EP3646418A2 - Système laser et procédé permettant de faire fonctionner un système laser - Google Patents

Système laser et procédé permettant de faire fonctionner un système laser

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Publication number
EP3646418A2
EP3646418A2 EP18740115.3A EP18740115A EP3646418A2 EP 3646418 A2 EP3646418 A2 EP 3646418A2 EP 18740115 A EP18740115 A EP 18740115A EP 3646418 A2 EP3646418 A2 EP 3646418A2
Authority
EP
European Patent Office
Prior art keywords
laser beam
laser
pulse
fiber amplifier
detector
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP18740115.3A
Other languages
German (de)
English (en)
Inventor
Christoph Bollig
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Abacus Laser GmbH
Original Assignee
Abacus Laser GmbH
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Abacus Laser GmbH filed Critical Abacus Laser GmbH
Publication of EP3646418A2 publication Critical patent/EP3646418A2/fr
Withdrawn legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/06754—Fibre amplifiers
    • H01S3/06758—Tandem amplifiers
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
    • G01S17/88—Lidar systems specially adapted for specific applications
    • G01S17/95—Lidar systems specially adapted for specific applications for meteorological use
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
    • G01S7/481—Constructional features, e.g. arrangements of optical elements
    • G01S7/4814—Constructional features, e.g. arrangements of optical elements of transmitters alone
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
    • G01S7/481—Constructional features, e.g. arrangements of optical elements
    • G01S7/4816—Constructional features, e.g. arrangements of optical elements of receivers alone
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
    • G01S7/481—Constructional features, e.g. arrangements of optical elements
    • G01S7/4818—Constructional features, e.g. arrangements of optical elements using optical fibres
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
    • G01S7/483—Details of pulse systems
    • G01S7/484—Transmitters
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
    • G01S7/483—Details of pulse systems
    • G01S7/486—Receivers
    • G01S7/4868—Controlling received signal intensity or exposure of sensor
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
    • G01S7/497—Means for monitoring or calibrating
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/10007—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers
    • H01S3/10015—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers by monitoring or controlling, e.g. attenuating, the input signal
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/10007—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers
    • H01S3/10023—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers by functional association of additional optical elements, e.g. filters, gratings, reflectors
    • H01S3/1003—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers by functional association of additional optical elements, e.g. filters, gratings, reflectors tunable optical elements, e.g. acousto-optic filters, tunable gratings
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/13—Stabilisation of laser output parameters, e.g. frequency or amplitude
    • H01S3/1301—Stabilisation of laser output parameters, e.g. frequency or amplitude in optical amplifiers
    • H01S3/13013—Stabilisation of laser output parameters, e.g. frequency or amplitude in optical amplifiers by controlling the optical pumping
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S2301/00—Functional characteristics
    • H01S2301/03—Suppression of nonlinear conversion, e.g. specific design to suppress for example stimulated brillouin scattering [SBS], mainly in optical fibres in combination with multimode pumping
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/0014—Monitoring arrangements not otherwise provided for
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
    • H01S3/0085—Modulating the output, i.e. the laser beam is modulated outside the laser cavity
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/10038—Amplitude control
    • H01S3/10046—Pulse repetition rate control
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/10069—Memorized or pre-programmed characteristics, e.g. look-up table [LUT]
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/13—Stabilisation of laser output parameters, e.g. frequency or amplitude
    • H01S3/1305—Feedback control systems
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/13—Stabilisation of laser output parameters, e.g. frequency or amplitude
    • H01S3/1306—Stabilisation of the amplitude
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
    • H01S3/16—Solid materials
    • H01S3/1601—Solid materials characterised by an active (lasing) ion
    • H01S3/1603—Solid materials characterised by an active (lasing) ion rare earth
    • H01S3/1608—Solid materials characterised by an active (lasing) ion rare earth erbium
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
    • Y02A90/00—Technologies having an indirect contribution to adaptation to climate change
    • Y02A90/10—Information and communication technologies [ICT] supporting adaptation to climate change, e.g. for weather forecasting or climate simulation

Definitions

  • the invention relates to a laser arrangement with at least one optical fiber amplifier and to a method for operating a 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).
  • SBS stimulated Brillouin scattering
  • the incident and scattered light produce a beat whose frequency exactly matches the frequency of the participating sound wave.
  • 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 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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 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.
  • 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.
  • the laser-specific parameter is at least one value selected from the group of pulse energy, pulse shape, radiation power and / or frequency.
  • such a controlled operation is realized on the basis of at least one of the aforementioned parameters.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • a backscattering of the light within the laser arrangement is measured by means of the detector.
  • 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.
  • 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.
  • 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.
  • the measurement takes place behind the last amplifier, provided that a plurality of fiber amplifiers are provided in the laser arrangement.
  • 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.
  • 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.
  • 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.
  • 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.
  • pulsed fiber amplifiers are much more limited in pulse energy than non-linear effects, such as stimulated Brillouin scattering, than in average power.
  • non-linear effects such as stimulated Brillouin scattering
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • the power of the fiber amplifier is changed by a change in the power of a pump laser of the fiber amplifier.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • Fig. 1 Schematic representation of a basic embodiment of a
  • Figure 2 Schematic representation of the execution of a coherent
  • Fig. 4 Schematic representation of the execution of a coherent
  • Figure 5 Schematic representation of the execution of a coherent
  • Fig. 6 Schematic representation of the execution of a coherent
  • FIG. 7 Graphical representation of measured pulse shapes
  • Fig. 8 Representation of recorded signals for the signal strength at a
  • Fig. 9 Representation of recorded signals for the wind speed at a pulse repetition rate of 20 kHz with noise caused by clouds.
  • FIG. 1 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.
  • 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.
  • EDFA erbium doped fiber amplifiers
  • the circulator 7 is typically a fiber optic circulator, but may also be embodied as a fiber optic polarizer with an additional ⁇ / 4 plate or as a free jet polarizer with an additional ⁇ / 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 ⁇ , which corresponds to a pulse repetition rate of 10 kHz and a pulse length of 500 ns.
  • no power is extracted from a fiber amplifier 5, 6.
  • 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.
  • 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.
  • 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.
  • 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.
  • the preset pulse shape generally only adapts to a very specific operating state with regard to pulse repetition rate, pulse length and power.
  • 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.
  • 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.
  • 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.
  • SBS stimulated Brillouin scattering
  • FIGs 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.
  • 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.
  • 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.
  • an open detector 16 is preferably placed, on which the edge of the beam falls.
  • 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.
  • the curve 22 shown in FIG. 7 shows a typical signal of the detector 16, which was recorded in uncontrolled mode in this case.
  • 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.
  • a common detector 16 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.
  • the control and evaluation unit 15 controls the time profile of the transmission of the optical switch 4.
  • 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.
  • 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.
  • 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.
  • 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.
  • EDFA Erbium Doped Fiber Amplifiers
  • 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.
  • the detector 16 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.
  • 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.
  • 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.
  • FIGS. 8 and 9 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.
  • 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.
  • a minimum distance between pulses 7 of 20 s results.
  • 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.
  • 7 has increased from 20 s to 30 s.
  • the signal of the cloud is thus "smeared out” over the range of 0.5 to 2 km.
  • 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).
  • SBS stimulated Brillouin scattering
  • 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.
  • the repetition rate from measurement period to measurement period.
  • the cloud is recognizable as a signal, but at a different apparent distance.
  • the spectra can be corrected accordingly.
  • 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.
  • 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.
  • 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.
  • the control and evaluation unit 15 varies the pulse repetition rate (PRF), as described in the above example.
  • PRF pulse repetition rate
  • 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.
  • the distance after each pulse is shortened by 5 ns, so that the PRF continuously increases again from 33.3 kHz to 50 kHz.
  • 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.
  • 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.

Abstract

L'invention concerne un système laser comportant au moins un amplificateur de fibre optique (5, 6), ainsi qu'un procédé permettant de faire fonctionner ledit système laser, selon lequel un rayonnement laser pulsé est produit au moyen d'une source lumineuse (1), le rayonnement laser est envoyé à un amplificateur de fibre optique (5, 6), au moins une caractéristique du rayonnement laser est détectée au moyen d'un détecteur (11, 13, 16) et transmise à une unité de commande (15), et selon lequel l'unité de commande (15) produit, sur la base de la caractéristique du rayonnement laser détectée par le détecteur (11, 13, 16), un signal de commande qui est transmis à l'amplificateur de fibre optique (5, 6) pour agir sur l'amplification de l'énergie d'impulsion du rayonnement laser produite par l'amplificateur de fibre optique (5, 6), ou à un commutateur optique (4) pour modifier une forme d'impulsion. La solution technique selon l'invention est caractérisée en ce qu'une valeur absolue ou une modification d'une énergie d'impulsion, d'une forme d'impulsion et/ou d'une fréquence du rayonnement laser est détectée par le détecteur (11, 13, 16) en tant que caractéristique, et en ce que l'unité de commande (15) produit le signal de commande en prenant en considération une comparaison entre la valeur détectée et une valeur caractéristique de modification de l'énergie d'impulsion, de la forme d'impulsion et/ou de la fréquence du rayonnement laser, une diffusion Brillouin stimulée (SBS) se produisant dans le système laser. (Fig. 3)
EP18740115.3A 2017-06-25 2018-06-25 Système laser et procédé permettant de faire fonctionner un système laser Withdrawn EP3646418A2 (fr)

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US7539231B1 (en) * 2005-07-15 2009-05-26 Lockheed Martin Corporation Apparatus and method for generating controlled-linewidth laser-seed-signals for high-powered fiber-laser amplifier systems
WO2007132182A2 (fr) * 2006-05-11 2007-11-22 Spi Lasers Uk Limited Appareil de rayonnement optique
US8964801B2 (en) * 2009-06-11 2015-02-24 Esi-Pyrophotonics Lasers, Inc. Method and system for stable and tunable high power pulsed laser system
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