Abstract:
The present invention relates generally to laser diodes, and particularly to an operational amplifier able to switch laser diodes on and off quickly without adversely affecting data transmission by the laser diodes. A differential amplifier included in an operational amplifier has a high transconductance when a laser diode is first turned on and a low, near constant transconductance when the laser diode is transmitting data. The operational amplifier is preferably incorporated in optoelectronic transceivers used in passive optical networks. Switching laser diodes on and off quickly enables more efficient use of network bandwidth in such passive optical networks.

Description:
[0001]    The present application claims priority, under 35 U.S.C.  120 , to a United States Non-Provisional Patent Application, which is incorporated herein by reference, entitled “SYSTEM FOR CONTROLLING BIAS CURRENT IN LASER DIODES WITH IMPROVED SWITCHING RATES,” filed on Jul. 2, 2002, and identified by attorney reference number 9775-085-999 and application Ser. No. 10/188,575.  
         [0002]    The present application also claims priority, under 35 U.S.C. 119(e), to a United States Provisional Patent Application, which is incorporated herein by reference, entitled “SYSTEM FOR CONTROLLING BIAS CURRENT IN LASER DIODES WITH IMPROVED SWITCHING RATES,” filed on Sep. 5, 2002, and identified by attorney reference number 9775-130-888 and serial No. 60/408,587. 
     
    
     
       BRIEF DESCRIPTION OF THE INVENTION  
         [0003]    The present invention relates generally to semiconductor lasers, and particularly to operational amplifiers configured to switch semiconductor lasers on and off.  
         BACKGROUND OF THE INVENTION  
         [0004]    Passive optical networks enable a plurality of optoelectronic transceivers to share one or more optical fibers while transmitting and receiving data in an optical form. Typically, passive optical networks employ a time division multiplexing access (TDMA) scheme to make this possible. In such schemes, the data transmission capabilities of the plurality of optoelectronic transceivers are operational only during separate, non-overlapping periods of time.  
           [0005]    When the turn-on and turn-off times of the optoelectronic transceivers decrease, the amount of time available to each optoelectronic transceiver in a passive optical network to transmit optical data increases. Prior art optoelectronic transceivers are able to turn a laser diode on and off within 100 microseconds to 1 millisecond.  
           [0006]    Persons skilled in the art, moreover, recognize that turning a laser diode on and off is a time consuming aspect of turning an optoelectronic transceiver on and off. Passive optical networks, therefore, require laser diodes to be turned on and off quickly to make efficient use of network bandwidth.  
           [0007]    A laser diode is typically embedded in a feedback loop of an optoelectronic transceiver. This feedback loop turns the laser diode on, and then maintains the laser diode in a linear operating range so that it is able to transmit data efficiently. Maintaining the operational efficiency of the laser diode includes adjustments to the output of an operational amplifier, which is a portion of the feedback loop. Persons skilled in the art recognize that the optical output power of a given laser diode may fluctuate in ways that are inconsistent with electrical input that is intended to modulate the optical output of the laser diode. The purpose of the feedback loop is to counteract these unwanted fluctuations.  
           [0008]    In particular, the operational amplifier produces a bias current to maintain the operational efficiency of the laser diode. However, feedback loops (e.g., operational amplifiers) with a large bandwidth tend to null out the electrical input that is intended to modulate the optical output of a corresponding laser diode. This is so because the bandwidth of a feedback loop may overlap some or all of the bandwidth of the electrical input. This is problematic in the context of passive optical networks because feedback loops with a large bandwidth are ideal for turning a laser diode on and off quickly (and thus enable optoelectronic transceivers in a passive optical network to transmit more optical data or the inclusion of additional optoelectronic transceivers in the passive optical network). More specifically, feedback loops with a large bandwidth are able to modulate the optical output strength of a given laser diode at a relatively high frequency.  
           [0009]    What is needed in the art, therefore, is an optoelectronic transceiver capable of turning a laser diode on and off within 0.1 to 1.0 microseconds that does not adversely affect electrical input.  
         SUMMARY OF THE INVENTION  
         [0010]    An embodiment of the present invention is directed to a three stage operational amplifier for controlling bias current in a laser diode. The first stage includes a differential amplifier configured to receive as input a reference voltage and a laser diode voltage. The laser diode voltage represents an optical output strength of a laser diode and the reference voltage corresponds to a desired magnitude of the laser diode voltage. A second stage includes a capacitor and is configured to integrate an output current produced by the first stage to generate a first output voltage. A third stage includes an output buffer configured to receive as input the first output voltage to generate a second output voltage that is approximately equal to the first output voltage. The second output voltage is applied to a voltage controlled current source to control the magnitude of a bias current for the laser diode. The differential amplifier of the first stage if formed from a symmetrical assembly of transistors such that a transconductance of the differential amplifier approaches a constant when a difference between the desired magnitude of the laser diode voltage and the actual laser diode voltage is substantially zero volts and increases exponentially as this difference increases.  
           [0011]    Another embodiment of the present invention is directed to a three stage operational amplifier for controlling bias current in a laser diode. The first stage includes a differential amplifier configured to receive as input a reference voltage and a laser diode voltage. The laser diode voltage represents an optical output strength of a laser diode and the reference voltage corresponds to a desired magnitude of the laser diode voltage. The second stage includes a capacitor configured to integrate an output current produced by the first stage to produce a first output voltage. The third stage includes an output buffer configured to receive as input the first output voltage to produce a second output voltage that is approximately equal to the first output voltage. The second output voltage is applied to a voltage controlled current source to control the magnitude of a bias current for the laser diode. The operational amplifier also includes a voltage comparator to compare the laser diode voltage to the reference voltage. The voltage comparator directs a boosting current from a current source to the second stage when a difference between the laser diode voltage and the reference voltage is greater than or equal to a predefined amount.  
           [0012]    Yet another embodiment of the present invention is directed to a three stage operational amplifier. The first stage includes a differential amplifier configured to receive as input a reference voltage and a laser diode voltage. The laser diode voltage represents an optical output strength of a laser diode and the reference voltage corresponds to a desired magnitude of the laser diode voltage. The second stage has a plurality of stages. Each of the stages includes at least a capacitor for integrating an output current produced by the first stage to produce a first output voltage. The third stage includes an output buffer configured to receive as input the first output voltage to produce a second output voltage that is approximately equal to the first output voltage. The second output voltage is applied to a voltage controlled current source to control the magnitude of a bias current for the laser diode. The operational amplifier also includes a voltage comparator to compare the laser diode voltage to the reference voltage. The voltage comparator selects one of the stages from the second stage by reference to a relationship between the laser diode voltage and the reference voltage.  
           [0013]    In still other embodiments, the present invention includes a plurality of optoelectronic transceivers, a coordinator, a controller, an optical combiner, and a shared communication line in a passive optical network. The coordinator is configured to assign each of the plurality of optoelectronic transceivers to a separate portion of a cyclical time period. The controller is configured to turn optical data transmit capabilities of the plurality of optoelectronic transceivers on and off during their respective separate portions of the cyclical time period. The optical combiner is configured to relay optical data received from the plurality of optoelectronic transceivers to the shared communication line. And each of the plurality of optoelectronic transceivers includes an operational amplifier consistent with one of the embodiments described in the preceding paragraphs. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    Additional objects and features of the invention will be more readily apparent from the following detailed description and appended claims when taken in conjunction with the drawings, in which:  
         [0015]    [0015]FIG. 1 is an illustration of an exemplary feedback loop for a laser diode.  
         [0016]    [0016]FIG. 2 is an illustration of a prior art operational amplifier used in feedback loops for laser diodes.  
         [0017]    [0017]FIG. 3A is an illustration of a prior art differential amplifier found in prior art operational amplifiers.  
         [0018]    [0018]FIG. 3B is a graph of the transconductance of the differential amplifier illustrated in FIG. 3A.  
         [0019]    [0019]FIG. 4A is an illustration of a differential amplifier consistent with an embodiment of the present invention.  
         [0020]    [0020]FIG. 4B is a graph of the transconductance of the differential amplifier illustrated in FIG. 4A.  
         [0021]    [0021]FIG. 5 is an illustration of an operational amplifier consistent with an embodiment of the present invention. FIG. 5A is an illustration of a charge switch consistent with an embodiment of the present invention.  
         [0022]    [0022]FIG. 6 is an illustration of another operational amplifier consistent with an embodiment of the present invention.  
         [0023]    [0023]FIG. 7 is an illustration of an operational amplifier that is a functional equivalent of the operational amplifier illustrated in FIG. 6 when in a particular mode of operation.  
         [0024]    [0024]FIG. 8 is an illustration of an optoelectronic transceiver that is consistent with an embodiment of the present invention.  
         [0025]    [0025]FIG. 9 is an illustration of a passive optical network that is consistent with an embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]    Referring to FIG. 1, there is shown an exemplary feedback loop, including a laser driver  10  and a transmitter optical subassembly (“TOSA”)  2 , for controlling a bias current in a laser diode  4  embedded in the TOSA  2 . Persons skilled in the art recognize that a bias current pushes the laser diode  4  beyond its threshold value into a linear operating range. Once in a linear operating range, the optical output strength of the laser diode  4  is modulated by electrical input.  
         [0027]    As shown in the cutout  3 , the TOSA  2  includes the laser diode  4  and a photodiode  6 . The TOSA  2  also includes other components that are not illustrated. The laser driver  10  includes a first capacitor  40 , a first resistor  50 , an operational amplifier  60 , a voltage source  70 , a transistor  80 , electrical input lines  81 , a modulation amplifier  82 , a second capacitor  84 , a second resistor  86 , a third resistor  88 , and other components not illustrated.  
         [0028]    The photodiode  6  detects light emitted from the back facet of the laser diode  4 , which is proportional to the optical output strength of the laser diode  4 . The photodiode  6  converts the detected light into the current I pd . The current I pd  passes from the TOSA  2  to the laser driver  10  and across the first resistor  50  and the first capacitor  40  to produce the voltage V pd . Voltage V pd  is an input to the operational amplifier  60 .  
         [0029]    The operational amplifier  60  produces an output that is a function of two inputs. The first input to the operational amplifier  60  is the voltage V pd  as noted above; the second input to the operational amplifier  60  is a reference voltage V ref . V ref  is produced by the voltage source  70 . The output voltage V out  of the operational amplifier  60  is connected to the gate of a transistor  80  (a.k.a., a voltage controlled current source). The output voltage VOU, thus controls the current I bias  that flows through the transistor  80  and thus through the laser diode  4  to bias the laser diode  4 . The current I bias , therefore, affects the optical output strength of the laser diode  4 .  
         [0030]    Also affecting the optical output strength of the laser diode  4  is the modulation amplifier  82 . The modulation amplifier  82  amplifies the electrical input received over the electrical input lines  81  to produce an electrical signal in the form of the current I data , which mixes with the current I bias  to control the optical output of the laser diode  4 . As illustrated in FIG. 1, the output of the laser driver  10  passes from the laser driver  10  to the TOSA  2  generally and to the laser diode  4  therein specifically.  
         [0031]    The second capacitor  84  and the second and third resistors  86 ,  88  are exemplary components included the laser driver  10 , and which form part of the laser diode feedback loop. Other circuit configurations are possible and within the scope of the present invention.  
         [0032]    Ideally, the optical output strength of the laser diode  4 , which as described above is controlled by I data  and I bias , results in a voltage V pd  that matches the voltage V ref . When this occurs, the laser diode  4  is in a linear operating range, and operating efficiently. The operational amplifier  60 , in preferred embodiments of the present invention, is configured, therefore, to adjust V out  so that V pd  matches the voltage V ref . In other embodiments of the invention, the operational amplifier  60  is configured to, for example, adjust V out , so that V pd  have a predefined relationship to each other, such as a predefined ratio or offset with respect to each other.  
         [0033]    [0033]FIG. 2 shows internal components of a three stage, prior art operational amplifier  60 . Included in the operational amplifier  60  is a differential amplifier  100 , a capacitor  110 , a buffer  120 , and a kill switch  130 . The first stage of the operational amplifier  60  is the differential amplifier  100 , which may amplify and convert a difference between the voltages V pd  and V ref  to produce the current I da .  
         [0034]    The second stage of the operational amplifier  60  includes the capacitor  110 . The current I da  flows through the capacitor  110  to ground (also called circuit ground). While this occurs, the capacitor  110  integrates the current I da  to produce the voltage V out . The voltage V out , across the capacitor  110  is given by the following equation:  
           1   C          ∫       I   da             t           ,                         
 
         [0035]    , where C is the capacitance value of the capacitor  110  and t is time.  
         [0036]    The third stage of the operational amplifier  60  includes the buffer  120 . The input to the buffer  120  is the voltage V out , and the output of the buffer  120  is V out  as well. The purpose of the buffer  120  is to electrically isolate the capacitor  110  from loads placed on the output of the operational amplifier  60 . In this way, a load (e.g., the transistor  80  in FIG. 1) does not change the characteristics of the operational amplifier  100  by drawing current from capacitor  110 .  
         [0037]    The kill switch  130  is an exemplary means for disabling and enabling the operational amplifier  60 . More specifically, the kill switch  130  typically is a transistor that can short the current I da  and drain any charge from the capacitor  110  in response to a kill signal from an external source. When the current I da  is shorted and any charge from the capacitor  110  is drained, the voltage V out  is pulled to ground. Additionally, a resistor  122  is preferably included between both the capacitor  110  and the differential amplifier  100  and the kill switch  130 . This resistor prevents damaging current spikes each time the kill switch  130  is activated to short the current I da  and drain any charge from the capacitor  110 .  
         [0038]    Referring now to FIG. 3A, there is shown a prior art differential amplifier  100 . The differential amplifier  100  includes a first and second transistor  310 ,  320 , a current source  330 , and a current mirror  340 . The first and second transistors  310 ,  320  are identical. The base of the first transistor  310  is connected to the voltage V pd  and the emitter of the first transistor  310  is tied to the emitter of the second transistor  320  and the current source  330 . The collector of the first and second transistors  310 ,  320  are separately connected to the current mirror  340 . And the base of the second transistor is connected to the voltage V ref . The two currents I pd  and I ref  flow through the first and second transistors  310 ,  320  respectively and into or out of the current mirror  340 . The current mirror  340  is an arbitrary circuit that subtracts the current I ref  from the current I pd  to produce the current I da ,  
         [0039]    Because the emitters of the first and second transistors  310 ,  320  are tied together, the voltage at both emitters is the same. As persons skilled in the art recognize, current flow through a transistor is controlled in part by the voltage across the base and emitter. And because the voltage at the emitters of the first and second transistors  310 ,  320  is identical and the current supplied by the current source  330  is shared by the first and second transistors  310 ,  320 , the current that flows through the first and second transistors  310 ,  320  is controlled by the relative values of the voltages V pd  and V ref . This may mean, for example, that when V pd  and V ref  are the same, the currents I pd  and I ref  are nearly identical (e.g., one half of the value of the current source  330 ) and cancel each other out (e.g., I da =I pd −I ref ). Further, if the voltage V pd  is, for example, greater than the voltage V ref , I pd  is greater than I ref  and vice versa. And finally, the current flow through one of the transistors equals the value of the current source  330  only when the other transistor turns off entirely. This occurs when the voltage V pd  or V ref  is not high enough to enable current flow through a respective transistor  310 ,  320 .  
         [0040]    The transconductance of the differential amplifier  100  (e.g., (I pd −I ref )/(V pd −V ref )) is illustrated in FIG. 3B. As shown in FIG. 3B, the slope of the transconductance, in an embodiment of the present invention, increases (e.g., a difference between I pd  and I ref  changes at a higher rate) as voltages V pd  and V ref  converge. Similarly, as a difference between V pd  and V ref  increases, in an embodiment of the present invention, the slope of the transconductance approaches a constant. Ultimately, the slope of the transconductance is effectively constant when a difference between I pd  and I ref  is equal to the value of the current source  330 . This may mean that beyond a certain threshold, increases in a difference between V pd  and V ref  have no significant effect on the value of the current I da  (e.g., the gain of the operational amplifier  60  approaches zero).  
         [0041]    In the context of a laser diode  4  feedback loop, the greatest amount of amplification by this operational amplifier  60  takes place when the laser diode  4  is in a linear operating range. This is problematic because the differential amplifier  100 , and thus the feedback loop as a whole, may null out low frequency signal components (e.g., frequency components below the bandwidth of the feedback loop) from the electrical signal (e.g., the current I data ) transmitted to the laser diode  4 . Additionally, the slope of the transconductance approaches a constant (e.g., the gain of the differential amplifier  100  is relatively low) just as the laser diode  4  is turned on (e.g., when a difference between V pd  and V ref  is greatest) and the operational amplifier  60  attempts to drive the laser diode  4  into a linear operating range.  
         [0042]    As noted above, operational amplifiers typically consist of three stages. FIG. 4A discloses an embodiment of the present invention in which the first stage of an operational amplifier is adjusted to address the problems with the prior art identified above. More specifically, the differential amplifier  400  illustrated in FIG. 4A is formed from a symmetrical configuration of transistors and current sources. With respect to the first transistor  410 , the base is connected to the voltage V pd , the collector is connected to a current mirror  340 , and the emitter is connected to the base of a second transistor  420 , a first current source  430 , and the emitter of a third transistor  440 . With respect to the second transistor  420 , the collector is connected to a voltage V cc  and the emitter is connected to the base of a fourth transistor  450  and a second current source  460 . With respect to the third transistor  440 , the collector is connected to circuit ground and the base is connected to a third current source  470  and the emitter of a fifth transistor  480 . With respect to the fourth transistor  450 , the collector is connected to circuit ground and the emitter is connected to a fourth current source  490 , the base of the fifth transistor  480 , and the emitter of a sixth transistor  492 . With respect to the fifth transistor  480 , the collector is connected to the voltage V cc . And with respect to the sixth transistor  492 , the base is set to the voltage V ref  and the collector is connected to the current mirror  340 .  
         [0043]    This configuration of transistors results in the transconductance profile illustrated in FIG. 4B. Note that the slope of the transconductance approaches a constant (i.e, the gain of the differential amplifier  400  is reduced) as the voltages V pd  and V ref  converge. When the laser diode  4  is in or close to a linear operating range, the differential amplifier  400  has relatively little effect on the electrical signal (e.g., the current I data ) applied to the laser diode  4 . Note that in other embodiments of the invention, the transconductance approaches a constant as the voltage V pd  approaches a value that is a predefined function (e.g., a multiple or offset) of the voltage V ref .  
         [0044]    The slope of the transconductance increases exponentially as a difference between V pd  and V ref  increases. This means that just as the laser diode  4  is turned on (e.g., when a difference between V pd  and V ref  is greatest), the differential amplifier  400 , and thus the feedback loop as a whole, has its greatest effect on the optical output strength of the laser diode  4 . Additionally, the current I da , the output of the differential amplifier  400 , is nearly maximized across a greater range of V pd  and V ref  differences. Thus, the differential amplifier  400 , can continue to have its greatest effect when the laser diode  4  is in or close to a linear operating range.  
         [0045]    Of particular importance in the differential amplifier  400  illustrated in FIG. 4A is that the transistor  410  is preferably an npn transistor while the transistor  440  is preferably a pnp transistor. Again, the emitter of the transistor  410  is connected to the emitter of the transistor  440 . As a result, the current flowing through the transistor  410  and the transistor  440  are nearly equal in magnitude and direction. In the differential amplifier  100  illustrated in FIG. 3A, the emitters of the two transistors  310 ,  320  are connected, but both are npn transistors, so the current flowing through each tends to offset and limit the current flowing through the other to the magnitude of the current source  330 . This is not the case for the differential amplifier  400  illustrated in FIG. 4A. Instead, a nearly infinite amount of current can flow through the two transistors  410 ,  440  as V pd  increases.  
         [0046]    And as noted above, the differential amplifier  400  illustrated in FIG. 4A is symmetrical, so as V pd  oscillates around V ref , the current flowing through the transistors  492  and  450  is nearly equal in magnitude to the current flowing through the transistors  410  and  440 , but flowing in the opposite direction. The current I da  produced by the current mirror  340  increases proportionately to changes in the current I pd  and the current I ref .  
         [0047]    Referring to FIG. 5, there is illustrated another embodiment of the present invention in which the second stage of an operational amplifier is adjusted to address the problems with the prior art identified above. The operational amplifier  500  illustrated in FIG. 5 includes a differential amplifier  100  or  400 , a voltage comparator  510 , a charge switch  520 , a current source  530 , a capacitor  110 , a kill switch  130 , and a buffer  120 . The operational amplifier  500  may be used in a laser bias current control system, such as the system shown in FIG. 1, in place of operational amplifier  60  shown in FIG. 1.  
         [0048]    The kill switch  130  is an exemplary means for disabling and enabling the operational amplifier  500 . More specifically, the kill switch  130  typically is a transistor that can short the current I da  and drain any charge from the capacitor  110  in response to a kill signal from an external source. When the current I da  is shorted and any charge from the capacitor  110  is drained, the voltage V out  is pulled to ground. Additionally, a resistor  122  is preferably included between both the capacitor  110  and the differential amplifier  100  and the kill switch  130 . This resistor prevents damaging current spikes each time the kill switch  130  is activated to short the current I da  and drain any charge from the capacitor  110 .  
         [0049]    The first stage of the operational amplifier  500  is the differential amplifier  100  or  400 , which as noted above amplifies and converts a difference between the voltages V pd  and V ref  to produce the current I da .  
         [0050]    The second stage of the operational amplifier  500  includes the capacitor  110 , the voltage comparator  510 , the charge switch  520 , and the current source  530 . Depending on the state of the current switch  520 , either the current I da , or the current I da  and the current I charge , flow through the capacitor  110  to circuit ground. While this occurs, the capacitor  110  integrates the current(s) to produce the voltage V out .  
         [0051]    The third stage of the operational amplifier  500  includes the buffer  120 . The voltage V out  is the input to the buffer  120 . The output of the buffer  120  is also the voltage V out . The purpose of the buffer  120  is to electrically isolate the capacitor  110  from loads placed on the output of the operational amplifier  500 . In this way, a load (e.g., the transistor  80 , FIG. 1) does not change the characteristics of the operational amplifier  500  by drawing current from the capacitor  110 .  
         [0052]    The voltage comparator  510  is configured to activate the charge switch  520  when, for example, a difference between the voltages V pd  and V ref  is beyond a defined threshold (e.g., when the laser diode  4  is not in or close to a linear operating range). The charge switch  520 , which preferably is formed using two transistors ( 522 ,  524 , FIG. 5A), either shunts current produced by the current source  530  to circuit ground or allows current produced by the current source  530  to mix with the current I da . More specifically, when the charge switch  520  is activated, the current I charge  flows through the charge switch  520  and mixes with the current I da  When the charge switch  520  is not activated, it shunts the current I charge  to ground.  
         [0053]    [0053]FIG. 5A shows a preferred embodiment of the charge switch. The gate of the first transistor  422  is connected to the voltage output of the voltage comparator  510 , the drain of the first transistor is connected to the second stage, and the source of the first transistor is connected to the current source  530  and to a source of the second transistor. The gate of the second transistor  524  is connected to a bias voltage and the drain of the second transistor  522  is connected to the circuit ground. The bias voltage is preferably set such that when the voltage output of the voltage comparator  510  indicates that a difference between the laser diode voltage and the reference voltage is greater than or equal to a predefined threshold (i.e, a predefined amount), substantially all of the boosting current flows from the current source  530  through the first transistor  522  to the second stage. Further, the bias voltage is set such that when the voltage output of the voltage comparator  510  indicates that a difference between the laser diode voltage and the reference voltage is less than the predefined threshold, substantially all of the boosting current flows from the current source  530  through the second transistor  524  to the circuit ground.  
         [0054]    Referring again to FIG. 5, if the kill switch  130  is not activated, the current I charge  mixes with the current I da  to produce a voltage across the capacitor  110 . As a result, the voltage output (e.g., the voltage V out ) of the second stage of the operational amplifier is boosted by a fixed amount, as determined by the magnitude of the current I charge , over a defined range of values for the voltage V pd . In other words, the voltage V out , which controls the current I bias  via a transistor  80  (FIG. 1), is increased when needed most (e.g., when the laser diode  4  is not in or close to a linear operating range).  
         [0055]    When, for example, a difference between the voltages V pd  and V ref  is within the defined threshold, the operational amplifier  500  can behave in much the same way as prior art operational amplifiers  60  depending on the configuration of the differential amplifier utilized (e.g., a prior art differential amplifier  100  or the novel differential amplifier  400  described above).  
         [0056]    Because the voltage V out  can be adjusted as needed by the use and magnitude of the current I charge , the operational amplifier  500  can be configured so that the gain is minimized when the laser diode  4  is in or close to a linear operating range.  
         [0057]    Referring to FIG. 6, there is illustrated another embodiment of the present invention in which the second stage of an operational amplifier is adjusted to address the problems with the prior art identified above. The operational amplifier  600  illustrated in FIG. 6 may be used in place of the operational amplifier  60  included in the feedback loop illustrated in FIG. 1.  
         [0058]    The operational amplifier  600  illustrated in FIG. 6 includes a set of switches to control the gain and frequency bandwidth of the operational amplifier  600 . In particular, when V pd  and V ref  are within a defined threshold of each other, the functionality of the second stage of the operational amplifier  600  is controlled primarily by capacitor  110 , much like the prior art operational amplifier  60 . But when V pd  and V ref  are not within the defined threshold or ratio of each other, the functionality of the second stage of the operational amplifier  600  is controlled by a capacitor  680  and a resistor  670  in series.  
         [0059]    In more detail now, the operational amplifier  600  includes a differential amplifier  100  or  400 , a voltage comparator  510 , a capacitor  110 , a kill switch  130 , a buffer  120 , a fast loop switch  640 , another capacitor  680 , a resistor  670 , a slow loop switch  660 , another fast loop switch  650 , and another buffer  690 . The first stage of the operational amplifier  600  is the differential amplifier  100  or  400 , which as noted above amplifies and converts a difference between the voltages V pd  and V ref  to produce the current I da .  
         [0060]    The second stage of the operational amplifier  600  includes the capacitor  110 , the voltage comparator  510 , the fast loop switch  640 , the other capacitor  680 , the resistor  670 , the slow loop switch  660 , and the other fast loop switch  650 . The current I da  flows through either the resistor  670  and the capacitor  680  or the capacitor  110  depending on the state of the two fast loop switches  640 ,  650  and the slow loop switch  660 .  
         [0061]    The third stage of the operational amplifier  600  includes the buffer  120 . The voltage V out  is the input to the buffer  120 . The output of the buffer  120  is also the voltage V out . The purpose of the buffer  120  is to electrically isolate the capacitor  110  from loads placed on the output of the operational amplifier  600 . In this way, a load (e.g., the transistor  80 , FIG. 1) does not change the characteristics of the operational amplifier  600  by drawing current from capacitor  110  and/or capacitor  680 .  
         [0062]    When a difference or ratio between V pd  and V ref , as measured by the voltage comparator  510 , is greater than a defined predefined threshold, the output of the voltage comparator  510  closes the two fast loop switches  640 ,  650  and opens the slow loop switch  660  (note the input to the two fast loop switches  640  is preferably inverted so that the single output of the voltage comparator  510  operates on the two fast loop switches  650 ,  660  and the slow loop switch  660 ). When the switches are in this state, the operational amplifier  600  is the functional equivalent of the operational amplifier  700  illustrated in FIG. 7.  
         [0063]    Note that the buffer  690  and the capacitor  10  are omitted from the operational amplifier  700  even though both are actually active elements of the operational amplifier  700 . The voltage output of the buffer  690  is equal to the input voltage despite the presence of the capacitor  110 . In other words, the buffer  690  drives the voltage across the capacitor  110  to match the voltage across the resistor  670  and the capacitor  680 . The capacitor does not, therefore, significantly affect the gain or frequency bandwidth of the operational amplifier  700  when the switches  640 ,  650 ,  660  are in this configuration. The voltage V out  of the operational amplifier  700  is, therefore, set by the resistor  670  and the capacitor  680 .  
         [0064]    Persons skilled in the art recognize that as the frequency of a current increases, the voltage the current produces across a given capacitor  680  decreases. However, the ratio of voltage to current for the resistor  670  is constant even as the frequency of the current increases. As a result, the resistor  670  provides a gain floor for the second stage of the operational amplifier  600  at higher frequencies.  
         [0065]    When a difference or ratio between V pd  and V ref , as measured by the voltage comparator  510 , is within the defined threshold, the output of the voltage comparator  510  opens the two fast loop switches  640 ,  650  and closes the slow loop switch  660 . In this state, the operational amplifier  600  is the functional equivalent of the operational amplifier  60  illustrated in FIG. 2.  
         [0066]    The kill switch  130  is an exemplary means for disabling the operational amplifier  600  regardless of the switch states. More specifically, the kill switch  130  typically is a transistor that shorts the output I da  and drains any charge from the capacitor  680  and/or the capacitor  110  in response to a kill signal from an external source. When the output I da  is shorted and the charge from the capacitor  680  and/or the capacitor  110  is drained, the voltage V out  is pulled to ground. Additionally, a resistor  122  is preferably included between the capacitor  110 , the capacitor  680 , and the differential amplifier  100  or  400  on the one side, and the kill switch  130  on the other side of the resistor  122 . This resistor  122  prevents damaging current spikes each time the kill switch  130  is activated to short the current I da  and drain any charge from the capacitor  680  and/or the capacitor  110 .  
         [0067]    In this embodiment of the present invention, the operational amplifier  600  can be configured to have two distinct gains depending on whether the laser diode  4  is in or close to a linear operating range (e.g., whether the voltages V pd  and V ref  have converged). More specifically, the capacitance value of the capacitor  110  is preferably selected so that the gain of the operational amplifier  600  is minimized when the laser diode  4  is in or close to a linear operating range (e.g., a difference between the voltages V pd  and V ref  is within a defined threshold). In this way, the operational amplifier  600  is unable to null out the electrical signal (e.g., the current I data ) meant to modulate the optical output of the laser diode  4 . In contrast, the capacitance value of the capacitor  680  and the resistance value of the resistor  670  are preferably selected so that the gain of the operational amplifier  600  is maximized when the laser diode  4  is not in or close to a linear operating range (e.g., a difference between the voltages V pd  and V ref  is beyond a defined threshold).  
         [0068]    Referring to FIG. 8, there is shown a TOSA  2  and a laser driver  10  consistent with an embodiment of the present invention (e.g., including a laser driver  10  with the differential amplifier  400  illustrated in FIG. 4A and/or the operational amplifier  500  illustrated in FIG. 5 or the operational amplifier  600  illustrated in FIG. 6) incorporated in an optoelectronic transceiver  800 . The optoelectronic transceiver  800  also includes a receiver optical subassembly (ROSA)  804 , post-amplifier (“postamp”)  806 , and an integrated circuit (“IC”)  812 . The ROSA  804  converts optical input received over an optical input line  802  (e.g., optical fiber) to an electrical signal that is fed to the postamp  806 , which amplifies and outputs this electrical signal over the electrical output lines  808 . As indicated above, the laser driver  10  processes electrical input received over the electrical input lines  81  and feeds the processed electrical input to the TOSA  2 , which converts this input to optical output that is transmitted through the optical output line  810 .  
         [0069]    The IC  812  interfaces with external components (e.g., a controller  912 , FIG. 9) through the control data lines  814  to exchange control signals and data. The control signals may include a Loss of Signal signal, a Transmitter Fault Indication signal, a Transmitter Disable Input signal, a Clock signal, and one or more other data signals. Preferably, the control signals may direct the IC  812  to turn optical transmit capabilities of the optoelectronic transceiver  800  on and off. More specifically, one or more of these control signals may direct the IC  812  to turn the laser diode  4  on and off (e.g., to drop V ref  to zero, manipulate kill switches, etc.) via the laser driver  10 . And as illustrated in FIG. 8, the IC  812  has one or more connections to the laser driver  10 , the ROSA  804 , and the postamp  806 . The IC  812  uses these connections to control the operation of, and to obtain operational data from, the TOSA  2 , the laser driver  10 , the ROSA  804 , and the postamp  806 .  
         [0070]    Referring to FIG. 9, there is shown a portion of an exemplary passive optical network  900 . Included in FIG. 9 are a plurality of optoelectronic transceivers  800 , a plurality of optical diplexers  902 , an optical combiner  904 , a plurality of communication lines  906 , a shared communication line  908 , a coordinator (e.g., an optical gateway, base station, etc.)  910 , and a controller  912 . The portion of the exemplary passive optical network  900  not illustrated may include elements identical to, and in communication with, some or all of those illustrated in FIG. 9.  
         [0071]    As noted above, a passive optical network  900  enables a plurality of optoelectronic transceivers  800  to share one or more optical fibers while transmitting and receiving data in an optical form. Each optoelectronic transceiver  800  illustrated in FIG. 9 represents a separate channel of communication (e.g., channel  1  through channel n). Electrical input that passes through the controller  912  to an optoelectronic transceiver  800  over the electrical input lines  81  is transmitted in an optical form to an optical diplexer  902  through the optical output lines  810 . Additionally, optical input received from an optical diplexer  902  by each optoelectronic transceiver  800  through the optical input lines  802  is transmitted in an electrical form over the electrical output lines  808  through the controller  912 . The source and destination of electrical input and output, respectively, may vary without departing from the scope of the present invention.  
         [0072]    The optical diplexers  902  are devices that exchange data with a first device over a single communication line and with one or more other devices over two or more other communication lines. In preferred embodiments, the optical diplexers  902  communicate with optoelectronic transceivers  800  over the optical input line  802  and the optical output line  810  and communicate with the optical combiner  904  over the communication line  906 . So in a preferred embodiment, optical input received from and transmitted to the optical combiner  904  shares the communication line  906 , which may be an optical fiber, as illustrated in FIG. 9. This sharing is facilitated by the use of a different wavelengths for optical data exchanged by the optical diplexers and the optical combiner  904 . Typically, a wave division multiplexing access (“WDMA”) scheme is used to simultaneously transmit a plurality of light signals through an optical fiber. Persons skilled in the art recognize that optical data transmitted in a given wavelength can travel through optical fiber without losing its identity or interfering with optical data transmitted in other wavelengths. Electronic equipment (e.g., an optical diplexer  902 ) receiving optical data formed by a plurality of wavelengths can distinguish different signals by their respective wavelengths.  
         [0073]    The optical combiner  904  (or alternatively, optical combiner and splitter  904 ) preferably transmits all data received from optoelectronic transceivers  800  to the shared communication line  908  without regard to wavelength or origin and vice versa. In other words, the optical combiner  904  preferably does not partition bandwidth of the shared communication line  908 . Instead, this partitioning is preferably handled by the controller  912  and the coordinator  910 .  
         [0074]    The coordinator  910  coordinates the activities of the optoelectronic transceivers  800  illustrated in FIG. 9. In particular, the coordinator  910  determines the timing and duration of optical data transmission by the optoelectronic transceivers  800 . Additionally, the coordinator  910  may also interact with other coordinators  910  and controllers  912  (to control other sets of optoelectronic transceivers  800 ) (not illustrated) in the passive optical network  900  as needed.  
         [0075]    Similarly, the controller  912  is an electronic device that controls the optoelectronic transceivers  800 . More specifically, the controller  912  turns the optical transmit capabilities (e.g., the laser diode  4 ) of the optoelectronic transceivers  800  on and off, thus enabling effective use of the shared communication line  908 . The controller  912  also monitors data received by the optoelectronic transceivers  800 . In particular, the controller  912  determines whether data received by a given optoelectronic transceiver  800  is intended for this optoelectronic transceiver  800 , in which case the data may pass through the controller  912  on corresponding electrical output lines  808 , whether data received by a given optoelectronic transceiver  800  includes setup or other commands from the coordinator  910  related to this optoelectronic transceiver  800 , in which case the data may not pass through the controller  912 , or whether data received by a given optoelectronic transceiver  800  is destined for another optoelectronic transceiver  800 , in which case the data may not pass through the controller  912 .  
         [0076]    Typically, when an optoelectronic transceiver  800  is added to the passive optical network  900  and turned on, the controller  912  communicates with the coordinator  910  through the optoelectronic transceiver  800  and the data paths illustrated in FIG. 9 to setup and synchronize the optoelectronic transceiver  800 . Typically, the controller  912  and the coordinator  910  use a predetermined channel or bits within transmitted data for such communication. The optoelectronic transceiver  800  does not distinguish between this communication and data received and transmitted after the setup is complete.  
         [0077]    Based on turn-on and turn-off capabilities of the optoelectronic transceivers  800  and the number of optoelectronic transceivers  800  included in the passive optical network  900 - 1 , the coordinator  910 , among other things, assigns the newly added optoelectronic transceiver  800  to a specific time slot (e.g., a specific portion of a given temporal cycle) and may adjust (e.g., shorten, lengthen, offset, etc.) the time slots to which other optoelectronic transceivers  800  in the passive optical network  900 - 1  are assigned. The controller  912  is subsequently responsible for turning the optical transmit capabilities of the newly added optoelectronic transceiver  800  on and off at the beginning and end of the newly added optoelectronic transceiver&#39;s  800  assigned time slot. Typically, the receive capabilities of a given optoelectronic transceiver  800  are not turned off while the optoelectronic transceiver  800  is part of a passive optical network  900 . As noted above, the controller  912  preferably determines whether data received by a given optoelectronic transceiver  800  is intended for this optoelectronic transceiver.  
         [0078]    While preferred embodiments of the present invention have been disclosed in connection with FIGS. 4A, 5,  5 A,  6 ,  7 ,  8 , and  9 , it will be understood that in view of the foregoing description, other configurations can provide one or more of the features of the present invention, and all such other configurations are contemplated to be within the scope of the present invention. Accordingly, it should be clearly understood that the embodiments of the invention described above are not intended as limitations on the scope of the invention, which is defined only by the claims that are now or may later be presented.  
         [0079]    For example, in some embodiments of the present invention, optical diplexers  902  are not used. In these embodiments, two channels of communication (e.g., two strands of optical fiber) connect the optoelectronic transceivers  800  and the optical combiner  904  and the shared communication line  908  consists of two channels of communication (e.g., two strands of optical fiber). In these embodiments, separate wavelengths need not be used to transmit optical data. In still other embodiments, a separate controller  912  is included in the passive optical network  900  for each optoelectronic transceiver.