LASER DIODE CIRCUIT

Description

[0001] The present invention relates to a laser diode circuit comprising a charging circuit which can be connected to a voltage source and has a capacitor, and a load circuit which is fed by the charging circuit and has a laser diode in the forward direction, wherein the load circuit is connected in parallel to the capacitor and has a controllable first switch in series with the laser diode, and wherein the charging circuit has in series a coil, an electronic element which passes current to the capacitor and blocks current in the opposite direction, and the said capacitor.

[0002] Laser diode circuits of this type are known from WO 2021 / 051762 A1, JP 2009-260030 A, or US Pat. No. 9,368,936 B1 and are used, for example, in high-performance laser scanners for topographical surveying of landscapes, streets, mines, tunnels, or bodies of water. The generation of high-energy, ultrashort laser pulses in the nanosecond range with a high repetition or clock rate is crucial for quickly surveying even distant surfaces with high spatial resolution.

[0003] Fig. 1 shows a typical laser diode circuit of this type according to the prior art. The laser diode circuit consists of a charging circuit 1 for a capacitor C1 and a load circuit 2, which is fed by charging circuit 1, or more precisely, its capacitor C1, and contains the laser diode LD. As can be seen, charging circuit 1 and load circuit 2 share capacitor C1, i.e., the voltage U1 across capacitor C1 and thus at the output of charging circuit 1 relative to ground potential N is simultaneously the input and supply voltage of load circuit 2 with the laser diode LD.

[0004] Charging circuit 1 can be connected to (or includes) a voltage source V1. Voltage source V1 charges capacitor C1 via a charging resistor R1. Load circuit 2 includes a controllable switch S1 connected in series with laser diode LD and having a control input E1. Closing switch S1, controlled by a corresponding signal at control input E1, closes load circuit 2, and capacitor C1 charged by charging circuit 1 is discharged via laser diode LD, causing it to emit a laser pulse.

[0005] The total inductance L par shown in series with the laser diode LD in load circuit 2 represents, for simplification, the sum of all inductances occurring in load circuit 2 and includes the parasitic component, line and stray inductances spatially distributed in load circuit 2 in reality. The diode D3, which is connected antiparallel to the series circuit of total inductance L par and laser diode LD, serves to protect the laser diode LD from the adverse effects of a negative voltage that is induced at L par towards the end of the discharge current pulse.

[0006] A major disadvantage of the known laser diode circuit of Fig. 1 is the energy loss occurring at the charging resistor R1. This is approximately equal to the energy stored in capacitor C1 at the end of the charging process, which limits the theoretical maximum efficiency of this circuit to about 50%. Particularly when using LD laser diodes with high repetition rates and high laser powers, the power loss of charging circuit 1 represents a significant portion of the total power requirement of the laser diode circuit and must be dissipated from the device by complex cooling measures. The energy stored in the circuit of Fig. 1 The necessary protective diode D3, which converts the residual energy stored in the total inductance L par at the end of the laser pulse into thermal power loss, significantly reduces the efficiency.

[0007] The invention aims to overcome the disadvantages of the known prior art and to provide a laser diode circuit with improved efficiency.

[0008] This aim is achieved with a laser diode circuit of the type mentioned at the outset, which is characterized according to the invention in that the load circuit has a second diode, which is not a laser diode, connected upstream or downstream of the laser diode in the same forward direction, or an isolating switch connected upstream or downstream of the laser diode, which is designed to be opened at the end of the charge reversal process when a negative potential would be applied to the laser diode in order to protect the laser diode.

[0009] According to the invention, the residual energy stored in the parasitic sum inductance of the load circuit, which in the prior art is stored in the protective diode D3 ( Fig. 1 ) must be destroyed, can be used or recycled for the operation of the laser diode. For this purpose, in a first embodiment, a second diode is connected upstream or downstream of the laser diode in the load circuit in the same forward direction. This second diode, because it is not a laser diode, can be equipped with a particularly high dielectric strength in the reverse direction and a particularly high blocking resistance, so that in that phase of the current flow in the load circuit when the magnetic field in the parasitic total inductance collapses and drives further current flow, which recharges the capacitor to a negative potential, the laser diode is protected from excessively high negative voltage. By omitting the protective diode (D3 in Fig. 1 ), the residual energy in the load circuit's series resonant circuit, formed by the capacitor and the parasitic total inductance, can be utilized to recharge the capacitor to a negative potential, so that the next charging process can begin from this negative potential. This, in turn, saves power loss and allows a voltage source with a lower output voltage or a smaller capacitor to be used for the same charging power, or achieves a higher charging power with the same capacitor volume.

[0010] Instead of the second diode mentioned in the load circuit, in a second embodiment, a isolating switch connected upstream or downstream of the laser diode can also be used, which is switched off or opened at exactly the right time, ie at the end of the charging process of the capacitor, when the negative potential would be applied to the laser diode, in order to protect the laser diode.

[0011] In the laser diode circuit, a series circuit consisting of a coil and an element acting as a diode is used in the charging circuit instead of a lossy charging resistor. This transforms the charging circuit into a series resonant circuit consisting of a coil and a capacitor limited to the first half-wave. This simultaneously creates a voltage doubling circuit because, once the capacitor has reached the potential of the voltage source, the magnetic energy stored in the coil induces a continued current flow to the capacitor upon collapse of the magnetic field, thus further charging it to approximately twice the potential of the voltage source. The power loss of the charging circuit according to the invention is only a fraction of that when using a charging resistor and is essentially due only to the losses at the diode element and to the line resistance and magnetization losses of the coil.By doubling the capacitor's charging voltage, for example, the output voltage of the voltage source can be reduced by half, thus allowing a smaller voltage source to be used. Alternatively, with an unchanged voltage source, the same charging energy can be achieved with a capacitor with a quarter of the capacity, or conversely, four times the charging energy can be achieved with the same capacitor, which significantly improves the space requirement and the circuit's performance compared to the state of the art.

[0012] The electronic element that allows current to flow to the capacitor and blocks current in the opposite direction can be implemented in any desired way, for example, in the form of a suitably controlled electronic switch. However, the electronic element is preferably a first diode, which reduces the number of components required.

[0013] In a particularly advantageous embodiment of the invention, the series resonant circuit in the charging circuit can also be used in the manner of a charge pump, preferably by arranging a controllable second switch in the charging circuit, which is connected in parallel to the series circuit comprising the electronic element and the capacitor. By closing the second switch at the beginning of the charging phase, the capacitor is, as it were, bridged, so that the potential applied to the coil remains unchanged during the build-up of the current flow and thus the magnetic field in the coil, corresponding to the voltage of the voltage source. The magnetic energy stored in the coil in this way, which then commutates into a charging current and thus charge transport into the capacitor when the second switch is opened, can thus be selected by the duration of the second switch being closed.By repeatedly closing the second switch, the charging voltage of the capacitor can be increased even further, in the manner of a charge pump.

[0014] In connection with the control of a laser diode, this adjustability of the capacitor's charging voltage also opens up the possibility of varying the amplitude or power of the laser pulses from pulse to pulse, e.g., according to a desired modulation. For example, the pulse power of a laser scanner that emits laser pulses across a scanning fan on a flat floor can be increased in the peripheral areas of the fan, where the target distance is greater, or the pulse power for the next laser pulse to be emitted can generally be adjusted to the target distance measured with a previous laser pulse.

[0015] A particularly preferred embodiment of the laser diode circuit according to the invention for this purpose has a pulse generator designed to control the first switch with a first sequence of control pulses at a clock rate and the second switch with a second sequence of control pulses leading the first, at the same clock rate or a multiple thereof, and each with a pulse width corresponding to the current amplitude of a modulation signal. The modulation signal thus controls the switch-on times of the second switch in the charging circuit and thus the charging voltage of the capacitor according to the amplitude of a modulation signal fed, for example, into the laser diode circuit, which follows, for example, the angular direction of the laser pulses in a scanning fan, previous distance measurements, or other specifications.

[0016] The aforementioned adjustability of the capacitor's charging voltage by varying its closing duration or the pulse width of its drive signal can also be used to compensate for fluctuations in the voltage of the voltage source. A further preferred embodiment of the invention is accordingly characterized by a compensation circuit designed to vary the modulation signal inversely proportional to the voltage of the voltage source. This not only compensates for fluctuations in an unreliable voltage source, but also allows the use of voltage sources with low power or high internal resistance, which, for example, are buffered only by a buffer capacitor connected in parallel with the voltage source.Such capacitor-buffered voltage sources suffer a voltage drop by design during each charging cycle with continued current draw, which can now be compensated for by the circuit according to the invention. As a result, particularly small voltage sources buffered solely by a buffer capacitor can be used.

[0017] It goes without saying that the pulse width control of the second switch can also be used exclusively to compensate for such voltage source fluctuations, i.e., the modulation signal itself does not follow any external specification but depends solely on a measured value of the voltage source. Alternatively, it is possible to use the pulse width control of the second switch for both measures simultaneously, i.e., to additionally modify an external modulation signal received at a modulation input of the pulse generator depending on the voltage source.

[0018] The charging circuit can be continuously operated, meaning that every time a laser pulse is delivered, charging of the capacitor immediately begins again as soon as the first switch is opened after the pulse is delivered. However, if desired, the charging circuit can also have a charging switch connected upstream or downstream of the coil or first diode, which is only closed when charging of the capacitor is to begin.

[0019] The invention is explained in more detail below with reference to exemplary embodiments illustrated in the accompanying drawings. In the drawings: Fig. 1 a laser diode circuit according to the prior art in a circuit diagram; Fig. 2 a first embodiment of the laser diode circuit of the present disclosure in a circuit diagram; Fig. 3 the current and voltage curve when charging the capacitor in the charging circuit of the laser diode circuit of Fig. 2 in a time diagram; Fig. 4 a second embodiment of the laser diode circuit of the present disclosure in a circuit diagram; the Fig. 5a und 5b the current and voltage curves in the load circuit of the laser diode circuit of Fig. 4 each in a time diagram; the Fig. 6a und 6b the current and voltage curves of the Fig. 5a und 5b over a longer period of time, which also includes the charging phase, after the emission of a laser pulse, each in a time diagram, where Fig. 6b also shows the current flow in the charging circuit; Fig. 7 a third embodiment of the laser diode circuit of the present disclosure in a circuit diagram; Fig. 8 the current and voltage curves in the charging circuit of the laser diode circuit of Fig. 7 in the charging phase in a time diagram; the Fig. 9a bis 9c Amplitude / time diagrams of control pulses for the first switch ( Fig. 9a ) and the second switch ( Fig. 9b ) of the laser diode circuit of Fig. 7 and laser pulses generated thereby ( Fig. 9c ); and the Fig. 10a and 10c show two inventive variants of the laser diode circuit of Fig. 4 to protect the laser diode from excessive blocking voltages, each in a circuit diagram.

[0020] Concerning Fig. 1 Please refer to the description in the introduction. Like reference numerals designate like parts throughout the figures. The description of the Fig. 2 bis 10 is based on the description of the Fig. 1 and in the following only the differences compared to Fig. 1 explained.

[0021] Fig. 2 shows a first embodiment of the laser diode circuit of the present disclosure. In the charging circuit 1, instead of the charging resistor R1 of Fig. 1 a series circuit comprising a coil L1 and a first diode D1, which is forward-biased towards the capacitor C1, is arranged. A charging switch SL can be interposed at any point in the series circuit comprising coil L1 and diode D1, for example before or after the coil L1 or diode D1. The charging switch SL has a control input for a control signal EL, with which the charging circuit 1 is closed for the purpose of charging the capacitor C1 from the voltage source V1 via the coil L1 and the diode D1. The switch S1 in the load circuit 1 is open when the capacitor C1 is charging. As discussed, the voltage source V1 can be part of the laser diode circuit or not (i.e. external).

[0022] Fig. 3 shows the curve of current I1 in charging circuit 1 and voltage U1 across capacitor C1 during the charging phase, i.e., after switch SL closes. The specified values ​​for current, voltage, and time t are, of course, only examples.

[0023] After a laser pulse has been emitted by the laser diode LD by closing the switch S1 at time t = 0 (in Fig. 3 not shown, see later Fig. 5a - 6b ) and the voltage U1 across capacitor C1 has dropped accordingly, the recharging process is initiated by closing the charging switch SL. The charging current I1 initially increases sinusoidally until the voltage U1 reaches the voltage U V1 of the voltage source V1 (first charging phase a). In the subsequent second charging phase b, the current I1 is driven further by the magnetic field collapsing at the coil L1 and drives the charging voltage U1 to approximately twice the voltage U V1 of the voltage source V1 until the current I1 has decayed.

[0024] As from Fig. 3 As can be seen, the charging current I1 through L1 is half-sinusoidal. The duration of this half-sinusoidal waveform is essentially determined by the inductance of coil L1 and the capacitance of capacitor C1. Since the capacitance of capacitor C1 is usually determined by the required pulse energy of the laser diode LD, the charging duration a + b can be easily adjusted to the maximum available charging time or the required repetition rate of the laser pulses by selecting L1.

[0025] Fig. 4 shows a simplified embodiment of the laser diode circuit of Fig. 2 without charging switch SL. The SL switch can be omitted, especially if the capacitor C1 can or should be recharged immediately after discharging (and not after a delay).

[0026] Fig. 4 also shows the optional variant where, instead of the first diode D1, any electronic element is used that allows current to flow toward the capacitor C1 and blocks it in the opposite direction. For example, this electronic element is an electronic switch SD that is controlled to perform this diode function. In another variant (not shown), the electronic element could also be the charging switch SL ( Fig. 2 ) itself if it is controlled accordingly.

[0027] The Fig. 5a und 5b Figures 6a and 6b show the timing of laser pulse delivery and recharging in detail. An exemplary pulse repetition rate of 1 MHz provides 1 µs of time to recharge capacitor C1, see Fig. 6b . The pulse duration of a laser pulse of the laser diode LD is approximately 8 ns in the example shown, see Fig. 5a . Switch S1 for triggering a laser pulse could in principle only be closed for the duration of the laser pulse, e.g., (here) 8 ns. However, to utilize the residual energy stored in the total inductance L par, switch S1 is closed until this residual energy, which continues to drive current I2 after the voltage U1 collapses, has recharged capacitor C1 to approximately the negative voltage U1, e.g., (here) -45 V.

[0028] Switch S1 is opened for the next charging process. This subsequent charging process begins at a negative voltage U1 of -45 V across capacitor C1, which is then inverted by charging circuit 1 as discussed above and increased by twice the voltage U V1 of voltage source V1, in this case to approximately 95 V. The usable voltage swing across capacitor C1 is thus approximately 140 V, a multiple of the voltage U V1 of voltage source V1 (here: 25 V).

[0029] Fig. 7 shows a further embodiment of the laser diode circuit with a controllable second switch S2, which is located in the charging circuit 1 in parallel with the series circuit comprising the electronic element (diode D1 or switch SD or SL) and capacitor C1. With the help of the switch S2, the diode D1 and the capacitor C1 can be bridged in the first phase a of the charging process of the capacitor C1, i.e., the coil L1 can be closed to ground, whereby the coil current I1 can increase linearly, so to speak, unhindered – within the system limits of the internal resistance of the voltage source V1 and the line resistance of the coil L1 – so that its final value I1 max depends on the closing duration a of the switch S2, see Fig. 8 The magnetic energy stored in coil L1 in this way is commutated into the charge of capacitor C1 after the opening of switch S2 in the subsequent phase b of the collapse of the magnetic field, thus further building up voltage U1. The charging voltage U1 of capacitor C1 at the end of charging phase a + b is thus proportional to the closing duration a of switch S2, i.e., the pulse width PW of a control pulse applied to the control input E2 of switch S2 to close switch S2.

[0030] If desired, the voltage U1 across capacitor C1 could be further increased by repeating charging phases a + b before discharging capacitor C1 for the next laser pulse, similar to a charge pump. In the simplest case, especially when only a single charging phase a + b is available between two laser pulses at a high pulse repetition rate, only one such charging phase is performed per laser pulse.

[0031] The laser diode circuit of Fig. 7 Due to the adjustability of the charging voltage U1 and thus the supply voltage of the laser diode LD per laser pulse, it can also be used to modulate a sequence of laser pulses in their amplitude or pulse power, as can be seen from the Fig. 9a bis 9c is explained in more detail.

[0032] For the delivery of a sequence F of laser pulses R 1 , R 2 , ..., in general R k , with a clock rate ("pulse repetition rate", PRR) f ( Fig. 9c ) a pulse generator 3 generates a first sequence F 1 of control pulses P 1 , P 2 , ..., generally P i ( Fig. 9a ). The control pulses P i are fed to the control input E1 of the switch S1 in order to emit a laser pulse R i through the laser diode LD.

[0033] To control the second switch S2, the pulse generator 3 simultaneously generates a second sequence F 2 of control pulses Q 1 , Q 2 , ..., generally Q j ( Fig. 9b ). The clock rate f of the control pulses Q j for the second switch S2 is equal to the clock rate f of the control pulses P i for the first switch S1 (or an integer multiple thereof if several charging phases a + b per laser pulse R i are desired to multiply the charging voltage U1). The control pulses Q j each lead the control pulses P i, symbolized by a time delay element 4 between a generator 5 for the clock f of the pulse generator 3 and the input E1 of the switch S1.

[0034] It is understood that the term "clock rate" used here does not necessarily refer to a time-constant clock rate; the clock rate f can also vary over time (so-called PRR variation).

[0035] The pulse width PW j of the control pulses Q j of the second switch S2 is predetermined in each case according to a desired amplitude A k of the laser pulses R k emitted by the laser diode LD in order to set the respective charging voltage U1 of the capacitor C1 for the next laser pulse P k . Thus, for example, a modulation signal M at a modulation input M1 of the pulse generator 3, amplitude-modulated onto the output pulses of the generator 5 in a mixer 6 and converted in an amplitude / pulse width converter 7 into the pulse width-modulated sequence F 2 of control pulses Q j , can modulate the charging voltage U1 for each laser pulse R k of the output sequence F and thus the laser power of the laser pulse R k via the charging circuit 1 in the manner mentioned, see Fig. 9c .

[0036] The pulse width variation of the control signal of the second switch S2 can—alternatively or in addition to the discussed following of a modulation signal M fed to the modulation input M1—also be used to compensate for fluctuations in the voltage U V1 of the voltage source V1. An optional compensation circuit 8 serves this purpose, which modifies the modulation signal M depending on the voltage U V1 of the voltage source V1, in particular inversely proportional thereto. The compensation circuit 8 has, for example, an inverse value generator 9, to which a measurement signal of the voltage U V1 of the voltage source V1 is fed at its input and which modifies the modulation signal M accordingly via a further mixer 10.

[0037] In the Fig. 7 In the example shown, an (external) modulation signal fed into the modulation input M1, which, for example, follows the angular direction of the laser pulses in a scanning fan, previous distance measurements, or other specifications, is in turn modulated by the further mixer 10 for the purpose of compensating for fluctuations in the voltage source V1. However, if only compensation for fluctuations in the voltage source V1 is desired, the modulation input M1 can be omitted and the compensation circuit 8 can directly control the pulse width of the control signal of the second switch S2, for example by feeding the output of the inverse value generator 9 directly into the mixer 6.

[0038] The compensation circuit 8 can also be realized in a manner other than by an inverse value generator 9 and mixer 10, for example by a signal processor that implements a control whose control objective is to keep constant the voltage U1 reached at the end of the charging phase a + b at the capacitor C1, independent of the voltage U V1 of the voltage source V1.

[0039] As explained above, at the end of each laser pulse emission, the parasitic total inductance L par in load circuit 2 causes a charge reversal phase of capacitor C1 to approximately the negative charging voltage U1. This voltage is applied to the laser diode LD in the reverse direction at the end of the charge reversal phase. If switch S1 is a semiconductor switch that can only block in one direction, namely the forward direction of the laser diode LD, and is permeable in the opposite direction, the reverse direction of the laser diode LD, the negative charging voltage of capacitor C1 is applied essentially entirely to the laser diode LD. Conventional laser diodes, which have a low reverse voltage strength, could be damaged as a result. The Fig. 10a und 10b The variants of protective circuits according to the invention in the load circuit 2 shown, which can be combined with all the aforementioned embodiments of the laser diode circuit, protect the laser diode LD from such an excessive reverse voltage at the end of the laser pulse emission phase and at the beginning of the charging phase a + b, as long as the voltage U1 on the capacitor C1 is still negative.

[0040] According to Fig. 10a For this purpose, a second diode D2 can be connected upstream or downstream of the laser diode LD in load circuit 2 in the same forward direction. A working diode with high blocking voltage strength and high reverse resistance is selected for the diode D2, so that the negative voltage U1 drops predominantly across the diode D2 and not across the laser diode LD.

[0041] Fig. 10bshows an alternative embodiment with a disconnect switch ST instead of the second diode D2, here a second semiconductor switch, which, when off, has a forward direction opposite to that of the first switch S1 (here embodied as a semiconductor switch). The disconnect switch ST is closed together with switch S1 to emit a laser pulse and opened precisely when the negative potential U1 occurring during the recharging of the capacitor C1 after the emission of a laser pulse could endanger the laser diode LD.

[0042] The invention is not limited to the embodiments shown, but includes all variants, modifications and combinations thereof that fall within the scope of the appended claims.

Claims

1. Laser diode circuit, comprising a charging circuit (1), which is connectable to a voltage source (V1) and has a capacitor (C1), and a load circuit (2), which is powered by the charging circuit (1) and has a laser diode (LD) in forward direction, wherein the load circuit (2) is connected in parallel with the capacitor (C1) and comprises a controllable first switch (S1) connected in series with the laser diode (LD), and wherein the charging circuit (1) comprises, in a serial connection, a coil (L1), an electronic element (D1, SD, SL) which allows current to pass to the capacitor (C1) and blocks current in the opposite direction, and the capacitor (C1), characterized in that the load circuit (2) has a second diode (D2), which is not a laser diode, connected in the same forward direction in series with the laser diode (LD), or has a disconnector (ST) connected in series upstream or downstream of the laser diode (LD), which is configured to be disconnected at the end of the recharging process when the laser diode (LD) would have a negative potential in order to protect the laser diode (LD).

2. Laser diode circuit according to claim 1, characterized in that the electronic element is a correspondingly controlled electronic switch (SD, SL).

3. Laser diode circuit according to claim 1, characterized in that the electronic element is a first diode (D1).

4. Laser diode circuit according to any one of claims 1 to 3, characterized in that the charging circuit (1) comprises a controllable second switch (S2) which is connected in parallel with the serial connection of the electronic element (D1, SD, SL) and the capacitor (C1).

5. Laser diode circuit according to claim 4, characterized by a pulse generator (3) which is configured to control the first switch (S1) with a first sequence (F1) of control pulses (Pi) at a clock rate (f) and the second switch (S2) with a second sequence (F2) of control pulses (Qj), that is ahead of the first sequence, at the same clock rate (f) or a multiple thereof, and at a respective pulse width (PWj) corresponding to the respective present amplitude (Aj) of a modulation signal (M) .

6. Laser diode circuit according to claim 5, characterized by a compensation circuit (8) configured to modify the modulation signal (M) inversely proportional to the voltage (UV1) of the voltage source (V1) .

7. Laser diode circuit according to any one of claims 1 to 6, characterized by a buffer capacitor (CP) connected in parallel with the voltage source (V1).

8. Laser diode circuit according to any one of claims 3 to 7, characterized in that the charging circuit (1) comprises a charging switch (SL) connected upstream of downstream of the coil (L1) or to the first diode (D1).