FR2481530A1 - METHOD AND DEVICE FOR PRODUCING ELECTRIC PULSES FOR PUMPING A LASER - Google Patents

Abstract

A pulse generator, particularly for pumping a gas laser, comprises a transformer, the primary coil of which is fed with a current via a control switch in such a manner that, when the switch is open, a voltage pulse occurs across the secondary winding of the transformer and is supplied to a capacitor via a diode. A further switch, for example a thyratron or a spark switch, discharges the capacitor and generates the output pulse. To control the energy transferred from the transformer (which can be an autotransformer) to the capacitor, the current supplied to the primary winding of the transformer is monitored and the control switch is opened when this current reaches a predetermined value. The power for driving the other switch is obtained by discharging a capacitor which obtains the stored energy from the primary winding of the transformer via a further diode.

Description

 The present invention relates to pulse generators and, more particularly but not exclusively, pulse generators for pumping a laser.

We know how to pump a laser by charging a capacitor via a resistor, then discharging this capacitor in the laser via a switch such as a thyristor If the resistance has a value too low, the thyristor may not shut off properly after discharge, and if the resistance value is too high, the required charge rate and corresponding pumping rate may not be obtained. English Patent No. 1,425,987 proposes, for a semiconductor laser, an intermediate energy storage in a transformer - a current and therefore the storage of a flux are established in the transformer the current is then cut and the resulting energy pulse is transmitted to a capacitor which in turn discharges into the laser.

According to one aspect of the present invention, there is provided a method for producing an electrical pulse comprising passing a current from a voltage source through an inductor, detecting the amplitude of the current, and when this amplitude has risen to a predetermined value, breaking the current so that a pulse having a predetermined energy is induced by the inductor.

 The pulses produced by the present method can be stored in a capacitor which is periodically discharged so as to control a device such as a laser.

According to a second aspect of the present invention, there is provided a method of pumping a laser comprising periodically cutting off a current flowing from a voltage source in the primary winding of a step-up autotransformer by means of a switch connected in the circuit of the primary winding and the passage of the high voltage pulses thus formed in a capacitor connected to the circuit of the secondary winding of the autotransformer, the method further comprising the periodic discharge of the capacitor in the laser.

According to a third aspect of the present invention, there is provided a device for producing pulses comprising a boosting autotransformer, a first switch means for periodically cutting the current flowing in the primary winding of the autotransformer, a capacitor connected to the circuit. of the secondary winding of the autotransformer and a second switch means for discharging the capacitor so that there is production of said pulses.

 According to a fourth aspect of the present invention, there is provided a device for producing pulses comprising an inductor, a first switch means connected to the inductor and a current detection means for detecting the amplitude of the current flowing in the inductor and to cause the current to be cut off by the first switch means when its amplitude is equal to a predetermined value, the device further comprising a capacitor connected to the inductor and a second switch means for discharging the capacitor in order to forming said pulses.

The present invention will be understood from the following description given in conjunction with the accompanying drawings in which
FIG. 1 is a simplified diagram of a first embodiment of a device for producing power pulses connected so as to control a gas laser;
Figure 2 is a series of diagrams showing waveforms produced at various points in the device of Figure 1; and
FIG. 3 is a simplified diagram of the circuit of a second embodiment of a device for producing pulses.

In connection with FIG. 1, a gas laser 1 requires for its operation a series of pulses, each pulse having a peak voltage of, for example 10 K volts. These pulses are produced by the device from a direct current source, the nominal voltage of which can be, for example, 24 volts. However, the actual supply voltage may be different from this
nominal value. One side of the source 2 is connected by means of a line 3 to a control pulse input terminal 4 of the laser 1, while the other side is connected to another input terminal 6 of the laser by through the total winding of an autotransformer 7 comprising an intermediate socket 5, a diode 8 and a capacitor 9. The socket 5 of the autotransformer 7 is connected by the collector / emitter path of a switching transistor 10 and a resistor 11 on line 3. The primary winding of the transformer, that is to say the part between the socket 5 and the source 2, has a number of turns in the ratio 1 / N, N being equal for example to 10, with the number of turns of the rest of the winding. The connection point between the emitter of transistor 10 and resistor 11 is connected to an input of a comparator amplifier 12, the other input of which is arranged so as to receive a reference voltage V ref via a resistor 13. The output of the amplifier 12 is connected to this other input via a reaction capacitor 14 and to the base of the transistor 10 by a resistor 15, the transistor being arranged so as to also receive a repetitive pulse signal VA from a pulse generator (not shown). A gas discharge switch 16 is connected between line 3 and the connection point between diode 8 and capacitor 9, the control electrode of this switch being arranged so as to receive a control pulse signal repetitive closing VB from another pulse generator circuit (not shown). A small inductor 17 is connected between the control terminals of the laser 1.

 Now in connection with FIGS. 1 and 2, the transistor 10 is made periodically conductive by the pulses VA and, when it is made conductive, a current i of increasing value flows from the source 2 in the primary winding of the transformer. 7, transistor island 10 and the resistancell. If we ignore the losses, the current i increases to a peak value I = ET / L, where E represents the voltage of the source 2, L the inductance of l primary winding of the transformer, and T the time during which the transistor 10 is conductive. It will thus be noted that I can be controlled by varying E or T, and, in a variant (not shown) of the illustrated circuit, the transistor 10 could be made conductive for fixed periods of time where, insofar as the voltage E would remain substantially constant, the peak current I reached during successive conduction periods would also be substantially constant. However, as already indicated, the circuit shown aims to be used with a source whose voltage can vary significantly. Consequently, in this case, the time T is varied so as to keep I constant. This is obtained by using the comparator 12 to detect the voltage appearing across the resistor 11, a voltage which is obviously proportional to i. When this voltage is equal to the predetermined constant reference voltage V ref, the output of the comparator 12 goes d '' a level to another level, change which has the effect of making the transistor 10 nonconductive through 17 through the resistor 15 and at the same time through the reaction capacitor 14 to produce a hysteresis effect which maintains the output of the comparator 12 to its new level despite the cessation of the voltage across the resistor 11 due to the fact that the transistor is made non-conductive.

When the transistor 10 is made nonconductive, the 0.5LI2 energy stored in the core of the transformer causes the circulation of a reduced current, originally
I / N, in the capacitor 9 via the diode 8. The transformer and the capacitor constitute a resonant circuit and when the current flowing in the capacitor 9 has fallen to 0, when the voltage of the capacitor 9 will have reached its peak value, the diode 8 is cut off, which leaves the capacitor 9 charged The voltage Vc then appearing across the capacitor 9 can be calculated by equalizing the energy stored by the capacitor 9, c 'is to say 0.5 CVc2, and 1 energy previously stored by the inductor, cgest i.e. 0.5L12 (since this energy has now been completely dissipated in the capacitor) and, to be exact, by adding the source voltage 2 This gives

que cette variation peut être négligée.We will now see why the value of
peak of current I is regulated by capacitor 12 as
this has been described previously - this is due to the fact that the
voltage Vc applied to capacitor 9 depends thereon. In fact, the voltage Vc will still vary slightly with the value of E, but E itself is so small compared to

that this variation can be overlooked.

de sorte que, si Vc est voisin de 10 K volts et N à 10, VT sera égal à environ 1 K volt, valeur se trouvant dans les caractéristiques des transistors disponibles.As a result of the elevation effect of the transformer 7, the peak voltage VT appearing across the terminals of the transistor 10 is substantially lower than the voltage applied to the capacitor. More, this voltage is equal to

so that, if Vc is close to 10 K volts and N to 10, VT will be equal to approximately 1 K volt, value found in the characteristics of the available transistors.

 The pulses are applied to the laser 1 by applying the control pulses VB to the control electrode of the gas discharge switch 16 which is consequently made conductive, which allows the capacitor 9 to be discharged in the laser. To this end, having been made conductive, it is not maintained in this state by the current coming from the source 2, each pulse VB is synchronized with the start of a pulse VA applied to the transistor 10 to make it conductive. When the transistor 10 is made conductive, the voltage VL becomes negative (- E x N volts). Consequently, the diode 8 is cut and the switch 16 remains conductive only during the period when the capacitor 9 discharges into the laser 1. It is not necessary for the capacitor 9 to be discharged each time the transistor 10 is made conductive. In fact, it is advantageous that, if this is not the case, since the energy stored in the capacitor is established during several switching cycles of transistor 10 and this for a given power to be released in the laser at each discharge of the capacitor 9, the peak current coming from the source 2, the dimensions of the various connection wires, the constraints concerning the internal resistance of the source, the radiated electromagnetic interference and the dimensions of the transformer core are all correspondingly reduced. On the other hand, the repetition rate of the discharge pulses is naturally reduced compared to its theoretical value which can be disadvantageous for certain applications.

 The inductor 17 forms a direct current path for the charging current of the capacitor. This is only necessary if the laser itself is such that it does not provide this route.

The gas discharge switch 16 could be replaced by any other suitable switch, for example by a high voltage semiconductor device.

 It will be noted that the use of the autotransformer 7, unlike that of a transformer having separate windings, results in appreciable savings in weight and volume.

The autotransformer 7 could be replaced by a single coil (not shown) and a separate transformer (not shown), with its primary winding connected in parallel with the coil. So, as the coil has no secondary winding, its core can be smaller than that of the transformer 7. Furthermore, the separate transformer, in parallel with the coil, does not need to store energy, but to simply transfer it, and therefore, this member can be relatively small with a high winding resistance and a low power core. The resulting combination can have an overall volume lower than that of the transformer 7.

 The embodiment of Figure 1 can be modified as shown in Figure 3, where the parts identical to those of Figure 1 have the same reference numbers. This variant is identical to the embodiment of Figure 1 except for the replacement of the autotransformer 7 by an ordinary transformer 30 having primary and secondary windings 31 and 32 separated. The primary winding 31 is connected between the collector of the transistor 10 and the source 2 as in the previous case. In addition, one side of the secondary winding 32 is connected to the diode 8 as previously, but its other side is connected to a part 33 of a ground connection line comprising the part 33 and another part 34 which is connected to part 33 via a resistor 35. This line replaces line 3 in Figure 1, and its function is to reduce the risk of damage and interference with electronic components connected to the low voltage side of the transformer 30 by transient phenomena and the like produced on the high voltage side. Consequently, the low voltage electronic parts are grounded by the part 34, while the laser 1, the secondary 32 of the transformer etc. are grounded by part 33. The function of the resistor 35 is to maintain part 33 at the ground voltage of the circuit while rejecting the transient phenomena and the like mentioned above. It will naturally be noted that the two parts could be isolated in another way, for example by bringing the parts to different respective points of earthing.

 As previously indicated, the gas discharge switch or "thyratron" in Figure 1 could be replaced by another type of switch and such a modification is shown in Figure 3 where the thyratron is replaced by a spark gap switch 36 comprising a discharge initiation electrode 37 and two main discharge electrodes 38 and 39, main electrodes which are connected to the part 33 of the ground line and to the point of interconnection between the diode 8 and the capacitor 9, respectively.

Whether a thyratron or a spark gap is used to discharge the capacitor 9, it should be noted that the energy required to control them can be quite large. For example, each time the capacitor 9 must be discharged, the switch may have to be ordered with a pulse of, say, 60 milli-Joules, for example 10
KV for a spark gap or 400 V for a thyratron. In the case of a thyratron, the pulse may require a duration of at least 10 seconds. Thus, the same problem can be raised for the control of the switch as in the case of the control of the laser itself - it is to say the need for a very powerful diet; some means for storing energy and then releasing it is desirable, but if such means includes a simple combination of capacitor / resistor / switch, it may be impossible to obtain both a sufficient charge rate of the condensa effective cut-off of the switch following discharge. The embodiment of FIG. 3 incorporates a modification of FIG. 1, which, if carried out in an appropriate manner, can make it possible to obtain the energy storage conditions mentioned above with regard to the switch, and can improve the overall efficiency of the circuit, i.e. the switch is controlled by the energy stored in the primary winding of the transformer, in particular by the energy associated with the leakage reactance of the winding primary, energy which is present both in the embodiment of FIG. 1 and in that of FIG. 3 following the normal operation of the circuit. In FIG. 1, this energy is simply wasted, but in FIG. 3, a connection is made between the transistor side of the primary winding 31 and one side of a capacitor 40 via a diode 41.

The other side of the capacitor 40 is connected to the side of the winding 31 which is connected to the source 2. When the transistor 10 is made non-conductive so as to transfer an energy pulse from the secondary winding 32 of the transformer 30 at the capacitor 9, a pulse appears at the terminals of the primary winding 31, which is partly due to the energy associated with the leakage reactance of the primary winding and partly to the reflection, in the primary, of the secondary pulse. This primary pulse charges the capacitor 40 via the diode 41. The capacitor 40 can then be discharged via the primary of a small pulse transformer A2 and of a thyristor 43 so to produce, from the secondary of the transformer 42, a control or ignition pulse of the spark gap switch 36. The ratio of the number of turns of the transformer 42 will depend on the nature of the switch to be controlled, it could for example be 1/50 to increase the pulse voltage for the spark gap switch shown. For a thyratron like the one in Figure 1, the ratio could be 1/1.

In the embodiment of Figure 1, if for some reason the capacitor 9 does not discharge or only partially discharges in the laser, for example, if the switch 16 does not always work correctly or if the laser 1 operates so as to leave a certain charge in the capacitor, the energy pulses subsequently coming from the transformer can cause the voltage of the capacitor to be established at a level where damage can occur. To avoid this, a voltage control device could be connected to the capacitor and arranged so as to stop the operation of the circuit if a certain predetermined threshold voltage was exceeded. However, such a throne device could tend to charge the capacitor and therefore reduce the efficiency of the circuit. Figure 3 shows a means which could also be incorporated in Figure 1, whereby the control of the voltage of the capacitor would be performed on the primary side of the transformer - this is possible given that, although the description of the previous circuit was made for reasons of clarity in relatively simple terms, and may have caused other suggestions, each illustrated circuit is dynamic in that the signals appearing at the output of transformers 7 and 30 are such that they are returned to the primary side. Consequently, the control means of FIG. 3 comprises a voltage divider 45 which is connected to the primary winding 31. The central socket of this divider is connected to an input of a comparator amplifier 46, the other input of which is connected to an appropriate reference voltage source. The output of amplifier 46 is connected to the control input of a thyristor 47, the terminals of the main current path of which are connected between line 34 and the collector of the transistor 10. In addition, a fuse 48 is connected in series with the source 2, and a diode 49 is connected in parallel with the source and the fuse so that, normally, it is polarized inversely. The amplifier 46 compares the reflection primary side of the voltage transformer at the capacitor 9 at the reference voltage which is chosen in accordance with the maximum voltage that one wishes to appear at the capacitor. If this maximum is exceeded, the amplifier 46 makes the thyristor 47 conductive so that the fuse 48 blows. The resulting energy peaks coming from the transformer 30 can circulate without danger in the thyristor 47 and the diode 49.

In FIG. 1, the value of the inductor 17 must be chosen with care so as to allow a sufficient charge rate of the capacitor 9 while not causing too great a bypass of the discharge current around the laser 1. In the figure 3, this need for compromise is eliminated by replacing the inductor 17 by a high voltage diode 61 which is arranged so as to allow the passage of the charging current but not the discharge current. In addition, in order to avoid floating at a value that may be too negative of the potential at the anode of diode 61 and at the side of the capacitor connected to the diode, a resistor 62 is connected in parallel with the diode. However, this resistance can have a very high value and therefore a negligible effect on the discharge current passing through the laser. It may not be necessary in all cases.

 The fuse -48 is not necessary if the source 2 comprises a supply device suitably limited in current so that, when the voltage of the capacitor 9 becomes excessive and the thyristor 47 is conductive, this supply device switches off. simply until it is reset.

où L est l'inductance du secondaire du transformateur et C la capacité du condensateur 9. Cependant, le temps est de préférence inférieur à 10 millisecondes, ou mieux encore à 5 millisecondes, de façon à éviter une décharge par effet corona. Les circuits représentés peuvent avoir à fournir des impulsions à des tensions supérieures à la tension de 10 KV citée précédemment, peut être même à des tensions de 30 à 40 KV ou plus et à ces valeurs élevées le problème de l'effet corona est important.Cependant, de telles décharges prennent du temps à s'établir et c'est un avantage du circuit représenté en figure 3 lorsqu'il fonctionne de la manière décrite, c'est-à-dire avec une décharge automatique du condensateur 9 peu après chaque transfert d'énergie à celui-ci que des hautes tensions ne sont présentes à l'intérieur du circuit que pendant des laps de temps courts et que par conséquent, avec un circuit convenable, l'effet corona peut être évité.In FIG. 3, the signal V8 for the discharge of the capacitor 9 and therefore, the pumping of the laser is obtained automatically. Thus, the supply of a pumping pulse is initialized simply by the application of a VA pulse. The automatic discharge is carried out by a time device 60 connected between the output of the amplifier 12 and the signal line Vg. At a predetermined time t after the initialization by the comparator 12 of the non-conduction of the transistor 10, the device 60, for example a simple monostable circuit, gives at its output a pulse making the thyristor 43 conductive and, consequently, the switch The time t could be chosen so as to allow sufficient energy transfer between the transformer 30 and the capacitor 9, that is to say it could preferably be at least

where L is the secondary inductance of the transformer and C is the capacitance of the capacitor 9. However, the time is preferably less than 10 milliseconds, or better still 5 milliseconds, so as to avoid a corona discharge. The circuits shown may have to supply pulses at voltages higher than the 10 KV voltage mentioned above, may even be at voltages from 30 to 40 KV or more and at these high values the problem of the corona effect is important. However, such discharges take time to establish and it is an advantage of the circuit shown in Figure 3 when it operates in the manner described, that is to say with an automatic discharge of the capacitor 9 shortly after each transfer of energy thereto that high voltages are present inside the circuit only for short periods of time and that consequently, with a suitable circuit, the corona effect can be avoided.

The time device 60 could receive a control input, not from the amplifier 12, but directly from the line receiving the pump initiation signal VA, the time interval t being suitably adjusted. This is not entirely satisfactory in the arrangement shown since t must be set to give the longest possible time for the required energy level to be established in the transformer - therefore, the pumping rate maximum achievable may not be very high.

The fact that in FIGS. 1 and 3 the amplitude of the current i is detected and that the transistor 10 is made non-conductive when it has reached a predetermined value and therefore when the energy stored in the transformer 7 or 30 has itself reaches a predetermined value, is not only important to take account of variations in the source voltage, but also to make irrelevant non-linearities in the characteristics of the transformers and to obtain optimum efficiency by enabling the transformer to operate closer to its saturation point. If transistor 10 was simply made non-conductive at a predetermined time according to the moment when it is made conductive, the value i reached by the current could be higher or lower than that causing saturation If i exceeds the level of saturation, it will increase suddenly without increasing the energy of the nucleus and therefore the energy will be wasted If it does not reach the saturation level, the circuit will be ineffective in the sense that the nucleus will be larger than necessary (in certain circumstances, this is a very important factor. important)
The present invention is not limited to the exemplary embodiments which have just been described, it is on the contrary liable to modifications and variants which will appear to those skilled in the art.

Claims (6)

 1 - Method for producing an electrical pulse, characterized in that it comprises the passage of a current from a voltage source in an inductor, the detection of the amplitude of the current and, when the amplitude has risen to a predetermined value, switching off the current so that a pulse having a predetermined energy is induced by the inductor. 2 - Method of pumping a laser, characterized in that it comprises the periodic interruption of a current flowing from a voltage source in the primary winding of a step-up autotransformer by means of a connected switch in the circuit of the primary winding and the transfer of the high voltage pulses thus formed to a capacitor connected to the circuit of the secondary winding of the autotransformer, the method further comprising periodically discharging the capacitor in the laser.  3 - Device for producing pulses, characterized in that it comprises a step-up autotransformer, a first switching means for periodically cutting the current flowing in the primary winding of the autotransformer, a capacitor connected to the winding circuit secondary of the autotransformer, and a second switching means for discharging the capacitor in order to produce the pulses.  4 - Pulse production device, characterized in that it comprises an inductor, a first switching means connected to the inductor and a current detection means for detecting the amplitude of the current flowing in the inductor and for causing the first current switching means to cut off when its amplitude is equal to a predetermined value, the device further comprising a capacitor connected to the inductor and a second switching means for discharging the capacitor in order to form the pulses.  5 - Device for producing pulses for pumping a laser, characterized in that here comprises - a transformer, a current supply means connected by means of a switch to the primary winding of the transformer to supply it with electric current, the switch means being able to operate to cut this current and cause the appearance of first and second respective voltage pulses across the primary and secondary windings of the transformer;  - first and second capacitors respectively connected to the primary and secondary windings via unidirectional current circulation devices so as to receive and store the first and second voltage pulses; and  - first and second semiconductor switches each comprising a control terminal, the first semiconductor switch being connected to the first capacitor and capable of operating so as to discharge this first capacitor in order to form a control pulse making the second conductive a semiconductor switch, and the second semiconductor switch being connected to discharge the second capacitor. 6 - Device for producing pulses for pumping a laser, characterized in that it comprises: a transformer, a current supply means connected by means of a switch means to the winding- primary transformer to provide it with an electric current, the switch means being able to operate so as to cut off the current and cause the appearance of a voltage pulse across the secondary winding of the transformer, a capacitor connected to the secondary winding through a unidirectional current flow device to receive and store the pulse, and switch means for discharging the capacitor, the device further comprising voltage control means connected to the primary and working winding, via the transformer to control the voltage of the capacitor.