Method and apparatus for generating laser pulses

Abstract

The present invention relates to a method for generating laser pulses (3a, 3b) by varying the quality factor of a resonator (4), the method comprising generating the laser pulses (3a, 3b) by switching, by means of a control signal (S), a light modulator (10) between a first operating state (B1) of the light modulator (10) for generating a first quality factor of the resonator (4) and a second operating state (B2) of the light modulator (10) for generating a second quality factor of the resonator (4). In order to generate a sequence (2) of laser pulses (3a, 3b) in which a first laser pulse (3a) alternates with a second laser pulse (3b) different from the first laser pulse, the light modulator (10) is alternately and respectively differently controlled by the control signal (S) to generate a corresponding first laser pulse (3a) and a corresponding second laser pulse (3b). The present invention also relates to a corresponding device (1) for generating laser pulses (3a, 3b).

Description

Technical Field

[0001] The present invention relates to a method for generating laser pulses by changing the quality factor (Güte) of a (laser) resonator, the method comprising: generating laser pulses by switching, by means of a manipulation of an optical modulator, between a first operating state of the optical modulator for generating a first quality factor of the resonator and a second operating state of the optical modulator for generating a second quality factor of the resonator, the second quality factor being different from the first quality factor. The present invention also relates to a related device for generating laser pulses, the device comprising: a resonator, an optical modulator arranged in the resonator, and a control device configured to generate a control signal for switching the optical modulator between a first operating state for generating a first quality factor of the resonator and a second operating state for generating a second quality factor of the resonator, the second quality factor being different from the first quality factor. Background Art

[0002] Laser pulse sequences with very short pulse durations (such as those used in material processing) can be generated in a laser resonator, for example, by means of a quality factor switch (Q-switch) or cavity dumping. In the case of pulse generation by cavity dumping, here, the coupling output degree or loss of the resonator is modulated by means of a quality factor switch, specifically typically between a first operating state and a second operating state. In the first operating state, the resonator for establishing the laser pulse is closed or almost completely closed (i.e., typically, the coupling output degree or loss is 0%-20%), and in the second operating state, the laser pulse is coupled out of the resonator (the coupling output degree or loss is typically 30-100%). The loss of the resonator is a dimensionless quantity that is inversely proportional (reziprok proportional) to the quality factor of the resonator (also known as the Q-factor).

[0003] In the case of a conventional quality factor switch, the loss in the first operating state of the optical modulator is high (i.e., approximately 40%-100%), and the quality factor is low in order to establish gain in the laser medium. In the second operating state, the quality factor is high and the loss is low, i.e., typically approximately 0%-60%, in order to establish the laser pulse and couple it out of the laser resonator. Different from the case of cavity dumping, in the case of a conventional quality factor switch, therefore, the laser pulse is both established and coupled out in the second operating state.

[0004] This modulation of the coupling-out degree or quality factor of the resonator can be achieved, for example, by means of an acousto-optic modulator or a delay device, which is, for example, a combination of a delay plate for generating a fixed phase delay and an optical modulator (for example, an electro-optic modulator) for generating a variable phase delay, combined with a polarization-selective coupling-out device in the form of a polarizer, for example. In the case of a conventional quality factor switch, the polarization-selective coupling-out device can be dispensed with, if necessary, i.e., the coupling-out can be achieved, for example, by means of a partially transmissive (end) mirror.

[0005] Due to laser dynamics, fluctuations in the pulse energy and / or mode characteristics can occur during pulsed operation of a laser oscillator or a laser resonator (for example, in the case of a quality factor switch or in the case of cavity dumping). The oscillating mode characteristics, i.e., the (transverse) modes excited in a multimode resonator when establishing the corresponding laser pulse, are typically not pre-given or controlled, so that the beam characteristics and the energy can fluctuate in an uncontrolled manner from laser pulse to laser pulse. Due to the different pulse establishment times of the mode set, in addition to the energy fluctuations, temporal fluctuations or temporal jitter also occur.

[0006] US 5,365,532 describes a device and a method for stabilizing the output amplitude of a laser in the case of pulse generation by means of cavity dumping. Therein, by means of a detector, the intensity of the pulse establishment or rise of the laser radiation in the resonator is monitored, and the coupling-out time of the laser pulse is triggered when a threshold value of the intensity is reached. The temporal jitter occurring due to the triggered coupling-out time can be reduced by other measures.

[0007] US 4,044,316 describes a stabilized Nd:YAG laser with cavity dumping, in which relaxation oscillations are suppressed. Relaxation oscillations occur if the power during the power build-up in the resonator is higher than its steady-state value, which generates oscillations with a damping time of the order of several hundred milliseconds. To reduce the damping time, an optical crystal for frequency doubling or second harmonic generation (SHG) is arranged in the resonator. To reduce the damping time, it is sufficient if the optical crystal generates a second harmonic power of the order of approximately 0.1% of the fundamental frequency power.

[0008] Object of the present invention

[0009] The object on which the present invention is based is to provide methods and devices which can reduce the temporal fluctuations and energy fluctuations of laser pulses generated by a quality factor switch or cavity dumping. Summary of the Invention

[0010] Subject matter of the present invention

[0011] According to the present invention, this task is achieved by a method of the type mentioned at the beginning, in which, in order to generate a laser pulse sequence with alternating first laser pulses and second laser pulses different from the first laser pulses, the optical modulator is alternately and differently controlled by means of a control signal to generate the corresponding first laser pulses and the corresponding second laser pulses.

[0012] According to the present invention, instead of reducing the fluctuations of the individual laser pulses, by alternately controlling the optical modulator in a targeted manner, the laser resonator is brought into a strong bistable state, i.e., the laser resonator oscillates between two states with stable mode characteristics or stable pulse energies, respectively. The time fluctuations described above occur particularly in the frequency range in which the period duration corresponds to the fluorescence lifetime of the respectively excited laser level (in the case of Yb:YAG, typically in the range of a few kHz). In other frequency ranges, especially at very low frequencies < 100 Hz or at very high frequencies > 1 MHz, due to two different sets of oscillating modes, uncontrolled fluctuations usually do not occur, so it is generally not necessary to alternately control the optical modulator in a targeted manner in these frequency ranges. The typical (pulse) frequency when generating the corresponding (first and second) laser pulses is between approximately 200 Hz and approximately 1000 kHz, preferably between approximately 1 kHz and approximately 100 kHz.

[0013] The first laser pulses and the second laser pulses typically differ in different pulse energies, especially different (maximum) pulse amplitudes. By alternately controlling the optical modulator described here, a pulse sequence can be generated in which the corresponding first laser pulses and the corresponding second laser pulses have a time jitter of less than approximately 1 ns. The laser pulse sequence typically includes a certain number of, for example, more than 1000 laser pulses, and if necessary, more than approximately 100,000 laser pulses, depending on the application-specific operating duration, which can be 10 seconds or longer, for example, during laser processing of a workpiece. By the bistable operation of the resonator, even at high average power, it is possible to set how much energy is contained in the corresponding first laser pulse or the corresponding second laser pulse. In addition, the corresponding first laser pulse or the corresponding second laser pulse has high energy stability.

[0014] It goes without saying that the laser pulse sequence can additionally have third, fourth, ... laser pulses alternating with the first, second, ... laser pulses, where the first, second, third, fourth, ... laser pulses are each different from one another. In this case, likewise, the optical modulators for generating the respective first, second, third, fourth, ... laser pulses are alternately and differently controlled by control signals, and stable laser operation is carried out, where the state repeats every three, four, ... laser pulses.

[0015] In one variant, the method includes: generating a sequence of first laser pulses by suppressing, preferably by means of another optical modulator arranged outside the laser resonator, the second laser pulses. The difference between the first laser pulses and the second laser pulses is arbitrary, so the above wording is equivalent to the wording "generating a sequence of second laser pulses by suppressing the first laser pulses". Typically, the group or sequence of (first or second) laser pulses having a lower maximum pulse energy is suppressed. By suppressing the sequence or group of (first or second) laser pulses, the frequency of the sequence of the non-suppressed (second or first) laser pulses is halved. In order to generate such a laser pulse sequence with a desired output frequency, it is therefore necessary to control the optical modulator with a control signal whose frequency corresponds to twice the desired output frequency. The suppression or masking of the second laser pulses is preferably achieved by means of another (external) optical modulator, but can also be achieved in some other way if necessary. It goes without saying that the suppression of the second laser pulses is only optional, as this is only required if the suppressed laser pulses, which usually have lower energy or power, have an interfering effect in the respective application.

[0016] In another variant, the optical modulator is controlled by a control signal having a constant control frequency, wherein a first laser pulse and a second laser pulse and optionally a third laser pulse, a fourth laser pulse, etc. are respectively generated during the period duration of the control signal. The control signal typically has a signal profile that switches between two or more discrete signal levels, i.e., the signal profile typically does not have a continuous profile. In order to generate two laser pulses during the period duration, it is necessary to switch back and forth between a first operating state and a second operating state twice. For alternating control, the following durations can be selected to be different when the first laser pulse and the second laser pulse are generated: the duration during which the control signal remains at the corresponding signal level during the period duration. Alternatively or additionally, in order to alternately control the optical modulator, the corresponding signal levels for generating the first laser pulse and for generating the second laser pulse can also be selected to be different. The control frequency of the optical modulator is preferably between 200 Hz and 1000 kHz, especially between 1 kHz and 100 kHz. In order to generate more than two laser pulses during the period duration, it is also possible to switch between the first operating state and the second operating state more than twice. As described above, in this case, the corresponding quality factors of the signal levels or operating states can also vary.

[0017] In an extended variant, the dwell duration of the optical modulator in the first operating state when the first laser pulse is generated and the dwell duration of the optical modulator in the first operating state when the second laser pulse is generated (and optionally the dwell duration of the optical modulator in the first operating state when the third laser pulse, the fourth laser pulse, etc. are generated) are selected to be different. In this variant, the gain times available for establishing the corresponding first and second (optionally third, fourth,...) laser pulses in the laser resonator are selected to be different.

[0018] In this variant, especially when the first laser pulse and the second laser pulse are generated, the total dwell durations of the optical modulator in the first operating state and the second operating state can be selected to have the same length, i.e., the total dwell durations respectively correspond to half of the period duration of the control signal. In this case, the different dwell durations of the optical modulator in the first operating state when the first laser pulse / second laser pulse is generated inevitably result in different dwell durations of the optical modulator in the second operating state when the first laser pulse / second laser pulse is generated.

[0019] In another extension scheme, the total residence duration of the optical modulator in the first operating state and the second operating state during the generation of the first laser pulse is selected to be different from the total residence duration of the optical modulator in the first operating state and the second operating state during the generation of the second laser pulse. In this case, the cycle durations available for pulse establishment and the coupled output of the corresponding laser pulses are alternating. Similarly, bistability of the laser resonator can be achieved in this way.

[0020] In one extension scheme, the first quality factor during the generation of the first laser pulse and the first quality factor during the generation of the second laser pulse are selected to be different, and / or the second quality factor during the generation of the first laser pulse and the second quality factor during the generation of the second laser pulse are selected to be different. In this case, the losses of the optical modulator (or the quality factor proportional to the reciprocal of the loss) in the first operating state and / or the second operating state during the generation of the first laser pulse and the second laser pulse are selected to be different. For this purpose, the control signals for manipulating the optical modulator in the corresponding first operating state and second operating state to generate the first laser pulse and the corresponding second laser pulse have two different signal levels. Generally, the signal level for generating the (first or second) laser pulse with higher pulse energy is selected such that the coupling output degree or loss of the laser resonator is 0%, that is, the laser resonator has the minimum loss in the first operating state. The signal level of the control signal during the generation of the laser pulse with lower pulse energy can be defined according to the gain in the laser medium of the resonator. For example, in the case of a disk laser with low gain, an optical modulator loss of less than approximately 5% is sufficient to significantly reduce the pulse energy during cavity dumping, while in the case of a slab laser with high gain, a loss higher than approximately 50% may be required.

[0021] In another variant scheme, the first quality factor of the resonator is generated in the first operating state to establish a laser pulse in the resonator, and a lower second quality factor is generated in the second operating state to couple the laser pulse out of the resonator. In this variant scheme, the resonator operates in cavity dumping, that is, a high quality factor and thus low loss of the resonator are generated in the first operating state, enabling the establishment of a laser pulse or laser power that can be coupled out of the resonator in the second operating state.

[0022] In an extended scenario, when a pre-given power threshold of the laser power established in the resonator is reached, the optical modulator switches from a first operating state to a second operating state, where a first intensity threshold is selected during the generation of a first laser pulse and a second intensity threshold different from the first intensity threshold is selected during the generation of a second laser pulse. In this variant, the switch from the first operating state to the second operating state is triggered by reaching the threshold of the power of the laser pulse established in the laser resonator, as described in US 5,365,532 cited in the introduction, the entire content of which is incorporated herein by reference. The power of the laser pulse established in the laser resonator can be measured, for example, by means of a detector, such as a photodiode. For power measurement (or equivalently, for the measurement of the intensity of the laser radiation in the laser resonator here), a fixedly pre-given small fraction of the laser radiation power propagating in the laser resonator is typically coupled out of the laser resonator. Optical components that are present in the resonator anyway, such as a partially transmissive end mirror, can be used for the coupling out.

[0023] By different selections of the corresponding power or intensity thresholds for switching from the first operating state to the second operating state, the laser resonator can also operate in a bistable manner, because during the establishment of the first laser pulse and the corresponding second laser pulse, the selection of two different power thresholds results in two different gain durations. In this case, the optical modulator can also be controlled by means of a control signal with a constant control frequency (i.e., the period duration of the control signal is constant), only the corresponding moments of switching from the first operating state to the second operating state when not only the first laser pulse but also the second laser pulse is generated are not precisely pre-given and may fluctuate slightly respectively. It goes without saying that in principle there is also the possibility that when the first laser pulse is generated, the switch from the first operating state to the second operating state occurs when the power threshold is reached, and when the second laser pulse is generated, the moment of switching from the first operating state to the second operating state is fixedly pre-given, or vice versa. In this case, the intensity thresholds can be selected such that the relevant residence duration in the first operating state when the first laser pulse is generated is different from the residence duration in the first operating state when the second laser pulse is generated. Furthermore, if the moment of switching from the first operating state to the second operating state is fixedly pre-given when the second laser pulse is generated, the total residence duration in the first operating state and the second operating state when the first laser pulse is generated (where the switch from the first operating state to the second operating state is triggered by reaching the intensity threshold) can be different from the total residence duration in the first operating state and the second operating state when the second laser pulse is generated.

[0024] In an alternative variant, a first quality factor is generated in a first operating state for building up a gain in the laser-active medium of the resonator, and a second, higher quality factor is generated in a second operating state for reducing the gain in the laser-active medium and coupling out the laser pulses. In this variant, a conventional quality factor switch is implemented in the resonator, wherein in the first operating state the gain is built up in the laser-active medium until a maximum gain is reached in the laser-active medium. In the second operating state, the gain is reduced by coupling out the laser pulses from the resonator.

[0025] In another variant, part of the laser power propagating at the fundamental frequency is converted into laser power at twice the fundamental frequency by means of a frequency doubling device in the resonator. The frequency doubling device is usually an optical, typically birefringent crystal designed for second harmonic generation (SHG). The optical crystal can be, for example, lithium triborate (LiB 3 O 5 ), β-barium borate (BaB 2 O 4 ), sodium barium niobate (Ba 2 Na(NbO 3 ) 5 ) or some other suitable optical crystal. Second harmonic generation has been shown to be beneficial for improving energy stability.

[0026] Another aspect of the invention relates to an apparatus of the type mentioned in the introduction, wherein, in order to generate a laser pulse sequence having a first laser pulse alternating with a second laser pulse different from the first laser pulse, a control device is constructed or configured / programmed to control the light modulator alternately and differently by means of a control signal to generate a corresponding first laser pulse and a corresponding second laser pulse. For example, the control device can be a control computer or an electronic control circuit (IC, programmable gate array, etc.) that generates the desired control signal. The control signal (more precisely its signal distribution) is constructed differently for generating the first laser pulse and generating the second laser pulse, as described above in conjunction with the method. The control device can be constructed in particular for generating a control signal in the form of a control voltage, which is applied to an electrode of an electro-optical modulator, for example in the form of a Pockels cell.

[0027] In one embodiment, the device further includes another optical modulator for suppressing the second laser pulse, and the other optical modulator is arranged outside the laser resonator. The optical modulator can be configured to deflect, for example, the second laser pulse from the beam path of the first laser pulse, as in the case of an acousto-optic modulator. It goes without saying that for this purpose, the first laser pulse or the beam path of the first laser pulse can also be deflected by the optical modulator, while the second laser pulse passes through the optical modulator without deflection. If necessary, the second laser pulse can also be suppressed by a rapidly switchable filter, or by a combination of another electro-optic modulator and a polarizer for separating the first laser pulse and the second laser pulse into different beam paths. The other optical modulator is only required when the second laser pulse has an interfering effect in the corresponding application where a laser pulse is needed. If this is the case, the frequency of the laser pulse sequence generated by the device is halved. In this case, the optical modulator needs to be controlled by a control signal whose control frequency is twice the desired frequency of the laser pulse sequence.

[0028] Preferably, the control device is configured or programmed to control the optical modulator by means of a control signal having a constant control frequency, and the control signal is used to generate the first laser pulse and the second laser pulse during the period duration of the control signal. It is advantageous if the control frequency of the control signal is between approximately 1 kHz and approximately 1000 kHz, preferably between approximately 1 kHz and approximately 100 kHz.

[0029] In one embodiment, the control device is configured to generate a first quality factor of the resonator in a first operating state to establish a laser pulse in the resonator, and to generate a lower second quality factor in a second operating state to couple the laser pulse out of the resonator. As described above in connection with the method, in this case the resonator operates in cavity dumping.

[0030] In another embodiment, the device has a detector for detecting the power of the laser pulse established in the laser resonator in the first operating state of the optical modulator. As already described above, the detector can involve, for example, a photodiode, etc., which detects the power of the laser radiation coupled out of the laser resonator during the first operating state. The measured power can be used to appropriately select the coupling-out moment, i.e., the moment of switching from the first operating state to the second operating state (see below).

[0031] In another embodiment, the control device is configured to switch the optical modulator between a first operating state and a second operating state when a pre-given power threshold of the laser power established in the laser resonator is reached, and the control device is configured to pre-give a first power threshold for generating a first laser pulse and a second power threshold different from the first power threshold for generating a second laser pulse. In this embodiment, the value of the power currently present in the laser resonator (which value can be measured, for example, in the manner described above in connection with the method) is compared with different power thresholds during the generation of the first laser pulse and the second laser pulse. In this way, strong bistability of the laser operation can also be generated.

[0032] In another embodiment, the control device is configured to generate a first quality factor in the first operating state for establishing gain in the laser-active medium of the resonator and a second, higher quality factor in the second operating state for reducing the gain in the laser-active medium and for coupling out the laser pulse. As described above in connection with the method, in this case the resonator operates with a conventional quality factor switch.

[0033] In another embodiment, a frequency doubling device is arranged in the resonator, which frequency doubling device is used to convert a part of the laser radiation propagating in the resonator at the fundamental frequency into laser radiation at twice the fundamental frequency. The frequency doubling device can in particular relate to a non-linear crystal, for example a birefringent crystal. As is generally common in the case of frequency conversion, phase matching is also required for the frequency conversion in this case, and the phase matching may require appropriate temperature adjustment of the optical crystal.

[0034] In another embodiment, the resonator additionally includes: a laser-active medium, in particular a polarization-selective coupling-out device (for example a polarizer for coupling out the laser pulse from the resonator), and preferably a phase delay device for generating a fixed phase delay. The laser-active medium typically relates to a solid-state medium, for example in the form of a laser crystal, for example in the form of Yb:YAG, Nd:YAG, Nd:YVO 4 … In the form of. The laser-active (solid-state) medium can be configured in the form of a laser disk, a laser rod, a laser slab, etc. In order to excite the laser-active medium, the latter is typically pumped by means of pump radiation, and for this purpose the device can include a pump light source, for example a pump laser source.

[0035] The cavity dumping and quality factor switching can also be achieved without a phase delay device, for example if an acousto-optic modulator is used as the optical modulator. However, a delay device consisting of an optical modulator and, if necessary, an additional delay plate is usually used for cavity dumping. Here, the modulator generates a time-variable phase delay, while the delay plate generates a fixedly pre-given phase delay. The delay plate can relate to, for example, a λ / 4 delay plate (or a λ / 2 plate in the case of a ring laser), but other delays are also suitable. The delay device usually generates its maximum phase delay in the second operating state, which results in the laser radiation being maximally delayed when passing through the delay device twice, such that the laser pulse can be coupled out of the laser resonator at the polarization-selective coupling output device. In the case of a linear resonator with a λ / 4 delay plate, the polarization of the laser radiation can be rotated by 90° when passing through the delay twice, which corresponds to maximum coupling out. The polarization-selective coupling output device can relate to, for example, a thin-film polarizer that transmits laser radiation having a first polarization direction and reflects laser radiation having a second polarization direction perpendicular to the first polarization direction. Other types of polarizers can also be used as the polarization-selective coupling output device in the laser resonator, for example a polarizer consisting of a birefringent medium that enables beam displacement of the polarization components (s polarization and the corresponding p polarization) in the birefringent medium, and thus enables separation of the polarization components, etc. The delay device for generating a fixed phase delay prevents the resonator from shutting down in case of a fault (i.e., in case of a fault of the optical modulator), such that the laser pulse cannot be coupled out and further amplified until it damages components in the resonator. Here, the fixed phase delay of the delay device is selected such that in case of a fault (i.e., in case of a fault of the optical switch), the laser pulse is automatically coupled out.

[0036] A resonator with a polarization-selective coupling output device and, if necessary, a delay device with a fixed phase delay can also be operated with a conventional quality factor switch. In this case, the laser pulse can be coupled out of the resonator without polarization selection, for example by coupling out the laser pulse from the resonator at a coupling output device in the form of a partially transmissive coupling output mirror (such as a partially transmissive end mirror). In this case, the losses in the resonator are generated by the optical modulator and the polarization-selective element.

[0037] Other advantages of the invention become apparent from the description and the drawings. Likewise, the above-described features and the features presented further below can be used separately on their own or in any desired combination as a plurality. The embodiments shown and described should not be understood as an exhaustive list, but rather have exemplary features for summarizing the invention. Description of the Drawings

[0038] Shown in the figures:

[0039] Figure 1A schematic diagram of an embodiment of a device for generating a sequence of laser pulses by cavity dumping or quality factor switching in a laser resonator is shown. The device has alternately controlled optical modulators to generate a sequence of alternating first and second laser pulses.

[0040] Figure 2 A diagram similar to Figure 1 is shown, where the device additionally includes a frequency doubling device in the resonator and an external modulator for suppressing the second laser pulse.

[0041] Figures 3a to 3d Four diagrams showing the time distribution of control signals for bistable control of an optical modulator in the case of cavity dumping are shown.

[0042] Figure 4 A diagram showing the time distribution of control signals for bistable control of an optical modulator in the case of quality factor switching, and

[0043] Figure 5 A diagram of a device similar to Figure 1 is shown, which is used to generate a sequence of laser pulses in the case of quality factor switching of a laser resonator.

[0044] In the following description of the drawings, the same reference numerals are used for the same or functionally identical components. Detailed Description

[0045] Figure 1 An exemplary construction of a device 1 for generating a sequence 2 of laser pulses 3a, 3b is shown. The device includes a laser resonator 4. The laser resonator 4 includes two end mirrors 5a, 5b and a disk-shaped laser active medium 6 (in this example, a Yb:YAG crystal) applied to a heat sink 7. The laser active medium 6 is reflectively coated (verspiegelt) on its side facing the heat sink 7 and is optically excited by the pump radiation of a pump laser (not shown). Thereby, laser radiation 8 with a laser wavelength λ of 1030 nm is generated in the laser resonator 4.

[0046] The laser resonator 4 has a plurality of folding mirrors 9a to 9d in order to cause the laser radiation 8 to pass through the laser active solid-state medium 6 multiple times. The laser radiation 8 generated in the laser resonator 4 or the laser active solid-state medium 6 is linearly polarized, for example, s-polarized.

[0047] The laser resonator 4 further includes an optical modulator 10 in the form of an electro-optical modulator (more precisely, a Pockels cell (Pockelszelle)) and a control device 11 for controlling the electro-optical modulator 10 by means of a control signal S. A delay device 12 and a polarization-selective coupling output device 13 are also arranged in the laser resonator 4. The delay device is, for example, in the form of a λ / 4 retardation plate for generating a constant phase delay of λ / 4, and the polarization-selective coupling output device is in the form of a thin-film polarizer and serves as a partial mirror. The laser pulses 3a, 3b generated in the laser resonator 4 are coupled out at the polarization-selective coupling output device, as will be described in detail below.

[0048] The optical modulator 10 operates in principle in two operating states B1, B2 for cavity dumping. The first operating state B1 is used to establish the laser pulses 3a, 3b in the resonator 4, while in the second operating state B2, the corresponding laser pulses 3a, 3b are coupled out of the resonator 4.

[0049] In the first operating state B1, the control signal S (in the form of a voltage signal) can be applied to the electro-optical modulator 10 by means of the control device 11, and this signal generates a (positive) quarter-wave voltage, i.e., a voltage that causes a +λ / 4 phase delay of the laser radiation 8. The retardation plate 12 generates a phase delay in the opposite direction of -λ / 4, so that the sum of the phase delays of the retardation plate 12 and the electro-optical modulator 10 is zero in the first operating state B1. Therefore, the polarization state of the s-polarized laser radiation 8 generated in the laser resonator 4 remains unchanged, and this laser radiation irradiates the thin-film polarizer 13 in an s-polarized manner and is deflected there, i.e., the laser radiation 8 is not coupled out at the thin-film polarizer 13. The determination of the sign of the phase delay is based on convention, where a positive / negative voltage applied to the electro-optical modulator 10 causes a phase delay with a positive / negative sign.

[0050] In the second operating state B2, a phase delay of zero is generated at the electro-optical modulator 10, i.e., there is no voltage difference or a control signal S with a voltage of 0V exists at the modulator. In this case, the laser radiation 8 passes through the retardation plate 12 twice, generating a phase delay of 2x(-λ / 4) = -λ / 2. This phase delay causes the polarization direction (E vector) of the linearly polarized laser radiation 8 to rotate by 90°, so that this linearly polarized laser radiation irradiates the coupling output device in the form of a thin-film polarizer 13 in a p-polarized manner and is coupled out of the laser resonator 4 at this polarizer. In the case of a properly designed and controlled electro-optical modulator 10, the retardation plate 12 can also have a (random) fixed phase delay different from ±λ / 4.

[0051] Figure 1The laser resonator 4 shown in [Figure] operates in a bistable state, where a sequence 2 of alternating first and second laser pulses 3a, 3b is generated, and the laser pulses differ from each other in at least one characteristic. As Figure 1 shown, here the first laser pulse 3a has a greater maximum pulse power or energy than the second laser pulse 3b. It goes without saying that, alternatively, the first laser pulse 3a can have a lower energy or a lower maximum pulse power than the second laser pulse 3b. In order to generate alternating first laser pulses 3a and second laser pulses 3b with different characteristics, the electro-optic modulator 10 is alternately controlled by means of a control signal S, and there are various possibilities for alternately controlling the electro-optic modulator 10, where, for example, Figures 3a to 3d shows four possibilities by way of example. It goes without saying that, in order to generate a sequence of third, fourth,... laser pulses that each have different characteristics from other (first, second,...) laser pulses, the electro-optic modulator 10 can be controlled accordingly to achieve stable laser operation in three, four,... states.

[0052] In Figures 3a to 3d all four examples shown, the control signal S has a constant control frequency f, which can be, for example, on the order of a few kHz, for example, between 200 Hz and 1000 kHz, preferably between 1 kHz and 100 kHz. During the period duration T of the control signal S, the optical modulator 10 is respectively controlled such that the first laser pulse 3a and the second laser pulse 3b are generated. Generating the corresponding first laser pulse 3a or second laser pulse 3b requires respectively switching back and forth once between a first operating state B1 and a second operating state B2. In Figures 3a to 3d the signal level or quality factor Q of the control signal S is respectively plotted, more precisely, it is 1 / Q (proportional to the loss L), between a maximum loss L (corresponding to a minimum quality factor Q) represented by "one" and a minimum loss L (corresponding to a maximum quality factor Q) represented by "zero". In the case of the minimum quality factor Q (and maximum loss L), in the example shown, the optical modulator 10 generates the zero phase delay described above, while in the case of the maximum quality factor Q (and minimum loss L), the optical modulator 10 generates a phase delay of +λ / 4 (see above).

[0053] In Figure 3a the example shown, the alternating control is achieved by means of the control signal S, where the durations for generating the corresponding first laser pulse 3a and the corresponding second laser pulse 3b are of the same magnitude and correspond to half of the period duration T / 2 of the control signal S. However, in Figure 3a the example shown, the dwell duration t in the first operating state B1 during the generation of the first laser pulse 3a B1,1The dwell duration t in the first operating state B1 when the second laser pulse 3b is generated B1,2 is different. The dwell duration t in the first operating state B1 when the first laser pulse 3a is generated B1,1 is greater than the dwell duration t in the first operating state B1 when the second laser pulse 3b is generated B1,2 . In this way, a longer time period can be used for the gain or pulse build-up of the respective first laser pulse 3a, which results in the first laser pulse 3a having a higher maximum power compared to the second laser pulse 3b, as Figure 1 shown.

[0054] In Figure 3b the example shown, the alternating control is also achieved in such a way that the switching between the first operating state B1 and the second operating state B2 during the generation of the two laser pulses 3a, 3b is carried out at different times. However, in Figure 3b the example shown, the respective dwell durations t B1,1 and t B1,2 in the first operating state B1 for the first laser pulse 3a and the second laser pulse 3b have the same length. However, in Figure 3b the total dwell duration t tot,1 of the optical modulator 10 in the first operating state B1 and the second operating state B2 during the generation of the first laser pulse 3a is different from the total dwell duration t tot,2 of the optical modulator 10 in the first operating state B1 and the second operating state B2 during the generation of the second laser pulse 3b. The sum of these two dwell durations t tot,1 and the corresponding t tot,2 corresponds to the constant period duration T of the control signal S. Since the total dwell duration t tot,1 of the optical modulator 10 during the generation of the first laser pulse 3a is greater, the first laser pulse has a greater maximum pulse power than the second laser pulse 3b. Importantly here, since the dwell duration of the optical modulator 10 in the second operating state B2 is longer during the generation of the first laser pulse 3a, more energy or gain is introduced into the laser-active medium 6 compared to during the generation of the second laser pulse 3b. Therefore, during the subsequent dwell duration in the first operating state B1, more energy can be extracted for the first laser pulse than for the second laser pulse.

[0055] It goes without saying that Figure 3a and Figure 3b the possibilities of alternating control shown in B1,1 can also be combined, i.e., the dwell duration t tot,1can be different from the dwell duration t in the first operating state B1 when the second laser pulse 3b is generated B1,2 and the total dwell duration t in the first operating state B1 and the second operating state B2, respectively tot,2 is different

[0056] In Figure 3c the example shown, the different manipulations for generating the two laser pulses 3a, 3b are not achieved by different dwell durations in the two operating states B1, B2, but by the quality factor Q of the optical modulator 10 in the first operating state B1 when the first laser pulse 3a is generated 1 and the quality factor Q of the optical modulator 10 in the first operating state B1' when the second laser pulse 3b is generated 1 'being different. The corresponding quality factors Q 1 、Q 1 'are dimensionless values and are inversely proportional to the losses L 1 、L 1 '. For the first operating state B1 when the first laser pulse 3a is generated, as in Figure 3a 、 Figure 3b the cases of the two examples, the loss L 1 of the laser resonator 4 is actually equal to zero and the quality factor Q 1 is at a maximum. In contrast, for the first operating state B1' in which the optical modulator 10 operates when the second laser pulse 3b is generated, L 1 ' = 0.2 applies, and depending on the type of the laser active medium 6, other values of the loss L 1 'are possible, for example, this other value can be between 0.01 and 0.5. When the second laser pulse 3b is generated in the first operating state B1', the optical modulator 10 is manipulated by the control signal S, and the signal level of this control signal causes a non-zero phase delay of the optical modulator 10. This results in the polarization direction (E vector) of the linearly polarized laser radiation 8 being rotated and having a component that is coupled out of the laser resonator 4 during the first operating state B1'. In this way, the second laser pulse 3b can extract and build up less energy, and as a result, its maximum pulse power is lower than in the case of the first laser pulse 3a

[0057] Finally Figure 3d a possibility of alternately manipulating the optical modulator 10 is shown, in which once the power P of the laser radiation 8 established in the laser resonator 4 is higher than a pre-given power threshold P S,1 、P S,2, the optical modulator 10 switches from the first operating state B1 to the second operating state B2, and the pre-given power threshold is selected to be different for generating the two laser pulses 3a, 3b with different sizes. In the illustrated example, the first power threshold P of the first laser pulse 3a is selected to be greater than the second power threshold P of the second laser pulse 3b. S,1 Accordingly, when generating the first laser pulse 3a, the optical modulator 10 switches from the first operating state B1 to the second operating state B2 at a later moment, that is, the residence duration t in the first operating state B1 when generating the first laser pulse 3a S,2 is greater than the residence duration t in the first operating state B1 when generating the second laser pulse 3b. B1,1 B1,2

[0058] The exact residence durations t in the first operating state B1 B1,1 、t B1,2 are determined by reaching the corresponding power thresholds P S,1 、P S,2 . When generating the sequence 2 of laser pulses 3a, 3b, in the case of successive first laser pulses 3a and corresponding second laser pulses 3b, the power thresholds fluctuate slightly respectively. Nevertheless, in this case, the control signal S also has a constant control frequency f, because the switching from the second operating state B2 to the first operating state B1 is carried out at a fixed pre-given moment within the corresponding cycle duration T respectively. Accordingly, Figure 3d the illustrated control signal S and Figure 3a the illustrated control signal S only differ in that the moment of switching from the first operating state B1 to the second operating state B2 is not fixed pre-given, but is triggered by reaching the corresponding power thresholds P S,1 、P S,2 .

[0059] In order to determine the (instantaneous) power P of the laser radiation 8 in the laser resonator 4 in the first operating state B1, Figure 1 the device 1 shown in has a detector 14 constructed in the form of a photodiode. The detector 14 is arranged outside the laser resonator 4. In order to couple out a small part of the laser radiation 8 propagating in the laser resonator 4 for detection, the second end mirror 5b of the laser resonator 4 is constructed for partial transmission, that is, for the laser radiation 8 propagating in the laser resonator 4, it has a transmittance of approximately 0.01% or less. If the optical modulator 10 is alternately controlled in the manner described in combination with Figures 3a to 3c , the detector 14 can be omitted if necessary.

[0060] Figure 2 shows the device 1 for generating the laser pulse sequence 2, and this device is the same as Figure 1 ​​The difference of the illustrated device 1 mainly lies in that it includes another optical modulator 15 arranged outside the laser resonator 4, for example in the form of an acousto-optic modulator. The other optical modulator 15 is used to couple out or suppress the second laser pulse 3b from the sequence 2 of the first laser pulse 3a and the second laser pulse 3b coupled out of the laser resonator 4. The acousto-optic modulator 15 deflects the second laser pulse 3b to an absorber (not shown diagrammatically). For the deflection, a phase diffraction grating is generated by the acousto-optic modulator 15 in the optical crystal by means of an ultrasonic generator having a preset switching frequency f / 2 corresponding to half of the control frequency f of the control signal S.

[0061] Alternatively, the external optical modulator 15 can be another electro-optic modulator for generating a phase shift or a phase delay, for example in the form of a Pockels cell. In both cases, the external optical modulator 15 can be controlled by the control device 11 at half of the control frequency f of the control signal, i.e., f / 2, so as to eliminate the second laser pulse 3b from the sequence 2 of the laser pulses 3a, 3b, such that only the first laser pulse 3a leaves the device 1. It goes without saying that, if necessary, a dedicated control device, for example in the form of an electronic control circuit, can be provided for the other optical modulator 15. In this case, the control of the optical modulator 10 and the other optical modulator 15 needs to be synchronized appropriately. For this purpose, for example, a common frequency generator can be provided in the device 1.

[0062] The laser radiation 8 generated in the laser resonator 4 has a fundamental frequency f proportional to the reciprocal of the laser wavelength λ. G . In order to additionally suppress time jitter, especially energy fluctuations, during the generation of the sequence of the laser pulses 3a, 3b, a frequency doubling device 16 in the form of a frequency doubling crystal (SHG crystal) is arranged in the Figure 2 laser resonator 4. In the SHG crystal 16, a small part (usually less than 10% or less than 1%) of the laser radiation 8 generated in the laser resonator 4 is converted into laser radiation 17 having twice the fundamental frequency 2f. G At one of the deflection mirrors 9a configured for wavelength-selective optical elements, the converted laser radiation 17 is transmitted and coupled out of the laser resonator 4.

[0063] Figure 4 The time distribution of the control signal S is shown when the laser resonator 4 is not operated in cavity dumping as shown, but in a conventional quality factor switching operation. In this case, in the first operating state B1, a gain V is established in the laser active medium 6 of the resonator 4 as follows: the resonator 4 has a high loss L Figures 3a to 3d almost equal to 1.0, or a quality factor Q close to zero. 1 =1.0 1 or 1Operation. Once the gain V in the laser active medium 6 assumes its maximum value (at a fixedly pre-given time), the optical modulator 10 switches from the first operating state B1 to the second operating state B2. In the second operating state B2, the optical modulator 10 generates a (second) quality factor Q that is greater than the first quality factor Q in the first operating state B1 1 of the (second) quality factor Q 2 (the losses L 2 、L 2 ‘being close to zero), in order to reduce the gain V in the laser active medium 6 and in order to couple out the laser pulses 3a, 3b from the laser resonator 4.

[0064] In Figure 4 the example shown, in the second operating state B2, the second quality factor Q when generating the first laser pulse 3a 2 and the second quality factor Q when generating the second laser pulse 3b 2 ‘are selected to be different, where the second quality factor Q when generating the first laser pulse 3a 2 is greater than the second quality factor Q when generating the second laser pulse 3b 2 ‘(and correspondingly the loss L 2 is less than the loss L 2 ‘). Accordingly, the corresponding first laser pulse 3a has a greater pulse energy than the corresponding second laser pulse 3b. It goes without saying that as an alternative or addition to the manipulation of the optical modulator 10 as shown in Figure 4 the first laser pulse 3a and the second laser pulse 3b, which are different from each other in at least one characteristic, can also be generated in a manner similar to that described above in connection with Figure 3a 、 3b the cavity dumping.

[0065] Figure 5 Shows an example of a device 1 for generating the laser pulses 3a, 3b, where the laser resonator 4 is also operated with a conventional quality factor switch. Figure 5 The device 1 shown differs from the Figure 1 device 1 shown mainly only in that the first end mirror 5a of the resonator 4 is constructed as a partially transmissive mirror (for example with a transmittance of 10%) and serves as a coupling-out device, while the thin film polarizer 13‘ does not serve as a coupling-out device, i.e., is not constructed to be partially transmissive. In this case, the retardation plate 12 can be omitted. In this case, the losses in the resonator 4 are generated by the acou-optical modulator 10. In Figure 5 the case of the device 1 shown, the manipulation of the acou-optical modulator 10 by means of the control signal S can also be carried out in the Figure 4 manner described in Figure 5 . Instead of the acou-optical modulator, an electro-optical modulator can also be used in the

[0066] In summary, by means of the alternating manipulation of the optical modulator 10, the laser resonator 4 can operate in a robust bistable state. In this way, a sequence 2 of laser pulses 3a, 3b can be generated by means of the device 1, for which sequence of laser pulses a very low temporal jitter as well as a high energy stability of the corresponding laser pulses 3a, 3b can be achieved.

Claims

1. A method for generating laser pulses (3a, 3b) by changing the quality factor (G) in a resonator (4), comprising: The laser pulses (3a, 3b) are generated by switching the optical modulator (10) between a first operating state (B1) and a second operating state (B2) of the optical modulator (10) by means of a control signal (S), the first operating state being used to generate a first quality factor (G 1 ) in the resonator (4), and the second operating state being used to generate a second quality factor (G 2 ) different from the first quality factor in the resonator (4). wherein, in order to generate a sequence (2) of laser pulses (3a, 3b), wherein a first laser pulse (3a) and a second laser pulse (3b) different from the first laser pulse alternate, the optical modulator (10) is alternately and differently controlled by means of the control signal (S) to generate a corresponding first laser pulse (3a) and a corresponding second laser pulse (3b), so as to bring the resonator (4) into a strong bistable state, wherein the resonator oscillates between two states each having a stable mode characteristic or a stable pulse energy, characterized in that the method further comprises: generating a sequence (2) of first laser pulses (3a) by suppressing the second laser pulse (3b).

2. The method according to claim 1, further comprising: generating a sequence (2) of first laser pulses (3a) by suppressing the second laser pulse (3b) by means of a filter or another optical modulator (15) arranged outside the resonator (4).

3. The method according to claim 1 or 2, wherein, the optical modulator (10) is controlled by means of a control signal (S) having a constant control frequency (f), wherein a first laser pulse (3a) and a second laser pulse (3b) are respectively generated during the period duration (T) of the control signal (S).

4. The method according to claim 1 or 2, wherein, The dwell duration (t B1,1 ) of the optical modulator (10) in the first operating state (B1) when the first laser pulse (3a) is generated is selected to be different from the dwell duration (t B1,2 ) of the optical modulator (10) in the first operating state (B1) when the second laser pulse (3b) is generated.

5. The method according to claim 1 or 2, wherein, The total residence duration (t tot,1 ) of the optical modulator (10) in the first operating state and the second operating state (B1, B2) when the first laser pulse (3a) is generated is selected to be different from the total residence duration (t tot,2 ) of the optical modulator (10) in the first operating state and the second operating state (B1, B2) when the second laser pulse (3b) is generated.

6. The method according to claim 1 or 2, wherein, The first quality factor (Q 1 , Q2) at the generation of the first laser pulse (3a) and the first quality factor (Q 1 ‘) at the generation of the second laser pulse (3b) are selected to be different, and / or, wherein the second quality factor (Q 2 ) at the generation of the first laser pulse (3b) and the second quality factor (Q 2 ‘) at the generation of the second laser pulse (3b) are selected to be different.

7. The method according to claim 1 or 2, wherein, The first quality factor (Q 1 ) of the resonator (4) is generated in the first operating state (B1) in order to establish laser pulses (3a, 3b) in the resonator (4), and wherein a lower second quality factor (Q 2 ) is generated in the second operating state (B2) in order to couple out the laser pulses (3a, 3b) from the resonator (4).

8. The method according to claim 7, wherein, When a preset power threshold (P S,1 , P S,2 ) of the laser power (P) established in the resonator (4) is reached, the optical modulator (10) is switched from the first operating state (B1) to the second operating state (B2), wherein a first intensity threshold (P S,1 ) is selected when the first laser pulse (3a) is generated, and a second intensity threshold (P S,2 ) different from the first intensity threshold is selected when the second laser pulse (3b) is generated.

9. The method according to claim 1 or 2, wherein, A first quality factor (Q 1 ) is generated in the first operating state (B1) in order to establish a gain (V) in the laser-active medium (6) of the resonator (4), and a higher second quality factor (Q 2 ) is generated in the second operating state (B2) in order to reduce the gain (V) in the laser-active medium (6) and to couple out laser pulses (3a, 3b).

10. The method according to claim 1 or 2, wherein, A part of the laser radiation (8) propagating in the resonator (4) at the fundamental frequency (f G ) is converted into laser radiation (17) having twice the fundamental frequency (2f G ) by means of a frequency doubling device (16).

11. An apparatus (1) for generating laser pulses (3a, 3b), comprising: a resonator (4), an optical modulator (10) arranged in the resonator (4), Control device (11), which is configured to generate a control signal (S) in order to cause the optical modulator (10) to operate in a first operating state (B1) for generating a first quality factor (G 1 ) of the resonator (4) and a second operating state (B2) for generating a second quality factor (G 2 ) of the resonator (4), the second quality factor being different from the first quality factor, wherein, the control device (11) is configured to, in order to generate a sequence (2) of laser pulses (3a, 3b), wherein a first laser pulse (3a) and a second laser pulse (3b) different from the first laser pulse alternate, alternately and differently control the optical modulator (10) by means of the control signal (S) to generate a corresponding first laser pulse (3a) and a corresponding second laser pulse (3b), so as to bring the resonator (4) into a strong bistable state, wherein the resonator oscillates between two states each having a stable mode characteristic or a stable pulse energy, characterized in that there is provided another optical modulator (15) or a filter for suppressing the second laser pulse (3b), and the other optical modulator is arranged outside the resonator (4).

12. The apparatus according to claim 11, wherein, The control device (11) is configured to control the optical modulator (10) by means of a control signal (S) having a constant control frequency (f) so as to generate a first laser pulse (3a) and a second laser pulse (3b) during the period duration (T) of the control signal (S).

13. The device according to claim 11 or 12, wherein, The control device (11) is configured to generate a first quality factor (Q 1 ) of the resonator (4) in the first operating state (B1) in order to establish laser pulses (3a, 3b) in the resonator (4), and to generate a lower second quality factor (Q 2 ) in the second operating state (B2) in order to couple out the laser pulses (3a, 3b) from the resonator (4).

14. The device according to claim 11 or 12, further comprising: a detector (14) for detecting the laser power (P) established in the resonator (4) in a first operating state (B1) of the optical modulator (10).

15. The device according to claim 14, wherein, The control device (11) is configured to switch the optical modulator (4) between the first operating state (B1) and the second operating state (B2) when a pre-given power threshold (P S,1 , P S,2 ) of the laser power (P) established in the resonator (4) is reached, and wherein the control device (11) is configured to pre-give a first power threshold (P S,1 ) for generating a first laser pulse (3a), and to pre-give a second power threshold (P S,2 ) different from the first power threshold for generating a second laser pulse (3b).

16. The device according to claim 11 or 12, wherein, The control device (11) is configured to: generate a first quality factor (Q 1 ) in the first operating state (B1) in order to establish a gain (V) in the laser-active medium (6) of the resonator (4), and generate a higher second quality factor (Q 2 ) in the second operating state (B2) in order to reduce the gain (V) in the laser-active medium (6) and couple out laser pulses (3a, 3b).

17. The device according to claim 11 or 12, further comprising: Frequency doubling device (16), which is arranged in the resonator (4) and is used to convert a part of the laser radiation (8) propagating in the resonator (4) at the fundamental frequency (f G ) into laser radiation (17) at twice the fundamental frequency (2f G ).

18. The device according to claim 11 or 12, wherein, the resonator (4) further comprises: a laser-active medium (6), a coupling output device (13, 5a), the polarization-selective coupling output device being configured to couple out the laser pulses (3a, 3b) from the resonator (4).

19. The device according to claim 18, wherein, the coupling output device (13, 5a) is polarization-selective.

20. The device according to claim 18, wherein, the resonator (4) further comprises a phase delay device (12).

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