Abstract:
A laser diode driver circuit uses transconductance amplifying devices, preferably FETs, in a balanced input configuration. First and second amplifying devices are arranged to receive respective inverting and non-inverting input signals on their respective control terminals (gates). The amplifying devices are arranged to drive a laser diode connected between the current output terminals (source terminals) of said first and second amplifying devices. In one embodiment, a first node connects a source terminal of a first amplifier FET, a first terminal of the laser diode, and a drain terminal of a biasing FET. In another embodiment, in addition to the circuitry of the first embodiment a second node connects the second amplifier FET, a drain of a second biasing FET, and a second terminal of the laser diode. Preferably, the first and (optionally) the second biasing FETs bias the circuit&#39;s outputs with an offset, relative to one another, of substantially the turn-on threshold of the laser diode. The invention provides fast transitions with low power consumption.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to electro-optics and optical communications generally and more specifically to electronic driver circuits for modulated laser diodes.  
           [0003]    2. Description of the Related Art  
           [0004]    Fiber optic communication systems such as the “Synchronous Optical Network” (SONET) commonly employ electro-optic transponders which translate signals from electrical to optical (and vice-versa). A typical fiber optic communication system might include a fiber transmission medium with multiple nodes at each of which a transponder launches and receives optical data and provides a data interface to electronic systems. To provide the appropriate optical/electronic data interface, transponders commonly include both an optical transmitter and an optical receiver in the same package.  
           [0005]    Currently, the semiconductor laser diode is the most prevalent device used in transponders in the transmitter section to directly convert electronic signals into pulses of light.  
           [0006]    A suitable electronic driver circuit is required to modulate or switch a laser diode in a transponder circuit. Such a laser diode driver should preferably have fast rise and fall times and low power dissipation. In addition, the driver should provide clean switching of the laser diode: no distortion or spurious content should be introduced into the optical signal (for example, from power supply line fluctuation). Furthermore, a laser diode driver circuit should preferably be relatively simple, economical, and operable from readily available power supply levels.  
           [0007]    A typical prior laser-diode driver circuit is shown in FIG. 1. A differential pair of transistors Q diff1  and Q diff2  is driven by a differential drive input ( + Vin and −Vin). The differential pair is biased by a current source I source  in the common “tail” of the emitter circuit, in the well known differential amplifier configuration. Typically the current source includes at least two transistors in a “current mirror” or similar configuration. The laser diode is connected in the collector circuit of one of the transistors (Q diff2  in FIG. 1). Typically such driver circuits are supplied in an open collector configuration, so that the laser diode LD is required to be connected via a pin out  2  as shown. Optionally, a current limiting resistor R limit  can be interposed between the supply voltage VCC and the laser diode LD.  
           [0008]    Although the prior circuit shown in FIG. 1 is adequate for many applications, it consumes power at an undesirably high rate, in part because of the large number of transistors (including multiple transistors included in the tail current source) . The typical driver circuit can be viewed as a switched current source with fairly high output impedance. Since laser diode impedance levels are relatively low (less than 10 ohms, typically), a high mismatch between the driver and load impedance levels exists. In order to reduce ringing and waveform distortion problems associated with the mismatch, a matching resistor is placed off-chip in series with the laser diode to provide a better matched load to the driver. The matched load, of course, comes at the price of useless dissipation of power.  
         SUMMARY OF THE INVENTION  
         [0009]    In view of the above problems, the present invention is a simple, economical laser diode driver circuit which provides fast switching times while maintaining a clean optical output, notwithstanding substantial power supply fluctuations. A key improvement provided by the present invention is the ability to drive low impedance loads directly without the need for external matching resistors which can greatly increase the power dissipation in transponder systems.  
           [0010]    The circuit of the invention includes an amplifier having inverting and non-inverting inputs and inverted and non-inverted outputs arranged to drive a laser diode connected between the inverted and the non-inverted outputs.  
           [0011]    The driver circuit uses transconductance amplifying devices, preferably FETs, in a balanced input configuration. First and second amplifying devices are arranged to receive respective inverting and non-inverting input signals on their respective control terminals (gates). The amplifying devices are arranged to drive a laser diode connected between the current output terminals (source terminals) of said first and second amplifying devices. In one embodiment, a first node connects a source terminal of a first amplifier FET, a first terminal of the laser diode, and a drain terminal of a biasing FET. In another embodiment, in addition to the circuitry of the first embodiment a second node connects the second amplifier FET, a drain of a second biasing FET, and a second terminal of the laser diode. Preferably, the first and (optionally) the second biasing FETs bias the circuit&#39;s outputs with an offset, relative to one another, of substantially the turn-on threshold of the laser diode.  
           [0012]    These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which: 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a schematic diagram of a prior art laser diode driver circuit;  
         [0014]    [0014]FIG. 2 is a block level schematic of a laser diode circuit in accordance with the invention;  
         [0015]    [0015]FIG. 3 is a component level schematic diagram of one embodiment in accordance with the invention;  
         [0016]    [0016]FIG. 4 a  is a graph of voltage versus time, showing typical voltage inputs Vin 1  and Vin 2  in the circuit of FIG. 3, representative of a balanced 5 Gigahertz square wave input;  
         [0017]    [0017]FIG. 4 b  shows the time variation of the current through the laser diode of FIG. 3, in response to the input shown in FIG. 4 a , for a variety of supply voltages;  
         [0018]    [0018]FIG. 4 c  shows the response of the circuit of FIG. 3 when excited by the input of FIG. 4 a , under the condition of an induced supply voltage (V dd ′) fluctuation of 0.2 V. (peak-to-peak);  
         [0019]    [0019]FIG. 4 d  shows the corresponding response of the circuit of FIG. 3 under the condition of an increased supply voltage (V dd ′) fluctuation of 0.4 V. (peak-to-peak);  
         [0020]    [0020]FIG. 4 e  shows the superimposition of the waveforms of FIGS. 4 c  and  4   d , to allow convenient comparison;  
         [0021]    [0021]FIG. 5 is a graph of the frequency response of the circuit of FIG. 3, with amplitude graphed on a linear scale and frequency on a logarithmic scale;  
         [0022]    [0022]FIG. 6 is a schematic diagram of an alternate embodiment of a driver circuit in accordance with the invention;  
         [0023]    [0023]FIG. 7 is a schematic diagram of a multistage laser diode driver circuit which incorporates an output stage in accordance with the invention;  
         [0024]    [0024]FIG. 8 a  is an accumulated response “eye diagram” which graphs output current (in amperes) as a function of time (in picoseconds) for the simulated circuit of FIG. 7 when driven by a pseudorandom binary excitation at a 10 Gigabit per second rate, where 0.2 nanohenries of bond lead inductance have been assumed associated with the each bond lead of the laser diode; and  
         [0025]    [0025]FIG. 8 b . is another “eye diagram” obtained under circumstances analogous to FIG. 8 a , with the single difference that no bond lead inductance has been assumed. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    As shown in FIG. 2, the laser diode driver amplifier  18  has inverting and non-inverting inputs ( 20  and  22 ) and inverted and non-inverted outputs ( 24  and  26 ), arranged to receive a laser diode LD connected between the differential outputs  24  and  26 . The amplifier  18  is preferably biased to create an offset voltage between the output branches, to set the operating point of the laser diode LD at a predetermined voltage: for example, it can be maintained at the turn on threshold (for switching applications) or it could alternatively be biased slightly on, in a more linear part of the laser diode&#39;s response curve (for applications which demand more linear response).  
         [0027]    The economical circuit of FIG. 3 embodies a laser diode driver in accordance with the invention. Differential input signals V in1  and V in2  are coupled to respective gates of FETs Q 1  and Q 2 , which are arranged in two parallel branches. Bias FET Q 3  provides a bias current which is shared (unequally) by the FETs Q 1  and Q 2 . The driven laser diode LD is connected between the branches: i.e., with its anode connected to the source of Q 1  (a first output terminal  28 ) and its cathode connected to the source of Q 2  (a second output terminal  30 ) as shown. Thus, the circuit in its simple form includes a differential source follower amplifier, having inverting and non-inverting inputs, inverted and non-inverted outputs ( 28  and  30 ), and a load (laser diode) connected between the inverted and non-inverted outputs.  
         [0028]    Bias to FETs Q 1  and Q 2  is set by the gate bias voltages of each FET as well as the bias voltage V gg1  applied to the gate of Q 3 . Preferably, the bias point is set so that the laser diode is “half-biased” near its turn-on threshold—for example, at approximately 1 volt (forward bias) with no input signals applied to V in1  and V in2  . This allows the circuit to drive the laser diode with a small input signal and achieve high efficiency and fast switching times.  
         [0029]    The circuit operates as follows. When V in =V in1 −V in2  is high, the diode is switched on and current flows through Q 1 , LD and Q 3 . When V in  is reduced (near zero or negative voltage), the current through Q 1  is switched off and instead diverted to the right hand branch of the circuit through Q 2  and Q 3 .  
         [0030]    The inductances L ss1  and L ss2  represent stray inductance associated with the laser diode LD and its circuit pathways. It is advantageous to provide a compensating circuit, preferably a series Resistor-Capacitor circuit to cancel the effects of the stray inductances at the anticipated frequency of operation. Typically (but not necessarily) the laser diode LD will not be integrated on the same chip with the driver circuit, but will instead be separately packaged. In such an arrangement, it has been found that the stray inductances L ss1  and L ss2  are typically on the order of  0.2  nanohenries. The compensating resistance and capacitance should be chosen to compensate appropriately. For example, for operation at  10  Ghz, the compensating resistance and capacitances (R c1 , R c2 , C C1  and C c2 ) can be chosen suitably to introduce a 3 decibel attenuation frequency is placed at approximately 11.5 Gigahertz, to reduce ringing. In practice, values of  — 5 ohms and 1 picofarad for each resistance R and capacitance have been found to produce desirably clean switching waveforms at a switching frequency of 10 Ghz.  
         [0031]    The network of R s1 , R s2 , L s1  L s2  and C s  has been introduced to simulate characteristics of a real, imperfect voltage source. For simulation purposes, a square wave fluctuation V fluc7  has also been added, representing a small square wave fluctuation on top of a DC voltage supply. The supply fluctuations V fluc7  typically arise due to coupling with other circuits on or nearby the chip with the circuit in FIG. 3. There may also be independent fluctuations in the supply voltage itself in a typical application. For purposes of simulation (the results of which are discussed below in connection with FIGS. 4 a - 4   e ) the following values were assumed: R s1 =0.5 Ohm, R s2 =2.0 Ohm, L s1=0.2  nH, L s2 =10 nH, and C s1 =20 nF.  
         [0032]    [0032]FIG. 4 a  shows typical drive input pulses as might be applied to V in1  and V in2 . V in1  and V in2  preferably have a DC offset of approximately 0.9 V DC for the off state (of the laser diode). The output current I LD  through the laser diode is shown in FIG. 4 b , for several values of supply voltage V dd . Curve  100  represents the response with V dd =3.5 V, curve  102  the response for V dd =3.0 V, curve  104  for V dd =2.5 V, and curve  106  represents the response for V dd =2.0 V. For purposes of this analysis, inherent supply inductance L s  and supply resistance R s  are assumed to be 10,000 nanohenrys and 0.001 Ohm, respectively. These values were chosen to represent realistic assumptions, but actual power supplies will have varying characteristics which will affect performance.  
         [0033]    The circuit of FIG. 3 provides excellent rejection of supply voltage fluctuations. FIG. 4 c  shows a typical case in which voltage V dd ′ fluctuates by 0.2 V peak-to-peak from a typical average value. Nevertheless, the voltage across the laser diode LD is not greatly affected by the V dd  fluctuation. Waveforms for the respective voltages v d1  and v d2  at the anode and cathode of the laser diode LD are shown. The voltage across the laser diode LD follows the input waveform, without significant effect from the supply fluctuation, as demonstrated by the figure.  
         [0034]    [0034]FIG. 4 d  shows the corresponding response for an increased (induced) power supply voltage fluctuation of 0.4 V. peak-to-peak. Voltages Vd1 and Vd2 show little deviation from the previous figure. The insensitivity to supply fluctuation is further emphasized by FIG. 4 e , which simply superimposed the two previous figures. No difference in the responses Vd1 and Vd2 is visible (the waveforms superimpose completely).  
         [0035]    In a typical application, laser diode current swings in the range of 80-100 milliamps are obtained without appreciable sensitivity to supply voltage fluctuations, even at very fast switching speeds (up to 10 Ghz).  
         [0036]    The frequency response of a driver circuit of FIG. 3 is shown in FIG. 5. Flat frequency response is displayed from 100 KHz to 10 GHz, with less than 3 db reduction in response even at frequencies as high as 100 GHz.  
         [0037]    The circuit of the invention can most suitably be fabricated with Psuedomorphic High Electron Mobility Transistors (PHEMT) for all the FETs. GaAs is a suitable material for the PHEMTs, and provides fast switching with low supply voltages, thus keeping power consumption low. Low power consumption is also facilitated by the reduced number of transistors as compared to prior art circuits.  
         [0038]    An alternate embodiment of the invention is shown in FIG. 6. The alternate embodiment includes the same essential circuit as the circuit of FIG. 3, with an additional FET Q 4  connected between the source of Q1 and ground. The additional FET Q 4  sinks some current from the source of Q 1 , in an amount which is determined by the bias voltage (V gg2 ). This additional FET provides more flexibility in setting the bias point of the laser diode LD, but at the expense of some additional power consumption. Thus, the alternate embodiment of FIG. 6 would be most appropriate in an application in which the laser diode must be biased at a precisely determined turn-on voltage.  
         [0039]    It is desirable in either of the above described embodiments that the current bias be provided by a simple single transistor (in the embodiment of FIG. 3) or dual transistor (in the embodiment of FIG. 6). More complex current source bias circuits such as a current mirror should preferably be avoided in order to reduce power consumption and heat generation by the driver circuit.  
         [0040]    [0040]FIG. 7 shows a multistage laser diode driver circuit which incorporates a driver circuit in accordance with one embodiment of the invention (the embodiment discussed above in connection with FIG. 3). FIG. 7 could also be modified to use a driver in accordance with the embodiment of FIG. 6. The circuit is well adapted for driving a laser diode LD at frequencies in the neighborhood of 10 Gigahertz from a supply voltage Vdd of 3.3 Volts. Balanced inputs IN+ and IN− are amplified by differential amplifier stage  100 , level shifted by level shifting stage  102  and further amplified by a second differential amplifier stage  104 . The amplified and level shifted signal is coupled to a load bias control circuit  106 , which offsets the + and − drive signals with respect to one another. The offset is adjustable by the voltages at V bias1  and V bias2 , and is preferably set to produce driver output at or near the turn on threshold for the laser diode LD, for zero input signal. The driver stage  108  is essentially the circuit discussed above in connection with FIG. 3. Note that the stray inductance L bond  is the inductance associated with bond and other wire pathways connecting the (typically external) laser diode LD. The components shown in block  110  are typically external to (not monolithically fabricated with) the driver circuit ( 100 - 108 ).  
         [0041]    Typical simulated time domain output responses of the circuit of FIG. 7 are shown in FIGS. 8 a  and  8   b  (“eye diagrams”) . The outputs show overlaid responses of the circuit when driven by a pseudorandom binary excitation at a 10 Gigabit per second rate. FIG. 8 a  assumes that the bond inductance in the laser diode branch (external laser diode) is equivalent to two inductors (in series), each with a value of 0.2 nanohenries. Output current in amperes is shown by curves  200  and  202  (phase shifted by one half period). Both rising and falling waveforms are shown superimposed in order to illustrate and emphasize the symmetry of the response. Excellent balance, low overshoot and fast rise time (approximately 30 picoseconds) are apparent. FIG. 8 b  shows the corresponding outputs where the bond inductance has been assumed to be 0.0 nanohenries. Very slight peak overshoot is seen in curves  200  and  202 . In both figures, compensating capacitance C out  is assumed to be 1 picofarad.  
         [0042]    While several illustrative embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Differing amounts of additional voltage gain, for example from a common source gain stage, can be incorporated to drive input FETs Q 1  and Q 2  from whatever input voltage levels are available. Bipolar transistors could be partially or completely substituted for the FETs. Transistors and supplies of complementary polarity could be employed to produce an equivalent circuit. Obviously, Vdd could be fixed at a nominal zero or ground potential, and the circuit ground could be biased at a negative potential such as −3.3 volts with respect to Vdd. Such variations and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the invention as defined in the appended claims.