# Patent

## Title
Voltage controlled optical directional coupler

## Abstract

A voltage controlled optical directional coupler (VCODC) having a coupling ratio that can be adjusted to any desired value through voltage tuning is disclosed. The VCODC may include a first optical hybrid coupler and a second optical hybrid coupler, which may be coupled with each other via one or more voltage controlled optical elements having a variable transparency depending on a voltage applied to the one or more voltage controlled optical elements. The VCODC may be configured to divert a portion of optical power received to a trunk input of the VCODC to a tap output of the VCODC based on the variable coupling ratio of the VCODC, which may be dependent on the variable transparency of the one or more voltage controlled optical elements.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of International Application No. PCT/IL14/050535 filed on Jun. 12, 2014, which claims the benefit of priority to U.S. Provisional Application No. 61/834,066, filed on Jun. 12, 2013, and Provisional Application No. 61/894,129, filed on Oct. 22, 2013, the contents of which are relied upon and incorporated herein by reference in their entireties.


BACKGROUND
The disclosure relates generally to couplers that can be used in optical communication technology and more particularly to a voltage controlled optical directional coupler and associated systems and operating methods, which may be used in optical communication networks, such as fiber optic networks.
No admission is made that any reference cited herein constitutes prior art. Applicant expressly reserves the right to challenge the accuracy and pertinency of any cited reference.
SUMMARY
A first embodiment of the disclosure relates to a voltage controlled optical directional coupler. The voltage controlled optical directional coupler (VCODC) of this embodiment may include a first optical hybrid coupler, which may include a trunk input of the voltage controlled optical directional coupler. The VCODC of the first embodiment may further include a second optical hybrid coupler, which may be coupled with a tap output of the VCODC, and one or more voltage controlled optical elements configured to couple the first optical hybrid coupler to the second optical hybrid coupler. The one or more voltage controlled optical elements may have a variable transparency depending on a voltage applied to the one or more voltage controlled optical elements. The VCODC may be configured to divert a portion of optical power received to the trunk input to the tap output based on a variable coupling ratio of the VCODC. The portion of optical power that is diverted may be dependent on the variable transparency of the one or more voltage controlled optical elements.
A second embodiment relates to a method for operating a VCODC. The VCODC that may be used in the method of the second embodiment may include a first optical hybrid coupler, which may include a trunk input of the VCODC, and a second optical hybrid coupler coupled with a tap output of the VCODC. The first optical hybrid coupler may be coupled with the second optical hybrid coupler via one or more voltage controlled optical elements having a variable transparency depending on a voltage applied to the one or more voltage controlled optical elements. The method may include setting a target optical power at the tap output (POPT TAP) for the VCODC. A portion of optical power received to the trunk input may be diverted to the tap output based on a variable coupling ratio of the VCODC. The portion of optical power that is diverted may be dependent on the variable transparency of the one or more voltage controlled optical elements. The method may further include observing an actual POPT TAP during operation of the VCODC, and determining whether the actual POPT TAP is equal to the target POPT TAP. The method may also include adjusting the variable coupling ratio of the VCODC to achieve the POPT TAP by tuning the voltage applied to the one or more voltage controlled optical elements in an instance in which the actual POPT TAP is not equal to the target POPT TAP.
A third embodiment relates to a system including a VCODC. The system may be, for example, an optical network, such as a fiber optic network in which a VCODC in accordance with various embodiments may be implemented. The VCODC included in the system of the third embodiment may be the VCODC of the first embodiment. The system of the third embodiment may further include a control loop that may be configured to adjust the variable coupling ratio of the VCODC to achieve a target optical power at the tap output (POPT TAP) by tuning a voltage that may be applied to one or more voltage controlled optical elements of the VCODC in an instance in which an observed POPT TAP is not equal to the target POPT TAP. The system may additionally include processing circuitry configured to set the target POPT TAP.
Additional features and advantages will be set forth in the detailed description, and will be readily apparent to those skilled in the art.
The foregoing general description and the detailed description are merely exemplary, and are intended to provide an overview or framework to understand the nature and character of the claims.
The accompanying drawings constitute a part of this specification. The drawings each illustrate one or more embodiments, and together with the description serve to explain principles and operation of the various embodiments.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates an optical daisy chain power distribution system based on optical directional couplers (VCODC).
FIGS. 2A-2D illustrates the operation of an optical hybrid coupler.
FIG. 3 is a block diagram of a VCODC according to some example embodiments.
FIG. 4 is a block diagram of another VCODC according to some example embodiments.
FIG. 5 is a block diagram of a system including a VCODC according to some example embodiments.
FIG. 6 illustrates a flowchart according to an example method for operating a VCODC according to some example embodiments.



DETAILED DESCRIPTION
An optical directional coupler diverts a fixed amount of optical power determined by a coupling ratio of the optical directional coupler from a trunk to a branch or tap location. Optical directional couplers are used for signal distribution of optical signals in a variety of distribution topologies, such as in a daisy chain topology. FIG. 1 illustrates an optical daisy chain power distribution system based on optical directional couplers. The optical directional couplers 102 are configured in a daisy chain topology where each optical directional coupler 102 diverts an amount of power from the main trunk 104 to a respective tap 106. In most daisy chain networks it is required to have an equal power at each tap. Because conventional optical directional couplers have a fixed coupling ratio (e.g., 3 db, 6 db, 10 db, etc.), design and configuration of a daisy chain can be very complicated, and in most cases less than optimal. In this regard, as an amount of power is removed from the main trunk 104 by each optical directional coupler 102, there is less optical power reaching each subsequent optical directional coupler 102 in the daisy chain. Accordingly, in order for each respective tap 106 to have an equal power, each optical directional coupler 102 in the daisy chain must typically have a different coupling ratio, thus introducing complications in the design and configuration of a daisy chain using conventional optical directional couplers. Moreover, as, in some instances, it may not be possible to determine the exact coupling ratio needed for a given tap position without experimentation, the time and associated costs required to deploy a daisy chain using conventional optical directional couplers can be prohibitive. As such, network topologies using conventional optical directional couplers are often practically limited in terms of daisy chain length and/or the number of taps. This limitation can increase the amount of fiber required in an overall network deployment, as the limitation on daisy chain length can increase the number of trunk lines needed to accommodate the required amount of taps in the network.
Various example embodiments disclosed herein provide a voltage controlled optical coupler that can address the design limitations inherent with usage of conventional optical directional couplers. In this regard, the coupling ratio of a voltage controlled optical coupler disclosed herein can be adjusted through voltage tuning to achieve a desired tap output power. As such, voltage tuned optical directional couplers disclosed herein can reduce design complications in deployment of daisy chain topologies, allowing for deployment of longer daisy chains having more taps than possible when using conventional optical directional couplers. Moreover, the increased daisy chain length possible with use of voltage controlled optical couplers disclosed herein can reduce the amount of fiber needed for an overall network deployment, thus reducing costs for network deployment.
FIGS. 2A-2D illustrate the operation of an optical hybrid coupler, also referred to as an optical hybrid combiner, which may be used in a voltage controlled optical directional coupler in accordance with various example embodiments. The transfer function of a basic optical hybrid coupler as shown in FIG. 2A can be described by the following transfer matrix:




  T
  :=
  
    
      (
      
        
          -
          1
        
        2
      
      )
    
    ·
    
      (
      
        
          
            0
          
          
            J
          
          
            1
          
          
            0
          
        
        
          
            J
          
          
            0
          
          
            0
          
          
            1
          
        
        
          
            1
          
          
            0
          
          
            0
          
          
            J
          
        
        
          
            0
          
          
            1
          
          
            J
          
          
            0
          
        
      
      )
    
  




In this operation, Ports 2 and 3 are assumed to have identical reflectors with reflection coefficient Γ, and optical power with a magnitude of P is injected to port 4. According to the transfer matrix, the optical power at port 2 will be one half of the input power and with an angle of 180 degrees, and the optical power at port 3 will be one half of the input power with an angle of 270 degrees (referring to the input). In this example, the phase shift between port 2 and port 3 optical powers is 90 degrees.
Since Ports 2 and 3 have identical reflectors with a reflection coefficient of 0<Γ<1,the same amount of optical power will be reflected from port 2 and port 3, as illustrated in FIG. 2B.
In a case where the reflected waves from ports 2 and 3 returns to port 4, (input port) the waves will return in a 180 degree phase shift between of them, and they will cancel each other as illustrated in FIG. 2C. In a case where one of the reflected waves from ports 2 and 3 returns to port 1, (output port) the waves will return in a 0° phase shift between them and they will sum, as illustrated in FIG. 2D. In a case of balanced reflectors in ports 2 and 3, the reflection will return to port 1 only. Since the reflection coefficient of the reflector




  
    Γ
    =
    
      
        P
        -
      
      
        P
        +
      
    
  
  ,




the magnitude of the reflected power to port 1 is PPORT1=Γ·P+.

FIG. 3 is a block diagram of a VCODC) 300 according to some example embodiments. The VCODC 300 has a coupling ratio that can be adjusted to any required coupling ratio through voltage tuning.
The VCODC 300 may include a first optical hybrid coupler 310 and a second optical hybrid coupler 314, which may be coupled (e.g., connected back-to-back) via one or more voltage controlled optical elements. The first optical hybrid coupler 310 and the second optical hybrid coupler 314 are coupled via two such voltage controlled optical elements—a first voltage controlled optical element 320 and a second voltage controlled optical element 324. It will be appreciated that while two voltage controlled optical elements are illustrated by way of example in FIG. 3, other arrangements are contemplated within the scope of the disclosure. For example, a single voltage controlled optical element may be used to span both coupled sets of ports between the first optical hybrid coupler 310 and the second optical hybrid coupler 314. As such, where reference is made herein to the first voltage controlled optical element 320 and the second voltage controlled optical element 324, it will be appreciated that such reference is by way of example, and not by way of limitation, such that other arrangements involving one or more voltage controlled optical elements may be substituted mutatis mutandis for the first voltage controlled optical element 320 and the second voltage controlled optical element 324 within the scope of the disclosure.
The voltage controlled optical element(s) (e.g., the first voltage controlled optical element 320 and the second voltage controlled optical element 324) used to couple the first optical hybrid coupler 310 and the second optical hybrid coupler 314 may be implemented via any optical element having a variable transparency depending on a voltage applied to the one voltage controlled optical elements. For example, a lens, such as a voltage controlled optical reflector may be used to implement the first voltage controlled optical element 320 and/or the second voltage controlled optical element 324.
An optical termination 330, such as may include black or opaque structure, may be connected to the lower output of the second optical hybrid coupler 314. The other port of the second optical hybrid coupler 324 may be coupled with a tap output of the VCODC 300. In the embodiment illustrated in FIG. 3, the second optical hybrid coupler 324 may be coupled with an optical coupler 340, which may provide the tap output 346 of the VCODC 300.
The first optical hybrid coupler 310 may include and/or otherwise be coupled with a trunk input 342, which may receive an input optical power, POPT IN. The VCODC 300 may be configured to divert a portion of power received to the tap input 342 (e.g., a portion of POPT IN) to the tap output 346. The portion of optical power diverted to the tap output 346 may be referred to as POPT TAP. The portion of optical power received to the trunk input 342 that is not diverted to the tap output 346 may be passed through to the trunk output 348, and may be referred to as POPT OUT.
The optical power value of POPT OUT our at the trunk output 348 varies with the coupling ratio of the VCODC 300, and may be described by the following equation:

POPT OUT=Γ·P+,

where the reflection coefficient Γ is correlated to the voltage controlled transparency of the voltage controlled optical element(s) (e.g., the first voltage controlled optical element 320 and the second voltage controlled optical element 324) used to couple the first optical hybrid coupler 310 and the second optical hybrid coupler 314.

The optical power value of POPT TAP likewise varies with the coupling ratio of the VCODC 300, and may be described by the following equation:

POPT TAP=POPT IN·(1−Γ).

The VCODC 300 may further include and/or otherwise be coupled with a control loop 348. The control loop 348 may be configured to adjust the variable coupling ratio of the VCODC 300 to maintain and/or otherwise achieve a target POPT TAP by tuning the voltage applied to the first voltage controlled optical element 320 and the second voltage controlled optical element 324 in an instance in which an actual POPT TAP observed during operation of the VCODC 300 is not equal to the target POPT TAP. In this regard, the control loop 348 may be configured to tune the voltage applied to the first voltage controlled optical element 320 and the second voltage controlled optical element 324 to increase transparency of the first voltage controlled optical element 320 and the second voltage controlled optical element 324 in an instance in which the observed POPT TAP is less than the target POPT TAP, and may be configured to tune the voltage applied to the first voltage controlled optical element 320 and the second voltage controlled optical element 324 to decrease transparency of the first voltage controlled optical element 320 and the second voltage controlled optical element 324 in an instance in which the observed POPT TAP is greater than the target POPT TAP.
The control loop 348 of some example embodiments may include a sensor 350 and a voltage controller 360. The sensor 350 may be embodied as any sensor configured to sense an actual POPT TAP during operation of the VCODC 300 and to generate a correction signal indicative of the value of the actual POPT TAP. The correction signal may comprise a correction voltage having a value corresponding to the optical power of the actual POPT TAP observed by the sensor 350. For example, such as that illustrated in and described below with respect to FIG. 4, the sensor 350 may be implemented as a photodiode, which may be positioned such that it may be illuminated by the tap output 346 (e.g., by output of the optical coupler 340), and may be configured to generate a correction voltage based on the illumination.
The correction signal generated by the sensor 350 may be passed to the voltage controller 360. The voltage controller 360 may also be provided with an indication of a target POPT TAP value 362. The target POPT TAP value 362 may be a tunable threshold, or setpoint, that may be adjusted based on a desired POPT TAP value. In some example embodiments, such as that illustrated in and described below with respect to FIG. 5, the target POPT TAP value 362 may be controlled by processing circuitry, which may be configured to control one or more VCODCs 300 implemented within a network.
The voltage controller 360 may be embodied as any circuit that may be configured to tune the voltage supplied to the first voltage controlled optical element 320 and the second voltage controlled optical element 324 based on the correction signal received from the sensor 350 to achieve the target POPT TAP value 362. In this regard, the voltage controller 360 may be configured to tune the voltage applied to the first voltage controlled optical element 320 and the second voltage controlled optical element 324 to increase transparency of the first voltage controlled optical element 320 and the second voltage controlled optical element 324 in an instance in which the observed POPT TAP is less than the target POPT TAP. In some embodiments, increasing the transparency of the first voltage controlled optical element 320 and the second voltage controlled optical element 324 may also decrease a reflectivity of the first voltage controlled optical element 320 and the second voltage controlled optical element 324. The voltage controller 360 may be further configured to tune the voltage applied to the first voltage controlled optical element 320 and the second voltage controlled optical element 324 to decrease transparency of the first voltage controlled optical element 320 and the second voltage controlled optical element 324 in an instance in which the observed POPT TAP is greater than the target POPT TAP. Decreasing the transparency of the first voltage controlled optical element 320 and the second voltage controlled optical element 324 may also increase a reflectivity of the first voltage controlled optical element 320 and the second voltage controlled optical element 324. The voltage controller 360 may accordingly be configured to close the control loop 348 by producing a voltage fed to the first voltage controlled optical element 320 and the second voltage controlled optical element 324.
In some embodiments, such as that illustrated in and described with respect to FIG. 4 below, the voltage controller 360 may comprise a loop filter. In such embodiments, the control signal that may be received from the sensor 350 may be a correction voltage, such as may be supplied by a photodiode, and the indication of the target POPT TAP value 362 that may be supplied to the voltage controller 360 may be a threshold voltage (e.g., a reference voltage) corresponding to the target POPT TAP. The voltage controller 360 may be configured to tune the voltage supplied to the first voltage controlled optical element 320 and the second voltage controlled optical element 324 based on a relationship between the correction voltage and the threshold voltage. In this regard, if the correction voltage and the threshold voltage are not equal, the voltage controller 360 may tune the voltage supplied to the first voltage controlled optical element 320 and the second voltage controlled optical element 324 until the correction voltage is substantially equivalent to the threshold voltage.
Accordingly the coupling ratio of the VCODC 300 can be adjusted through voltage tuning to achieve a target POPT TAP, which may be required and/or otherwise desired for deployment within a network topology. As such, a substantially constant POPT TAP may be maintained by the VCODC 300. According to one aspect, the VCODC 300 can be used to build optimized, easy to design daisy chain networks with reduced (e.g., minimal) power loss compared to conventional optical directional couplers.
FIG. 4 is a block diagram of another VCODC 400 according to some example embodiments. In this regard, the VCODC 400 can be an embodiment of the VCODC 300. In accordance with some embodiments, the VCODC 400 has a coupling ratio that can be adjusted to any required coupling ratio through voltage tuning.
The VCODC 400 may include a first optical hybrid coupler 410 and a second optical hybrid coupler 414, which may, respectively, be embodiments of the first optical hybrid coupler 310 and the second optical hybrid coupler 314. The first optical hybrid coupler 410 and the second optical hybrid coupler 414 may be coupled (e.g., connected back-to-back) via a first voltage controlled optical reflector 420 and a second voltage controlled optical reflector 424. The first voltage controlled optical reflector 420 and the second voltage controlled optical reflector 424 may, for example, be embodiments of the first voltage controlled optical element 320 and the second voltage controlled optical element 324.
An optical termination 430, such as may include black or opaque structure, may be connected to the lower output of the second optical hybrid coupler 414. The other port of the second optical hybrid coupler 424 may be coupled with an optical coupler 440, which may provide the tap output 446 of the VCODC 400.
The first optical hybrid coupler 410 may include and/or otherwise be coupled with a trunk input 442, which may receive an input optical power, POPT IN. The VCODC 400 may be configured to divert a portion of power received to the tap input 442 (e.g., a portion of POPT IN) to the tap output 446. The portion of optical power diverted to the tap output 446 may be referred to as POPT TAP.The portion of optical power received to the trunk input 442 that is not diverted to the tap output 446 may be passed through to the trunk output 448, and may be referred to as POPT OUT.
Similarly to the VCODC 300, the optical power value of POPT OUT at the trunk output 448 varies with the coupling ratio of the VCODC 400, and may be described by the following equation:




  
    
      P
      
        OPT
        ⁢
        
          
        
        ⁢
        OUT
      
    
    =
    
      Γ
      ·
      
        P
        +
      
    
  
  ,




where the reflection coefficient 1′is correlated to the voltage controlled transparency of the first voltage controlled optical reflector 420 and the second voltage controlled optical reflector 424.

The optical power value of POPT TAP likewise varies with the coupling ratio of the VCODC 400, and may be described by the following equation:

POPT TAP=POPT IN·(1−Γ).

The VCODC 400 may further include a photodiode 450 and a loop filter 460, which may form a control loop that may be configured to tune the voltage applied to the first voltage controlled optical reflector 420 and the second voltage controlled optical reflector 424 to tune POPT TAP to a target value. The photodiode 450 may, for example, be an embodiment of the sensor 350. The loop filter 460 may, for example, be an embodiment of the voltage controller 360. In this regard, the photodiode 450 and the loop filter 460 may collectively form an embodiment of the control loop 348.
The output of the optical coupler 440 may illuminate the photo diode 450, which may be configured to produce a correction voltage related to the POPT TAP observed via the output of the optical coupler 440. The generated correction voltage may be passed to the loop filter 460, which may also be supplied with a tunable threshold voltage 462 (e.g., a reference voltage), which may correspond to a target POPT TAP. In some example embodiments, such as that illustrated in and described below with respect to FIG. 5, the threshold voltage 462 may be supplied and/or otherwise controlled by processing circuitry, which may be configured to control one or more VCODCs 400 implemented within a network.
The loop filter 460 may be configured to tune the voltage applied to the first voltage controlled optical reflector 420 and the second voltage controlled optical reflector 424 based at least in part on the relationship between the threshold voltage 462 and the correction voltage produced by the photodiode 450. The loop filter 460 can accordingly close the control loop for the VCODC 400 by producing the voltage fed to the first voltage controlled optical reflector 420 and the second voltage controlled optical reflector 424 based on the relationship between the threshold voltage 462 and the correction voltage produced by the photodiode 450.
The coupling ratio of the VCODC 400 can accordingly be adjusted through voltage tuning to achieve a target POPT TAP, which may be required and/or otherwise desired for deployment within a network topology. According to one aspect, the VCODC 400 can be used to build optimized, easy to design daisy chain networks with reduced (e.g., minimal) power loss compared to conventional optical directional couplers.
FIG. 5 is a block diagram of a system 500 including a VCODC 502 according to some example embodiments. The VCODC 502 may be embodied as any VCODC disclosed herein, such as the VCODC 300 or the VCODC 400. The VCODC 502 may include and/or otherwise be coupled with a control loop 548, which may, for example, comprise an embodiment of the control loop 348.
The control loop 548 may include a sensor 550 and a voltage controller 560. The sensor 550 may be configured to observe an actual POPT TAP of the VCODC 502 and generate a correction signal indicative of the actual POPT TAP. In this regard, the sensor 550 may, for example, be an embodiment of the sensor 350. The voltage controller 560 may be configured to receive the correction signal generated by the sensor 550. The voltage controller 560 may also be supplied with a tunable indication of a target POPT TAP 562, which may, for example, be a threshold voltage corresponding to the target POPT TAP. The voltage controller 560 may be configured to adjust a coupling ratio of the VCODC 502 through voltage tuning that may be performed based on the tunable indication of the target POPT TAP 562 and the correction signal to achieve the target POPT TAP. In this regard, the voltage controller 560 may, for example, be an embodiment of the voltage controller 360.
The target POPT TAP 562 may be supplied and/or otherwise controlled by processing circuitry 570. In some example embodiments, the processing circuitry 570 may include a processor 572 and, in some embodiments, such as that illustrated in FIG. 5, may further include memory 574.
The processor 572 may be embodied in a variety of forms. For example, the processor 572 may be embodied as various hardware processing means such as a microprocessor, a coprocessor, a controller or various other computing or processing devices including integrated circuits such as, for example, an ASIC (application specific integrated circuit), an FPGA (field programmable gate array), some combination thereof, or the like. Although illustrated as a single processor, it will be appreciated that the processor 572 may comprise a plurality of processors. In some example embodiments, the processor 572 may be configured to execute instructions that may be stored in the memory 574 and/or that may be otherwise accessible to the processor 572. As such, whether configured by hardware or by a combination of hardware and software, the processor 572 may be configured to control the target POPT TAP for one or more VCODCs 502 in accordance with various example embodiments.
In some example embodiments, the memory 574 may include one or more memory and/or other storage devices. Memory 574 may include fixed and/or removable memory devices. In some embodiments, the memory 574 may provide a non-transitory computer-readable storage medium that may store computer program instructions that may be executed by the processor 572. In this regard, the memory 574 may be configured to store information, data, applications, instructions and/or the like for enabling the processor 572 to control the target POPT TAP for one or more VCODCs 502.
The system 500 may be implemented within a network topology including one or more VCODCs. For example, in some deployments, the system 500 may be implemented within a daisy chain topology including a plurality of VCODCs 502. The processing circuitry 570 in such deployments may be interfaced with the control loops of multiple VCODCs 502 to control the target POPT TAP for each respective VCODC 502. In such deployments, the processing circuitry 570 may be configured to individually set a target POPT TAP for each respective VCODC 502, or may be configured to set a universal target POPT TAP for all of the VCODCs 502 depending on the particular network topology and/or other network design specifications/needs.
FIG. 6 illustrates a flowchart according to an example method for operating a VCODC, such as VCODCs 300 400, and/or 500, having a variable coupling ratio.
Operation 600 may include setting a target optical power at the tap output (POPT TAP) for a VCODC, and may be performed by processing circuitry, such as circuitry 570. Operation 600 may include providing an indication of the target POPT TAP to an element of a control loop, such as a voltage controller (e.g., voltage controller 360, loop filter 460, and/or voltage controller 560) as a setpoint to enable the control loop to adjust the coupling ratio of the VCODC to achieve the target POPT TAP. For example, operation 600 may include supplying a threshold, or reference, voltage corresponding to the target POPT TAP to a voltage controller.
Operation 610 may include observing an actual POPT TAP at the tap output of the VCODC. Operation 610 may be performed by a sensor, such as sensor 350, photodiode 450, and/or sensor 550, which may be implemented within the control loop. Operation 610 may include the sensor generating a correction signal indicative of the actual POPT TAP.
Operation 620 may include determining whether the actual POPT TAP is equal to the target POPT TAP. Operation 620 may be performed by a voltage controller as, such voltage controller 360, loop filter 460, and/or voltage controller 560. Operation 620 may be performed based on a control signal indicative of the actual POPT TAP that may be supplied to the voltage controller attendant to performance of operation 610. For example, in embodiments in which the correction signal is a correction voltage and the target POPT TAP is indicated via a threshold voltage, operation 620 may be performed by determining a relationship between the correction voltage and the threshold voltage.
In an instance in which it is determined at operation 620 that the actual POPT TAP does not equal the target POPT TAP, the method may proceed to operation 630, which may include the control loop adjusting the variable coupling ratio of the VCODC to achieve the target POPT TAP. In this regard, operation 630 may include tuning a voltage applied to one or more voltage controlled optical elements (e.g., voltage controlled optical elements 320, 324; voltage controlled optical reflectors 420, 424; and/or the like) within the voltage controlled optical coupler to increase transparency of the voltage controlled optical elements in an instance in which the actual POPT TAP is less than the target POPT TAP, or to decrease transparency of the voltage controlled optical elements in an instance in which the actual POPT TAP is greater than the target POPT TAP. Operation 630 may be performed by a voltage controller, such as voltage controller 360, loop filter 460, and/or voltage controller 560, which may be implemented within the control loop.
If, however, it is determined at operation 620 that the actual POPT TAP is equal (e.g., substantially equal within a margin of error that may vary with design specifications) to the target POPT TAP, the method may instead proceed to operation 640, which may include maintaining the current coupling ratio of the VCODC.
In some embodiments, the method may return to operation 610 after performance of operation 630 and/or after operation 640. In this regard, operations 610-640 may be performed on an ongoing basis by the control loop during operation of the VCODC in order to maintain the target POPT TAP.
Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not actually recite an order to be followed by its steps or it is not otherwise specifically stated in the claims or descriptions that the steps are to be limited to a specific order, it is no way intended that any particular order be inferred.
It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the spirit or scope of the invention. Since modifications combinations, sub-combinations and variations of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and their equivalents.

## Claims

1. A voltage controlled optical directional coupler, comprising:
a first optical hybrid coupler comprising a trunk input;
a second optical hybrid coupler coupled with a tap output; and
one or more voltage controlled optical elements configured to couple the first optical hybrid coupler to the second optical hybrid coupler, the one or more voltage controlled optical elements having a variable transparency depending on a voltage applied to the one or more voltage controlled optical elements;
wherein the voltage controlled optical directional coupler is configured to divert a portion of optical power received to the trunk input to the tap output based on a variable coupling ratio of the voltage controlled optical directional coupler, the variable coupling ratio being dependent on the variable transparency of the one or more voltage controlled optical elements; wherein a first output of the second optical hybrid coupler is coupled to the tap output, a second output of the second hybrid optical coupler is coupled to an optical termination, and the voltage controlled optical directional coupler is configured to pass through to a trunk output of the first hybrid optical coupler a remaining portion of optical power received to the trunk input and not diverted to the tap output.

2. The voltage controlled optical directional coupler of claim 1, wherein the one or more voltage controlled optical elements comprise one or more voltage controlled optical reflectors.

3. The voltage controlled optical directional coupler of claim 1, further comprising:
a control loop configured to adjust the variable coupling ratio of the voltage controlled optical directional coupler to achieve a target optical power at the tap output (POPT TAP) by tuning the voltage applied to the one or more voltage controlled optical elements in an instance in which an observed POPT TAP is not equal to the target POPT TAP.

4. The voltage controlled optical directional coupler of claim 3, wherein the control loop comprises:
a sensor configured to generate a correction signal indicative of the observed POPT TAP; and
a voltage controller configured to tune the voltage applied to the one or more voltage controlled optical elements based at least in part on the correction signal.

5. The voltage controlled optical directional coupler of claim 4, wherein the correction signal comprises a correction voltage, and wherein the voltage controller is configured to tune the voltage applied to the one or more voltage controlled optical elements based at least in part on a relationship between the correction voltage and a threshold voltage corresponding to the target POPT TAP.

6. The voltage controlled optical directional coupler of claim 5, wherein the voltage controller comprises a loop filter.

7. The voltage controlled optical directional coupler of claim 4, wherein the sensor comprises a photodiode, the photodiode being positioned to be illuminated by output from the tap output and being configured to generate a correction voltage based on illumination from the tap output, and wherein the correction signal comprises the correction voltage generated by the photodiode.

8. The voltage controlled optical directional coupler of claim 3, wherein the control loop is configured to tune the voltage applied to the one or more voltage controlled optical elements to increase transparency of the one or more voltage controlled optical elements in an instance in which the observed POPT TAP is less than the target POPT TAP.

9. The voltage controlled optical directional coupler of claim 3, wherein the control loop is configured to tune the voltage applied to the one or more voltage controlled optical elements to decrease transparency of the one or more voltage controlled optical elements in an instance in which the observed POPT TAP is greater than the target POPT TAP.

10. The voltage controlled optical directional coupler of claim 1, wherein the one or more voltage controlled optical elements comprises a plurality of voltage controlled optical elements.

11. The voltage controlled optical directional coupler of claim 1, wherein the second optical hybrid coupler is coupled to a separate optical coupler, which is configured to provide the tap output.

12. The voltage controlled optical directional coupler of claim 1, wherein at least one of the first optical hybrid coupler and the second optical hybrid coupler comprises four ports, a second port and a third port of the four ports having identical reflectors having reflection coefficient Γ, and wherein, when an optical power with a magnitude of P is injected to a fourth port, an optical power at a second port will be one half of the input power and with an angle of 180 degrees, and the optical power at a third port will be one half of the input power with an angle of 270 degrees, and a same amount of optical power will be reflected from the second port and the third port.

13. A method for operating a voltage controlled optical directional coupler comprising:
a first optical hybrid coupler comprising a trunk input; and
a second optical hybrid coupler coupled with a tap output of the voltage controlled optical directional coupler, the first optical hybrid coupler being coupled with the second optical hybrid coupler via one or more voltage controlled optical elements having a variable transparency depending on a voltage applied to the one or more voltage controlled optical elements, the method comprising:
setting a target optical power at the tap output (POPT TAP) for the voltage controlled optical directional coupler, wherein a portion of optical power received to the trunk input is diverted to the tap output based on a variable coupling ratio of the voltage controlled optical directional coupler, the variable coupling ratio being dependent on the variable transparency of the one or more voltage controlled optical elements;
observing an actual POPT TAP;
determining whether the actual POPT TAP is equal to the target POPT TAP; and
adjusting the variable coupling ratio of the voltage controlled optical directional coupler to achieve the target POPT TAP by tuning the voltage applied to the one or more voltage controlled optical elements in an instance in which the actual POPT TAP is not equal to the target POPT TAP; and passing through to a trunk output of the first hybrid optical coupler a remaining portion of optical power received to the trunk input and not diverted to the tap output.

14. The method of claim 13, wherein adjusting the variable coupling ratio of the voltage controlled optical directional coupler comprises:
tuning the voltage applied to the one or more voltage controlled optical elements to increase transparency of the one or more voltage controlled optical elements in an instance in which the actual POPT TAP is less than the target POPT TAP; and
tuning the voltage applied to the one or more voltage controlled optical elements to decrease transparency of the one or more voltage controlled optical elements in an instance in which the actual POPT TAP is greater than the target POPT TAP.

15. The method of claim 13, wherein:
observing the actual POPT TAP comprises a sensor detecting the actual POPT TAP and generating a correction signal indicative of the actual POPT TAP; and
adjusting the variable coupling ratio of the voltage controlled optical directional coupler comprises a voltage controller tuning the voltage applied to the one or more voltage controlled optical elements based at least in part on the correction signal.

16. The method of claim 15, wherein the correction signal comprises a correction voltage, and wherein adjusting the variable coupling ratio of the voltage controlled optical directional coupler comprises the voltage controller tuning the voltage applied to the one or more voltage controlled optical elements based at least in part on a relationship between the correction voltage and a threshold voltage corresponding to the target POPT TAP.

17. The method of claim 13, wherein:
setting the target POPT TAP comprises providing a target voltage corresponding to the target POPT TAP to a control loop; and
adjusting the variable coupling ratio of the voltage controlled optical directional coupler comprises the control loop tuning the voltage applied to the one or more voltage controlled optical elements.

18. The method of claim 13, wherein setting the target POPT TAP comprises processing circuitry setting the target POPT TAP for the voltage controlled optical directional coupler.

19. A system comprising:
a voltage controlled optical directional coupler comprising:
a first optical hybrid coupler comprising a trunk input;
a second optical hybrid coupler coupled with a tap output; and
one or more voltage controlled optical elements configured to couple the first optical hybrid coupler to the second optical hybrid coupler, the one or more voltage controlled optical elements having a variable transparency depending on a voltage applied to the one or more voltage controlled optical elements;
wherein the voltage controlled optical directional coupler is configured to divert a portion of optical power received to the trunk input to the tap output based on a variable coupling ratio of the voltage controlled optical directional coupler, the variable coupling ratio being dependent on the variable transparency of the one or more voltage controlled optical elements;

a control loop configured to adjust the variable coupling ratio of the voltage controlled optical directional coupler to achieve a target optical power at the tap output (POPT TAP) by tuning the voltage applied to the one or more voltage controlled optical elements in an instance in which an observed POPT TAP is not equal to the target POPT TAP; and
processing circuitry configured to set the target POPT TAP; wherein a first output of the second optical hybrid coupler is coupled to the tap output, a second output of the second hybrid optical coupler is coupled to an optical termination, and the voltage controlled optical directional coupler is configured to pass through to a trunk output of the first hybrid optical coupler a remaining portion of optical power received to the trunk input and not diverted to the tap output.

20. The system of claim 19, wherein the control loop is configured to:
tune the voltage applied to the one or more voltage controlled optical elements to increase transparency of the one or more voltage controlled optical elements in an instance in which the observed POPT TAP is less than the target POPT TAP; and
tune the voltage applied to the one or more voltage controlled optical elements to decrease transparency of the one or more voltage controlled optical elements in an instance in which the observed POPT TAP is greater than the target POPT TAP.

21. The system of claim 19, wherein the control loop comprises:
a sensor configured to generate a correction signal indicative of the observed POPT TAP; and
a voltage controller configured to tune the voltage applied to the one or more voltage controlled optical elements based at least in part on the correction signal.

22. The system of claim 21, wherein the correction signal comprises a correction voltage, and wherein:
the processing circuitry is configured to set a threshold voltage corresponding to the target POPT TAP; and
the voltage controller is configured to tune the voltage applied to the one or more voltage controlled optical elements based at least in part on a relationship between the correction voltage and the threshold voltage.

23. The system of claim 22, wherein the sensor comprises a photodiode, the photodiode being positioned to be illuminated by output from the tap output and being configured to generate the correction voltage based on illumination from the tap output.