Laser diode driving circuit

Abstract

A laser diode driving circuit (101) includes a loop (LP1) configured to include a laser diode (LD1), a driving capacitor (C1) that accumulates driving charge, and a switching element (Q1); a first inductor (L1) connected in series with the laser diode (LD1); a parallel capacitor (C2) connected in parallel with a series circuit including the laser diode (LD1) and the first inductor (L1); and a first diode (D1) connected in parallel with the series circuit in a polarity opposite to that of the laser diode (LD1).

Description

TECHNICAL FIELD

[0001] The present application relates to a circuit for driving a laser diode, and particularly, to a driving circuit for a laser diode that emits a short pulse laser. BACKGROUND

[0002] Figure 14 is a circuit diagram of a laser diode driving circuit disclosed in Patent Literature 1. In the laser diode driving circuit, a switching element 16 shorts a capacitor 15 that is charged with a high voltage via a laser diode 4. If a light pulse is generated thereby, the capacitor 15 is charged again via a charging element (resistance element) 18. A driver 17 drives the switching element 16. A diode 19 has a function of discharging a charging current of the capacitor 15 and a function of returning a pulse current of the laser diode 4. The diode 19 suppresses oscillation of a current flowing through a circuit formed by the laser diode 4, the capacitor 15, and the switching element 16, and prevents a positive voltage applied via the laser diode 4. A resistance element 20 is selected as a resistance value that makes the pulse current of the laser diode 4 disappear rapidly.

[0003] Figure 15 is a circuit diagram of a laser diode driving circuit disclosed in Patent Literature 2. The laser diode driving circuit 14 includes: a series circuit 30 in which a direct current power source VI, an inductor 22, a reverse current prevention diode 24, a capacitor 26, and a laser diode 28 that emits light by a discharge current of the capacitor 26 are connected in series; a diode 32 connected in parallel with the laser diode 28; a switching element 34 having one end connected between the diode 24 and the capacitor 26 and the other end grounded, and switching a current flowing through the inductor 22 by on / off; and a control circuit 36 that controls on / off of the switching element 34. When the capacitor 26 is charged, the control circuit 36 makes the switching element 34 off.

[0004] PRIOR ART DOCUMENTS

[0005] PATENT LITERATURE

[0006] Patent Literature 1: Japanese Patent Application Laid-Open No. 2009-544022

[0007] Patent Literature 2: Japanese Patent Application Laid-Open No. 2016-152336 SUMMARY

[0008] PROBLEMS TO BE SOLVED BY THE INVENTION

[0009] In the laser diode driving circuit described in Patent Literature 1, a laser diode driving circuit that emits a short pulse laser of several ns to several tens of ns is assumed. The switching element 16 requires at least several ns to several tens of ns in switching of impedance. During this period, the impedance of the switching element 16 cannot be said to be sufficiently low compared with the impedance of the laser diode 4, and the voltage of the capacitor 15 is divided by the impedances of the laser diode 4 and the switching element 16 and applied to both. As a result, the applied voltage to the laser diode 4 and the emitted light power are decreased.

[0010] The above decrease in the emitted light power can be eliminated by increasing the input voltage (VH) in the circuit of FIG. 1. However, if a circuit that generates the high voltage is additionally provided, the circuit is complicated, the number of components increases, and this becomes a main cause of cost increase. Furthermore, since the pulse width of the emitted light is widened by the application of the high voltage, this becomes a problem for uses that require a short pulse width and a high instantaneous peak value. Figure 14

[0011] In the laser diode driving circuit described in Patent Literature 2, the applied voltage to the laser diode 28 and the emitted light power are also decreased based on the same reason as the circuit shown in Patent Literature 1. The above decrease in the emitted light power can be eliminated by increasing the voltage of the node Vo in the circuit of FIG. 2. However, the pulse width of the emitted light is widened by the increase in the voltage of the node Vo, and this still becomes a problem for uses that require a short pulse width and a high instantaneous peak value. Figure 15

[0012] Therefore, an object of the present application is to provide a laser diode driving circuit that emits a short pulse laser of a short pulse width and a high instantaneous peak value.

[0013] Technical Solution for Solving the Problem

[0014] (A) As one example of the present disclosure, a laser diode driving circuit is characterized by comprising: a loop configured to include a laser diode, a drive capacitor that accumulates a drive charge, and a switching element; a first inductor connected in series to the laser diode; a parallel capacitor connected in parallel to a series circuit including the laser diode and the first inductor; and a first diode connected in parallel to the series circuit in a relationship of opposite polarity to the laser diode, the laser diode driving circuit taking both ends of the switching element as an input portion of a direct-current power supply.

[0015] ​​According to the above structure, in addition to a current path formed by the drive capacitor, the switching element, the first inductor, and the laser diode, a current path formed by the drive capacitor, the switching element, and the parallel capacitor is formed. Further, immediately after the switching element is turned on, the first inductor functions as an obstacle to the rise of current flowing through the laser diode. Therefore, compared to a case where there is no parallel capacitor and first inductor, immediately after the switching element is turned on, the current flowing through the laser diode is small. Then, the energy charged to the parallel capacitor is supplied to the laser diode, and therefore, compared to a case where there is no parallel capacitor, the current flowing through the laser diode is large.

[0016] (B) A laser diode driving circuit according to one embodiment of the present disclosure is characterized by including: a laser diode and a switching element, which constitute a loop together with a direct-current power supply; a first inductor connected in series to the laser diode; and a parallel capacitor connected in parallel to a series circuit including the laser diode and the first inductor.

[0017] According to the above structure, in addition to a current path formed by the drive capacitor, the switching element, the first inductor, and the laser diode, a current path formed by the drive capacitor, the switching element, and the parallel capacitor is formed. Further, immediately after the switching element is turned on, the first inductor functions as an obstacle to the rise of current flowing through the laser diode. Therefore, compared to a case where there is no parallel capacitor and first inductor, immediately after the switching element is turned on, the current flowing through the laser diode is small. Then, the energy charged to the parallel capacitor is supplied to the laser diode, and therefore, compared to a case where there is no parallel capacitor, the current flowing through the laser diode is large.

[0018] (C) A laser diode driving circuit according to one embodiment of the present disclosure is characterized by including: a loop configured to include a laser diode, a drive capacitor that accumulates drive charge, and a switching element; a first inductor connected in series to the laser diode; and a parallel capacitor connected in parallel to a series circuit of the laser diode and the first inductor, the laser diode driving circuit taking both ends of the drive capacitor as an input of a direct-current power supply.

[0019] According to the above structure, as in the case of the structure of the above (A), immediately after the switching element is turned on, the current flowing through the laser diode is small, and then, the current flowing through the laser diode is large.

[0020] Effects of Invention

[0021] According to the present invention, a laser diode driving circuit capable of emitting short-pulse laser light of a short pulse width and a high instantaneous peak value can be obtained. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 is a circuit diagram of the laser diode drive circuit 101 according to the first embodiment.

[0023] Figure 2 is a waveform chart showing a current flowing through the laser diode LD1 after the switching element Q1 of the laser diode drive circuit 101 is turned on.

[0024] Figure 3 is a chart showing an example of a waveform of a current I LD1 flowing through the laser diode LD1 and a current I C2 flowing through the parallel capacitor C2.

[0025] Figure 4 (A) of FIG. 10, Figure 4 (B) of FIG. 11, Figure 4 (C) of FIG. 12 are circuit diagrams of other laser diode drive circuits according to the first embodiment.

[0026] Figure 5 is a circuit diagram of the laser diode drive circuit 102 according to the second embodiment.

[0027] Figure 6 is a circuit diagram of the laser diode drive circuit 103A according to the third embodiment.

[0028] Figure 7 is a circuit diagram of another laser diode drive circuit 103B according to the third embodiment.

[0029] Figure 8 is a circuit diagram of the laser diode drive circuit 104 according to the fourth embodiment.

[0030] Figure 9 is a circuit diagram of the laser diode drive circuit 105 according to the fifth embodiment.

[0031] Figure 10 (A) of FIG. 18, Figure 10 (B) of FIG. 19 are circuit diagrams of the laser diode drive circuit 106A according to the sixth embodiment.

[0032] Figure 11 is a circuit diagram of another laser diode drive circuit 106B according to the sixth embodiment.

[0033] Figure 12 is a circuit diagram of the laser diode drive circuit 107 according to the seventh embodiment.

[0034] Figure 13 is a circuit diagram of the laser diode drive circuit 108 according to the eighth embodiment.

[0035] Figure 14 is a circuit diagram of a laser diode driving circuit disclosed in Patent Literature 1.

[0036] Figure 15 is a circuit diagram of a laser diode driving circuit disclosed in Patent Literature 2. DETAILED DESCRIPTION

[0037] Hereinafter, referring to the drawings and citing several specific examples, a plurality of modes for carrying out the present application will be shown. In each drawing, the same reference numeral is assigned to the same part. In consideration of the easiness of explanation or understanding of the gist, for the convenience of explanation, the modes are shown in a plurality of modes, but partial substitution or combination of the structures shown in different modes can be made. After the 2nd embodiment, the description about the matters common to the 1st embodiment will be omitted, and only the different points will be explained. In particular, regarding the same effect based on the same structure, it will not be mentioned successively in each mode.

[0038]

[0039] Figure 1 is a circuit diagram of a laser diode driving circuit 101 related to the 1st embodiment. The laser diode driving circuit 101 is provided with a 1st loop LP1 configured to include a laser diode LD1, a drive capacitor C1 that accumulates a drive charge, and a switching element Q1. The 1st inductor L1 is connected in series to the laser diode LD1. Further, a parallel capacitor C2 is connected in parallel to a series circuit including the laser diode LD1 and the 1st inductor L1. Further, the 1st diode D1 is connected in parallel to the series circuit of the laser diode LD1 and the 1st inductor L1 in the opposite polarity relation to the laser diode LD1. The both ends of the switching element Q1 are input portions of a direct current power supply, and a resistance element R1 is connected in series to the direct current power supply V1. The 2nd loop LP2 is configured by the switching element Q1, the drive capacitor C1, and the parallel capacitor C2, and the 3rd loop LP3 is configured by the parallel capacitor C2, the laser diode LD1, and the 1st inductor L1.

[0040] In standby, the switching element Q1 is kept in the off state. In this standby, in the path of the direct current power supply V1 → the resistance element R1 → the drive capacitor C1 → the 1st diode D1, a charging current flows through the drive capacitor C1, and the drive capacitor C1 is charged with the direct current voltage of the direct current power supply V1. Further, in this standby, in the path of the direct current power supply V1 → the resistance element R1 → the drive capacitor C1 → the parallel capacitor C2, a charging current flows through the parallel capacitor C2, but since the 1st diode D1 is connected in parallel to the parallel capacitor C2, the parallel capacitor C2 is only charged with the forward voltage of the 1st diode D1. ​

[0041] When the laser diode LD1 is driven, the switching element Q1 is turned on, and the charge of the drive capacitor C1 is discharged through the path of the first loop LP1, whereby the laser diode LD1 is driven. Further, the parallel capacitor C2 is charged through the path of the second loop LP2.

[0042] Then, the discharge current of the parallel capacitor C2 flows through the laser diode LD1 in the path of the third loop LP3.

[0043] Then, the charge of the drive capacitor C1 disappears, whereby the current of the laser diode LD1 becomes 0.

[0044] Figure 2 is a waveform chart showing the current flowing through the laser diode LD1 after the switching element Q1 of the laser diode driving circuit 101 is turned on. In Figure 2 , the horizontal axis is the elapsed time from when the switching element Q1 is turned on, and the vertical axis is the current flowing through the laser diode LD1. In Figure 2 , the waveform CW0 is the waveform based on the laser diode driving circuit of the comparative example, and the waveform CW1 is the waveform based on the laser diode driving circuit 101 according to the first embodiment. The laser diode driving circuit of the comparative example is a circuit without the first inductor L1 and the parallel capacitor C2.

[0045] As will be described later, the time period T1 in Figure 2 may be called "driving current suppression period", and the time period T2 may be called "driving current enhancement period", respectively.

[0046] In the present embodiment, in addition to the current path (the first loop LP1) formed by the drive capacitor C1, the switching element Q1, the first inductor L1, and the laser diode LD1, there is also provided a current path (the second loop LP2) formed by the drive capacitor C1, the switching element Q1, and the parallel capacitor C2, so that the charge accumulated in the drive capacitor C1 immediately after the switching element Q1 is turned on is discharged through the path of the first loop LP1, and is also discharged through the second loop LP2. Therefore, the rise of the current flowing through the laser diode LD1 in the time period T1 immediately after the switching element is turned on can be suppressed. The parallel capacitor C2 is charged by the current flowing through the above-mentioned second loop LP2.

[0047] Further, the first inductor L1 hinders the rise of the current flowing through the laser diode LD1 immediately after the switching element Q1 is turned on by its inductance. Therefore, by the action of the first inductor L1, the rise of the current flowing through the laser diode LD1 in the time period T1 immediately after the switching element is turned on can also be suppressed.

[0048] The energy charged to the shunt capacitor C2 is supplied to the laser diode LDl through the path of the loop LP3 during the time period T2, so the current flowing through the laser diode LDl becomes large compared to the case where there is no shunt capacitor C2.

[0049] By the above action of the shunt capacitor C2 and the first inductor LI, the time period T2 as the driving current enhancement period is shortened, and the peak value of the driving current flowing through the laser diode LDl is increased.

[0050] When the current flowing through the circuit formed by the shunt capacitor C2, the first inductor LI, the laser diode LDl and the first diode Dl is a decaying oscillation current, and the peak value of the current and the peak value of the current flowing from the driving capacitor Cl to the first loop LP1 overlap, the current enhancement effect during the time period T2 becomes maximum.

[0051] Therefore, when the capacitance of the shunt capacitor C2 is denoted by C2, the inductance of the first inductor LI is denoted by LI, and the resistance component of the laser diode LDl is denoted by RLDl, respectively, it is preferable to satisfy the following conditions:

[0052] RLDl < 2π2L1 / C2 LD1 2 LD1 <4L1 / C2.

[0053] This is the same for the embodiments to be shown later.

[0054] Figure 3 is a graph showing an example of the waveforms of the current I LD1 flowing through the laser diode LDl and the current I C2 flowing through the shunt capacitor C2. Here, the direction of the current charging the shunt capacitor C2 through the loop LP2 shown by Figure 1 is "positive", and the direction of the current discharging from the shunt capacitor C2 through the loop LP3 is "negative".

[0055] In Figure 3 , the time point tzl is the time point at which the current I C2 changes from positive to negative, tp is the time point at which the current flowing through the laser diode LDl becomes maximum, and tz2 is the time point at which the current I C2 ​The time point when the current changes from negative to positive. As in this example, it is preferable that the current flowing through the laser diode LD1 reaches its maximum at a time tp during the period when the current in the parallel capacitor C2 is negative. In other words, it is preferable to satisfy the condition tz1 < tp < tz2. This is because, while the magnitude relationship between tz1, tz2, and tp varies depending on the value of the parallel capacitor C2, satisfying this condition allows the discharge current of the parallel capacitor C2 to enhance the drive current of the laser diode LD1. This also applies to the second and subsequent embodiments described below.

[0056] Figure 4 (A) Figure 4 (B) Figure 4 (C) is a circuit diagram of another laser diode driving circuit according to the first embodiment.

[0057] Figure 4 The laser diode drive circuit 101A shown in (A) is replaced with Figure 1 The laser diode drive circuit 101A and the first inductor L1 are shown in FIG. Figure 1 The laser diode driving circuit 101 shown is equivalent in circuitry.

[0058] Figure 4 The laser diode driving circuit 101B shown in (B) is a modified Figure 1 The loops LP1 and LP2 including the driving capacitor C1 are equivalent to the laser diode driving circuit 101, so the laser diode driving circuit 101B and Figure 1 The laser diode driving circuit 101 shown is equivalent in circuitry.

[0059] Figure 4 The laser diode driving circuit 101C shown in (C) is changed Figure 1 The example of the position of the resistor element R1 is shown. The charging current path of the driving capacitor C1 of the laser diode driving circuit 101C is equivalent to the charging current path of the driving capacitor C1 of the laser diode driving circuit 101, so the laser diode driving circuit 101C and Figure 1 The laser diode driving circuit 101 shown is equivalent in circuitry.

[0060] Second Implementation Method

[0061] In the second embodiment, a laser diode driving circuit including a circuit for boosting the charging voltage of the driving capacitor C1 is exemplified.

[0062] Figure 5is a circuit diagram of a laser diode driving circuit 102 according to the second embodiment. The laser diode driving circuit 102 includes a laser diode LD1, a driving capacitor Cl, a switching element Ql, a first inductor LI, a parallel capacitor C2, and a first diode Dl. A series circuit of a second inductor L2 and a second diode D2 is inserted between a direct current power supply VI and the switching element Ql. The structure of the series circuit of the second inductor L2 and the second diode D2 is different from the laser diode driving circuit 101 shown in the first embodiment.

[0063] In the laser diode driving circuit 102 according to the second embodiment, by turning on the switching element Ql, a current flows in a path of the direct current power supply VI → the second inductor L2 → the second diode D2 → the switching element Ql, and a magnetizing energy is accumulated in the second inductor L2. Then, at the time of turning off the switching element Ql, a charging current of the driving capacitor Cl flows in a path of the direct current power supply VI → the second diode D2 → the driving capacitor Cl, the first diode Dl. At this time, by the same action as the step-up chopper circuit, a voltage that is stepped up charges the driving capacitor Cl.

[0064] According to the present embodiment, it is possible to drive the laser diode LDl at a voltage higher than that of the direct current power supply VI. That is, it is possible to perform high voltage driving of the laser diode LDl with a small number of components without additionally providing a special step-up circuit.

[0065]

[0066] In the third embodiment, a laser diode driving circuit having a structure different from the circuit forming the second loop LP2 in the first and second embodiments is exemplified.

[0067] Figure 6 is a circuit diagram of a laser diode driving circuit 103A according to the third embodiment. The laser diode driving circuit 103A includes a laser diode LDl, a driving capacitor Cl, a switching element Ql, a first inductor LI, a parallel capacitor C2, a first diode Dl, and a resistive element Rl.

[0068] In the laser diode driving circuit 103A, unlike the laser diode driving circuit 101 shown in the first embodiment, a parallel circuit of a resistive element R2 and a third diode D3 is inserted between the switching element Ql and the parallel capacitor C2.

[0069] ​In the 2nd loop LP2 including the switching element Ql, the drive capacitor Cl, and the parallel capacitor C2, there is a parasitic inductance. By the action of this parasitic inductance, sometimes the voltage of the parallel capacitor C2 becomes higher than the voltage of the drive capacitor Cl. At this time, the 3rd diode D3 blocks the discharge current of the parallel capacitor C2 from flowing to the switching element Ql side. Thereby, as shown in the 3rd loop LP3 in FIG. 1, more current flows through the laser diode LDl, and a larger instantaneous peak current can be obtained. The resistance element R2 forms a charging current path CP of the drive capacitor Cl. In order to ensure the above-mentioned action of the 3rd diode D3, this resistance element R2 needs to be sufficiently high compared with the impedance of the laser diode LDl. Figure 6

[0070] Figure 7 is a circuit diagram of another laser diode drive circuit 103B to which the 3rd embodiment relates. This laser diode drive circuit 103B is a circuit diagram in which the resistance element R2 of the laser diode drive circuit 103A shown in FIG. 1 is replaced by a 3rd inductor L3. Even with the structure of this laser diode drive circuit 103B, the 3rd diode D3 blocks the discharge current of the parallel capacitor C2 from flowing to the switching element Ql side. Further, the 3rd inductor L3 suppresses the transient current of the discharge current of the parallel capacitor C2 from flowing to the switching element Ql side, and thus the above-mentioned action of the 3rd diode D3 can be ensured. Figure 6

[0071]

[0072] In the 4th embodiment, a laser diode drive circuit whose topology is different from the circuits shown in each of the 1st, 2nd, and 3rd embodiments is exemplified.

[0073] Figure 8 is a circuit diagram of a laser diode drive circuit 104 to which the 4th embodiment relates. This laser diode drive circuit 104 is a circuit provided with: a 1st loop LP1 configured to include a laser diode LDl, a drive capacitor Cl, and a switching element Ql; a 1st inductor LI connected in series with the laser diode LDl; and a parallel capacitor C2 connected in parallel with the series circuit of the laser diode LDl and the 1st inductor LI, and this laser diode drive circuit 104 takes both ends of the drive capacitor Cl as an input portion of a direct-current power source.

[0074] This laser diode drive circuit 104 operates as follows.

[0075] In standby, the switching element Ql remains in an off state. In this standby, the drive capacitor Cl is charged by the voltage of the direct-current power source VI.

[0076] ​​​When the laser diode LD1 is driven, the switching element Q1 is turned on, and the charge of the drive capacitor Cl is discharged through the path of the first loop LP1, whereby the laser diode LD1 is driven. Further, the parallel capacitor C2 is charged through the path of the second loop LP2.

[0077] Then, the discharge current of the parallel capacitor C2 flows through the path of the third loop LP3 via the laser diode LD1.

[0078] <5th Embodiment>

[0079] In the 5th embodiment, a laser diode driving circuit in which an element that limits the discharge current of the parallel capacitor C2 is provided to the laser diode driving circuit of the topology shown in the 4th embodiment is exemplified.

[0080] Figure 9 is a circuit diagram of the laser diode driving circuit 105 to which the 5th embodiment is applied. The laser diode driving circuit 105 is a circuit in which Figure 8 the fourth diode D4 is provided between the drive capacitor Cl and the parallel capacitor C2 in the laser diode driving circuit 104 shown in

[0081] In the second loop LP2 that includes the switching element Q1, the drive capacitor Cl, and the parallel capacitor C2, there is a parasitic inductance. Due to the effect of this parasitic inductance, sometimes the voltage of the parallel capacitor C2 becomes higher than the voltage of the drive capacitor Cl. At this time, the fourth diode D4 prevents the discharge current of the parallel capacitor C2 from flowing to the drive capacitor Cl side. Due to this, the discharge current of the parallel capacitor C2 flows entirely through the laser diode LD1, and a larger instantaneous peak current flows through the laser diode LD1.

[0082] <6th Embodiment>

[0083] In the 6th embodiment, a laser diode driving circuit in which the structure of the drive capacitor Cl of the laser diode driving circuit in each of the 1st, 2nd, and 3rd embodiments is different is exemplified.

[0084] Figure 10 (A) of Figure 10 (B) of Figure 10 (A) of Figure 1 is a circuit in which the drive capacitor Cl in the laser diode driving circuit 101 shown in Figure 10 (B) is a diagram that expresses the circuit shown in Figure 10 (A) in a general form.

[0085] The laser diode driving circuit 106A operates as follows.

[0086] When laser diode LD1 is driven, switching element Q1 turns on, and the drive current of laser diode LD1 flows through the path (first loop LP1) from DC power supply V1 to switching element Q1 to first inductor L1 to laser diode LD1. Furthermore, in the path (second loop LP2) from DC power supply V1 to switching element Q1 to shunt capacitor C2, charging current flows through shunt capacitor C2. The discharge current from shunt capacitor C2 then flows through the third loop LP3.

[0087] Then, the switching element Q1 is turned off, whereby the current of the laser diode LD1 becomes zero.

[0088] Figure 11 This is a circuit diagram of another laser diode driving circuit 106B according to the sixth embodiment. While the DC power supply in the aforementioned laser diode driving circuit 106A is a negative power supply, this laser diode driving circuit 106B uses a positive power supply. The circuit operation is the same as that of the aforementioned laser diode driving circuit 106A.

[0089] Seventh Implementation Method

[0090] In the seventh embodiment, a laser diode driving circuit is exemplified in which the structure of the driving capacitor C1 of the laser diode driving circuit in the fourth embodiment is different.

[0091] Figure 12 FIG. 1 is a circuit diagram of a laser diode driving circuit 107 according to the seventh embodiment. The laser diode driving circuit 107 is a circuit that converts Figure 8 The driving capacitor C1 in the laser diode driving circuit 104 shown is replaced by a DC power supply V1.

[0092] The laser diode driving circuit 107 operates as follows.

[0093] When laser diode LD1 is driven, switching element Q1 turns on, and the drive current for laser diode LD1 flows through the path (first loop LP1) from DC power supply V1 to first inductor L1 to laser diode LD1 to switching element Q1. Furthermore, in the path (second loop LP2) from DC power supply V1 to shunt capacitor C2 to switching element Q1, charging current flows through shunt capacitor C2. The discharge current from shunt capacitor C2 then flows through the third loop LP3.

[0094] Then, the switching element Q1 is turned off, whereby the current of the laser diode LD1 becomes zero.

[0095] Implementation Example 8

[0096] In the eighth embodiment, a laser diode driving circuit different in structure of the drive capacitor Cl of the laser diode driving circuit in the fifth embodiment is exemplified.

[0097] Figure 13 is a circuit diagram of the laser diode driving circuit 108 involved in the eighth embodiment. The laser diode driving circuit 108 is a circuit in which the drive capacitor Cl in the laser diode driving circuit 105 shown in Figure 9 is replaced by the direct-current power supply VI.

[0098] The operation of the laser diode driving circuit 108 is as follows.

[0099] When the laser diode LDl is driven, the switching element Ql is turned on, and the drive current of the laser diode LDl flows in the path of the direct-current power supply VI → the fourth diode D4 → the first inductor LI → the laser diode LDl → the switching element Ql. Further, in the path of the direct-current power supply VI → the fourth diode D4 → the parallel capacitor C2 → the switching element Ql, the charging current flows through the parallel capacitor C2. Then, the discharge current of the parallel capacitor C2 flows through the laser diode LDl.

[0100] Then, the switching element Ql is turned off, whereby the current of the laser diode LDl becomes 0.

[0101] In the second loop LP2 including the switching element Ql, the drive capacitor Cl, and the parallel capacitor C2, there is a parasitic inductance. Due to the action of this parasitic inductance, sometimes the voltage of the parallel capacitor C2 becomes higher than the voltage of the drive capacitor Cl. At this time, the fourth diode D4 prevents the discharge current of the parallel capacitor C2 from flowing to the direct-current power supply VI side.

[0102] Finally, the present application is not limited to the above-described embodiments. Modification and change can be appropriately made by those skilled in the art. The scope of the present application is not shown by the above-described embodiments, but is shown by the claims. Further, the scope of the present application includes modification and change from the embodiments within the scope equivalent to the claims.

[0103] For example, the first inductor LI shown in each of the embodiments can be constituted by a parasitic inductance possessed by a wiring section associated with the laser diode LDl. Further, a synthetic inductance of an inductor and a parasitic inductance can be used as the first inductor LI.

[0104] Further, the parallel capacitor C2 shown in each of the embodiments can be constituted by a parasitic capacitance possessed by a wiring section associated with the laser diode LDl. Further, a synthetic capacitance of a capacitor and a parasitic capacitance can be used as the parallel capacitor C2.

[0105] Reference Signs

[0106] C1: drive capacitor

[0107] C2: parallel capacitor

[0108] CP: charge current path

[0109] D1: first diode

[0110] D2: second diode

[0111] D3: third diode

[0112] D4: fourth diode

[0113] L1: first inductor

[0114] L2: second inductor

[0115] L3: third inductor

[0116] LD1: laser diode

[0117] LP1: first loop

[0118] LP2: second loop

[0119] LP3: third loop

[0120] Q1: switching element

[0121] R1, R2: resistance element

[0122] V1: direct current power supply

[0123] 101, 101A, 101B, 101C, 102, 103A, 103B, 104, 105, 106A, 106B, 107, 108: laser diode drive circuit

Claims

1. A laser diode drive circuit comprising: a loop circuit including a laser diode, a driving capacitor for accumulating driving charge, and a switching element; a first inductor connected in series with the laser diode; a parallel capacitor connected in parallel with the series circuit including the laser diode and the first inductor; and A first diode is connected in parallel to the series circuit with a polarity opposite to that of the laser diode. The first end of the switching element is connected to the first end of the DC power supply, the second end of the switching element is connected to the second end of the DC power supply, the second end of the DC power supply has a higher potential than the first end of the DC power supply, and the cathode of the laser diode is connected to the second end of the DC power supply.

2. The laser diode driving circuit according to claim 1, wherein: A series circuit of a second inductor and a second diode is inserted between an input portion of the DC power supply and the switching element.

3. The laser diode driving circuit according to claim 1, wherein: A parallel circuit of a resistor element and a third diode for backflow prevention, or a parallel circuit of an inductor and a third diode for backflow prevention, is inserted between the switching element and the parallel capacitor.

4. A laser diode drive circuit comprising: The laser diode and the switching element form a loop together with the DC power supply; a first inductor connected in series with the laser diode; and A parallel capacitor is connected in parallel with the series circuit including the laser diode and the first inductor.

5. A laser diode drive circuit comprising: a loop circuit including a laser diode, a driving capacitor for accumulating driving charge, and a switching element; a first inductor connected in series with the laser diode; and a parallel capacitor connected in parallel with the series circuit of the laser diode and the first inductor, The laser diode driving circuit uses both ends of the driving capacitor as input parts of a DC power supply.

6. The laser diode driving circuit according to claim 5, wherein: A fourth diode for backflow prevention is provided between the parallel capacitor and the drive capacitor.

7. The laser diode driving circuit according to any one of claims 1 to 6, wherein: The current flowing through the circuit formed by the parallel capacitor, the first inductor, and the laser diode is a damped oscillating current, and the peak value of the damped oscillating current overlaps with the peak value of the current flowing through the loop including the laser diode and the switching element.

8. The laser diode driving circuit according to any one of claims 1 to 6, wherein: C2 represents the capacitance of the parallel capacitor, L1 represents the inductance of the first inductor, and R LD1 represents the resistance component of the laser diode. At this time, it is in the following relationship: R 2 LD1 <4L1 / C2。 9. The laser diode driving circuit according to any one of claims 1 to 6, wherein: Tz1 represents the time point when the current flowing through the parallel capacitor changes from the positive direction to the negative direction, Tz2 represents the time point when the current flowing through the parallel capacitor changes from the negative direction to the positive direction, and Tp represents the time point when the current flowing through the laser diode reaches its peak. At this time, the capacitance of the parallel capacitor has the following relationship: Tz1<Tp<Tz2.

10. The laser diode driving circuit according to any one of claims 1 to 6, wherein: The first inductor is formed by parasitic inductance of a wiring portion associated with the laser diode.

11. The laser diode driving circuit according to any one of claims 1 to 6, wherein: The parallel capacitor is formed by parasitic capacitance of a wiring portion associated with the laser diode.

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