Abstract:
A three-dimensional optical switch using three-dimensional rewritable waveguides in a photosensitive substrate. At least two light beams are used to illuminate the substrate in the region where the waveguide is to be written. The refractive index of the substrate changes at the points where the beams intersect. Therefore, the light beams can be directed to create a three-dimensional waveguide in the photosensitive substrate. A plurality of inlet ports in one surface of the substrate are thereby connected to a plurality of outlet ports on another surface of the substrate. Alternatively, the inlet ports and the outlet ports may be on the same surface of the substrate. The invention also provides methods of creating waveguides in photosensitive media using the device described herein.

Description:
BACKGROUND  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to optical waveguide based switches and more particularly to a three-dimensional, rewriteable, waveguide switch in a photosensitive medium.  
           [0003]    2. Discussion of the Related Art  
           [0004]    Fiber optic networks for transmission of optical signals have come into wide use for telecommunications and data communications in recent years. These fiber optic networks need rapid, efficient and effective switching apparatus to switch one or more incoming channels to one or more other channels within the network. Several switching techniques and structures have been developed in recent years that are satisfactory for many applications for switching one or more incoming channels from one or more paths to one or more other paths. However, in general these are not entirely satisfactory for many applications, including large cross-connect switches.  
           [0005]    Several cross-connect switches using a waveguide matrix have recently been developed. However these optical waveguide based matrix switches are based on two-dimensional, planar lightwave circuits (bubble switches) or free space switches using microelectromechanical systems (MEMS) (tilting mirrors). The waveguide approach overcomes some of the problems of the MEMS approach in that the device is minimally sensitive to mechanical vibrations and thermal fluctuations. However, the MEMS approach takes advantage of 2N(1XN) switching architecture and free space beam propagation in order to minimize losses that increase with port count. A number of switch structures of this type are disclosed in U.S. Pat. Nos. 5,699,462 and 5,960,131. In general, an optical signal must traverse between one and (2* N)−1 points in an N*N matrix switch.  
           [0006]    Accordingly, there is a need for an improved waveguide based switch that has the advantages of both of the aforementioned approaches. The present invention satisfies this need and provides other related advantages as well.  
         SUMMARY OF THE INVENTION  
         [0007]    It is one purpose of the present invention to provide a waveguide based matrix switch that overcomes the above problems of the prior art by providing a rewriteable, three-dimensional switch.  
           [0008]    In one aspect of the invention, an optical switch has a photosensitive substrate with a plurality of surfaces, a plurality of inlet ports in one of the plurality of surfaces, and a plurality of outlet ports in another of the plurality of surfaces. It also has a source of at least two light beams, and a means for directing the light beams at a selected line of intersection drawn between one inlet port and one outlet port, thereby creating a waveguide.  
           [0009]    In additional embodiments, the invention has a photosensitive substrate with at least one surface, at least one inlet port in one surface and at least one outlet port in the same surface of the substrate. It also has a source of one light beam, a means for splitting the light beam into at least two light beams, and a means for focusing the two light beams at a selected location line drawn between one inlet port and one outlet port, thereby creating at least one waveguide in the photosensitive substrate.  
           [0010]    The invention also provides a method of forming a waveguide in a substrate by taking a three-dimensional photosensitive substrate having a plurality of planar surfaces, the photosensitive substrate comprising a plurality of inlet ports in one of the plurality of surfaces, and a plurality of outlet ports in another one of the plurality of surfaces. The method also includes adding a source of at least two light beams and focusing the light beams at a selected line of intersection drawn between one inlet port and one outlet port to change the refractive index of the photosensitive substrate at the line of intersection, thereby creating a waveguide in the substrate. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0011]    The above and other objects, features and advantages of the invention will become more apparent from the following detailed description, when read in conjunction with the accompanying drawing, in which:  
         [0012]    [0012]FIG. 1 is a perspective diagramatic illustration of a waveguide based matrix switch configured in accordance with the present invention; and  
         [0013]    [0013]FIG. 2 is a plan view of one surface of a photosensitive body according to one embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0014]    Referring to FIG. 1 of the drawing, a perspective view of a waveguide based matrix switch in accordance with an exemplary embodiment of the invention is illustrated and designated generally by reference numeral  10 . The illustrated embodiment comprises a generally rectangular three-dimensional body  12  preferably made of a photosensitive crystal. Body  12  may also be referred to as a substrate. In a preferred embodiment, the photosensitive crystal is comprised of a photorefractive material. The photorefractive material enables the forming of waveguides within the material using at least two beams. As used herein the term “photosensitive” refers to the characteristic of being reactive or sensitive to radiant energy, especially light. Also as used herein, the term “photorefractive” is applied to materials, which exhibit a change in index of refraction in response to incident radiant energy, especially light.  
         [0015]    In the present invention, the focusing of laser beams within the material changes the refractive index of the photorefractive material where the lasers are focused. Thus, lasers can be used to write or draw a waveguide from an inlet point or port on one side of the body to another point or port on another side of the body. Preferably two light beams of different wavelengths are used herein to write or draw the waveguide. However, it will be understood that one light beam, or two light beams of the same wavelength, may also be used. The photorefractive material is also preferably erasable and rewritable so that the connections between ports on one side can be changed to other ports on the other side. One material that can be erased is lithium niobate, although it is understood by those skilled in the art that other suitable materials may be used, as long as they exhibit photorefractive properties and are erasable and rewritable. While preferred, erasability and rewritability are not essential or necessary aspects of the invention.  
         [0016]    For example, in the case of lithium niobate, only one beam is required to change the refractive index. However, in this configuration the index change is relatively difficult to localize in three dimensions. Other more suitable photorefractive materials may exhibit no, or relatively little, refractive index change when only one laser beam or wavelength of light is present, but when two intersecting laser beams or two different wavelengths of light are employed, a relatively large index change may be produced where they cross. The typical index of refraction change between air and the photosensitive material is preferably about 0.5.  
         [0017]    The embodiment illustrated in FIG. 1 shows a photorefractive material or body  12  having a plurality of substantially flat surfaces  14 ,  16 ,  18  and  20 . Body  12  is shown as having a three-dimensional, rectangular, box-like configuration but it would be understood by those skilled in the art that the body may have any number of other configurations, as long as similar functionality is present. A plurality of optical fibers, for example glass fibers, which may be inputs or outputs  22 ,  24 ,  26 ,  28  and  30 , are shown positioned to connect to ports (not shown) in surface  18 . Another plurality of optical fibers, which may be inputs or outputs  32 ,  34 ,  36 ,  38  and  40 , are shown connected to ports in surface  20 . The ports will be constructed and sized accordingly. In one embodiment, fibers may be spaced at least  125  microns on center. However, in preferred embodiments the input and output fibers are about nine microns in diameter and are spaced about 250 microns apart from one another.  
         [0018]    First laser  42  located on one side of body  12  is positioned to direct light beam  44  onto tiltable mirror  46  for directing the light beam along a path between a port on one side (for example, surface  18 ) of the body to a port on another side (for example, surface  20 ) of the body. The light beam is shown tracing path  48 , for example, between input optical fiber  26  on side  18  of the photosensitive body to output optical fiber  38  on side  20  of the body. It is also contemplated herein that the radius of curvature of the waveguides is a factor in choosing the length of body  12 . It is understood that the radius of the bend of the waveguide is inversely proportional to the amount of bending losses. At the bends, a certain amount of input energy is lost due to radiation or reflection. Therefore, it is desirable to reduce these effects by keeping the radii of the bends within optimal ranges. Those skilled in the art understand that these optimal ranges depend on the index of refraction difference between the waveguide and the surrounding material. For example, in embodiments where the index of refraction difference is relatively high, the radii of the waveguides may be less than 1 mm. However, in a preferred embodiment, the radii of the bends are between 1 and 5 mm. Thus, those skilled in the art are able to construct different length bodies without undue experimentation and determine whether functionality is retained. Other approaches to reducing bending losses are known, such as described, for example, in U.S. Pat. No. 6,243,525. It should also be noted that by designating one fiber as an “input” fiber, the possibility that it could function as an output fiber continues to exist.  
         [0019]    Second laser  50 , positioned on another side of photosensitive body  12 , is positioned to direct light beam  52  onto tiltable mirror  54  for directing the light beam along a path between a port on one side of the body to a port on another side of the body, in a manner similar to the manner the beam from laser  42  is directed. A tiltable mirror is an example of a means for directing a light beam. Those skilled in the art will understand that other structures such as prisms may comprise a means for directing a light beam. This process can then be repeated such that the switch is “rewritable.” In most instances the beams will be directed to a common point to simultaneously trace along a common path. Where the light beams intersect, an area of higher refractive index is formed, which remains after the light beams are removed, either until it is erased or until it dissipates or fades over a period of time. Paths may be traced in some photorefractive materials, such as lithium niobate, with a single beam of light, while other requires two beams of different frequency. Particularly preferable photorefractive materials for body  12  are selected so that the refractive index will change where two of the light beams intersect, but will not change under the influence of a single focused light beam. This operation may be controlled by control system  63 , shown in FIG. 1, which may comprise, for example, a computer system. Control system  63  may interact with laser  42 , laser  50 , as well as mirror  46  and mirror  54 .  
         [0020]    Alternative illumination schemes may be employed such as two-dimensional (2-D) arrays of lasers positioned on adjacent sides of the body. Similarly, 2-D arrays of LEDs may also be positioned on adjacent sides of the photosensitive material for directing the light and tracing the respective waveguides.  
         [0021]    In the embodiment illustrated in FIG. 1, the optical fibers and the ports appear to lie in a common plane. However, it is contemplated herein that the optical fibers and the ports lie in three dimensions. In such an arrangement, care may be taken to ensure that when connections are made, the traces or waveguides do not intersect and interfere with one another. In some constructions it may be possible to position the ports so that interference between the connections is easily avoided. During construction of the waveguides, control system  63  may direct the particular waveguide being written so that it does not cross or intersect an existing waveguide. In addition, it is possible to direct the waveguide being written to cross or intersect an existing waveguide at any particular angle. It is preferable however, for the waveguides to cross at an angle that minimizes cross-talk or intersection loss. For example, if the angle of intersection is small, there is a greater chance for cross-talk and intersection loss than if the angle of intersection is greater. In particularly preferred embodiments where waveguides do cross or intersect, the angle of intersection is about 90°. However, in other embodiments, the angle of intersection may be between about 90° and 45°. In other embodiments, it may be between about 45° and 20°. However, the angle of intersection of crossed waveguides is not limited to the ranges specified. The invention contemplates any angle of intersection. Furthermore, in some situations it may be desirable to allow the waveguides to cross or intersect, due to size constraints, or even where cross-talk and intersection loss is desired.  
         [0022]    It the exemplified embodiment, path  56  is shown traced between upper fiber  22  on face  18  to lower fiber  40  on face  20 , traversing all other traces in the body. This requires movement of the trace or waveguide out of the plane of the other traces in order to cross without interference. Trace or waveguide  58  extends directly between fiber  24  on side  18  to fiber  34  on side  20 , which may lie in a common plane with the respective ports. Waveguide  48  is the one shown in the process of being formed in the illustration. Another waveguide  60  is shown formed between fiber  28  and fiber  32 . This waveguide crosses three other waveguides between the ports. Finally, waveguide  62  extends from the fiber  30  to the fiber  36 , crossing two other waveguides.  
         [0023]    The switch structure as described enables the switch to be changed on an “as needed” basis. Assuming that one side of the switch (for example, the left side in FIG. 1) is the input, any one of the inputs can be connected to any one of the outputs that is available. Once a connection is no longer required, it may be erased by any one of a number of ways. For example, the waveguides may be erased by blanket exposure of a different wavelength, polling with an external field, or waiting a predetermined amount of time until the waveguides naturally fade. In one embodiment, the waveguide may not fade until erased by an erasing means. FIG. 1 shows an exemplary erasing means  64 .  
         [0024]    In another embodiment, the waveguide may fade naturally within a predictable period of time, for example one second. In this embodiment, the waveguides could be rewritten, or refreshed every second in order to maintain the switch. In this way the user could determine whether the refresh rate should or should not coincide with the fade rate of the waveguides, to accomplish a stable switch, or one that changes with varying frequency.  
         [0025]    The present invention therefore enables the construction of three-dimensional (3-D) switching structures having a high number of inputs and outputs. Referring to FIG. 2, an end view of a three-dimensional switching assembly is illustrated wherein multiple rows and columns of ports are shown. While only ten rows and columns are shown, an array may be formed with as many as thirty rows and thirty columns, yielding approximately 1000 inputs and outputs. This is a significant advantage over the typical 2-D array. The illustrated embodiment shows a photosensitive body  66  having vertically spaced horizontal rows  68 ,  70 ,  72 ,  74 ,  76 ,  78 ,  80 ,  82 ,  84  and  86 . Any one of the ports in row  68  may be connected to any port in any one of any other rows on a different surface of the body. In addition, any one of the ports in row  68 , for example, may be connected to any port in any one of any other rows on the same surface of the body. Thus, a huge number of interconnections can be made with such a construction. It will be apparent that the columns and rows may vary and be unequal in number. Although it is preferred that each single input will have a waveguide to a single output, it is also contemplated herein that a single input could have a waveguide coupled to more than one output, if desired.  
         [0026]    It should be understood that the terms “light” and “optical” as used herein do not limit them to wavelengths in the visible spectrum. Two separate laser sources are shown in FIG. 1, but satisfactory results may be achieved with a single laser and appropriately placed optical devices (splitters or reflectors) to create and control two distinct light beams within the substrate.  
         [0027]    While the invention has been illustrated and described by means of specific embodiments it is to be understood that numerous changes and modifications maybe made thereto without departing from the scope of the invention as defined in the appended claims.