Patent Publication Number: US-2022224075-A1

Title: Laser diode drive circuit

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of PCT/JP2020/023943 filed Jun. 18, 2020, which claims priority to Japanese Patent Application No. 2019-180617, filed Sep. 30, 2019, the entire contents of each of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a laser diode drive circuit. 
     BACKGROUND 
     Light detection and ranging (LiDAR) has recently be used for an automobile system or a weather observation system. The LiDAR refers to a system that analyzes a distance to a remote object or a property of the object by irradiating the object with light (e.g., a pulse of light) from a laser diode that emits pulsed light and measuring light scattered by the object. 
     Various laser diode drive circuits that can be used for the LiDAR have been developed, for example, in Japanese Patent Laying-Open No. 2016-152336 (hereinafter “PTL 1”) and Japanese Patent Laying-Open No. 2009-170870 (hereinafter “PTL 2”). PTL 1 discloses a laser diode drive circuit including a series circuit in which a direct-current (DC) power supply, an inductor, a current backflow prevention element, a capacitor, and a laser diode are connected in series. Moreover, the laser diode emits light by using an emission current from the capacitor, a switching element, and a control circuit. The switching element in PTL 1 has one end connected between the current backflow prevention element and the capacitor, and switches a current that flows to the inductor based on ON and OFF states controlled by the control circuit. 
     PTL 2 discloses a configuration in which a switching element for charging has a drain electrode connected to an anode side of each laser diode and has a source electrode connected a power supply. In PTL 2, a switching element for driving has a drain electrode connected to a cathode side of each laser diode, and a capacitor is connected between the anode of each laser diode and a GND line. 
     In the configuration in PTL 1, however, in driving a plurality of laser diodes, irradiation with short pulses of light by sequential light emission from the plurality of laser diodes cannot be carried out, and a circuit configuration for individually controlling the plurality of laser diodes cannot be adopted. 
     In addition, in the configuration in PTL 2, when switching between on and off states of a switching element corresponding to one laser diode is made in order to drive the same, a current flows also to another laser diode through a parasitic capacitance that exists between the drain electrode and the source electrode of the switching element and that laser diode emits light. Therefore, in the configuration in PTL 2, a laser diode may emit light at timing when light emission is not desired. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present disclosure provides a laser diode drive circuit configured for individually having only a laser diode desired to emit light at timing when light emission therefrom is desired. 
     In an exemplary aspect, a laser diode drive circuit is provided that includes at least one laser diode, a driving switching element connected in series to an anode side of the laser diode, with the driving switching element switching between an ON state in which a current is supplied to the laser diode and an OFF state in which the current is not supplied, a driving capacitor having one end connected to the driving switching element, with the driving capacitor supplying a current to the laser diode, a drive circuit that drives the driving switching element, and a boot strap circuit that supplies a voltage for driving the driving switching element to the drive circuit. The boot strap circuit includes a capacitor connected to the drive circuit and has one end connected to a source electrode of the driving switching element and another end connected to a gate electrode of the driving switching element, Moreover, the capacitor is charged with prescribed charges, and a current suppression element connected to the one end of the capacitor suppresses a flow of a current that flows to the capacitor and to the laser diode connected in parallel during charging of the capacitor. 
     According to the exemplary aspect of the present disclosure, when a laser diode having a cathode connected is driven, undesired light emission from a laser diode at timing when light emission therefrom is prevented. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a circuit diagram showing a laser diode drive circuit in a first exemplary embodiment. 
         FIG. 2  is a circuit diagram showing a laser diode drive circuit in a modification of the first exemplary embodiment. 
         FIG. 3  is a circuit diagram showing a laser diode drive circuit in a modification of the first exemplary embodiment. 
         FIG. 4  is a circuit diagram showing a laser diode drive circuit in a modification of the first exemplary embodiment. 
         FIG. 5  is a circuit diagram showing a laser diode drive circuit in a second exemplary embodiment. 
         FIG. 6  is a timing chart showing timing of switching of a switching element and a driving switching element. 
         FIG. 7  is a circuit diagram showing a laser diode drive circuit in a modification of the second exemplary embodiment. 
         FIG. 8  is a circuit diagram showing a laser diode drive circuit in a modification of the second exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Each embodiment will be described in detail below with reference to the drawings. The same or corresponding elements in the drawings have the same reference characters allotted and description thereof will not be repeated. 
     First Exemplary Embodiment 
     A laser diode drive circuit in a first exemplary embodiment will be described with reference to  FIG. 1 . As shown,  FIG. 1  is a circuit diagram showing a laser diode drive circuit  10  in the first embodiment. Laser diode drive circuit  10  is adopted in a system that provides short pulses of light, such as light detection and ranging (LiDAR), and drives laser diodes LD 1  and LD 2 . Though the laser diode drive circuit that drives two laser diodes is described in the embodiment below, without being limited thereto, the laser diode drive circuit may be configured to drive a single laser diode or three or more laser diodes in alternative embodiments. 
     Moreover, laser diode drive circuit  10  includes laser diodes LD 1  and LD 2 , a driving power supply Vd, a driving capacitor Cd, driving switching elements Swd 1  and Swd 2 , drive circuits Dr 1  and Dr 2 , boot strap circuits Cr 1  and Cr 2 , and a drive circuit power supply Va. 
     Laser diode LD 1 , driving switching element Swd 1 , drive circuit Dr 1 , and boot strap circuit Cr 1  form an arm Ar 1 , and laser diode LD 2 , driving switching element Swd 2 , drive circuit Dr 2 , and boot strap circuit Cr 2  form an arm Ar 2 . Though two laser diodes (laser diodes LD 1  and LD 2 ) are connected to laser diode drive circuit  10 , at least one laser diode should only be connected and an arm is provided for each connected laser diode in the exemplary aspect. 
     In operation, laser diodes LD 1  and LD 2  emit light by receiving a current supplied from driving capacitor Cd. Timing of light emission from laser diodes LD 1  and LD 2  is controlled by driving switching elements Swd 1  and Swd 2 . Driving switching elements Swd 1  and Swd 2  can have laser diodes LD 1  and LD 2  emit short pulses of light by switching between an ON state and an OFF state at a high speed. Laser diodes LD 1  and LD 2  have anodes connected to source electrodes of driving switching elements Swd 1  and Swd 2 , respectively, and have cathodes connected the ground. 
     Driving power supply Vd is a DC power supply. Driving power supply Vd supplies a current to driving capacitor Cd. 
     Charges supplied from driving power supply Vd are stored in driving capacitor Cd, which supplies the current to laser diodes LD 1  and LD 2 . Driving capacitor Cd is provided between driving power supply Vd and the ground. Driving capacitor Cd has one end connected to first ends of driving switching elements Swd 1  and Swd 2 . 
     Driving switching elements Swd 1  and Swd 2  switch between the ON state in which the current is supplied to laser diodes LD 1  and LD 2  and the OFF state in which the current is not supplied. In exemplary aspects, driving switching elements Swd 1  and Swd 2  are N-type switching elements, such as a MOSFET or a GaN FET, and connected in series to anode sides of respective laser diodes LD 1  and LD 2 . Though driving switching elements Swd 1  and Swd 2  may be P-type switching elements, by adopting the N-type switching elements, a high current to be fed to laser diodes LD 1  and LD 2  can be switched fast. 
     Drive circuits Da and Dr 2  control switching between the ON state and the OFF state of driving switching elements Swd 1  and Swd 2  based on input control signals from LiDAR where laser diode drive circuit  10  is employed. When drive circuits Da and Dr 2  receive an ON signal, they switch driving switching elements Swd 1  and Swd 2  to the ON state, and when they receive an OFF signal, they switch driving switching elements Swd 1  and Swd 2  to the OFF state. 
     Charges supplied from drive circuit power supply Va (DC power supply) are stored in boot strap circuits Cr 1  and Cr 2 , and boot strap circuits Cr 1  and Cr 2  supply voltages necessary for driving the driving switching elements Swd 1  and Swd 2  to drive circuits Da and Dr 2 , respectively. Driving switching elements Swd 1  and Swd 2  are thus driven. Boot strap circuits Cr 1  and Cr 2  include current backflow prevention elements Da 1  and Da 2 , capacitors Ca 1  and Ca 2 , and inductors La 1  and La 2 , respectively. 
     In the exemplary aspect as shown, current backflow prevention elements Da 1  and Da 2  are diodes that prevent a current from flowing toward drive circuit power supply Va. 
     Moreover, charges supplied from drive circuit power supply Va are stored in capacitors Ca 1  and Ca 2 , and capacitors Ca 1  and Ca 2  supply voltages necessary for driving the driving switching elements Swd 1  and Swd 2  to drive circuits Dr 1  and Dr 2 , respectively. Capacitors Ca 1  and Ca 2  are charged while driving switching elements Swd 1  and Swd 2  are in the OFF state. Capacitors Ca 1  and Ca 2  are connected to drive circuits Dr 1  and Dr 2  having first one ends connected to source electrodes of driving switching elements Swd 1  and Swd 2  and having the other (or second) ends connected to gate electrodes of driving switching elements Swd 1  and Swd 2 , respectively. 
     When laser diodes LD 1  and LD 2  are controlled to emit short pulses of light, switching between the ON state and the OFF state of driving switching elements Swd 1  and Swd 2  is made at a high speed. Therefore, a voltage and a current at a high frequency are applied to laser diodes LD 1  and LD 2 . When laser diodes LD 1  and LD 2  are controlled to emit short pulses of light, however, with inductors La 1  and La 2  that function to cut off a current at a high frequency, a current does not flow to inductors La 1  and La 2 . Since the current that flows to capacitors Ca 1  and Ca 2  during charging of capacitors Ca 1  and Ca 2  is a current at a low frequency, the current flows to inductors La 1  and La 2  but does not flow to laser diodes LD 1  and LD 2 . In other words, inductors La 1  and La 2  operate as current suppression elements that suppress a flow of the current that flows to capacitors Ca 1  and Ca 2  to laser diodes LD 1  and LD 2  during charging of capacitors Ca 1  and Ca 2 . Inductors La 1  and La 2  have first ends connected to ends of capacitors Ca 1  and Ca 2  and connected in parallel to laser diodes LD 1  and LD 2 , respectively. 
     Specifically, in laser diode drive circuit  10 , a frequency (e.g., a pulse frequency) of the current that flows to laser diodes LD 1  and LD 2  is within a range not lower than 100 MHz and not higher than 1 GHz and a frequency (e.g., an emission frequency) of the current that flows to capacitors Ca 1  and Ca 2  is not higher than 500 kHz. In laser diode drive circuit  10 , at the pulse frequency of laser diodes LD 1  and LD 2 , absolute values of impedances of laser diodes LD 1  and LD 2  (an impedance of a path including laser diodes LD 1  and LD 2  between points of connection A 1  and A 2  and points of connection B 1  and B 2 ) are configured to be smaller than impedance values of inductors La 1  and La 2 . Inductances L of inductors La 1  and La 2  have a value within a range where an amount of charges released from capacitors Ca 1  and Ca 2  can be provided within a time period of one cycle at an emission frequency fsw (a reciprocal of a cycle of emission of pulses of light). Inductances L of inductors La 1  and La 2  have a value at which the current that flows to laser diodes LD 1  and LD 2  during charging of capacitors Ca 1  and Ca 2  is smaller than an amount of the current necessary for light emission from laser diodes LD 1  and LD 2 . 
     According to an exemplary aspect, L represents inductances of inductors La 1  and La 2 , fld represents a pulse frequency of pulses of light emitted from laser diodes LD 1  and LD 2 , fsw represents an emission frequency of emission of pulses of light from laser diodes LD 1  and LD 2 , and Zld represents impedances of laser diodes LD 1  and LD 2 . For purposes of this disclosure, pulse frequency fld is defined as a ½ cycle of a sin wave of pulses of light emitted from laser diodes LD 1  and LD 2 . Emission frequency fsw is defined, with an interval of pulses of light emitted from laser diodes LD 1  and LD 2  being defined as one cycle. Impedance Zld is defined as an impedance of a path including laser diodes LD 1  and LD 2  between points of connection A 1  and A 2  shown in  FIG. 1  to points of connection B 1  and B 2  shown in  FIG. 1 . Naturally, so long as the impedances of laser diodes LD 1  and LD 2  themselves are higher than impedances of other components, the impedances of laser diodes LD 1  and LD 2  themselves can be defined as impedance Zld. 
     Inductances L of inductors La 1  and La 2  and impedances Zld of laser diodes LD 1  and LD 2  satisfy relation in an expression 1 below: 
       | Zld|&lt; 2π fld×L   (Expression 1)
 
     With such frequency characteristics of inductors La 1  and La 2 , during charging of capacitors Ca 1  and Ca 2 , currents I 1  and I 2  flow from drive circuit power supply Va through current backflow prevention elements Da 1  and Da 2 , capacitors Ca 1  and Ca 2 , and inductors La 1  and La 2  to the ground and does not flow to laser diodes LD 1  and LD 2 . 
     In addition, with such frequency characteristics of inductors La 1  and La 2 , while laser diodes LD 1  and LD 2  are driven, the current flows from driving capacitor Cd through driving switching elements Swd 1  and Swd 2  to laser diodes LD 1  and LD 2 , but does not flow to inductors La 1  and La 2 . 
     Thus, during charging of capacitors Ca 1  and Ca 2  (i.e., while driving switching elements Swd 1  and Swd 2  are in the OFF state), the current flows to inductors La 1  and La 2  and substantially no voltage is applied across the anode and the cathode of each of laser diodes LD 1  and LD 2 . Therefore, laser diodes LD 1  and LD 2  do not emit light in this state. Thus, light emission from laser diodes LD 1  and LD 2  at timing when light emission is not desired is also suppressed. 
     In addition, during charging of capacitors Ca 1  and Ca 2  (i.e., while driving switching elements Swd 1  and Swd 2  are in the OFF state), the current flows to inductors La 1  and La 2  and substantially no voltage is applied across the anode and the cathode of each of laser diodes LD 1  and LD 2 . Therefore, a voltage substantially as high as drive circuit power supply Va is applied across opposing ends of each of capacitors Ca 1  and Ca 2 . A high voltage necessary for driving the driving switching elements Swd 1  and Swd 2  can thus be secured. Therefore, when driving switching elements Swd 1  and Swd 2  are set to the ON state, a high current can abruptly flow to laser diodes LD 1  and LD 2 . 
     While laser diodes LD 1  and LD 2  are driven (i.e., driving switching elements Swd 1  and Swd 2  are in the ON state), the current flows from driving capacitor Cd through driving switching elements Swd 1  and Swd 2  to laser diodes LD 1  and LD 2 , and does not flow to inductors La 1  and La 2 . 
     Laser diodes LD 1  and LD 2  emit light by switching of driving switching elements Swd 1  and Swd 2  from the OFF state to the ON state. Thus, by switching the driving switching element corresponding to the laser diode desired to emit light at timing when light emission therefrom is desired from the OFF state to the ON state, the laser diode desired to emit light at timing when light emission therefrom is desired can individually be controlled to emit light. 
     A modification of the first embodiment will be described with reference to  FIGS. 2 to 4 . In general, it is noted components similar to those in the first embodiment have the same reference characters allotted and description thereof will not be repeated. 
       FIG. 2  is a circuit diagram showing a laser diode drive circuit  11  in a modification of the first exemplary embodiment. As shown, laser diode drive circuit  11  is different from laser diode drive circuit  10  in including a resistive element Rd. Resistive element Rd is provided to suppress an amount of charges supplied from driving power supply Vd to driving capacitor Cd during drive with pulses of laser diodes LD 1  and LD 2 . In this aspect, resistive element Rd has one end connected to driving power supply Vd and the other end connected to driving capacitor Cd. 
     Thus, the current that flows from driving power supply Vd to driving capacitor Cd when driving switching elements Swd 1  and Swd 2  are set to the ON state and then charges stored in driving capacitor Cd are supplied to laser diodes LD 1  and LD 2  can be restricted, and laser diodes LD 1  and LD 2  can be driven with short pulses. 
       FIG. 3  is a circuit diagram showing a laser diode drive circuit  12  in a modification of the first exemplary embodiment. Laser diode drive circuit  12  is different from laser diode drive circuit  10  in that it includes driving capacitors Cd 1  and Cd 2  and current backflow prevention elements Dd 1  and Dd 2  for respective arms Ar 1  and Ar 2 . 
     Charges supplied from driving power supply Vd are stored in driving capacitor Cd 1 , and driving capacitor Cd 1  supplies the current to laser diode LD 1 . Similarly, charges supplied from driving power supply Vd are stored in driving capacitor Cd 2 , and driving capacitor Cd 2  supplies the current to laser diode LD 2 . Driving capacitors Cd 1  and Cd 2  are provided between driving power supply Vd and the ground. Driving capacitors Cd 1  and Cd 2  have first ends connected to ends of driving switching elements Swd 1  and Swd 2 , respectively. 
     Current backflow prevention elements Dd 1  and Dd 2  are diodes that prevent, when the current (charges) stored in driving capacitors Cd 1  and Cd 2  flows to laser diodes LD 1  and LD 2 , the current from flowing to a laser diode connected to another arm. Driving power supply Vd is connected to anode sides of current backflow prevention elements Dd 1  and Dd 2 , and driving capacitors Cd 1  and Cd 2  are connected to cathode sides of current backflow prevention elements Dd 1  and Dd 2 , respectively. 
     While laser diodes LD 1  and LD 2  are driven (i.e., while driving switching elements Swd 1  and Swd 2  are in the ON state), a current may also flow to a laser diode other than a laser diode desired to emit light due to a parasitic capacitance, a parasitic inductance, or a parasitic resistance of driving switching elements Swd 1  and Swd 2 , laser diodes LD 1  and LD 2 , or an interconnection between components. In laser diode drive circuit  12 , however, driving capacitors Cd 1  and Cd 2  and current backflow prevention elements Dd 1  and Dd 2  are provided for respective arms Ar 1  and Ar 2 . Therefore, a flow of the current emitted from driving capacitors Cd 1  and Cd 2  to a laser diode in a different arm, in particular a laser diode not desired to emit light, is suppressed. Since current backflow prevention elements Dd 1  and Dd 2  should only be able to suppress a flow to a different arm, of the current emitted from driving capacitors Cd 1  and Cd 2 , they may be resistive elements or inductors. 
       FIG. 4  is a circuit diagram showing a laser diode drive circuit  13  in a modification of the first exemplary embodiment. Laser diode drive circuit  13  is different from laser diode drive circuit  10  in that it includes resistive elements Ra 1  and Ra 2  for respective arms Ar 1  and Ar 2 . 
     Resistive elements Ra 1  and Ra 2  are connected in series to capacitors Ca 1  and Ca 2  between capacitors Ca 1  and Ca 2  and the ground, respectively. In laser diode drive circuit  13 , the pulse frequency of laser diodes LD 1  and LD 2  is within a range not lower than 100 MHz and not higher than 1 GHz and the emission frequency of laser diodes LD 1  and LD 2  is not higher than 500 kHz. In laser diode drive circuit  13 , at the pulse frequency of laser diodes LD 1  and LD 2 , absolute values of impedances of laser diodes LD 1  and LD 2  (an impedance of a path including laser diodes LD 1  and LD 2  between points of connection A 1  and A 2  and points of connection B 1  and B 2 ) are configured to be smaller than a value of the sum of the impedances of inductors La 1  and La 2  and the impedances of resistive elements Ra 1  and Ra 2 . 
     In this aspect, with R representing resistance values of resistive elements Ra 1  and Ra 2 , L representing inductances of inductors La 1  and La 2 , fid representing a pulse frequency of laser diodes LD 1  and LD 2 , fsw representing an emission frequency of laser diodes LD 1  and LD 2 , and Zld representing impedances (an impedance of a path including laser diodes LD 1  and LD 2  between points of connection A 1  and A 2  and points of connection B 1  and B 2 ) of laser diodes LD 1  and LD 2 , laser diode drive circuit  13  satisfies an expression 2 below. 
       | Zld|&lt; 2π fld×L+R   (Expression 2)
 
     During charging of capacitors Ca 1  and Ca 2 , ringing (e.g., noise) may be produced between capacitors Ca 1  and Ca 2  and inductors La 1  and La 2 . In laser diode drive circuit  13 , by providing resistive elements Ra 1  and Ra 2  between capacitors Ca 1  and Ca 2  and inductors La 1  and La 2 , respectively, ringing (e.g., noise) produced during charging of capacitors Ca 1  and Ca 2  can be suppressed. 
     Resistive elements Ra 1  and Ra 2  should only be connected in series to capacitors Ca 1  and Ca 2 , respectively, and a position of connection does not have to be located between capacitors Ca 1  and Ca 2  and the ground. 
     Second Exemplary Embodiment 
     A laser diode drive circuit in a second embodiment will be described. In the first embodiment, inductors La 1  and La 2  are provided to suppress a flow of a current to laser diodes LD 1  and LD 2  during charging of capacitors Ca 1  and Ca 2 . In contrast, in the second embodiment, a flow of the current during charging of the capacitor is controlled by means of a switching element. Components similar to those in the first embodiment have the same reference characters allotted and description thereof will not be repeated. 
       FIG. 5  is a circuit diagram showing a laser diode drive circuit  20  in the second embodiment.  FIG. 6  is a timing chart showing timing of switching of switching elements Swf 1  and Swf 2  and driving switching elements Swd 1  and Swd 2 . 
     Referring to  FIG. 5 , switching elements Swf 1  and Swf 2  instead of inductors La 1  and La 2  shown in the first embodiment are connected to laser diode drive circuit  20 . Switching elements Swf 1  and Swf 2  are exemplary current suppression elements. While switching elements Swf 1  and Swf 2  are in the ON state, the current flows through switching elements Swf 1  and Swf 2 , and while switching elements Swf 1  and Swf 2  are in the OFF state, the current does not flow through switching elements Swf 1  and Swf 2 . Switching between ON and OFF of switching elements Swf 1  and Swf 2  is made by drive circuits Drf 1  and Drf 2 . Switching elements Swf 1  and Swf 2  are controlled to the ON state while capacitors Ca 1  and Ca 2  are charged, and when charging of capacitors Ca 1  and Ca 2  is completed, they are controlled to the OFF state. 
     Referring to  FIGS. 5 and 6 , switching elements Swf 1  and Swf 2  (denoted as “Swf(i)” in  FIG. 6  (i being an integer)) is controlled to the ON state at any timing during the period of the OFF state of driving switching elements Swd 1  and Swd 2  (denoted as “Swd(i)” in  FIG. 6  (i being an integer)). When switching elements Swf 1  and Swf 2  are controlled to the ON state, currents I 1  and I 2  flow from drive circuit power supply Va through current backflow prevention elements Da 1  and Da 2 , capacitors Ca 1  and Ca 2 , and switching elements Swf 1  and Swf 2  to the ground so that capacitors Ca 1  and Ca 2  are charged. At this time, the current does not flow to laser diodes LD 1  and LD 2 . After lapse of a sufficient time period for completion of charging of capacitors Ca 1  and Ca 2 , switching elements Swf 1  and Swf 2  are controlled to the OFF state. Thereafter, driving switching elements Swd 1  and Swd 2  are controlled to the ON state for a required time period so that the current flows to laser diodes LD 1  and LD 2 . 
     Timing (a) and timing (c) shown in  FIG. 6  indicate timing of completion of charging of capacitors Ca 1  and Ca 2  (timing of switching of switching elements Swf 1  and Swf 2  from the ON state to the OFF state), and timing (b) and timing (d) shown in  FIG. 6  indicate timing of switching of driving switching elements Swd 1  and Swd 2  from the OFF state to the ON state. Timing of switching of switching elements Swf 1  and Swf 2  from the ON state to the OFF state and timing of switching of driving switching elements Swd 1  and Swd 2  from the OFF state to the ON state are controlled not to coincide with each other. A period for which switching elements Swf 1  and Swf 2  are controlled to the ON state should only be sufficient for charging of capacitors Ca 1  and Ca 2 . Moreover, a period for which driving switching elements Swd 1  and Swd 2  are controlled to the ON state should only be sufficient for having laser diodes LD 1  and LD 2  emit light. In  FIG. 6 , laser diode drive circuit  20  controls driving switching element Swd(i) and switching element Swf(i) such that timing ( 0  of switching of switching element Swf(i) from the OFF state to the ON state comes after timing (e) of switching of driving switching element Swd(i) from the ON state to the OFF state. When emission of pulses of light shorter than a period for which driving switching element Swd(i) is controlled to the ON state by laser diode Ld(i) (i being an integer) is desired, however, laser diode drive circuit  20  may control driving switching element Swd(i) and switching element Swf(i) such that timing (e) comes after timing ( 0 . In other words, laser diode drive circuit  20  forcibly has charges in driving capacitor Cd released to the ground by setting switching element Swf(i) to the ON state in the middle of the period for which driving switching element Swd(i) is controlled to the ON state, so as to be able to realize pulses of light shorter than the period for which driving switching element Swd(i) is controlled to the ON state. 
     Thus, in charging of capacitors Ca 1  and Ca 2  (i.e., switching elements Swf 1  and Swf 2  being in the ON state and driving switching elements Swd 1  and Swd 2  being in the OFF state), the current flows to switching elements Swf 1  and Swf 2  and substantially no voltage is applied across the anode and the cathode of each of laser diodes LD 1  and LD 2 . Therefore, laser diodes LD 1  and LD 2  do not emit light. Moreover, undesired light emission from laser diodes LD 1  and LD 2  can thus be suppressed. 
     In charging of capacitors Ca 1  and Ca 2  (i.e., switching elements Swf 1  and Swf 2  being in the ON state and driving switching elements Swd 1  and Swd 2  being in the OFF state), the current flows to switching elements Swf 1  and Swf 2  and substantially no voltage is applied across the anode and the cathode of each of laser diodes LD 1  and LD 2 . Therefore, a voltage substantially as high as drive circuit power supply Va is applied across the opposing ends of each of capacitors Ca 1  and Ca 2 . A high voltage necessary for driving the driving switching elements Swd 1  and Swd 2  can thus be secured. Therefore, when driving switching elements Swd 1  and Swd 2  are set to the ON state, a high current can abruptly flow to laser diodes LD 1  and LD 2 . 
     When laser diodes LD 1  and LD 2  are driven (i.e., switching elements Swf 1  and Swf 2  being in the OFF state and driving switching elements Swd 1  and Swd 2  being in the ON state), the current flows from driving capacitor Cd through driving switching elements Swd 1  and Swd 2  to laser diodes LD 1  and LD 2 , and does not flow to switching elements Swf 1  and Swf 2 . 
     Laser diodes LD 1  and LD 2  emit light by switching of driving switching elements Swd 1  and Swd 2  from the OFF state to the ON state. By switching a driving switching element corresponding to a laser diode desired to emit light at timing at which light emission therefrom is desired from the OFF state to the ON state, the laser diode desired to emit light at timing at which light emission therefrom is desired can individually be controlled to emit light. 
     In general, it is noted that although two laser diodes (e.g., laser diodes LD 1  and LD 2 ) are connected to laser diode drive circuit  20 , at least one laser diode should only be connected and an arm is provided for each connected laser diode. 
     A modification of the second embodiment will be described with reference to  FIGS. 7 and 8 . Components similar to those in the first or second embodiment have the same reference characters allotted and description thereof will not be repeated. 
       FIG. 7  is a circuit diagram showing a laser diode drive circuit  21  in a modification of the second exemplary embodiment. Laser diode drive circuit  21  is different from laser diode drive circuit  20  in including resistive element Rd. Resistive element Rd suppresses an amount of charges supplied from driving power supply Vd to driving capacitor Cd during drive with pulses of laser diodes LD 1  and LD 2 . Resistive element Rd has one end connected to driving power supply Vd and the other end connected to driving capacitor Cd. 
     Thus, the current that flows from driving power supply Vd to driving capacitor Cd when driving switching elements Swd 1  and Swd 2  are set to the ON state and then charges stored in driving capacitor Cd are supplied to laser diodes LD 1  and LD 2  can be restricted, and laser diodes LD 1  and LD 2  can be driven with short pulses. 
       FIG. 8  is a circuit diagram showing a laser diode drive circuit  22  in a modification of the second embodiment. Laser diode drive circuit  22  is different from laser diode drive circuit  20  in including driving capacitors Cd 1  and Cd 2  and current backflow prevention elements Dd 1  and Dd 2  for respective arms Ar 1  and Ar 2 . 
     Charges supplied from driving power supply Vd are stored in driving capacitor Cd 1 , and driving capacitor Cd 1  supplies the current to laser diode LD 1 . Similarly, charges supplied from driving power supply Vd are stored in driving capacitor Cd 2 , and driving capacitor Cd 2  supplies the current to laser diode LD 2 . Driving capacitors Cd 1  and Cd 2  are provided between driving power supply Vd and the ground. Driving capacitors Cd 1  and Cd 2  have first ends connected to ends of driving switching elements Swd 1  and Swd 2 , respectively. 
     Current backflow prevention elements Dd 1  and Dd 2  are diodes that prevent, when the current (charges) stored in driving capacitors Cd 1  and Cd 2  flows to laser diodes LD 1  and LD 2 , the current from flowing to a laser diode connected to another arm. Driving power supply Vd is connected to the anode sides of current backflow prevention elements Dd 1  and Dd 2 , and driving capacitors Cd 1  and Cd 2  are connected to the cathode sides of current backflow prevention elements Dd 1  and Dd 2 , respectively. 
     While laser diodes LD 1  and LD 2  are driven (i.e., while driving switching elements Swd 1  and Swd 2  are in the ON state), a current may flow also to a laser diode other than a laser diode desired to emit light due to a parasitic capacitance, a parasitic inductance, or a parasitic resistance of driving switching elements Swd 1  and Swd 2 , laser diodes LD 1  and LD 2 , or an interconnection between components. In laser diode drive circuit  22 , however, driving capacitors Cd 1  and Cd 2  and current backflow prevention elements Dd 1  and Dd 2  are provided for respective arms Ar 1  and Ar 2 . Therefore, a flow of the current emitted from driving capacitors Cd 1  and Cd 2  to a laser diode in a different arm (a laser diode not desired to emit light) can be suppressed. Since current backflow prevention elements Dd 1  and Dd 2  should only be able to suppress a flow of the current emitted from driving capacitors Cd 1  and Cd 2  to a different arm, they may be resistive elements or inductors. 
     [Additional Modifications of the Exemplary Embodiments] 
     For the laser diode drive circuits described above, configurations for light emission from two laser diodes LD 1  and LD 2  are described. A configuration for light emission from three or more laser diodes can also similarly be applied. For example, in a laser diode drive circuit that drives n laser diodes LD 1 , LD 2 , LDn, inductors La 1 , La 2 , . . . , Lan are connected between points of connection A 1 , A 2 , . . . , An and points of connection B 1 , B 2 , Bn. Relation between an inductance of each inductor and an impedance of each laser diode satisfies an expression 3 below. L(i) represents an inductance of each inductor La(i) and Zld(i) represents an impedance of each laser diode LD(i), i being an integer from 1 to n. 
       | Zld ( i )|&lt;2π fld×L ( i )  (Expression 3)
 
     Inductance L(i) of inductor La(i) has such a value that the current that flows to laser diode Ld(i) during charging of capacitor Ca(i) is smaller than an amount of current necessary for light emission from laser diode Ld(i). 
     A configuration of the laser diode drive circuit in which a resistive element is connected in series to each of inductors La 1 , La 2 , . . . , Lan can also similarly be applied. In this case, relation between the inductance of each inductor and the impedance of each laser diode satisfies an expression 4 below. R(i) represents a resistance value of each resistive element Ra(i), i being an integer from 1 to n. 
       | Zld ( i )|&lt;2π fld×L ( i )+ R ( i )  (Expression 4)
 
     In general, it should be understood that the embodiments disclosed herein are illustrative and non-restrictive in every respect. 
     REFERENCE SIGNS LIST 
       10 ,  11 ,  12 ,  13 ,  20 ,  21 ,  22  laser diode drive circuit; Ar 1 , Ar 2  arm; Ca 1 , Ca 2  capacitor; Cd, Cd 1 , Cd 2  driving capacitor; Cr 1 , Cr 2  boot strap circuit; Da 1 , Da 2 , Dd 1 , Dd 2  current backflow prevention element; Dr 1 , Dr 2 , Drf 1 , Drf 2  drive circuit; LD 1 , LD 2  laser diode; La 1 , La 2  inductor; Ra 1 , Ra 2 , Rd resistive element; Swd 1 , Swd 2  driving switching element; Swf 1 , Swf 2  switching element; Va drive circuit power supply; Vd driving power supply