Patent Publication Number: US-2018045893-A1

Title: Integrated optical switching and splitting for optical networks

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Patent Application Ser. No. 62/094,506, filed on Dec. 19, 2014, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The present invention is generally directed to optical transmission networks, and more particularly to systems that permit flexible configuration of optical components in the field. 
     Passive optical networks are becoming prevalent in part because service providers want to deliver high bandwidth communication capabilities to customers. Passive optical networks are a desirable choice for delivering high-speed communication data because they may not employ active electronic devices, such as amplifiers and repeaters, between a central office and a subscriber termination. The absence of active electronic devices may decrease network complexity and/or cost and may increase network reliability. 
       FIG. 1  illustrates one embodiment of a network  100  deploying fiber optic lines. In the illustrated embodiment, the network  100  can include a central office  101  that connects a number of end subscribers  105  (also called end users  105  herein) in a network. The central office  101  can additionally connect to a larger network such as the Internet (not shown) and a public switched telephone network (PSTN). The network  100  can also include fiber distribution hubs (FDHs)  103  that distribute signals to the end users  105 . The various lines of the network  100  can be aerial or housed within underground conduits. 
     The portion of the network  100  that is closest to central office  101  is generally referred to as the F 1  region, where F 1  is the “feeder fiber” from the central office  101 . The portion of the network  100  closest to the end users  105  can be referred to as an F 2  portion of network  100 . The network  100  includes a plurality of break-out locations  102  at which branch cables are separated out from the main cable lines. Branch cables are often connected to drop terminals  104  that include connector interfaces for facilitating coupling of the fibers of the branch cables to a plurality of different subscriber locations  105 . 
     FDHs receive signals fiber distribution hubs may include and input fiber that receives an incoming signal from the central office  101 . The incoming signal may then be split at the FDH  103 , using one or more optical splitters (e.g., 1×8 splitters, 1×16 splitters, or 1×32 splitters) to generate different user signals that are directed to the individual end users  105 . In typical applications, an optical splitter is provided prepackaged in an optical splitter module housing and provided with a splitter output in pigtails that extend from the module. The optical splitter module provides protective packaging for the optical splitter components in the housing and thus provides for easy handling for otherwise fragile splitter components. This modular approach allows optical splitter modules to be added incrementally to FDHs  103  as required. 
     The number of end users may change, however, for example through the addition of new customers to the network or by customers dropping out of the network, and so occasions arise where the splitter in the FDH  103  may need to be replaced. In the case where more customers are added to the network, a splitter may need to be replaced by one having more outputs, for example a 1×16 splitter may need replacing by a 1×32 splitter. In other situations, for example where the number of customers drops, it may be useful to replace a splitter with one having fewer outputs. The replacement of a splitter at an FDH  103  requires that a technician travel to the FDH  103  to physically swap out the splitter. This can be costly and time-consuming. Also, a technician visit may be necessary when taking other actions, such as switching over to more OLTs when the number of customers increases, or when switching users between different service levels, such as different bitrates or video channels. 
     Furthermore, the splitters that are conventionally used in optical networks are passive devices whose configuration cannot be changed, which can lead to difficulties in monitoring the performance of the optical network. For example, one way of tracking down the cause of a signal loss at one or more end users is to use optical time-domain reflectometry (OTDR), which involves transmitting a pulsed optical signal along the fiber. Breaks, cracks or other issues with the fiber can result in a portion of the optical pulse being reflected to the source of optical pulses. The arrival times of the reflected pulses can be recorded and the time-of-flight measurement can be correlated with the position in the fiber where the reflection occurred. If there is a problem with transmission of signals to a particular end user, a technician has to set up the OTDR equipment downstream of the splitter output in the FDH  103  in order to isolate the end user&#39;s fiber from other fibers. This requires that the technician travels to the FDH  103  and physically disconnects the end user&#39;s fiber from the splitter in order to initiate the OTDR measurements. Again, this can be costly and time-consuming 
     Therefore, there is a need for remote access to the FDH for changing the configuration of the splitter to add or drop fibers to end users, or to reconfigure the optical network to allow monitoring of one or more end users&#39; fibers. 
     SUMMARY 
     According to some embodiments of the invention, an optical device has a waveguide splitter cascade comprising at least first and second tiers of waveguide splitter nodes. Each waveguide splitter node has a respective input waveguide coupled to two respective output waveguides. At least one output waveguide of the first tier of waveguide splitters comprises an input waveguide of a waveguide splitter of the second tier of waveguide splitters. An active optical switch having two or more inputs and an output is connected as an input to one of the waveguide splitter nodes. 
     According to other embodiments of the invention, an active optical switch device includes a first switch waveguide, a second switch waveguide and a coupling region formed between the first and second switch waveguides for coupling light between the first and second switch waveguides. One or more microfluidic droplets are controllably movable relative to the coupling region to change the amount of light couplable between the first and second switch waveguides. A first configuration of the one or more microfluidic droplets corresponds to a minimum level of optical coupling between the first and second switch waveguides. A second configuration of the one or more microfluidic droplets corresponds to a maximum level of optical coupling between the first and second switch waveguides, while at least a third configuration of the one or more microfluidic droplets corresponds to at least an intermediate level of optical coupling between the first and second switch waveguides. 
     According to another embodiment of the invention, an active optical switch device includes a first switch waveguide supported on a first substrate that has a first optical circuit and a second switch waveguide supported on a second substrate that has a second optical circuit. A coupling region is formed between the first and second switch waveguides for coupling light between the first and second switch waveguides. One or more microfluidic droplets are controllably movable relative to the coupling region to change the amount of light couplable between the first and second switch waveguides. 
     According to another embodiment of the invention, an optical device includes a first active optical switch having a first waveguide and a second waveguide. A first coupling region is formed between the first and second waveguides for coupling light between the first and second waveguides. At least one microfluidic droplet is controllably movable relative to the first coupling region. The optical device also has a second active optical switch that includes the first waveguide and a third waveguide. A second coupling region is formed between the first and third waveguides for coupling light between the first and third waveguides. At least one microfluidic droplet is controllably movable relative to the second coupling region. The optical device also has a third active optical switch that includes the second waveguide and a fourth waveguide. A third coupling region is formed between the second and fourth waveguides for coupling light between the second and fourth waveguides. At least one microfluidic droplet is controllably movable relative to the third coupling region. 
     Other embodiments of the invention include an end user optical transceiver device that includes an input waveguide coupled to receive optical data transmitted from a fiber distribution hub and a transceiver unit coupled to receive an optical signal from the input waveguide. A second waveguide is coupled to a waveguide reflector that reflects light at a wavelength of light received at the input waveguide. An optical switch is located at a coupling region between the input and second waveguides to selectively switch light from the input waveguide to the waveguide reflector. 
     Other embodiments of the invention are directed to a switchable, wavelength-dependent optical device that includes a first waveguide couplable to receive an optical signal at at least a first wavelength and a second wavelength. A wavelength selective reflector on the first waveguide transmits light within the first waveguide at the first wavelength, in a first reflective state, reflects light within the waveguide at the second wavelength, and in a second reflective state transmits light within the waveguide at the second wavelength. A microfluidic arrangement is configured to control the reflective state of the wavelength selective reflector. 
     The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
         FIG. 1  schematically illustrates various elements of an optical data distribution and communication network; 
         FIG. 2  schematically illustrates an embodiment of elements of a fiber distribution hub according to an embodiment of the present invention; 
         FIGS. 3A-3D  schematically illustrate switched optical splitters according to embodiments of the present invention; 
         FIGS. 4A-4D  schematically illustrate switched optical circuits according to additional embodiments of the present invention; 
         FIGS. 5A-5B  schematically illustrate an optical switch according to an embodiment of the present invention; 
         FIGS. 6A-6C  schematically illustrate an optical switch according to another embodiment of the present invention; 
         FIGS. 7A-7D  schematically illustrate an optical switch according to another embodiment of the present invention; 
         FIGS. 8A-8D  schematically illustrate an optical switch according to another embodiment of the present invention; 
         FIGS. 9A-9B  schematically illustrate an optical switch according to another embodiment of the present invention; 
         FIGS. 10A-10B  schematically illustrate an optical switch according to another embodiment of the present invention; 
         FIG. 11  schematically illustrates an optical circuit that includes two optical switches in series according to an embodiment of the present invention; 
         FIGS. 12A-12B  schematically illustrate an optical circuit that includes two optical switches in series according to another embodiment of the present invention; 
         FIGS. 13A-13B  schematically illustrate a two layer optical circuit according to another embodiment of the present invention; 
         FIG. 14  schematically illustrates part of an optical network using optical switches according to an embodiment of the present invention; 
         FIGS. 15A-15B  schematically illustrate a wavelength selective switch according to an embodiment of the present invention; and 
         FIGS. 16A-16B  schematically illustrate a wavelength selective switch according to another embodiment of the present invention. 
     
    
    
     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     The present invention is directed to various optical devices and systems that can provide benefit in optical networks by providing for remote configuration, thus reducing the need for technician visits to a fiber distribution hub (FDH) and allowing various operations to be carried out more quickly than using conventional passive optical components. 
     In an illustrated embodiment of the invention, the optical network  100  includes a cable  110  that connects to an FDH  103 . The cable  110  includes at least an optical data transmission fiber and an FDH control channel, which may be optical or electrical. 
     An illustrated embodiment of the FDH  103  and cable  110  are seen in greater detail in  FIG. 2 . The cable  110  entering the FDH  103  includes an optical data channel  212  and an FDH control channel  214 . The optical data channel is typically one or more optical fibers and may include optical data transmission, such as cable television signals which are typically unidirectional in the fiber, and optical communications, for example internet traffic which are typically bidirectional in the fiber. The control channel  214  provides a control signal to the optical circuit  216  located within the FDH  103 . The optical circuit  216  contains any optical elements that are used in the FDH  103  to distribute an optical signal to the end users  105 . For example, the optical circuit may contain one or more optical splitters, optical switches, amplifiers, optical circulators, multiplexers, and other such elements that are typically used in optical data transmission networks. In the illustrated embodiment, the optical circuit  216  includes one or more splitters so that the optical signal is split into a number of different output channels  218  that are fed to end users  105 . The output channels  218  may be optical channels, such as optical fibers, or may be electrical channels, for example coaxial electrical cables. In the case where the output channels  218  are electrical channels, the optical circuit  216  may also include optical-electrical converters for data transmission. 
     According to an embodiment of the present invention the optical circuit  216  includes one or more remotely-controlled active optical elements that may be used, for example, to change the configuration of the optical circuit or the ratio of signal split into different output channels. Different approaches may be used to provide active control of the optical signals within the optical circuit  216  including, for example microfluidic, micromechanical (e.g. MEMS), and electro-optic. An advantage of the microfluidic and micromechanical approaches over an electro-optical approach is that a microfluidically-controlled optical circuit can be manufactured on a glass substrate, which is relatively inexpensive, whereas the electro-optical approach requires the use of electro-optic crystals are more expensive than glass. A remotely-controllable optical circuit (RCOC) may, for example include one or more switches that can change a splitter from a configuration having a first number of outputs to a splitter having a second number of outputs. In another example of an RCOC, an optical switch is able to provide multiple levels of coupling between two waveguides, thus allowing a user to control the amount of light that is coupled from one waveguide into one or more other waveguides. The following description provides some examples of RCOCs that may be incorporated in an FDH. 
     A first exemplary embodiment of an RCOC  300  is schematically illustrated in  FIG. 3A . The RCOC  300  includes first and second input waveguides  302   a  and  302   b,  labeled Input  1  and Input  2  respectively, and has eight outputs  304 , labeled Output  1 -Output  8 . Outputs  1 - 4  are directly connected to Input  1  via a first splitting network  306  and Outputs  5 - 8  are directly connected to Input  2  via a second splitting network  308 . In many cases, the splitting networks  306 ,  308  may include a number of symmetric splitting nodes  310  that split the input optical power into two equally powered outputs, although this need not be the case and some of the splitting nodes  310  may be asymmetric, with more of the incoming optical power being directed to one of its outputs than the other. A splitting node  310  includes one input waveguide that splits into two output waveguides. 
     An optical switch  312  is positioned to allow coupling of light between Input  1  and Input  2 . The optical switch may be, for example, microfluidic, micromechanical or electro-optical. In some embodiments the optical switch  312  may be adjustable between only two switching states and in other embodiments the optical switch may be adjustable over a number of switching states. The term “switching state” refers to the amount of light coupled between waveguide  302   a  and  302   b  in the switch  312 . Thus, in a first switching state a first amount of light is coupled between the waveguides  302   a,    302   b.  In a second switching state, a second amount of light is coupled between the waveguides. In some cases, a two-state optical switch will have a “bar state”, in which non light is coupled between the waveguides and a “cross state” in which approximately 100% of the light is coupled between the waveguides. A control signal may be applied to the switch  312  via a control channel  314 . The control signal on control channel  314  may be optical or electrical. In the illustrated embodiment the splitting nodes  310  are arranged in two tiers, with the first tier including the nodes  310  immediately following the switch  312 , splitting from two to four waveguides and a second tier of nodes  310  splitting from four to eight waveguides. 
     In one configuration of the RCOC  300 , schematically illustrated in  FIG. 3B , the switch  312  is in the so-called “bar state,” in which 100% of the light traveling along a waveguide remains in that waveguide and 0% of the light is coupled into the other waveguide. In  FIGS. 3B-3D , the two numbers below the switch correspond to the percent amount transmitted along a waveguide without coupling and to the percent amount coupled between the waveguides. Thus, the number 100/0 means that 100% of the light is transmitted along the respective waveguide and no light is coupled to the other waveguide. Thus, in the embodiment illustrated in  FIG. 3B  substantially all the light entering Input  1  is transmitted to Outputs  1 - 4  and substantially all the light entering Input  2  is directed to Outputs  5 - 8 . In this embodiment it has been assumed that all the splitter nodes  310  are symmetrical, so the light from Input  1  is split equally among outputs  1 - 4  and the light from Input  2  is split equally among Outputs  5 - 8 . Of course, it will be appreciated that one or more of the splitter nodes  310  may be asymmetrical in this embodiment or in other embodiments discussed below. 
     In another configuration of the RCOC  300 , schematically illustrated in  FIG. 3C , the switch  312  is in a so-called “cross-state,” in which 100% of the light travelling along a waveguide is coupled to the other waveguide. This configuration may be referred to as 0/100. Thus, substantially all the light entering Input  1  is coupled over to the second splitting network  308  and transmitted to Outputs  5 - 8 , while substantially all the light entering Input  2  is coupled over to the first splitting network  306  and transmitted to Outputs  1 - 4 . 
     In another configuration of the RCOC  300 , schematically illustrated in  FIG. 3D , the switch  312  is in an intermediate state, in which some light is transmitted along the waveguide and some is coupled to the other waveguide. In the illustrated embodiment 50% of the light travelling along a waveguide is coupled to the other waveguide. This configuration may be referred to as 50/50. Thus, 50% of the light entering Input  1  is transmitted to the first splitting network  306  and 50% is coupled over to the second splitting network  308  and transmitted to Outputs  5 - 8 . If no signal is applied to Input  2 , then Outputs  1 - 8  all transmit a fraction of the signal applied to Input  1 . Where the splitting nodes  310  are all symmetrical, each output contains about 12.5% of the light signal applied at Input  1  (ignoring losses). In this 50/50 configuration, the RCOC  300  acts as a 2×8 splitter, whereas the RCOC  300  configurations shown in  FIGS. 3B and 3C  act as two 1×4 splitters. 
     Input  2  may also be used to inject a signal into the RCOC  300 , for example at a different wavelength from that injected into input  1 , as might be used for the simultaneous transmission of a video signal and a data signal or a network test signal. In the illustrated embodiment, with the switch  312  in a 50/50 configuration, light injected at input  2 , as well as light injected at input  1 , is spread evenly among all outputs. 
     Another exemplary embodiment of a RCOC  400  is schematically illustrated in  FIG. 4A . The RCOC  400  has four inputs  402 , labelled Input  1 -Input  4  and four outputs  404 , labelled Output  1 -Output  4 . Output  1  is directly connected along a waveguide  406 - 1  to Input  1 , and likewise Outputs  404  are directly connected along waveguides  406 - 2 - 406 - 4  to Inputs  2 - 4 , respectively. In this embodiment, a first optical switch  412   a  is positioned to couple light between the second and third waveguides  406 - 2  and  406 - 3 . Also, downstream of the first optical switch  412   a,  a second optical switch  412   b  is positioned to couple light between the first and second waveguides  406 - 1  and  406 - 2 , and a third optical switch  412   c  is positioned to couple light between the third and fourth waveguides  406 - 3  and  406 - 4 . 
     The configurations of the switches  412   a - 412   c  may be selected to determine different output conditions from the RCOC  400 . For example, if all three switches  412   a - 412   c  are in the 100/0 configuration, then no light is coupled from one waveguide to another, and so the signal at Output  1  is simply the input signal at Input  1 , and the signals at Outputs  2 - 4  are the respective signals at Inputs  2 - 4 . 
     In another configuration, schematically illustrated in  FIG. 4B  the switches  412   a - 412   c  are set as 50/50 switches. Thus a signal at Input  2  is split equally between the second and third waveguides  406 - 2  and  406 - 3 . The signal propagating along the second waveguide  406 - 2  to the second switch  412   b  is again split equally into two signals respectively propagating along waveguides  406 - 1  and  406 - 2  to Outputs  1  and  2 . Likewise, the signal propagating along the third waveguide  406 - 3  to the third switch  412   c  is split equally into two signals respectively propagating along waveguides  406 - 3  and  406 - 4  to Outputs  3  and  4 . Thus, the reconfiguration of the switches  412   a - 412   c  as 50/50 switches results in the RCOC  400  operating as a 1×4 splitter, splitting the signal at Input  2  equally into four output signals at Outputs  1 - 4 . It will be appreciated that the switch configuration illustrated in  FIG. 4B  will also lead to the RCOC operating as a 1×4 splitter for signals applied at Input  3 . 
     In some configurations there may be no signal on Inputs  1 ,  3  and/or  4 . In other configurations, there may be input signals present at any combination of Inputs  1 - 4 . For example, a configuration with an input signal applied to at Input  2  and an input signal applied to Inputs  1  and/or  4 , may be useful where the signal on Input  2  can undergo more splitting than the signal(s) on Inputs  1  and/or  4 , e.g. a video broadcast signal at 1550 nm is input at Input  2  and data signals at 1310 nm/1490 nm are input to Inputs  1  and/or  4 . 
     Another configuration is schematically illustrated in  FIG. 4C . This configuration is similar to that described above for  FIG. 4B , except that the first switch  412   a  is set as a 60/40 switch, with the result that Outputs  1  and  2  each transmit a signal of 30% of the signal at Input  2 , whereas Outputs  3  and  4  each transmit a signal of 20% of the signal at Input  2 . Such a configuration may be useful, for example, where the end users who receive signals from Outputs  1  and  2  are located at a greater distance from the FDH than the users who receive signals from Outputs  3  and  4 , and are therefore subject to greater signal transmission losses. It will be appreciated that the switching ratios of each of the switches  412   a - 412   c  may be adjusted to achieve any particular desired balance in the magnitude of signals at Outputs  1 - 4 , for example to account for downstream transmission losses in a situation where it is desirable for each end user to receive a signal of the same magnitude. 
     In another configuration, schematically illustrated in  FIG. 4D , the first switch  412   a  is set as a 33.3/66.7 switch, so that one third of the signal at Input  2  propagates along the second waveguide  406 - 2  to the second switch  412   b  and two thirds of the signal propagates along the third waveguide  406 - 3  to the third switch  412   c.  The third switch  412   c  is set as a 50/50 switch, so that half of the signal entering the third switch  412   c  is transmitted to Output  3  and the other half is transmitted to Output  4 . The second switch  412   b  is set as a 100/0 switch, so that all of the signal reaching the second switch  412   b  from the first switch  412   a  is transmitted to Output  2 . In this manner, the signals at Outputs  2 - 4  are each ⅓ of the signal input at Input  2 . Thus, in this configuration, the RCOC  400  operates as a 1×3 splitter. Since the second switch  412   b  is set as a 100/0 switch, all of the signal at Input  1  propagates to Output  2 , thus permitting point-to-point communications between a first user coupled to Input  1  and a second user coupled to Output  1 . 
     It will be appreciated that the RCOC may operate differently under other configurations of the switches. For example, in the case where the first switch  412   a  is set as a 100/0 switch and the second and third switches  412   b,    412   c  are each set as 50/50 switches, the RCOC operates as two 1×2 splitters. Thus, Outputs  1  and  2  will each receive 50% of a signal input at Input  1  (or Input  2 ) while Outputs  3  and  4  will each receive 50% of the signal at Input  3  (or Input  4 ). Furthermore, the second and third switches  412   b  and  412   c  may be set to provide for asymmetric division of optical signals. For example, the second switch  412   b  may be set for a 33.3/66.7 switching ratio. 
     One approach to implementing an optical switch having multiple switching states is to use a microfluidic optical switch. Microfluidic switches are generally based on changing the effective refractive index experienced by light propagating within a waveguide. This can be achieved, for example, by moving a droplet of liquid of a first refractive index liquid surrounded by a liquid of a second refractive index liquid in microfluid channels disposed close to waveguides. Examples of microfluidic switches are described in C. Lerma Arce et al. “Silicon Photonic Sensors Incorporated in a Digital Microfluidic System,” Analytical and Bioanalytical Chemistry, 404(10) 2887-94 (2012) and C. Lerma Arce, PhD Thesis: “Novel Microfluidic Platforms Incorporating Photonic Ring Resonator Sensors,” Photonics Research Group, INTEC University of Gent, 2014, and U.S. Pat. No. 7,283,696, incorporated herein by reference. The microfluidic change in the effective refractive index can affect the coupling coefficient between a waveguide along which the light is propagating and a neighboring waveguide. Thus, it is possible to microfluidically control the coupling coefficient and, therefore, the amount of light propagating along the two waveguides. 
     One embodiment of a multiple state, microfluidic optical switch is schematically illustrated in  FIGS. 5A-B . Such a switch is capable of switching among more than two, i.e. it has a minimum coupling state, a maximum coupling and one or more intermediate coupling states.  FIG. 5A  schematically shows a microfluidic switch  500  formed on a substrate  502  having a first waveguide  504  and a second waveguide  506 . A coupling region  508  is a region where the first and second waveguides  504 ,  506  are spaced closely together to permit optical coupling between the waveguides  504 ,  506 . In the illustrated embodiment, light propagates along the first waveguide in the direction shown by the arrows, entering the switch at the input and exiting the switch at the outputs. According to the coordinate system of the figure, the light propagates along the y-direction. In this embodiment, the switch  500  includes four activatable microfluidic droplets  510 , labelled  510   a,    510   b,    510   c  and  510   d,  near the coupling region  508 . The four droplets  510  are independently movable in the ±x direction. When a droplet  510  is moved in a position above the waveguides  504  and  506 , the coupling coefficient is changed so that a fraction of the light propagating in the first waveguide  504  is coupled into the second waveguide  506 . In this embodiment the droplets  510  can have one of two positions, namely i) away from the coupling portion, for example as shown for droplets  510   c  and  510   d,  in a position that does not contribute to the coupling coefficient, and ii) over the coupling portion, as is shown for droplets  510   a  and  510   b,  in a position that does contribute to the coupling coefficient. Thus, in the droplet configuration illustrated in  FIG. 5A , only droplets  510   a  and  510   b  affect coupling of light from the first waveguide  504  to the second waveguide  506 . 
     The amount by which a droplet  510  affects the coupling coefficient can depend on a number of different factors including the size of the droplet and the magnitude of the change in the effective refractive index experienced by light in the waveguide. When the droplet is larger, for example when it extends further along the waveguide in the y-direction, the coupling coefficient is increased. The change in the effective refractive index of the waveguide is dependent on the refractive index of the droplet  510 . Generally, when the difference between the refractive indices of the droplet  510  and the waveguides is smaller, the coupling coefficient increases. 
     In one example of the embodiment illustrated in  FIG. 5A , each droplet  510  has the same effect on coupling coefficient, and increases the coupling coefficient by 25%. Thus, for each droplet  510  positioned in the coupling region  508  to couple light between the waveguides  504 ,  506 , the amount of light coupled from the first waveguide  504  to the second waveguide  506  is increased by 25%. In the illustrated embodiment two droplets  510   a,    510   b  are positioned in the coupling region  508  to couple light between the waveguides  504 ,  506 , so 50% of the light is coupled from the first waveguide  504  to the second waveguide  506 . In another droplet configuration, schematically illustrated in  FIG. 5B , three droplets  510   a,    510   b  and  510   d  are positioned in the coupling region  508  the coupling region to couple light between the waveguides  504 ,  506 . In this case, 75% of the light is coupled from the first waveguide  504  into the second waveguide  506 , with 25% of the light propagating in the first waveguide  504  beyond the coupling region. 
     While the waveguides  504 ,  506  are shown in  FIGS. 5A-B  are shown sitting on top of the substrate  502 , it is not intended that this be a limitation of the invention herein. The waveguides of this and other embodiments may be formed in any conventional manner, including growing the waveguides on a substrate or in the substrate via diffusion or implantation or other suitable technique. Thus, waveguides may be formed on and/or in a substrate. 
     An advantage of optical microfluidic switched optical circuits discussed herein is that a control signal need only be applied to change a switch state, to move the droplet from one position to another but need not be continually applied to maintain the switch in that state. The microfluidic switches can persist in a selected state after being switched to that state without continued application of the control signal, since an activation signal is only required to move a droplet from one position to another. Once a droplet has reached a desired position, it remains in that position until another activation signal is applied to remove it. Microfluidic droplets can typically be moved using electrostatic or hydrostatic forces. 
     It will be understood that the droplets need not all contribute the same amount of coupling, and different droplets may contribute respectively different amounts of coupling. The amount of coupling contributed by each droplet may be selected so that the user can select a number of different coupling values. For example, a first droplet may provide 6.25% coupling between the waveguides, while a second droplet provides 12.5% coupling, a third droplet provides 25% coupling and a fourth droplet provides 50% coupling. Various arrangements of these four droplets will provide up to  16  different values of coupling. To illustrate, in another exemplary embodiment, schematically shown in  FIG. 6A , a first waveguide  604  is located on a substrate  602  of a microfluidic optical switch  600 , along with a second waveguide  606 . A coupling region  608  is provided where optical coupling between the waveguides  604 ,  606  may take place. The first droplet  610   a  provides 6.25% coupling between the two waveguides  604 ,  606 , the second droplet  610   b  provides 12.5% coupling between the two waveguides  604 ,  606 , the third droplet  610   c  provides 25% coupling between the two waveguides  604 ,  606  and the fourth droplet  610   d  provides 50% coupling between the two waveguides. Thus, in the droplet configuration illustrated in  FIG. 6A , droplets  610   a  and  610   b  are in position to optically couple light between the waveguides  604  and  606 , and so 18.75% (12.5%+6.25%) of the light is coupled from the first waveguide  604  to the second waveguide  606 , and 81.25% is left to propagate along the first waveguide  604 . In the droplet configuration illustrated in  FIG. 6B , the first, second and third droplets  610   a,    610   b  and  610   c  are in position to couple light between the waveguides  604 ,  606 . In this case, 43.75% (6.25%+12.5%+25%) of the light is coupled into the second waveguide  606  from the first waveguide  604 , with 56.25% of the light remaining in the first waveguide  604 . In another droplet configuration illustrated in  FIG. 6C , the first, second and fourth droplets  610   a,    610   b  and  610   d  are in position to couple light between the waveguides  604 ,  606 . In this case, 68.75% (6.25%+12.5%+50%) of the light is coupled into the second waveguide  606  from the first waveguide  604 . It will be appreciated that other configurations will result in different amounts of light being coupled from the first waveguide  604  to the second waveguide  606 . It will further be appreciated that different numbers of droplets may be incorporated in a multi-state switch and that amount of coupling attributable to each droplet may be selected to have different values from those discussed in the example above. 
     The description of optical switches herein ignores optical losses due to, for example, impurities, fabrication errors and the like. Accordingly, values of light transmission, coupling etc. given as a percentage or fraction should be understood to cover an ideal embodiment, while actual devices may not operate with the same values are exemplified herein. In illustration, the droplets in a real device of the above embodiment may not produce the exact same values of coupling as described, which are provided for illustration purposes only, but may operate within an approximate range of these values. 
     In another approach to a multi-state optical switch, a microfluidic droplet may be controllably moved to one of several different positions relative to the coupling region between two waveguides, resulting in respectively different levels of coupling when the droplet is in the different positions. One embodiment of such an optical switch is schematically illustrated in FIGS. 7 A- 7 D. The switch includes a first waveguide  704  and a second waveguide  706 . Portions of the first and second waveguides  704 ,  706  are positioned closely together to form a coupling portion  708  where light is coupled between the waveguides  704 , 706 . In this embodiment, the droplet  710  is moved in a transverse direction across the waveguides  704 ,  706 . In  FIG. 7A  the droplet  710  is in a first position removed from the coupling portion  708  so that no coupling takes place between the waveguides  704 ,  706 . In  FIG. 7B  the droplet  710  is in a second position closer to the coupling portion  708  than the first position to couple a first amount of light from the first waveguide  704  to the second waveguide  706 . In the illustration, 30% of the light in the first waveguide  704  is coupled to the second waveguide  706  when the droplet  710  is in the second position. In  FIG. 7C  the droplet  710  is in a third position closer to the coupling portion  708  than the second position to couple a second amount of light from the first waveguide  704  to the second waveguide  706 , that is larger than the first amount of light. In the illustration, 60% of the light in the first waveguide  704  is coupled to the second waveguide  706  when the droplet  710  is in the third position, leaving 40% of the light in the first waveguide  704 . In  FIG. 7D  the droplet  710  is in a fourth position closer to the coupling portion  708  than the third position to couple a third amount of light from the first waveguide  704  to the second waveguide  706 . In the illustration, 100% of the light in the first waveguide  704  is coupled to the second waveguide  706  when the droplet  710  is in the fourth position, leaving no light in the first waveguide  704 . 
     In a variation of the embodiment shown in  FIGS. 7A-7D , the droplet  710  may have a refractive index that is non-uniform over the range of light wavelengths that pass along the waveguides. For example, the refractive index of the fluid may be tailored using an additive such as semiconductor quantum dots or the like. Thus, the switch may be able to demonstrate a wavelength-dependent switching ability, and be able to couple light at a first wavelength relatively strongly while coupling light at a second wavelength either relatively weakly, if not at a zero level. Such a switch is referred to as a wavelength-dependent microfluidic switch. It will be appreciated that such wavelength dependence may be included into the other embodiments of microfluidic switch, and optical circuits including such switches, described herein. 
     Another embodiment of an optical switch having a multiple switching states is schematically illustrated in  FIGS. 8A-8D . The switch includes a first waveguide  804  and a second waveguide  806 . Portions of the first and second waveguides  804 ,  806  are positioned closely together to form a coupling portion  808  where light is coupled between the waveguides  804 ,  806 . In this embodiment, the droplet  810  is moved in a longitudinal direction approximately across the waveguides  804 ,  806 . In  FIG. 8A  the droplet  810  is in a first position removed from the coupling portion  808  so that no coupling takes place between the waveguides  804 ,  806 . In  FIG. 8B  the droplet  810  is in a second position closer to the coupling portion  808  than the first position to couple a first amount of light from the first waveguide  804  to the second waveguide  806 . In the illustration, 30% of the light in the first waveguide  804  is coupled to the second waveguide  806  when the droplet  810  is in the second position. In  FIG. 8C  the droplet  810  is in a third position closer to the coupling portion  808  than the second position to couple a second amount of light from the first waveguide  804  to the second waveguide  806 . In the illustration, 60% of the light in the first waveguide  804  is coupled to the second waveguide  806  when the droplet  810  is in the third position, leaving 40% of the light in the first waveguide  804 . In  FIG. 8D  the droplet  810  is in a fourth position closer to the coupling portion  808  than the third position to couple a third amount of light from the first waveguide  804  to the second waveguide  806 . In the illustration, 100% of the light in the first waveguide  804  is coupled to the second waveguide  806  when the droplet  810  is in the fourth position, leaving no light in the first waveguide  804 . 
     Another approach to changing the effective refractive index experienced by light passing through an optical switch is now discussed with regard to the embodiment schematically illustrated in  FIGS. 9A and 9B . In this embodiment, a substrate supports a first waveguide  904  and a second waveguide  906 . The two waveguides  904 ,  906  are located closely to one another to form a coupling region  908  where optical coupling can take place between the waveguides  904 ,  906 , depending on the effective refractive index experienced by light propagating along the waveguides  904 ,  906 . 
     A capsule  910  above the coupling region  908  contains particles  912  in a fluid  914 . The particles  912  have a different refractive index from the fluid  914  and are moveable within the fluid under the application of an external force. For example, the particles  912  may be magnetic and, therefore, moveable under the application of a magnetic field. Magnetic particles  912  may include a magnetic core that is coated with another material or may include a magnetic component that is attached to another component, for example a relatively small magnetic component may be attached to a relatively large nonmagnetic component that has a refractive index different from the refractive index of the fluid. 
     An external magnetic field applied to the capsule  910  can result in the particles  912  moving within the fluid  914 . The magnetic field source  916  may be placed under the substrate  902 , as illustrated, or may be placed above the substrate  902  if it is desired to move the particles in a direction perpendicular to the substrate  902 . In  FIG. 9A  the particles  912  are located towards the top of the capsule  910 , so that they have little effect on the effective refractive index in the coupling region  908 . This may be termed a first coupling state. In  FIG. 9B  the particles are located towards the bottom of the capsule  910 , so as to affect the effective refractive index of the coupling region. This may be termed a second coupling state. In some embodiments the refractive indices of the fluid  914  and particles  912  may be selected so that the first coupling state is a bar state, in other words, light propagating along the first waveguide  904  is not coupled to the second waveguide  906 , and appears at the first waveguide output  918 . In these embodiments, the second coupling state may be a cross state, in which light propagating along the first waveguide  904  is coupled to the second waveguide  906 . Depending on the degree of coupling at the coupling region  908 , all the light may be coupled to the second waveguide  906  or a certain fraction of the light may be coupled from the first waveguide  904  to the second waveguide  906 . In other embodiments, the refractive indices of the fluid  914  and particles  912  may be selected so that the first coupling state is a cross state, in other words, light propagating along the first waveguide  904  is coupled to the second waveguide  906 . Depending on the degree of coupling at the coupling region  908 , all the light may be coupled to the second waveguide  906  or a certain fraction of the light may be coupled from the first waveguide  904  to the second waveguide  906 , with the remainder appearing at the first waveguide output  918 . In these embodiments, the second coupling state may be a bar state, in which light propagating along the first waveguide  904  is not coupled to the second waveguide  906  and appears at the first waveguide output  918 . 
     It will be appreciated that the particles  912  need not be restricted to moving only in a vertical direction, perpendicular to the substrate  902 , to change the amount of optical coupling between the first and second waveguides  904 ,  906  at the coupling region  908 . In other embodiments, the particles  912  may be moved transverse across the waveguides  904 ,  906  or along the waveguides  904 ,  906  to affect the amount of optical coupling at the coupling region  908 . The latter case, where the particles  912  are moved along the waveguides  904 ,  906 , is illustrated in  FIGS. 10 a    and  10 B. In  FIG. 10A  the particles  912  are located in a position removed from the coupling region and so have little effect on the effective refractive index experienced at the coupling region  908 . In  FIG. 10B , the particles  912  are located above the coupling region  908  and do affect the effective refractive index at the coupling region. 
     An embodiment of an optical circuit  1100  that includes more than one optical switch placed along a waveguide is schematically illustrated in  FIG. 11 . The circuit  1100  has a substrate  1102  which supports a first waveguide  1104 . A first switch  1110   a  is formed at a first coupling region  1108   a  between the first waveguide  1104  and a second waveguide  1106   a.  A second switch  1110   b  is formed at a second coupling region  1108   b  between the first waveguide  1104  and a third waveguide  1106   b.  In the illustrated embodiment, the first and second switches  1110   a,    1110   b  are shown with a fluid droplet  1112   a,    1112   b  over the respective coupling regions  1108   a,    1108   b.  It will be appreciated that any of the different types of optical switch discussed above may be used as the switches  1110   a,    1110   b,  including embodiments of optical switch that permit various levels of optical coupling between two waveguides. 
     In operation, the optical circuit  1100  may be used to generate three output different signals having selected power levels. In illustration, consider that an optical signal is input to the first waveguide  1104  at the input  1114 . If the first switch  1110   a  is set to couple a fraction X (between 0 and 1) of the input light, I 0 , into the second waveguide  1106   a,  then the amount of light passing to the second switch  1110   b  is (1−X) I 0 , while XI 0  passes out of the second waveguide output  1116   a.  If the second switch  1110   b  is set to couple a fraction Y(between 0 and 1) of the passing light into the third waveguide  1106   b,  then Y(1−X) I 0  is coupled out of the third waveguide output  1116   b,  and (1−X)(1−Y)I 0  passes to the output  1118  of the first waveguide  1104 . It will be appreciated that, according to some of the switch embodiments discussed above, different values of X and Y may be selected so that desired relative amounts of light are obtained at the outputs. 
     Additionally, it will be appreciated that additional switches may be used in the circuit  1100 , for example switches may be added on the second or third waveguides  1106   a,    1106   b  to couple light into additional waveguides, or may be positioned on the first waveguide  1004  downstream of the second switch  1110   b.    
     Another embodiment of an optical circuit arrangement that arrangement that permits the switching of light from a single to multiple waveguides is schematically illustrated in  FIGS. 12A and 12B . This embodiment shows an approach to switching light out of one waveguide into two waveguides, but this approach can be extended to cover switching into more than two waveguides. A substrate  1202  supports a first waveguide  1204 . Second and third waveguides  1206   a,    1206   b  are positioned so as to form coupling regions  1208   a,    1208   b  with the first waveguide  1204 . In this embodiment, a number of droplets  1210  are positionable over the coupling regions  1208   a,    1208   b.  The positions of the droplets  1210  relative to the coupling regions  1208   a,    1208   b  can affect the effective refractive index at the coupling regions  1208   a,    1208   b,  resulting in coupling light from the first waveguide  1204  to the second and third waveguides  1206   a,    1206   b.  The droplets  1210  may be moved in position, for example as shown in  FIGS. 12A and 12B , in a manner that changes the amount of optical coupling at the coupling regions  1208   a,    1208   b.  In some embodiments, the refractive index of one or more droplets  1210  may be different from the refractive indices of other droplets, which may result in a greater change in the amount of optical coupling when the position of the droplets  1210  is changed. Thus, in the configuration shown in  FIG. 12A , if an amount of light I 0  enters the first waveguide input  1212 , and the light exiting the second waveguide output  1214   a  is XL while the light exiting the third waveguide output  1214   b  is YI 0 , then the amount of light leaving the first waveguide output  1216  is (1−X−Y)I 0 , where (X+Y)≦1. In the configuration shown in  FIG. 12B , the amount of light exiting the second and third waveguide outputs  1214   a,    1214   b  is X′I 0  and Y′I 0  and the amount of light exiting the first waveguide output  1216  is (1−X′−Y′)I 0 , where (X′+Y′)≦1 and X≠X′ and Y≠Y′. The values of X, Y and X′, Y′ are selectable by changing the position of the droplets  1210 . 
     In the embodiments discussed so far, light is switched between waveguides present on the same substrate. In other embodiments, light may be switched from a first waveguide on a first substrate to a second waveguide on a second substrate. An optical circuit that uses more than one substrate may be useful in reducing the footprint required to achieve certain optical functions. For example, as is schematically illustrated in  FIGS. 13A and 13B , an optical circuit  1300  may include first and second substrates  1302   a,    1302   b,  with the first substrate  1302   a  containing circuit A  1312  and the second substrate  1302   b  containing circuit B  1314 . The first substrate  1302   a  supports a first waveguide  1304  connected to circuit A  1312  and the second substrate  1302   b  supports a second waveguide  1306  connected to circuit B  1314 . A coupling region  1308  is formed between the first and second waveguides  1304 ,  1306  and an optical switch  1310  formed at the coupling region  1308  allows light to be coupled between the first and second waveguides  1304 ,  1306 . The optical switch  1310  may include any suitable embodiment of optical switch discussed hereon. In the illustrated embodiment, a single fluid droplet  1311  is used for switching. 
     Although the figures show that the first waveguide  1304  crosses the second waveguide  1306 , this is not a necessary requirement, and is shown only for clarity. It will be appreciated that the coupling region  1308  may be formed between regions of the waveguides  1304 ,  1306  that are substantially parallel, in a manner similar to certain of the embodiments discussed earlier. Any suitable arrangement of the waveguides  1304 ,  1306  may be used to form the coupling region  1308 . However, in this embodiment, when light is coupled between the first waveguide  1304  and the second waveguide  1306  the light passes from a waveguide on one substrate to a waveguide on another substrate. 
     If light is input to the circuit  1300  via the first waveguide  1304 , then light can pass to circuit A  1312  if the optical switch  1310  is in the bar state, as illustrated in  FIG. 13A , or to circuit B  1314  if the optical switch  1310  is in the cross state. It will be appreciated that the switch  1310  may permit light to pass to both circuits A and B  1312 ,  1314  if the switch  1310  couples a fraction of the incoming light to the second waveguide  1306  while at the same time permitting some of the light to pass along the first waveguide  1304 . 
     The switches and optical circuits discussed above may be used in the FDH. In other applications, switches may be used elsewhere, such as at the end user&#39;s location. For example, an end user&#39;s location may be supplied with not only a transceiver for receiving and sending optical signals, it may also be provided with an optical time domain reflectometry (OTDR) facility that permits the operator to test the optical fiber all the way up to the individual end user. One example of an implementation of this is schematically illustrated in  FIG. 14 , which shows three different transceivers  1402 ,  1404  and  1406  coupled to respective waveguides  1408   a,    1408   b,    1408   c.  Each transceiver  1402 ,  1404 ,  1406  may be at a different end user&#39;s location. The input end  1410   a,    1410   b,    1410   c  of each waveguide  1408   a - c  may be coupled to separate optical fibers from an FDH. Each waveguide  1408   a - c  is provided with a respective optical switch  1412   a,    1514   b,    1412   c  that includes a second waveguide  1414   a,    1414   b,    1414   c  that form coupling regions  1416   a,    1416   b,    1416   c  where light can be coupled between the waveguides  1408   a - c  and respective second waveguides  1414   a - c.  The switches  1412   a - c  may be activated in a manner as discussed above, for example using one or more microfluidic droplets. In the illustrated embodiment the switches  1412  are activated by a microfluidic droplet  1418   a,    1418   b,    1418   c.  The second waveguides  1414   a - c  are provided with OTDR reflectors  1420   a,    1420   b,    1420   c  that reflect light at the wavelength received from the FDH or Central Office, for example distributed Bragg reflectors (DBRs) or the like. 
     When the switches  1410   a - c  are in the bar state, no incoming light is coupled to the OTDR reflector, and the light is detected by the transceivers  1402 ,  1404 ,  1406 . If a switch is set to the cross state, light is coupled to the OTDR reflector associated with that switch and a large portion of the incoming light is reflected back to the FDH or Central Office. Also, the amount of light reaching the transceiver is reduced, perhaps almost to zero if the coupling is very high in the optical switch. Such a situation is schematically illustrated in  FIG. 14 . The switches  1412   a,    1412   b  associated with the first and second transceivers  1402 ,  1404  respectively are in the bar state, since the droplets  1418   a,    1418   b  are removed from the coupling regions  1416   a,    1416   b.  The third switch  1412   c,  associated with the third transceiver  1406 , however, is in the cross state, and so light input at  1410   c  is efficiently reflected by the OTDR reflector  1420   c.  At the same time, little or no optical signal reaches the third transceiver  1406 . Such an arrangement permits the operator to remotely test the optical transmission path from the Central office all the way to the end user&#39;s facility. It also permits the operator to turn off a dysfunctional transceiver at the user&#39;s end. 
     In another embodiment, one or more optical switches may be located on optical paths leading out of an FDH. For those optical paths whose switches are in the bar state, the optical signals are transmitted out of the FDH to the respective end users. However, a switch in the cross-state may be used to direct an optical signal out of the network, e.g. into an optical dump, so as to prevent the output of that optical signal. This may be useful for the operator, for example, in controlling which end users receive signals and which do not, or for preventing the transmission of an optical signal along a fiber that is in place but is not yet connected to an end user. 
     The reflection spectrum of the OTDR reflectors  1420   a - c  may be selected to be specific to the laser spectrum used for the OTDR measurement, or may be broader band to reflect light over a large range of wavelengths. In addition, the reflection spectrum may be tailored to be effective over a range of operating temperatures, both the operating temperature of the OTDR reflector and the laser used for the OTDR measurement. 
     An embodiment of a wavelength dependent optical switch  1500  is schematically illustrated in  FIGS. 15A and 15B . A substrate  1502  supports an optical waveguide  1504  into which light is supplied at an input  1506 . The waveguide  1504  includes a grating reflector  1508 , for example a DBR, that is particularly reflective at a select wavelength, for example λ 2 . Thus, if light at three different wavelengths, λ 1 , λ 2 , λ 3 , is input to the waveguide  1504 , the light at λ 2  will be reflected while the light at λ 1  and λ 3  will be transmitted. The transmitted light at λ 1  and λ 3  may be detected, for example, by a transceiver  1510 . 
     The wavelength dependent optical switch  1500  includes one or more microfluidic droplets  1512 . The illustrated embodiment shows only a single droplet  1512  for simplicity. In  FIG. 15A  the droplet  1512  is positioned away from the reflector  1508  so as to have a reduced, or minimal, effect on the reflector  1508 . In  FIG. 15B  the droplet  1512  is positioned close to the reflector  1508 . With appropriate selection of the refractive index and the extent of the droplet  1512 , the effective refractive index of the reflector  1508  can be changed so as to reduce the reflectivity at λ 2 , thus permitting the light at λ 2  to be transmitted and reach the transceiver  1510 . In some embodiments, the microfluidic droplet  1512  has the same refractive index as the waveguide  1504 . When the droplet  1512  is moved into a position over the reflector  1508 , the refractive index of the fluid affects the refractive indices of the grating reflector  1508 . In some embodiments, the grating reflector  1508  is formed with recesses  1514  which may be filled by the fluid. Where the refractive indices of the fluid droplet  1512  and waveguide  1504  are the same, the grating is effectively erased when the droplet  1512  is over the grating reflector  1508 , permitting light at λ 2  to be transmitted. 
     Another embodiment of a wavelength dependent optical switch  1600  is schematically illustrated in  FIGS. 16A and 16B . A substrate  1602  supports a waveguide  1604  that has a grating reflector  1606 . The grating reflector  1606  may be formed as a grating in the waveguide  1604 . For example, the waveguide  1604  may be formed from a material having a refractive index of n 1 . The grating is formed from repeated regions  1608  in the waveguide  1604  having a different refractive index, n 2 . The reflection spectrum of the grating reflector  1606  depends on a number of factors, including the relative spacing between the repeated regions  1608  having the different refractive index. As in the example discussed above, the reflection spectrum of the grating reflector  1606  may be selected for operation under a number of different conditions. 
     In the illustrated embodiment, the grating reflector  1606  normally reflects light at a wavelength λ 2  and transmits light at other wavelengths, for example λ 1  and λ 3 . Reflection by the grating reflector can be controlled by shape-controlled microfluidics. A reservoir  1610  contains a fluid having a refractive index different from n 2 , for example n 3 . One or more ducts  1612  lead from the reservoir  1610  to the repeated regions  1608 , permitting the refractive index of the repeated regions  1608  to be changed from n 2 , when the fluid is absent from the repeated regions  1608 , to n 3  when the fluid is present in the repeated regions  1608 . When the refractive index of the repeated regions  1608  is changed from n 2  to n 3 , for example by moving the liquid into the repeating regions  1608 , the reflection properties of the reflecting grating can be changed. For example, where the liquid has a refractive index, n 3 , that is approximately equal to that of the waveguide material, n 1 , the reflectivity of the grating reflector  1606  can be reduced to close to zero, effectively allowing light at λ 2  to be transmitted through the grating reflector  1606 , as is schematically illustrated in  FIG. 16B . 
     One example where such an optical switch  1600  may be used is in a situation like that shown in  FIGS. 15A-15B , where the wavelength dependent optical switch is used to isolate light at a selected wavelength from a transceiver while permitting other wavelengths to be transmitted to the transceiver. This might be useful, for example, if light at λ 2  is used for OTDR measurements. Under normal circumstances it may be preferred to permit all wavelengths to reach the transceiver, but to have a reflector at λ 2  active just in front of the transceiver when OTDR measurements are to be made. 
     While various examples were provided above, the present invention is not limited to the specifics of the examples. For example, various combinations of elements shown in different figures may be combined together in various ways to form additional optical circuits not specifically described herein. 
     As noted above, the present invention is applicable to fiber optical communication and data transmission systems. Accordingly, the present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. The claims are intended to cover such modifications and devices. 
     The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.