Abstract:
A push-pull thermooptic interferometer switch can be controlled using only one control lead and one drive signal per switch. Advantageously, using only control signal lead and one electrical driver per switch can greatly reduce the complexity of the electronics needed to drive a switch array. The single control lead push-pull thermooptic interferometer switch can be driven by controllable voltage or current signals. The connections of the control and other signal leads to an array of these switched can be made in a planar manner without the need for crossover paths.

Description:
RELATED APPLICATION  
       [0001]    Related subject matter is disclosed in my previously filed application entitled “PUSH-PULL THERMOOPTIC SWITCH” by C. R. Doerr, Ser. No. 09/810,135, filed on Mar. 16, 2001, and assigned to the same Assignee. 
     
    
     
       TECHNICAL FIELD OF THE INVENTION  
         [0002]    This invention relates generally to optical communication devices and arrangements, and, in particular, to a push-pull thermooptic switch which is driven from a single control signal.  
         BACKGROUND OF THE INVENTION  
         [0003]    Wavelength Division Multiplexing (WDM) control devices, such as wavelength add-drops (WADs), wavelength selective cross connects (WSCs), and dynamic gain equalization fllters (DGEFs), often consist of a demultiplexer and a multiplexer connected by an array of switches. A low-loss, compact, mass-produceable way to make the switches is to use a planar arrangement of thermooptic Mach-Zehnder (M-Z) interferometer switches in silica waveguides.  
           [0004]    As discussed in my above-identified patent application, a push-pull thermooptic Mach-Zehnder interferometer switch has several advantages over conventional thermooptic Mach-Zehnder switches, such as reduced power consumption and polarization dependence and constant power dissipation. With reference to FIG. 1, there is shown a push-pull thermooptic Mach-Zehnder interferometer switch as described my above-identified patent application. As shown, each waveguide arm  101  and  102  includes a thermooptic phase shifter. A thermooptic phase shifter is simply a heater, (e.g.,  110  and  112 ) deposited over the waveguide arm ( 101  and  102 , respectively) that causes the refractive index of the waveguide arm material to change via a temperature change when electrical signal (CONTROL  1 , CONTROL  2 , respectively) is applied to the heater. Usually the two path lengths (i.e., the lengths of arms  101  and  102 ) between the input coupler  120  and the output coupler  130  are designed to be equal when the thermo-optic phase shifter is undriven, although sometimes there is about a quarter-wavelength element  140  to provide a phase bias in the switch.  
           [0005]    Notwithstanding the improvements of my previous push-pull thermooptic Mach-Zehnder interferometer switch, that design required a separate control lead ( 111 ,  112 ) and drive signal [CONTROL  1 , CONTROL  2 , respectively] for each arm. Since WDM control devices, often consists of a demultiplexer and a multiplexer connected by an array of such push-pull thermooptic interferometer switches, it would be desirable to reduce the number of control leads and drive signals needed to operate each push-pull thermooptic interferometer switch.  
         SUMMARY OF THE INVENTION  
         [0006]    In accordance with the present invention, I have recognized that a push-pull thermooptic interferometer switch can be controlled using only one control lead and one drive signal per switch. Advantageously, using only control signal lead and one electrical driver per switch can greatly reduce the complexity of the electronics needed to drive a switch array. My single control lead push-pull thermooptic interferometer switch can be driven by controllable voltage or current signals. The connections of the control and other signal leads to an array of these switched can be made in a planar manner without the need for crossover paths.  
           [0007]    More particularly, my invention is a push-pull driven two-arm thermooptic interferometer switch comprising  
           [0008]    a first two-terminal phase control element to control the optical phase in a first arm, a first terminal of the first phase control element being connected to a fixed electrical signal,  
           [0009]    a second two-terminal phase control element to control the optical phase in a second arm, where a second terminal of the second phase control element is connected to the second terminal of the first phase control element, and  
           [0010]    a single controllable electrical signal applied across the first and second terminals of the second phase control element, wherein the level of the controllable electrical signal controls the optical phase in a first and second arms by controlling the current flow in the first and second arms, respectively.  
           [0011]    In one embodiment, the single controllable electrical signal includes a controllable voltage source applied to the second terminal of the second phase control element and a fixed voltage source applied to the first terminal of the second phase control element. In a second embodiment, the controllable electrical signal includes a fixed current source applied to the second terminal of the second phase control element and a controllable current source applied to the first terminal of the second phase control element. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The present invention will be more fully appreciated by consideration of the following Detailed Description, which should be read in light of the accompanying drawing in which:  
         [0013]    [0013]FIG. 1 illustrates the push-pull thermooptic Mach-Zehnder interferometer switch shown and described in my above-identified patent application.  
         [0014]    [0014]FIG. 2 shows, in accordance with the present invention, an illustrative push-pull thermooptic Mach-Zehnder interferometer switch which uses only one control lead for providing one voltage drive signal per switch.  
         [0015]    [0015]FIG. 3 shows another embodiment of my invention, where an illustrative push-pull thermooptic Mach-Zehnder interferometer switch uses only one control lead for providing one current drive signal per switch.  
         [0016]    [0016]FIG. 4 shows an illustrative design of an array of push-pull thermooptic interferometer switches and the connection of control leads and drive signals to the switches of the array. 
     
    
       [0017]    In the following description, identical element designations in different figures represent identical elements. Additionally in the element designations, the first digit refers to the figure in which that element is first located (e.g.,  120  is first located in FIG. 1).  
       DETAILED DESCRIPTION  
       [0018]    With reference to FIG. 1, there is shown a diagram illustrating the arrangement of a push-pull thermo-optic Mach-Zehnder interferometer switch for switching or attenuating optical signals as described in my above-identified patent application. One or two input waveguides, such as waveguides  105 ,  106 , are connected to a first coupler  120  that connects the optical signal to two waveguides or paths  101 ,  102  that advantageously have a path-length difference between one eighth and three eighths of an optical wavelength. This path length difference can be obtained, for example, by including an “extra” path length, shown illustratively as element  140  in FIG. 1, in path  102 . Waveguides or paths  101 ,  102  are connected to a second coupler  130  that in turn couples the coupler outputs to one or two output waveguides, such as waveguides  107 ,  108 . In accordance with the invention, two electric heaters  110 ,  112  are disposed, one on each of the waveguides  101 ,  102 . The heaters  110 ,  112  are both driven, not necessarily simultaneously, by electrical control (drive) signals received on control inputs  111  and  113 , respectively, to control the optical transmissivity through the device in such a manner that one arm is driven or switched to one state, and the other arm is driven or switched to the other state. In this way, the overall device operates in a push-pull mode. There are many benefits of a push-pull M-Z switch including reduced power consumption, constant power dissipation, reduced polarization dependence and constant phase.  
         [0019]    For a thermooptic switch, the electric power consumed is proportional to the applied phase shift. To switch a conventional M-Z switch, where only one thermooptic phase shifter is used (only one arm, e.g.,  101 , has a heater), a phase shift of 180 degrees is required. With no control signal applied to the conventional M-Z switch, each arm has the same phase shift (since in this state the thermooptic phase shifter has a phase shift of zero degrees) and the signals from the arms are combined to form the switch output signal. This is referred to as the “bar” state of the M-Z switch. When the control signal is applied the thermooptic phase shifter has a phase shift of 180 degrees and when the 180 degree out of phase arm signals are combined at the output coupler (e.g.,  130 ), they cancel each other producing essentially no switch output signal. This is referred to as the “cross” state of the M-Z switch.  
         [0020]    In comparison, to switch a push-pull M-Z switch, where a thermooptic phase shifter is used in each arm, the phase shift in each arm is switched only 90 degrees. When a control signal (CONTROL  1 ) is applied on lead  111  and no control signal (CONTROL  2 ) is applied on lead  113 , arm  101  and arm  102  (because of the element  140 ) both add a +90 degree phase shift to the applied input optical signals at leads  105  and/or  106 . Since the resulting differential phase shift between the two arms  101 ,  102  is zero, the signals from the arms are combined to form a switch output signal ( 107 ,  108 ). This is the bar state where optical signals pass through the M-Z switch. When no control signal (CONTROL  1 ) is applied on lead  111  and a control signal (CONTROL  2 ) is applied on lead  113 , arm  101  has a zero degree phase shift and arm  102  has a +180 degree phase shift (+90 degrees from the thermooptic phase shifter  110 ,  101  and +90 degrees from element  140 ). In this condition, the differential phase shift between the signals of arms  101  and  102  is 180 degrees and they cancel and produce no substantial switch output signal ( 107 ,  108 ). This is the cross state where no optical signals pass through the M-Z switch. Since in a push-pull M-Z switch of FIG. 1, the thermooptic phase shifter in each arm is switched only 90 degrees, rather than 180 degrees in the conventional M-Z switch, the power consumption of the push-pull M-Z switch is reduced by up to a factor of two.  
         [0021]    With reference to FIG. 2 there is shown, in accordance with the present invention, an illustrative push-pull thermooptic Mach-Zehnder interferometer switch which uses only one control lead and one voltage drive signal per switch. In the FIG. 2 description, element designations that are identical to those of FIG. 1 function in the same manner as those described in FIG. 1. In FIG. 2, the control signal (V)  201  is applied via a single control lead  201  that connects to leads  111  and  113  at one end of heaters  110  and  112 , respectively. The second lead  210  of heater  110  connects to ground potential, while the second lead  211  of heater  112  is connected to a fixed voltage V 0 . Thus, a single controllable electrical signal, including control signal (V) and fixed voltage V 0 , is applied across the terminals  113  and  211  of heater  112 . The operation of the M-Z interferometer switch of FIG. 2 is as follows.  
         [0022]    When the control signal  201  is at a first voltage level (e.g., zero volts), there is no current flow through heater  110  and the thermooptic phase shifter  1  (heater  110  and arm  101 ) of the first arm  101  produces a phase shift of zero degrees. However, the first voltage level (e.g., zero volts) causes a current flow through heater  112  and, hence, the thermooptic phase shifter  2  (heater  112  and second arm  102 ) of arm  102  produces a phase shift of +90 degrees. The resulting optical phase shift through arm  102  is 180 degrees which is the sum of the +90 degrees of the “extra” path element  140  plus the +90 degrees of the thermooptic phase shifter  2  of arm  102 . Since, the optical signals through arms  101  and  102  are out of phase, 180 degrees different, they essentially cancel each other at output coupler  130  thereby forming no switch output signal ( 107 ,  108 ). This condition is the “cross” or “shuttered” state of M-Z interferometer switch of FIG. 2.  
         [0023]    When the control signal  201  is at a second voltage level (e.g., V 0  volts), there is a current flow through heater  110  and the thermooptic phase shifter  1  (heater  110  and arm  101 ) of arm  101  produces a phase shift of +90 degrees. However, when the first voltage level (e.g., V 0  volts) is applied to lead  113  of heater  112  no current flows since lead  211  is also at the V 0  volts. Hence, the thermooptic phase shifter  2  (heater  112  and arm  102 ) of arm  102  produces a phase shift of zero degrees. The resulting optical phase shift through arm  102  is 90 degrees which is the sum of the +90 degrees of the “extra” path element  140  plus the zero degrees of the thermooptic phase shifter  2  of arm  102 . Since the optical signals through arms  101  and  102  both undergo a phase shift of +90 degrees they remain in-phase and hence are combined at output coupler  130 , thereby forming a switch output signal ( 107 ,  108 ). This condition is the “bar” state of M-Z interferometer switch of FIG. 2.  
         [0024]    In this manner, the M-Z interferometer switch of FIG. 2 uses a single control lead  201  to apply a control drive signal V of either zero or V 0  volts to switch (or shutter) the optical signal received over input leads  105  and  106  between the “cross” and “bar” states, respectively. While FIG. 2 shows the use of positive voltage levels for V and V 0 , it should be noted that negative voltages could also be used. Since the M-Z interferometer switch of FIG. 2 uses only one control lead, only one drive circuit (not shown) is needed to provide the controllable voltage drive signal V over that one control lead. Note that the total power to the M-Z switch is constant when V is at either zero volts or V 0  volts. However, when the voltage V is between 0&lt;V&lt;V 0 , the power is not constant. Thus, the M-Z switch power consumption is not constant while V is being switched between the zero and V 0  volt levels. Note also that the overall phase in arm  101  varies from 0 to +90 degrees while the overall phase in arm  102  goes from +180 to +90. Thus while the change in phase is 90 degrees for both arm  1  and arm  2 , the overall phase of the M-Z switch is not constant.  
         [0025]    With reference to FIG. 3 there is shown, another illustrative embodiment of my push-pull thermooptic M-interferometer switch that uses only one control lead and one current drive signal per switch. In the FIG. 3 description, element designations that are identical to those of FIG. 1 function in the same manner as those described in FIG. 1. In FIG. 3 the control signal  302  is a current signal rather than voltage control signal  201  used in FIG. 2. In FIG. 3, the single control current signal (I) is applied in series with the heater  112 . The fixed current signal (I 0 ) is applied to lead  111  of heater  110  and lead  113  of heater  112 . The second lead  310  of heater  110  is connected to ground. Thus in FIG. 3, a single controllable electrical signal, including control current signal (I) and fixed current signal (I 0 ), is applied across the terminals  113  and  211  of heater  112 . The operation of the M-Z interferometer switch of FIG. 3 is described below.  
         [0026]    When the control current signal source (I)  301  is at a first current level (e.g., zero amps), the current flow through heater  110  is equal to I 0 , from the fixed current source (I 0 )  302 . Hence the thermooptic phase shifter  1  (heater  110  and arm  101 ) of arm  101  produces a phase shift of +90 degrees. However, since control current signal source (I)  301  is at zero amps (the first current level) no current flow through heater  112  and, hence, the thermooptic phase shifter  2  (heater  112  and arm  102 ) of arm  102  produces no phase shift. The resulting optical phase shift through arm  102  is +90 degrees which is the sum of the +90 degrees of the “extra” path element  140  plus the zero degrees of the thermooptic phase shifter  2  of arm  102 . Since the optical signals through arms  101  and  102  both undergo a phase shift of +90 degrees they remain in-phase and hence are combined at output coupler  130  thereby forming a switch output signal ( 107 ,  108 ). This condition is the “bar” state of M-Z interferometer switch of FIG. 3.  
         [0027]    When the current signal source (I)  301  is at a second current level (e.g., I 0  amps) it receives all of the current  10  from fixed source  302 , and hence, no current flows through heater  110 . With no current flow through thermooptic phase shifter  1  (heater  110  and arm  101 ) of arm  101  produces a phase shift of zero degrees. Since the current signal source (I)  301  is at I 0  amps, this current flows through heater  112  and, hence, thermooptic phase shifter  2  (heater  112  and arm  102 ) of arm  102  produces a +90 degree phase shift. The resulting optical phase shift through arm  102  is +180 degrees which is the sum of the +90 degrees of the “extra” path element  140  plus the +90 degrees of the thermooptic phase shifter  2  of arm  102 . Since the optical signals through arms  101  has a zero degree phase shift while arm  102  undergoes a +180 degree phase shift, these signals are out of phase and cancel each other at output coupler  130  thereby producing no switch output signal ( 107 ,  108 ). This condition is the “cross” state of M-Z interferometer switch of FIG. 3.  
         [0028]    In this manner, the M-Z interferometer switch of FIG. 3 uses a single control lead  311  to apply a control current drive signal of zero amps or I 0  amps to switch (or shutter) the optical signal received over input leads  105  and  106  between the “cross” and “bar” states, respectively. While FIG. 3 shows the use of positive current levels for 1 and I 0 , it should be noted that negative voltages could also be used. Since the M-Z interferometer switch of FIG. 3 uses only one control lead, only one current drive circuit (not shown) is needed to provide the controllable current drive signal I over the one control lead  311 . Note that the total power to the M-Z switch is constant when I is at either zero or I 0  amps. However, when the voltage I is between 0&lt;I&lt;I 0 , the power is not constant. Thus, the M-Z switch power consumption is not constant while the current I is being switched between the zero and  10  current levels. Note also that the overall phase in arm  101  varies from 0 to +90 degrees while the overall phase in arm  102  goes from +180 to +90. Thus the overall phase of the M-Z switch of FIG. 3 is not constant.  
         [0029]    [0029]FIG. 4 shows an illustrative design of an arrangement for providing the connections of voltage control leads and drive signals to each of the push-pull thermooptic interferometer switches  401 - 403  of a planar array  400 . As shown, the array includes three M-Z switches  401 ,  402 ,  403 , which are controlled, respectively, by control voltage leads V1, V2, V3. A common fixed voltage lead V 0  and a common ground lead  404  are used to provide the fixed voltages needed for the operation of the switch array. Note that the lead connection layout of FIG. 4 is planar since no lead crossovers are needed. If the array of M-Z switches utilized current drive, as shown in FIG. 3, the arrangement of FIG. 4 may be adapted in a similar manner (where Vo becomes I 0 ; V1, V2, V3 become I1, I2, and I3; etc.) to produce a planar lead connection layout without crossovers.  
         [0030]    Various modifications of this invention will occur to those skilled in the art. Nevertheless all deviations from the specific teachings of this specification that basically rely upon the principles and their equivalents through which the art has been advanced are properly considered within the scope of the invention as described and claimed.  
         [0031]    While this invention has been described above as a digitally controllable device (i.e., control signal  201  is switched between ground and V 0 , and control signal  311  is switched between zero and I 0 ), one could also use intermediate electrical control signal values to produce partial switching (or attenuation) of the two input optical signals (on input waveguides  105 ,  106 ). Such an arrangement would operate as a variable optical attenuator.