PROTECTION FOR MEMS CROSS-BAR SWITCH

Abstract: A protection system for a micro electro-mechanical system (MEMS) cross-bar switch (200) is described. Optical signals that are cross-connected by a NxN optical matrix switch are routed through alternative protection paths using protection switch elements (210, 220, 230). The protection switch elements (210, 220, 230) are incorporated as part of the silicon wafer based fabricated structure that forms the NxN optical matrix. The protection switch elements (210, 220, 230) enable the NxN optical matrix switch to recover from one or more failures in switch elements of the NxN optical matrix using alternative protection paths that have the same path length as the original optical path.

Description: PROTECTION FOR MEMS CROSS-BAR SWTCH BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to optical matrix switches, and more specifically, to protection mechanisms for micro electro-mechanical system (MEMS) cross-bar switches. Discussion of the Related Art Fiber optic technology has continued to expand across today's data communication networks. Having replaced many of the long-haul connections and other inter-office facilities, fiber optics technology has begun to replace transmission facilities and network elements used in intra-office communication. One of the primary network elements used in intra-office communication is the digital cross-connect. Generally, digital cross-connects link any of several incoming lines to any of several outgoing lines. Today's digital cross-connects switch digital signals on the electrical level. Thus, a fiber optic terminal that receives an optical signal must convert the optical signal to an electrical signal before it sends it to the digital cross-connect. Optical cross-connects are envisioned as the replacement for the conventional digital cross-connect. Optical cross-connects switch signals at the optical level and therefore obviate the need for optical-to-electrical conversions. The elimination of unnecessary components can lower the overall cost of the network while also increasing the reliability of the network. Reliability is a paramount concern to network planners and bandwidth providers. For optical cross-connects to be considered as viable replacements for digital cross-connects, the reliability of the optical cross-connects must meet reasonable reliability expectations. SUMMARY OF THE INVENTION The present invention addresses reliability concerns of micro electro- mechanical system (MEMS) cross-bar switches by providing alternative protection paths for optical signals that are cross-connected by a N x N MEMS cross-bar switch. The N x N optical cross-bar switch receives, on a first side, input optical signals from an array of one or more input lines (i), and outputs, on a second side, output optical signals to an array of one or more output lines (j). Because of the physical distance the light must propagate between the input and output lines, the input lines are typically provided with lenses to convert the signal on an input fiber into a collimated beam, and the output lines are similarly provided with lenses to focus the output collimated beam onto the output fiber. The design of such collimating and focusing lenses is critically dependent on the required optical path length between the two lenses. Cross connection within the N x N switch matrix is effected through the activation of selected switch elements. For example, input line (i) can be connected to output line (j) by activating switch element (i,j) in the N x N cross-bar switch. In addition to the N x N matrix of switch elements, the MEMS cross-bar switch of the present invention includes a N x M matrix of protection switch elements, a M x N matrix of protection switch elements, and a set of at least M protection switch elements. The N x M matrix of protection switch elements is positioned between the array of input lines (i) and the first side of the N x N optical cross-bar switch such that the N rows of the N x M matrix of protection switch elements are aligned with the N rows of the N x N optical cross-bar switch. The M x N matrix of protection switch elements is positioned between the second side of the N x N optical cross-bar switch and the array of one or more output lines (j) such that the N columns of the M x N matrix of protection switch elements are aligned with the N columns of the N x N optical cross-bar switch. Finally, each of the M protection switch elements in the set of at least M protection switch elements is aligned with one of the M columns of the N x M matrix of protection switch elements and one of the M rows of the M x N matrix of protection switch elements. This configuration of protection switch elements in combination with the N x N optical cross-bar switch enables a portion of an original path of an optical signal from an input line (i) that is connected to an output line (j) via switch element (i,j) in the N x N optical cross-bar switch to be switched to an alternative protection path that traverses a path between a first protection switch element in row (i) of the N x M matrix of protection switch elements, a second protection switch element in column (j) of the M x N matrix of protection switch elements, and a protection switch element in the set of at least M protection switch elements that is aligned with the first and second protection switch elements. It is a feature of the present invention that the path length of the alternative protection path is the same as the original path of the optical signal. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention that together with the description serve to explain the principles of the invention. In the drawings: Fig. 1 illustrates a micro electro-mechanical systems (MEMS) cross-bar switch; and Fig. 2 illustrates a protection mechanism for a MEMS cross-bar switch. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. The field of micro electro-mechanical systems (MEMS) is a revolutionary, enabling technology. Using an ever-expanding set of fabrication processes and materials, MEMS will provide the advantages of small size, low power, low mass, low-cost and high-functionality to integrated electro-mechanical systems. One application of MEMS technology is to opto-electro-mechanical devices such as fiber-optic cross-bar switches. Fiber-optic cross-bar switches are envisioned as playing a key role in the evolution of data communication networks. Today, electrical digital signal cross-connects are used to link a set of incoming lines to a set of outgoing lines. This type of electrical cross-connection requires additional optical-to-electrical and electrical-to-optical conversion components between a set of network elements. Fiberoptic cross-bar switches would obviate the need for these conversion elements. An example of a MEMS cross-bar switch is illustrated in Fig. 1. MEMS cross-bar switch 110 is a silicon wafer based fabricated structure that includes a 4 x 4 matrix of switch elements. Each of the switch elements represents a mirror that is operatively moved into the center of the optical path junction at a 45 angle. When the mirror is moved into the optical path, the switch operates in the "reflection" mode; and when the mirror is moved out of the optical path, the switch operates in the "through" mode. Various micro electro-mechanical structures can be used to effect a movement of the mirror into and out of the optical path. In one embodiment, the mirror is pushed into or pulled out of the optical path. In another embodiment, the mirror is moved into and out of the optical path through a pivot mechanism. The movement of the mirror into the optical path is referred to below as the activation of the switch element. As illustrated in Fig. 1 , the switch elements serve to connect input optical lines 102A-102D to output optical lines 104A-104D. Because of the physical distance the light must propagate between the input and output lines, the input lines are typically provided with lenses (not shown) to convert the signal on an input fiber into a collimated beam, and the output lines are similarly provided with lenses (not shown) to focus the output collimated beam onto the output fiber. The design of such collimating and focusing lenses is critically dependent on the required optical path length between the two lenses. In the MEMS cross-bar switch 110 illustrated in Fig. 1, input line 102A is connected to output line 104B, input line 102B is connected to output line 104C, input line 102C is connected to output line 104D, and input line 102D is connected to output line 104A. These specific connections are enabled through the activation of switch element (1,2), switch element (2,3), switch element (3,4), and switch element (4,1), respectively, where switch element (i,j) refers to the switch element in row i, column j of the 4 x 4 MEMS cross-bar switch 110. One of the key concerns regarding the adoption and use of MEMS cross-bar switches is the reliability of the switching mechanisms. Once a mirror is stuck in either the activated or inactivated position, the entire MEMS cross-bar switch must be replaced because there is no way to do an in-service repair of the defective actuator that controls the faulty switch element. The effect of a faulty switch element is dependent upon whether the switch element is stuck in either the activated or inactivated position. If the switch element is stuck in the inactivated position (i.e., the mirror is stuck outside the optical path), only one connection will be affected. For example, if switch element (1,2) was stuck in an inactivated position, input line 102A would not be able to be connected to output line 104B. No other connection would be affected. On the other hand, if a switch element is stuck in the activated position (i.e., mirror is stuck in the optical path), two connections could potentially be disrupted. Fig. 1 illustrates a potential fault in switch element (3,3). If this switch element is stuck in an activated position, the connection between input line 102B and output line 104C and the connection between input line 102C and output line 104D would both be disrupted. As noted, a switch element cannot be repaired in service. Accordingly, MEMS cross-bar switch 110 would have to be replaced in its entirety should switch element (3,3) fail. The replacement of MEMS cross-bar switch 110 would cause a significant disruption in the data communications network. Upon replacement of MEMS cross-bar switch 110, all optical signals that traverse MEMS cross-bar switch 110 would either have to be transferred to an external protection path or taken off line, thereby leaving a substantial part of the network either unprotected or in a failed state. The present invention addresses the potential reliability issues of MEMS cross-bar switches by incorporating internal protection paths within the MEMS cross-bar switch. In other words, the protection paths are included as part of the silicon wafer based fabricated structure that forms the MEMS cross-bar switch. An example of a MEMS cross-bar switch that incorporates a protection structure is illustrated in Fig. 2. MEMS cross-bar switch 200 includes the 4 x 4 matrix of switch elements that formed MEMS cross-bar switch 110. Additionally, MEMS cross-bar switch 200 includes protection switch element sections 210, 220, and 230. In this particular example, protection switch element section 210 is a 2 x 4 switch matrix, protection switch element section 220 is a 2 x 2 switch matrix, and protection switch element section 230 is a 2 x 4 switch matrix. As described below, protection switch element sections 210, 220, and 230, in combination, enable MEMS cross-bar switch 200 to recover from failures in switch elements of the original 4 x 4 matrix of switch elements 110. To demonstrate the protection feature of MEMS cross-bar switch 200, Fig. 2 focuses on the connection between input line 102B and output line 104C and the connection between input line 102C and output line 104D. As noted, both of these connections would be disrupted if switch element (3,3) in matrix switch section 110 became stuck in an activated position. In this failure scenario, switch elements in protection switch element sections 210, 220, and 230 can be selectively activated to re-route the failed connections. This re- routing is handled internally by MEMS cross-bar switch 200. Accordingly, one or more failures in matrix switch section 110 would not require the replacement of MEMS crossbar switch 200. As illustrated, a portion of the original path of the connection between input line 102B and output line 104C is re-routed using switch element (2,2) in protection switch element section 210, switch element (1,2) in protection switch element section 220, and switch element (1,3) in protection switch element section 230; and a portion of the original path of the connection between input line 102C and output line 104D is re-routed using switch element (3,1) in protection switch element section 210, switch element (2,1) in protection switch element section 220, and switch element (2,4) in protection switch element section 230. Significantly, each of the protection paths that are set up to replace a portion of the failed connection does not change the total path length of the connection between the input and output lines. For example, the path length between switch element (2,2) in protection switch element section 210, switch element (1,2) in protection switch element section 220, and switch element (1,3) in protection switch element section 230 would be equivalent to the path length between switch element (2,2) in protection switch element section 210, switch element (2,3) in matrix switch section, and switch element (1,3) in protection switch element section 230. This feature of the present invention eliminates the need for compensating lenses in the protection path. More importantly, the inclusion of a protection mechanism within the MEMS cross-bar switch itself reduces the likelihood and immediacy of replacement of the entire MEMS cross-bar switch 200. It should be noted that MEMS cross-bar switch 200 can be configured to accommodate any amount of protection paths. In the example of Fig. 2, MEMS cross-bar switch 200 is capable of accommodating two protection paths. Conversely, if protection switch element sections 210, 220, and 230 are each configured as a 4 x 4 matrix of switch elements, then MEMS cross-bar switch 200 could accommodate four protection paths, one each for the four connections that can be cross-connected through matrix switch section 110. In the embodiment discussed above, the switch elements in protection switch element section 220 are switchable based on some form of system control. In an alternative embodiment, selective switch elements in protection switch element section 220 can be fixed in an activated position. For example, as illustrated in Fig. 2, switch elements (1,2) and (2,1) of protection switch element section 220 can be fixed in an activated position to accommodate the two illustrated protection paths. Alternatively, switch elements (1,1) and (2,2) of protection switch element section 220 can be fixed in an activated position. In this case, the original path of the connection between input line 102B and output line 104C is re-routed using switch element (1,2) in protection switch element section 210, switch element (1,1) in protection switch element section 220, and switch element (1,3) in protection switch element section 230; and a portion of the original path of the connection between input line 102C and output line 104D is re-routed using switch element (3,2) in protection switch element section 210, switch element (2,2) in protection switch element section 220, and switch element (2,4) in protection switch element section 230. It should be noted that most network architectures assume that all paths are two- fiber, two-way. In cross-connecting such 2-way signals, one has to switch the signals in both directions. This is accomplished by having two identical cross-connects, one for each direction. If the 2-way paths are always cross-connected in the same way, a considerable simplification and cost saving in the cross-connect can be achieved by using a dual design. For a dual design, each input port is replaced by a pair of ports, which would have different collimators, but would be reflected by the same switch element. Similarly, each output port is replaced by a pair of output ports. In this dual design, the physical size of the switch matrix would need to be increased to accommodate the additional collimators, but the total number of switch elements (and protection switch elements) is unchanged. While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims: WHAT IS CLAIMED IS: 1. An optical cross-bar switch system, comprising: a N x N matrix of switch elements, a first side of said N x N matrix of switch elements receiving input optical signals from an array of one or more input lines (i), a second side of said N x N matrix of switch elements transmitting output optical signals to an array of one or more output lines (j), wherein input line (i) can be connected to output line (j) by activating switch element (i,j) in said N x N matrix of switch elements; a N x M matrix of protection switch elements, said N x M matrix of protection switch elements being positioned between said array of one or more input lines (i) and said first side of said N x N optical cross-bar switch such that the N rows of said N x M matrix of protection switch elements are aligned with the N rows of said N x N matrix of switch elements; a M x N matrix of protection switch elements, said M x N matrix of protection switch elements being positioned between said second side of said N x N matrix of switch elements and said array of one or more output lines (j) such that the N columns of said M x N matrix of protection switch elements are aligned with the N columns of said N x N matrix of switch elements; a set of at least M protection switch elements, each of said M protection switch elements in said set being aligned with one of the M columns of said N x M matrix of protection switch elements and one of the M rows of said M x N matrix of protection switch elements, wherein a portion of an original path of an optical signal from an input line (i) that is connected to an output line (j) via switch element (i,j) in said N x N matrix of switch elements can be switched to an alternative protection path that traverses a path between a first protection switch element in row (i) of said N x M matrix of protection switch elements, a second protection switch element in column (j) of said M x N matrix of protection switch elements, and a protection switch element in said set of at least M protection switch elements that is aligned with said first and second protection switch elements, such that a path length of said alternative protection path is the same as said portion of said original path of said optical signal. 2. The optical cross-bar switch of claim 1 , wherein said set of at least M protection switch elements consists of M protection switch elements. 3. The optical cross-bar switch of claim 2, wherein said M protection switch elements are fixed in an activated position. 4. The optical cross-bar switch of claim 1 , wherein said set of at least M protection switch elements consists of a M x M matrix of protection switch elements, wherein the M columns of said M x M matrix of protection switch elements are aligned with the M columns of said N x M matrix of protection switch elements, and the M rows of said M x M matrix of protection switch elements are aligned with the M rows of said M x N matrix of protection switch elements. 5. The optical cross-bar switch of claim 4, wherein M protection switch elements in said M x M matrix of protection switch elements are fixed in an activated position. 6. The optical cross-bar switch of claim 1 , wherein said switch elements are silicon wafer based. 7. The optical cross-bar switch of claim 6, wherein said switch elements and said protection switch elements are on the same silicon substrate. 8. An optical cross-bar switch system, comprising: an optical cross-bar switch for receiving a plurality of optical input signals, said optical cross-bar switch having a plurality of optical outputs and a plurality of optical switch elements for selectively connecting at least one of said optical input signals to a selected one of said optical outputs; and protection optical switching elements for selectively connecting an optical input signal to the selected optical output in the event of failure of one or more of said optical switch elements. 9. The optical cross-bar switch system of claim 8, wherein a length of an optical path traversed by said at least one optical input signal through said optical crossbar switch is substantially the same as a length of an optical path traversed by said at least one optical input signal using said protection optical switching elements. 10. The optical cross-bar switch system of claim 8, wherein said optical crossbar switch consists of a N x N matrix of switch elements, and wherein said protection optical switching elements includes a N x M matrix of protection switch elements, said N x M matrix of protection switch elements being positioned such that the N rows of said N x M matrix of protection switch elements are aligned with the N rows of said N x N matrix of switch elements; a M x N matrix of protection switch elements, said M x N matrix of protection switch elements being positioned such that the N columns of said M x N matrix of protection switch elements are aligned with the N columns of said N x N matrix of switch elements; and a set of at least M protection switch elements, each of said M protection switch elements in said set being aligned with one of the M columns of said N x M matrix of protection switch elements and one of the M rows of said M x N matrix of protection switch elements. 11. The optical cross-bar switch system of claim 10, wherein said set of at least M protection switch elements consists of M protection switch elements. 12. The optical cross-bar switch system of claim 11, wherein said M protection switch elements are fixed in an activated position. 13. The optical cross-bar switch system of claim 10, wherein said set of at least M protection switch elements consists of a M x M matrix of protection switch elements, wherein the M columns of said M x M matrix of protection switch elements are aligned with the M columns of said N x M matrix of protection switch elements, and the M rows of said M x M matrix of protection switch elements are aligned with the M rows of said M x N matrix of protection switch elements. 14. The optical cross-bar switch system of claim 13, wherein M protection switch elements in said M x M matrix of protection switch elements are fixed in an activated position. 15. The optical cross-bar switch system of claim 8, wherein said switch elements are silicon wafer based. 16. The optical cross-bar switch system of claim 15, wherein said switch elements and said protection switch elements are on the same silicon substrate. 17. A telecommunications network, comprising: a first plurality of network elements, said first plurality of network elements generating a first plurality of optical signals on a plurality of input lines; a second plurality of network elements, said second plurality of network elements operative to receive said first plurality of optical signals on a plurality of output lines; and an optical cross-bar switch system that provides connectivity between said first plurality of network elements and said second plurality of network elements, said optical cross-bar switch system including a N x N matrix of switch elements, a first side of said N x N matrix of switch elements receiving input optical signals from an array of one or more input lines (i), a second side of said N x N matrix of switch elements transmitting output optical signals to an array of one or more output lines (j), wherein input line (i) can be connected to output line (j) by activating switch element (ij) in said N x N matrix of switch elements; a N x M matrix of protection switch elements, said N x M matrix of protection switch elements being positioned between said array of one or more input lines (i) and said first side of said N x N optical cross-bar switch such that the N rows of said N x M matrix of protection switch elements are aligned with the N rows of said N x N matrix of switch elements; a M x N matrix of protection switch elements, said M x N matrix of protection switch elements being positioned between said second side of said N x N matrix of switch elements and said array of one or more output lines (j) such that the N columns of said M x N matrix of protection switch elements are aligned with the N columns of said N x N matrix of switch elements; a set of at least M protection switch elements, each of said M protection switch elements in said set being aligned with one of the M columns of said N x M matrix of protection switch elements and one of the M rows of said M x N matrix of protection switch elements, wherein a portion of an original path of an optical signal from an input line (i) that is connected to an output line (j) via switch element (i,j) in said N x N matrix of switch elements can be switched to an alternative protection path that traverses a path between a first protection switch element in row (i) of said N x M matrix of protection switch elements, a second protection switch element in column (j) of said M x N matrix of protection switch elements, and a protection switch element in said set of at least M protection switch elements that is aligned with said first and second protection switch elements, such that a path length of said alternative protection path is the same as said portion of said original path of said optical signal. 18. A telecommunications network, comprising: a first plurality of network elements, said first plurality of network elements generating a first plurality of optical signals on a plurality of input lines; a second plurality of network elements, said second plurality of network elements operative to receive said first plurality of optical signals on a plurality of output lines; and an optical cross-bar switch system that provides connectivity between said first plurality of network elements and said second plurality of network elements, said optical cross-bar switch system including an optical cross-bar switch for receiving a plurality of optical input signals, said optical cross-bar switch having a plurality of optical outputs and a plurality of optical switch elements for selectively connecting at least one of said optical input signals to a selected one of said optical outputs; and protection optical switching elements for selectively connecting an optical input signal to the selected optical output in the event of failure of one or more of said optical switch elements.

Inventors: BALA KRISHNA, TOMLINSON W JOHN, BALA, KRISHNA, TOMLINSON, W., JOHN
Applicants: TELLIUM INC, TELLIUM, INC.
Application year: 2000
Main classification: H01M
IPC codes: G02B6, G02B26