Method for frequency stabilization of a Q-switched narrowband laser and laser arrangement designed for this purpose

Abstract

In a method for frequency stabilization of a narrowband Q-switched laser, laser radiation from a seed laser with a wavelength at which the laser pulses are to be generated, and / or from a measurement laser with a different wavelength, is coupled into the laser resonator. The temporal intensity profile of the laser radiation exiting the laser resonator from the seed and / or measurement laser is recorded, and the optical resonator orbit length is varied in the period between two laser pulses and beyond, across several half or whole wavelengths of the seed and / or measurement laser, so that multiple resonance signals are detected during the measurement period. The time interval between a first and an nth resonance signal is measured and compared with a pre-determined predicted value.The change in the optical resonator length is adjusted immediately after reaching the nth resonance signal if the measured time interval deviates from the pre-determined predicted value, in such a way as to counteract the deviation. The Q-switch is then activated at a trigger time determined based on the predicted resonance times to generate a laser pulse. This method allows for very precise prediction of the emission time of the laser pulses and can therefore be used for synchronizing other devices or applications.

Description

Technical Field of ApplicationThe present invention relates to a method for the frequency stabilization of a narrowband laser, which has a laser resonator of a variable optical resonator length with an active Q-switch for generating laser pulses, in which laser radiation of 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.Q-switched lasers generally have a spectral bandwidth that includes many longitudinal modes of the laser resonator. Exceptions are e.g. laser beam sources which, due to a short resonator, 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 of an active Q-switch alone normally requires a resonator length which makes the above-mentioned condition unsatisifiable.If the application requires spectral narrowbandness, the laser must be manipulated such that fewer longitudinal modes are excited and amplified. This is often done by seeding the laser oscillator with a narrow band laser, the so-called seed laser. This seed laser provides an effective photon density of the preferred wavelength in the resonator of the laser to be frequency stabilized, which is several orders of magnitude above the natural, fluorescence-induced one and excites the resonator at this wavelength. If only the one longitudinal mode in the resonator is to be excited and amplified, which corresponds to the frequency of the seed laser, 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 in order to ensure single-frequency operation of the laser given a free spectral range and time bandwidth limit. A seed laser having a bandwidth that is below about 10-20% of the time-bandwidth limit of the oscillator is generally used. For stable single-frequency or narrow-band operation of a Q-switched laser at wavelengths in the VIS to NIR of about 400 nm to about 3 μm, the resonator length may therefore only fluctuate by fractions of a nanometer. Natural thermal drift, but in particular vibrations, however, already generate larger fluctuations. It is therefore necessary to actively control and thus stabilize the resonator length of the laser.Prior ArtA proven method for regulating the resonator length for a single-frequency or correspondingly narrow-band 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. One of the mirrors of the laser resonator is axially displaced, so that the resonator can be considered a scanning Fabry-Perot interferometer. This has proven to be effective, for example. Piezoelectric actuators, with which a stroke of a few μm is possible on a time scale of 10 μs to 500 μs. During seeding of the laser resonator, in the ramp-fire method, the resonator round trip length is changed approximately linearly by at least one wavelength and the transmissivity of the resonator at the wavelength of the seed laser is measured (qualitatively) using a photodiode. A typical transmission signal of a scanning Fabry-Perot interferometer for a mono-frequency signal results. In the transmission maxima, the resonator is resonant 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 length change must be compensated for, since the switched resonator is no longer resonant with the same geometric length as the resonant non-switched resonator. This is the case, for example, when using a Pockels cell as Q-switch, which consists of a birefringent crystal which changes the refractive index in one axis during switching. The compensation takes place with a fixed delay of the switching process to the previously detected resonance, in which the piezoelectric actuator geometrically displaces the corresponding change in the optical resonator length and compensates the change caused during the 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 likewise possible, for example based on acousto-optical or electro-optical modulators instead of piezo actuators.The ramp fire method has the great advantage that the time interval between the information about the resonance and the laser pulse is only very small. It is typically only a few nanoseconds to a few microseconds depending on the configuration. As a result, disturbing external influences have only a very small effect on the system, so that the ramp fire method generally enables a very stable mono-frequency operation. Furthermore, a fixed, approximately linear ramp is driven with the piezoelectric actuator, and the actuator position is not used as a controlled variable. As a result, the inertia of the actuator due to mass and its hysteresis do not represent a direct limitation for the control accuracy. However, the accuracy of the method increases with the slope of the ramp, i.e. the speed of the linear change in length by the piezoelectric actuator, since, with a higher speed, on the one hand the time period between the information about the resonance and the laser pulse is smaller and, on the other hand, the greater sharpness of the resonance signal makes the (electronic) detection more accurate. Here, however, the mass-induced inertia of the actuator indirectly represents a limit for the control accuracy because it limits the steepness of the ramp.However, the ramp fire method also has basic disadvantages. Thus, the exact time of the resonance and thus the exact time of the emission of the laser pulse are unknown a priori. It is variable about a free spectral range or about the corresponding time equivalent given by the travel speed of the piezoelectric actuator, typically a time period of 10 μs and greater. This leads, on the one hand, to the gain in the active medium generally being different for each pulse, since it changes on a μs time scale with constant pumping or non-pumping. The result is undesirable pulse-to-pulse energy fluctuations. On the other hand, the synchronization of other devices associated with the laser system is made considerably more difficult or even impossible, in particular if these other devices are triggered in a fixed time reference to the laser pulse, but must already be triggered by internal delays before the laser pulse.From the prior art, further methods of resonator length control are known which completely or partially obviate the above-described disadvantages of the ramp fire method.In the cavity dither method, the resonator length is continuously harmonically changed ("dithered") by the piezo movement. The control electronics determine, on the basis of the transmission signal, the piezo position in which the laser resonator is resonant with respect to the seed signal. The Q-switch of the laser resonator is triggered at a fixed, known time, wherein the resonance position (plus compensation offset) has to be approached immediately before the Q-switch. This works for a steady state. The information about the piezo position to be adjusted must be obtained before the beginning of the pumping operation, since the optical resonator length changes during the pumping, and a dither period is generally at least on the order of magnitude of the pumping time. The time duration between obtaining the information for setting the actuator during Q-switching and Q-switching is thus on the order of 100 μs or longer, which is why the cavity dither method under vibration is more error-prone than the ramp fire method.In the case of the Pound-Drever-Hall (PDH) and the Hansch-Couillaud (HC) methods, as described in Drever, R. W. P. 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 Hansch, 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)) is obtained for control almost in real time. The piezoelectric actuator is continuously readjusted here; its inertia represents the limiting factor for the band width of the adjustable disturbances. This is therefore in the order of magnitude of a few 100 Hz. Specifically in the pump phase, the optical resonator length changes dynamically within a time variable order of 100 μs, so that control frequencies significantly greater than 10 kHz would be required.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 piezoelectric actuator ramps up 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, typically a few 10 μs must be waited until the Q-switch is triggered, which makes the method more susceptible to disturbances than the classic ramp fire method.DE 10 2007 003 759 B4 describes a modified ramp fire method in which the time of the occurrence of the resonance during the driving of the piezo ramp is predicted with a lead time of a few 10 μs, so that other devices can be synchronized with the laser pulse. This makes use of the fact that for a fixed starting value of the actuator, a one-to-one association between the times at which the first resonance signal occurs and the further resonance signals can be assumed. This assignment is measured and stored in the electronics. In the case of a system that is undisturbed during the ramp travel time, the further times at which the resonator is resonant with the seed laser can thus be predicted on the basis of the time of the occurrence of the first resonant signal. It is then possible, when the first resonance signal occurs, to define the triggering time of the Q-switch on the basis of the predicted resonance times in the further ramp profile and at the same time to achieve good spectral stability from pulse to pulse. External disturbances which occur between the first resonance signal and the triggering time of the Q-switch cannot, however, be compensated for in this way.If the deflection of the actuator were to depend strictly linearly on the applied voltage, the detection of the temporal occurrence of two resonance signals in the absence of external disturbances could predict the temporal occurrence of each further resonance signal with any desired accuracy. Furthermore, it would be possible to reduce the above-described fluctuation of the laser pulse caused by thermal drift or vibration by the temporal equivalent of a free spectral range by stabilizing the temporal occurrence of the resonance signals by regulating the voltage offset of the piezoelectric actuator. Adjusting the temporal fluctuation of the laser pulse by the time equivalent of a free spectral width by a piezoactuator offset adjustment has the advantage that the triggering time of the Q-switch can be kept constant relative to the pumping time of the oscillator. As a result, pulse energy fluctuations, triggered by gains in the active medium that differ from pulse to pulse, are significantly reduced. However, the control frequency is equal to the pulse repetition rate here, so that only small interference bandwidths can be compensated. Furthermore, a different starting value of the piezoelectric deflection changes the deflection characteristic of the actuator via the voltage on account of the hysteresis, so that the association stored in the electronics no longer matches between the time of the occurrence of the first resonance and the times of the following resonances. The characteristic would have to be remeasured, but this is not possible since the voltage offset generally changes from pulse to pulse. One approach would be to extend the mapping to be learned by the voltage offset as another independent variable. Independently of this, the pulse energy fluctuations can also be reduced without voltage offset control, in that the amplification in the active medium is kept constant for the time interval of the possible pulse triggering by adjusting the pump rate. This is purchased at a somewhat lower efficiency and, in the case of a solid state laser, with a somewhat stronger thermal lens of the gain medium. Furthermore, the pulse triggering time fluctuates once again by a time equivalent of a free spectral range.The deflection characteristic of the actuator over the voltage is, however, not linear and additionally subject to hysteresis. In addition, both the piezoelectric driver (generally a more or less regulated voltage amplifier) and the actuator itself have an inertia with corresponding acceleration characteristics at the beginning and at the end of the ramp. Perfect linearization of the piezo actuator ramp could solve a major part of the problems outlined. Since the deflection of the piezoelectric actuator is linear to the charge flowing into the actuator, a piezoelectric driver, which generates a constant current, could linearize the ramp. The piezoelectric drivers available at present are either pure voltage amplifiers or they regulate the current or the charge flowing into the piezoelectric against a reference charge. However, the maximum control frequency of the available devices is about 10 kHz. With typical ramp durations of the order of 100 μs, the current and charge drivers also function only as voltage amplifiers.From C. Lemmerz et al., "Frequency and timing stability of an airborne injection-seeded Nd: YAG laser system for direct-detection wind lidar", Applied Optics, 2017, 56th Jg., No. 32, pp. 9057-1268 a method for the frequency stabilization of a narrow-band laser is known, in which the optical resonator round trip length of the laser resonator is changed in each case in a measurement period between two laser pulses and moreover over several whole or half wave lengths of a seed laser, so that during the measurement period several resonance signals are detected in the temporal intensity profile. In this case, a time interval between a first and an nth resonance signal is measured in the measurement period and compared with a prediction value determined in advance for the time interval between the first and the nth resonance signal. A Q-switch in the laser resonator is then switched to generate a laser pulse at a triggering time which is established on the basis of the prediction values for the occurrence of the resonance.The object of the present invention is to specify a method and an arrangement for the frequency stabilization of a Q-switched, narrowband or single-frequency laser, with which a high frequency stability and a higher accuracy of the prediction of the time of emission of the respective laser pulse are simultaneously achieved. A narrowband laser is understood to mean a laser with a laser emission whose bandwidth is less than the free spectral range of the laser resonator. For example, the laser emission can have a full width at half maximum of less than 50 MHz, preferably less than 20 MHz. The frequency stabilization takes place around the center frequency of this spectral emission.SUMMARY OF THE INVENTIONThe object is achieved with the method and the laser arrangement according to claims 1 and 8. Advantageous embodiments of the method and of the laser arrangement are the subject matter of the dependent patent claims or can be gathered from the following description and from the exemplary embodiment.In the proposed method, the laser radiation of a seed laser having a wavelength at which the laser pulses of the laser to be stabilized in frequency are to be generated, and / or of a measurement laser having a different, preferably shorter, wavelength is coupled into the laser resonator. A temporal intensity profile of laser radiation of the seed and / or measurement laser emerging from the laser resonator is detected and the optical resonator round trip length of the laser resonator is changed, i.e. enlarged or reduced, in each case in a time period between two laser pulses, referred to below as measurement time period, and moreover over a plurality of half or entire wavelengths of the seed and / or measurement laser, such that a plurality of resonance signals are detected in the temporal intensity profile during the measurement time period. The method is characterized in that, on the one hand, at least one time interval between a first and an nth resonance signal (n≥2) is measured in the measurement period and is compared with a reference or prediction value determined beforehand for the time interval between the first and the nth resonance signal. On the other hand, the change in the optical resonator length in the event of a deviation of the measured time interval from the previously determined prediction value, which exceeds a predefinable threshold value, is adapted immediately after reaching the nth resonance signal in such a way that it counteracts the deviation. For this purpose, the change in the optical resonator length takes place either at a speed which is regulated as a function of the deviation of the measured temporal distance from the previously determined prediction value, or an offset of the change in the optical resonator length is regulated as a function of the deviation of the measured temporal distance from the previously determined prediction value. The Q-switch is then switched at a triggering time determined in the further ramp course on the basis of the predicted resonance times or at a predefined time interval therefrom for generating a laser pulse. The predefined time interval corresponds to a time delay for compensating for changes in the optical resonator length caused by the circuit during switching of the Q-switch itself.In the proposed method, the change in the optical resonator length during operation of the laser is thus readjusted in such a way that forecast values determined in advance for the occurrence of the resonance are observed as accurately as possible. The optical resonator length is increased or decreased in the time period between two laser pulses and beyond, i.e., beyond the generation of the respective laser pulse. As in the method of DE 10 2007 003 759 B4, also referred to as a ramp delay fire method, explained in the introduction to the description, a ramp is run over several free spectral ranges and the triggering time of the Q-switch is determined on the basis of the predicted resonance times. In contrast to the procedure in the ramp delay fire method, however, the ramp parameters are readjusted here on the basis of the information about the resonant states. This compensates for errors and disturbances in the temporal pulse prediction.The method is described below with reference to the resonator length change with the aid of a piezoelectric actuator. This description can also be applied to other means of changing the optical resonator length, i.e. applies equally to the correction of non-linearities of other methods for varying the optical path length, both for the geometric change and also via the change in the refractive index, for example by phase shifters.In the proposed method, the respectively measured time intervals between a first resonance signal in the measurement period, preferably the resonance signal first occurring in the measurement period, and at least one further resonance signal are compared with prediction values determined in advance. These forecast values are determined beforehand using the laser used via one or more tests in which the laser is operated as free of interference as possible. In this case, the temporal occurrence of the resonance signals is measured as a function of the ramp parameters, in particular the starting time of the ramp or another time referenced at the starting time, ramp slope and ramp offset. The values measured in advance are stored in such a way that they are available to the laser or the control electronics during operation of the laser. These values serve as a reference for later operation. If necessary, measured values of the experiments carried out beforehand can also be processed before storage. For example, it is possible to use a switch. The average values can be formed over several measurements or the like. The time intervals between the resonance signals for the respective parameters of the change in the optical resonator length can be determined from the stored values; they are thus predicted points in time or predicted values for the occurrence of the resonances. During the later operation of the laser, the time intervals then measured in the measurement periods are compared with these prediction values determined in advance. In the event of a deviation of the temporal occurrence of a resonance signal from its predicted value, the ramp offset or the ramp slope is adapted in such a way that the deviation would be compensated for when the next resonance occurs. The change in the optical resonator length, the ramp, is thus readjusted. This regulation can be carried out using any desired regulation method. External disturbing influences and internal errors (e.g. discretizing errors) which cause temporal deviations of the occurrence of the resonances from the prediction values are thus compensated. The spectral properties, in this case in particular the frequency stability, of the laser pulses are thus improved.In the proposed method, during the operation of the laser, the time intervals between all resonances occurring in the measurement period or between a first and all subsequent resonances can be measured in the respective measurement period and in each case compared with forecast values determined in advance. A corresponding adaptation of the change in the optical resonator length takes place after each individual resonance if a corresponding deviation has been detected. However, the method can also be carried out in such a way that the change in the optical resonator length is not readjusted at each individual resonance. In the simplest case, a correction after the last resonance is sufficient before the triggering of the laser pulse. In this case, only the distance of this last resonance from one of the preceding resonances is then measured as the first resonance and compared with the prediction value determined accordingly in advance.The change or variation of the optical resonator length of the laser is preferably effected by mechanical movement of a mirror of the resonator. For this purpose, a piezoelectric actuator is preferably used. However, other methods for changing the optical resonator length are also possible, for example by varying the refractive index, for example by means of a Pockels cell. The change in the optical resonator length can be effected linearly, i.e. the resonator length is changed in a ramp-like manner as in the ramp-fire method described at the beginning.The proposed laser arrangement for carrying out the method accordingly comprises a laser resonator with an active medium and an active Q-switch and an element by means of which the optical resonator length can be changed in the range of some wavelengths, for example by moving one of the mirrors of the laser resonator with an actuator. Preferably, a piezoelectric actuator is used for this purpose. If necessary, further optical elements can also be arranged in the laser resonator. Furthermore, the laser arrangement comprises a narrowband seed and / or measurement laser, the laser radiation of which is coupled into the laser resonator. A portion of the radiation of the seed and / or measurement laser circulating in the resonator is decoupled again and directed onto a suitable photodetector. For this purpose, corresponding coupling-in and coupling-out elements can also be arranged in the laser resonator if the coupling-in and coupling-out is not effected via one of the mirrors. The arrangement comprises a control device which receives the signal detected by the photodetector and actuates the element, for example the actuator, according to the proposed method, in particular measures the time intervals between individual resonances, compares them with the stored prediction values and, if required or if there is a deviation beyond a defined threshold value (≥ 0), correspondingly adjusts the change in the optical resonator length. Furthermore, this control device controls the active Q-switch accordingly for switching.The method and the associated laser arrangement achieve an improvement in the accuracy of prognosis of the times of occurrence of the resonances and thus of the laser pulses in the ramp delay fire method. The advantage is an improvement of the spectral properties of the laser pulse, here in particular the stability of the center frequency from pulse to pulse with simultaneously accurate temporal relationship between any point in time within the ramp travel time and the emission of the laser pulse.The method can generally be used advantageously for all applications, preferably in harsh or harsh environments, in which narrowband pulsed laser beam sources are required, the occurrence of which over time must be very accurately known, e.g. for synchronizing other devices or sequences. Furthermore, they are relevant for the stabilization of lengths both geometrically and temporally referenced by means of optical methods utilizing the optical path length and resonance.Brief Description of the DrawingsThe proposed method and the associated laser arrangement are briefly explained again below with reference to an exemplary embodiment in conjunction with the drawings. The following are shown here: FIG. 1 shows an exemplary design of the proposed laser arrangement in a highly schematic illustration; and FIG. 2 shows a schematic illustration of an example of the resonator length change according to the proposed method.WAYS OF CARRYING OUT THE INVENTIONFIG. 1 shows a highly schematic representation of an example of the proposed laser arrangement for the frequency stabilization of a narrowband, Q-switched laser. The laser to be stabilized in frequency in this example has a linear laser resonator made up of two mirrors 2 with an active medium 3 and a Q-switch 4. As Q-switch 4, for example, an AOM (acousto-optical modulator), e.g. a Pockels cell, an EOM (electro-optical modulator) or an EOM plus A / 4 plate, can be used. The pump arrangement for pumping the active medium 3 is not shown in FIG. 1. Depending on the active medium, the pumping can be carried out, for example, optically via suitable pumping diodes. The laser arrangement also has a seed laser 5, the narrowband laser radiation of which 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 tuned to this wavelength to be stabilized with respect to the resonance condition. However, external effects, for example vibrations or also thermal effects, alter the resonator length of the laser. Therefore, a suitable regulation of the resonator length must take place for stabilization. For this purpose, in the present example, one of the mirrors 2 is moved axially via a piezoelectric 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 avoid feedback into the seed laser 5 of the laser radiation exiting from the laser in the direction of the seed laser 5. In addition, a λ / 4 plate 1 can also be arranged on both sides of the active medium 3 in order to avoid spatial hole burning. A seed signal transmitted through the laser resonator of the laser is directed via a beam splitter onto an optical detector 8, for example a photodiode, which detects the temporal intensity profile of the seed laser radiation transmitted through the laser resonator. In addition to the seed laser 5, a measurement laser can also be arranged, for example, at the same position in order to couple a measurement laser beam into the resonator if the optical components in the resonator are also designed for its wavelength for resonance in the oscillator.In the proposed laser arrangement and the associated method, the piezoelectric actuator 6 in the present example is controlled via a control device 9 in the time interval between two laser pulses in such a way that it increases the resonator length approximately linearly over a plurality of free spectral ranges. As a result, a plurality of resonances occur in the transmitted seed signal which is detected by the detector 8.This is exemplified in the schematic illustration of FIG. 2. The illustration shows the ramp-shaped change in the resonator length in this case between two laser pulses 10, the occurrence of which is indicated in the figure over time. As a result of the change in the optical resonator length over a plurality of free spectral ranges, a plurality of resonances 11 occur within the measurement period, that is to say the period between the two laser pulses 10. The change ΔL of the resonator length takes place here approximately linearly in the form of the ramp 12, at least until the adaptation according to the proposed method. The ramp does not have to extend over the entire time period between two laser pulses, but can advantageously be started only at a shorter time interval before a laser pulse occurs. Thus, the ramp can start at a pulse period of, for example, 10 ms first 100-200 μs before the respective laser pulse. As in the ramp delay fire method, prediction values are determined beforehand for the parameters of the change in the resonator length, which prediction values represent the distance of the resonances 11 or resonance signals at the corresponding ramp parameters.In the present example, the process is carried out in the following manner. After the first resonance signal has been detected in the measurement period and the time of occurrence has been measured, an adjustable number of resonance signals is ignored, in the present example the second and the third resonance signals. The occurrence of the fourth resonance signal is then predicted on the basis of the prediction values determined in advance. These forecast values are stored and are only read out by the control electronics. The time of occurrence of this resonance signal, in the present example relative to the occurrence of the first resonance signal (time period Δt), is compared with its predicted value and the difference is formed. In the case of a corresponding difference, i.e. a deviation, in the present example the offset of the ramp 12 is adjusted by this difference, multiplied by an adjustable proportionality factor (p controller). The following resonance then occurs at a point in time which is substantially closer to the prediction value previously determined for this resonance. At this prediction time, the Q-switch is then controlled to generate the laser pulse. FIG. 2 shows the change in the ramp offset immediately after the deviation has been determined, i.e. after the corresponding resonance signal.In the present example, the fourth resonance signal occurs approximately 24 μs before the triggering time of the Q-switch. These 24 μs are caused by a fixed delay between the detection of the fourth signal and the offset reaction of the ramp (13 μs) and the time period caused by the time equivalent of the free spectral range until the next resonance (here approximately 11 μs). By omitting the second and third resonance, the ramp can be driven more steeply in this example without placing the time of the laser pulse closer than several 10 μs after the occurrence of the first resonance. Several free spectral ranges are then traversed, which requires ignoring some resonance signals. The advantage of this approach with a steeper ramp is that the resonance signals are sharper, facilitating resonance signal detection. The accuracy of the method may increase with the number of resonances on a ramp. With the method illustrated by way of example with reference to FIG. 2, the prediction accuracy of approximately 65 ns (according to the ramp delay fire method) could be improved to approximately 35 ns with vibration.List of reference characters1 λ / 4 plate 2 resonator mirror 3 active medium 4 Q-switch 5 seed laser 6 piezoactuator 7 optical isolator 8 optical detector 9 control device 10 laser pulse 11 resonance 12 ramp

Claims

Method for the frequency stabilization of a narrowband laser, which has a laser resonator of a variable optical resonator length with an active Q-switch (4) for generating laser pulses (10), in which - laser radiation of a seed laser (5) with a wavelength at which the laser pulses (10) are to be generated and / or of a measurement laser with a different wavelength is coupled into the laser resonator, - a temporal intensity profile of laser radiation of the seed and / or measurement laser (5) emerging from the laser resonator is detected, - the optical resonator circulation length of the laser resonator is changed in each case in a measurement period between two laser pulses (10) and moreover over a plurality of whole or half wavelengths of the seed and / or measurement laser (5), so that during the measurement period several resonance signals (11) are detected in the temporal intensity course, wherein - at least one time interval between a first and an nth resonance signal is measured in the measurement period and compared with a previously determined prediction value for the time interval between the first and the nth resonance signal, wherein n≥2, - the change of the optical resonator length is adapted to counteract the deviation in the case of a deviation of the measured time interval from the previously determined prediction value immediately after reaching the nth resonance signal, and - the Q-switch (4) is subsequently switched to generate a laser pulse (10) at a triggering time determined on the basis of the prediction values for the occurrence of the resonance or at a predetermined time interval thereto, wherein - the change of the optical resonator length takes place at a speed, which is controlled as a function of the deviation of the measured time interval from the previously determined predictive value, or an offset of the change in the optical resonator length is controlled as a function of the deviation of the measured time interval from the previously determined predictive value.Method according to Claim 1, characterized in that a plurality of time intervals between resonance signals are measured in the measurement period and are compared with previously determined prediction values for these time intervals, and in each case the change in the optical resonator length in the event of a deviation of the respectively measured time interval from the previously determined prediction value is adapted in such a way that it counteracts the deviation.Method according to Claim 1 or 2, characterized in that the change in the optical resonator length of the laser resonator is effected by displacement of a resonator mirror using an actuator.Method according to Claim 3, characterized in that the change in the optical resonator length of the laser resonator is effected by displacement of a resonator mirror (2) using a piezoactuator.Method according to Claim 1 or 2, characterized in that the change in the optical resonator length of the laser resonator is effected by a phase shifter.Method according to one of Claims 1 to 5, characterized in that the optical resonator length of the laser resonator is changed linearly or at least approximately linearly in the measurement period between two laser pulses (10) and moreover.Method according to one of Claims 1 to 6, characterized in that the laser is pumped in a pulsed manner, the measurement period corresponding approximately to one pumping phase.Laser arrangement having at least - a laser resonator with an active medium (3) and an active Q-switch (4) and an element (6) via which an optical resonator length of the laser resonator can be varied, - a seed and / or measurement laser (5) whose laser radiation is coupled into the laser resonator, - a photodetector (8) which detects laser radiation of the seed and / or measurement laser (5) coupled out of the laser resonator, and - a control device (9) to which the photodetector (8) is connected and which is designed for driving the element (6) and the Q-switch (4) according to the method of one or more of the preceding patent claims.

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