Abstract:
A laser diode optical transmitter and a method of operating the optical transmitter utilize a transistor to quickly enable and disable a laser diode by selectively shorting the laser diode. In an embodiment, the transistor is separated from the laser diode by an inductor. The inductor provides a high impedance between the transistor and the laser diode to at least reduce the effect of a parasitic capacitance associated with the transistor when the laser diode is driven to generate optical signals of different power levels. The use of the transistor and the inductor (i) reduces the amount of leakage light generated by the laser diode when in a disabled state, (ii) shortens the enable and disable times of the optical transmitter, and (iii) allows the laser diode to be modulated at a high rate of speed.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to optical transmitters, and more particularly to an optical transmitter for a time division multiplex access (TDMA) system. 
     BACKGROUND OF THE INVENTION 
     The explosion of the Internet and the desire to provide multiple communications and entertainment services to end users have created a need for a broadband network architecture that improves access to end users. One broadband network architecture that improves access to end users is a point-to-multipoint passive optical network (PON). A point-to-multipoint PON is an optical access network architecture that facilitates broadband communications between an optical line terminal (OLT) and multiple remote optical network units (ONUs) over a purely passive optical distribution network. A point-to-multipoint PON utilizes passive fiber optic splitters and couplers to passively distribute optical signals between the OLT and the remote ONUs. 
       FIGS. 1A and 1B  illustrate the management of network traffic in a point-to-multipoint PON. As an example, the PON is shown to include an OLT  102  and three ONUs  104 ,  106  and  108 , although the PON may include additional ONUs. Referring to  FIG. 1A , the OLT includes an optical transmitter  110  that sends downstream traffic containing ONU-specific information blocks  1 ,  2  and  3  to the ONUs. The downstream traffic is optically broadcasted by a passive optical splitter  112  into three separate signals that each carries all of the ONU-specific information blocks. The ONUs  104 ,  106  and  108  include optical receivers  114 ,  116  and  118 , respectively, that receive all the information blocks transmitted by the OLT. Each ONU then processes the information blocks that are intended for that ONU and discards the information blocks that are intended for the other ONUs. For example, ONU- 1  receives information blocks  1 ,  2 , and  3 . However, ONU- 1  only delivers information block  1  to end user  1 . Likewise, ONU- 2  only delivers information block  2  to end user  2  and ONU- 3  only delivers information block  3  to end user  3 . 
     Referring to  FIG. 1B , the ONUs  104 ,  106  and  108  also include optical transmitters  120 ,  122  and  124 , respectively, to transmit upstream traffic to OLT  102 . The upstream traffic is managed utilizing time division multiplexing, in which specific transmission time slots are dedicated to individual ONUs. The ONU-specific time slots are synchronized so that upstream information blocks from the ONUs do not interfere with each other once the information blocks are combined onto the common fiber. For example, ONU- 1  transmits information block  1  in a first ONU-specific time slot, ONU- 2  transmits information block  2  in a second ONU-specific time slot, and ONU- 3  transmits information block  3  in a third ONU-specific time slot. The time division multiplexed upstream traffic is then received by an optical receiver  126  of the OLT. 
     There are a number of factors that contribute to the efficiency of a point-to-multipoint PON. One such factor is the length of guard bands between combined information blocks of the upstream traffic. These guard bands, or dark spaces, provide safety zones between information blocks to prevent collision of adjacent information blocks when they are combined onto the common fiber. However, the guard bands can occupy a significant amount of bandwidth and consequently, reduce the overall bandwidth of the PON for data transmission. Thus, minimizing the length of the guard bands will increase the bandwidth of the PON. However, in order to reduce the length of the guard bands, the optical transmitters of the ONUs must be able to more quickly start and stop sending optical signals, i.e., faster enable and disable times, to ensure that combined upstream information blocks are properly separated by the guard bands. 
     Another factor that contributes to the efficiency of the PON is the speed with which the optical transmitters of the ONUs can emit binary optical signals. That is, the speed with which the optical transmitters can modulate between “1” signals and “0” signals. Using optical transmitters with increased modulation speed, the PON can increase the rate with which data is transmitted between the OLT and the ONUs. 
     Still another factor that contributes to the efficiency of the PON is the amount of light leakage from disabled optical transmitters of the ONUs. Any leakage of light from a single disabled optical transmitter may increase the background noise. This is critical in a point-to-multipoint PON where light leakage from one disabled ONU will combine with light leakage from other disabled ONUs of the PON. The overall effect is that significant background noise may be created by the combined light leakage, which may affect the OLT from differentiating legitimate data from the background noise. This effect is amplified as the number of ONUs supported by the PON is increased. 
     In view of these factors, there is a need for an optical transmitter with fast enable and disable times, increased modulation speed, and reduced light leakage. 
     SUMMARY OF THE INVENTION 
     A laser diode optical transmitter and a method of operating the optical transmitter utilize a transistor to quickly enable and disable a laser diode by selectively shorting the laser diode. In an embodiment, the transistor is separated from the laser diode by an inductor. The inductor provides a high impedance between the transistor and the laser diode to at least reduce the effect of a parasitic capacitance associated with the transistor when the laser diode is driven to generate optical signals of different power levels. The use of the transistor and the inductor (i) reduces the amount of leakage light generated by the laser diode when in a disabled state, (ii) shortens the enable and disable times of the optical transmitter, and (iii) allows the laser diode to be modulated at a high rate of speed. 
     In an exemplary embodiment, the optical transmitter includes a light emitting device, a switching device, a modulation circuit and an inductive element. The light emitting device may be a laser diode that can generate light of different optical power levels. The switching device may be a transistor that is connected in parallel to the light emitting device to selectively short the light emitting device to disable the optical transmitter. In an embodiment, the transistor is a PNP bipolar transistor. The modulation circuit is configured to drive the light emitting device to generate light of different optical power levels. In an embodiment, the modulation circuit include a pair of differential transistors that are connected to the light emitting device and a modulation current source. The inductive element of the optical transmitter may be an inductor that provides an impedance between the light emitting device and the switching device. The impedance provided by the inductive element to at least reduce the effect of parasitic capacitance associated with the switching element when the light emitting device is driven by the modulation circuit. 
     In an embodiment, the optical transmitter further includes a controller that is connected to the switching device. The controller is configured to control the conductive state of the switching device. In one embodiment, the controller includes an input that receives a patterned signal. The controller is configured to activate the switching device to a conducting state when the patterned signal is not received by the controller. 
     In an exemplary embodiment, the method of operating an optical transmitter includes a step of driving a light emitting device to generate optical signals of different power levels and a step of providing an inductive impedance between the light emitting device and at least a switching device that can electrically short the light emitting diode. The inductive impedance reduces at least the effect of parasitic capacitance associated with the switching device when the light emitting device is driven to generate the optical signals. In an embodiment, the method also includes a step of receiving an indicator that is associated with a predefined condition to disable the optical transmitter and a step disabling the optical transmitter by activating the switching device to short the light emitting device in response to the indicator. In one embodiment, the step of receiving the indicator includes not receiving a patterned signal from an external source. 
     Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrated by way of example of the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates the downstream flow of traffic from an OLT to multiple ONUs in a point-to-multipoint PON. 
         FIG. 1B  illustrates the upstream flow of traffic from the ONUs to the OLT in the point-to-multipoint PON. 
         FIG. 2  is a schematic diagram of a laser diode optical transmitter that can be used in a point-to-multipoint PON in accordance with the present invention. 
         FIG. 3  illustrates the power levels of optical signals generated by the optical transmitter of FIG.  2 . 
         FIG. 4  illustrates the parasitic capacitors associated with some of the electrical components of the optical transmitter of FIG.  2 . 
         FIG. 5  is process flow diagram of a method of operating the optical transmitter of  FIG. 2  in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 2 , a laser diode optical transmitter  202  for use in a passive optical network (PON) system that utilizes time division multiplex access (TDMA) is shown. The optical transmitter is characterized by fast enable and disable times so that the length of the guard bands used by the system can be minimized. In addition, the optical transmitter is designed to prevent, or significantly reduce, light leakage when disabled. Furthermore, the optical transmitter is designed to modulate output optical signals with sufficient speed for data transmission in the Gigabit per second range. 
     The optical transmitter  202  includes a laser diode  204  that is driven to emit binary optical signals. As illustrated in  FIG. 3 , the laser diode can be driven to generate light signals having an optical power level of P 1 , which are optical representations of “1” signals. Alternatively, the laser diode can be driven to generate light signals having an optical power level of P 0 , which are optical representations of “ 1 ” signals. 
     As shown in  FIG. 2 , the anode of the laser diode  204  is coupled to a supply voltage terminal  206 , while the cathode of the laser diode is coupled to an inductor  208 . In parallel to the laser diode, a PNP bipolar transistor  210  is connected between the supply voltage terminal and the inductor. The emitter of the PNP transistor is coupled to the supply voltage terminal, while the collector of the PNP transistor is coupled to the inductor and a bias current source  212 , which is connected to electrical ground. The bias current source conducts bias current through the laser diode when the PNP transistor is in a non-conducting state. However, when the PNP transistor is in a conducting state, the laser diode is shorted by the transistor and is disabled. 
     The laser diode  204  of the optical transmitter  202  is also coupled to the collector of an NPN bipolar transistor  214 . The transistor  214  is part of a differential pair of NPN bipolar transistors  214  and  216 . The collector of the transistor  216  is coupled to a resistor  218 , which is connected to the supply voltage terminal  206 . The bases of these transistors  214  and  216  are connected to a differential amplifier  220  having inputs  222  and  224 . Signals applied to the inputs of the differential amplifier control the conducting state of the transistors  214  and  216 . The emitters of the transistors  214  and  216  are connected to a modulation current source  226 , which is connected to ground. The modulation current source conducts modulation current that flows through the laser diode then the transistor  214  is switched to a conducting state. 
     The optical transmitter  202  further includes a micro-controller unit  228  that controls various components of the transmitter. The micro-controller unit utilizes a digital-to-analog converter  230  to control the bias current source  212 . Similarly, the micro-controller unit utilizes a second digital-to-analog converter  232  to control the modulation current source  226 . The micro-controller unit can also control the PNP transistor by sending a disable signal through path  234  to an OR gate  236 . The OR gate can also receive a disable signal from an external source. The OR gate is connected to an inverter  238  that outputs a signal to the base of the PNP transistor  210  via a resistor  240 . Thus, a single disable signal to the OR gate will turn “on” the PNP transistor to disable the optical transmitter. The output of the OR gate is also connected to the differential amplifier  220  to disable and enable the differential amplifier. 
     The micro-controller unit  228  includes a security input  242 , which is used to disable the optical transmitter  202  under certain conditions. These conditions may include the initialization period of an ONU embodying the optical transmitter  202 , a failure of critical ONU components, and other conditions when the optical transmitter should not output any light. Under normal operation, a special pattern is received by the micro-controller unit through the security input. However, when the special pattern is lost due to a specified condition, the micro-controller unit sends a disable signal to the OR gate  236 , disabling the optical transmitter to ensure that light is not emitted by the laser diode  204 . Thus, under these conditions, the optical transmitter will not erroneously transmit light, which may interfere with the operation of a PON system that embodies the optical transmitter. 
     In operation, the micro-controller unit  228  provides a bias control signal to the bias current source  212 , so that a predefined amount of current is flowing through the bias current source. Similarly, the micro-controller unit provides a modulation control signal to the modulation current source  226 , so that a predefined modulation current is flowing through the modulation current source. In a disabled state, the PNP transistor  210  is activated to a conducting state by a disable signal to the OR gate  236 , which may be from an external source or the micro-controller unit. The output signal from the OR gate is inverted by the inverter  238  and then applied to the base of the PNP transistor, turning “on” the PNP transistor. The output signal from the OR gate also disables the differential amplifier  220 . When activated, the PNP transistor provides a current path that diverts current away from the laser diode  204 , i.e. electrically shorts the laser diode. 
     The optical transmitter  202  is switched to an enabled state by removing the disable signal to the OR gate  236 , which turns “off” the PNP transistor  210  and enables the differential amplifier  220 . Consequently, the bias current is routed though the laser diode  204 , which then generates light having an output power level of P 0 , as illustrated in FIG.  3 . The light output of the laser diode is modulated between the power levels P 0  and P 1  by the signals on the inputs  222  and  224  of the differential amplifier  220 . When a “high” signal is applied to the input  222  and a “low” signal is applied the input  224 , the transistor  214  is turned “on” and the transistor  216  is turned “off”. The activation of the transistor  214  increases the amount of current through the laser diode  204  by allowing the modulation current to flow through the laser diode. The increased current drives the laser diode to increase the output power level to P 1 , as illustrated in FIG.  3 . However, when the signals to the inputs of the differential amplifier are reversed, the transistor  214  is turned “off” and the transistor  216  is turned “on”. In this situation, the modulation current is not conducted through the laser diode. Thus, only the bias current is conducted through the laser diode. Hence, the light output of the laser diode falls back to the power level P 0 . 
     The speed in which the laser diode  204  can be modulated to output light between the levels P 0  and P 1  is affected by the parasitic capacitance of the PNP transistor  210  and the bias current source  212 . As illustrated in  FIG. 4 , the PNP transistor has an associated parasitic capacitor  402  and the bias current source has an associated parasitic capacitor  404 . The laser diode also has an associated parasitic capacitor  406 . These parasitic capacitors impact the rise and fall times of the laser diode to emit optical signals of power levels P 0  and P 1 . The inductor  208  of the optical transmitter  202  operates to reduce the effects of the parasitic capacitors  402  and  404  so that the laser diode can be driven to modulate the optical signals between the output power levels P 0  and P 1  at a high rate of speed. 
     An impedance of an inductor is defined by the following equation.
 
I=2πfL, 
 
where f is the frequency of the current through the inductor and L is the inductance of the inductor. Thus, as the frequency increases, the impedance of an inductor becomes greater. The optical transmitter  202  utilizes this fact to provide a high impedance between the laser diode  204  and both the PNP transistor  210  and the bias current source  212  at times when it is needed the most. That is, when the laser diode is modulated at a high rate of speed, the inductor  208  provides a high impedance due to the high frequency of the current through the inductor. Therefore, the effects of the parasitic capacitors  402  and  404  associated with the PNP transistor and the bias current source are significantly reduced. Consequently, the laser diode can be modulated at a higher rate of speed.
 
     A method of operating the laser diode optical transmitter  202  in accordance with the present invention is described with reference to  FIGS. 2 and 5 . At step  502 , the optical transmitter is initialized. During this step, control signals are transmitted from the micro-controller unit  228  to the bias current source  212  and the modulation current source  226  such that bias current is flowing through the bias current source and modulation current is flowing through the modulation current source. In addition, a disable signal is transmitted to the OR gate  236 , turning “on” the PNP transistor  210  to ensure that the optical transmitter is in a disabled state, i.e., the laser diode  204  is not emitting any light. The disable signal also deactivates the differential amplifier  220 . At step  504 , the optical transmitter is enabled by removing the disable signal from the OR gate  236 , which turns “off” the PNP transistor to allow the bias current to flow through the laser diode. The removal of the disable signal also activates the differential amplifier. 
     Next, at step  506 , the laser diode  204  is driven to generate binary optical signals of power levels P 0  and P 1  by applying appropriate signals to the inputs  222  and  224  of the differential amplifier  220 . At step  508 , a high AC impedance is maintained between the laser diode  204  and both the PNP transistor  210  and the bias current source  212  by the inductor  208 . As stated above, the impedance of the inductor reduces the effect of parasitic capacitance associated with the PNP transistor and the bias current source so that the optical signals generated by the laser diode can be switched between the power levels P 0  and P 1  at a high rate of speed. At step  510 , the optical transmitter is disabled by applying a disable signal to the OR gate  236 , which turns “on” the PNP transistor. The disable signal also deactivates the differential transmitter  220 . The disable signal may be from an external source. Alternatively, the disable signal may be from the micro-controller unit  228  in response to loss of a special pattern at the security input  242  of the micro-controller unit due to a specified condition.