Abstract:
A free-space optical switch includes a micro-electro-mechanical system (MEMS) mirror array for switching an optical transmission path having an input MEMS mirror array and an output MEMS mirror array confronting an input fiber port array and an output fiber port array, respectively. The input MEMS mirror array and the output MEMS mirror array are integral with each other and divided from each other by at least two intersecting lines.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a free-space optical switch for switching an optical signal path in fields of optical communication, information processing, etc. 
     It is to be noted that in this specification, the term “micromirror” represents a micro-electro-mechanical system (MEMS) mirror. 
     2. Description of the Prior Art 
     FIG. 11 shows an arrangement of a free-space optical switch described at page 168 in a tutorial “Optical-Layer Networking: Opportunities for and Progress in Lightwave Micromachines” by L. Y. Lin et al. in proceedings of the 25th Optical Fiber Communication Conference (OFC 2000). In FIG. 11, a signal light ray incident from an input port array  11  is reflected at a desired angle by an input micromirror array  21  and is propagated to an output port array  12  through angular control of an output micromirror array. Namely, a signal light array  31  incident from a port  111  in the input port array  11  is initially reflected at a desired angle by a micromirror  211  in the input micromirror array  21  and is propagated to a desired port  121  in the output port array  12  through angular control of a micromirror  221  in the output micromirror array  22 . 
     Then, as a switching function of the optical switch, the output port  121  is switched to an output port  122 . To this end, the micromirror  211  of the input micromirror array  21  undergoes angular change so as to propagate to a micromirror  222  of the output micromirror array  22  the signal light ray  31  from the input port  111  and the micromirror  222  performs corresponding angular control of the signal light ray  31  so as to propagate the signal light ray  31  to the output port  122  such that the optical path is switched. In angular control of the micromirrors in switching of the optical path, when the optical path proceeds from the micromirror  211  at one diagonal end of one micromirror array  21  in the opposing micromirror arrays  21  and  22  to the micromirror  222  at the other diagonal end of the other micromirror array  22 , the micromirror  211  undergoes a maximum angular change. 
     Meanwhile, FIG. 12 shows an arrangement of a free-space optical switch described at page 167 of the above mentioned tutorial. In FIG. 12, a fixed mirror  4  is arranged and the input port array  11  and the output port array  12  of FIG. 11 are integrally formed into a port array  13 , while the input micromirror array  21  and the output micromirror array  22  of FIG. 11 are integrally formed into a micromirror array  23  such that a function similar to that of FIG. 11 is fulfilled. Also in the arrangement of FIG. 12, when the optical path proceeds from the micromirror  211  to a micromirror  223  which are, respectively, disposed at one diagonal end and the other diagonal end in the integral micromirror array  23 , the micromirror  211  undergoes a maximum angular change. 
     In the conventional optical switches of FIGS. 11 and 12, the micromirrors  211 ,  221 ,  222  and  223  should be subjected to angular control. However, the micromirror produced by micromachine technology has such a drawback that it is structurally difficult to cause a large angular change owing to difficulty in materializing a large scale in a vertical direction of the micromirror in comparison with that in a horizontal direction of the micromirror in thin film deposition technology, etching technology or the like. 
     Meanwhile, the micromirror is disadvantageous in that since it is difficult to produce a large driving force such as electrostatic force, magnetic field of the like in the vertical direction of the micromirror in terms of its driving principle, it is difficult to effect a large angular change. 
     SUMMARY OF THE INVENTION 
     Accordingly, an essential object of the present invention is to provide, with a view to eliminating the above mentioned drawbacks of prior art free-space optical switches, a free-space optical switch which is highly reliable by minimizing amount of angular change of an input micromirror or is more compact by minimizing an overall optical path length in case the amount of angular change of the input micromirror is identical on the contrary. 
     In order to accomplish this object of the present invention, a free-space optical switch according to the present invention comprises: a micro-electro-mechanical system (MEMS) mirror array for switching an optical transmission path, in which an input MEMS mirror array and an output MEMS mirror array confronting an input fiber port array and an output fiber port array, respectively are provided, wherein the input MEMS mirror array and the output MEMS mirror array are formed integrally with each other; wherein instead of equally dividing the MEMS mirror array into the input MEMS mirror array and the output MEMS mirror array simply by a single boundary line, one or both of the input MEMS mirror array and the output MEMS mirror array are further divided so as to be arranged. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This object and features of the present invention will become apparent from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings in which: 
     FIG. 1 is a front elevational view of a micromirror array employed in a free-space optical switch according to a first embodiment of the present invention; 
     FIG. 2 is a front elevational view of a micromirror array which is a first modification of the micromirror array of FIG. 1; 
     FIG. 3 is a front elevational view of a micromirror array which is a second modification of the micromirror array of FIG. 1; 
     FIG. 4 is a front elevational view of a micromirror array employed in a free-space optical switch according to a second embodiment of the present invention; 
     FIG. 5 is a side elevational view of a free-space optical switch according to a third embodiment of the present invention; 
     FIG. 6 is a view of a micromirror array of the free-space optical switch of FIG. 5 as observed in the direction of the arrow VI in FIG. 5; 
     FIG. 7 is a side elevational view of a free-space optical switch which is a modification of the free-space optical switch of FIG. 5; 
     FIG. 8 is a schematic top plan view of the free-space optical switch of FIG. 7; 
     FIG. 9 is a perspective view showing a micromirror employed in a free-space optical switch according to a fourth embodiment of the present invention; 
     FIG. 10 is a perspective view showing a micromirror acting as a comparative example of the micromirror of FIG. 9; 
     FIG. 11 is a perspective view showing an arrangement of a prior art free-space optical switch; and 
     FIG. 12 is a perspective view showing an arrangement of another prior art free-space optical switch. 
    
    
     Before the description of the present invention proceeds, it is to be noted that like parts are designated by like reference numerals throughout several views of the accompanying drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments of the present invention are described with reference to the drawings. 
     (First Embodiment) 
     FIG. 1 shows an N-row×M-column MEMS mirror array, i.e., micromirror array  40  employed in a free-space optical switch according to a first embodiment of the present invention. This optical switch has an arrangement similar to that of the prior art free-space optical switch of FIG.  12 . N×M/2 ports constitute an input micromirror array corresponding to input ports, while the remaining N×M/2 ports constitute an output micromirror array corresponding to output ports. Since each port can be regarded as a point of coordinates, each port is handled as a point below for simplification of the following description. 
     A signal light ray reflected by an input micromirror proceeds to an output micromirror via a fixed mirror (not shown). If the input micromirror and the output micromirror are disposed diagonally in the micromirror array  40 , the input micromirror needs to carry out a maximum angular change. Namely, a maximum angular displacement of the input micromirror corresponds to a case in which a distance between the micromirrors on the micromirror array  40  assumes a maximum value. Therefore, in order to minimize the maximum angular displacement of the input micromirror, comparative study for minimizing the distance between the micromirrors on the micromirror array  40  may be made. 
     In FIG. 1, if the micromirror array  40  is divided into regions for a group of input micromirrors and a group of output micromirrors most instinctively, the micromirror array  40  will be divided into halves by a vertical centerline  51  or a lateral centerline  52  such that the group of the input micromirrors are arranged in one of the halves and the group of the output micromirrors are arranged in the other of the halves. In the optical switch, an arbitrary input port should be switched to an arbitrary output port. Thus, when a signal light ray is propagated from an input micromirror  231  disposed at one diagonal end of the micromirror array  40  to an output port  232  disposed at the other diagonal end of the micromirror array  40 , angular displacement of the input micromirror  231  reaches a maximum value. Supposing that characters “a” and “b” denote a vertical length and a lateral length of the N-row×M-column micromirror array  40 , respectively, a distance between the input micromirror  231  and the output micromirror  232  is expressed as follows. 
     
       
         {square root over (a 2 +b 2 )}  (1)  
       
     
     In this embodiment, by optimizing division of the micromirror array  40  into the regions for the group of the input micromirrors and the group of the output micromirrors, a maximum value of necessary angular displacement of the input micrometer is lessened. Namely, the micromirror array  40  is equally divided into four regions A, B, C and D by the vertical centerline  51  and the lateral centerline  52  of FIG. 1 such that the group of the input micromirrors are provided in the regions A and D and the group of the output micromirrors are provided in the regions B and C. 
     As a result, a path leading to a maximum angular displacement traces from the input micromirror  231  in the region A to an output micromirror  233  in the region B or an output micromirror  234  in the region C. At this time, a distance between the input micromirror  231  and the output micromirror  233  or  234  is given as follows.                      a   2     4     +     b   2                       Or           (   2   )                   a   2     +       b   2     4               (   3   )                                
     Thus, the above distance (2) or (3) between the input micromirror  231  and the output micromirror  233  or  234  can be made smaller than the conventional distance (1) between the input micromirror  231  and the output micromirror  232 . Therefore, by lessening a maximum angular displacement of the input micromirror  231 , reliability of the optical switch can be upgraded. Alternatively, if amount of angular change of the input micromirror  231  is identical, an optical system of the optical switch can be made compact by minimizing its overall optical path length. 
     Meanwhile, if the vertical centerline  51  and the lateral centerline  52  in FIG. 1 are rotated through, for example, an arbitrary angle θ about a point P of intersection between the vertical centerline  51  and the lateral centerline  52  by maintaining the identical area of the regions A, B, C and D as shown in FIG. 2 so as to divide the micromirror array  40  equally, the distance between the input micromirror and the output micromirror can be made smaller than the above conventional distance (1). Namely, a path leading to a maximum angular displacement traces from the input micromirror  231  in the region A to an output micromirror  235  in the region B or an output micromirror  236  in the region C. At this time, a distance between the input micromirror  231  and the output micromirror  235  or  236  is given as follows.                      (       a   2     +         b   2     ·   tan                   θ       )     2     +     b   2                       Or           (   4   )                   a   2     +       (       b   2     -         a   2     ·   tan                   θ       )     2               (   5   )                                
     Thus, the above distance (4) or (5) between the input micromirror  231  and the output micromirror  235  or  236  can be made smaller than the conventional distance (1) between the input micromirror  231  and the output micromirror  232 . Therefore, by lessening a maximum angular displacement of the input micromirror  231 , reliability of the optical switch can be upgraded. Alternatively, if amount of angular change of the input micromirror  231  is identical, an optical system of the optical switch can be made compact by minimizing its overall optical path length. However, when the rotational angle θ assumes arctan(a/b), the distance (4) or (5) is not smaller than the distance (1). Hence, in order to make the distance (4) or (5) smaller than the distance (1), a condition (0≦θarctan(a/b)) should be satisfied. 
     In this embodiment, region boundary lines are formed by the straight lines. However, even if region boundary lines  53  and  54  have arbitrary shapes as shown in FIG. 3, the same effect can be gained as long as the identical area of the areas A, B, C and D is maintained. 
     Meanwhile, each input micromirror and each micromirror on the micromirror array  40  corresponds, by one-to-one, to each input port or each output port and initial inclination is set such that a signal light ray reflected by each micromirror is reflected by the fixed mirror orthogonally to an optical axis extending from each input port or each output port to each micromirror and returns on its original path. 
     (Second embodiment) 
     FIG. 4 shows a micromirror array  41  employed in a free-space optical switch according to a second embodiment of the present invention. The micromirror array  41  is divided into two regions A and B by a rectangular boundary line  55  in place of the boundary lines  51  and  52  and a group of input micromirrors and a group of output micromirrors are, respectively, provided in the regions A and B such that a necessary maximum angular displacement of the input micromirror is lessened. Namely, supposing that characters “c” and “d” in FIG. 4 denote a vertical length and a lateral length of the rectangular region B bounded by the boundary line  55 , respectively, the vertical length c and the lateral length d should satisfy the following conditions (6) and (7). 
     
       
           a·b= 2 c·d    (6)  
       
     
     
       
         c≦a, d≦b   (7)  
       
     
     If the group of the input micromirrors and the group of the output micromirrors are, respectively, provided in the regions A and B or vice versa, a path leading to a maximum angular displacement traces from the input micromirror  221  in the region A to an output micromirror  237  in the region B. At this time, a distance between the input micromirror  231  and the output micromirror  237  is given as follows.                    (       a   2     +     c   2       )     2     +       (       b   2     +     d   2       )     2               (   8   )                                
     Thus, the above distance (8) between the input micromirror  231  and the output micromirror  237  can be made smaller than then conventional distance (1) between the input micromirror  231  and the output micromirror  232 . Therefore, by lessening a maximum angular displacement of the input micromirror  231 , reliability of the optical switch can be upgraded. Alternatively, if amount of angular change of the input micromirror  231  is identical, an optical system of the optical switch can be made compact by minimizing its overall optical path length. 
     Meanwhile, in this embodiment, the region B is formed into a rectangular shape and the above equation (6) corresponds to the rectangular region B. However, if the regions A and B have an identical area, the region B may have an arbitrary shape. 
     (Third embodiment) 
     FIGS. 5 and 6 show a micromirror array  24  employed in a free-space optical switch according to a third embodiment of the present invention. As shown in FIG. 6, the micromirror array  24  has a width q. A group  25  of input micromirrors and a group  26  of output micromirrors on the micromirror array  24  do not have an identical reference angle and are angularly arranged so as to confront each other towards a fixed mirror  4  about an axis orthogonal to a drawing sheet of FIG. 5 such that a necessary maximum angular displacement of the input micromirror is lessened. 
     Namely, by inclining the group  25  of the input micromirrors, the group  26  of the output micromirrors and the fixed mirror  4  at angles θ1, θ2 and θ3 relative to a propagation optical axis, respectively, each input port forms a propagation path to each corresponding output port in reference angle state of each micromirror. Here, the following condition (9) should be satisfied. 
     
       
         π+2θ3=2(θ1+θ2)   (9)  
       
     
     By the above described arrangement, a distance between the input micromirror and the output micromirror, which corresponds to a path leading to a maximum angular displacement, is given as follows.                    (     p   2     )     2     +     q   2               (   10   )                                
     On the other hand, in case micromirrors of reference angle are arranged such that an input signal light ray  32  is returned to the input micromirror by the fixed mirror  4 , a distance between the input micromirror and the output micromirror is given as follows. 
     
       
         {square root over (p 2 +q 2 )}  (11)  
       
     
     Thus, the distance (10) between the input micromirror and the output micromirror can be made smaller than the distance (11) between the input micromirror and the output micromirror. Therefore, by lessening a maximum angular displacement of the input micromirror, reliability of the optical switch can be upgraded. Alternatively, if amount of angular change of the input micromirror is identical, an optical system of the optical switch can be made compact by minimizing its overall optical path length. Meanwhile, in FIG. 5, an input signal light ray  33  is illustrated in addition to the input signal light ray  32 . 
     As shown in FIGS. 7 and 8, the group  25  of the input micromirrors and the group  26  of the output micromirrors on the micromirror array  24  may be angularly arranged so as to confront each other towards the fixed mirror  4  about an axis parallel to a drawing sheet of FIG. 7 such that a necessary maximum angular displacement of the input micromirror is lessened. In the micromirror array  24  of FIG. 7, a ridgeline along which the group  25  of the input micromirrors and the group of the output micromirrors intersect with each other is inclined at an angle θ1 relative to a propagation optical axis. 
     By the above described arrangement, a distance between the input micromirror and the output micromirror, which corresponds to a path leading to a maximum angular displacement, is given as follows.                  p   2     +       (     q   2     )     2               (   12   )                                
     Thus, the above distance (12) between the input micromirror and the output micromirror can be made smaller than the distance (11) in which the micromirrors of reference angle are arranged such that the input signal light ray  32  is returned to the input micromirror by the fixed mirror  4 . Therefore, by lessening a maximum angular displacement of the input micromirror, reliability of the optical switch can be upgraded. Alternatively, if amount of angular change of the input micromirror is identical, an optical system of the optical switch can be made compact by minimizing its overall optical path length. 
     In this embodiment, each of planes of the group  25  of the input micromirrors and the group  26  of the output micromirrors is arranged so as to be inclined as a whole. However, if position of reference angle of each micromirror can be set independently of the planes of the group  25  of the input micromirrors and the group  26  of the output micromirrors and operating angle of each micromirror can be set arbitrarily within a range of a relative value from its reference position, the same effect can be achieved by setting an input micromirror  25   n  and an output micromirror  26   n  to predetermined angles similar to those of this embodiment. 
     (Fourth embodiment) 
     FIG. 9 shows an input micromirror or an output micromirror  10  employed in a free-space optical switch according to a fourth embodiment of the present invention. The input micromirror  10  includes an optically effective micromirror portion  6  for reflecting a signal light ray  31  and an optically ineffective portion  7  forming a structure. Since the signal light ray  31  passes through an axially symmetric collimator optical system from an optical fiber, the signal light ray  31  has a shape  8  symmetrical with respect to a propagation optical axis. Therefore, in order to minimize eclipse loss at the micromirror portion  6 , the micromirror portion  6  may be formed into an elliptic shape in which a dimension of an inclined major axis and a dimension of a mirror axis free from inclination are set at a ratio of (1/sinθ1) to 1 in accordance with an angle θ1 of inclination of the input micrometer  10 . As a result, transmission loss can be reduced. 
     If a nonelliptic micromirror portion, for example, a circular micromirror portion  61  is provided in an input micromirror  15  as shown in FIG. 10, a hatching portion in FIG. 10 suffers eclipse loss. 
     Even if the elliptic shape of the micromirror portion  6  in FIG. 9 is replaced by a shape including a projected image of the shape  8  of the signal light ray  31 , for example, a rectangular shape in which a dimension of a short side and a dimension of a long side are set at a ratio of 1 to (1/sinθ1), the same effect can be obtained. 
     As is clear from the foregoing description, the following marked effects (1) to (6) can be gained in the free-space optical switch of the present invention. 
     (1) Since the input MEMS mirror array and the output MEMS mirror array are formed integrally with each other and instead of equally dividing the MEMS mirror array into the input MEMS mirror array and the output MEMS mirror array simply by signal boundary line, one or both of the input MEMS mirror array and the output MEMS mirror array are further divided so as to be arranged, the maximum angular displacement of the input micromirror can be lessened. Alternatively, the optical system of the optical switch can be made compact. 
     (2) Since the input MEMS mirror array and the output MEMS mirror array are formed integrally with each other and the input MEMS mirror array and the output MEMS mirror array are arranged such that one of the input MEMS mirror array and the output MEMS mirror array surrounds the other of the input MEMS mirror array and the output MEMS mirror array, the maximum angular displacement of the input micromirror can be lessened. Alternatively, the optical system of the optical switch can be made compact. 
     (3) Since the MEMS mirror array is separated into the input MEMS mirror array and the output MEMS mirror array and the input MEMS mirror array and the output MEMS mirror array are angularly arranged so as to confront each other towards the fixed reflector, the maximum angular displacement of the input micromirror can be lessened. Alternatively, the optical system of the optical switch can be made compact. 
     (4) Since the input MEMS mirror array, the output MEMS mirror array and the fixed reflector are, respectively, inclined at the angles θ1, θ2 and θ3 relative to the propagation optical axis such that the angles θ1, θ2 and θ3 have the relation of {π+2θ3=2(θ1+θ2)}, the maximum angular displacement of the input micromirror can be lessened. Alternatively, the optical system of the optical switch can be made compact. 
     (5) Since each MEMS mirror of the input MEMS mirror array and the output MEMS mirror array is inclined at the predetermined angle relative to the propagation optical axis and is formed long in the direction of inclination of each MEMS mirror in accordance with the predetermined angle such that the shape of each MEMS mirror is symmetrical with respect to the propagation optical axis when projected towards a corresponding one of the input fiber ports and the output fiber ports, eclipse loss can be minimized and thus, transmission loss can be reduced. 
     (6) Since the predetermined angle is θ1 and the dimension of each MEMS mirror in the direction of inclination of each MEMS mirror is set to be a product of (1/sinθ1) and the dimension of each MEMS mirror in the direction other than the direction of inclination of each MEMS mirror, eclipse loss can be minimized and thus, transmission loss can be reduced.