Abstract:
Systems and methods for making a self-aligning optical system are provided, Briefly described, in architecture, one such system for making an optical system, among others, can be implemented as follows. The system comprises a grating substrate supporting a holographically-formed diffraction grating and an array mount for defining relative locations of point sources of light. The array mount comprises recording points that define locations of point sources of recording light used to illuminate the grating substrate during fabrication of the holographically-formed diffraction grating and use points that define locations of light apertures used in operation of the holographically-formed diffraction grating. Other systems and methods are also provided.

Description:
TECHNICAL FIELD  
       [0001]     The present disclosure is generally related to optics, and, more particularly, is related to wavelength division multiplexers.  
       DESCRIPTION OF THE RELATED ART  
       [0002]     A typical optical multiplexing/demultiplexing system couples different wavelengths of light from different sources into a single optical fiber. After transmission on the single fiber, another multiplexing/demultiplexing system separates the different wavelengths and provides them to different optical fibers. The typical multiplexing/demultiplexing system consists of a fiber array mount and a holographic diffraction grating. The holographic diffraction grating is produced through the effects of the interference of two coherent optical beams projected onto a photosensitive material from two point sources.  
         [0003]      FIG. 1  illustrates a typical method  100  for manufacturing a holographic diffraction grating  110 . First, a single coherent beam is generated by a suitable laser source  120 . The single coherent beam is then split into two coherent beams by a beamsplitter  130 . This directs the light into two separate optical fibers  141 ,  142 . The output of the fibers is positioned with respect to the diffraction grating substrate  150  so that an interference pattern  160  is projected onto a photosensitive layer  170  that covers the substrate  150 . Development of the photosensitive layer removes the exposed regions of the photosensitive layer and leaves the unexposed regions in place. This creates the surface relief pattern of the diffraction grating  110 . In the case of a reflection grating, the photosensitive layer is overcoated by a reflecting mirror coating to enhance grating efficiency.  
         [0004]     The diameters of the optical fibers  141 ,  142  used to record and manufacture the holographic diffraction grating  110  typically are on the order of 10 micrometers (μm) in diameter. The physical spacing between individual fibers  141 ,  142  typically is on the order of 25 to 50 millimeters (mm). Accordingly, the manufacturing of the holographic grating  110  involves precise alignment of the optical fibers  141 ,  142  in relation to the diffraction grating  110  and the laser beams.  
         [0005]      FIG. 2  depicts the utilization  200  of the holographic diffraction grating  110  produced via the method  100  illustrated in  FIG. 1  in an optical multiplexing demultiplexing system. First, a fiber array mount  230  secures the positioning of the ends of the input optical fiber  210  and output optical fibers  242 - 246 . Then, the fiber array mount  230  and diffraction grating  10  are precisely positioned so that light (composed of optical signals of different wavelengths) emitted by the input optical fiber  210  illuminates the diffraction grating  110 . The positioning is such that the optical signals of different wavelength  222 ,  224 ,  226  are separated via diffraction and imaged onto the fiber array mount  230 . In this way, individual optical signals of different wavelengths  222 ,  224 ,  226  are focused into the output optical fibers  242 ,  244 ,  246 . Since typical fiber cores are of the order of 10 μm in diameter and typical fiber arrays with cladding are as long as 25 mm in length, small misalignments of the parts in the multiplexer or demultiplexer can lead to significant optical loss in the optical system.  
       SUMMARY OF THE INVENTION  
       [0006]     Systems and methods for making a self-aligning optical system are provided. Briefly described, in architecture, one such system for making a self-aligning optical system, among others, can be implemented as follows. The system comprises a grating substrate supporting a holographically-formed diffraction grating and an array mount for defining relative locations of point sources of light. The array mount comprises recording points that define locations of point sources of recording light used to illuminate the grating substrate during fabrication of the holographically-formed diffraction grating and use points that define locations of light apertures used in operation of the holographically-formed diffraction grating.  
         [0007]     Briefly described, one embodiment of a method for making a self-aligning optical system comprises the steps of: determining locations of recording points from design parameters of a holographic diffraction grating; determining a positional relationship between locations of use points and the locations of the recording points; determining the locations of the use points based on the positional relationship; and fabricating a holographic diffraction grating using recording light emitted by point sources of light located at the recording points such that such that the light apertures at the use points are capable of optical communication via the holographic diffraction grating.  
         [0008]     Other features and/or advantages will be or may become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and/or advantages be included within the description and be protected by the accompanying claims.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The components in the drawings are not necessarily to scale. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.  
         [0010]      FIG. 1  is a block diagram illustrating a method for manufacturing a holographic diffraction grating of the prior art.  
         [0011]      FIG. 2  is a block diagram illustrating a method for utilizing the prior art holographic grating of  FIG. 1 .  
         [0012]      FIG. 3  is a block diagram illustrating an embodiment of a system for aligning an optical system employing a holographic diffraction grating.  
         [0013]      FIG. 4  is a flowchart illustrating an embodiment of a method for making the self-aligning optical system of  FIG. 3 .  
         [0014]      FIG. 5  is a flowchart diagram illustrating an embodiment of a method for making the self-aligning optical system of  FIG. 3 .  
         [0015]      FIG. 6  is a flowchart diagram illustrating an embodiment of a method for using the self-aligning optical system of  FIG. 3 .  
         [0016]      FIG. 7  is a flowchart illustrating an embodiment of a method for positioning the self-aligning optical system of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION  
       [0017]     The invention summarized above and defined by the enumerated claims may be better understood by referring to the following detailed description, which should be read in conjunction with the accompanying drawings.  
         [0018]     Reference is now directed to  FIG. 3 , which provides an illustration of an improved system for manufacturing a multiplexing/demultiplexing system containing a holographic diffraction grating  330 . In the example shown in  FIG. 3 , a fiber array mount  320  defines a pair of recording points r 1 , r 2 , one input use point u 1 , and M output use points u 2 -u 4 .  
         [0019]     The recording points r 1 , r 2  are fixed locations for recording point sources of light, such as the ends of recording optical fibers  310 ,  311 , and are arranged in a one-dimensional array disposed in the x-direction. The input use point u 1  is located in the fiber array mount  320  on same plane as the recording points r 1 , r 2 . The input use point u 1  is a fixed location for an input aperture, such as the end of an input optical fiber  342  or an entrance slit, among others.  
         [0020]     The output use points u 2 -u 4  are also located in the same plane as the recording points and the input use point. Each is offset from the input use point u 1  in the x-direction. The output use points u 2 -u 4  are fixed locations for output apertures, such as ends of output optical fibers  344 - 348  or exit slits, among others. Locating each optical fiber  310 ,  311 ,  342 - 348  at its respective use point in the fiber array mount  320  before fabrication of the holographic diffraction grating  330  provides a self-aligning holographic optical system  300 .  
         [0021]     The system  300  also includes a coherent laser source  120  that generates a laser beam that is split by a beamsplitter  130 . Alternatively, separate laser beams can be provided by an optical coupler (not shown). Each of the laser beams is directed into a respective one of the two recording optical fibers  310 ,  311 . The positions of the recording optical fiber outputs are precisely determined to provide an interference pattern  160  on the photosensitive layer  170  of a grating substrate  150 . Particularly, the end of the recording optical fibers  310 ,  311  are the point sources of light that create the interference pattern. Accordingly, the end of the recording optical fibers  310 ,  311  are positioned at the recording points r 1 , r 2  in the fiber array mount  320 .  
         [0022]     The interference pattern  160  exposes the photosensitive layer  170 . Typically, the photosensitive layer  170  is more sensitive to some wavelengths of light than others. Therefore, the wavelength of the light generated by the laser source  120  is in the sensitivity range of the type of photosensitive layer material used on the substrate  150 .  
         [0023]     Development of the photosensitive layer  170  removes the exposed regions of the photosensitive layer and leaves the unexposed regions in place. This creates a corrugated surface relief pattern that has sinusoidal ridges on the substrate  150 . This corrugated surface provides a transmission holographic grating. The corrugated surface is typically coated with a thin metallic layer to produce a reflection holographic grating  330 .  
         [0024]     To determine the proper placement of the recording points r 1 , r 2  within the fiber array mount  320  for developing the holographic diffraction grating  330 , a commercially available ray-tracing program (such as the programs sold under the trademarks ZEMAX® or CODE V®, etc.) can be used. Given design parameters of the holographic diffraction grating  330 , such as the wavelength of the recording light, the shape or curvature (if any) of the substrate  150 , the diffraction order, and the desired groove frequency, the ray-tracing program calculates the location of the recording points r 1 , r 2 .  
         [0025]     In operation of the holographic diffraction grating  330  as a wavelength filter, optical signals of different wavelengths are emitted from an input aperture (located at the input use point u 1 ), such as the end of an input optical fiber  142 , towards the diffraction grating  330 . The diffraction grating  330  separates the light into individual optical signals of different wavelengths and reflects the optical signals into respective output apertures (located at output use points u 2 -u 4 ), such as the end of output optical fibers  344 - 348 . The relative placement of the use points u 1 -u 4  is theoretically related to the position in space of the diffraction grating  330 , the curvature of the diffraction grating  330 , the groove frequency, and the wavelengths of the light emitted by the input optical fiber  342 . Accordingly, the locations of the use points u 1 -u 4  are based on the same positional relationships as the recording points r 1 , r 2 . Therefore, the locations of the use points u 1 -u 4  in the fiber array mount  320  are determined from the locations of the recording points r 1 , r 2  and the wavelengths of the recording light and use light.  
         [0026]     A point source of light is located at each recording point r 1 , r 2 . A small aperture in the fiber array mount  320  may act as a point source of light. With an aperture width at a few or more wavelengths of the recording light, the small aperture can also act as a spatial filter. However, the wavelength the aperture width can be smaller or larger than a few or more wavelengths of the recording light. In the example of  FIG. 3 , the small aperture is provided by the end of a recording optical fiber  310 ,  311 . In alternative embodiments, the small aperture may be a pinhole.  
         [0027]     At a recording light wavelength of approximately 400 nanometers (nm), the typical dimensions of the small aperture for exposing the wavelengths of light are approximately 2 to 10 μm. Optical fibers  310 ,  311  that have a single propagation mode at this wavelength (400 nm) will typically have a core diameter smaller than 4 μm and thus can be used as point sources without additional spatial filters.  
         [0028]     The imaging properties of the diffraction grating  330  are determined by the shape of the concave substrate  150  and the spacing and curvature of the grooves in the diffraction grating  330 . The spacing and curvature of the grooves on the diffraction grating  330  vary across the concave surface of the diffraction grating  330 , as does the local incidence angle. Therefore, the directions of diffracted rays over the surface of the grating  330  is not parallel. Accordingly, the holographic diffraction grating  330  acts as both a dispersive element and a focusing element.  
         [0029]     Once the holographic diffraction grating  330  is made using the recording points r 1 , r 2 , the positional relationships that determine the use points u 1 -u 4  are also defined. Therefore, before fabrication of the grating  330 , the recording points r 1 , r 2  and the use points u l -u 4  can be precisely determined, as shown in  FIG. 3 .  
         [0030]     Relative alignment between optical fibers in an array can be achieved with submicrometer precision with relative ease. Therefore, the placements of the input optical fiber  342  and output optical fibers  344 - 348  can be defined relative to the placements of the recording optical fibers  310 ,  311 , so that the input optical fiber  342  and output optical fibers  344  are self-aligned to the diffraction grating  330  that is formed from the recording optical fibers  310 ,  311 . Therefore, the same fiber array mount  320  that is used to locate recording optical fibers  310 ,  311  during fabrication of the grating  330  can also be used to locate input optical fiber  342  and output optical fibers  344 - 348 . After fabrication of the grating  330 , the input optical fiber  342  and output optical fibers  344 - 348  are capable of optically communicating with each other via the holographic diffraction grating  330  without the need for re-alignment. This saves significant time and cost by removing a very critical and sensitive alignment step.  
         [0031]      FIG. 4  shows one embodiment  400  of a method for making a self-aligning optical system. First, a positional relationship is determined ( 410 ) between the relative locations of use points u 1 -u 4  and recording points r 1 , r 2  with respect to a holographic diffraction grating  330 . For example, the locations of the recording points r 1 , r 2  can be defined in space in relation to the grating substrate  150  of the holographic diffraction grating  330 , and the locations of the use points u 1 -u 4  can be defined in relation to the locations of the recording points r 1 , r 2 . Therefore, the positional relationship between the relative locations of the use points u 1 -u 4  and locations of the recording points r 1 , r 2  can be determined from the locations of the recording points r 1 , r 2 , design parameters of the holographic diffraction grating  330 , and the wavelengths of use light utilized in operation of the diffraction grating  330 . The design parameters include the wavelength of the recording light, the shape or curvature (if any) of the substrate  150 , the diffraction order, and the desired groove frequency for the diffraction grating  330 . After the positional relationship is ascertained, a fiber array mount  330  having the recording points r 1 , r 2  and use points u 1 -u 4  at locations that satisfy the positional relationship is provided ( 420 ).  
         [0032]     As discussed previously, the recording points r 1 , r 2  are the fixed locations for the ends of the recording optical fibers  310 ,  311  (or pinholes) and are defined in space in relation to the diffraction grating substrate  150 . The ends of the recording optical fibers  310 ,  311  launch the coherent light that creates the interference pattern  160  on the substrate  150  covered with photosensitive material (e.g., photoresist). The use points u 1 -u 4  are the locations of the ends of the input optical fiber  342  (or an entrance slit) and output optical fibers  344 - 348  (or exit slits) that are used in operation of the holographic diffraction grating  330 . The recording points r 1 , r 2  and the use points u 1 -u 4  are at defined locations precisely within the fiber array mount  320 .  
         [0033]     In an embodiment, the recording points r 1 , r 2  additionally serve as two of the use points u 1 -u 4  in the fiber array mount  320 . Because of this added constraint, some other design parameter might need to be allowed to vary, such as the wavelengths of recording and use light, the curvature of the diffraction grating  330 , the groove frequency, or the diffraction order at which the diffraction grating  330  is used.  
         [0034]     In the next step, using the recording light emitted from the recording optical fibers  310 ,  311  at the recording points r 1 , r 2  in the fiber array mount  320 , the holographic diffraction grating  330  is fabricated ( 430 ). Fabricating the holographic diffraction grating  330  using recording light emitted by the recording optical fibers  310 ,  311  self aligns the input and output optical fibers  342 - 348  at the respective use points u 1 -u 4  in the fiber array mount  320  with the holographic diffraction grating  330  so that the input optical fiber  342  at the input use point u 1  can optically communicate with output optical fibers  344 - 348  at the output use points u 2 -u 4  via the holographic diffraction grating  330 . Typically, after fabrication of the holographic diffraction grating  330 , the recording optical fibers  310 ,  311  may be cut and removed, since the recording optical fibers  310 ,  311  are not used after the diffraction grating  330  has been fabricated. However, in some embodiments, the recording optical fibers  310 ,  311  additionally serve as input and output optical fibers  342 - 348  and thus are not removed.  
         [0035]     Correspondingly,  FIG. 5  shows one exemplary embodiment  500  of a method for making a self-aligning optical system. First, a positional relationship is determined ( 510 ) between the relative locations of use points U 1 -u 4  and recording points r 1 , r 2  with respect to a holographic diffraction grating  330 . After the positional relationship is determined, a fiber array mount  330  having the recording points r 1 , r 2  and use points u 1 -u 4  at locations that satisfy the positional relationship is provided ( 520 ).  
         [0036]     Next, a grating substrate  150  is provided ( 530 ), and the ends of the recording optical fibers  310 ,  311  are fixed ( 540 ) at the predetermined recording points r 1 , r 2  with respect to the grating substrate  150 . Using the recording light emitted from the recording optical fibers  310 ,  311  at the recording points r 1 , r 2  in the fiber array mount  320 , the holographic diffraction grating  330  is fabricated ( 550 ). The recording optical fibers  310 ,  311  are then removed ( 560 ) from the fiber array mount  320 .  
         [0037]     One embodiment  600  of a method for using a self-aligning optical system will be described next. After fabrication of the holographic diffraction grating, the ends of input optical fiber  342  and output optical fibers  344 - 348  are fixed ( 610 ) at the predetermined use points u 1 -u 4  with respect to the recording points r 1 , r 2 , as shown in  FIG. 6 . Accordingly, the fiber array mount  320  is employed to secure the positioning of the input and output optical fibers at the predetermined use points.  
         [0038]     Fabricating the holographic diffraction grating  330  using recording light emitted by the recording optical fibers  310 ,  311 , self aligns the input and output optical fibers  342 - 348  at the respective use points u 1 -u 4  in the fiber array mount  320  with the holographic diffraction grating  330  so that the input optical fiber  342  at the input use point u 1  can optically communicate ( 620 ) with output optical fibers  344 - 348  at the output use points u 2 -u 4  via the holographic diffraction grating  330 .  
         [0039]     One embodiment  700  of a method according to the invention for aligning the positioning of optical fibers  342 - 348  in a fiber array mount  320  that is used in operation of the holographic diffraction grating  330  but was not used to fabricate the holographic diffraction grating  330  will now be described with reference to  FIG. 7 . First, a positional relationship is determined ( 710 ) between the relative locations of use points u 1 -u 4  and recording points r 1 , r 2  with respect to the holographic diffraction grating  330 . Then, a fiber array mount  320  is provided ( 720 ) that has use points u 1 -u 4  and recording points r 1 , r 2  at locations that satisfy the positional relationship. Next, the recording points are aligned ( 730 ) with the holographic diffraction grating  330 .  
         [0040]     To align the recording points, a laser of the same wavelength as the recording laser can be used to illuminate the holographic diffraction grating  330  through the ends of recording optical fibers  310 ,  311  at the recording points in the fiber array mount  320 . When the diffraction grating  330  is in the same position relative to the ends of recording optical fibers  310 ,  311  that it was during its exposure process, the interference pattern on the grating  330  will disappear. Note, this is very similar to the pattern one gets in an interferometer. Therefore, once the alignment between the recording optical fibers  310 ,  3 . 11  and the diffraction grating  330  produces less than one interference fringe, the ends of recording optical fibers  310 ,  311  at the recording points r 1 , r 2  are properly positioned, and are aligned to the diffraction grating  330 .  
         [0041]     The proper placement of the ends of recording optical fibers  310 ,  311  at the recording points r 1 , r 2  with respect to the holographic diffraction grating means that the ends of use optical fibers  342 - 248  at the use points u 1 -u 4  are also properly aligned with the holographic diffraction grating  330 . Therefore, by aligning the recording points with the holographic diffraction grating  330 , the use points are self-aligned ( 740 ) with the holographic diffraction grating  330 . Advantageously, the alignment of optical fibers  342 - 248  at the use points u 1 -u 4  can be performed at the same time as the alignment of the recording optical fibers. In this way, sophisticated instrumentation does not have to be used to separately align optical fibers at the use points u 1 -u 4  with respect to the holographic diffraction grating  330 .  
         [0042]     By positioning the input optical fiber  342  and output optical fibers  344 - 348  at the respective use points u 1 , u 2 -u 4  whose locations are accurately defined relative to those of the recording points, the optical system  300  is automatically aligned. Therefore, this reduces the cost and complexity of manufacture a multiplexing/demultiplexing system containing a holographic diffraction grating  330 . Typically, a multiplexing/demultiplexing system is designed with identical input and output optical fibers. Accordingly, when light propagation is reversed in the system  300 , the demultiplexing system becomes a multiplexing system.  
         [0043]     It should be emphasized that the above-described embodiments, particularly, any “preferred” embodiments, are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiments without departing substantially from the invention defined by the claims. For example, optical pinholes may be used in lieu of optical fibers to form the interference pattern. Also, a holographic grating may be employed as a transmission grating. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims.