Abstract:
A drive circuitry that drives a number of vertical cavity surface emitting lasers having a common cathode. The drive circuitry includes a modulator and a dummy laser. The modulator controls the vertical cavity surface emitting lasers. A summed modulation and bias current is directed to one of the vertical cavity surface emitting lasers to turn on the laser. The modulation current is pulled away from the vertical cavity surface emitting laser to turn off the laser.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuing application of U.S. patent application Ser. No. 10/012,783 filed Nov. 6, 2001 which claims the benefit of U.S. provisional application No. 60/246,325 filed Nov. 6, 2000 which is hereby incorporated by reference as if set forth in full herein. 
     
    
     
       BACKGROUND  
         [0002]    The present invention relates generally to semiconductor lasers, and, more particularly, to methods and circuits for modulating data communication lasers.  
           [0003]    Semiconductor lasers are widely used in high speed data communications. Modulated light from the lasers are used to carry information through fiber optic lines. For some data formats, generally, when a laser emits light the data value is considered a logical one and when the laser is largely off the data value is considered a zero.  
           [0004]    Vertical cavity surface emitting lasers (VCSELs) are one type of laser used in data communication networks. VCSELs are generally relatively easy to manufacture using semiconductor processes and light from VCSELs is emitted from the VCSELs&#39; surfaces, rather than from their edges. Arrays of VCSELs are able to be relatively easily manufactured on a common substrate, with the common cathode.  
           [0005]    Typically, drive circuitry for VCSELs provide a VCSEL with sufficient current to turn “on”, i.e., cause the VCSEL to emit light. Likewise, the drive circuitry removes or prevents current from flowing to the VCSEL to turn the VCSEL “off”, i.e., cause the VCSEL to not emit light, or more generally, emit light at a reduced intensity. In high speed data communications, for directly modulated VCSELs, the drive circuitry should be able to drive the individual anodes of the individual VCSELs rapidly in order to switch the VCSEL on and off at high rates of speed.  
           [0006]    However, competing desired performance factors, such as speed, low power, and jitter, often causes difficulties in supplying a high speed current to the VCSEL. Other considerations that causes difficulties include a low power supply voltage, a high VCSEL forward voltage threshold, varying bias voltage and temperature and variations in the manufacturing of the VCSEL. Also, the VCSEL array having a common cathode and being able to control each individual VCSEL separately further introduces difficulties.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides a system and method for driving a number of semiconductor lasers such as a vertical cavity service emitting laser. In one embodiment, a drive circuitry is provided that drives a plurality of semiconductor lasers with each laser having a cathode and each cathode of the plurality of semiconductor lasers being common to a substrate. The driver circuitry includes a modulator which is coupled to one of the plurality of semiconductor lasers and controls the one of the plurality of semiconductor lasers and generates a modulation current. A dummy laser is also provided that is coupled to the one of the modulator. The modulator is configured to generate a bias current and a summed modulation and bias current. In one aspect of the invention, a transistor switch is provided that directs the summed modulation and bias current to flow to one of the plurality of semiconductor lasers. In another aspect of the invention, the transistor switch directs the modulation current to flow to the dummy laser. In a further aspect of the invention, a capacitor provides a discharge path for the transistor switch. The capacitor is added for higher speed.  
           [0008]    In another embodiment, a method of driving the plurality of semiconductor lasers each having a cathode is provided. Each cathode of the plurality of semiconductor lasers are common to a substrate. Also, a bias current is supplied to one of the plurality of semiconductor lasers. A modulation current is supplied. A summed modulation and bias current is provided to one of the plurality of semiconductor lasers via a first transistor switch to turn on the one of the plurality of semiconductor lasers. Also, a bias current is provided to the one of the plurality of semiconductor lasers via a second transistor switch to turn off the one of the plurality of semiconductor lasers. 
       
    
    
       [0009]    Many of the attendant features of this invention will be more readily appreciated as to the same becomes better understood by reference to the following detailed description and considered in connection with the accompanying drawings.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 illustrates a block diagram of one embodiment of a modulator; and  
         [0011]    [0011]FIG. 2 illustrates a circuit diagram of one embodiment of the modulator of FIG. 1. 
     
    
     DETAILED DESCRIPTION  
       [0012]    [0012]FIG. 1 illustrates a block diagram of one embodiment of a modulator for driving a semiconductor laser having a cathode connected to a substrate. The modulator includes current sources  3 ,  5 ,  7 ,  9  and  11 , switches  13  and  15 , dummy laser  17 , and a laser  19 . A capacitor, C 1 , connects current source  3  to current source  5  for speed improvement.  
         [0013]    When switches or switch circuits  13  and  15  are set at the left position as shown in FIG. 1, the current source  3  provides a summed bias and modulation current to the laser  19 . As such, the laser will emit light corresponding to a logic one. Control inputs A and C determines the direction of switch  13  while control inputs E and F control switch  15 . A modulation current from current source  5  flows to current source  9  through switch  13 . A differential current flows into dummy laser  17 . The current from current source  7  flows into current source  11  through switch  15 .  
         [0014]    When the switch  13  is flipped to the right position and the switch  15  remains at left position, the current from current source  3  minus the current pulled by current source  9  flows in to laser  19 . The current source  9  provides a modulation current. The laser is turned off since only bias current is being provided to the laser. Thus, the laser emits a dim light into a fiber optical cable corresponding to logic zero. During this turn-off transient, switch  15  is flipped to the right position and stays there for a short period and returns back to left position. This dynamic pulls current from laser  19  with the help of current source  11 . As such, a fast turn-off transient by removing the stored charge from the laser  19  forcibly is provided.  
         [0015]    [0015]FIG. 2 illustrates a circuit diagram of one embodiment of the modulator of FIG. 1. The modulator includes  7  P-channel FETs  21 ,  23 ,  25 ,  27 ,  29 ,  31  and  33 . The sources of FETs  21 ,  23 ,  25 ,  27  and  33  are coupled to a reference voltage V cc . The sources of FETs  29  and  31  are coupled to the drain of FET  33 . The gates of FETS  21  and  23  are coupled together and the drain of FET  23  is coupled to the gates of FETs  21  and  23 . As such, FETs  21  and  23  act as a current mirror providing a negative peaking current to bipolar junction transistor (BJT)  41  or dummy laser  11 , both coupled to FET  21  via its drain.  
         [0016]    Similarly, the gates of FETs  25  and  27  are coupled together and the drain of FET  25  is coupled to the gates of FETs  25  and  27 . FETs  25  and  27  act as a current mirror providing a modulation current to BJT  45  or dummy laser  11 , both being coupled to FET  27  via its drain. The bases of BJTs  41  and  45  are respectively coupled to control inputs C 1  and C 3 . In one embodiment, the value of control input C 1 , e.g., high or low, is generally high, transitioning to low for brief periods when control input C 3  goes low. When control inputs C 1  and C 3  are high, respective BJTs  41  and  45  turn on creating paths to current sources, I npk  and I modulation . Thus, negative peaking current flows to BJT  41  and modulation current flows to BJT  45 . On the other hand, when control input C 1  and C 3  are low, respective BJTs  41  and  45  turn off, and thus modulation current and negative peaking current flows to resistor  49  of dummy laser  11 . Resistor  49  is also coupled to diode  51  which is coupled to diode  53 .  
         [0017]    Collectors of BJTs  43  and  47  are also coupled together and to the anode of laser diode  13 . Also, the drain of FET  29  is coupled to the collectors of BJTs  43  and  47  and laser diode  13 . The gate of FET  29  is coupled to the gate and drain of FET  31 . Sources of FETs  29  and  31  are also coupled together. Together FETs  29  and  31  act as a current mirror providing a summed modulation and bias current. The bases of BJTs  43  and  47  are respectively coupled to control inputs C 2  and C 4  which form with control inputs C 1  and C 3 , respectively, differential inputs. In one embodiment, control input C 2  is briefly set high right after control input C 4  is set from low to high. When control input C 2  is high, BJT  43  turns on creating a path to ground and thus draws negative peaking current from laser diode  13 . When control input C 4  is high, BJT  47  turns on creating a path to I modulation  and thus draws modulation current from FET  29 . However, when control input C 2  is low, BJT  43  turns off and thus no negative peaking current is drawn from laser diode  13 . Also, when control input C 4  is low, BJT  47  turns off and thus modulation current is not drawn from FET  29 . As such, when BJTs  43  and  47  are off, a summed modulation and bias current flows to laser diode  13  thus turning laser diode  13  on, i.e., laser diode  13  emits light.  
         [0018]    On the other hand, when both BJTs  43  and  47  turn on, modulation current and negative peaking current is drawn away from laser diode  13 . As modulation and negative peaking current is drawn away from laser diode  13 , laser diode  13  turns off although bias current still flows to laser diode  13 . BJT  43  by drawing negative peaking current away from the laser diode  13 , assists in increasing the turn off transient. In other words, the laser diode  13  when turned on stores an electric charge. Removing the stored charge affects the turn off time of the laser. The amount of time or time period required to remove the charge from the laser diode, i.e., the turn off transient, is reduced by the BJT  43  drawing or pulling the negative peaking current from laser diode  13 . During the turn off transient, BJT  41  is off and thus current from FETs  21  and  23  flows to the dummy laser  11 .  
         [0019]    In one embodiment, the capacitor  55  is coupled to gates of FETs  25  and  27  and gates of FETs  29  and  31 . As such, gates of FETs  25  and  27  are coupled to gates of FETs  29  and  31 , via the capacitor  55 . The capacitor provides an AC discharge path through which charge built up at the gate of FET  29  flows. When the laser diode is turning on, voltage at the laser diode rises rapidly and thus sends charge into the gate of FET  29 . This charge lowers the source to gate voltage experienced by FET  29  which limits the drain to source current of FET  29 . Capacitor  55  thus provides a path for the charge sent by BJT  47  to be discharged by BJT  45 .  
         [0020]    In one embodiment, the drain of FET  33  is coupled to the sources of FETs  29  and  31 . The source of FET  33  is coupled to a reference voltage and its gate is coupled to a shutdown input. As such, when the shutdown input is high, the FET  33  turns off thus severing the path of the sources of the FETs  29  and  31  to the reference voltage. Hence, no current is able to be supplied to laser diode  13  and thus laser diode  13  turns off. On the other hand, when the shutdown input is low, FET  33  turns on and thus current is able to flow to laser diode  13  via FETs  29  and  31 .  
         [0021]    Accordingly, the present invention provides a method and system of controlling the modulation of a vertical cavity surface emitting laser array with a common-cathode. Although this invention has been described in certain specific embodiments, many additional modifications and variations would be apparent to those skilled in the art. It is therefore to be understood that this invention may be practiced otherwise than as specifically described. Thus, the present embodiments of the invention should be considered in all respects as illustrative and not restrictive. The scope of the invention to be determined by the appended claims, their equivalents, and claims supported by the specification, rather than the foregoing description.