EP3954005A1

EP3954005A1 - Method and device for generating laser pulses

Info

Publication number: EP3954005A1
Application number: EP20710870.5A
Authority: EP
Inventors: Rainer Flaig; Oliver Rapp; Christian Stolzenburg; Montasser Bouzid
Original assignee: Trumpf Laser GmbH
Current assignee: Trumpf Laser Se
Priority date: 2019-04-12
Filing date: 2020-03-06
Publication date: 2022-02-16
Also published as: DE102019205285A1; CN113678329A; US20220029374A1; WO2020207676A1; KR102541235B1; KR20210142185A

Abstract

The invention relates to a method for generating laser pulses (3a, 3b) by changing the quality of a resonator (4), having the step of: generating laser pulses (3a, 3b) by actuating an optical modulator (10) using a control signal (S) in order to switch over between a first operating state (B1) of the optical modulator (10) for generating a first quality of the resonator (4) and a second operating state (B2) of the optical modulator (10) for generating a second quality of the resonator (4). In order to generate a sequence (2) of laser pulses (3a, 3b) in which first laser pulses (3a) alternate with second laser pulses (3b) differing from the first laser pulses, the optical modulator (10) is actuated in a respectively alternating manner by the control signal (S) in order to generate a respective first laser pulse (3a) and a respective second laser pulse (3b). The invention also relates to a corresponding device (1) for generating laser pulses (3a, 3b).

Description

Method and device for generating laser pulses

The present invention relates to a method for generating laser pulses by changing the quality of a (laser) resonator, comprising: generating the laser pulses by driving an optical modulator to switch between a first operating state of the optical modulator for generating a first quality of the resonator and a second Operating state of the optical modulator for generating a second quality of the resonator that differs from the first. The invention also relates to an associated device for generating

Laser pulses comprising: a resonator, an optical modulator arranged in the resonator, and a control device which is designed to generate a control signal to switch 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 to switch from the first different quality of the resonator. Sequences of laser pulses with very short pulse durations, such as those used in material processing, can be used in a laser resonator

for example with the help of a Q-switch or cavity dumping. In the case of pulse generation with cavity dumping, the degree of decoupling or the loss of the resonator is modulated by means of a Q-switch, typically between a first operating state in which the resonator is closed or almost completely closed to build up a laser pulse (i.e. typically degree of decoupling or . Loss 0% -20%) and a second

Operating state in which the laser pulse is decoupled from the resonator (decoupling degree or loss typically 30-100%). The loss of the resonator is a dimensionless quantity that is inversely proportional to the quality of the resonator (also known as the Q factor).

In the classical Q-switching, the loss in the first operating state of the optical modulator is high, i.e. about 40% -100% and the goodness low to one

To build up gain in the laser medium. In the second operating state the quality is high and the loss is low, i.e. typically approx. 0% -60% to build up a laser pulse and couple it out of the laser resonator. In contrast to cavity dumping, with classic Q-switching, the laser pulse is therefore both built up and coupled out in the second operating state.

Such a modulation of the decoupling degree or the quality of the resonator can for example with an acousto-optical modulator or a

Delay device, e.g. a retardation plate, for generating a fixed phase delay in connection with an optical modulator, for example an electro-optical modulator, for generating a variable one

Phase delay can be realized with a polarization-selective

Coupling device is combined, for example, in the form of a polarizer. With the classic Q-switch, a polarization-selective decoupling device can be dispensed with, i.e. the decoupling can e.g. by means of a partially transparent (end) mirror.

Due to the laser dynamics, laser oscillators or laser resonators can fluctuate in pulsed operation (eg with Q-switching or with cavity dumping) the pulse energy and / or the mode profile. Typically, the starting mode profile, ie the (transverse) modes that are excited in a multimode resonator when a respective laser pulse is built up, is not specified or controlled, which is why the beam profile and the energy can fluctuate uncontrollably from laser pulse to laser pulse. Through the different

In addition to the energy fluctuation, there are also temporal fluctuations or temporal jitter.

US Pat. No. 5,365,532 describes a device and a method for stabilizing the output amplitude of lasers during pulse generation by means of cavity dumping. There, the pulse structure or the increasing intensity of the laser radiation in the resonator is monitored by means of a detector and the

Triggered the decoupling time of the laser pulse when a threshold value of the intensity is reached. The temporal jitter that occurs due to the triggered coupling-out time can be reduced by other measures.

US Pat. No. 4,044,316 describes a stabilized Nd: YAG laser with cavity dumping, in which relaxation oscillations are suppressed. The

Relaxation oscillations occur when the power builds up inside the resonator and the power exceeds its steady-state value, resulting in a

Oscillation leads whose damping time is on the order of a few hundred milliseconds. In order to reduce the damping time, there is an optical crystal in the resonator for frequency doubling or for generating the second

Harmonics ("second harmonic generation", SHG) arranged. For the

To reduce the damping time, it is sufficient if the optical crystal generates a power of the second harmonic which is in the order of magnitude of approximately 0.1% of the power at the fundamental frequency.

Object of the invention

The invention is based on the object of methods and devices

provide, which enable a reduction of temporal fluctuations as well as energy fluctuations of the laser pulses generated by Q-switching or cavity dumping. Subject of the invention

This object is achieved according to the invention by a method of the type mentioned at the outset, in which the optical modulator for generating a respective first laser pulse and a respective second laser pulse is used to generate a sequence of laser pulses in which first laser pulses alternate with second laser pulses that are different from the first Laser pulse alternating with the control signal

is controlled differently.

According to the invention it is proposed, instead of reducing the fluctuations of the individual laser pulses, to bring the laser resonator into a strong bistable state by means of a targeted alternating control of the optical modulator, i.e. the laser resonator oscillates between two states, each with a stable mode profile or stable pulse energy. The temporal fluctuations described above occur in particular in the range of frequencies whose period duration corresponds to the fluorescence lifetime of the respectively excited laser level (for Yb: YAG typically in the range of a few kHz). In other

Frequency ranges, especially at very low frequencies of <100 Hz or at very high frequencies of> 1 MHz, there are generally no uncontrolled fluctuations due to two different oscillating mode sets, so that targeted alternating control of the optical modulator in these

Frequency ranges are usually not required. Typical (pulse) frequencies at which a respective (first and second) laser pulse are generated are between approx. 200 Hz and approx. 1000 kHz, preferably between approx. 1 kHz and approx. 100 kHz.

The first laser pulses and the second laser pulses typically differ by a different pulse energy, in particular by a different (maximum) pulse amplitude. As a result of the alternating control of the optical modulator described here, a pulse sequence can be generated in which the respective first and respectively second laser pulses have a time jitter of less than approximately 1 ns. The sequence of laser pulses typically comprises a number of e.g. more than 1000 laser pulses, possibly more than approx. 100000 laser pulses, depending on the application-specific operating time, e.g.

Laser processing of a workpiece can be 10 seconds or more. The bistable operation of the resonator makes it possible to set how much energy is contained in the respective first and second laser pulses, even with a high average power. In addition, the first and second laser pulses in each case have a high level of energy stability.

It goes without saying that the sequence of laser pulses also includes third, fourth, ...

May have laser pulses that alternate with the first, second, ... laser pulses, the first, second, third, fourth, ... laser pulses differing from each other. In this case, too, the optical modulator for generating a respective first, second, third, fourth, ... laser pulse is alternately controlled differently with the control signal and a stable laser operation takes place in which the states all three, four, ... repeat laser pulses.

In a variant, the method comprises: generating a sequence of first

Laser pulses by suppressing the second laser pulses, preferably by means of a further optical modulator arranged outside the laser resonator. The distinction between first and second laser pulses is arbitrary, which is why the above formulation and the formulation “Generating a sequence of second laser pulses by suppressing the first laser pulses” are equivalent. Typically that group or sequence of (first or second) laser pulses is suppressed which have a lower maximum pulse energy. By suppressing a sequence or group of (first or second) laser pulses, the frequency of the sequence of unsuppressed (second or first) laser pulses is halved. To generate such a sequence of laser pulses with a desired output frequency, it is therefore necessary for the optical modulator to be controlled with a control signal whose frequency is twice the desired

Output frequency. The second laser pulses are preferably suppressed or masked out by a further (external) optical modulator, but can also be done in a different way, if necessary. It goes without saying that the

Suppression of the second laser pulses is only optional, since this is only necessary is when the suppressed laser pulses, which generally have the lower energy or power, are disruptive in the respective application.

In a further variant, the optical modulator is controlled with a control signal with a constant control frequency, a first laser pulse and a second laser pulse, and possibly a third laser pulse, a fourth laser pulse, ... being generated during a period of the control signal. The control signal typically has a waveform that is typically switched between two or more discrete signal levels, i. E. the waveform shows

typically does not show a continuous course. To generate two laser pulses during the period, two times between the first

Operating state and the second operating state switched back and forth. For the alternating control, the time during which the control signal remains at a respective signal level in the period can be selected differently when generating the first and the second laser pulse. Alternatively or additionally, a respective signal level for the generation of the first laser pulse and for the generation of the second laser pulse can be selected differently in order to control the optical modulator alternately. The

The control frequency of the optical modulator is preferably between 200 Hz and 1000 kHz, in particular between 1 kHz and 100 kHz. To generate more than two laser pulses during a period, it is also possible to switch between the first operating state and the second operating state more than twice. As described above, the

The signal level or the respective quality of the operating states vary.

In a further development, a dwell time of the optical modulator in the first operating state when generating the first laser pulse and a

Residence time of the optical modulator in the first operating state

Generating the second laser pulse (and possibly a dwell time of the optical modulator in the first operating state when generating a third laser pulse, a fourth laser pulse, etc.) is selected to be different. In this variant, the amplification time that is available for building up a respective first or second (possibly third, fourth, ...) laser pulse in the laser resonator is selected differently. In this variant, in particular, the total dwell time of the optical modulator in the first and second operating states when generating the first laser pulse and when generating the second laser pulse can be selected to be the same, ie the total dwell time corresponds to half of the time

Period of the control signal. In this case a different one leads

Dwell time of the optical modulator in the first operating state when generating the first / second laser pulse necessarily results in a different dwell time of the optical modulator in the second operating state when generating the first / second laser pulse.

In a further development, a total dwell time of the optical modulator in the first and second operating state when generating the first laser pulse and a total dwell time of the optical modulator in the first and second operating state when generating the second laser pulse

differently chosen. In this case, the period is alternated, which is available for the pulse build-up and for the decoupling of a respective laser pulse. A bistable state of the laser resonator can also be achieved in this way.

In a further development, the first quality when generating the first

Laser pulse and the first quality when generating the second laser pulse

differently and / or the second quality when generating the first laser pulse and the second quality when generating the second laser pulse are selected differently. In this case, the loss of the optical modulator or the quality - proportional to the reciprocal of the loss - is selected differently in the first and / or second operating state when generating the first laser pulses and when generating the second laser pulses. To this end, the

Control signal for controlling the optical modulator in the respective first or second operating state for generating the first or second laser pulses has two different signal levels. As a rule, a signal level that is used to generate (first or second) laser pulses with higher pulse energy is selected so that the degree of decoupling or the loss of the laser resonator is 0%, ie that the laser resonator is in the first Operating state has minimal losses. The signal level of the control signal when generating the laser pulse with lower pulse energy can be determined depending on the amplification in the laser medium of the resonator. For example, with a disk laser with a low gain, losses of the optical modulator of less than approx. 5% are sufficient to significantly reduce the pulse energy during cavity dumping, while with a slab laser with a high gain, losses of more than approx. 50% may be required.

In a further variant, a first quality of the resonator is generated in the first operating state to build up a laser pulse in the resonator, and in the second operating state a second, smaller quality is generated for coupling the laser pulse out of the resonator. In this variant, the resonator is operated with cavity dumping, i.e. In the first operating state, a high quality and thus a low loss of the resonator is generated, so that a laser pulse or laser power can build up, which is decoupled from the resonator in the second operating state.

In a further development, the optical modulator is when a

predetermined power threshold of built in the resonator

Laser power switched from the first operating state to the second operating state, a first power threshold value being selected when generating a first laser pulse and a second power welding value different from the first being selected when generating a second laser pulse. In this variant, switching from the first operating state to the second operating state is triggered by reaching a threshold value for the power of the laser pulse that is building up in the laser resonator, as is described, for example, in US Pat. No. 5,365,532 cited at the beginning, which by reference in its entirety is made part of this application. The power of the laser pulse building up in the laser resonator can for example be measured by means of a detector, for example by means of a photodiode. For the power measurement (or, equivalently, for the measurement of the intensity of the laser radiation in the laser resonator), a fixed, predetermined, low proportion of the power of the laser radiation propagating in the laser resonator is typically decoupled from the laser resonator. For the decoupling, one can anyway in the resonator Existing optical component, for example a partially transmissive end mirror, can be used.

By a different choice of the respective power or intensity threshold value for switching from the first operating state to the second

In the operating state, the laser resonator can also be operated in a bistable state, because the selection of two different power threshold values results in two different amplification times when building up the first and the second laser pulses. In this case too, the optical modulator can be driven with a control signal having a constant control frequency, i.e. the period duration of the control signal is constant, only the respective switching point in time from the first to the second operating state when generating both the first and the second laser pulse is not precisely specified and can fluctuate slightly. It goes without saying that, in principle, there is also the possibility of generating the first laser pulse when the power threshold value is reached from the first operating state to the second

To switch the operating state and to set the switching time from the first to the second operating state when the second laser pulse is generated, or vice versa. In this case, the intensity threshold value can be selected so that the associated dwell time in the first operating state when generating the first laser pulse differs from the dwell time in the first operating state when generating the second laser pulse. The total length of stay in the first and second operating states when generating the first laser pulse, in which switching from the first operating state to the second operating state is triggered by reaching the intensity threshold value, can also differ from the total length of stay in the first and second Different operating state when generating the second laser pulse when the switchover from the first to the second operating state when generating the second

Laser pulse is fixed.

In an alternative variant, a first quality is generated in the first operating state to build up a gain in a laser-active medium of the resonator, and in the second operating state a second, larger quality is generated to reduce the gain in the laser-active medium and to couple a laser pulse Goodness creates. In this variant, a classic Q-switch is implemented in the resonator, in which in the first operating state in the

laser-active medium a gain is built up to a maximum

Reinforcement is achieved in the laser-active medium. In the second

In the operating state, the gain is reduced by emitting a laser pulse from the

Resonator is decoupled.

In a further variant, by means of a frequency doubling device in the resonator, a portion of laser power propagating at a fundamental frequency is converted into laser power with twice the fundamental frequency. In the

Frequency doubling device is usually one

optical, typically birefringent crystal, which is designed to generate the second harmonic (SHG). The optical crystal can be, for example, lithium tri-borate (LiB ₃ 0 ₅ ), beta-barium borate (BaB ₂ 04), barium sodium niobate (Ba2Na (Nb03) ₅ ) or another suitable optical crystal Act crystal. The generation of the second harmonic has proven to be advantageous for improving the energy stability.

Another aspect of the invention relates to a device of the type mentioned at the beginning, in which the control device is designed or configured /

is programmed to generate a sequence of laser pulses, in which the first laser pulses alternate with second laser pulses different from the first, to control the optical modulator to generate a respective first laser pulse and a respective second laser pulse by means of the control signal alternately differently. The control device can be

for example a control computer or an electronic control circuit (IC, programmable gate array etc.) which generates the desired control signal. The control signal, more precisely its signal profile, is designed differently for the generation of the first laser pulses and for the generation of the second laser pulses, as described above in connection with the

Procedure is described. The control device can in particular

be designed to generate a control signal in the form of a control voltage that is applied to an electrode of an electro-optical modulator, for example in the form of a Pockels cell. In one embodiment, the device additionally comprises a further optical modulator arranged outside the laser resonator for suppressing the second laser pulses. The optical modulator can be designed, for example, to deflect the second laser pulses from the beam path of the first laser pulses, as is the case with an acousto-optical modulator. It goes without saying that for this purpose the first laser pulses or the beam path of the first laser pulses can also be deflected by the optical modulator, while the second laser pulses pass through this without deflection. If necessary, the second laser pulses can also be suppressed by means of a rapidly switchable optical filter or by means of a further electro-optical modulator in

Combination with a polarizer for splitting the first and second laser pulses into different beam paths. The further optical modulator is only required if the second laser pulses interfere with the respective application for which the laser pulses are required. If this is the case, the frequency of the sequence of laser pulses generated by the device is halved. In this case it is necessary to control the optical modulator with a control signal, the control frequency of which is twice the desired frequency of the sequence of

Laser pulses.

The control device is preferably designed or configured / programmed to control the optical modulator with a control signal with a constant control frequency that is used to generate a first laser pulse and a second

Laser pulse is used during a period of the control signal. It is favorable if the control frequency of the control signal is between approx. 1 kHz and approx. 1000 kHz, preferably between approx. 1 kHz and approx. 100 kHz.

In one embodiment, the control device is designed to generate a first quality of the resonator in the first operating state for building up a laser pulse in the resonator and in the second operating state to produce a second, smaller quality for coupling the laser pulse out of the resonator. As described above in connection with the method, the resonator is operated with cavity dumping in this case. In a further embodiment, the device has a detector for

Detecting a power of the laser resonator in the first

Operating state of the optical modulator building up laser pulse. As has been described above, the detector can be, for example, a photodiode or the like, which detects the power of laser radiation coupled out from the laser resonator during the first operating state. The measured power can be used to determine the time of decoupling, i.e. the time of switching from the first operating state to the second operating state should be selected appropriately (see below).

In a further embodiment, the control device is designed, the optical modulator between the first operating state and the second

To switch the operating state when a predetermined power threshold value of the laser power built up in the laser resonator is reached, and the

The control device is designed to specify a first power threshold value for generating a first laser pulse and a second power threshold value that is different from the first for generating a second laser pulse. In this embodiment, the value of the power currently present in the laser resonator, which can be measured, for example, in the manner described above in connection with the method, is compared with a power threshold value that is different when the first and second are generated Laser pulse differs. In this way, too, a strong bistable state of laser operation can be established. In a further embodiment, the control device is designed to generate a first quality for building up a gain in a laser-active medium of the resonator in the first operating state and in the second

Operating state to generate a second, higher quality for reducing the gain in the laser-active medium and for coupling out a laser pulse. As described above in connection with the method, in this case the resonator is operated in a classic Q-switch.

In another embodiment, there is one in the resonator

Frequency doubling device for converting a portion of the Resonator arranged with a fundamental frequency propagating laser radiation in laser radiation at twice the fundamental frequency. In the

Frequency doubling device can in particular be a

act nonlinear, for example birefringent crystal. As with the

Frequency conversion is common practice in this case too

Frequency conversion requires phase adjustment, which may require suitable temperature control of the optical crystal.

In another embodiment, the resonator additionally comprises: a

laser-active medium, a particularly polarization-selective decoupling

Device, for example a polarizer, for decoupling the laser pulses from the resonator, and preferably a phase delay device for

Create a fixed phase delay. The laser-active medium is typically a solid-state medium, for example in the form of a laser crystal, for example in the form of Yb: YAG, Nd: YAG, Nd: YV0 ₄ , .... The laser active

(Solid) medium can be designed in the form of a laser disk, a laser rod, a laser lab, etc. For the excitation of the laser-active medium, it is typically pumped with the aid of pump radiation, for which purpose the device can have a pump light source, for example a pump laser source.

The cavity dumping and the Q-switching can also take place without a phase delay device, for example if an acousto-optical modulator is used as the optical modulator. As a rule, however, a delay device is used for cavity dumping, which consists of an optical modulator and possibly an additional delay plate. The modulator generates a time-variable phase delay, while the

Delay plate generates a fixed predetermined phase delay. The retardation plate can be, for example, a ¼ retardation plate (or, in the case of ring lasers, a ½ plate), but other delays are also useful. The delay device generally generates its maximum phase delay in the second operating state, which means that the

Laser radiation is maximally delayed when it passes through the delay device twice, so that a laser pulse at the polarization-selective

Decoupling device can be decoupled from the laser resonator. At In a linear resonator with a 1/4 retardation plate, the polarization of the laser radiation can be rotated by 90 ° when it passes through the retardation twice, which corresponds to the maximum decoupling. The polarization-selective coupling-out device can be, for example, a thin-film polarizer that transmits laser radiation with a first polarization direction and laser radiation with a second direction perpendicular to the first

Direction of polarization reflected. Other types of polarizers can also be used as polarization-selective coupling-out devices in the laser resonator, e.g. Polarizers made of birefringent media, which enable a beam offset of the polarization components (s or p polarization) in the birefringent medium and thus a separation of the polarization components, etc. The delay device for generating the fixed phase delay prevents that in the event of an error, i.e. in the event of failure of the optical modulator, the resonator is closed so that the laser pulse cannot be decoupled and is further amplified until it damages components in the resonator. The fixed phase delay of the delay device is chosen so that in the event of an error, i.e. if the optical switch fails, the laser pulse is automatically decoupled. A resonator with a polarization-selective coupling-out device and possibly a delay device with a fixed phase delay can also be operated with a classic Q-switch. In this case, the laser pulses can be decoupled from the resonator without polarization selection.

for example by the laser pulses being decoupled from the resonator at a decoupling device in the form of a partially transparent output mirror, for example a partially transparent end mirror. In this case, the losses in the resonator are generated by the optical modulator and a polarization-selective element. Further advantages of the invention emerge from the description and the drawing. The features mentioned above and those listed below can also be used individually or collectively in any combination. The embodiments shown and described are not intended to be conclusive ßende enumeration to understand, but rather have an exemplary character for the description of the invention.

Show it:

Fig. 1 is a schematic representation of an embodiment of a

Device for generating a sequence of laser pulses by cavity dumping or a Q-switch in a laser resonator which has an alternately controlled optical modulator for generating a sequence of alternating first and second laser pulses,

FIG. 2 shows a representation analogous to FIG. 1, in which the device additionally comprises a frequency doubling device in the resonator and an external modulator for suppressing the second laser pulses,

3a-d four representations of the time course of a control signal for

bistable control of the optical modulator during cavity dumping,

4 shows a representation of the time profile of a control signal for the bistable

Control of the optical modulator with a Q-switch, as well as

FIG. 5 shows an illustration analogous to FIG. 1 with a device for generating a sequence of laser pulses with Q-switching of the laser resonator.

In the following description of the drawings, the same or

functionally identical components used identical reference numerals.

1 shows an exemplary structure of a device 1 for generating a sequence 2 of laser pulses 3 a, 3 b, which has a laser resonator 4. The laser resonator 4 comprises two end mirrors 5 a, 5 b and a disk-shaped laser-active medium 6, in the present example a Yb: YAG crystal, which is applied to a heat sink 7. The laser-active medium 6 is mirrored on its side facing the heat sink 7 and becomes one with the pump radiation (not shown) pump laser optically excited, whereby laser radiation 8 is generated in the laser resonator 4 at a laser wavelength l of 1030 nm.

The laser resonator 4 has a plurality of folding mirrors 9a-d in order to generate a multiple passage of the laser radiation 8 through the laser-active solid-state medium 6. In the laser resonator 4 or in the laser-active

Laser radiation 8 generated by solid-state medium 6 is linearly polarized, e.g. s-polarized.

The laser resonator 4 further comprises an optical modulator 10 in the form of an electro-optical modulator, more precisely a Pockels cell, and a control device 11 for controlling the electro-optical modulator 10 with a control signal S. In the laser resonator 4 is also a

Delay device 12, for example in the form of a 1/4 retardation plate for generating a constant phase delay of 1/4 and a

polarization-selective decoupling device 13 in the form of a thin-film

Arranged polarizer, which acts as a partially transmissive mirror and on which the laser pulses 3a, 3b generated in the laser resonator 4 are coupled out, as will be described in detail below. The optical modulator 10 is basically divided into two for cavity dumping

Operating states B1, B2 operated. The first operating state B1 is used to build up a laser pulse 3a, 3b in the resonator 4, while in the second

Operating state B2 a respective laser pulse 3a, 3b from the resonator 4

is decoupled.

In the first operating state B1, a control signal S (in the form of a

Voltage signal) which generates a (positive) quarter-wave voltage, i.e. a voltage which causes a phase delay of the laser radiation 8 of + 1/4. The retardation plate 12 generates an oppositely directed

Phase delay of - l / 4, so that the sum of the phase delays of the retardation plate 12 and the electro-optical modulator 10 in the first

Operating state B1 is zero. Therefore, the s-polarized laser radiation 8 generated in the laser resonator 4 is not changed in its polarization state. polarized onto the thin-film polarizer 13 and is deflected there, ie 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 a convention in which a positive / negative voltage applied to the electro-optical modulator 10 causes a phase delay with a positive / negative sign.

In the second operating state B2, a phase delay of zero is generated at the electro-optical modulator 10, i. there is no voltage difference on this or a control signal S with a voltage of 0V. In this case, the double passage of the laser radiation 8 through the retardation plate 12 leads to a phase delay of 2 x (−1 / 4) = −1 / 2. This phase delay causes the polarization direction (E-vector) of the linearly polarized laser radiation 8 to be rotated by 90 ° so that it strikes the decoupling device in the form of the thin-film polarizer 13 in p-polarized form and from the laser resonator 4 is decoupled. If the electro-optical modulator 10 is suitably designed and controlled, the retardation plate 12 can also be any (any) of ± K! 4 have a different fixed phase delay.

The laser resonator 4 shown in FIG. 1 is operated in a bistable state, in which a sequence 2 of alternating first and second laser pulses 3a, 3b is generated which differ from one another in at least one property

distinguish. As can be seen in FIG. 1, the first laser pulses 3a have a greater maximum pulse power or energy than the second laser pulses 3b. It goes without saying that the first laser pulses 3a can alternatively have a lower energy or a lower maximum pulse power than the second

Laser pulses 3b. To generate the alternating first and second laser pulses 3a, 3b with different properties, the electro-optical modulator 10 is controlled alternately with the control signal S, with the alternating

There are several possibilities for controlling the electro-optical modulator 10, of which four possibilities are exemplified in FIGS. 3a-d.

It goes without saying that in order to generate a sequence with third, fourth, ... laser pulses each with different laser pulses from the other (first, second, ...) Properties of the electro-optical modulator 10 can be controlled accordingly in order to realize a stable laser operation in three, four, ... states.

In all four examples shown in Figs. 3a-d, the control signal S has a constant control frequency f, which can for example be of the order of a few kHz, e.g. between 200 Hz and 1000 kHz, preferably between 1 kHz and 100 kHz. During a period T of the control signal S, the optical modulator 10 is activated in each case in such a way that a first laser pulse 3a and a second laser pulse 3b are generated. To generate a respective first or second laser pulse 3a, 3b, it is necessary to switch back and forth between the first operating state B1 and the second operating state B2 once in each case. In Fig. 3a-d, the signal level of the control signal S or the quality Q, more precisely 1 / Q (proportional to the loss L), between a maximum loss L (corresponding to a minimum quality Q), which is denoted by a one and a minimum loss L (corresponding to a maximum quality Q), which is denoted by zero. At the minimum quality Q (and maximum loss L), in the example shown, the optical modulator 10 generates that described above

Phase delay of zero, while with the maximum quality Q (and minimum loss L) the optical modulator 10 generates a phase delay of + 1/4 (see above).

In the example shown in FIG. 3a, the alternating activation takes place with a control signal S, in which the time duration for the generation of a respective first laser pulse 3a and a respective second laser pulse 3b is the same and corresponds to half the period T / 2 of the control signal S. . In the example shown in FIG. 3a, however, the dwell time t _{Bi, i} in the first operating state B1 when the first laser pulse 3a is generated differs from the dwell time tei, 2 in the first operating state B1 when the second laser pulse 3b is generated: the dwell time tsi _{, i} in the first operating state B1 is greater when the first laser pulse 3a is generated than the dwell time t _Bi , 2 in the first operating state B1 when the second laser pulse 3b is generated. In this way, a longer period of time is available for the amplification or for the pulse build-up of a respective first laser pulse 3a, which is the same as that shown in FIG. 1 higher maximum power of the first laser pulse 3a compared to the second laser pulse 3b leads.

In the example shown in FIG. 3b, the alternating control is also implemented by switching between the first and the second operating state B1, B2 when the two laser pulses 3a, 3b are generated at different times. In the example shown in FIG. 3b, however, the respective dwell time t _ß -i _, i or tei, 2 in the first operating state B1 for the first laser pulse 3a and for the second laser pulse 3b is the same length. In FIG. 3b, however, the total dwell time t _{tot, i of} the optical modulator 10 in the first operating state B1 and in the second operating state B2 when generating the first laser pulse 3a differs from the total dwell time t _{tot, 2 of} the optical modulator 10 in the first operating state B1 and in the second operating state B2 when the second laser pulse 3b is generated. The sum of the two total residence times t _{tot, i} and t _tot , 2 corresponds to the constant one

Period T of the control signal S. Due to the greater total dwell time t _{tot, i of} the optical modulator 10 when the first laser pulse 3a is generated, the latter has a greater maximum pulse power than the second laser pulse 3b.

It is essential here that, due to the longer dwell time of the optical modulator 10 in the second operating state B2, more energy or amplification is introduced into the laser-active medium 6 when the first laser pulse 3a is generated than when the second laser pulse 3b is generated. Accordingly, more energy can be extracted for the first laser pulse than for the second laser pulse during the subsequent dwell time in the first operating state B1.

It _{goes without saying} that the possibilities of alternating control shown in FIG. 3a and in FIG. 3b can also be combined, ie the dwell time t _{ßi, i} in the first operating state B1 and the total dwell time t _{tot, i} in the first and second operating state B1, B2 when generating the first

Laser pulse 3a can differ from the dwell time tsi _{, 2} in the first operating state B1 or from the total dwell time t _tot , 2 in the first and in the second

Operating state B1, B2 when generating the second laser pulse 3b

distinguish. In the example shown in Fig. 3c, the different control for generating the two laser pulses 3a, 3b is not done by different dwell times in the two operating states B1, B2, but by a quality Qi of the optical modulator 10 in the first operating state B1 when generating the first

Laser pulse 3a which has a quality Qi ^'of the optical modulator 10 in the first operating state B1' when generating the second laser pulse 3b

differs. The respective quality Qi, Qi ‘represents a dimensionless value (Q factor) and is inversely proportional to the loss L |, Li‘. For the first

Operating state B1 during the generation of the first laser pulse 3a, as in the two examples of FIGS. 3a, b, the fact that the loss Li of the laser resonator 4 is practically zero and the quality Qi is at a maximum. For the first operating state B1 ‘, in which the optical modulator 10 is operated when generating the second laser pulse 3b, the following applies: Li‘ = 0.2, depending on the type of

laser-active medium 6 other values for the loss L-i ‘are possible, the

for example between 0.01 and 0.5. When the second laser pulse 3b is generated, the optical modulator 10 is controlled in the first operating state B1 ‘with a control signal S with a signal level which causes a phase delay of the optical modulator 10 other than zero. This leads to the polarization direction (E-vector) being linearly polarized

Laser radiation 8 is rotated and has a portion which is decoupled from 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, so that its maximum pulse power is lower than in the case of the first laser pulse 3a.

Finally, FIG. 3d shows a possibility for alternating control of the optical modulator 10, in which the optical modulator 10 differs from the first

Operating state B1 is switched to the second operating state B2 as soon as the power P of the laser radiation 8 built up in the laser resonator 4 exceeds a predetermined power threshold P _{s, i} , Ps _{, 2} , which is necessary for the generation of the two laser pulses 3a, 3b different sizes are chosen. Im shown

For example, the first power threshold value Ps _{, i} for the first laser pulse 3a is selected to be greater than the second power threshold value Ps _{, 2} for the second laser pulse 3b. Accordingly, the optical modulator 10 in the generation of the first

Laser pulse 3a at a later point in time from the first operating state B1 to second operating state B2 is switched, ie the dwell time t _{Bi, i} in the first operating state B1 is greater when generating the first laser pulse 3a than the dwell time t _ßi , 2 in the first operating state B1 when generating the second laser pulse 3b.

The exact dwell time t _{ßi, i} , t _ß -i, 2 in the first operating state B1 is determined by the achievement of the respective power threshold value Ps _{, i} , Ps, 2, which occurs when the sequence 2 of laser pulses 3a, 3b is generated successive first laser pulses 3a and second laser pulses 3b each fluctuates slightly. The control signal S nevertheless has a constant control frequency f in this case as well, since the switchover from the second operating state B2 to the first

Operating state B1 takes place at fixed predetermined times within a respective period T. Correspondingly, the control signal S shown in FIG. 3d differs from the control signal S shown in FIG. 3a only in that the switching point in time from the first operating state B1 to the second

Operating state B2 is not fixed, but is triggered by the achievement of the respective power threshold value Ps _{, i} , Ps, 2.

To determine the (instantaneous) power P of the laser radiation 8 in the laser resonator 4 in the first operating state B1, the one shown in FIG. 1 has

Device 1 has a detector 14 which is designed in the form of a photodiode. The detector 14 is arranged outside the laser resonator 4. To a small proportion of the propagating in the laser resonator 4 laser radiation 8 for the

To couple out detection, the second end mirror 5b of the laser resonator 4 is partially transparent, i.e. this has a transmission of approximately 0.01% or less for the laser radiation 8 propagating in the laser resonator 4. The detector 14 can be dispensed with if the alternating control of the optical modulator 10 takes place in the manner described in connection with FIGS. 3a-c.

FIG. 2 shows a device 1 for generating a sequence 2 of laser pulses which essentially differ from the device 1 shown in FIG. 1

differs in that it has a further optical modulator 15, for example in the form of an acousto-optical modulator, which is located outside the laser resonator 4 is arranged. The further optical modulator 15 serves to decouple the second laser pulses 3b from the sequence 2 of first and second laser pulses 3a, 3b decoupled from the laser resonator 4 or to suppress them. The acousto-optical modulator 15 deflects the second laser pulses 3b to an absorber (not shown). For the deflection, a phase diffraction grating is generated by the acousto-optical modulator 15 in an optical crystal with the aid of an ultrasonic generator with a predetermined switching frequency f / 2, which corresponds to half the control frequency f of the control signal S.

Alternatively, the external optical modulator 15 can be another electro-optical modulator, e.g. act in the form of a Pockels cell to generate a phase shift or phase delay. In both cases, the external optical modulator 15 can be controlled with the aid of the control device 11 with half the control frequency f of the control signal f / 2 in order to eliminate the second laser pulses 3b from the sequence 2 of laser pulses 3a, 3b, so that only the first Laser pulses 3a leave the device 1. It goes without saying that, if necessary, a separate control device, e.g. can be provided in the form of an electronic control circuit. In this case it is necessary to suitably synchronize the control of the optical modulator 10 and the further optical modulator 15. For this purpose, for example, a common frequency generator can be provided in the device 1.

The laser radiation 8 generated in the laser resonator 4 has a fundamental frequency fc which is proportional to the reciprocal of the laser wavelength l. For the additional suppression of temporal jitter and in particular of energy fluctuations in the generation of the sequence of laser pulses 3a, 3b, a frequency doubling device 16 in the form of a is in the laser resonator 4 of FIG

frequency-doubling crystal (SHG crystal) arranged. In the SHG crystal 16, a small proportion (generally less than 10% or less than 1%) of the laser radiation 8 generated in the laser resonator 4 is converted into laser radiation 17 with twice the fundamental frequency 2 fe. The converted laser radiation 17 is transmitted at one of the deflecting mirrors 9 a, which is designed as a wavelength-selective optical element, and is coupled out of the laser resonator 4. FIG. 4 shows the time course of the control signal S when the laser resonator 4 is not operated with cavity dumping as shown in FIGS. 3a-d, but with a conventional Q-switch. In this case, in the first

Operating state B1 a gain V in the laser-active medium 6 des

Resonator 4 constructed by operating the resonator 4 with a high loss L-i of almost Li = 1.0 or with a quality Qi close to zero. As soon as the

Gain V in the laser-active medium 6 has assumed its maximum value (at a fixed predetermined point in time), the optical modulator 10 is switched 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 Q2 which is greater than the first quality Qi in the first operating state B1 (the loss L ₂ , L ₂ 'is close to zero) in order to increase the gain V in the to reduce laser-active medium 6 and to remove the laser pulses 3a, 3b from the laser resonator 4

decoupling.

In the example shown in FIG. 4, the second quality Q ₂ when generating the first laser pulse 3a and the second quality Q ₂ 'when generating the second

Laser pulse 3b in the second operating state B2 differently selected, the second quality G ₂ when generating the first laser pulse 3a is greater than the second quality Q ₂ 'when generating the second laser pulse 3b (and accordingly the loss L _{2 is} less than the loss L ₂ '). Correspondingly, a respective first laser pulse 3a has a greater pulse energy on a respective second laser pulse 3b. It goes without saying that, as an alternative or in addition to the control of the optical modulator 10, as shown in FIG. 4, first and second laser pulses 3a, 3b, which differ from one another in at least one property, are also analogous to that described above 3a, b for the cavity dumping described manner can be generated.

5 shows an example of a device 1 for generating laser pulses 3a, 3b, in which the laser resonator 4 is also operated with a conventional Q-switch. The device 1 shown in FIG. 5 differs

objectively of the device 1 shown in Fig. 1 only in that the first end mirror 5a of the resonator 4 as a partially transparent mirror (for example with a Transmission of 10%) and serves as a coupling-out device, while the thin-film polarizer 13 does not act as a coupling-out device, ie is not designed to be partially transmissive. The delay plate 12 can be dispensed with in this case. The losses in the resonator 4 are generated by the acousto-optical modulator 10 in this case. The control of the acousto-optical modulator 10 with the control signal S can also be carried out in the case of that shown in FIG

Device 1 take place in the manner described in FIG. Instead of an acousto-optical modulator, an electro-optical modulator can also be used in the device of FIG.

In summary, with the aid of the alternating control of the optical modulator 10, the laser resonator 4 can be operated in a robust bistable state. In this way, by means of the device 1, a sequence 2 of

Laser pulses 3a, 3b are generated, in which both a very low temporal jitter and a high energy stability of the respective laser pulses 3a, 3b can be realized.

Claims

1. A method for generating laser pulses (3a, 3b) by changing the quality (G) in a resonator (4), comprising:

Generation of the laser pulses (3a, 3b) by controlling an optical modulator (10) with a control signal (S) for switching between a first operating state (B1) of the optical modulator (10) for generating a first quality (Gi) in the resonator (4) ) and a second operating state (B2) of the optical modulator (10) for generating a second quality (G2) different from the first in the resonator (4),

characterized

that to generate a sequence (2) of laser pulses (3a, 3b), in which the first laser pulses (3a) alternate with second laser pulses (3b) different from the first, the optical modulator (10) for generating a respective first laser pulse ( 3a) and a respective second laser pulse (3b) with the

Control signal (S) is alternately driven differently.

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

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

4. The method according to any one of the preceding claims, wherein a

_Residence time (t _{ßi, i} ) of the optical modulator (10) in the first

Operating state (B1) when generating the first laser pulse (3a) and a Dwell time (tsi _, 2) of the optical modulator (10) in the first operating state (B1) when generating the second laser pulse (3b)

be chosen differently.

5. The method according to any one of the preceding claims, in which a total dwell time (t _tot. 1) of the optical modulator (10) in the first and second operating states (B1, B2) when generating the first laser pulse (3a) and a total Dwell time (t _tot, 2) of the optical modulator (10) in the first and second operating states (B1, B2) when generating the second laser pulse (3b) can be selected differently.

6. The method according to any one of the preceding claims, wherein the first quality (Oi, Ch) when generating the first laser pulse (3a) and the first quality (Qi ') when generating the second laser pulse (3b) are selected differently and / or in the second quality (Q ₂ ) when generating the first laser pulse (3b) and the second quality (V) when generating the second laser pulse (3b) are selected differently.

7. The method according to any one of the preceding claims, wherein in the first

Operating state (B1) a first quality (Qi) of the resonator (4) for building up a laser pulse (3a, 3b) is generated in the resonator (4) and in which in the second operating state (B2) a second, smaller quality (Q ₂ ) for decoupling the laser pulse (3a, 3b) from the resonator (4).

8. The method of claim 7, wherein the optical modulator (10) at

Reaching a predetermined power welding value (Ps _{, i} , Ps _{, 2} ) of the laser power (P) built up in the resonator (4) is switched from the first operating state (B1) to the second operating state (B2)

Generating a first laser pulse (3a) a first intensity threshold value (P _{s, i} ) and when generating a second laser pulse (3b) a second intensity threshold value (Ps _{, 2} ) different from the first is selected.

9. The method according to any one of claims 1 to 6, wherein in the first

Operating state (B1) for building up a gain (V) in a laser-active one Medium (6) of the resonator (4) a first quality (Qi) is generated and in which in the second operating state (B2) to reduce the gain (V) in the laser-active medium (6) and to couple a laser pulse (3a, 3b) a second, higher quality (Q ₂ ) is generated.

10. The method according to any one of the preceding claims, in which, by means of a frequency doubling device (16), a portion of laser radiation (8) propagating in the resonator (4) with a fundamental frequency (fc) in

Laser radiation (17) is converted with twice the basic frequency (2 fc).

11. Device (1) for generating laser pulses (3a, 3b), comprising:

a resonator (4),

an optical modulator (10) arranged in the resonator (4),

a control device (11) which is designed to generate a control signal (S) to the optical modulator (10) between a first

To switch the operating state (B1) for generating a first quality (Gi) of the resonator (4) and a second operating state (B2) for generating a second quality (G ₂ ) of the resonator (4) different from the first,

characterized,

that the control device (11) is designed to generate a sequence (2) of laser pulses (3a, 3b) in which first laser pulses (3a) alternate with second laser pulses (3b) different from the first, the optical

Controlling the modulator (10) for generating a respective first laser pulse (3a) and a respective second laser pulse (3b) alternately and differently by means of the control signal (S).

12. The apparatus of claim 11 further comprising: one external to the

Resonator (4) arranged further optical modulator (15) for

Suppression of the second laser pulses (3b).

13. The device according to claim 11 or 12, in which the control device (11) is designed to provide the optical modulator (10) with a control signal (S) with a constant control frequency (f) for generating a first laser pulse (3a) and a second laser pulse ( 3b) during a period (T) of the Control signal (S).

14. Device according to one of claims 11 to 13, wherein the

Control device (11) is designed to generate a first quality (Qi) of the resonator (4) for building up a laser pulse (3a, 3b) in the resonator (4) in the first operating state (B1) and in the second operating state (B2) to generate a second, lower quality (Q2) for coupling out the laser pulse (3a, 3b) from the resonator (4).

15. The apparatus of any one of claims 11 to 14, further comprising: a

Detector (14) for detecting in the resonator (4) in the first

Operating state (B1) of the optical modulator (10) built up laser power

(P).

16. The device according to claim 15, wherein the control device (11)

is formed, the optical modulator (4) between the first

Operating state (B1) and the second operating state (B2) when a predetermined power threshold value (Ps _{, i} , Ps _{, 2} ) of the laser power (P) built up in the resonator (4) is reached, and the

Control device (11) is designed to generate a first

Laser pulse (3a) a first power threshold value (Ps _{, i} ) and for generating a second laser pulse (3b) a second, different from the first power threshold value (Ps _{, 2} ).

17. Device according to one of claims 11 to 13, wherein the

Control device (11) is designed to generate a first quality (Qi) for building up a gain (V) in a laser-active medium (6) of the resonator (4) in the first operating state (B1) and in the second

Operating state (B2) a second, greater quality (Q2) to reduce the

To generate amplification (V) in the laser-active medium (6) and for coupling out a laser pulse (3a, 3b).

18. Device according to one of claims 11 to 17, further comprising: a frequency doubling device (16) arranged in the resonator (4) for Conversion of a portion of the laser radiation (8) propagating in the resonator (4) with a fundamental frequency (fc) into laser radiation (17) at twice the fundamental frequency (2 f _G ).

19. Device according to one of claims 11 to 18, in which the resonator (4) further comprises:

a laser-active medium (6),

an in particular polarization-selective decoupling device (13, 5a) for decoupling the laser pulses (3a, 3b) from the resonator (4), and

preferably a phase delay device (12).