Abstract:
The crosstalk-inhibiting arrangement is used on a switching cell in which an optical signal from an input waveguide is alternatingly directed to first and second output waveguides, depending upon whether there is fluid in a region between the input waveguide and the first output waveguide. The fluid has a refractive index selected to promote propagation of light energy through the fluid. However, in the absence of fluid, the light is reflected toward a second switching mechanism via an intermediate waveguide. The intermediate waveguide is at an angle greater than 90° relative to the input waveguide. The second switching mechanism is also fluid-manipulable and is used to inhibit leakage reflection from passing to the second output waveguide. Preferably, the angle between the intermediate waveguide and the input waveguide is in the range of 95° to 150°.

Description:
TECHNICAL FIELD 
     The invention relates generally to optical switching arrangements and more particularly to switching arrangements for inhibiting crosstalk among optical waveguides. 
     BACKGROUND ART 
     While signals within telecommunications and data communications networks have been traditionally exchanged by transmitting electrical signals via electrically conductive lines, an alternative medium of data exchange is the transmission of optical signals through optical fibers. Information is exchanged in the form of modulations of laser-produced light. The equipment for efficiently generating and transmitting the optical signals has been designed and implemented, but the design of optical switches for use in telecommunications and data communications networks is problematic. As a result, switching requirements within a network that transmits optical signals is often satisfied by converting the optical signals to electrical signals at the inputs of a switching network, then reconverting the electrical signals to optical signals at the outputs of the switching network. 
     Recently, reliable optical switching systems have been developed. U.S. Pat. No. 5,699,462 to Fouquet et al., which is assigned to the assignee of the present invention, describes a switching matrix that may be used for routing optical signals from one of a number of parallel input optical fibers to any one of a number of parallel output optical fibers. Another such matrix of switching elements is described in U.S. Pat. No. 4,988,157 to Jackel et al. An isolated switching element  10  is shown in FIG. 1, while a 4×4 matrix  32  of switching elements is shown in FIG.  2 . The optical switch of FIG. 1 is formed on a substrate. The substrate may be a silicon substrate, but other materials may be used. The optical switch  10  includes planar waveguides defined by a lower cladding layer  14 , a core  16 , and an upper cladding layer, not shown. The core is primarily silicon dioxide, but with other materials that achieve a desired index of refraction for the core. The cladding layers should be formed of a material having a refractive index that is substantially different from the refractive index of the core material, so that optical signals are guided along the waveguides. 
     The material core  16  is patterned to form an input waveguide  20  and an output waveguide  26  of a first optical path and to define a second input waveguide  24  and a second output waveguide  22  of a second optical path. The upper cladding layer is then deposited over the patterned core material. A gap  28  is formed by etching a trench through the core material and the two cladding layers to the substrate. The waveguides intersect the trench at an angle of incidence greater than the critical angle of total internal reflection (TIR) when the location  30  aligned with the waveguides is filled with a vapor or gas. Thus, TIR diverts light from the input waveguide  20  to the output waveguide  22 , unless an index-matching fluid resides within the location  30  between the aligned waveguides  20  and  26 . The trench  28  is positioned with respect to the four waveguides such that one sidewall of the trench passes through or slightly offset from the intersection of the axes of the waveguides. 
     The above-identified patent to Fouquet et al. describes a number of alternative approaches to switching the switching element  10  between a transmissive state and a reflective state. The element includes at least one heater that can be used to manipulate fluid within the gap  28 . One approach is illustrated in FIG.  1 . The switching element  10  includes two microheaters  50  and  52  that control the position of a bubble within the fluid-containing gap. The fluid within the gap has a refractive index that is close to the refractive index of the core material  16  of the four waveguides  20 - 26 . Fluid fill-holes  54  and  56  may be used to provide a steady supply of fluid, but this is not critical. In the operation of the switching element, one of the heaters  50  and  52  is brought to a temperature sufficiently high to form a gas bubble. Once formed, the bubble can be maintained in position with a reduced current to the heater. In FIG. 1, the bubble is positioned at the location  30  of the intersection of the four waveguides. Consequently, an input signal along the waveguide  20  will encounter a refractive index mismatch upon reaching the gap  28 . This places the switching element in a reflecting state, causing the optical signal along the waveguide  20  to be redirected to the output waveguide  22 . However, even in the reflecting state, the second input waveguide  24  is not in communication with the output waveguide  26 . 
     If the heater  50  at location  30  is deactivated and the second heater  52  is activated, the bubble will be attracted to the off-axis heater  52 . This allows index-matching fluid to fill the location  30  at the intersection of the waveguides  20 - 26 . The switching element  10  is then in a transmitting state, since the input waveguide  20  is optically coupled to the collinear waveguide  26 . 
     In the 4×4 matrix  32  of FIG. 2, any of the four input waveguides  34 ,  36 ,  38  and  40  may be optically coupled to any one of the four output waveguides  42 ,  44 ,  46  and  48 . The switching matrix is sometimes referred to as a “non-blocking” matrix, since any free input fiber can be connected to any free output fiber regardless of which connections have already been made through the switching matrix. Each of the sixteen optical switches has a gap that causes TIR in the absence of a fluid at the location between collinear waveguides, but collinear waveguides of a particular waveguide path are optically coupled when the locations between the waveguides are filled with the fluid. Trenches that are in the transmissive state are represented by fine lines that extend at an angle through the intersections of the optical waveguides in the matrix. On the other hand, trenches of switching elements in a reflecting state are represented by broad lines through points of intersection. 
     In FIGS. 1 and 2, the input waveguide  20  is in optical communication with the output waveguide  22 , as a result of TIR at the empty location  30  of the gap  28 . Since all other cross points for allowing the input waveguide  34  to communicate with the output waveguide  44  are in a transmissive state, a signal that is generated at input waveguide  34  will be received at output waveguide  44 . In like manner, the input waveguide  36  is optically coupled to the first output waveguide  42 , the third input waveguide  38  is optically coupled to the fourth output waveguide  48 , and the fourth input waveguide  40  is optically coupled to the third output waveguide  46 . 
     One concern with optical switching elements  10  of this type is that in the transmissive state, there is a small but potentially objectionable amount of reflection. If the index of refraction of the fluid is different than that of the core material  16 , reflections occur. A precise match between the indices of refraction is problematic, since there are other considerations in the selection of a fluid. For example, since the fluid is manipulated using thermal energy, the thermal properties of the liquid must be considered. The greater the mismatch between the index of refraction of the fluid and the index of refraction of the core material  16 , the greater the intensity of leakage to the second output waveguide  22  when the switching element is in the transmissive state for optically coupling the collinear waveguides  20  and  26 . This leakage will cause crosstalk among the waveguides. 
     What is needed is a switching arrangement that achieves greater isolation among waveguides of an optical switch. Particularly, what is needed is a switching arrangement that inhibits crosstalk among waveguides. 
     SUMMARY OF THE INVENTION 
     A crosstalk-inhibiting arrangement for a switching cell includes using more than one fluid-manipulable switching mechanism within the cell. An input waveguide is in communication with a first output waveguide when fluid is aligned within a gap between the waveguides. On the other hand, when no fluid is aligned with the input waveguide, an optical signal along the waveguide is reflected at the first switching mechanism to an intermediate waveguide that connects the first switching mechanism to a second fluid-manipulable switching mechanism. By locating fluid of the second switching mechanism into alignment with the intermediate waveguide, an optical signal within the intermediate waveguide will propagate through the fluid to a second output waveguide. The two switching mechanisms are operated in a push-pull manner, so that when one is in a transmissive state, the other is in a reflecting state. 
     The intermediate waveguide has an angle that is non-perpendicular with respect to the input waveguide. Preferably, the angle of the intermediate waveguide relative to the input waveguide is in the range of 95° to 150°. More preferably, the angle is in the range of 96° to 135°. However, there may be applications in which an angle of less than 90° provides acceptable results. 
     The input waveguide and the first output waveguide have ends at opposite sides of the first switching mechanism. Since an optical signal from the input waveguide will undergo refraction as it enters and exits a fluid for which the refractive indices are closely but not precisely matched, optimal performance does not require that the two waveguides be collinear. In addition to inducing refraction, a non-precise matching of refractive indices between the waveguide material and the fluid causes some reflection to occur at the interface of the input waveguide with the fluid. The reflected energy is leakage that becomes crosstalk if the energy enters the second output waveguide. However, by operating the two switching mechanisms in the push-pull manner, the leakage is reflected at the second switching mechanism, thereby preventing crosstalk into the second output waveguide. 
     When the first switching mechanism is changed from the transmissive state to the reflecting state, the switching mechanism is changed from the reflecting state to the transmissive state. The switching mechanisms are total internal reflection (TIR) devices. An optical signal along the input waveguide is reflected at the first switching mechanism. The reflected signal enters the intermediate waveguide and propagates through the fluid within the second switching mechanism to the second output waveguide. The optimal alignment of the second output waveguide to the intermediate waveguide is dependent upon the amount of refraction that occurs as a result of any mismatch between the refractive indices of the waveguides and the fluid. 
     Preferably, there is a monitoring waveguide that intersects the second switching mechanism on a same side as the intermediate waveguide and at an angle to receive any light energy reflected at the second switching mechanism. The intensity of the reflected energy can be used for detecting the most likely forms of failure with respect to placing the two switching mechanisms in the desired states for directing an input optical signal to either the first or the second output waveguide. 
     Also in the preferred embodiment, there is a second input waveguide that is coupled to the second output waveguide when the second switching mechanism is in the reflecting state. Thus, the cell can be only one cell within a switching matrix of the type described above with reference to FIG.  2 . 
     An advantage of the invention is that by using two TIR switching mechanisms within a single cell, crosstalk among waveguides is effectively controlled. Moreover, the use of the monitoring waveguide allows the crosstalk-inhibiting arrangement to be self-diagnosing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top view of an optical switching element that utilizes total internal reflection in accordance with the prior art. 
     FIG. 2 is a 4×4 matrix of switching elements of FIG. 1 to allow connection of any one of four input waveguides to any one of four output waveguides in accordance with the prior art. 
     FIG. 3 is a schematic view of a 3×3 matrix of switching cells having a crosstalk-inhibiting arrangement in accordance with the invention. 
     FIG. 4 is a schematic view of one of the switching cells of FIG.  3 . 
     FIG. 5 is a schematic view of a second embodiment of a 3×3 matrix of switching cells having a crosstalk-inhibiting arrangement in accordance with the invention. 
     FIG. 6 is a schematic view of one of the switching cells of FIG.  5 . 
    
    
     DETAILED DESCRIPTION 
     With reference to FIG. 3, a matrix  58  of optical switches employs bistable cells for selectively coupling any one of three input waveguides  60 ,  62  and  64  to any one of three output waveguides  66 ,  68  and  70 . Optionally, additional columns of switching cells may be connected along waveguides  72 ,  74  and  76  and additional rows of switching cells may be connected via waveguides  78 ,  80  and  82 . 
     Each one of the optical switches in the matrix  58  of nine switches utilizes a push-pull operation to direct optical signals with little or no crosstalk among waveguides. Referring to FIGS. 3 and 4, each optical switch  84  includes three total internal reflection (TIR) elements. Two of the TIR elements are trenches  86  and  88  that form gaps at intersections of waveguides, while the third element is a fixed state TIR mirror  90 . As described above with reference to FIG. 1, a thermal optical switch may be formed by etching a trench  86  and  88  at a crosspoint of optical waveguides. One or more heaters may be used to create bubbles at a crosspoint. When a bubble within the trench  86  is aligned with the input waveguide  60 , the first switching mechanism within the optical switch  84  will be in a reflecting state, causing an optical signal from the input waveguide to reflect to an intermediate waveguide  92 . On the other hand, if a suitable fluid resides in alignment with the input waveguide  60 , the first switching mechanism is in a transmissive state, causing the optical signal to propagate through the fluid to a waveguide  94 . In a similar manner, fluid aligned with the intermediate waveguide  92  within the trench  88  will cause optical communication between the intermediate waveguide and a waveguide  96 , while a bubble aligned with the intermediate waveguide will cause any light from the intermediate waveguide to be reflected into a monitoring waveguide  98 . 
     The input waveguide  60  intersects the trench  86  at an angle of incidence in the range of 45° to 60°, but this is not critical. More importantly, the intermediate waveguide  92  is at an angle greater than 90° and less than 150° relative to the input waveguide  60 . A more preferred range is 96° to 135°. The angle of the monitoring waveguide  98  to the intermediate waveguide  92  is preferably the same as the angle between the input waveguide  60  and the intermediate waveguide  92 , since the design and fabrication of the matrix is simplified by aligning the waveguides in parallel, when possible. Thus, the angle of a waveguide  100  to the output waveguide  96  and to the waveguide  78  is preferably the same as the angle of the input waveguide  60  to the intermediate waveguide  92 . 
     Opposite ends of each of the trenches  86  and  88  are connected to fluid fill-holes  102  and  104 . The fluid fill-holes connect to a liquid reservoir, allowing liquid to be replenished as needed. Many alternative liquids may be used, with each having associated advantages and disadvantages. Water and light alcohols do not have indices of refraction that match the index of the waveguides as closely as other liquids, but they do not decompose as quickly. There is also a concern that water and light alcohols may be susceptible to the buildup of bacteria or viruses within the liquid supply. A disinfectant may be added to the supply to retard the formation of bacterial or viral contamination, but the refractive index effect of any disinfectant must be considered in the design of a switch that is to include the disinfectant. 
     As shown in FIG. 4, the waveguide  94  is offset from collinearity with the input waveguide  60 . The offset is designed to compensate for the mismatch in the refractive indices of the waveguide material and fluid that is within the trench  86 . When fluid is aligned with the input waveguide  60 , an optical signal will propagate through the trench  86 , but some refraction will occur. The degree of offset of the waveguide  94  is selected based upon the amount of refraction as a result of the mismatch in refractive indices. Similarly, the waveguide  96  is offset from the intermediate waveguide  92  by a distance that is selected to compensate for refraction incurred as an optical signal propagates through fluid within the trench  88 . 
     The optical switch  84  includes a first switching mechanism that is toggled by manipulating fluid within the trench  86  and includes a second switching mechanism that is toggled by manipulating fluid within the trench  88 . A concern with the operation of the first switching mechanism is that when fluid is aligned between the input waveguide  60  and the waveguide  94 , a small but potentially objectionable amount of reflection occurs at the interface of the input waveguide with the trench. The reflection occurs even if the refractive index of the fluid is closely matched to the refractive index of the waveguide material. The greater the difference in refractive indices, the greater the intensity of reflected light, even though the first switching mechanism is in the transmissive state. The addition of the second switching mechanism removes the potentially objectionable leakage reflection from reaching the waveguide  96 . This is accomplished by operating the two switching mechanisms in the push-pull arrangement, such that the two switching mechanisms are always in opposite states. Another concern is the reliability of the optical switch  84 . The addition of the monitoring waveguide  98  allows the push-pull operation to be monitored in real time. If a fault is detected, repeated activation pulses can be applied to replace and/or remove liquid as required to correct the fault. 
     When the first switching mechanism is in the transmissive state, an input optical signal along the input waveguide  60  propagates to the waveguide  94  with only a limited amount of leakage reflection into the intermediate waveguide  92 . The leakage light will reach the second trench  88 , which is in the reflecting state. The switching mechanism is a TIR element, so the leakage light is reflected to the monitoring waveguide  98 . Therefore, no light exits the waveguide  96  that leads to the next cell of the switching matrix  58  of FIG.  3 . Instead, the leakage light along the monitoring waveguide  98  may be input to a monitor circuit, not shown. The monitor circuit may detect the most likely forms of failure of the optical switch  84 . If the two switching mechanisms are operating properly, the intensity of light along the monitoring waveguide  98  will be weak when the first switching mechanism is in a transmissive state. The intensity will be primarily dependent upon the index difference between the waveguide material and the fluid. In a fault condition in which both of the switching mechanisms of the switch  84  are in a transmissive state, the intensity of the light along the monitoring waveguide  98  will be below the anticipated intensity, thereby demonstrating one type of failure. On the other hand, if the switching mechanisms are both in a reflecting state, effectively all of any input signal along the input waveguide  60  will be directed to the monitoring waveguide  98 , thereby demonstrating the second type of failure. 
     Again referring to the condition in which the first switching mechanism is in the transmissive state and the second switching mechanism is in the reflecting state, the waveguide  94  is optically coupled to the input waveguide  60 , but the waveguide  96  is decoupled from the input waveguide. In this condition, an optical signal can be transmitted from an “above” switch via the waveguide  78 . The light traveling through the waveguide  78  will not interfere with light traveling through waveguide  94 . Instead, the light from the above switch will strike the TIR mirror  90  and be reflected to the interface of the waveguide  100  with the trench  88 . Since the trench  88  is in the reflecting state, the light will be reflected into the waveguide  96  for propagation to the optical switch “below” the switch  84 . 
     In order to direct an input optical signal from the input waveguide  60  to the waveguide  96 , the states of the two switching mechanisms are reversed. That is, the first switching mechanism is changed to a reflecting state by removing liquid from alignment between the input waveguide  60  and the waveguide  94 , while the second switching mechanism is changed to the transmissive state by filling the gap between the waveguides  92  and  96  with liquid. The means for manipulating the liquid within the two trenches  86  and  88  is not critical to the invention. It is possible that the two trenches can be replaced with a single trench in which a single gas bubble is manipulated to achieve the push-pull operation of the switch  84 . 
     With the first switching mechanism in the reflecting state, an optical signal along the input waveguide  60  is reflected by the trench  86  into the intermediate waveguide  92 . Since the second switching mechanism is in the transmissive state, the optical signal propagates through the liquid within the second trench  88  and exits into the waveguide  96 . Some reflection will occur at the interface of the intermediate waveguide with the trench  88 . This leakage current enters the monitoring waveguide  98  and may be used to confirm that the two switching mechanisms of the optical switch  84  are in opposite states. 
     Referring to FIG. 3, the push-pull operation of the optical switch  84  determines whether an input optical signal along input waveguide  60  remains in the same row of switches in the matrix  58  or is directed “downwardly” to the next row for output via the output waveguide  66 . The monitoring waveguide  98  is shown in dashed lines, since the error detection capability is not critical to the operation of the matrix. 
     A second embodiment of a crosstalk-inhibiting arrangement for a switching cell will be described with reference to FIGS. 5 and 6. In FIG. 5, a 3×3 switching matrix  106  includes three input waveguides  108 ,  110  and  112  and includes three reflecting state output waveguides  114 ,  116  and  118 . Waveguides  120  may be used to add additional rows of switches, and waveguides  122  are through guides and may be used to add additional columns. 
     First and second trenches  124  and  126  are formed to provide first and second switching mechanisms for push-pull operation of an optical switch  128 . The input waveguide  112  is optically coupled to a waveguide  130  when the first switching mechanism is in a transmissive state, i.e., when liquid is aligned with the input waveguide to allow propagation to be generally aligned with waveguide  130 . Any leakage reflection at the wall of the trench  124  is directed into an intermediate waveguide  132 . The intermediate waveguide  132  is at an angle in the range of 95° to 150° relative to the input waveguide  112 . A more preferred range is 96° to 135°, inclusive. When the first switching mechanism is in the transmissive state, the second switching mechanism is in the reflecting state. A gas bubble at the intersection of the intermediate waveguide with the second trench  126  causes any light propagating down the intermediate waveguide to be reflected into a monitoring waveguide  134 . The monitoring waveguide may be used in the same capacity as described with reference to FIGS. 3 and 4. By placing a monitoring circuit at the output of the monitoring waveguide  134 , the intensity of leakage reflection can be measured and used for confirming proper functioning of the push-pull operation. If both of the switching mechanisms are in a transmissive state, the intensity of light along the monitoring waveguide will be substantially less than the anticipated intensity. Corrective action can then be triggered by the monitoring circuit. On the other hand, if both of the switching mechanisms are in the reflecting state, the intensity along the monitoring waveguide  134  will be substantially greater than the anticipated intensity. Again, corrective action may be triggered by the monitoring circuit. 
     When the first switching mechanism is in the reflecting state, the second switching mechanism is changed to the transmissive state, providing optical communication between the input waveguide  112  and the output waveguide  114  via the intermediate waveguide  132  and the liquid within the second trench  126 . The input signal is reflected at the wall of the first trench  124 , but propagates through the liquid within the second trench  126 . 
     Alternatively, the output waveguide  114  may be coupled to one of the other two input waveguides  108  and  110  by means of a waveguide  136  and a fixed TIR mirror  138 . For example, if the input waveguide  110  is to be coupled to the output waveguide  114 , bubbles are properly aligned to induce TIR at trenches  140  and  126 , while fluid is aligned to allow light transmission through a trench  142 . With this setup, an input signal from the input waveguide  110  is reflected at the trench  140 , propagates through the trench  142 , and is reflected at both the TIR mirror  138  and the trench  126 . 
     The invention may be used in matrices other than those shown in FIGS. 3 and 5. In fact, the crosstalk-inhibiting arrangement may be used in applications having a single switching cell. Moreover, while the invention has been described and illustrated in the clearly preferred embodiment in which waveguides intersecting at a trench are an angle in the range of 95° to 150°, there may be applications in which an angle of less than 90° is desirable.