Abstract:
A fiber optic switch using MEMS is scalable by the use of a matrix of cross-points located at the intersection of all possible input and output light paths. Cross-points are formed by a MEMS procedure where a digitally movable mirror intersects a light path to provide a digital switching action with the remaining cross-point mirrors being moved out of position to provide through transmission.

Description:
INTRODUCTION 
     The present invention is directed to a fiber optic switch using MEMS (micro electro mechanical systems), and more specifically to a scalable n×n switch. 
     BACKGROUND OF THE INVENTION 
     The great demand for data-centric services brought on by the explosive growth of Internet has led service providers to dramatically increase their capacity. All optical networks (AON) utilizing wave division multiplexing (WDM) is expected to satisfy the bandwidth requirements. As more networking is required at the optical layer, an all photonic switch is emerging as an enabling technology. While most switching in communication systems today is accomplished electronically, emerging AON will require switches to route signals purely in the optical realm to achieve higher bit rates. These network applications require switching matrices from 8×8 to 1024×1024. 
     Conventional optomechanical switches are mostly available in 1×2 and 2×2 configurations and rely on mature optical technologies. Large scale matrix switches are difficult to realize because of their complexity, size, and the number of moving mechanical parts requiring assembly. 
     Other attempts at silicon MEM switches are based upon torsional or hinged mirrors which are limited in angular excursion; and require angular sensors for feedback servo control to slew the mirrors into required angular positions. The difficulty in precise angular control and servo mechanisms which limits switching speed have prevented these analog techniques from realizing useful optical switching. 
     OBJECT AND SUMMARY OF THE INVENTION 
     It is a general object of the present invention to provide a fiber optic switch which is scalable. 
     In accordance with the above object, there is provided an optical matrix switch having a plurality of cross-points for switching a plurality of information carrying light beams between any one of a plurality of input beams to any one of a plurality of output beams by choosing the appropriate cross-point of the matrix. Each cross-point is a micro electromechanical (MEM) type mirror having a first position where the mirror reflects the selected input beam to provide a selected one output beam and a second position where it provides a through path for transmission of said light beam. Means are provided for actuating a selected cross-point mirror to a first position to reflect the input beam to the output beam and for causing the remaining mirrors in the path of such beam to remain in the second position to allow through transmission. 
     In addition, a method is also provided of switching a selected one of a plurality of input optical signal paths to a selected one of a plurality of output optical signal paths comprising the steps of providing a matrix of optical mirrors at all cross-points of the input and output optical paths, selectively and digitally moving a mirror into an optical path to allow a selected input optical path to be reflected to a selected output optical path, and allowing the remaining mirrors in the optical path to provide through transmission. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view in simplified form of a fiber optic switch embodying the present invention. 
     FIG. 2 is a partial cross-section of a portion of FIG.  1 . 
     FIG. 3 is a plan view of a portion of FIG. 1 demonstrating its operation. 
     FIGS. 4A,  4 B and  4 C is an alternative embodiment of FIG.  3 . 
     FIG. 5A is a perspective view illustrating a solenoid type portion of FIGS. 4A,  4 B and  4 C. 
     FIG. 5B is a diagrammative view of a portion of FIG.  5 A. 
     FIG. 6 is a perspective view illustrating one embodiment for mounting the portions shown in either FIG. 3 or FIGS. 4A,  4 B, and  4 C. 
     FIGS. 7A through 7I illustrate an alternative process for forming a portion of FIG.  1  and mounting it. 
     FIG. 8 is a perspective view of a lens used in FIG.  1 . 
     FIG. 9 is a simplified side view if an alternative lens usable in FIG.  1 . 
     FIG. 10 is a perspective view of an optical platform. 
     FIG. 11 is a perspective view of another optical platform. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a simplified optical matrix switch constructed in accordance with the present invention which is based on micro electromechanical systems. (MEMS). The device includes a silicon or other semiconductor substrate  10  on which is placed a matrix of cross-points  11 . These are located at all possible cross-points or intersections of a plurality of input information carrying light beams  12   a - 12   d  (many more are possible, of course) and a plurality of output beams  13   a - 13   d . Typically, these would be of fiber optic lines. From a switching standpoint, this is similar to a mechanical cross-bar switch where it is desired to allow any one selected input  12   a - 12   d  to be connected to any one selected output  13   a - 13   d . Thus, the cross-points  11  located at every possible switching junction are in the form of mirrors having a first position where the mirror reflects a selected input beam to provide a selected output beam. The mirrors are plasma etched on a mono-crystalline silicon wafer which has been polished to optical flatness. 
     Thus, in the case of the input  12   a , a beam  14  is reflected by a mirror  16  at a right angle and continues on path to the output  13   b . Similarly, the input  12   b  is reflected at the mirror cross-point  17 , its light path  18  is redirected to the output  13   d . The cross-points  11  provide that the light beams  14  and  18  have clear transmission through all the intervening cross-points  11 . As illustrated in FIG. 3, a typical cross-point  11  in one embodiment includes, for example, a movable mirror  16  on a substrate  21  to move in the direction shown by arrow  22  to selectively cover the aperture  23  which, for example, might be in the beam path  14 . This mirror is digitally movable between first position to reflect the input beam to the desired output beam path and in a second position where it allows through transmission. Thus, the mirror  16  serves as a type of shutter. Shutter  16  is movable in a single plane, along the surface of the semiconductive base to provide a very stable reflecting surface to provide an exact desired 90° angle of reflection as illustrated in the preferred embodiment. The mirror is movable as illustrated in FIG. 3 by a comb-type actuator  26 , which is formed on the semiconductive substrate  21  along with the mirror  16 . This comb-type actuator is formed by MEMS technology. Its construction is illustrated in a co-pending application Ser. No. 09/299,472 filed Apr. 26, 1999, entitled Method of Fabricating Angular Rate Sensor From A Structural Wafer Of Single Crystal Silicon and assigned to the present assignee. Also there are several technical articles relating to such MEMS technology. With the comb-type actuator drive an effective spring suspension  27  is provided. 
     Another type of drive actuation also illustrating the movement of the mirror  16  over the aperture  23  is in FIGS. 4A,  4 B, and  4 C. Here an effective solenoid MEMS type device  28  is provided. In FIG. 4A the light beam  14  is transmitted through the aperture  23 . FIG. 4B shows mirror  16  in a partly operative condition and then FIG. 4C shows light beam  14  being reflected as is illustrated in FIG.  1 . The structure for a solenoid type operation  28  is illustrated in FIG.  5 A. Using a MEMS type construction on two silicon components, there is a lower component  31  and an upper component  32  with a U-shaped opening  33  which contains moving core of silicon  34 . On top of core  34 , designated  36 , is a Permalloy (trademark) coating or other suitable magnetic material. This interacts with the electroplated coil  37  to provide a solenoid type movement. Coil  37  includes (see FIG. 5B) an upper set of conductive traces  37   a  connected by vias  38  to lower traces  37   b  to form an effective coil for the solenoid. 
     To mount mirror or shutter  11  on a substrate  10  (see FIG.  1 ), FIG. 6 illustrates one technique where the cross-point  11  is erected and supported by a pair of side supports  41   a  and  41   b  which maintains the mirror or shutter assembly  11  in the desired fixed vertical position. Another technique is shown in FIGS. 7A through 7I where as illustrated in FIG. 7H the mirror assembly  11  is placed in the indicated vertical slot  42 . FIGS. 7A through 7I show the MEMS type construction process where in FIG. 7A the initial shutter and mirror assembly is formed as shown by the top view of FIG.  7 C and this is mated to a bottom portion illustrated in FIG.  7 B. In FIG.  7 D and also illustrated in FIG. 7E, the mirror portion is broken out from the substrate  10  to form the final vertical mirror section  11  and in FIG. 7F as shown at  46  the beginning of the vertical slot is formed with a riser  47 . FIG. 7G is a top view. Finally in FIGS. 7H and 7I the cross-point shutter mirror assembly  11  is moved into the vertical slot  42 . 
     To form a more effective switching unit, it is useful for the light beams (which of course are highly collimated) to be focused. Thus as illustrated in FIG. 8, a Fresnel lens that is integrally fabricated in the silicon may be provided at the end of the fiber optic transmission line, or in FIG. 9 a glass ball type or GRIN (gradient index) lens  52  are placed in precisely registered grooves ( 51   a  and  52   a , FIGS. 10 and 11) on the optical platform. Such platform includes V-shaped slots. The ball and GRIN lenses are separate elements that would require a pick, place, and secure process during assembly. 
     Finally to position the input and output light beams, for example  12   a - 12   d  so that the critical aperture and mirror portion of a cross-point  11  is intersected, the semiconductive substrate  10  as illustrated in FIG. 2 includes a type of raised border or scaffold  52  on which are mounted the inputs and outputs as illustrated. As illustrated in FIG. 2 the end of the fiber optic light beam path of  12   a , with a Fresnel lens has been mounted. 
     In summary, the present invention is an improvement over prior switches because: 
     1. It is digital rather than analog; 
     2. It eliminates having to slew mirrors to different angles; and 
     3. It achieves higher speed switching while maintaining accurate reflective angles. 
     The foregoing is accomplished in the present invention by: 
     1. Fixing the mirrors in an n×n matrix at precise stationary planes at precise angles; and 
     2. The mirrors are only moved in the plane of the fixed angle in digital on-off fashion.