EP3646418A2

EP3646418A2 - Laser arrangement and method for operating a laser arrangement

Info

Publication number: EP3646418A2
Application number: EP18740115.3A
Authority: EP
Inventors: Christoph Bollig
Original assignee: Abacus Laser GmbH
Current assignee: Abacus Laser GmbH
Priority date: 2017-06-25
Filing date: 2018-06-25
Publication date: 2020-05-06
Also published as: WO2019002229A3; WO2019002229A2

Abstract

The invention relates to a laser arrangement comprising at least one optical fibre amplifier (5, 6) and to a method for operating said type of laser arrangement in which a pulsed laser beam is generated using a light source (1), the laser beam is sent to an optical fibre amplifier (5, 6), at least one characteristic of the laser beam is detected by the detector (11, 13, 16) and is transmitted to a control unit (15), and in which the control unit (15) generates a control signal based on the characteristic of the laser beam detected by the detector (11, 13, 16), the control signal is transmitted to the fibre amplifier (5, 6) for reacting on the amplification of the pulse energy of the laser beam generated by the fibre amplifier (5, 6), or to an optical switch (4) for modifying a pulse form. The technical solution described is characterized in that an absolute value or a change of a pulse energy, pulse form and/or frequency of the laser beam is detected by the detector (11, 13, 16) as a characteristic, and that the control unit (15) produces a control signal by taking into account a comparison of the detected value with a characteristic value of the pulse energy, pulse form, frequency and/or frequency modification of the laser beam, in which a stimulated Brillouin scattering (20) (SBS) is generated in the laser arrangement.

Description

Laser arrangement and method for operating a laser arrangement

The invention relates to a laser arrangement with at least one optical fiber amplifier and to a method for operating a laser arrangement. With the described laser arrangement, a pulsed laser beam is generated which can be used for different technical applications, in particular in a lidar (English: light detection and ranging).

From the prior art, different laser arrangements are known, which are used in various technical fields of application. Often, such laser arrays have optical fiber amplifiers to raise the radiant power of the laser beams generated by the laser light source to the required value. Generic laser arrays are widely used in lidars, which are systems that use laser beams for optical distance and velocity measurement and for remote measurement of atmospheric parameters.

Depending on the application, laser beams of different frequency and power are generated by laser arrangements. Often, laser radiation with a narrow optical frequency bandwidth and high peak power is needed. In many cases, there is a risk that stimulated Brillouin scattering (SBS) will occur in the light-conducting components of a laser arrangement. In stimulated Brillouin scattering, the incident and scattered light produce a beat whose frequency exactly matches the frequency of the participating sound wave. Through electro-restrictions, the vibration frequency can increase the intensity of the sound waves. This in turn increases the intensity of the scattered light and the positive feedback causes an exponential increase in the backscattered light.

Stimulated Brillouin scattering is a non-linear effect in which a high frequency acoustic wave forms in the fiber and scatters back a portion of the light. The laser beam or the pulse of a laser beam thereby deforms and becomes irregular, the pulse power fluctuates, the frequency bandwidth of the pulse is increased and the backscattered light can cause damage in the laser arrangement.

The power threshold at which SBS occurs, the so-called SBS threshold, is inversely proportional to the length of the optical fiber through which the light passes and proportional to the mode diameter of the light in the fiber.

When a laser array is required for generating pulsed laser beams for wind lidar applications, laser beams with a narrow optical frequency bandwidth are required needed. In this case, stimulated Brillouin scattering can be avoided only by using short fibers, large diameter fibers, or limiting optical power. In particular, since the reinforcing fiber in the last stage of a fiber amplifier requires a certain length for efficient operation, it is not possible to make the fibers arbitrarily short. In addition, the fiber diameter can not be made arbitrarily large, otherwise reduce the beam quality in an inadmissible manner. For the above reasons, the optical power that can be extracted from the fiber amplifier is regularly limited. For a laser arrangement, this means that the radiation power is limited to a maximum value, and since the pulse length is determined by the requirements of the spatial resolution when using pulsed laser beams, a limitation of the peak power also means a limitation of the pulse energy, which can be achieved ,

In existing systems, it is common to set the energy per pulse so that the radiant power is below the SBS threshold. However, since both the efficiency of the fiber amplifier and the SBS threshold changes, such as temperature, during the service life of the laser array, accurate adjustment is difficult and not reliable in the long term. In addition, a variation of the operating parameters is problematic, since this requires a new setting.

Furthermore, DE 10 2012 017 363 A1 discloses a laser arrangement for suppressing stimulated Brillouin scattering. The technical solution described is based on the fact that in a cw laser, a polarization controller with a birefringent delay element is used to adjust the radiation power of a laser beam. The laser beam generated in a laser light source is directed to a birefringent element, the laser beam being emitted with a delay between two orthogonal axes from the birefringent element. A polarization controller receives the laser beam from the birefringent element and emits it with a desired polarization. Subsequently, a fiber amplifier amplifies the laser beam and sends it to a compensating birefringent element which almost eliminates the transmission delay between the two axes of the laser beam and emits an output beam whose polarization is detected by a polarization detector giving feedback to the polarization controller it is ensured that the polarization of the output beam is approximately equal to a desired output polarization. With the aid of the described technical solution, a polarization splitting and an associated splitting of a laser beam into two sub-beams, the radiation power is below the SBS threshold, so that taking into account both partial beams, a higher overall optical power than with the total laser beam before the division can be achieved.

Based on the known from the prior art laser arrangements with fiber amplifier and the problems described above, the invention is based on the object to provide a laser array with fiber amplifier, with which a pulsed laser beam in the respectively required frequency band can be provided The performance of the individual pulses should be kept at a level that is almost maximum and at the same time the occurrence of stimulated Brillouin scattering is reliably avoided. At the same time it should be ensured that the system can be operated for a long time without having to change settings, in particular the system should be independent of temperature changes and material aging. The technical solution to be specified should be able to be implemented with comparatively simple technical components and at the same time ensure economic operation of a corresponding laser arrangement. Another aspect of the invention is to develop a laser arrangement such that the pulse shape of a pulsed laser beam can be adjusted as needed. Here, an adjustment or regulation of the pulse shape in response to a predetermined, for a specific application as at least almost optimally regarded pulse shape with high quality and at the same time done comparatively quickly.

The above object is achieved by laser arrangement according to claims 1 and 16 and a method according to claim 19. Advantageous embodiments of the invention are the subject of the dependent claims and will be explained in more detail in the following description with partial reference to the figures.

The invention relates to a laser arrangement with at least one optical fiber amplifier, which is passed via a light source for generating a pulsed laser beam, via an optical waveguide through which the laser beam from the light source to the optical fiber amplifier and at least one detector for detecting a property of the laser beam features. According to the invention, the laser arrangement has been developed in such a way that the detector generates a measured value on the basis of the detected characteristic of the laser beam and transmits it to a control unit which is set up to generate a control signal taking into account the detected measured value and a laser-beam-specific parameter and to the fiber amplifier and / or to transmit an optical switch by which a pulse energy of the laser beam is changed while passing through the fiber amplifier and / or a pulse shape of the laser beam as it passes through the optical switch on the basis of the control signal. Preferably, the laser-specific parameter is at least one value selected from the group of pulse energy, pulse shape, radiation power and / or frequency. Advantageously, such a controlled operation is realized on the basis of at least one of the aforementioned parameters.

According to a particular embodiment of the invention, the laser-specific parameter has at least one value of a property of the laser beam, which is characteristic when a stimulated Brillouin scattering occurs. Preferably, particular values or manifestations of a pulse energy, pulse shape, frequency, frequency change and / or power, which are characteristic of the occurrence of a stimulated Brillouin scattering in the laser arrangement, are taken into account in the generation of the control signal.

The essential idea of the invention is therefore to provide a regulation within the laser arrangement such that the light source and the fiber amplifier of the laser arrangement are operated in such a way that stimulated Brillouin scattering does not occur within the laser arrangement. The advantage here is that depending on the, for example, in a data storage attached to the control unit laser beam specific limit, the pulse energy and / or pulse shape are set such that just no or at least a very small stimulated Brillouin scattering occurs and the laser array thus very is operated close to the maximum level of performance.

In a first specific embodiment, it is provided that the control unit is adapted to control the control signal taking into account a comparison between the measured value generated by the detector by detecting a pulse shape of the laser beam and at least one value of a pulse shape characteristic of the occurrence of the stimulated Brillouin scattering is to produce. According to this technical embodiment, the potential risk of the occurrence of the stimulated Brillouin scattering is detected on the basis of a specific pulse shape and likewise the pulse shape is adjusted on the basis of a correspondingly generated control signal in such a way as to ensure that no or only a very small stimulated Brillouin Scattering occurs. In general, it is conceivable to detect the pulse power of the laser beam in addition to or as an alternative to the pulse shape and to set it with a corresponding limit value at the occurrence of which a stimulated Brillouin scattering is to be expected.

In general, different variants for controlling the light source are conceivable in connection with the laser arrangement according to the invention. On the one hand, the occurrence of stimulated Brillouin scattering can already be detected near the SBS threshold and on the basis of a suitable control signal, the pulse energy laser beam in the fiber amplifier are controlled to a value such that the fiber amplifier is working at the SBS threshold, so no or only a very small stimulated Brillouin scattering occurs. Alternatively or in addition, it is conceivable to set the pulse shape and / or the pulse energy of the respective laser beam to a predetermined value. In this case, not a characteristic value for achieving the SBS threshold, but a specific pulse shape and / or a specific value of the pulse energy is the command variable of a corresponding regulation. According to a specific development, it is provided that the pulse energy and another property simultaneously the laser beam, which is characteristic of the occurrence of the stimulated Brillouin scattering, are monitored and the fiber amplifier is controlled such that it either keeps the set pulse energy stable or amplifies the laser beam in such a way that stimulated Brillouin scattering does not occur. thus just below the SBS threshold.

In a particular embodiment of the invention, a backscattering of the light within the laser arrangement, in particular in an optical fiber and / or the fiber amplifier, is measured by means of the detector. Preferably, this measurement takes place behind the last amplifier stage of a laser arrangement, since here the beam power with respect to the laser arrangement assumes the highest value and the risk of the occurrence of the stimulated Brillouin scattering is greatest.

Preferably, a laser arrangement has at least two amplifier stages, wherein the choice of the number of amplifier stages depends on the required radiation power of the laser beam.

In a special way, the detection of the radiation power takes place in front of an amplifier stage, provided that several amplifier stages are provided before the last amplifier stage, backscattered light or its intensity is measured in an optical fiber and / or in the fiber amplifier. According to a particular embodiment, the measurement takes place behind the last amplifier, provided that a plurality of fiber amplifiers are provided in the laser arrangement.

In a particular embodiment, light which is conducted backwards through an optical fiber and / or a fiber amplifier, separated, which is preferably carried out using a fiber optic circulator. The separated light is then passed through an optical fiber to a detector where the optical power is converted to an electronic signal. The electronic signal is passed through an electrical connection to a control unit which, depending on the separated backscattered Light controls the amplifier stage such that a stimulated Brillouin scattering, which causes the backscatter of the light, at least largely avoided. A corresponding variation of the amplifier power is advantageously achieved in that the power of one or more pump lasers of the fiber amplifier is changed by a corresponding control. If a plurality of amplifier stages are provided in a laser arrangement, a corresponding regulation of the amplifier power can be realized at an amplifier stage, at at least two amplifier stages or at all amplifier stages.

By means of the above-described targeted activation of at least one amplifier stage of a laser arrangement designed according to the invention, it is possible to operate the respective fiber amplifier just below or only slightly above the SBS threshold.

In a further embodiment of the invention, it is provided that a frequency or a frequency change of the laser beam is detected with the aid of the detector, and with the aid of the control unit a control signal for controlling at least one fiber amplifier taking into account a frequency or a frequency change, from which the occurrence of a stimulated Brillouin Scattering is expected to be generated. It should be noted that the stimulated Brillouin scattering leads to a frequency shift of the backscattered light compared to the laser light guided in the normal direction by a few gigahertz. It is therefore alternatively or in addition to the previously described detection of the intensity of the backscattered light possible to use this frequency shift as a reference variable for controlling the gain of a gain stage.

Incidentally, stimulated Brillouin scattering also causes fluctuations in pulse power and pulse shape. For this reason, it is further conceivable to detect the pulse power and / or the pulse shape of the pulses generated by the laser arrangement and to compare these with corresponding characteristic values at which the occurrence of a stimulated Brillouin scattering is to be expected, and thus for to use the control or regulation of at least one fiber amplifier.

A further embodiment of the invention is based on the fact that the change in the frequency bandwidth of a pulsed laser beam is detected and a difference to a desired value, which is then the generation of a control variable and the control signal derived therefrom for adjusting the gain caused by at least one fiber amplifier determined. As the above discussion has shown, pulsed fiber amplifiers are much more limited in pulse energy than non-linear effects, such as stimulated Brillouin scattering, than in average power. The same applies in part to other lasers, for example pulsed diode lasers. Therefore, it is advantageous for a laser arrangement that generates a pulsed laser beam, such as is used in a lidar for wind measurement, to choose the repetition rate such that the distance between the pulses is sufficient, at least temporarily, just for the expected measurement distance. In some applications in wind energy, such as for positioning or for predictive control, only ranges up to 300 m are needed. Theoretically, a laser arrangement could be used to generate a laser beam with a frequency of up to 400 kHz. This value results from a circulation time of 2 s for the light at a measuring distance of 300 m plus one time reserve each for the pulse. Deviating from this, however, currently laser beams are used with a maximum frequency of 20 kHz, since at higher repetition rates, the strong signal from low clouds can lead to erroneous readings. At a repetition rate of 100 kHz, for example, a cloud with lower edge at a distance of 1.8 km would cause a signal which is not or only very difficult to distinguish in the evaluation of a signal from a distance of 0.3 km. This is the case since the light needs a transit time of 12 s for the distance of the object of 1, 8 km for the outward and the return journey, while after 10 s the next pulse has already been generated and therefore the signal appears as if it were at 2 s. Even at a repetition rate of 20 kHz, a cloud at a distance of 7.8 km could cause a similar disturbance, and at a repetition rate of 10 kHz, a cloud could be at a distance of 15.3 km. On the basis of the problems described above, it would thus be advantageous to design a laser arrangement with its control and evaluation unit in such a way that errors caused by reflections of the emitted laser beams on clouds are largely avoided or at least considerably reduced during wind measurements. Furthermore, the invention also relates to a laser arrangement with at least one optical fiber amplifier, with a light source for generating a pulsed laser beam whose beam power is at least temporarily increased by the at least one fiber amplifier, with an optical unit downstream of the fiber amplifier in the direction of the beam path, with which the laser beam is emitted into an environment and at least one of the environment by reflection of the emitted laser beam generated light beam is received and with a control and evaluation, in which determines a frequency shift of the reflected light beam relative to the emitted laser beam on the basis of a superposition of the reflected light beam with a reference laser beam and from the values of the frequency shift averaged over a measurement period, at least one velocity of an object moved in the environment is determined. According to the invention, such a laser arrangement has been further developed in that a pulse repetition rate of the laser beam is changed at least once within the measurement period. With the aid of the technical solution described above, it is possible in a comparatively simple manner to suppress interference signals caused by clouds in laser arrangements which are used for measurements in the atmosphere. Likewise, this technique can be used in other situations where high scattering exists behind an atmosphere.

The essential idea in this case is to vary the pulse repetition rate during a measurement period so that a signal caused by a cloud is washed out. The structure must not be changed compared to a commercially available laser arrangement in a lidar.

According to a particular embodiment it is provided that between the light source and the optical unit at least one detector is arranged, which is adapted to detect a pulse energy of the laser beam and transmit a measured value corresponding to the detected pulse energy to the control and evaluation unit and that they control and evaluation unit is designed to generate on the basis of the measured value, a control signal for changing the gain caused by the fiber amplifier beam power.

Preferably, the power of the fiber amplifier is changed by a change in the power of a pump laser of the fiber amplifier. In addition to a laser arrangement, the invention also relates to a method for operating a laser arrangement in which a pulsed laser beam is generated with a light source, the laser beam is fed to an optical fiber amplifier, detected by a detector at least one property of the laser beam and transmitted to a control unit and at in that the control unit generates a control signal on the basis of the property of the laser beam detected by the detector, which is transmitted to the fiber amplifier for influencing the amplification effected by the amplifier. According to the invention, the method has been developed such that the detector detects an absolute value or a change in a pulse energy, pulse shape and / or frequency of the laser beam as a property and the control unit, taking into account a comparison of the detected value with a characteristic value of the pulse energy, pulse shape, Frequency and / or frequency change of the laser beam, in which a stimulated Brillouin scattering of the laser array occurs, generates the control signal. In this case, the control signal is preferably generated in such a way that at least in a predetermined time interval during the operation no or only a small Brillouin scattering occurs in the laser assembly. Furthermore, it is advantageously conceivable for the control signal to be generated in such a way that the pulse energy, pulse shape and / or frequency of the laser beam leaving the fiber amplifier and / or of the laser beam generated by the light source travel at least within a predetermined time interval during the operation of the laser arrangement Assumes a value which is within a tolerance interval below the characteristic value of the pulse energy, pulse shape, frequency and / or frequency change of the laser beam at which stimulated Brillouin scattering occurs in the laser array. According to this embodiment, it is therefore conceivable to provide a distance to the so-called SBS threshold, so that a laser arrangement is operated just below or only slightly above the so-called SBS threshold.

In a preferred manner, it is provided that the detector detects an absolute value or a change in the pulse energy, pulse shape and / or frequency of the laser beam as a property and the control unit, taking into account the detected value, generates the control signal such that the light source and / or the fiber amplifier Laser beam with a predetermined pulse energy, pulse shape / and / or frequency leaves. In this embodiment, the regulation of the operation of a laser arrangement is not based on the consideration of the SBS threshold, but rather a pulse energy, pulse shape and / or frequency are used as setpoints for the operation of the laser arrangement and in particular for determining the gain caused by an amplifier stage.

In the following, the invention will be explained in more detail without limiting the general idea of the invention on the basis of exemplary embodiments with reference to the figures. Showing:

Fig. 1: Schematic representation of a basic embodiment of a

coherent laser arrangement;

Figure 2: Schematic representation of the execution of a coherent

Laser arrangement for SBS control;

Figure 3: Schematic representation of the execution of a coherent

Laser arrangement for controlling the pulse shape;

Fig. 4: Schematic representation of the execution of a coherent

Laser arrangement for controlling the pulse energy;

Figure 5: Schematic representation of the execution of a coherent

Laser arrangement for controlling the pulse shape and pulse energy;

Fig. 6: Schematic representation of the execution of a coherent

Laser arrangement for wind measurement; Fig. 7: Graphical representation of measured pulse shapes;

Fig. 8: Representation of recorded signals for the signal strength at a

Pulse repetition rate of 20 kHz with interference signals caused by clouds as well

Fig. 9 Representation of recorded signals for the wind speed at a pulse repetition rate of 20 kHz with noise caused by clouds.

A typical construction of a fiber amplifier coherent lidar, as known in the art and forming the basis for the embodiments of the invention described in further detail below, is shown in FIG. The light of a light source 1 designed as a master oscillator MO is directed with an optical waveguide 2, also referred to as an optical fiber, to a fiber coupler 3, where it is divided into two parts. In one part, an optical switch 4, which is typically designed as an acousto-optical modulator, AOM, a pulse is cut out and at the same time frequency-shifted. This can also happen in a double pass through the AOM. This low-energy pulse is amplified by a first and a second fiber amplifier 5, 6, which are usually erbium doped fiber amplifiers (EDFA) with two to three amplification stages, to form a pulse with the highest possible pulse energy. The light then passes through a circulator to separate the exiting light from the light backscattered from the environment.

The circulator 7 is typically a fiber optic circulator, but may also be embodied as a fiber optic polarizer with an additional λ / 4 plate or as a free jet polarizer with an additional λ / 4 plate. The emerging light is then collimated with an optical unit 8 to a laser beam or focused at a greater distance. Objects 9 in the environment, in particular aerosols 9 in the atmosphere, scatter a small portion back, which is focused by the optical unit 8 back into the fiber 2. This small part is directed by the circulator 8 to a fiber coupler 10, in which it is superimposed with a part of the reference light of the master oscillator 1. The superposition is detected in a detector 11 and converted into an electronic signal.

A fiber amplifier 5, 6 is typically pumped continuously, thus storing pump energy between pulses. This energy is extracted during the pulse. A typical time span between two pulses is 100 μβ, which corresponds to a pulse repetition rate of 10 kHz and a pulse length of 500 ns. In the 99.5 s between two pulses, no power is extracted from a fiber amplifier 5, 6. In the 500 ns during the pulse, a large part of the previously stored energy is extracted. This has one Fiber amplifier 5, 6 at the beginning of the pulse a much higher gain than at the end of the pulse. This causes the pulse to become unbalanced, with much higher power at the beginning of the pulse than at the end. In the prior art, this problem is met with the optical switch 4, which cuts out a part of the pulse with a specific pulse shape, which has a low power at the beginning of the pulse and a high at the end.

Fig. 7 shows an example of a measured pulse. The curve 20 represents the signal with which the transmission of the optical switch 4 was controlled. The curve 21 shows the power curve of the pulse after the preamplifier 5 and the curve 22 shows the power curve of the pulse at the output of the laser array, which was measured near the optical unit 8. Further, the curve 23 represents the measured power of the light propagating backward through the fiber 2 between 6 and 5. There is a very small proportion of stimulated Brillouin scattering to detect, which is still harmless. The time delay between the control signal 20 and the measured signals 21, 22 and 23 results from a time delay of a few seconds in the optical switch 4, which in this case is an acousto-optic modulator.

It can be clearly seen in FIG. 7 that the pulse 22 has the highest power shortly after the beginning, although the control signal 20 is set in such a way that the optical switch 4 has the highest transmission toward the end. With a more complex form of the control signal 20, the pulse could be adjusted somewhat more symmetrically. Since, however, the gain can change by more than one order of magnitude during a pulse, it is difficult to achieve a good compensation with a permanently set form of the control signal 20. In addition, the temporal gain profile changes with the operating parameters, in particular with the temperature and the power of the pump laser, so that fluctuations in the pulse shape can occur.

Furthermore, the preset pulse shape generally only adapts to a very specific operating state with regard to pulse repetition rate, pulse length and power.

According to the invention, the performance of a laser arrangement for generating pulsed laser beams is increased by specifically setting a specific pulse shape and / or operating the laser arrangement, in particular the at least one fiber amplifier, just below or only slightly above the SBS threshold, so at least almost no stimulated Brillouin scattering occurs. One possible arrangement for the detection of stimulated Brillouin scattering near the SBS threshold is shown in FIG. The stimulated Brillouin scattering (SBS) leads to a backscatter of the amplified light. The detection can therefore be carried out as shown in FIG. 2 by measuring how much light is scattered back in the fiber 2. Since the power after the last amplifier stage 6 is the highest, it makes sense to measure the backscattered by SBS by the last amplifier stage 6 radiation or the corresponding radiation power. This can be done by separating 6 light, which is passed backwards through the fiber 2, before the last amplifier stage, here with a fiber optic circulator 12th

The light is then passed through an optical fiber 2 to a detector 13 where the optical power is converted to an electronic signal, typically the current being proportional to the optical power. The electronic signal is passed through an electrical connection 14 to a control and evaluation unit 15. The control and evaluation unit 15 controls the gain of the last amplifier stage 6 in such a way that only a small part SBS is produced which is harmless. In this way, a closed-loop control is generated in which the optical power of the backscattered light, at which no or only little stimulated Brillouin scattering occurs, represents the reference variable.

This regulation can be implemented, for example, by controlling the power of one or more pump lasers of the last stage of the amplifier 6 as required. Alternatively, it is conceivable to regulate the gain of the amplifier 5 or both amplifiers 5, 6 accordingly. This will ensure that the fiber amplifier is operating at or just above the SBS threshold. For this purpose, the curve 23 shown in FIG. 7 shows a typical signal of the detector 13 at an advantageous operating point, in which only a very small proportion SBS is produced.

Besides the embodiment described above, it is conceivable to measure SBS in other ways or elsewhere. For example, it is possible to measure past the last amplifier stage 6, at certain points in the last amplifier stage 6 or in other amplifier stages 5.

Furthermore, it should be noted that stimulated Brillouin scattering (SBS) leads to a frequency shift of the backscattered light by a few GHz. It is therefore also possible to detect this frequency shift, or both the intensity and the frequency shift of the backscattered light.

In addition, since SBS results in fluctuations in the pulse power and the pulse shape, it is possible to detect SBS only by detecting the outgoing pulses, and to perform the control on the basis of this measurement, or to combine the information from the detection of the outgoing pulse with the detection of the backscattered light.

Incidentally, stimulated Brillouin scattering also leads to a frequency broadening of the pulse, so that SBS are also extracted in the signal of the overlay detector 11. Due to the back reflection at the optics during the pulse, light passes through the fiber 2 to this detector 1 1 and leads to the superposition with the coming of the light source 1 reference light. Figures 3, 4 and 5 show technical embodiments in which the pulse shape or the pulse energy of a laser beam is detected and based on these measurements, the central control and evaluation unit 15, the light source 1 and / or at least one existing fiber amplifier 5, 6 so controls, that the desired pulse shape and / or the desired pulse energy is kept stable. Here it makes sense, it makes sense to measure the power of the outgoing laser beam. A good position for this is at the edge of the beam in the beam path before or after the optical unit 8. There, an open detector 16 is preferably placed, on which the edge of the beam falls.

Since in a Gaussian beam with only one transverse mode, as is customary for use in coherent laser arrangements or lidar systems, the intensity is also proportional to the total power of the beam at the edge of the beam, in this case the intensity is measured and recorded there the basis of this measurement in the control and evaluation unit 15 determines the pulse shape. The detector 16 converts the measured intensity into an electronic signal, usually a current, which is passed through an electrical connection 14 to the control and evaluation unit 15. For this exemplary embodiment, the curve 22 shown in FIG. 7 shows a typical signal of the detector 16, which was recorded in uncontrolled mode in this case.

Alternatively, a portion of the light from the fiber 2 may also be coupled out to the fiber amplifier 6, for example between the fiber amplifier 6 and the circulator 7 or between the circulator 7 and the end of the fiber 2.

For the demand-controlled regulation of the pulse shape and / or the pulse energy, a common detector 16 will preferably be used, as described above. It is also conceivable to use separate detectors or a plurality of detectors which contribute together to the control.

The pulse shape can also be extracted from the signal of the overlay detector 1 1, since due to the reflection of the emitted light via the optical unit 8 during the pulse light passes through the fiber to the overlay detector 1 1. For control of the pulse shape, as shown in FIG. 3, the control and evaluation unit 15 controls the time profile of the transmission of the optical switch 4.

For controlling the pulse energy, as shown in FIG. 4, the control and evaluation unit 15 controls the gain of the last amplifier stage 6 or alternatively also the amplification of the other amplifier stage 5 or both amplifier stages 5, 6.

A control variant in which the control and evaluation unit 15 controls the timing of the transmission of the optical switch 4 and / or for controlling the pulse energy, the gain of the last amplifier stage 6 for controlling the pulse shape, is shown in Figure 5.

In all the control circuits described above, the pulse shape and / or the pulse energy of a single pulse can be determined or the pulse shape and / or pulse energy of several pulses can be averaged to correct the time course and the energy of subsequent pulses. A very particularly preferred embodiment of the invention is shown in FIG. 6. The light of the master oscillator MO, which operates continuously as light source 1 and has a wavelength of approximately 1550 nm and an optical bandwidth of <200 kHz, is conducted with an optical fiber 2 to a fiber coupler 3 divided there into two parts. The greater part, here about 90%, is guided with a fiber 2 to an optical switch 4, here an acousto-optic modulator (AOM). The AOM 4 cuts out a pulse whose length, shape and repetition rate are determined by the electronic control and evaluation unit 15. At the same time, the AOM shifts the frequency of the light by 80 MHz in this case.

This cut-out low-energy pulse is amplified by a first fiber amplifier 5, a fiber preamplifier, and a main fiber amplifier 6. Both amplifier stages 5, 6 are "Erbium Doped Fiber Amplifiers" (EDFA) in which an erbium-doped fiber is excited with a pump laser and amplifies the light in the region around 1550 nm, which is scattered back by stimulated Brillouin scattering in the main amplifier 6 or which is backscattered on the fiber 2 to the main amplifier 6 passes through the fiber optic circulator 12 to the detector 13, which detects the light with a good temporal resolution of about 10 ns The signal of the detector 13 is sent to the control and Evaluation unit 15 passed, which receives the signal and evaluates.

The light from the fiber amplifier 6 is passed through a fiber-optic circulator 7 and guided over a short piece of fiber to an optical unit 8 with a telescope. There, the exiting light is collimated with the optical unit 8 to form a laser beam. In the telescope, Near the optical unit 8 is the detector 16, which detects the power of the pulse with good temporal resolution of about 10 ns. The signal from the detector 16 is also sent to the control and evaluation unit 15, which receives and evaluates the signal.

Aerosols 9 in the atmosphere scatter a small portion of the light back, which is focused back into the fiber 2 by the optical unit 8. This small part is directed by the circulator 7 to a fiber coupler 10 and superimposed in the fiber coupler 10 with a part of the reference light of the light source 1. The superposition is detected in a superposition detector 11 and converted into an electronic signal. A data acquisition unit (not shown here) measures and analyzes this signal to determine the Doppler shift as a function of distance and calculate the wind speed therefrom. The control and evaluation unit 15 determines from the signals of the detectors 13 and 16 whether the system operates at the optimum operating parameters. Deviations from the optimum operating parameters are corrected by adjusting the time profile of the transmission of the AOM 4 and the gain of the main amplifier 6. These control circuits set the optimum pulse energy at which very little SBS occurs, and the optimum, symmetrical pulse shape is achieved.

The control and evaluation unit 15 includes an interface to a user program, via which the optimal pulse shape for the application as well as other operating parameters, such as pulse repetition rate, pulse length, and possibly other parameters, can be set.

Pulsed fiber amplifiers are limited in pulse energy by non-linear effects, in particular stimulated Brillouin scattering, less in average power. The same applies in part to other lasers, eg pulsed diode lasers. Therefore, especially for a laser arrangement providing pulsed laser beams for a wind lidar, it is attractive to choose the repetition rate such that the distance between the pulses is just sufficient for the expected measurement distance. In some applications in wind energy, such as for positioning or for predictive control, only ranges up to 300 m are needed. Theoretically, a laser arrangement could be used to generate a laser beam with a frequency of up to 400 kHz. This value results from a circulation time of 2 s for the light at a measuring distance of 300 m plus one time reserve each for the pulse. Deviating from this, however, currently laser beams are used with a maximum frequency of 20 kHz, since at higher repetition rates the strong signal from low clouds too can lead to erroneous readings. At a repetition rate of 100 kHz, for example, a cloud with lower edge at a distance of 1.8 km would cause a signal which is not or only very difficult to distinguish in the evaluation of a signal from a distance of 0.3 km. This is the case since the light needs a transit time of 12 s for the distance of the object of 1, 8 km for the outward and the return journey, while after 10 s the next pulse has already been generated and therefore the signal appears as if it were at 2 s. Even at a repetition rate of 20 kHz, a cloud at a distance of 7.8 km could cause a similar disturbance, and at a repetition rate of 10 kHz, a cloud could be at a distance of 15.3 km.

A typical example at 20 kHz pulse repetition rate can be seen in FIGS. 8 and 9. Between 8:15 and 8:27, the signal of a cloud appears to be moving between -2 km and 0 km distance to the location of the laser array, although it is actually at a distance of -9.5 km to -7 , 5 km distance was. From -8: 32 o'clock it is then seen at the actual distance from there 7.4 to 7.0 km, mind you with a short exception against -8: 37 o'clock, where the cloud again signals, which are a distance around -0 , 5 km represent, caused.

In order to advantageously avoid false signals caused by clouds, it is advantageously provided to design the control and evaluation unit and the other components of the laser arrangement, in particular the light source and the fiber amplifier, in such a way that the pulse repetition rate in FIG a measuring interval is varied. In this connection, a particularly suitable embodiment will be described below

Assuming that it is to be measured up to a distance of about 3 km from the laser array, a minimum distance between pulses 7 of 20 s results. Assuming that 2000 pulses are averaged, one possible approach is to extend the distance between pulses 7 by a fixed time A7 for each pulse. An example of a possible A7 is 5 ns. After 2000 pulses, 7 has increased from 20 s to 30 s. After 2000 pulses, the next averaging is started again with 7 = 20 s, or alternatively at 7 = 30 s backwards, the time after each pulse is reduced by 5 ns.

Assuming a cloud is 5 km away, which corresponds to a orbital period of 33.33 s, the signal appears at T = 20 μβ, as if the cloud were 2 km away, at T = 30 s, as if it were in 500 m distance. During a measurement interval or an averaging period of 2000 pulses, the signal of the cloud is thus "smeared out" over the range of 0.5 to 2 km. As the time between pulses changes, so does the energy stored in the fiber. This changes the pulse energy, which can lead to problems. This is especially true for stimulated Brillouin scattering (SBS).

In order to avoid this problem, the gain in the fiber amplifier can be adjusted in parallel by adjusting the power of the pump fibers of the fiber amplifier stages during the change of the repetition rate, in order to achieve as constant a pulse energy as possible.

The pump power can be adjusted both directly according to a stored calculation rule, as well as via a control with respect to an undershooting of the SBS threshold, as described above, as well as via the pulse energy control. Likewise, combinations of different control strategies are conceivable.

Alternatively or additionally, it is conceivable to vary the repetition rate from measurement period to measurement period. In this case, the cloud is recognizable as a signal, but at a different apparent distance. Thus, the spectra can be corrected accordingly.

Normally, the light source 1, which is designed as a master oscillator MO and has already been described above, has a fixed and very stable frequency of light. This is important during the time that a pulse is being generated and, subsequently, when the backscattered light of that pulse is being received. The same frequency is usually used for the next pulse, although this is not absolutely necessary. An advantageous solution consists in generating a small frequency jump in the light source, in particular the master laser, directly in front of a pulse.

If the desired range is 600 m, then the data must be recorded for a period of 4 s. If a frequency hopping of +2 MHz is generated in the master laser directly after these 4 s, then all the signals from a distance between 600 and 1200 m are shifted by +2 MHz. After the next pulse, a jump of - 3 MHz could occur. When collecting data for this pulse, all signals from a distance between 600 and 1200 m are shifted by -3 MHz, all signals from a distance between 1200 and 1800 m by +2 MHz. Then the frequency could then be shifted by +4 MHz and the shift continued accordingly.

Since in a lidar with fiber amplifier the signal is typically averaged out of a few thousand pulses, the variable frequency shift can cause the signal from Distances> 600 m in this example are widened very well over a large frequency range, so that there is only a very weak background.

This method can be used alternatively or in addition to the variable pulse repetition rate.

For implementing the aforementioned methods for suppressing signals generated by clouds during the measurement of atmospheric parameters, the arrangements shown in Figs. 1 and 3 to 6 may be used. In that regard, reference is expressly made at this point to the components shown in the cited figures and described above.

If the laser arrangements described above are operated in a mode for suppressing cloud signals, the control and evaluation unit 15 varies the pulse repetition rate (PRF), as described in the above example. Within 2,000 pulses, the distance between pulses is increased from 20 ns to 30 ns by increasing the gap by 5 ns for each pulse, thus reducing the PRF continuously from 50 kHz to 33.3 kHz. During the next 2,000 pulses, the distance after each pulse is shortened by 5 ns, so that the PRF continuously increases again from 33.3 kHz to 50 kHz.

At the same time, in the fiber preamplifier 5, the power of the pump laser is adjusted linearly with respect to the PRF in order to keep the energy of the pulses approximately constant at the output of the fiber preamplifier 5.

The control and evaluation unit 15 further determines from the signals of the detectors 13 and 16 the occurrence of SBS, the pulse shape and the pulse energy and adjusts the time profile of the transmission of the AOM 4 and the gain of the main amplifier 6 accordingly. In this case, a combination of control loops and the information of the varying PRF is used. As a result, a stable pulse energy and a stable pulse shape can be achieved despite the varying PRF.

LIST OF REFERENCE NUMBERS

1 light source

2 optical fibers

3 fiber couplers

4 optical switch

5 fiber preamplifiers

6 fiber main amplifiers

7 circulator

8 optical unit

9 objects in the area

10 fiber couplers

1 1 overlay detector

12 circulator

13 Detector for detecting backscattered light

14 electrical connection

15 control unit

16 Detector on the optical unit

Claims

claims

1. A laser arrangement with at least one optical fiber amplifier (5, 6), which has a light source (1) for generating a pulsed laser beam, via an optical waveguide (2) through which the laser beam from the light source (1) to the optical fiber amplifier (5, 6), and has at least one detector (1 1, 13, 16) for detecting a property of the laser beam,

characterized in that the detector (11, 13, 16) generates at least one measured value based on the detected characteristic of the laser beam and transmits it to a control unit (15) arranged to take into account the detected measured value and a laser-beam specific Parameters, to generate a control signal and to send to the fiber amplifier (5, 6) and / or an optical switch (4) through which a pulse energy and / or a pulse shape of the laser beam as it passes through the fiber amplifier (5, 6) and / or the optical switch (4) is changed on the basis of the control signal.

2. Laser arrangement according to claim 1,

characterized in that the laser-beam-specific parameter has at least one property of the laser beam selected from the group of pulse shape, pulse energy, radiation power and frequency.

3. Laser arrangement according to claim 1 or 2,

characterized in that the laser beam specific parameter has at least one value for a characteristic of the laser beam which is characteristic when a stimulated Brillouin scattering (SBS) occurs.

4. Laser arrangement according to one of claims 1 to 3,

characterized in that the control unit (15) is adapted to the control signal taking into account a comparison between the measured value generated by the detector (1 1, 13, 16) by detecting a pulse shape of the laser beam and at least one value of a pulse shape which is characteristic of the Occurrence of stimulated Brillouin scattering is to produce.

5. Laser arrangement according to one of claims 1 to 4,

characterized in that the control unit (15) is arranged to generate the control signal in consideration of a comparison between that generated by the detector (1 1, 13, 16) by detecting a pulse energy of the laser beam And at least one value of pulse energy characteristic of the occurrence of stimulated Brillouin scattering.

6. Laser arrangement according to one of claims 1 to 5,

characterized in that the detector (1 1, 13, 16) is arranged to generate a measured value taking into account an intensity of the backscattered light in the optical waveguide (2) and / or in the fiber amplifier (5, 6).

7. Laser arrangement according to claim 6,

characterized in that the control unit (15) is adapted to the control signal taking into account a comparison between the measured value generated by the detector (1 1, 13, 16) for an intensity in the optical waveguide (2) and / or in the fiber amplifier (5, 6) and a value of intensity of light backscattered in the optical fiber (2) and / or fiber amplifier (5, 6) characteristic of the occurrence of the stimulated Brillouin scattering.

8. Laser arrangement according to claim 6 or 7,

characterized in that at least two fiber amplifiers (5, 6) are provided and the detector (1 1, 13, 16) is arranged to generate a measurement signal taking into account an intensity of the backscattered in the last reached by the laser beam fiber amplifier (6) light.

9. Laser arrangement according to one of claims 6 to 8,

characterized in that in the optical waveguide (2) and / or in the fiber amplifier (5, 6) backscattered light is diverted into a separate measuring path in which the detector (13) is arranged.

10. Laser arrangement according to one of claims 1 to 9,

characterized in that the detector (1 1, 13, 16) is arranged to detect a frequency shift in the laser beam located in the optical waveguide (2) and / or in the fiber amplifier (5, 6).

1 1. Laser arrangement according to one of claims 1 to 10,

characterized in that the detector (1 1, 13, 16) is arranged to detect a bandwidth of the frequency of the in the optical waveguide (2) and / or in the fiber amplifier (5, 6) located laser beam.

12. Laser arrangement according to one of claims 1 to 1 1,

characterized in that the detector (1 1, 13, 16) is arranged to detect the at least one property of the laser beam in the edge region of the laser beam.

13. Laser arrangement according to one of claims 1 to 12,

characterized in that between the light source (1) and the fiber amplifier (5, 6) a fiber coupler (3) for dividing the laser beam is arranged in at least two partial beams.

14. Laser arrangement according to claim 13,

characterized in that at least one of the at least two partial beams is directed into the optical switch (4), which is adapted to reject a pulse and to shift a frequency of the cut-out pulse relative to a frequency of the at least one of the at least two partial beams.

15. Laser arrangement according to one of claims 13 to 15,

in that one of the two partial beams is fed to the at least one fiber amplifier (5, 6).

16. Laser arrangement according to one of claims 1 to 15,

characterized in that in the direction of the beam path behind the fiber amplifier (5, 6) at least one optical unit (8) is arranged, via which the laser beam is emitted into an environment.

17. Laser arrangement according to one of claims 1 to 16,

characterized in that the optical switch (4) has at least one acousto-optical modulator (AOM).

18. A laser arrangement with at least one optical fiber amplifier (5, 6), with a light source (1) for generating a pulsed laser beam whose pulse energy is at least temporarily increased by the at least one fiber amplifier (5, 6), with a fiber amplifier (5, 6) in the direction of the beam path downstream optical unit (8), with which the laser beam is emitted into an environment and at least one of the environment by reflection of the emitted laser beam generated light beam is received and with a control and evaluation unit (15), in which a frequency shift the reflected light beam relative to the emitted laser beam based on an overlay the reflected light beam is determined with a reference laser beam and at least one speed of an object moved in the surroundings is determined from the values of the frequency shift averaged over a measuring period,

characterized in that a pulse repetition rate of the laser beam is changed at least once within the measurement period.

Laser arrangement according to claim 18,

characterized in that between the light source (1) and the optical unit (8) at least one detector (1 1, 13, 16) is arranged, which is adapted to detect a pulse energy of the laser beam and a measured value corresponding to the detected pulse energy to the Control and evaluation unit (15) to transmit and that the control and evaluation unit (15) is designed to generate on the basis of the measured value, a control signal for changing the gain of the pulse energy caused by the fiber amplifier (5, 6).

Laser arrangement according to claim 19,

characterized in that on the basis of the control signal, a power of a pump laser of the fiber amplifier (5, 6) is changed.

Method for operating a laser arrangement, in which a pulsed laser beam is generated with a light source (1), the laser beam is fed to an optical fiber amplifier (5, 6), with a detector (1 1, 13, 16) detects at least one property of the laser beam and to a control unit (15) and in which the control unit (15) generates a control signal on the basis of the property of the laser beam detected by the detector (1 1, 13, 16), which is used to influence the signal passing through the fiber amplifier (5, 6 ) is transmitted amplification of the pulse energy of the laser beam to the fiber amplifier (5, 6) or to change a pulse shape to an optical switch (4),

characterized in that the detector (1 1, 13, 16) an absolute value or a change of a pulse energy, pulse shape and / or frequency of the laser beam is detected as a property and the control unit (15) taking into account a comparison of the detected value with a characteristic Value of the pulse energy, pulse shape, frequency and / or frequency change of the laser beam generates the control signal.

Method according to claim 21, characterized in that a characteristic value of a property of the laser beam is used at which stimulated Brillouin scattering (SBS) occurs in the laser array.

A method according to claim 21 or 22,

characterized in that the control signal is generated in such a way that, at least in a predetermined time interval during operation, no Brillouin scattering (SBS) occurs in the laser arrangement. 24. The method according to any one of claims 21 to 23,

characterized in that the control signal is generated in such a way that, at least in a predetermined time interval during operation of the laser arrangement the

Pulse energy, pulse shape and / or frequency of the laser beam generated by the light source assumes a value within a tolerance interval below the characteristic value of the pulse energy, pulse shape, frequency and / or frequency change of the laser beam, in which a stimulated Brillouin scattering (SBS) in the Laser arrangement occurs, is.

Method according to one of claims 21 to 24,

characterized in that the detector (1 1, 13, 16) detects an absolute value or a change of a pulse energy, pulse shape and / or frequency as a property of the laser beam and the control unit (15) generates the control signal in such a way taking account of the detected value, the light source (1) and / or the fiber amplifier (5, 6) leaves a laser beam with a predetermined pulse energy, pulse shape and / or frequency.