Abstract:
A circuit apparatus for driving short current pulses through a laser diode is disclosed. The circuit allow fast recovery time, comparable to the pulse duration. This enables high duty cycle pulse trains and bursts. The fast recovery is achieved by a passively self gated charging of the pulse circuit.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to electrical pulse generation, and more particularly optical pulse generation by driving current pulses through a laser diode. 
         [0003]    One application is within seeding of high power amplifiers, e.g. fiber optical amplifiers. This find application in master oscillator power amplifier laser systems. 
         [0004]    The advantage of the disclosed pulse drive technique is its fast recovery time which among other things enable high duty cycle pulse bursts. 
         [0005]    2. Description of Related Art 
         [0006]    A widely used method for generating short i.e. less than a few tens of ns (1E-9 seconds) current pulse through a load, often a laser diode rely on fast discharge of a capacitor coupled to the laser diode. Using a small capacitance value on the order of 100 pF (1E-10 Farads) gives a pulse spike with good immunity to impedance mismatch. This is often used for driving large junction laser diodes to peak powers much above CW average power rating, i.e. several A (Ampere) current peaks. 
         [0007]    In the conventional capacitor discharge pulse drive arrangement, see  FIG. 1  the capacitor  104  is charged through a large value resistor  103  such that current through the charge path  108  from the supply rail  102  is negligible compared to that from the capacitor  104  positive (+) terminal which is what causes the current through the laser diode  106  when the fast discharge switch  101  is conducting to the lower supply rail  107  connected to the laser diode anode via the supply rail,  107  and  109  being at the same potential. 
         [0008]    An electrical power supply means may in general be said to have a positive rail, also named positive pole or terminal, or just supply rail. For direct current to flow the power supply means must in addition have a negative rail, pole or terminal which in many cases is at the system ground potential. 
         [0009]      FIG. 2  show the voltage evolution in trace a at the positive (+) terminal of capacitor  104 , and the resulting current through the laser diode  106  is shown in trace b. At timing instance t 1  the switch  101  is brought from open circuit to conducting state. The voltage on the capacitor  104  drops abruptly causing the negative terminal (−) to swing negative, with a notch (not shown) below the negative supply rail. This drives a forward current through the laser diode  106  as illustrated in trace b. The switch is in conduction state from time instance t 1  to time instance t 2  where it is brought in open circuit i.e. non conduction state. The positive side (+) of the capacitor  104  is charged through the resistor  103  in the time t 2  to t 3 . The charge time, also called recovery time, sets the minimum pulse repetition period (T). The value of the resistor  103  needs to be large compared to the ON-resistance of the switch  101  for any voltage drop on the pulse forming capacitor  104  and resistor  105  to develop. As a consequence recharge time (t 3 -t 2 ) is long leading to a minimum pulse repetition period which is long compared to the current pulsewidth. Such drivers are currently used for ns (1E-9 seconds) pulsewidths and ms (1E-3 seconds) pulse repetition period. With typical values for charge resistors on the order of 1K (1000 ohms), and capacitor  104  with capacitance on the order of 100 pF (1E-10 Farads) with resistor  105  of value on the order of 1R (1 ohm). For higher pulse repetition rates, the same as shorter pulse repetition periods the circuit does not have enough time to fully charge the capacitor. This conventional drive circuit has been adequate in numerous applications utilizing high peak current at low duty cycle, defined as pulse width divided by pulse repetition period. The attainable duty cycle with such driver is up to around 0.001% (1E-5). 
         [0010]    Other, well known current pulse drive circuits include those using direct modulation with avalanche or normal mode transistor drive. It can be AC or DC coupled, and employ active push-pull drive. In the direct modulation scheme the current pulse follow the electrical switching. Unlike the capacitive discharge, as described in detail above where the current pulse duration, pulsewidth is limited by the discharge of a capacitor. 
         [0011]    It is possible to use push-pull drive in a capacitive charge and discharge circuit, but it would require sub ns timing control of the charge path gating, which additionally needs to be smooth and free from ringing which otherwise would drive ghost pulses following the intended pulse. The shortcomings in attainable duty cycle of the conventional capacitor discharge pulse drive circuit, and the serious design challenges of actively timed discharge and charge circuit impose is the background for the disclosed novel pulse drive circuit. 
       SUMMARY OF THE INVENTION 
       [0012]    The invention solves the shortcoming of the prior art by taking advantage of the capacitive discharge pulse circuit and providing a passively self gating switched charge part to significantly reduce the charge time. 
         [0013]      FIG. 3  shows a block diagram illustrating the operation of the present invention. Comparing with the conventional capacitive discharge circuit in  FIG. 1  the circuit of the invention replaces the charge resistor with a charge control circuit block  303 . The charge control block is connected to the positive terminal of the capacitor  305  by a charge path  308  and additionally by a feedback connection  302 . The functioning of the charge control of the invention is detailed in  FIG. 4 . The upper supply rail connection  401  is connected via a current limiting circuit means  402  to a switch  403 . The charge current is supplied on the connection  408  and the charge voltage is sensed on line  404 . The decision circuit block  405  compares the voltage with a threshold and communicates a signal representing the threshold crossing instance to a time delay means  406 , which in turn communicates a delayed version of the instance via the connection  407  to the charge current switch  403 . The switch commutes from open circuit state to conducting state upon reception of the delayed signal. In this way the charge control circuit block  303  exhibits a high impedance allowing the switch  301  to effect an abrupt voltage drop at the positive (+) terminal of the capacitor  305  producing a current spike through the pulse forming resistor  306  and the load illustrated as a laser diode  307 . The charge control detects the voltage drop and commutes the switches  403  to its conducting state allowing current to flow in the charge path through the current limiter  402  connected via  401  to a supply rail. When the charge voltage, e.g. the supply rail voltage minus a diode drop is reached the charge switch  403  returns to its high resistance, OFF or open circuit state ready for the next trigger pulse. 
         [0014]    The capacitor  305  is referred to as having a positive (+) terminal, the most relevant type of capacitor is ceramic capacitor which a priori is polarity undesignated. The terminal is referred to as positive (+) because it, inside the circuit must be charged to a higher potential than the other terminal of the same capacitor. 
         [0015]    The power supply connected to the laser diode anode can be any well buffered supply rail  309  including the circuit ground. 
         [0016]    The rapid charging is what enables the fast recovery and short pulse repetition periods. As illustrated in  FIG. 5  the pulse circuit of the invention has an evolution of the voltage at the positive terminal (+) of capacitor  305 , trace c in  FIG. 5 , which exhibits a fast drop with undershoot  501  at the time instance t 1 . After the time delay on the order of the pulse width the recharge starts ( 403  switched to conducting state), possibly while the switch  301  is still in the conducting state leading to the dynamic balance at the plateau  502 . When switch  301  is brought to the non conducting state the voltage recovers rapidly following the curve segment  503  to reach the charge voltage smoothly, as the voltage difference driving the current approaches zero. 
         [0017]    The switch  301  may be any electrical switching means which can be commuted between a conducting and a non conducting or open circuit state by a command signal  310 . The conduction state may also be referred to as low resistance or ON state, while the non conducting state may be refereed to as high resistance or OFF state and may not necessarily provide galvanic isolation. Switching may also be referred ON-OFF gating action. The same comments apply to the switch  403  commanded by the signal  407 . The most relevant switching means are MOSFET or bipolar junction transistors. 
         [0018]    The current limiting means can be a current limiting diode, or a feature of other circuit elements in the path or it can be any of the well known current clamping or constant current circuit arrangements from the established art of electronics. 
         [0019]    The limit level of the current limiter  402  can allow currents near the pulse current such that pulse trains with duty cycle near 50% can be produced. This, among other things enable new modes of operation where long pulses of several 100 ns (1E-7 seconds) can be replaced with a train of closely spaced short pulses. This allow spectral broadening arising from the short pulses modulation of high energy pulse trains. 
         [0020]    The short pulsewith drive capability with fast recovery has particular value in master oscillator fiber power amplifier laser systems where it can suppress nonlinear scattering e.g. stimulated Brillouin Scattering in the power amplifier. The high peak power of the short pulses are also advantageous in nonlinear frequency conversion, e.g. by four photon mixing of the laser output, directly from the laser diode or after power amplification. 
         [0021]    Other applications include communications and coding of pulse bursts for laser ranging, for example for distance ambiguity resolution. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    For a more complete understanding of the present invention, and the advantage it brings reference is now made to the ensuing description taken in connection with the accompanying drawings, briefly described as follows: 
           [0023]      FIG. 1  Simplified schematic diagram of prior art capacitor discharge pulse driver. 
           [0024]      FIG. 2  Waveform graphs illustrating prior art charge cycle. 
           [0025]      FIG. 3  Simplified schematic of the invention. 
           [0026]      FIG. 4  Detailed block diagram of the charge control of the invention. 
           [0027]      FIG. 5  Waveform graphs illustrating the charge cycle in the pulse driver of the present invention. 
           [0028]      FIG. 6  Process flow chart of the method of the invention 
           [0029]      FIG. 7  Circuit diagram of exemplary embodiment of the invention. 
           [0030]      FIG. 8  Circuit diagram of preferred fully featured embodiment of the invention. 
           [0031]      FIG. 9  Illustrative waveforms from the operation of the preferred fully featured embodiment of the invention. 
           [0032]      FIG. 10  Illustrates use of inductive boost to produce pulse train with increasing peak level. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0033]    Further features and advantages of the invention as well as the structure and operation of exemplary embodiments of the invention are described in detail below, with reference to the accompanying  FIGS. 3-10 , wherein reference numerals have the FIG. number as the leading number. 
         [0034]    One exemplary embodiment of the present invention is illustrated in  FIG. 7 . A capacitive discharge pulse circuit comprises the capacitor  705 , pulse forming resistor  704  and the laser diode supply  702 , which is buffered to ground  717  by the decoupling capacitor  703 . By switching the N-type MOSFET transistor  707  to its ON-state by driving the TRIG signal line  715  high, the falling voltage edge on the capacitor  705  drives a forward current spike through the laser diode load  701 . The capacitor recharge circuit of the invention is embodied by the P-channel MOSFET  708  as switching and current limiting element and the delay element formed by the resistors  706  and  709  in combination with the capacitor  710  and the gate capacitance of the P-channel MOSFET. The gate  719  of the P-channel MOSFET is biased to the supply rail  713  with the pull up resistor  711  and the diode  712 . The function of diode  712  is to match the bias voltage to the source to drain voltage drop of the P-channel MOSFET  708 . With no pulse trigger, i.e. TRIG line  715  is low, the transistor  707  is OFF, in this state the gate voltage of the P-channel MOSFET is pulled to the supply rail  713  and the transistor  708  is in the OFF state. When a pulse trigger signal is applied as a rising edge from low to high on line  715  to the gate of the N-channel MOSFET  707  it goes from OFF state to ON state with a low, on the order of 1 R (1 ohm) on resistance. This causes the drain voltage (line  720 ) to drop abruptly since at this instance the connection to the supply rail  713  is switched to its OFF state and the charge path connected to the capacitor  705  via the resistor  706  exhibits a high impedance. The falling edge of the voltage at  720  is what caused the voltage on the capacitor  705  to drop and drive a current through the laser diode  701 . The voltage drop also discharge the gate of the P-channel MOSFET  708 , labeled  719  but at a rate set by the resistor  709  and  706 . When the gate voltage at  708  reaches the switching voltage of the transistor, the supply rail  713  voltage plus the Gate Threshold Voltage (VGSth typically around −2 V) the charge path switch transistor  708  will go into its conducting ON state, exhibiting low impedance for the recharge. The transistor  708  will limit the charge current since it has a finite ON-State Drain Current on the order of 1 A (1 Ampere). When the TRIG signal  715  is brought low the transistor  707  goes to its OFF state and the capacitor  705  charges through the P-channel transistor  708 . The drain to gate connection via the resistor  709  assures that the transistor  708  switches to its OFF state when the charge voltage reaches the switching voltage of the transistor. The pull up resistor  711  allow a designed balance between switch on delay set mainly by the resistor  709  and switch-off response set by the combined effect of resistor  709  and  711 . The resistor  706  assures that the charge voltage is reached smoothly without overshoot. The described charge cycle is now returned to the state where the pulse driver is ready to receive the next rising edge on TRIG  715  commanding the firing of another pulse. 
         [0035]    It is noted that the single element, the P-channel MOSFET  708  embodies the functions of current limiter  402  in  FIG. 4 , charge path switch  403  and voltage threshold crossing instance detection  405 . The time delay  406  is embodied by the resistor capacitor network dominated by resistor  709  and capacitor  710  and MOSFET gate capacitance of  708 . 
         [0036]    The more elaborated embodiment of  FIG. 8  adds some features important for the practical utilization of the invention. A capacitive discharge sub-circuit is embodied by the capacitor  821  and N-channel MOSFET  827  which can be an RF Power Field Effect Transistor for fast switching. The discharge of  821  drives a current spike through the laser diode load  802  via the pulse forming resistor  805  from the supply rail  803  buffered by the capacitor  804  to ground  819 . 
         [0037]    The supply rail  803  can be at any voltage or at the ground level. The power supply of the preferred embodiment has the ground terminals  819  and  828  connected with low impedance as they close the fast pulse discharge path around the capacitor  821 . The ground terminal  824  is at the same average potential, but may have transient isolation, e.g. a ferrite bead on its connection to  828 . The potential of the supply rail  801  must be above that of terminal  828  for the charging and discharging cycle to take place. The preferred embodiment has the supply rails  801  and  803  at a high voltage and separate decoupling capacitor banks  815  and  804 . 
         [0038]    The pulse forming resistor  805  acts together with the capacitance value of  821  to set the pulse duration, e.g. large component values gives longer pulse duration. 
         [0039]    The branch comprising the bias isolation inductor coil  822  and current sink  806 , e.g. embodied by a current limiting diode, and the N-channel transistor  818 , allow a small bias current through the laser diode to be turned ON and OFF. Some laser diodes of interest may gainswitch with an optical output pulsewidth much smaller than the applied current spike. To control, among other things this phenomenon a pre-bias of the laser diode is useful. 
         [0040]    The pulse trigger input TRIG  816  connects to a logic gate oscillator  810 . This allow a slow pulse trigger signal on  816  to set the duration of a pulse train, modulating the pulse duration into individual pulses at the oscillation frequency of the gated oscillator  810 . 
         [0041]      FIG. 9  shows typical timing diagrams for the different signals. Trace e is the pulse trigger, or pulse duration signal applied to TRIG  816 , this signal gates the oscillator which supplies the oscillator signal f to the buffer  823 . A bias gating signal, trace g is independently applied to the BiasGate line  825  control to pre-bias the laser diode before the current pulses, triggered by each rising edge of the oscillator signal f, arrive. The voltage on the capacitor  821  at  826  is shown in trace h, it drops with a fast falling edge, producing the current pulses shown in trace i, at the rising edge of the signal f and recovers as the capacitor  821  recharges through the charge control embodied by the charge switching P-channel transistor  809 , with its MOSFET gate biased by the diode coupled P-channel transistor  820  and resistor  813 . 
         [0042]    A diode coupled transistor is used to match the transistor source to drain voltage drop of  809  across operating temperatures. 
         [0043]    The inductor coil  808  has two main functions, first it adds to the impedance of the resistors  812  and  807  giving delay in the switching of  809 , second it gives an inductive voltage boost, swinging the voltage at  826  higher than that at the supply rail  801  when the transistor  827  is turned OFF. This action is illustrated in  FIG. 10  where trace j represent the voltage at  826 . The pulse by pulse increased in voltage for each pulse in the pulse train gives the effect of increasing pulse peak current as shown in trace k. The peak power will settle at a dynamic equilibrium after some pulses. 
         [0044]    The increase of peak power from the level of the first pulses in a pulse burst is an advantage for laser diode output pulse trains which are to be amplified in optical amplifiers exhibiting gain saturation. In such amplifiers the first few pulses will experience higher gain than subsequent pulses. The ramping of the laser diode pulse peak current and thus peak optical output from the laser diode will counter act the gain saturation such that the amplified pulse train will exit the amplifier with leveled peak powers. This so called predistortion, preemphasis or first pulse suppression may be use in combination with known optical domain predistortion techniques to enhance the performance e.g. it may be used in arrangements including an optical saturable absorber means between the pulse drive laser diodes output and the optical amplifier input. 
         [0045]    The embodiments in  FIGS. 7 and 8  have the simplicity of letting a single element,  709  and  809  respectively, perform the voltage threshold crossing detection, current limiting and charge switching. 
         [0046]    It is obvious that several elements could be combined to performed the functionalists of the block diagram in  FIG. 4 . A compactor integrated circuit (IC) or operational amplifier could be used to detect the voltage drop, a digital delay line or RC (resistor capacitor) based timer IC could perform the timing delay and several switch elements exist which could implement the charge path switching. It also follows from the description of the invention, that a functioning circuit can be constructed by using only part of the functional blocks. In particular a current limiter alone, i.e. no switching element in the charge path, would be less than optimal but would work. Likewise the current limiter can be omitted if the time delay for the switching ON, of the recharge is longer than the ON-time of the pulse switch. 
         [0047]    The invention has been described above using specific embodiments for the purpose of illustration. It will be readily apparent to one of ordinary skills in the art, however that the principles of the invention can be embodied in other ways, for example other transistor types that the MOSFET may be used and current limitation can be implemented in a number of well know ways other than a current limiting diode, oscillator circuits may be constructed in ways alternative to the mentioned logic gate oscillator, for example utilizing crystal oscillators or digital counters based on a higher frequency clock. The oscillator may be always oscillating and its output gated or the gating action may turn the oscillator ON and OFF. The laser diode pre-bias arrangement may be connected in alternative ways providing a limited current low through the laser diode and include a switch or omit it. Therefore the invention should not be regarded as being limited in scope to the specific embodiments disclosed herein, but instead as being fully commensurate in scope with the following claims.