Abstract:
A 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 a waveguide. The crossing angles of the writing beams may be adjusted. In one embodiment, circular wedges in each writing beam may be rotated about their axes to alter the crossing angle. In another embodiment, acousto-optic Bragg cells may be utilized to change the writing angle of the writing beams.

Description:
BACKGROUND 
   This invention relates to optical networks and, particularly, to waveguides including Bragg gratings. 
   An optical 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 optical add/drop multiplexer uses a fiber or waveguide 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 better ways 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 graph of Bragg wavelength in nanometers versus rotational angle in degrees in accordance with one 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, and other potential error sources. For example, the apparatus  100  permits control of grating period and grating phase in the manufacture of Bragg gratings in any waveguide including planar and fiber waveguides. 
   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 S ranging from about 80%-120% of Λ pm /2 in some embodiments. While the gratings  108 ,  109  are illustrated as separate components they may be integrated as a single component. An undiffracted component  111  of the beam  102  may be blocked by a baffle  110  in some embodiments. 
   A diffraction order  114  (a−1 order) produced by the grating  108  and a diffraction order  115  (a+1 order) produced by the grating  109  are directed to movable, light transmissive devices  118  and  119 , which in one embodiment may be circular, rotatable wedges or prisms. However, the devices  118 ,  119  may be any movable device. For example, devices having two non-parallel light transmissive surfaces, such as lenses may be used as the devices  118 ,  119 . In one embodiment, the devices  118  and  119  are arranged so that their bisector is normal to the direction of propagation of light from a grating  108 ,  109  to a waveguide  112 . 
   The intersecting angle of the two orders  114 ,  115  (at the interference pattern  116 ) and, thus, the spatial frequency, of a Bragg grating being written in a waveguide  112  is modified by rotating the devices  118  and  119 . In an embodiment, the axis of rotation is nominally along the direction of propagation of the light, although other rotational directions are possible. In some embodiments, the rotations of the two wedges are in opposite directions, as indicated by the arrows  121 ,  122 . Advantageously, movement of the devices  118 ,  119  moves the writing beams back and forth across the target. 
   In some embodiments only one of the devices  118 ,  119  may be used. However, using both devices  118 ,  119  and moving them by the same amount may be advantageous. For example, using both devices  118 ,  119  may aid in maintaining the direction of the interference pattern. 
   This process introduces a small amount of translation in the position of the beam intersection which may be compensated by translation of the waveguide transverse to the scanning direction. This translation may be perpendicular to the plane defined by the intersection of the beams or within the plane of intersection. 
   The tuning of the Bragg wavelength, for example, as a function of rotational angle for 0.5 degree devices  118 ,  119 , is shown in  FIG. 3  in one embodiment. 
   Referring to  FIG. 4 , the tuning devices  118 ,  119  may be replaced by acousto-optic Bragg cells  124 ,  125  in another embodiment of the present invention. The acoustooptic Bragg cells  124 ,  125  create a Bragg grating using an acoustic wave. Application of a radio frequency wave to the acousto-optic Bragg cells  124 ,  125  causes a sound wave to move through a crystal included in each cell  124 ,  125 . The sound wave develops regions of compression and expansion that act like an optical grating. The period of the effective grating is a function of the radio wave frequency so that by charging the frequency of the radio frequency wave, the diffraction of the effective grating may be modified accordingly. 
   In this case, the intersection angle is modified by changing the frequency of the radio frequency signals  126 ,  127  used to drive the Bragg cells  124 ,  125 . Exposure by other orders of the Bragg cells  124 ,  125 , such as the undeflected zero order is advantageously avoided. 
   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 Bragg grating exposure times and increase manufacturing throughput, blazed gratings can be used to deliver power to a selected diffraction order or orders. 
   The apparatus  100  of  FIG. 1  can be arranged in several ways for Bragg grating fabrication. According to one embodiment, shown in  FIG. 2 , a waveguide  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 mounted on the stage  156 . Beams  171 ,  172  produced by the gratings  158 ,  159 , respectively, produce an interference pattern in the waveguide  152  after passing through devices  118 ,  119  mounted on devices  173 ,  174 . 
   The waveguide  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. In some embodiments 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 waveguide  152 . Gratings  158  and  159  may be fixed with respect to the translation stage  162  in some embodiments. 
   A Bragg grating is conveniently written in the waveguide  152  section by section or by continuous scanning. In an embodiment, 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 movement of the devices  118 ,  119 . Alternatively, motion of the wedges or phase shifts in the RF driving of acoustooptic cells could be used to set the phase of the interference pattern. After the phase and period of the interference pattern are determined, exposure of the waveguide  152  to form a 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 the waveguide (e.g., the amplitude of a periodic refractive index change produced by exposure to the interference pattern) can be adjusted by dithering GI along an axis  174  (partially washing out fringes of the interference pattern) or through other exposure control means. For embodiments utilizing wedges, control of the amplitude can also be accomplished through appropriate dithering of the wedges. For embodiments utilizing acoutooptic deflectors, appropriate modulation of the phase, amplitude, and/or frequency of the driving RF can be used. 
   After exposure of a selected waveguide segment is complete, the translation stage  156  is adjusted to position another waveguide 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 Bragg grating to be produced. In additional embodiments, the waveguide and gratings are stationary while both the laser beam and tuning elements are 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. 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 fiber Bragg gratings (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 waveguides are described above. In additional embodiments, a recording schedule can include one or more exposures of at least some portions of the waveguide. For example, one section of the waveguide can receive multiple exposures if the writing beam has a width greater than an incremental translation distance. Another embodiment includes exposing a waveguide to a writing beam having a width ω and translating the writing beam continuously across the waveguide. 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 waveguide, 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 waveguide. 
   Insertion of tuning elements (cells  124 ,  125 ) into the resulting interferometer may be carried out to ensure that additional phase error is not introduced upon scanning the laser beam with respect to the gratings  108 ,  109  and the waveguide  112 . In particular, if the tuning elements are small and fixed with respect to the laser beam  101 , scanning of the interferometric apparatus  100  results in a spatially dependent phase shift in the resulting waveguide  112  grating. 
   The magnitude of the phase shift is proportional to the detuning of the interferometer from its natural spatial wavelength. The phase shift may be compensated by the application of an appropriate spatially dependent phase correction term, either through the motion of a grating  104 ,  108 ,  109 , or through the application of a phase shift to the radio frequency signal  126 ,  127  used to drive the Bragg cells  124 ,  125 . The phase shift may be developed from any variable radio frequency wave received from the radio frequency source  130  by controllably applying a phase shift in a shifter  132 . 
   Interferometric fabrication apparatus such as the apparatus  100  of  FIG. 1  permits fabrication of Bragg gratings having selectable refractive index modulations by selective application of an interference pattern to a waveguide. Such apparatus provide interference patterns that are readily accessible so that waveguides to be exposed can be situated in the interference patterns. 
   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 cover all such modifications and variations as fall within the true spirit and scope of this present invention.