Method for stabilising the frequency of a q-switched narrow-band laser

Abstract

The invention relates to a method for stabilising the frequency of a narrow-band Q-switched laser, in which method laser radiation from a seed laser (5) with a wavelength at which the laser pulses are to be generated is coupled into the laser resonator. A temporal intensity profile of laser radiation from the seed laser (5) coupled out of the laser resonator is detected, and, on the basis of the simultaneously detected temporal intensity profile, the optical resonator length of the laser resonator is controlled, by periodically varying same, to an approximately constant temporal average value in a control period between two laser pulses. During the time range in which the laser pulse is being generated, the optical resonator length of the laser resonator is changed from an initial value, that is known due to the previous control process, to a resonance value at which the laser resonator is in resonance with the wavelength of the seed laser (5) when the Q-switch (4) is switched, and, when said resonance value is reached, the Q-switch (4) is switched in order to generate a laser pulse. The method protects the element used to change the resonator length and yet makes it possible to stabilise frequency in a highly accurate manner with good a priori knowledge of the trigger time.

Description

[0001] Method for frequency stabilization of a Q-switched narrowband laser

[0002] Technical application area

[0003] The present invention relates to a method for frequency stabilization of a narrowband laser having a laser resonator of variable resonator length with an active Q-switch for generating laser pulses, in which laser radiation from a seed laser having a wavelength at which the laser pulses are to be generated is coupled into the laser resonator. The invention also relates to a laser arrangement for carrying out the method.

[0004] Q-switched lasers generally have a spectral bandwidth that encompasses many longitudinal modes of the laser cavity. Exceptions include, for example, laser beam sources that, due to a short cavity, have such a large longitudinal mode spacing that no more than one mode can be amplified in the active medium. These are usually sources with a passive Q-switch, since the installation space alone of an active Q-switch normally requires a cavity length that makes the above condition impossible to fulfill.

[0005] If the application requires spectral narrowband, the laser must be manipulated so that fewer longitudinal modes are excited and amplified. This is often achieved by seeding the laser oscillator with a narrowband laser, the so-called seed laser. This seed laser provides an effective photon density of the preferred wavelength in the resonator of the laser whose frequency is to be stabilized. This photon density is several orders of magnitude higher than the natural, fluorescence-induced wavelength and excites the resonator at this wavelength. If only the one longitudinal mode in the resonator, which corresponds to the frequency of the seed laser, is to be excited and amplified, the round-trip length of the laser resonator must be a multiple of the wavelength of the seed laser. The seed laser must be sufficiently narrowband to ensure monofrequency operation of the laser for a given free spectral range and time-bandwidth limit.Typically, a seed laser with a bandwidth below approximately 10-20% of the oscillator's time-bandwidth limit is used. For stable monofrequency or narrowband operation of a Q-switched laser at wavelengths in the VIS to NIR from approximately 400 nm to approximately 3 m, the resonator length may therefore only fluctuate by fractions of a nanometer. Natural thermal drift alone, and especially vibrations, generate larger fluctuations. It is therefore necessary to actively control and thus stabilize the laser's resonator length.

[0006] State of the art

[0007] A proven method for controlling the resonator length for monofrequency or correspondingly narrowband operation of a Q-switched laser is the ramp-fire method, as described, for example, in S. W. Henderson et al., "Fast resonance-detection technique for single-frequency operation of injection-seeded Nd:YAG lasers", Optics Letters, Vol. 11, No. 11, Nov. 1986. In this method, one of the mirrors of the laser resonator is axially displaced so that the resonator can be regarded as a scanning Fabry-Perot interferometer. Piezo actuators, for example, have proven successful for this purpose, as they enable a stroke of a few pm on a time scale of 10 ps to 500 ps. During seeding of the laser resonator in the ramp-fire method, the resonator revolution length is changed approximately linearly by at least one wavelength and the transmittance of the resonator at the wavelength of the seed laser is measured (qualitatively) with a photodiode.The result is the typical transmission signal of a scanning Fabry-Perot interferometer for a monofrequency signal. At the transmission maxima the resonator is in resonance with the seed signal and the Q-switch of the laser resonator can be triggered. If the optical resonator length is changed during Q-switching, the resulting change in length must be compensated for, since the switched resonator is no longer in resonance with the resonant unswitched resonator when the geometric length is the same. This is the case, for example, when a Pockels cell is used as a Q-switch. This cell consists of a birefringent crystal that changes its refractive index along one axis when switching. Compensation is achieved with a fixed delay of the switching process to the previously detected resonance, during which the piezo actuator geometrically moves the corresponding change in the optical resonator length and compensates for the change caused by Q-switching.The laser resonator then emits a correspondingly narrowband or monofrequency laser pulse. Other methods for tuning the optical path length of the resonator to the seed laser are also possible, for example, based on acousto-optical or electro-optical modulators instead of piezo actuators.

[0008] The ramp-fire method has the great advantage that the time period between the information about the resonance and the laser pulse is very short. Depending on the configuration, it is typically only a few nanoseconds to a few microseconds. As a result, disruptive external influences have only a very small impact on the system, so that the ramp-fire method generally enables very stable monofrequency operation. Furthermore, the piezo actuator follows a fixed, almost linear ramp; the actuator position is not used as a controlled variable. As a result, the mass-related inertia of the actuator and its hysteresis do not represent a direct limitation on the control accuracy.However, the accuracy of the method increases with the ramp steepness, i.e., the speed of the linear length change through the piezo actuator. This is because, with higher speeds, the time between the information about the resonance and the laser pulse is shorter, and, secondly, the greater sharpness of the resonance signal makes the (electronic) detection more precise. However, the mass-related inertia of the actuator indirectly represents a limit to the control accuracy because it restricts the steepness of the ramp.

[0009] However, the ramp-fire method also has fundamental disadvantages. For example, the exact time of resonance and thus the exact time of emission of the laser pulse is not known a priori. It is variable by a free spectral range or by the corresponding time equivalent given by the travel speed of the piezo actuator, typically a period of 10 s or more. This means, on the one hand, that the gain in the active medium is generally different for each pulse, since it changes on a s time scale with constant pumping or non-pumping. The result is undesirable energy fluctuations from pulse to pulse. On the other hand, the synchronization of other devices belonging to the laser system is made considerably more difficult or even impossible, especially if these other devices are fired at a fixed time relative to the laser pulse but have to be triggered before the laser pulse due to internal delays.A further disadvantage is the mechanical load on the actuator used for the length change. The linear ramp and in particular the strong accelerations at the start and end of the ramp place a very heavy mechanical load on the actuator. This load shortens its service life and is undesirable for applications where the actuator can only be replaced with great effort or not at all. The load on the actuator increases with the ramp steepness, but this also increases the accuracy of the process and thus the frequency stability of the resonator. Frequency stability and load on the actuator are therefore in conflict with the objectives. A flatter ramp also increases the problem of pulse energy fluctuations because the time interval between laser pulse triggering increases with decreasing ramp steepness. The last-described disadvantage is particularly relevant for satellite applications.In an electro-optically or acousto-optically Q-switched laser with an amplifier and frequency converter, there are no mechanically stressed or moving components without control of the resonator length. However, if the resonator is controlled with a piezo actuator, the piezo actuator, as the only mechanically moving element, has one of the highest failure probabilities of all laser components and thus represents a limiting factor for the duration of a satellite-supported mission. Redundancy by installing an additional piezo actuator on a different resonator mirror is undesirable for reasons of the optomechanical stability of the oscillator.

[0010] Other methods of resonator length control are known from the state of the art which completely or partially avoid the disadvantages of the ramp-fire method described above, but at the expense of frequency stability.

[0011] For example, DE 102007003759 B4 describes a modified ramp-fire method in which the time of occurrence of resonance when moving the piezo ramp is predicted with a lead time of a few tens of ps. The prediction is made based on the measured time of occurrence of the first detected resonance, which is related to the time of occurrence of a subsequent resonance at which the Q-switch is triggered. This is then known with a lead time of a few tens of ps, so that other devices can be synchronized to the laser pulse. However, this leaves unsolved the problems of the uncertain triggering time in the amplification curve and the high load on the piezo actuator. Furthermore, the deviation from the linearity of the piezo travel path to the voltage reduces the accuracy.

[0012] In the Pound-Drever-Hall (PDH) and Hänsch-Couillaud (HC) methods, as described in Drever, RWP et al., "Laser phase and frequency stabilization using an optical resonator," Appl. Phys. B Photophysics and Laser Chemistry, Springer Science and Business Media LLC, 1983, 31, 97-105, and in Hänsch, T. et al., "Laser frequency stabilization by polarization spectroscopy of a reflecting reference cavity," Opt. Comm., Elsevier BV, 1980, 35, 441-444, the error signal (electronic detection of modulated sidebands (PDH) or polarization change (HC)) for control is obtained almost in real time. The piezo actuator is continuously adjusted; its inertia represents the limiting factor for the bandwidth of the controllable disturbances. This is therefore in the order of a few 100 Hz.Especially in the pumping phase, the optical resonator length changes dynamically within a time scale of 100 ps, ​​so that control frequencies significantly greater than 10 kHz would be required.

[0013] In the ramp-hold-fire method, as described in T. Walther et al., "Generation of Fourier-transform-limited 35-ns pulses with a ramp-hold-fire seeding technique in a Ti: sapphire laser," Appl. Opt. 40, 3046-3050 (2001), the piezo actuator follows a ramp and is stopped when resonance occurs. The pulse can then be triggered at a previously known time. Due to the ringing of piezo elements and mechanics, it is typically necessary to wait several tens of ps before the Q-switch is triggered, which makes the method more susceptible to interference than the classic ramp-fire method. In this case, the influence of the change in resonator length due to the change in gain in the laser crystal cannot be taken into account until the end, which results in poorer stability. Furthermore, the load on the actuator is the same as in the ramp-fire method.

[0014] Another method known to the inventors for controlling the resonator length is the so-called cavity dithering method. In this method, the resonator length is constantly harmonically changed ("dithering") by the piezo movement. The control electronics use the transmission signal to determine the piezo position at which the laser resonator is resonant to the seed signal. The Q-switch of the laser resonator is triggered at a fixed, known time, whereby the resonance position (plus compensation offset) must be reached immediately before Q-switching. This works for a steady state. If pumping occurs during dithering, the dithering, i.e. the harmonic change in the resonator length, must compensate for the optical resonator length change induced by the pumping. In order to keep the load on the piezo actuator low, the dithering frequency should not exceed the order of 1 kHz.If the resonator is pulsed pumped due to the ratio of the storage time of the active medium to the pulse repetition rate and the storage time is much shorter than 1 ms (example: Nd:YAG oscillator, storage time 230 s, pulse repetition rate 100 Hz), the influence of the pumping can then no longer be compensated for by dithering. The influence of the pumping on the optical resonator length must then be regarded as (almost) constant and taken into account when determining the compensation offset mentioned above. The load on the piezo actuator is significantly lower compared to the ramp-fire method, but so is the frequency stability, since the time period between the information about the state of the resonator and the triggering of the Q-switch is relatively long at dither frequencies around 1 kHz.

[0015] The object of the present invention is to provide a method and an arrangement for frequency stabilization of a Q-switched, narrowband or longitudinally monofrequency laser, which enables a reduced mechanical load on the element regulating the resonator length with high frequency stability and in which the time of emission of the respective laser pulse is known a priori.

[0016] Description of the invention

[0017] This object is achieved by the method and the laser arrangement according to patent claims 1 and 11. Advantageous embodiments of the method and the laser arrangement are the subject of the dependent patent claims or can be derived from the following description and the exemplary embodiment. In the proposed method, the laser radiation of a narrowband seed laser with a wavelength at which the laser pulses of the laser whose frequency is to be stabilized are to be generated is coupled into the laser resonator.A temporal intensity profile of laser radiation of the seed laser coupled out of the laser resonator is recorded and the optical resonator length of the laser resonator is controlled in a period between two laser pulses, hereinafter referred to as the control period, on the basis of the recorded temporal intensity profile by periodic variation and adaptation of an offset of the variation to an approximately constant temporal average. The resonator length is therefore periodically dithered in the control period. The transmitted seed signal is also periodic, but drifts with, for example, thermal changes in the resonator length. By controlling the offset of the dither movement, the response signal and thus the resonator length are stabilized. The periodic variation is carried out in such a way that the mechanical load on the actuator is as low as possible, in particular lower than in the ramp-fire method.The variation is preferably harmonic or at least approximately harmonic. The coupling and decoupling of the laser radiation from the seed laser can be carried out in the same way as in the known methods of controlling the resonator length of the prior art mentioned in the introduction to the description. The optical resonator length of the laser resonator is then changed in each case in the time range of the generation of a laser pulse, hereinafter also referred to as the triggering period, starting from an initial value known from the previous control to a value (and beyond), hereinafter referred to as the resonance value, at which the laser resonator is in resonance with the wavelength of the seed laser after switching the Q-switch. The Q-switch is switched to generate a laser pulse when this resonance value is reached.A resonance before the Q-switch is switched is detected on the basis of the transmitted seed signal, and the Q-switch is switched, possibly with a time delay to compensate for circuit-related changes in the optical resonator length. The change or variation in the optical resonator length of the laser is preferably carried out by mechanical movement of a mirror of the resonator. A piezo actuator is preferably used for this purpose. For an Nd:YAG laser to be stabilized, the triggering period can be in the range of 1 Ops to 10 Ops, for example, and the control period can be in the range of 10 ms to 20 ms, for example. An exemplary dither frequency is approximately 1 kHz.

[0018] The switching time of the Q-switch should, if possible, coincide approximately with the time of maximum pump inversion. At the beginning of the triggering phase, due to the control, the initial value of the optical resonator length is known in relation to the resonance value to be achieved. The rate of change or the variation of the optical resonator length is then selected in the triggering phase such that the resonance value is at least approximately reached at the desired switching time. As explained in more detail below, this can be achieved by a linear change in the resonator length, but preferably by suitable synchronization of the periodic variation such that the resonance value and thus the switching time each occur in a phase of the preferably harmonic variation that is as linear as possible. The corresponding values ​​(initial value and resonance value or their difference) can be determined by preliminary tests with the laser arrangement.Changes in the optical resonator length that may occur during switching are taken into account.

[0019] The optical resonator length is preferably controlled during the control periods by maintaining a fixed relationship between the variation in the optical resonator length and the temporal intensity profile of the laser radiation of the seed laser coupled out of the laser resonator. For this purpose, the voltage value by which the voltage applied to the piezo actuator is varied accordingly, preferably harmonically, is suitably adjusted or changed during a variation performed by moving a resonator mirror using a piezo actuator.

[0020] The change in the optical resonator length in the triggering phase can be linear; that is, the resonator length is changed in a ramp-like manner, as in the ramp-fire method described above. The ramp can be selected to be flatter than in the ramp-fire method, with better predictability of the triggering time and lower actuator load. Alternatively, the change in the resonator length in this phase can also occur in a different form, preferably while maintaining a harmonic or at least approximately harmonic movement. The harmonic movement is then preferably selected such that the resonance value lies in the region of an inflection point of the harmonic movement.

[0021] A narrowband laser is defined as a laser with a laser emission whose bandwidth is narrower than the free spectral range of the laser resonator. For example, the laser emission can have a half-width of less than 50 MHz, preferably less than 20 MHz. Frequency stabilization occurs around the center frequency of this spectral emission. In the following, such a laser is also referred to as a monofrequency laser.

[0022] Two cases can be distinguished: the case of pulsed pumping of the laser and the case of continuous pumping (continuous wave) of the laser.

[0023] If the laser is pumped in a pulsed manner, the resonator length is varied periodically during the pause phase between two pump pulses, also known as dithering. The control period therefore corresponds to the pause phase. The periodic variation is preferably achieved by a corresponding harmonic or nearly harmonic movement of one of the mirrors using voltage, current or charge modulation with a piezo actuator. The signal transmitted through the laser resonator from the narrowband seed laser as a reference laser is detected with a suitable detector, preferably a photodiode. If the dither amplitude, i.e. the amplitude of the periodic variation of the resonator length, is smaller than a free spectral range of the laser resonator, the detection signal oscillates at twice the dither frequency. The signal curve fluctuates relative to the dither movement due to changes in the resonator length, which can be caused e.g.caused by vibrations or thermal drift. When using a piezo actuator, the signal curve of the detection signal can be controlled by means of the voltage offset of the dither. It is stabilized to the dither movement using control electronics. The dither, i.e. the periodic change in the resonator length or movement of one of the mirrors, thus occurs by a value that is continuously adjusted during dithering in order to maintain a constant relationship between the dither movement and the photodiode signal. The dither frequency in the proposed method is preferably no more than 10 kHz. Shortly before the desired triggering time of the Q-switch, the variation in the resonator length, i.e. in particular the movement of the piezo actuator, is changed from dithering to a linear ramp, for example. The voltage interval of the ramp must contain at least one resonance state.The Q-switch is triggered at the time of resonance plus any temporal compensation for circuit-related changes in the optical resonator length. In the case of a continuous-wave pumped laser, the gain and thus the optical resonator length changes continuously due to the pumping. Accordingly, the dither period in the control period must be chosen sensibly in order to be able to compensate for these changes. With a constant pumping rate and thus a known rate of change of the optical resonator length, the change can be compensated for independently of the dither control, e.g. by a continuous, static offset adjustment. The triggering phase then proceeds in the same way as in the case of pulsed pumping.

[0024] Dithering ensures that the state of the laser resonator is very well known at the start of the triggering phase, particularly during a ramp run. Disturbances with frequencies below the dither period can be compensated for. This makes it very easy to predict when the resonator will be resonant with the seed laser during the ramp run. The dither and ramp parameters can be selected such that the laser resonator is resonant exactly at the gain maximum. Changes in the optical resonator length caused by pumping the active medium during the ramp run are the same in each period and can be easily compensated for. The ramp can then be run comparatively flat and thus gentle on the actuator, without causing temporal fluctuations and thus energy fluctuations in the laser pulse. The protection of the actuator can be further improved by ensuring that the transitions from dithering to linear movement and vice versa are as harmonious as possible in order to avoid large accelerations.In an alternative embodiment of the method, instead of a linear movement (ramp travel), the dithering can be retained in the triggering phase. Ideally, the dither parameters are adjusted before the triggering phase in such a way that the dither movement is at an inflection point at the switching time in the triggering phase. At the inflection points, a harmonic movement is approximately linear for a short time. In addition, the first derivative with respect to time is greatest, which maximizes the resonance signal sharpness. If necessary due to the change in resonator length, the dither amplitude can also be briefly increased in the triggering phase in order to improve the steepness at the inflection point and thus the signal sharpness. The steeper section achieved in this way (at the inflection point) is uncritical compared to a correspondingly steep ramp in the ramp-fire method because the accelerations for the actuator are smaller due to the harmonic movement.

[0025] The proposed laser arrangement for carrying out the method accordingly comprises a laser resonator with an active medium and active Q-switch, as well as an actuator by means of which the resonator length can be varied in the range of a few meters by moving one of the mirrors of the laser resonator. This is preferably a piezo actuator. If required, further optical elements can of course also be arranged in the laser resonator. Furthermore, the laser arrangement comprises a narrow-band seed laser, the laser radiation from which is coupled into the laser resonator. Part of the radiation from the seed laser circulating in the resonator is coupled out again and directed onto a suitable photodetector. For this purpose, corresponding coupling and decoupling elements can also be arranged in the laser resonator if the coupling and decoupling does not take place via one of the mirrors.The arrangement comprises a control device that receives the signal detected by the photodetector and, according to the proposed method, controls the actuator during the control periods to generate a dither movement of the respective mirror and control it to an approximately constant temporal average of the optical resonator length, and during the triggering phases to change the resonator length to the resonance value. Furthermore, this control device controls the active Q-switch accordingly for switching when the resonance value is reached.

[0026] The proposed method achieves lower loading of the element that changes the resonator length in a narrowband or monofrequency laser compared to the state of the art cited in the introduction, while maintaining high frequency stability. This significantly reduces the failure probability of frequency-stable narrowband, Q-switched lasers, as the element that changes the resonator length is subjected to considerably less loading than with previously established methods. The proposed method shows its advantages particularly in applications in which a piezo actuator is used as the element that changes the resonator length and this element can only be replaced with great effort or not at all. Examples of such applications are aircraft or satellite-based laser systems, for example for LIDAR applications. The costs of a flight measurement campaign aborted due to a piezo actuator failure are very high.In a satellite mission, replacing the piezo actuator is virtually impossible. This also means that the development and qualification costs for a piezo actuator used in a satellite mission can be significantly reduced, as the requirements placed on it are considerably lower due to the lower load caused by the proposed method. The method can generally be used advantageously for all applications that require high frequency stability of the emitted laser radiation in an unstable environment while at the same time placing low load on the element that changes the resonator length. With this method, the time of emission of the laser pulse is also known more precisely, as the resonator length is regulated accordingly between the triggering phases. This enables the synchronization of other devices belonging to the laser system that must have a fixed time reference to the laser pulse.

[0027] Short description of the drawings

[0028] The proposed method and the associated laser arrangement are briefly explained below using an exemplary embodiment in conjunction with the drawings. Herein:

[0029] Fig. 1 shows an exemplary structure of the proposed laser arrangement in a highly schematic representation; and Fig. 2 shows an example of the control of a piezo actuator for changing the resonator length according to the proposed method and an exemplary profile of the photodiode signal of the seed laser transmitted through the resonator during the control period.

[0030] Ways to implement the invention

[0031] Figure 1 shows a highly schematic representation of an example of the proposed laser arrangement for frequency stabilization of a narrowband, Q-switched laser. In this example, the laser whose frequency is to be stabilized has a linear laser resonator consisting of two mirrors 2 with an active medium 3 and a Q-switch 4. A Pockels cell, for example, can be used as the Q-switch 4. The pump arrangement for pumping the active medium 3 is not shown in Figure 1. Depending on the active medium, the pumping can be done optically, for example, using suitable pump diodes. The laser arrangement also has a seed laser 5, whose narrowband laser radiation is coupled into the laser resonator of the laser. The center wavelength of the narrowband seed laser radiation corresponds to the wavelength of the laser to be stabilized.The resonator length of the laser must be matched to the wavelength to be stabilized with regard to the resonance conditions. However, external influences, such as vibrations or thermal effects, change the resonator length of the laser. Therefore, suitable control of the resonator length must be carried out for stabilization. For this purpose, in the present example, one of the mirrors 2 is moved axially via a piezo actuator 6. In the present example, an optical isolator 7, for example a Faraday isolator, is arranged between the seed laser 5 and the laser in order to prevent feedback of the laser radiation emerging from the laser in the direction of the seed laser 5 into the seed laser 5. In addition, an λ / 4 plate 1 can be arranged on either side of the active medium 3 in order to avoid spatial hole burning.Via this optical isolator 7, a seed signal transmitted through the laser resonator of the laser is directed to an optical detector 8, for example a photodiode, which detects the temporal intensity profile of the seed laser radiation transmitted through the laser resonator.

[0032] In the proposed laser arrangement and the associated method, the piezo actuator 6 is harmonically controlled by a control device 9 in the period between two laser pulses, so that the resonator length is increased and decreased approximately harmonically. The value by which the resonator length varies, i.e. by which the harmonic movement occurs, is controlled during this time via the detection signal from the detector 8, which is fed to the control device 9, such that a fixed relationship occurs between the detection signal from the detector 8 and the harmonic movement of the resonator length. In this way, slow changes in the optical resonator length compared to the frequency of the harmonic movement can be compensated.In the region of laser pulse generation, i.e., in the triggering phase, the resonator length is then changed approximately linearly via the piezo actuator 6 in the present example to a value (and beyond) at which the resonator is in resonance with the wavelength of the seed laser—also taking into account any optical length changes during the switching process. Upon reaching this value, referred to in the present patent application as the resonance value, the Q-switch 4 is switched. This switching of the Q-switch 4 is also carried out via the control device 9.

[0033] Fig. 2 shows, in the upper part, an example of the voltage U applied to the piezo actuator 6 as a function of time, starting in the period between two laser pulses (control period) through the triggering phase to the subsequent control period for the next laser pulse. The harmonic curve of the voltage 10 leads to a corresponding variation in the resonator length. In the area of ​​the triggering of a laser pulse, a ramp 11, i.e. an almost linear increase in the voltage at the piezo actuator - corresponding to a linear change in the resonator length - is then applied and when the value corresponding to a resonance is reached (at time t p) a laser pulse is triggered within this ramp. Then, once a steady state has been reached, the system switches back to harmonic control. The lower part of the figure shows an example of a detection signal 12 that is recorded by the detector 8 of the laser arrangement. This wave-shaped detection signal obtained by the harmonic movement is used to regulate changes in the resonator length that are not due to the harmonic control of the piezo actuator. For this purpose, the center value of the voltage 10 at the piezo actuator, around which the harmonic variation occurs, is regulated in such a way that a fixed relationship is maintained between the detection signal 12 and the voltage 10 at the piezo actuator.

[0034] Due to the harmonic control between two laser pulses and the corresponding control of the resonator length, the load on the piezo actuator is relatively low in this case, and the state of the laser resonator at the beginning of the triggering phase is very well known. This makes it very easy to predict when the resonator will be resonant with the seed laser during the subsequent movement, especially the ramp movement. The good a priori knowledge of the resonance point allows for a flatter ramp, so that this movement can also be performed in a way that protects the actuator, without temporal fluctuations and thus energy fluctuations of the laser pulse.

[0035] Reference symbol list

[0036] 1 X / 4 plate 2 resonator mirrors

[0037] 3 active medium

[0038] 4 Q-switches

[0039] 5 seed lasers

[0040] 6 Piezo actuator 7 optical isolator

[0041] 8 optical detector

[0042] 9 Control device

[0043] 10 harmonic voltage signal

[0044] 11 ramp-shaped voltage signal 12 detection signal

[0045] 13 Polarization beam splitter t p Switching time

Claims

Patent claims 1 . Method for frequency stabilization of a narrowband laser having a laser resonator of variable optical resonator length with an active Q-switch ( 4 ) for generating laser pulses , in which - laser radiation from a seed laser ( 5 ) with a wavelength at which the laser pulses are to be generated is coupled into the laser resonator, - a temporal intensity profile of laser radiation of the seed laser (5) coupled out of the laser resonator is recorded and the optical resonator length of the laser resonator is controlled in a control period between two laser pulses on the basis of the simultaneously recorded temporal intensity profile by periodic variation and adaptation of an offset of the variation to an approximately constant temporal average, - the optical resonator length of the laser resonator is changed in each case in the temporal range of the generation of a laser pulse, starting from an initial value known by the preceding control, beyond a resonance value at which the laser resonator will be in resonance with the wavelength of the seed laser (5) after switching the Q-switch (4), and - the Q-switch ( 4 ) when this resonance value is reached to generate a laser pulse is switched on.

2. Method according to claim 1, characterized in that the control of the optical resonator length of the laser resonator in the control period to the approximately constant time average value is carried out by maintaining a fixed relationship between the periodic variation of the optical resonator length and the detected temporal intensity profile of the laser radiation coupled out of the laser resonator. 3 . Method according to claim 1 or 2, characterized in that the optical resonator length of the laser resonator is changed linearly from the initial value beyond the resonance value in the temporal range of the generation of a laser pulse. 4 . Method according to claim 1 or 2, characterized in that the optical resonator length of the laser resonator is changed in the temporal range of the generation of a laser pulse by the periodic variation from the initial value to the resonance value. 5 . Method according to claim 4 , characterized in that an amplitude of the periodic variation in the temporal range of the generation of a laser pulse is increased.

6. Method according to claim 4 or 5, characterized in that the periodic variation is harmonic or approximately harmonic and is selected such that the resonance value lies in the region of an inflection point of the harmonic or approximately harmonic variation.

7. Method according to one of claims 1 to 6, characterized in that the change in the optical resonator length of the laser resonator is carried out by displacement of a resonator mirror (2) with an actuator (6).

8. Method according to claim 7, characterized in that the change in the optical resonator length of the laser resonator is carried out by displacement of a resonator mirror (2) with a piezo actuator.

9. Method according to one of claims 1 to 6, characterized in that the change in the optical resonator length of the laser resonator is carried out by a phase shifter.

10. Method according to one of claims 1 to 9, characterized in that the laser is pumped in a pulsed manner, wherein the time range of the generation of a laser pulse a pumping phase and the control period corresponds to the period between the pumping phases.

11. Laser arrangement with at least - a laser resonator with an active medium (3) and an active Q-switch (4) and an actuator (6) via which a resonator length of the laser resonator can be changed by moving a mirror (2) of the laser resonator, - a seed laser (5) whose laser radiation is coupled into the laser resonator, - a photodetector (8) which detects laser radiation of the seed laser (5) coupled out of the laser resonator, and - a control device (9) to which the photodetector (8) is connected and which is used to control the actuator (6) and the Q-switch (4) is designed according to the method of one or more of the preceding claims.

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