Abstract:
The present disclosure relates to a high voltage switch which may comprise a chain of MOS field-effect transistors (MOSFETs). The current of the individual MOSFETS, and hence the chain, can be controlled by means of adding a current measuring resistance into the source path of the transistors and transmitting the voltage arising there via a capacitor to a gate connector of the transistors.

Description:
RELATED APPLICATION 
       [0001]    This application claims priority to German Patent Application No.: 10 2009 006 154.1, filed on Jan. 26, 2009, and PCT Application No.: PCT/DE2010/075009, filed on Jan. 26, 2010, respectively entitled “HIGH VOLTAGE SWITCH WITH ADJUSTABLE CURRENT”, at least some of which may be incorporated herein. 
     
    
     BACKGROUND 
       [0002]    For switching laser pulses, optical switches and/or modulators can be used and may be arranged either inside or outside a laser resonator in a beam path. Essentially, optical modulators may be differentiated between electro-optic modulators and acousto-optic modulators. A significant factor in the use of optical modulators is the achievable switching time. In acousto-optic modulators, the achievable switching time is generally defined by the sound velocity and the laser beam diameter. In some cases switching flanks of acousto-optic modulators are too slow to effectively suppress pre-pulses and/or post-pulses, for example, at a short distance from the main pulse. In comparison therewith, electro-optic modulators have been recently developed to achieve higher clock speeds. Thus, electro-optic modulators may be able to replace acousto-optic modulators in specific laser applications, as they are characterised by substantially shorter, electronically induced switching flanks. 
         [0003]    Electro-optic modulators are generally designed so that they have a Pockels cell as an actual optical switching element with modifiable optical properties and a polarization-selective element, such as, for example, a reflecting analyser with unmodifiable optical properties. A Pockels cell generally has a birefringent crystal which is directed in an appropriate manner towards an incident monochromatic and polarized light beam and to which an electrical voltage is applied in the order of several 100 V up to several kilovolts. In combination with the polarization-dependent optical element, the Pockels cell is able to:
       (a) switch on or switch off the light depending on the electrical voltage applied thereto, and/or   (b) to deflect the light on two different paths through an optical system.       
 
         [0006]    The Pockels cell may be switched to-and-fro by a suitable switchable high voltage supply between two states in which the laser beam emerging from the Pockels cell (e.g., with polarization directions located perpendicular to one another) is polarized in a linear manner. The voltage that is required in order to achieve the two aforementioned states is a function of the crystal parameters and the wavelength of the light to be switched. Applications of Pockels cells exist in which the Pockels cells have to be switched on and off rapidly such that one or both of the transition times have to be in the range of a few nanoseconds. Where merely one of these transition times has to be short (e.g., either the switching-on or switching-off process), the other transition time may be in the range of microseconds. 
         [0007]    Such an electro-optic modulator constructed from a Pockels cell and a suitable switchable high voltage supply may be used, for example, in order to switch optically short laser pulses having a duration of a few nanoseconds (ns) or ultra-short laser pulses having a duration of picoseconds (ps) and/or femtoseconds (fs) (e.g., to alter the intensity or the beam direction of the laser pulses). Such ultra-short laser pulses may be generated by a mode-locking method known to those skilled in the art, for example. Thus laser beam sources for ultra-short pulses, in principle, have relatively high repetition rates (e.g., typically 40-200 MHz for solid state lasers) and low pulse energy (e.g., typically 0.1-50 nJ). If individual pulses or pulse groups of ps-laser pulses and/or fs-laser pulses are required, frequently a Pockels cell is used in order to select said pulses. In this case, initially the voltage has to be fully switched on between two pulses (e.g., which the laser beam source typically transmits at a time interval of 5-25 ns) in order to be fully switched off again after allowing a single laser pulse through 5-25 ns later. 
         [0008]    In many applications, therefore, it is a case of achieving more rapid switching times. However, there are also situations in which it is advantageous if at least one of the two switching times is slower than might be possible by the switching element itself. A Pockels cell, for example, may be used inside a laser resonator in a manner known per se as a cavity dumper. In such an embodiment, it may be desirable to make the pulse, which is decoupled from the resonator by the cavity dumper, longer and to reduce the peak intensity. This may be achieved by extending the switching times of the control circuit of the Pockels cell. Accordingly, high voltage circuits may be required in which at least one of the switching flanks is made slower than the switching elements would normally permit. 
       SUMMARY 
       [0009]    It is an object of the present disclosure to provide a switching unit which makes it possible to delay at least one switching flank. 
         [0010]    Switching units or high voltage switches of the type described in this application are generally constructed as a transistor chain, arranged in series relative to their load paths. In particular, the switching units are typically provided by a chain made up of metal-oxide-semiconductor field-effect transistors (MOSFETs), which are arranged in series relative to their source/drain paths. 
         [0011]    The currents of the MOSFET switches and the switching flanks thereof are determined by internal impedances such as circuit inductors and/or transistor impedances of the design of the circuit, for example. In such MOSFET switches, in principle the switching flanks may be decelerated by one or more resistors, R_load, being introduced into the current path to the load, such as the Pockels cell. A drawback with such an arrangement, as shown in  FIG. 8 , however, would be the relatively high amount of power generated on the resistor (e.g., which would have to be cooled where possible and would require a relatively large amount of space). 
         [0012]    The present disclosure therefore recognizes that it may be advantageous to control the current via the transistors themselves. To this end, a well-defined impedance may be introduced into the load path, on which a well-defined voltage drops, according to the current that flows therethrough. This voltage may be transmitted by means of a capacitor to the control electrode gate of the transistor. 
         [0013]    The disclosure, therefore, relates to a switching unit comprising a plurality of transistors arranged in series relative to their load paths, at least one transistor with its load path being connected to a current measurement impedance, the current measurement impedance being connected to a capacitor, and the capacitor being connected to a control electrode of the transistor. 
         [0014]    Otherwise, the transistor may be connected to a charging/discharge circuit in a manner known to those skilled in the art. The capacitor may be charged and/or discharged by the arrangement at the same time with the gate source capacity of the MOSFET through the charging/discharge circuit. In this manner, power directly accumulates on the transistor where it may be easily discharged. Moreover, the current may not merely be controlled by the measurement impedance but also by the supplied voltage and/or charge from the charging/discharge circuit. 
         [0015]    In one embodiment, a plurality of the transistors of the switching unit (e.g., preferably all transistors) are connected with their load path to a current measurement impedance, which in turn is connected to a capacitor. The capacitor may be connected to a control electrode of the transistor. 
         [0016]    In one embodiment, the transistors are provided by field-effect transistors (e.g., such as MOS transistors), and the current measurement impedance is connected to the source of the transistor. 
         [0017]    In one embodiment, the current measurement impedance has a real resistor. 
         [0018]    In one embodiment, the control electrode of the transistor is connected to a charging/discharge circuit. 
         [0019]    In one embodiment, a diode is arranged in parallel with the current measurement impedance. 
         [0020]    The switching unit according to the present disclosure may be used in different ways for voltage switches, in particular high voltage switches. 
         [0021]    A relatively simple arrangement comprises a so-called push-pull circuit comprising a circuit node, a first switching unit as provided herein with a first terminal connected to a first potential and with a second terminal connected to the circuit node, and a second switching unit as provided herein connected by a first terminal to the circuit node and by a second terminal to a second potential. 
         [0022]    In such an embodiment, the first potential may be provided by a pole of a voltage source and the second potential may be provided by earth, for example. Similarly, the first potential may be provided by earth and the second potential may be provided by the other pole of the voltage source, for example. 
         [0023]    Such a push-pull circuit may be designed as a Pockels cell control circuit, with a first terminal of a Pockels cell being connected to the circuit node and a second terminal of the Pockels cell being connected to a fixed potential. 
         [0024]    A slightly more complex arrangement comprises a bridge circuit comprising a first circuit node and a second circuit node. The first circuit node may be connected by a first line to a first potential and by a second line, via a first switching unit as described herein, to a second potential. The second circuit node may be connected by a third line to the first potential and by a fourth line, via a second switching unit as described herein, to the second potential. At least one further switching unit as described herein may be connected to the first potential via one of the two circuit nodes. 
         [0025]    Such a bridge circuit may be designed as a Pockels cell control circuit, with a first terminal of a Pockels cell being connected to the first circuit node and a second terminal of the Pockels cell being connected to the second circuit node. 
         [0026]    A Pockels cell controlled as described above may thus be used as a cavity dumper inside a resonator of a laser beam source. By extending at least one of the switching flanks of the electrical switching pulses supplied to the Pockels cell, the laser pulse decoupled from the Pockels cell may be chronologically or timely extended and its peak intensity may be reduced, for example. 
         [0027]    Those of ordinary skill in the art will appreciate still other aspects of the present application upon reading and understanding the appended description. 
     
    
     
       FIGURES 
         [0028]    Exemplary embodiments are described in more detail hereinafter with reference to the drawings, in which: 
           [0029]      FIG. 1  illustrates an example schematic view of a switching unit; 
           [0030]      FIG. 2  illustrates an example schematic view of a switching unit; 
           [0031]      FIG. 3  illustrates an example schematic view of a switching unit; 
           [0032]      FIG. 4  illustrates an example schematic view of an embodiment of a push-pull circuit with switching units as set forth herein; 
           [0033]      FIG. 5  illustrates an example schematic view of an embodiment of a bridge circuit comprising a switching unit as set forth herein; 
           [0034]      FIG. 6  illustrates an example schematic view of an embodiment of a bridge circuit comprising a switching unit as set forth herein; 
           [0035]      FIG. 7  illustrates an example schematic view of an embodiment of a bridge circuit comprising a switching unit as set forth herein; 
           [0036]      FIG. 8  illustrates an example schematic view of a switching unit; 
           [0037]      FIG. 9  illustrates an example schematic view of an embodiment of a laser beam source with a Pockels cell inside a resonator as a cavity dumper. 
       
    
    
     DESCRIPTION 
       [0038]    As used herein, “connected,” “electrically connected,” and/or “coupled” together when used to describe circuit elements can mean that the aforementioned elements are coupled directly together (e.g., without further elements located therebetween). However, unless otherwise stated, this can also mean that the aforementioned elements are not directly coupled together and that further elements may be coupled between the aforementioned elements. 
         [0039]    Insofar as the same reference numerals are used in the figures, they relate to elements which are the same or substantially the same or functionally equivalent so that in these cases the description of these elements is not repeated. 
         [0040]      FIG. 1  illustrates a schematic diagram of one embodiment of a switching unit  10 . The switching unit  10  substantially comprises a transistor chain made up of N MOS field effect transistors (MOSFETs) (e.g., referred to in the figures as merely T N ), which are arranged in series relative to their source/drain paths, where N is a positive integer. The MOSFETs are switched on and/or off by known charging/discharge circuits CL N , which are connected to a gate terminal and a source terminal of the MOSFETs. Such an arrangement can be configured as a high voltage switch. High voltage switches can switch voltages up to several kilovolts or several tens of kilovolts. The switching times of such high voltage circuits may be in the range of a few nanoseconds and may, for example, be controlled by TTL signals with a signal level of a few volts. The internal design of these high voltage switches, in particular the charging/discharge circuits CL N , is, for example, disclosed in DE 3630775 C2, and thus does not need to be explained in more detail in the present case. 
         [0041]    According to one embodiment of the instant disclosure, the source terminal of respective MOSFETs is connected to an impedance X N . The impedance X N  is connected to the gate terminal of the MOSFET via a capacitor C N . The impedance X N  has the function of a current measurement impedance. The voltage drop on the impedance X N  may be transmitted via the capacitor C N  to the gate terminal of the MOSFET. The capacitor C N  is real in terms of circuit technology, whilst respective MOSFETs additionally have a merely virtual or parasitic (illustrated in dotted lines) gate source capacitor C GS . The real capacitor C N  and the parasitic capacitor C GS  are also charged and/or discharged by the charging/discharge circuit CL N . 
         [0042]    The voltage drop on the impedance X N  and the voltage representing the instantaneous value of the current flowing through the MOSFET is transmitted to the gate terminal of the MOSFET, and thus, in turn, controls the current through the MOSFET and the total current of the transistor chain. The current may, moreover, be controlled by the voltage and/or charge supplied by the charging/discharge circuit CL N . A switching flank delivered by the charging/discharge circuit CL N  to the MOSFET may be decelerated by the arrangement. Power may be directly applied to the MOSFETs and the impedances X N , from where it may be relatively easily discharged. 
         [0043]    The impedance X N  may, in one embodiment, be provided by an individual real resistor R N . The impedance X N  may, however, also be provided by a combination of a resistor and a capacitor. Moreover, a combination of one or more resistors and/or capacitors and/or inductors and/or non-linear components is conceivable. It may also be provided that a combination of these components may have a controllable or adjustable impedance or resistance value. In this manner, the proportion of the voltage fed back to the gate may be set in a variable manner. 
         [0044]      FIG. 2  illustrates an example schematic of another embodiment of a switching unit  20 , with the current measurement impedance having non-linear properties. This embodiment of a switching unit  20  is similar to the switching unit  10  illustrated in  FIG. 1 , except that the impedance X N  shown in the embodiment of  FIG. 1  is shown as a real resistor R N  in  FIG. 2 . Additionally, in respective MOSFETs of the switching unit  20  with a diode D N , a non-linear resistor is provided which is arranged in parallel with the impedance X N . The diode D N  may be arranged according to desired properties of current control in the non-conducting direction or conducting direction and may act as a current limiter for the current flowing in the parallel branch through the resistor R N . 
         [0045]      FIG. 3  illustrates an example schematic of another embodiment of a switching unit  30 . It will be appreciated that this embodiment of a switching unit  30  is similar to the switching unit  20  illustrated in  FIG. 2 , except that diodes D 1  to D N  have been omitted. Additionally, in the switching unit  30 , circuits XC N  and/or XG N  are inserted between adjacent MOSFETs of the transistor chain and/or in the gate lines of the transistors. Such circuits XC N  and/or XG N  may make it possible to effect further alterations to the temporal characteristics when switching the current, for example. In one embodiment, the circuits XC N  and/or XG N  may be a resistor or a resistor with a capacitor arranged in parallel and/or an inductor arranged in parallel. In order to effect non-linear properties, diodes or other non-linear components may also be used in these switching units. 
         [0046]      FIG. 4  illustrates an example schematic of one embodiment of a push-pull circuit with one or more of switching units  50 ,  60 , such as the switching units  10 ,  20  and/or  30  described with respect to  FIGS. 1-3 . The push-pull circuit  100  has a first switching unit  50  and a second switching unit  60 . The switching units  50  and  60  are arranged in series between a first potential terminal and a second potential terminal, the first potential terminal being able to be provided by a high voltage terminal (HV) and the second potential terminal by an earth terminal (e.g., ground). 
         [0047]    The two switching units  50  and  60  are, for the sake of clarity, shown in block form. However, when shown schematically, they may resemble the schematics of one or more of the switching units  10 ,  20 , and/or  30  shown in  FIGS. 1-3  and described above. 
         [0048]    The first switching unit  50  is connected by its first terminal to the high voltage terminal and by its second terminal to a circuit node  55 . The second switching unit  60  is connected by its first terminal to the circuit node  55  and by its second terminal to the earth terminal. The circuit node  55  is, in turn, connected to a first terminal of a load  70 , such as a Pockels cell. The second terminal of the load  70  is connected to the earth terminal. 
         [0049]    The push-pull circuit  100  is operated so that a switching unit is closed, whilst the respective other switching unit is open. If the switching unit  60  is closed and the switching unit  50  is open, no voltage is applied to the load  70 . In the reverse case, all or substantially all of the high voltage is applied to the load  70 . Both switching units  50  and  60  may be provided by switching units as described herein with respect to  FIGS. 1-3  or merely one of the switching units  50  and  60  may be provided by switching units as described herein with respect to  FIGS. 1-3 . This may depend on whether both switching flanks of the voltage pulse applied to the load  70  are intended to be delayed or merely one thereof, for example. If, for example, merely a rising flank of the voltage pulse is intended to be delayed, the switching unit  50  may be designed as set forth herein. If merely a falling flank of the voltage pulse is intended to be delayed, the switching unit  60  may be designed as set forth herein. If both switching flanks are intended to be delayed, both switching units  50  and  60  may be designed as set forth herein. A switching unit not designed as set forth herein may have a transistor chain as shown in  FIG. 1  but with the impedance X N  and the capacitor C N  being omitted, for example. 
         [0050]    It is also conceivable to design both switching units  50  and  60  as switching units as set forth in  FIGS. 1-3 , but to take care that delays provided by the switching units are modifiable, namely by variable impedances X N  and/or variable capacitors C N  or variable voltages of the charging units CL N . 
         [0051]    A further embodiment of a control circuit is provided by a bridge circuit which in terms of circuit technology is more complex than the push-pull circuit, but the drawbacks thereof when controlling a Pockels cell in terms of dynamic circuits may be able to be avoided, for example. 
         [0052]      FIG. 5  illustrates a schematic of an embodiment of a bridge circuit with switching units as set forth in the instant disclosure (e.g., such as with respect to  FIGS. 1-3 ). The bridge circuit  200  comprises a so-called H-configuration. This H-configuration has two circuit nodes  205  and  215  which are to be attached to the electrical terminals of a load  270  such as a Pockels cell. The switching nodes  205  and  215  are the starting point of four lines which correspond to the four limbs of the letter H. The first circuit node  205  is connected by a first line via a first recharging resistor  230  to a first potential terminal such as a (high) voltage terminal HV and by a second line via a first switching unit  210  to a second potential terminal such as an earth terminal, whilst the second switching node  215  is connected by a third line via a second recharging resistor  240  to the first potential terminal and by a fourth line via a second switching unit  220  to the earth terminal. 
         [0053]    The two switching units  210  and  220  are, for the sake of clarity, shown in block form and may be provided by switching units  10 ,  20 , and/or  30 , as shown in  FIGS. 1-3 , which have been described above. In this regard, the same applies as in the above-described push-pull circuit. The two switching units  210  and  220  are controlled in order to provide a voltage pulse to the load  270 . If merely one switching flank of the voltage pulse is intended to be delayed, merely one of the switching units  210  and  220  may be designed as a switching unit as set forth herein. If both switching flanks are intended to be delayed, both switching units  210  and  220  may be designed as switching units as set forth herein. 
         [0054]    Further embodiments of bridge circuits may be formed by at least one further switching unit as set forth herein being added, which is arranged in parallel with one or both of the recharging resistors  230 ,  240 , and/or replaces one or both recharging resistors  230 ,  240 . 
         [0055]    Accordingly, in  FIG. 6  a further embodiment of a bridge circuit with switching units as set forth herein is shown schematically. The bridge circuit  300  substantially corresponds to the bridge circuit  200  of  FIG. 5 , but moreover has a further switching unit  250  which is arranged between the two circuit nodes  215  and the high voltage terminal and thus is arranged in parallel with the second recharging resistor  240 . The further switching unit  250  is shown in block form for the sake of clarity. It may be provided by a switching unit such as one of the switching units  10 ,  20 , and/or  30  described with respect to  FIGS. 1-3 . 
         [0056]    In  FIG. 7  a further embodiment of a bridge circuit with switching units as set forth herein shown schematically. The bridge circuit  400  substantially corresponds to the bridge circuit  200  of  FIG. 5 , but additionally has two further switching units  280  and  290 , which replace the recharging resistors  230  and  240  of the bridge circuit of  FIG. 5 . The two further switching units  280  and  290  are, for the sake of clarity, shown in block form and may be provided by switching units as set forth herein, such as one or more of the switching units  10 ,  20 , and/or  30  described with respect to  FIGS. 1-3 . 
         [0057]    In  FIG. 9 , an embodiment of a laser beam source with a cavity dumper is shown schematically. The laser beam source  500  has a cw-resonator which is arranged between the (highly) reflective mirrors HR 1  and HR 2 . A gain medium  510  is located inside the resonator. A Pockels cell  520  is arranged inside the resonator in relative proximity to the first (highly) reflective end mirror HR 1 . A lambda/4 plate  540  is arranged in front of the Pockels cell  520 . A polarization-dependent mirror P reflects, for example, s-polarized light completely, whilst it is completely permeable to p-polarized light. The Pockels cell  520  is connected to a control circuit  530  as set forth here, such as a push-pull circuit or a bridge circuit as described above. 
         [0058]    Between two output pulses, no voltage may be applied to the Pockels cell  520 , which is thus optically inactive, so that laser radiation coming from the gain medium  510  and which is, for example, s-polarized, is reflected on the mirror P and after passing twice through the Pockels cell  520 , lambda/4 plate  540  and reflection on the HR 1  is now p-polarized and decoupled from the mirror P. Thus no gain occurs when no voltage is applied to the Pockels cell  520 . For generating the pulse, a voltage is applied to the Pockels cell  520 , such that said Pockels cell when passed through twice, effects a rotation of the polarization by 90°, which together with the polarization rotation by the lambda/4 plate  540  adds up to 0° or 180°. In this state, the resonator is closed and the gain occurs on the gain medium  510 . If the voltage is slowly removed from the Pockels cell  520   e,  the Pockels cell  520  together may act with the mirror P as a time-dependent decoupling. The control circuit  530  as set forth herein and connected to the Pockels cell  520  has the effect that at least one of the two switching flanks and/or the voltage pulse is extended, so that the laser pulse decoupled from the resonator obtains a desired chronological length and a correspondingly reduced peak intensity. 
         [0059]    It should be noted that the features and elements described in the exemplary embodiments are also interchangeable between the exemplary embodiments as far as this is possible and expedient. Further exemplary embodiments may come within the scope of the invention, as defined by the following claims.