Abstract:
The present invention provides a device and method for achieving protection in a free-space micromachined optical switching fabric. The present invention provides two protection schemes for mirror failures, which utilize either existing switching mirrors or additional integrated mirrors without extra fabrication effort or performance-degradation of the switch fabric. A switch fabric includes a plurality of micromachined free-rotating mirrors arranged in a rectilinear matrix configuration. The mirrors are reflective on both front and back sides. According to one embodiment of the present invention, upon the detection of a failed mirror associated with a transmission between an input and an output, a transmission path is determined that includes a backside of a pivot mirror. A transmission path is then established between the input and the output that utilizes the backside of the pivot mirror.

Description:
PRIOR PROVISIONAL PATENT APPLICATION 
     The present application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application No. 60/112,111 filed Dec. 14, 1998. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to optical networks. In particular, the present invention relates to a device and method for achieving protection in the case of mirror failure in free-space micromachined optical switches. 
     BACKGROUND INFORMATION 
     With the growing capacity demand for optical fiber communications, optical crossconnects (“OXCs”) with hundreds of ports are expected to become essential components of future optical transport networks within three to five years. This requirement in port count far outstrips the demonstrated capabilities of current deployable technology. Conventional mechanical switches suffer from large size, large element mass and slow switching time. On the other hand, guided-wave solid-state switches show limited expandability due to their high loss, high crosstalk and long device length. 
     Recently, free-space micromachined optical switching technology has been proposed as a means of building large optical crossconnects. This technology features the advantages of free-space interconnection, chiefly low loss and low crosstalk, while retaining the compactness and batch-fabrication economy of monolithic integration. Furthermore, the sub-millisecond switching times are well matched to the needs of OXCs in optical transport networks. 
     With the increasing capacity demand and complexity of optical networks, restoring network traffic promptly in the event of fiber failure becomes an important issue for network control and management. Optical crossconnects have been proposed as promising candidates for provisioning and restoration in optical networks at wavelength levels. In addition to protecting fiber failures, the optical crossconnect should also have protection schemes for itself so that its functionality will not be interrupted when one or more of its switching elements malfunctions. 
     Recent developments have focused on free-space micro-machined optical switches to achieve the optical performance and capacity requirement for optical crossconnects in multi-wavelength optical networks. The switch fabric includes micromachined free-rotating mirrors as switching elements. Backsides of the mirrors have also been used to achieve connection-symmetry in optical networks. 
     SUMMARY OF THE INVENTION 
     The present invention provides a device and method for achieving protection in a free-space micromachined optical switching fabric. The present invention provides two protection schemes for mirror failures, which utilize either existing switching mirrors or additional integrated mirrors without extra fabrication effort or performance-degradation of the switch fabric. A switch fabric includes a plurality of micromachined free-rotating mirrors arranged in a rectilinear matrix configuration. The mirrors are reflective on both front and back sides. According to one embodiment of the present invention, upon the detection of a failed mirror associated with a transmission between an input and an output, a transmission path is determined that includes a backside of a pivot mirror. A transmission path is then established between the input and the output that utilizes the backside of the pivot mirror. 
     According to one embodiment, the switching fabric utilizes M×N mirrors arranged in a rectilinear matrix configuration of M columns and N rows. Upon the detection of a mirror failure associated with a column Fc and a row Fr (mirror (Fc, Fr)), a pivot mirror is determined, wherein the pivot mirror is any actuated mirror having located in a column Pc and row Pr, wherein Pc&lt;Fc and Pr&lt;Fr. A first mirror at location (Fc,Pr) and a second mirror at location (Pc,Fr) are then actuated to perform the necessary routing. 
     According to an alternative embodiment, at least one protection row of switch mirrors and at least one protection column of switch mirrors is coupled to a rectilinear switch fabric. Upon detection of a mirror failure, a transmission path associated with the failed mirror is established by actuating mirrors in the protection row and protection column. In particular, according to one alternative embodiment, the present invention is applied to a rectilinear switch fabric of M×N micromirrors arranged in M columns and N rows. A series of input ports are provided along a first side of the switch fabric and a series of output ports are provided along a second side of the switch fabric perpendicular to the first side. According to the present invention, at least one protection row of mirrors is coupled between the input ports and the switch fabric and at least one protection column of mirrors is coupled between the output ports and the switch fabric, wherein the protection row includes a pivot mirror that is also co-located in the protection column. The pivot mirror is placed in a permanently actuated state. Upon detection of a mirror failure at location (Fc,Fr), a mirror in the protection row at location Fc is actuated and a mirror in the protection column at location Fr is actuated to form a transmission path from the input to the output via the pivot mirror. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a micromachined optical switch according to one embodiment of the present invention. 
     FIG. 2 depicts a switch mirror according to one embodiment of the present invention. 
     FIG. 3 depicts a block diagram of a mirror switching matrix according to one embodiment of the present invention. 
     FIG. 4 is a flowchart for a protection operation according to one embodiment of the present invention. 
     FIG. 5 depicts an exemplary protection operation in a micromachined mirror switching matrix according to one embodiment of the present invention. 
     FIG. 6 also depicts an exemplary protection operation utilizing a protection row and protection column according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 depicts a matrix free-space micromachined optical switch (“FS-MOS”) according to one embodiment of the present invention. FS-MOS  101  includes silicon (Si) substrate  105 , collimators  107 ( 1 , 0 )- 107 (M, 0 ) arranged on a first perimeter of silicon substrate  105  and collimators  107 ( 0 , 1 )- 107 ( 0 ,N) arranged on a second perimeter of silicon substrate  105 . Collimators  107 ( 1 , 0 )- 107 (M, 0 ) each respectively receive an input signal from an input port  180 ( 1 )- 180 (M) on FS-MOS  101 . Likewise, collimators  107 ( 0 , 1 )- 107 ( 0 ,N) each respectively provide an output signal to an output port  170 ( 1 )- 107 (N) on FS-MOS  101 . FS-MOS  101  further includes lenses  109 ( 1 , 0 )- 109 (M, 0 ), each arranged, respectively between a collimator  107  and switch matrix  131 . Likewise, FS-MOS  101  includes lenses  109 ( 0 , 1 )- 109 ( 0 ,N) arranged respectively between collimators  107 ( 0 , 1 )- 107 ( 0 ,N) and switch matrix  131 . Switch matrix  131  includes M×N switch mirrors arranged in M columns and N rows within switch matrix  131 . 
     For purposes of consistency, a switch mirror is referred to herein as (Mc,Mr), wherein Mc refers to the column where the mirror is located (beginning with the rightmost column with increasing column numbers to the left) and Mr refers to the row in which the switch mirror  120  is located (beginning with the row at the bottom of the switch matrix with increasing column numbers toward the top of switch matrix  131 ). Thus, for example, switch mirror ( 1 , 1 ) is located in the bottom right of switch matrix  131  and switch mirror (M,N) is located in the top left of switch matrix  131 . 
     Switch mirrors  120  are coupled to substrate  105  in a pivoting configuration described in more detail below. According to one embodiment, each mirror  120  in matrix  131  is associated with a transmission path between a single input  180  and single output  170 . Thus, for example mirror  120 ( 1 , 2 ) is associated with a transmission path from input  180 ( 1 ) to output  170 ( 2 ). The collimated light is switched to the desired output port  180  by rotating a selected mirror with microactuators as described in more detail below. 
     FIG. 1 also shows switch controller  190  coupled to switch matrix  131 , which controls the switching of switch mirrors  120  in switch matrix  131 . In particular, according to one embodiment, switch controller  190  is a processor, which executes a process to perform switching decisions for mirrors in switch matrix  131 . Switch controller  190  runs a process to detect mirror failures in switch matrix  131  and perform protection operations (described in more detail below). According to one embodiment of the present invention, switch controller  190  is coupled to actuators (described in detail below) at each switch mirror  120 . Switch controller  190  may cause a mirror  120  to actuate by transmitting a signal to the actuator, which causes the respective mirror to actuate. 
     FIG. 2 depicts a switch mirror according to one embodiment of the present invention. FIG. 2 shows reflecting mirror  203 , which includes reflecting surface  260 . Although not depicted in FIG. 2, each reflecting mirror  203  is reflective on both sides. Reflecting mirror  203  is coupled to translation plate  240  via pushrod  210  and hinge joint  220 . Switch mirror  120  also includes spring  230 . Translation plate  240  includes scratch drive actuator  250 . Reflecting mirror  203  is pivoted on substrate  105  via hinge joint  220 . Pushrod  210  couples switch mirror  203  with translation plate  240  through hinge joints  220  and converts plate translation into mirror rotation efficiently. Translation plate  240  is integrated with high-precision scratch drive actuators  250 . 
     Scratch drive actuators  250  are controlled by mirror actuation control unit  214 , based upon switching decisions determined by switch controller  190 . In particular, upon receiving a signal from switch control processor  190 , mirror actuation control unit  214  applies a bias voltage via scratch drive actuators  250 , which causes that particular mirror  120  to actuate. Conversely, to de-actuate a mirror  120 , mirror actuation control unit  260  couples scratch drive actuators  250  to ground. Translation plate  240  translation distance and therefore switch mirror  203  rotation angle is determined by the number of bias pulses applied to scratch drive actuator  250 . 
     FIG. 3 depicts a block diagram of a mirror switching matrix according to one embodiment of the present invention. Switch matrix  131  is coupled to M inputs and N outputs (via collimators  107  and lenses  109  not shown in FIG.  3 ). Furthermore, switch matrix  131  includes M×N microactuated switch mirrors  120  arranged respectively in M parallel columns and N parallel rows in switching matrix  131 . Thus, for example, mirror ( 3 , 5 ) is located in column  3  and row  5 . Note that columns are numbered from right to left and rows are numbered from bottom to top. These numbering conventions are merely illustrative and are not intended to limit the scope of the claims appended hereto. Each mirror located in column Mc and row Mr is associated with a transmission path between input Mc and output Mr. Thus, for example, if mirror ( 3 , 5 ) were actuated, it would establish a transmission path between input  3   180  and output  5   180 . Note that FIG. 3 does not depict the actuation status of any mirrors ( 1 , 1 )-(M,N). The actuation of a mirror within switching matrix  305  is accomplished as described above with respect to FIG.  2 . 
     During the lifetime of an optical switch, certain switching elements  120  may fail. According to a first embodiment of the present invention, rather than requiring replacement of an entire switch  101  in the event of a mirror failure, an alternate protection path is determined to connect the desired input to the desired output using the backside of a reflecting mirror  203  of an already actuated mirror  120 . In particular, upon the detection of a mirror failure that is associated with a transmission path between input M and output N, switch controller  190  determines a protection path connecting the input and output associated with the failed mirror  120  that utilizes the backside of a reflecting mirror  203  of an already actuated mirror  120  (referred to herein as a pivot mirror  120 ) in switching matrix  131 . Then, switch controller  190  actuates additional mirrors  120  to establish the transmission path from the desired input  180  to the backside of the pivot mirror  120  to the output  170 . 
     FIG. 4 is a flowchart for a protection operation according to one embodiment of the present invention. In step  405 , the process is initiated. In step  410 , it is determined whether a mirror failure has occurred in switching matrix  131 . If not (‘no’ branch of step  410 ), mirror failure is checked again (step  410 ). If a mirror failure is detected (‘yes’ branch of step  410 ), in step  420 , the location of the failed mirror are determined (Fc,Fr). Then in step  425  a pivot mirror is determined where the pivot mirror is a actuated mirror at location (Pc,Pr) where Pc&lt;Fc and Pr&lt;Fr. In steps  430  and  440  mirrors  120  at locations (Fc,Pr) and (Pc,Fr) are actuated (turned on). 
     FIG. 5 depicts an exemplary protection operation in a micromachined mirror switching matrix according to one embodiment of the present invention. FIG. 5 utilizes the definition of mirror (M,N) as the mirror at the intersection of input M and output N. FIG. 5 depicts an example of a failure of mirror ( 3 , 5 )  120 . It is assumed that prior to the failure of mirror ( 3 , 5 ), this mirror was actuated to connect signals from input  3  to output  5 . It is further assumed that mirror ( 1 , 2 )  120  was also actuated prior to failure of mirror ( 3 , 5 )  120  to provide a transmission path from input  1  to output  2 . Upon the detection of the failure of mirror ( 3 , 5 )  120  a transmission path is determined from input  3  using the backside of already actuated mirror ( 1 , 2 )  120  (pivot mirror) to mirror ( 1 , 5 )  120 . Then, mirrors ( 3 , 2 ) and ( 1 , 5 ) are actuated to establish the transmission path. 
     The approach depicted in FIGS. 3-4 has the limitation in the case in which mirror (Fc,Fr)  120  fails and all inputs  180  I where I&lt;Fc are connected to output J  170 , where J&gt;Fr. In this case, utilizing the backside of any switch mirror (Pc,Pr)  120 , where Pc&lt;Fc and Pr&lt;Fr will cause blocking of the optical path from input Pc  180 . 
     According to an alternative embodiment, which overcomes the above limitation, at least one additional row of mirrors  120  and at least one additional column of switch mirrors are integrated into the switch fabric  131 . FIG. 6 depicts the integration of a protection row and a protection column into a switch fabric according to one embodiment of the present invention. In particular, FIG. 6 shows protection row  610  and protection column  620  integrated with switch fabric  131 . Protection row  610  includes mirrors  120 ( 0 , 0 )- 120 (M, 0 ). Protection column  620  includes mirrors  120 ( 0 , 0 )- 120 ( 0 ,N). In particular, protection row  610  is integrated between inputs  180  and bottom row of switch fabric  131 . Protection column  620 , on the other hand, is integrated between the rightmost column of switch fabric  131  and outputs  170 . Note also that protection row  610  and protection column  620  share a single mirror  120 ( 0 , 0 ). This mirror  120 ( 0 , 0 ) is permanently actuated and forms a potential path between any mirror in protection row  610  and protection column  620 . 
     FIG. 6 also depicts an exemplary protection operation utilizing the protection row  610  and protection column  620 . In particular, it is assumed that mirror  120 ( 3 , 5 ) fails. It is further assumed that mirror  120 ( 1 , 2 ) is actuated to connect input  1   180  with output  2   170 . In this case mirror  120 ( 3 , 0 ) in protection row  610  and mirror ( 0 , 5 ) in protection column are actuated to connect input  3   180  with output  5   170 . In general, upon the failure of a mirror  120 (Fc,Fr), mirror (Fc, 0 ) in protection row  610  and mirror ( 0 ,Fr) in protection column  620  are actuated.