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 modulation and bias current is directed to one of the vertical cavity surface emitting lasers to turn on the laser. A modulation current is directed away from the vertical cavity surface emitting laser to turn off the laser.

Description:
[0001]    CROSS-REFERENCE TO THE RELATED APPLICATIONS  
         [0002]    This application is a continuing application of U.S. patent application Ser. No. 10/012,787 which claims the benefit of U.S. provisional application No. 60/246,301 filed Nov. 6, 2000 which are hereby incorporated by reference as if set forth in full herein. 
     
    
     
       BACKGROUND  
         [0003]    The present invention relates generally to semiconductor lasers, and, more particularly, to methods and circuits for modulating data communication lasers.  
           [0004]    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.  
           [0005]    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. Hence, VCSELs are able to be manufactured on a common substrate and thus a common cathode. Conversely, other lasers in which light is emitted from their edges or sides, only a single laser or a comparatively small number of lasers are able to be constructed on a common substrate.  
           [0006]    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.  
           [0007]    Operating with a five volts or less power supply and the limitations of typical transistors, driving the individual anodes of the individual VCSELs at high rates of speed is often difficult. Also, in order to maintain high data rates, the drive circuitry should supply a high speed current to the VCSELs to drive the VCSELs. However, wiring associated with the VCSEL and/or the drive circuitry introduces parasitic inductance and resistance. As a result, the high speed current through the associated wiring often generates noise to adjacent circuitry or distorts the current driving the VCSELs and/or the light emitted by the VCSELs.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention provides methods and circuits that control the modulation of a vertical cavity surface emitting laser. In one embodiment, a drive circuitry is provided that drives a plurality of semiconductor lasers with each laser having a cathode. Each cathode of the plurality of semiconductor lasers are common to a substrate. The drive circuitry includes a modulator and a dummy laser. The modulator is coupled to one of the plurality of semiconductor lasers and generates an output signal to control the one of the plurality of semiconductor lasers. The dummy laser is coupled to the modulator. The modulator also includes a steering circuit that directs current to one of the dummy laser and the one of the plurality of semiconductor lasers.  
           [0009]    In another embodiment, the drive circuitry is provided that drives a plurality of semiconductor lasers with each laser having a cathode. Each cathode of the plurality of semiconductor lasers are common to a substrate. The drive circuitry includes a modulator coupled to one of the plurality of semiconductor lasers and controls the one of the plurality of semiconductor lasers. A dummy laser is provided and is coupled to the modulator. The modulator also includes a steering circuit which directs current to one of the dummy laser and the one of the plurality of semiconductor lasers. The modulator also includes a first modulation and bias current source configured to generate a first modulation and bias current. The first modulation and bias current source is also coupled to the steering circuit and the dummy laser.  
           [0010]    Many of the attendant features of this invention may be more readily appreciated as 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  
       [0011]    [0011]FIG. 1 illustrates a block diagram of one embodiment of a modulator;  
         [0012]    [0012]FIG. 2 illustrates a circuit diagram of one embodiment of the modulator of FIG. 1;  
         [0013]    [0013]FIG. 3 illustrates a block diagram of another embodiment of a modulator; and  
         [0014]    [0014]FIG. 4 illustrates a circuit diagram of one embodiment of the modulator of FIG. 3. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    [0015]FIG. 1 illustrates a block diagram of one embodiment of a modulator driving a vertical cavity surface emitting laser. In FIG. 1, the modulator includes a modulation current source  5 , a bias circuit  7 , a steering circuit  9 , and a laser  101 . The bias circuit provides a bias current to the laser  5  so that the laser does not completely turn off. Such a bias current is useful in allowing the laser to more rapidly go from a decreased light emitting level to an increased light emitting level. The modulation current source provides the modulation current to the laser, depending on the state of the steering circuit. The state of the steering circuit is based on a control input C, which corresponds to data desired for transmission using the laser.  
         [0016]    The modulator also includes a dummy modulation current source  3  and a dummy laser. The dummy modulation current source is coupled to a power supply  103 , the steering circuit  9  and the dummy laser  11 . The dummy modulation current source is also coupled to the modulation current source  5 . The dummy modulation current source mirrors the current output from the modulation current source and thus generates a similar modulation current, i.e., a dummy modulation current. The dummy modulation current is supplied to either the dummy laser  11  or the steering circuit  9 .  
         [0017]    The steering circuit connects a modulation current sink to the modulation current source or the dummy modulation current source, depending on the state of the control input. For example, in one embodiment, the steering circuit pulls current from the modulation current source when the control input indicates a logical one, with the current from the dummy modulation current source going through the dummy laser. Conversely, when the control input is a logical zero, current from the modulation current source is provided to the laser and current from the dummy modulation current source is passed through the steering circuit. With the dummy laser configured to largely match the impedance of the laser, the current generated by the power source is largely constant.  
         [0018]    [0018]FIG. 2 illustrates a circuit diagram of one embodiment of a modulator and laser  101 . The modulator includes 5 P-channel FETs  21 ,  23 ,  25 ,  27  and  29 . The sources of FETs  21 ,  23 ,  25 ,  27  and  29  are coupled to a power supply  103 . The power supply is also coupled to the VCSEL substrate. The FET  21  is driven by a modulation current  20  from a sink (not shown).  
         [0019]    The gates of FETs  27  and  29  are coupled together and to the drain of FET  29 . The drain of FET  27  is coupled to the laser  101 . FETs  27  and  29  act as a current mirror and thus supply a bias current  30  to laser  101 . FETs  21 ,  23 , and  25  are gate coupled, with the gates coupled to the drain of FET  21 . FETs  21 ,  23  and  25  act as a current mirror. As such, a modulation current  20  is provided to a dummy laser  11  or a bipolar junction transistor (BJT)  57 , both coupled to FET  23  via its drain. Likewise, a modulation current is provided to laser  101  or BJT  59 , both coupled to FET  25  via its drain. The dummy laser  11  includes resistor  51  and diodes  53  and  55 .  
         [0020]    Control voltage or inputs C 1  and C 2  are provided between the bases of BJTs  57  and  59  of such a magnitude as to completely switch the modulation current  20  through BJT  57  or BJT  59 . Accordingly, current from FET  25  flows to the laser when Cl is more positive then C 2  or through transistor BJT  59  when C 2  is more positive then C 1 . Similarly, current from FET  23  flows to the dummy laser when C 2  is more positive then C 1  or through BJT  57  when C 1  is more positive then C 2 .  
         [0021]    [0021]FIG. 3 illustrates a block diagram of another embodiment of a modulator for driving vertical cavity surface emitting lasers of the present invention. In FIG. 1, the modulator includes a dummy modulation and bias current source  31 , a modulation and a bias current source  33 , a steering circuit  35  and a dummy laser  11 . The dummy modulation and bias current source  31  is coupled to the steering circuit  35  and the dummy laser  11 . The dummy modulation and bias current source  31  is also coupled to the modulation and bias current source  33 . The modulation and bias current source is coupled to steering circuit  35  and a vertical cavity surface emitting laser  101 . The modulation and bias current source and the dummy modulation and bias current source are coupled to one terminal of a power supply  103 . The other terminal of power supply  103  is coupled to the substrate of the laser  101  and to the dummy laser  11 .  
         [0022]    The dummy modulation and bias current source  31  provides a dummy summed modulation and bias current. Steering circuit  35  allows the dummy summed modulation and bias current to flow into dummy laser  11  or removes the modulation current to leave only the bias current. The modulation and bias current source  33  coupled to the dummy modulation and bias current source  31  also provides a summed modulation and bias current that mirrors the dummy summed modulation and bias current. Likewise, the steering circuit  35  allows the summed modulation and bias current to flow into the laser  101  or removes the modulation current to leave only the bias current. The steering circuit  35  also receives a control input C that directs the dummy summed modulation and bias current towards the dummy laser  11  or directs the summed modulation and bias current towards the laser  101 .  
         [0023]    [0023]FIG. 4 illustrates a circuit diagram of one embodiment of the modulator of FIG. 3. The modulator includes 3 P-channel FETs  41 ,  43  and  45 . The sources of FETs  41 ,  43  and  45  are coupled to a power supply  103 . The power supply, in one embodiment, is  5  volts or less. The power supply is coupled to the laser  101  and the dummy laser  11 . Gates of FETs  41  and  43  are coupled together and the drain of FET  41 . As such, FETs  41  and  43  act as a current mirror which provides a summed modulation and bias current  40  to dummy laser  11  or a modulation current  20  to BJT  57  and a bias current only to dummy laser  11 . The gates of FETs  41  and  43  are also coupled to the gate of FET  45 . FETs  41  and  45  act as a current mirror to provide a summed modulation and bias current to laser  101  or a modulation current to BJT  59  and a bias current only to laser  101 . The dummy laser  11  includes a resistor  51  which is coupled to diode  53 . The cathode of diode  53  is coupled to the anode of diode  55 . The cathode of diode  55  is coupled to laser  101  and power supply  103 .  
         [0024]    Control voltage or inputs C 3  and C 4  are provided between the bases of BJT  57  and BJT  59  of such a magnitude as to completely switch the modulation current  20  through BJT  57  or BJT  59 . Accordingly, the summed modulation and bias current from FET  45  flows to laser  101  when C 3  is more positive then C 4  or only the bias current flows to the laser  101  while the modulation current is diverted through BJT  59  when C 4  is more positive then C 3 . Similarly, the summed modulation and bias current from FET  43  flows to the dummy laser when C 4  is more positive then C 3  or only the bias current flows to the dummy laser while the modulation current is diverted through BJT  57  when C 3  is more positive then C 4 . As such, a modulation and bias current is supplied to laser  101  or to dummy laser  11 , but not to both the dummy laser  11  and laser  101  at the same time. Thus, the modulation currents through and from the power supply  103  remains constant. As such, parasitic inductance and resistance associated with wiring from the power supply to the other components, e.g., to modulation and bias current source  33  (FIG. 3), and any mutual inductance and capacitance to other wiring does not cause the modulation and bias current to produce voltage noise in adjacent circuitry or distort the signal current required by laser  101  to output light.  
         [0025]    Accordingly, the present invention provides methods and systems that control the modulation of vertical cavity surface emitting lasers. 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 this specification.