures if the writing beam has a width greater than an incremental translation distance. Another embodiment includes exposing a fiber to a writing beam having a width ω and translating the writing beam continuously across the fiber. The translation can be at a substantially constant rate, or can vary. With such a recording schedule, the total exposure can be represented as a convolution of the beam width ω with the translation profile. By recording the position of exposure of the fiber, a predetermined amount of dithering, intensity, or beam angle detuning can be applied during exposure. In this way, it is possible to programmably apply an arbitrary index modulation profile to the fiber. 
     Interferometric fabrication apparatus such as the apparatus  100  of  FIG. 1  permits fabrication of FBGs having selectable refractive index modulations by selective application of an interference pattern to a fiber or other waveguide. Such apparatus provide interference patterns that are readily accessible so that fibers or waveguides to be exposed can be situated in the interference patterns. Other advantages of some embodiments, compared to, for example, the single phase mask contact printing method, is that the since all the beams are accessible it is possible to block spurious diffraction orders and thereby increase the effective modulation contrast at the fundamental period allowing for more efficient use of the dynamic range of the photosensitivity of the exposed waveguides. Another advantage of some embodiments of the interferometric technique is that it is possible to achieve perfect beam overlap of the interfering beams at the waveguide. This is not possible in the single phase mask approach because as the beams diffract from the phase mask the overlap of the beams decreases with increasing separation between the waveguide and the phase mask. For example, for a telecommunications fiber that has a core diameter of 9 microns and a cladding diameter of 125 microns, the smallest separation between the phase mask and waveguide is about 62.5 microns. The beam overlap becomes very important when trying to use small beam widths in order to write fine-featured index profiles into the waveguide. 
     As shown in  FIG. 2 , a phase of an interference pattern is selectable based on rotations of the gratings  158 ,  159 . However, rotations of the gratings  158 ,  159  can adversely effect the insensitivity of the interference pattern to misalignments and source imperfections. A fabrication apparatus  200  shown in  FIG. 3  includes mirrors  224 ,  225  (M 1  and M 2 , respectively) situated downstream of gratings  208 ,  209 . The mirrors  224 ,  225  are configured to direct beams  215 ,  216  to form an interference pattern  221  at a fiber  230 . By using the mirrors  224 ,  225  to direct the beams  215 ,  216 , the gratings  208 ,  209  need not be rotated. By rotating the mirrors  224 ,  225  in opposite directions, the period of the refractive index modulation can be selected. By rotating the mirrors  224 ,  225  in the same direction, the phase of the refractive index modulation is selectable. Rotations of the mirrors  224 ,  225  change the position of an intersection of the beams  215 ,  216  but such position changes can be compensated by moving the mirrors towards or away from the fiber. Alternatively, pivot points of the mirrors M 1  and M 2  can be selected to reduce the displacement of the point of intersection. 
     In another embodiment, an interferometric exposure apparatus  300 , shown in  FIG. 4 , similar to that of  FIGS. 1 and 2 , includes prisms  324 ,  325  that are rotatable about angles α r  and α o  to change an angle at which beams  314 ,  315  intersect and thereby change the spatial frequency of an interference pattern  331  to which a fiber  330  is exposed. In some embodiments, the prisms  324 ,  325  are rotatable in equal amounts but in opposite directions or in the same direction. 
     It should be noted finally, when beam scanning is employed to write a long grating into waveguides, then the choice of the relative motions of the beams, mirrors, gratings, and waveguide needs to be carefully considered when angle tuning of the interferometer is used. For example, in  FIG. 3 , if mirrors M 1  and M 2  are kept fixed with respect to the writing beams, while gratings  204 ,  208  and  209  and the waveguide  221  are moved so as to effectively scan the beams across a length of waveguide, then it is important to ramp the phase difference between the beams as a function of scan length. The phase ramp depends on the angle deviation of the mirrors and should compensate for the lack of phase ramp that would ordinarily be seen if the mirrors were moved with the rest of the interferometer. The same applies to  FIG. 4  for the case where the wedges are held fixed with respect to the beam during waveguide scanning. 
     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims &amp;over all such modifications and variations as fall within the true spirit and scope of this present invention.