Abstract:
The invention is generally directed to integrated optical devices, such as switches and variable optical attenuators. A solid-state actuation mechanism (e.g., heat) is used to switch, attenuate, and/or trim the devices. The devices have a known state in the case of electrical power failure. Some disclosed designs direct optical signals out an output port of the device in the absence of power, while the other designs direct optical signals out a separate exhaust port in the absence of power.

Description:
PRIORITY INFORMATION  
       [0001]    This application claims priority from provisional application Ser. No. 60/294,941 filed May 31, 2001. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The invention relates to the field of optical components.  
           [0003]    One method of increasing the transportable bandwidth in optical communications networks is a technique known as wavelength division multiplexing (WDM). WDM is a technology that combines two or more wavelengths of light for transmission along a single optical waveguide. Each wavelength represents a channel that can carry a bit stream, i.e. content. Transporting two or more wavelengths on a waveguide effectively increases the aggregate bandwidth of the waveguide. For example, if 40 wavelengths, each capable of 10 Gb/s are used on a single fiber, the aggregate bandwidth of the fiber becomes 400 Gb/s.  
           [0004]    A similar manner of increasing transportable bandwidth has been termed dense wavelength division multiplexing (DWDM). DWDM generally involves combining a denser number of wavelengths onto a fiber than WDM. While DWDM deals with more difficult issues associated with multiplexing a larger number of wavelengths on a fiber, such as cross-talk and non-linear effects, WDM and DWDM are typically used interchangeably.  
           [0005]    Optical space switches and variable optical attenuators (VOA) based on planar lightwave circuit (PLC) technology are becoming important optical components in optical networks such as WDM networks. Optical switches switch optical signals from one optical waveguide to another, while VOAs attenuate the intensity of the optical signal in an optical waveguide.  
           [0006]    Optical switches and VOAs typically require power in order to be in a specific state. Mechanically actuated switches and attenuators, e.g. based on moving fibers or MicroElectroMechanical Systems (MEMS), can maintain their state upon loss of power because they can be latching.  
           [0007]    In contrast to mechanically actuated switches and attenuators, latching is not practical in solid-state optical switches or VOA components. Solid-state switches or VOA components are made as planar layers of a material, e.g. silica glass or polymer-based, on a silicon wafer or similar substrate. Thermo-optic, electro-optic, magneto-optic, or stress-optic effects, or any combination thereof, are typically used to actuate the device. Electrical power is typically used to operate the components implementing these effects for actuation. Latching is not practical in typical solid-state switches or VOAs because no mechanical motion occurs during actuation. Some possibilities do exist to introduce latching in solid state switches or VOAs, such as poling of polymer molecules, which is a process that consists of changing the molecule orientation by applying a large voltage (e.g., 1000 Volts). The change in orientation remains in effect after the voltage is turned off. Yet, such approaches are not practical and are not used in optical communication components. There is a need, therefore, for solid-state switches and VOAs in which the state is known upon power failure (i.e., where an optical signal is directed), even if the state is independent of the pre-failure state.  
         SUMMARY OF THE INVENTION  
         [0008]    In one aspect, the present invention provides an optical device comprising an optical component and at least a first bypass path. The optical component has at least a first input port to receive at least a first optical signal. A portion of the first bypass path is formed near the first input port to create a first coupler. The first coupler is designed and fabricated to provide essentially 100% coupling of the first optical signal to the first bypass path in the absence of power such that the first bypass path routes the optical signal around the optical component to a known location.  
           [0009]    Another aspect of the present invention provides an optical device comprising an interferometric switching component having at least a first input port to receive at least a first optical signal and first and second output ports. The first optical signal is output to either the first output port or the second output port depending on the state of an actuation mechanism coupled to the switching component. The switching component is designed and fabricated so that essentially 100% of the first optical signal is output the first output port in the absence of power to the actuation mechanism. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIGS. 1 a - b  and  2   a - c  illustrate embodiments, according to the present invention, of optical space switches and VOAs, respectively, in which a bypass path is used to route an input optical signal to a specified output port in the absence of any actuation;  
         [0011]    [0011]FIGS. 3 a - 6   b  illustrate specific embodiments of the switching or attenuating components to achieve the functions as described with respect to FIGS. 1 a - b  and  2   a - c;    
         [0012]    [0012]FIGS. 7 a - b  generally illustrate embodiments of optical space switches, according to the present invention, that use interferometric switching components in which the interferometric switching components are designed and fabricated to route an input optical signal to a specified output port in the absence of any actuation;  
         [0013]    FIGS.  8 - 11  illustrate specific embodiments of the switching or attenuating components to achieve the functions as described with respect to FIGS. 7 a  and  7   b;    
         [0014]    [0014]FIGS. 12 a - b  and  13   a - c  illustrate embodiments, according to the present invention, of optical space switches and VOAs, respectively, in which a bypass path is used as an exhaust port to output an input optical signal in the absence of any actuation; and  
         [0015]    [0015]FIGS. 14 a - 17   b  illustrate specific embodiments of the switching or attenuating components to achieve the functions as described with respect to FIGS. 12 a - b  and  13   a - c.   
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    As described above, it is desirable to know the state of optical devices, such as optical space switches and VOAs, in the absence of actuation due to an electrical power failure. The present invention provides optical space switches and VOAs that are designed and fabricated to have a known state by directing optical signals to a known location in the absence of any actuation. That location can be an output port of the device, as illustrated in the embodiments of FIGS.  1 - 11 , or a separate exhaust port, as illustrated in the embodiments of FIGS.  12 - 17 .  
         [0017]    [0017]FIGS. 1 a - b  and  2   a - c  illustrate embodiments of optical space switches and VOAs, respectively, in which a bypass path is used to route an input optical signal around the optical component to a specified output port in the absence of any actuation (i.e., when there is no electrical power).  
         [0018]    [0018]FIG. 1 a  generally illustrates an embodiment of a 1×2 optical space switch in which an input optical signal bypasses the switching component of the device and goes straight to an output port in the absence of electrical power. As shown, a 1×2 switching component  114  is formed on a substrate  100 , including an actuation mechanism. Switching component  114  has an input port  102  that receives an optical signal, a first output port  104  that outputs the received optical signal when the actuation mechanism is actuated to place switching component  114  in a first state, and a second output port  106  that outputs the received optical signal when the actuation mechanism is actuated to place switching component  114  in a second state. Each port is formed as an optical waveguide fabricated on substrate  100 .  
         [0019]    A bypass path  108  is also formed as an optical waveguide on substrate  100 . A portion of bypass path  108  is formed close to input port  102  to create a first coupler  110 . An actuation mechanism (illustrated by the arrow) is formed as part of coupler  110 . When electrical power is applied, the actuation mechanism causes coupler  110  to couple essentially 0% of the optical power received at input port  102  to bypass path  108 . In the absence of power for the actuation mechanism, however, coupler  110  couples essentially 100% of the optical power received at input port  102  to bypass path  108 .  
         [0020]    Similarly, a different portion of bypass path  108  is formed close to second output port  106  so as to create a second coupler  112 . An actuation mechanism (illustrated by the arrow) is also formed as part of coupler  112 . Coupler  112  is designed and fabricated such that, when electrical power is applied, the actuation mechanism causes coupler  112  to couple essentially 100% of the optical power on bypass path  108  to second output port  106 . In the absence of power for the actuation mechanism, however, coupler  110  couples essentially 0% of the optical power output at output port  102  to bypass path  108 .  
         [0021]    Thus, when electrical power is applied and an optical signal is received at input port  102 , the optical signal is not coupled to bypass path  108  and switching component  114  operates as normal, outputting the optical signal to first output port  104  or second output port  106  depending on the state of switching component  114 . In the absence of electrical power, however, an optical signal received at input port  102  is coupled into bypass path  108  by coupler  110 , bypasses switching component  114 , and is coupled into second output port  106  by coupler  112 .  
         [0022]    To couple essentially 100% of the received optical signal into and out of bypass path  108  in the absence of electrical power, couplers  110  and  112  are fabricated with precise and predictable coupling without the need for active trimming (i.e., without need to adjust the coupling ratio by applying electrical power). In a preferred embodiment, couplers  110  and  112  are directional couplers. Prior art directional couplers are not fabricated with precise and predictable coupling absent electrical power because of the difficulty in precisely patterning the gap between the waveguides in the coupling region. Rather, the coupling ratio is adjusted by applying a small bias to the actuation mechanism to couple optical signals out one of the outputs, while a lager bias is used to actuate the coupler to couple optical signals out the other output. In accordance with the present invention, however, the gap between the waveguides in the coupling region is precisely patterned by applying one of the following design rules:  
         [0023]    1) The gap has an aspect ratio of at least 1:1 (i.e., the width is at least as large as the height); or  
         [0024]    2) No gap exists (i.e., the two waveguides merge as an essentially double-width waveguide in the coupling region).  
         [0025]    Therefore, one of the above design rules is used to design couplers  110  and  112 . Couplers  110  and  112  are then precisely fabricated on substrate  100  so that they do not require trimming to provide essentially 100% coupling in the absence of electrical power.  
         [0026]    [0026]FIG. 1 b  generally illustrates an embodiment of a 2×2 optical space switch in which input optical signals bypass the switching component of the device and go straight to an output port in the absence of electrical power. As shown, a 2×2 switching component  130  is formed on a substrate  120 , including an actuation mechanism. Switching component  130  has first and second input ports,  122  and  124  respectively, that receive optical signals. Switching component  130  also has first and second output ports  140  and  142  respectively, that output the received optical signals depending on the state of switching component  130 . When switching component  130  is actuated to be in a first state, the optical signal received by first input port  122  is output to second output port  142 , while the optical signal received on second input port  124  is output to first output port  140 . Conversely, when switching component  130  is actuated to be in a second state, the optical signal received by first input port  122  is output to first output port  140 , while the optical signal received on second input port  124  is output to second output port  142 . Each port is formed as an optical waveguide fabricated on substrate  100 .  
         [0027]    A first bypass path  108  connecting first input  122  and first output  140  is also formed as an optical waveguide on substrate  100 . A portion of first bypass path  132  is formed close to first input port  122  to create a first coupler  126 , while a different portion of bypass path  132  is formed close to first output port  140  so as to create a second coupler  136 . Actuation mechanisms (illustrated by the arrow) are formed as part of couplers  126  and  136 .  
         [0028]    Similarly, a second bypass path  134  connecting second input port  124  and second output port  142  is also formed as an optical waveguide on substrate  100 . A portion of second bypass path  134  is formed close to second input port  124  to create a third coupler  128 , while a different portion of bypass path  134  is formed close to second output port  142  so as to create a second coupler  138 . Actuation mechanisms (illustrated by the arrow) are formed as part of couplers  128  and  138 .  
         [0029]    The couplers  126 ,  136 ,  128 , and  138  are designed and fabricated in the same manner as described with respect to FIG. 1 a  such that, in the absence of electrical power, the bypass paths  132  and  134  route optical signals around switching component  130 . Thus, when electrical power is applied and an optical signal is received at first input port  122 , the optical signal is not coupled to first bypass path  132  and switching component  130  operates as normal, outputting the optical signal to first output port  140  or second output port  142  depending on the state of switching component  130 . In the absence of electrical power, however, an optical signal received at first input port  122  is coupled into first bypass path  132  by coupler  126 , bypasses switching component  130 , and is coupled into first output port  140  by coupler  136 . Likewise, when electrical power is applied and an optical signal is received at second input port  124 , the optical signal is not coupled to second bypass path  134  and switching component  130  operates as normal, outputting the optical signal to first output port  140  or second output port  142  depending on the state of switching component  130 . In the absence of electrical power, however, an optical signal received at second input port  124  is coupled into second bypass path  134  by coupler  128 , bypasses switching component  130 , and is coupled into second output port  142  by coupler  138 .  
         [0030]    [0030]FIG. 2 a - c  generally illustrate embodiments of a VOA in which an input optical signal bypasses the attenuating component of the device and goes straight to an output port in the absence of electrical power. FIG. 2 a  illustrates an embodiment in which the attenuating component is based on a 1×1 design. FIG. 2 b  illustrates an embodiment in which the attenuating component is based on a 1×2 design. FIG. 2 c  illustrates an embodiment in which the attenuating component is based on a 2×2 design.  
         [0031]    In each embodiment, as shown, an attenuating component  206  is formed on a substrate  200 . Attenuating component  206  has an input port  202  that receives an optical signal and an output port  210  that outputs the received optical signal after it is attenuated by attenuating component  206 . Each port is formed as an optical waveguide fabricated on substrate  200 .  
         [0032]    Similar to the embodiment of FIG. 1 a , a bypass path  208  connecting input port  202  and output port  210  is also formed as an optical waveguide on substrate  200 . A portion of bypass path  208  is formed close to input port  202  to create a first coupler  204 , while a different portion of bypass path  208  is formed close to output port  210  so as to create a second coupler  212 . Actuation mechanisms (illustrated by the arrow) are formed as part of couplers  204  and  212 .  
         [0033]    The couplers  204  and  212  are designed and fabricated in the same manner as described with respect to FIG. 1 a , such that, in the absence of electrical power, bypass path  208  routes optical signals around attenuating component  206 . Thus, when electrical power is applied and an optical signal is received at input port  202 , the optical signal is not coupled to bypass path  208  and attenuating component  206  operates as normal, attenuating the optical signal and outputting the attenuated signal to output port  210 . In the absence of electrical power, however, an optical signal received at input port  202  is coupled into bypass path  208  by coupler  204 , bypasses attenuating component  206 , and is coupled into output port  210  by coupler  212 .  
         [0034]    [0034]FIGS. 3 a - 6   b  illustrate specific embodiments of the switching or attenuating components to achieve the functions as described with respect to FIGS. 1 a - b  and  2   a - c.    
         [0035]    [0035]FIG. 3 a  illustrates an embodiment of the 1×2 designs of FIGS. 1 a  and  2   b  in which the switching or attenuating component is based on a Y-branch switch  314 . When used as a VOA, only one input and one output is used. Similarly, FIG. 3 b  illustrates an embodiment of the 2×2 designs of FIGS. 1 b  and  2   c  in which the switching or attenuating component is based on a 2×2 DOS design  330 . When used as a VOA, only one input and one output is used.  
         [0036]    [0036]FIG. 4 a  illustrates an embodiment of the 1×2 designs of FIGS. 1 a  and  2   b  in which the switching or attenuating component is based on a 1×2 directional coupler  414 . When used as a VOA, only one input and one output is used. Similarly, FIG. 4 b  illustrates an embodiment of the 2×2 designs of FIGS. 1 b  and  2   c  in which the switching or attenuating component is based on a 2×2 directional coupler  430 . When used as a VOA, only one input and one output is used.  
         [0037]    [0037]FIG. 5 a  illustrates an embodiment of the 1×2 designs of FIGS. 1 a  and  2   b  in which the switching or attenuating component is based on a 1×2 multi-mode interference (MMI) coupler  514 . When used as a VOA, only one input and one output is used. Similarly, FIG. 5 b  illustrates an embodiment of the 2×2 designs of FIGS. 1 b  and  2   c  in which the switching or attenuating component is based on a 2×2 MMI coupler  530 . When used as a VOA, only one input and one output is used.  
         [0038]    [0038]FIG. 6 a  illustrates an embodiment of the 1×2 designs of FIGS. 1 a  and  2   b  in which the switching or attenuating component is based on a 1×2 Mach-Zehnder Interferometer (MZI)  614 . When used as a VOA, only one input and one output is used. Similarly, FIG. 6 b  illustrates an embodiment of the 2×2 designs of FIGS. 1 b  and  2   c  in which the switching or attenuating component is based on a 2×2 Mach-Zehnder Interferometer  630 . When used as a VOA, only one input and one output is used.  
         [0039]    [0039]FIGS. 7 a - b  generally illustrate embodiments of optical space switches that use interferometric switching components in which the interferometric switching components are designed and fabricated to route an input optical signal to a specified output port in the absence of any actuation (i.e., when there is no electrical power).  
         [0040]    [0040]FIG. 7 a  illustrates a switch in which a 1×2 interferometric switching component  704  is formed on substrate  700 , including an actuation mechanism coupled to switching component  704 . Switching component  704  has an input port  702  that receives an optical signal, a first output port  706  that outputs the received optical signal when the actuation mechanism is actuated to place switching component  704  in a first state, and a second output port  708  that outputs the received optical signal when the actuation mechanism is actuated to place switching component  704  in a second state. Each port is formed as an optical waveguide fabricated on substrate  700 . Switching component  704  is designed and fabricated such that, in the absence of electrical power, an input optical signal traversing the device is routed interferometrically to one output of the device.  
         [0041]    Thus, when electrical power is applied and an optical signal is received at input port  702 , the optical signal is switched normally, i.e. to first output port  706  or second output port  708  depending on the actuation of switching component  704 . In the absence of electrical power, however, an optical signal received at input port  702  is routed by switching component  704  to, for example, first output port  706 .  
         [0042]    [0042]FIG. 7 b  illustrates a switch in which a 2×2 interferometric switching component  730  is formed on substrate  720 , including an actuation mechanism coupled to switching component  704 . Switching component  730  has first and second input ports,  722  and  724  respectively, that receive optical signals. Switching component  730  also has first and second output ports,  726  and  728  respectively, that output the received optical signals depending on the state of switching component  730 . When the actuation mechanism is actuated to place switching component  730  in a first state, the optical signal received by first input port  722  is output to second output port  728 , while the optical signal received on second input port  724  is output to first output port  726 . Conversely, when the actuation mechanism is actuated to place switching component  730  in a second state, the optical signal received by first input port  722  is output to first output port  726 , while the optical signal received on second input port  724  is output to second output port  728 . Each port is formed as an optical waveguide fabricated on substrate  700 . Switching component  730  is designed and fabricated such that, in the absence of electrical power, an input optical signal received at first input port  722  and traversing the device is routed interferometrically to one output of the device, while an input optical signal received at second input port  724  and traversing the device is routed interferometrically to the other output of the device.  
         [0043]    Thus, when electrical power is applied and an optical signal is received at first input port  722 , the optical signal is switched normally, i.e. to first output port  726  or second output port  728  depending on the actuation of switching component  730 . In the absence of electrical power, however, an optical signal received at first input port  722  is routed by switching component  730  to, for example, second output port  728 . Likewise, when electrical power is applied and an optical signal is received at second input port  724 , the optical signal is switched normally, i.e. to first output port  726  or second output port  728  depending on the actuation of switching component  730 . In the absence of electrical power, however, an optical signal received at second input port  724  is routed by switching component  730  to the other output port, for example, first output port  726 .  
         [0044]    FIGS.  8 - 11  illustrate specific embodiments of the switching or attenuating components to achieve the functions as described with respect to FIGS. 7 a  and  7   b.    
         [0045]    [0045]FIG. 8 a  illustrates an embodiment of the 1×2 design of FIG. 7 a  in which the switching component is based on a 1×2 directional coupler  804  with an actuation mechanism formed therewith. To achieve the predictable routing of essentially 100% of a received optical signal to one of the output ports in the absence of electrical power, one of the design rules described above in relation to FIG. 1 a  is used to design coupler  804 . Coupler  804  is then precisely fabricated on substrate  800  so that it does not require trimming to provide essentially 100% coupling in the absence of electrical power. Similarly, FIG. 8 b  illustrates an embodiment of the 2×2 design of FIG. 7 b  in which the switching component is based on a 2×2 directional coupler  830  with an actuation mechanism formed therewith. One of the design rules described above in relation to FIG. 1 a  is also used to design and fabricate coupler  830  so as to achieve the predictable routing of essentially 100% of a received optical signal to one of the output ports in the absence of electrical power.  
         [0046]    [0046]FIG. 9 a  illustrates an embodiment of the 1×2 design of FIG. 7 a  in which the switching component is based on a 1×2 MMI coupler  904  with an actuation mechanism formed therewith. MMI couplers of the prior art suffer from similar disadvantages as directional couplers. Therefore, in accordance with the present invention, a design rule is followed in order to produce MMI couplers with precise and predictable coupling without the need to adjust the coupling ratio by applying electrical power. To obtain the predictable routing of essentially 100% of a received optical signal to one of the output ports in the absence of electrical power, the output waveguides,  906  and  908 , of MMI coupler  904  should have practically no evanescent coupling between them. This is achieved by designing the gap between them to have an aspect ratio of at least 2:1 (i.e., the gap width has to be at least twice as large as the height). When designed accordingly, the MMI can be fabricated to route essentially 100% of a received optical signal to one of the output ports in the absence of electrical power. Similarly, FIG. 9 b  illustrates an embodiment of the 2×2 design of FIG. 7 b  in which the switching component is based on a 2×2 MMI coupler  930  with an actuation mechanism formed therewith. The design rule described above in relation to FIG. 9 a  is also used to design and fabricate MMI coupler  930 , however, the input waveguides,  922  and  924 , should also have practically no evanescent coupling between them, which is achieved by designing the gap between them to have an aspect ratio of at least 2:1. By substantially eliminating the evanescent coupling, the predictable routing of essentially 100% of a received optical signal to one of the output ports in the absence of electrical power can be achieved.  
         [0047]    [0047]FIG. 10 a  illustrates an embodiment of the 1×2 design of FIG. 7 a  in which the switching component is based on a 1×2 MZI  1004  with an actuation mechanism formed in one arm thereof. In order to obtain the predictable routing of essentially 100% of a received optical signal to one of the output ports in the absence of electrical power, the coupling regions,  1001  and  1003 , of the input 3 dB coupler and the coupling regions,  1005  and  1007 , of the output 3 dB coupler are designed and fabricated in the same manner as the directional couplers described in the embodiment of FIG. 1 a . Similarly, FIG. 10 b  illustrates an embodiment of the 2×2 design of FIG. 7 b  in which the switching component is based on a 2×2 MMI coupler  1030  with an actuation mechanism formed in one arm thereof. As with MZI  1004 , the coupling regions,  1021  and  1023 , of the input 3 dB coupler and the coupling regions,  1025  and  1027 , of the output 3 dB coupler are also designed and fabricated in the same manner as the directional couplers described in the embodiment of FIG. 1 a  to obtain the predictable routing of essentially 100% of a received optical signal in the absence of electrical power.  
         [0048]    [0048]FIGS. 11 a - c  illustrate embodiments of the 1×2 design of FIG. 7 a  in which the switching component is a 1×2 DOS that is asymmetric by design such that, in the absence of electrical power, an input optical signal traversing the device is routed to one output of the device. One method of achieving such asymmetry is by having the angle of one arm of the Y-branch with respect to the input port be smaller than the angle of the other arm with respect to the input port. FIG. 11 a  shows a particular embodiment of achieving the asymmetry this way. As shown, arm  1101  has a non-zero angle with respect to input port  1102 , while arm  1103  has a zero angle with respect to input port  1102 . Another method of achieving such asymmetry is by having the width of one arm be smaller than the width of the other arm. In two specific cases of this embodiment, one arm has a uniform width similar to that of the input and output waveguides and the other arm (i) starts with a smaller width and tapers out to essentially the width of the first arm; or (ii) has a uniformly thin width for some length and then tapers out to essentially the width of the first arm. FIG. 11 b  illustrates the case in which first arm  1123  has a uniform width and second arm  1121  starts with a smaller width and tapers out to essentially the width of first arm  1123 . A third method of achieving such asymmetry is by having both the angle asymmetry of FIG. 11 a  and the width asymmetry of FIG. 11 b . This is illustrated in FIG. 11 c , which shows arm  1131  with a non-zero angle and tapered width, while arm  1133  has a zero angle and uniform width.  
         [0049]    [0049]FIGS. 12 a - b  and  13   a - c  illustrate embodiments of optical space switches and VOAs, respectively, in which a bypass path routes an input optical signal around the optical component and is used as an exhaust port to output the input optical signal in the absence of any actuation (i.e., when there is no electrical power).  
         [0050]    [0050]FIG. 12 a  generally illustrates an embodiment of a 1×2 optical space switch in which, in the absence of electrical power, an input optical signal bypasses the switching component via a bypass path and is output by the bypass path. As shown, a 1×2 switching component  1208  is formed on a substrate  1200 . Switching component  1208  has an input port  1202  that receives an optical signal, a first output port  1210  that outputs the received optical signal when switching component  1208  is actuated to be in a first state, and a second output port  1212  that outputs the received optical signal when switching component  1208  is actuated to be in a second state. Each port is formed as an optical waveguide fabricated on substrate  1200 .  
         [0051]    As with the embodiment of FIG. 1 a , a bypass path  1206  is also formed as an optical waveguide on substrate  1200 . A portion of bypass path  1206  is formed close to input port  1202  to create a first coupler  1204 . An actuation mechanism (illustrated by the arrow) is formed as part of coupler  1204 . Instead of being formed into a second coupler as with the embodiment of FIG. 1 a , bypass path  1206  is routed to the edge of substrate  1200  and acts as an exhaust port to output the optical signal.  
         [0052]    The coupler  1204  is designed and fabricated, however, in the same manner as described with respect to FIG. 1 a  such that, in the absence of electrical power, the bypass path  1206  routes optical signals around switching component  1208 . Thus, when electrical power is applied and an optical signal is received at first input port  1202 , the optical signal is not coupled to bypass path  1206  and switching component  1208  operates as normal, outputting the optical signal to first output port  1210  or second output port  1212  depending on the state of switching component  1208 . In the absence of electrical power, however, an optical signal received at first input port  1202  is coupled into bypass path  1206 , which acts as an exhaust port to output the optical signal.  
         [0053]    [0053]FIG. 12 b  generally illustrates an embodiment of a 2×2 optical space switch in which, in the absence of electrical power, input optical signals bypass the switching component of the device via a respective bypass path and are output by the bypass path. As shown, a 2×2 switching component  1234  is formed on a substrate  1220 . Switching component  1234  has first and second input ports,  1222  and  1224  respectively, that receive optical signals. Switching component  1234  also has first and second output ports,  1236  and  1238  respectively, that output the received optical signals depending on the state of switching component  1234 . When switching component  1234  is actuated to be in a first state, the optical signal received by first input port  1222  is output to second output port  1238  while the optical signal received on second input port  1224  is output to first output port  1236 . Conversely, when switching component  1234  is actuated to be in a second state, the optical signal received by first input port  1222  is output to first output port  1240 , while the optical signal received on second input port  1224  is output to second output port  1236 . Each port is formed as an optical waveguide fabricated on substrate  1220 .  
         [0054]    A first bypass path  1230  is also formed as an optical waveguide on substrate  1220 . A portion of first bypass path  1230  is formed close to first input port  1222  to create a first coupler  1226 . An actuation mechanism (illustrated by the arrow) is formed as part of coupler  1226 . Instead of being formed into a second coupler as with the embodiment of FIG. 1 b , bypass path  1230  is routed to the edge of substrate  1200  and acts as an exhaust port to output the optical signal.  
         [0055]    Similarly, a second bypass path  1232  is formed as an optical waveguide on substrate  1220 . A portion of second bypass path  1232  is formed close to second input port  1224  to create a second coupler  1228 . An actuation mechanism (illustrated by the arrow) is formed as part of coupler  1228 . Like bypass path  1230 , bypass path  1232  is routed to the edge of substrate  1200  and acts as an exhaust port to output the optical signal.  
         [0056]    The couplers  1226  and  1228  are designed and fabricated in the same manner as described with respect to FIG. 1 a  such that, in the absence of electrical power, the bypass paths  1230  and  1232  route optical signals around switching component  1234 . Thus, when electrical power is applied and an optical signal is received at first input port  1222 , the optical signal is not coupled to first bypass path  1230  and switching component  1234  operates as normal, outputting the optical signal to first output port  1236  or second output port  1238  depending on the state of switching component  1234 . In the absence of electrical power, however, an optical signal received at first input port  1222  is coupled into first bypass path  1230  by coupler  1226 , which acts as an exhaust port to output the optical signal. Likewise, when electrical power is applied and an optical signal is received at second input port  1224 , the optical signal is not coupled to second bypass path  1232  and switching component  1234  operates as normal, outputting the optical signal to first output port  1236  or second output port  1238  depending on the state of switching component  1234 . In the absence of electrical power, however, an optical signal received at second input port  1224  is coupled into second bypass path  1232  by coupler  1228 , which acts as an exhaust port to output the optical signal.  
         [0057]    [0057]FIG. 13 a - c  generally illustrate embodiments of a VOA in which, in the absence of electrical power, an input optical signal bypasses the attenuating component of the device via a bypass path, which acts as an exhaust port to output the optical signal. FIG. 13 a  illustrates an embodiment in which the attenuating component is based on a 1×1 design. FIG. 13 b  illustrates an embodiment in which the attenuating component is based on a 1×2 design. FIG. 13 c  illustrates an embodiment in which the attenuating component is based on a 2×2 design.  
         [0058]    In each embodiment, as shown, an attenuating component  1310  is formed on a substrate  1300 . Attenuating component  1310  has an input port  1302  that receives an optical signal and an output port  1308  that outputs the received optical signal after it is attenuated by attenuating component  1310 . Each port is formed as an optical waveguide fabricated on substrate  1300 .  
         [0059]    Similar to the embodiment of FIG. 12 a , a bypass path  1306  is also formed as an optical waveguide on substrate  1300 . A portion of bypass path  1306  is formed close to input port  1302  to create a coupler  1304 . An actuation mechanism (illustrated by the arrow) is formed as part of coupler  1304 .  
         [0060]    Coupler  1304  is designed and fabricated in the same manner as described with respect to FIG. 1 a , such that, in the absence of electrical power, bypass path  1304  routes optical signals around attenuating component  1310 . Thus, when electrical power is applied and an optical signal is received at input port  1302 , the optical signal is not coupled to bypass path  1306  and attenuating component  1310  operates as normal, attenuating the optical signal and outputting the attenuated signal to output port  1308 . In the absence of electrical power, however, an optical signal received at input port  1302  is coupled into bypass path  1306 , which outputs the optical signal.  
         [0061]    [0061]FIGS. 14 a - 17   b  illustrate specific embodiments of the switching or attenuating components to achieve the functions as described with respect to FIGS. 12 a - b  and  13   a - c.    
         [0062]    [0062]FIG. 14 a  illustrates an embodiment of the 1×2 designs of FIGS. 12 a  and  13   b  in which the switching or attenuating component is based on a Y-branch switch  1408 . When used as a VOA, only one input and one output is used. Similarly, FIG. 14 b  illustrates an embodiment of the 2×2 designs of FIGS. 12 b  and  13   c  in which the switching or attenuating component is based on a 2×2 DOS design  1434 . When used as a VOA, only one input and one output is used.  
         [0063]    [0063]FIG. 15 a  illustrates an embodiment of the 1×2 designs of FIGS. 12 a  and  13   b  in which the switching or attenuating component is based on a 1×2 directional coupler  1508 . When used as a VOA, only one input and one output is used. Similarly, FIG. 15 b  illustrates an embodiment of the 2×2 designs of FIGS. 12 b  and  13   c  in which the switching or attenuating component is based on a 2×2 directional coupler  1534 . When used as a VOA, only one input and one output is used.  
         [0064]    [0064]FIG. 16 a  illustrates an embodiment of the 1×2 designs of FIGS. 12 a  and  13   b  in which the switching or attenuating component is based on a 1×2 MMI coupler  1608 . When used as a VOA, only one input and one output is used. Similarly, FIG. 16 b  illustrates an embodiment of the 2×2 designs of FIGS. 12 b  and  13   c  in which the switching or attenuating component is based on a 2×2 MMI coupler  1634 . When used as a VOA, only one input and one output is used.  
         [0065]    [0065]FIG. 17 a  illustrates an embodiment of the 1×2 designs of FIGS. 12 a  and  13   b  in which the switching or attenuating component is based on a 1×2 Mach-Zehnder Interferometer (MZI)  1708 . When used as a VOA, only one input and one output is used. Similarly, FIG. 17 b  illustrates an embodiment of the 2×2 designs of FIGS. 12 b  and  13   c  in which the switching or attenuating component is based on a 2×2 Mach-Zehnder Interferometer  1734 . When used as a VOA, only one input and one output is used.  
         [0066]    Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.