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    • 专利摘要:
    In one aspect, the present disclosure relates to a laser source for transmitting a group of pulses comprising: a primary laser source (22) adapted for emitting at least one primary laser pulse; at least one interferometer (24) adapted for forming, from said primary laser pulse, a plurality of secondary laser pulses, each interferometer comprising at least one delay line for temporally separating two secondary laser pulses, a delay of between 50 ps and 10 ns; and a single-mode amplifying optical fiber for receiving the secondary laser pulses, for outputting a group of spatially superimposed pulses.
    公开号:FR3063395A1
    申请号:FR1751653
    申请日:2017-02-28
    公开日:2018-08-31
    发明作者:Ammar Hideur；Adil HABOUCHA；Arnaud BULTEL；Said IDLAHCEN；Thomas GODIN
    申请人:Centre National de la Recherche Scientifique CNRS；Institut National des Sciences Appliquees de Rouen；Universite de Rouen Normandie；
    IPC主号:
    专利说明:

    Holder (s): NATIONAL CENTER FOR SCIENTIFIC RESEARCH, NATIONAL INSTITUTE OF APPLIED SCIENCES OF ROUEN - INSA, UNIVERSITY OF ROUEN NORMANDY.
    Extension request (s)
    Agent (s): MARKS & CLERK FRANCE General partnership.
    FR 3 063 395 - A1 (54) LASER SOURCE FOR THE EMISSION OF A GROUP
    ©) According to one aspect, the present description relates to a laser source for the emission of a group of pulses comprising: a primary laser source (22) adapted for the emission of at least one primary laser pulse; at least one interferometer (24) suitable for forming, from said primary laser pulse, a plurality of secondary laser pulses, each interferometer comprising at least one delay line making it possible to temporally separate two secondary laser pulses, a delay between 50 ps and 10 ns; and a single-mode amplifying optical fiber intended to receive the secondary laser pulses, to form at the output a group of pulses superimposed spatially.

    Laser source for the emission of a group of pulses
    STATE OF THE ART
    Technical area
    The present description relates to a laser source for the emission of a group of pulses, for example a doublet or a triplet of pulses, in particular for Laser Induced Plasma Spectroscopy (LIBS). The present description also relates to a method for generating a group of pulses and to a LIBS analysis system comprising such a laser source.
    State of the art
    Laser Induced Plasma Spectroscopy (LIBS) is a technique for qualitative and quantitative analysis of the chemical composition of different solid materials, gaseous liquids or aerosols. The technique is based on the interaction of a laser pulse with a material to be analyzed. The light / matter interaction induces the vaporization of the material in the form of a plasma. The atoms and the ions of the material present in the plasma are brought to excited energy levels and emit, while being de-excited, light presenting a spectrum made up of atomic lines, whose wavelengths make it possible to identify the elements present. The intensity of each line is proportional to the concentration of the emitting atoms. The elementary composition of the material is thus deduced therefrom.
    However, the characteristics of the emission depend not only on the material but also on the ambient air; lines coming from the excited atoms of the ambient air can then mask compounds of the material and make imprecise the analysis of the LIBS signal, in particular in the case where the pulses of the laser used are nanoseconds. To overcome this drawback, LIBS can use lasers with femtosecond pulses (<1 ps). The use of femtosecond pulses reduces the interaction between the ambient gas and the laser pulses that can cause the emission of a continuum. Reducing the emission from the continuum provides better contrast of the plasma emission lines. The use of femtosecond pulses also makes it possible to minimize the dimensions of the heated zone and the mechanical deformations of the material to be analyzed. This results in better spatial resolution and precise machining as well as better reproducibility of the measurement.
    However, the intensities of the spectral lines of the plasma induced by the femtosecond pulses are lower than those produced by nanosecond pulses. Although it has been shown that the intensities of the exploitable lines of the plasma increase with the energy of the femtosecond pulses, the high cost of high-speed ultra-fast lasers and the complexity of their technology make the use of this technique difficult in particular. industrial environment outside the laboratory.
    One attempt to improve the performance of LIBS using femtosecond pulsed lasers is the use of the LIBS double pulse technique (DPLIBS) and more precisely in collinear configuration. The DP-LIBS in collinear configuration consists in using two femtosecond collinear pulses delayed between them by a few tens of picoseconds to a few nanoseconds. These two pulses interact with the material to be analyzed in order to increase the intensity of the atomic lines.
    It is known for the formation of two femtosecond pulses delayed between them, to use a programmable acousto-optical filter. For example, the article by Roberts et al. “Femtosecond laser ablation of silver foil with single and double puises” (Applied Surface Science 256, pp. 1784-1792 (2010)) describes DP-LIBS experiments using a programmable acousto-optical filter for the shaping of a pulse doublet with a delay between 0.2 and 6 ps. The drawback of this shaping approach is that the induced inter-pulse delay is only a few picoseconds and hardly reaches a few tens of picoseconds. However, recent studies show that the optimal conditions for DP-LIBS in ultra-fast regime are reached for inter-pulse delays situated in the range of 100 ps to a few ns.
    This range of delays is accessible through the use of an optical delay line. The article by Mildner et al. "Emission signal enhancement of laser ablation of metals (aluminum and titanium) by time delayed femtosecond double puises from femtoseconds to nanosecond" describes a scheme for implementing this technique with a delay line. The device described in this article is shown in FIG. 1. The device comprises a femtosecond FSL laser source followed by a delay line of the Mach-Zehnder interferometer type to form, from a pulse, two pulses delayed between them by 500ps which will interact with the material S to be analyzed. . To extend the delay (beyond 3ns) between pulses, a delay line AD is added to one of the arms of the MachZehnder interferometer. The spectrum of radiation from the interaction is then detected by a spectrometer combined with a camera C. One of the drawbacks of this technique lies in a difficult and restrictive alignment of the device which can lead to a quality of measurement which is not optimal. . Another drawback lies in the high sensitivity of the device in free space which makes it difficult to deploy it in an industrial environment.
    The present description presents a laser source for the emission of multiple pulses which can be used in the DP-LIBS technique, which has, compared to known sources, a simple, effective alignment to allow in particular a better sensitivity of the analysis by LIBS.
    ABSTRACT
    According to a first aspect, the present description relates to a laser source for the emission of a group of pulses comprising: a primary laser source suitable for the emission of at least one primary laser pulse; one or more interferometers suitable for forming, from said primary laser pulse, a plurality of secondary laser pulses, each interferometer comprising at least one delay line making it possible to temporally separate two secondary laser pulses, with a delay included between 50 ps and 10 ns; and a single mode amplifying optical fiber intended to receive the secondary laser pulses, to form at the output a group of pulses superimposed spatially.
    The laser source thus described makes it possible to generate a group of collinear pulses delayed with respect to one another and spatially superimposed. The applicants have thus shown a spatial recovery rate of the pulses greater than 90%, advantageously greater than 95%, advantageously greater than 99%. The recovery rate of two or more than two pulses is for example defined from a cross-correlation function of the images of said pulses, the images being for example formed in the focal plane of a focusing lens arranged at the output of the single mode amplifying optical fiber. The applicants have shown that the single-mode amplifying optical fiber of the laser source thus described plays the role of spatial filter of the secondary laser pulses by projecting them onto a single mode at the output of the optical fiber. Thus, the laser source automatically ensures very good spatial overlap between the pulses of the group of pulses emitted up to distances of several meters. Such a laser source also has the advantage of being compact and of greatly simplifying the adjustment of the optics of the devices described in the prior art.
    By “interferometer”, is understood in the present description any device, fiber or not, comprising means of spatial separation of incident light pulses, two arms in which each of the pulses propagates after separation, and means of spatial recombination of the pulses after propagation; and wherein one of the arms forms a delay line.
    According to one or more exemplary embodiments, at least one of said interferometers comprises means for spectral and / or temporal shaping of at least one of the secondary pulses. The spectral and / or temporal shaping means comprise for example a spectral filter and / or a temporal stretching device such as a variable pitch Bragg grating.
    According to one or more exemplary embodiments, at least one of said interferometers comprises means for controlling the relative optical power of the secondary laser pulses. The means for controlling the optical power include, for example, a variable attenuator arranged on one of the arms of the interferometer.
    It is thus possible to control the optical powers, the pulse duration and / or the spectrum of each of the pulses of the group of pulses generated by the laser source. In the case of LIBS measurements, this allows, for example, depending on the material to be analyzed, or even the focusing system on the material, to adjust the pulse parameters to optimize the creation of the plasma necessary for LIBS measurements.
    According to one or more exemplary embodiments, the laser source further comprises:
    a stretcher arranged upstream of the single-mode amplifying optical fiber so that the primary laser pulse is stretched at a pulse duration greater than 50 ps, and
    - a compressor downstream of the single mode amplifying optical fiber to compress the pulses of the pulse group in time so that the pulses of the pulse group have a pulse duration of less than 500 fs.
    It is thus possible to generate a group of high energy pulses without damaging the single mode amplifying optical fiber nor to cause nonlinear effects in the single mode amplifying optical fiber.
    According to one or more exemplary embodiments, the laser source further comprises at least one optical pre-amplifier disposed upstream of the single-mode amplifying optical fiber. It is thus possible to use a primary laser source providing lower energy pulses while guaranteeing the necessary optical power in the optical fiber.
    According to one or more exemplary embodiments, the laser source further comprises a laser diode for optically pumping the single-mode amplifying optical fiber.
    According to one or more exemplary embodiments, the laser diode is arranged so as to optically pump the single-mode amplifying optical fiber in counterpropagative mode.
    According to one or more exemplary embodiments, the single-mode amplifying optical fiber is a fiber with modal filtering.
    According to one or more exemplary embodiments, the single-mode amplifying optical fiber is a wide-pitch rod fiber.
    According to one or more exemplary embodiments, the single-mode amplifying optical fiber is intended to operate in a saturated gain regime. The saturated gain regime reduces the dependence on variations in the external environment (mechanical, thermal variations, etc.) of the amplitude of the pulses generated, which makes the laser source more stable from measurement to measurement. 'other. In fact, when the single-mode amplifying optical fiber operating in saturated gain mode, a variation in intensity of the pulses or a variation in the injection of the secondary laser pulses into the fiber does not modify the amplitude of the pulses of the pulse group in output of the single mode amplifier optical fiber.
    According to one or more exemplary embodiments, to reduce the bulk and simplify the optical adjustments, the path of the pulses between the primary laser source and the input of the single-mode amplifying optical fiber is fiberized.
    According to one or more exemplary embodiments, the primary laser source is adapted to the emission of a train of primary laser pulses temporally separated by a delay of at least 2 ps. Thus, the primary laser source emits primary laser pulses spaced in time between them by a delay greater than or equal to 2 ps to allow the emission of groups of pulses which will be separated by a delay greater than or equal to 2 ps. Thus, advantageously, during a LIBS measurement, the excited ions of the plasma will be able to completely relax between two groups of pulses, which will reduce the noise.
    According to one or more exemplary embodiments, the primary laser pulse is subpicosecond.
    According to one or more exemplary embodiments, one or less of the interferometers is an interferometer of the Mach Zehnder type, the delay line being formed by an arm of the interferometer.
    According to a second aspect, the present description relates to a laser induced plasma spectroscopic analysis system (LIBS) comprising a laser source for the emission of a group of pulses as described according to the first aspect; a collector for collecting a beam resulting from the interaction between an object to be analyzed and the group of pulses emitted by the laser source; and a spectrometer allowing, from the spectral analysis of the collected beam, to obtain the LIBS spectrum of the object to be analyzed.
    According to a third aspect, the present description relates to a method for generating a group of pulses comprising the following steps:
    emission of at least one primary laser pulse,
    generation from said primary pulse of a plurality of secondary laser pulses temporally separated by a delay of between 50 ps and 10 ns, by means of one or more interferometers, injection of the secondary laser pulses into an optical fiber single-mode amplifier to form a group of spatially superimposed pulses.
    According to one or more exemplary embodiments, the step of generating a plurality of secondary laser pulses further comprises the step of spectral and / or temporal shaping of at least one of the secondary pulses.
    According to one or more exemplary embodiments, the method for generating a group of pulses further comprises the following steps:
    temporal stretching of the primary laser pulse such that the primary laser pulse is stretched to a pulse duration greater than 50 ps, and temporal compression of the pulses of the group of pulses superimposed spatially so that the pulses of the group of spatially superimposed pulses have a pulse duration of less than 500 fs.
    According to one or more exemplary embodiments, the method for generating a group of pulses further comprises the step of saturation of the gain of the single-mode amplifying optical fiber.
    According to one or more exemplary embodiments, the method for generating a group of pulses further comprises the step of adjusting the delay between the secondary laser pulses.
    BRIEF DESCRIPTION OF THE DRAWINGS
    Other advantages and characteristics of the object of the description will appear on reading the description, illustrated by the following figures:
    FIG. 1, a diagram of a DP-LIBS system in collinear configuration according to the prior art (already described);
    FIG. 2, a diagram of a laser source for the emission of a group of pulses according to an example of the present description;
    FIG. 3, a diagram of a laser source for the emission of a group of pulses according to an example of the present description comprising a stretcher and a compressor;
    FIG. 4, a diagram of another example of a laser source for the emission of a group of pulses according to an example of the present description, said source being fiberized;
    FIG. 5 and FIG. 6, diagrams of a laser source for the emission of a group of pulses composed of more than two pulses according to examples of the present description;
    FIG. 7, a diagram of a laser source for the emission of a group of pulses composed of more than two pulses according to an example of the present description, in which the laser source is fibered;
    FIG. 8, a diagram of a spectroscopic analysis system by LIBS according to an example of the present description;
    FIGS. 9A and 9B, characteristic spectra of Aluminum obtained in single-pulse LIBS and in DP-LIBS using a laser source for the emission of a group of pulses according to an example of the present description, the pulses of the source laser with 50 ps and 750 ps inter-pulse delay, respectively.
    DETAILED DESCRIPTION
    In the figures, identical elements are identified by the same references.
    FIG. 2 illustrates a first example of a laser source 20 for the emission of a group of pulses according to the present description. It comprises a primary laser source 22, an interferometer 24 suitable for the formation of two secondary laser pulses 1, 2, and an amplification module 26 comprising a single-mode amplifying optical fiber 262 intended to receive said secondary laser pulses 1, 2, for form a group of pulses 3, 4 spatially superimposed.
    In the example shown in FIG. 2, the primary laser source 22 is for example suitable for the emission of at least one primary laser pulse of duration, for example subpicosecond, for example sub-500 fs. It can also emit a primary laser pulse train with an adjustable rate between 1 kHz and 500 KHz. In this case, the pulses are spaced from 2 ps to 1 ms. The primary laser source can for example comprise a laser with standard mode blocking operating at a fixed rate, for example of a few tens of MHz, which can be followed by a pulse selector which can reduce the rate of the primary source for example to less than 500 KHz. The primary source may include a preamplifier to compensate for loss of insertion into the pulse selector.
    In the example in FIG. 2, the laser source comprises an interferometer suitable for the formation, from the primary laser pulse, of two secondary laser pulses. The laser source can also include several interferometers in series to generate a grouping of an even number of pulses, as will be described later. It is also possible to nest the interferometers together to generate odd groupings of pulses.
    In the example in FIG. 2, the interferometer is of the Mach-Zehnder type. It comprises two mirrors 243, 244 and two semi-reflecting mirrors 241 and 242. With the aid of a semi-reflecting mirror 241, the primary laser pulse is divided into two secondary laser pulses 1, 2 which traverse two optical paths different ; the optical path of pulse 1 is elongated relative to the optical path of pulse 2 by means of two reflecting mirrors 243, 244. Then the two secondary laser pulses are recombined using a semi-reflecting mirror 242 In this way, the two secondary laser pulses are separated in time by a delay At = δ / c with c the speed of propagation of the light and δ the difference between the two optical paths traversed by the two secondary laser pulses 1, 2. This delay can be adjusted by modifying the difference of the optical paths δ and consequently the length of one of the optical paths. To modify the difference of the optical paths, it is possible for example to move the position of the two mirrors 243 and 244. It may be noted that the delay At is chosen to be greater than or equal to the duration of the primary pulse so that the two secondary laser pulses 1, 2 cannot interfere with each other.
    For LIBS measurements, the delay At is preferably adjustable between ten picoseconds to a few nanoseconds, for example between 50 ps and 10 ns. This allows the inter-pulse delay to be adapted to optimize the heating of the plasma produced by the first pulse of the doublet, which delay varies from one material to another.
    The interferometer can also be of the Michelson type or any other type of interferometer. For example, a Michelson-type interferometer may include two mirrors and a separating plate. The primary laser pulse is sent to the semi-reflective plate which separates the primary pulse into two secondary laser pulses. One is sent to one of the mirrors and the other to the other mirror. The two mirrors are at a different distance from the separating plate, thus making it possible to introduce a difference δ of optical path and therefore to introduce a delay At between the two secondary laser pulses. By moving the position of the mirrors, the delay between the two secondary laser pulses can be adjusted.
    ίο
    According to one or more exemplary embodiments, the interferometer is fiber-reinforced and in this case, it may for example comprise one or more optical fibers arranged around a piezoelectric drum, as will be described below.
    As illustrated in FIG.2, at the output of the interferometer 24, the spatial energy distributions of the two secondary laser pulses 1, 2 (referenced 1 ’and 2’) are not perfectly spatially superimposed. In fact, there is an offset due to the difficulty in aligning the interferometer and to disturbances linked to the environment.
    The two secondary laser pulses are then injected into an amplification module 26. The amplification module comprises a single-mode amplifying optical fiber 262 in which the secondary laser pulses 1, 2 are coupled, for example using a lens 261 or by splicing (by welding two fibers) in the case of a monolithic fiber configuration (see FIG. 4). The lens 261 thus makes it possible to couple the secondary laser pulses with the single mode of the single-mode amplifying fiber. It is also possible to use 262 "and 262" glass tips welded to the ends of the fiber to widen the size of the output beam and thus push the threshold of optical damage at high intensities.
    The mode of a waveguide - such as a single-mode optical fiber - indicates how light travels within the guide. In a single-mode fiber, there is only one propagation mode. The single-mode character of the single-mode amplifying optical fiber thus ensures a spatial superposition of the secondary laser pulses at the output of the single-mode amplifying optical fiber. In other words, the single mode amplifying optical fiber acts as a spatial filter by projecting the two secondary laser pulses in a single mode and thus by "cleaning" the secondary laser pulses according to the single mode of the fiber. The applicants have shown that a spatial recovery rate greater than 90% and more can thus be obtained between the pulses of the pulse group thus formed.
    The spatial overlap of the pulses can for example be measured by means of a cross-correlation function of the images of the pulses formed in the focal plane of a focusing lens arranged downstream of the single-mode amplifying optical fiber. For example, the cross-correlation function r (u, v) is calculated between the two images of the beams formed at the focus of a focusing lens placed at the output of the single-mode amplifying fiber. This function is given by the equation:
    r (u, v)
    Z%, y [(/ Ooy) ~ /) * (ff (* -u, yv) - g)] Σχ, γ J (/ Where y) - f) 2 Σχ, γ ~ U, y -v) - g) 2
    Where f (x, y) and g (x, y) are the images corresponding to the two pulses of the doublet and f and g the mean intensities of the two images. The maximum value of the “r” function gives the degree of resemblance between the two images which is all the better when the degree of resemblance is close to 1. Furthermore, the position of the maximum peak of “r” indicates the degree of overlap between the two images. The overlap is all the better as the maximum is centered on the point of coordinates u = 0, v = 0. Thus, we can search for a cross autocorrelation function "r" has a peak whose maximum value is close to 1 , for example greater than 0.99, and centered at the point of coordinates (0, 0) or near the point of coordinates (0, 0).
    The single-mode amplifying optical fiber also makes it possible to amplify the intensity of the secondary laser pulses and thus allows the use of a weak primary laser source.
    The single-mode amplifier fiber can be optically pumped with a light beam emitted by a pumping means such as a diode or a laser in order to cause population inversion. When the secondary laser pulses pass through the single-mode amplifying optical fiber, their intensities are increased tenfold by the stimulated emission effect induced by the active ions. Total gains over the entire length of the active fiber of 20 to 30 dB can thus be achieved.
    The intensities of the secondary pulses at the input of the single-mode amplifying optical fiber are adjusted so as to preferably ensure operation in the latter's saturated gain regime. The pulse intensity at the input of the single mode amplifier optical fiber is adjusted upstream by the primary source or by inserting a preamplifier. The saturated gain regime has the advantage of making the laser source insensitive to the outside environment. In particular, during the operation of the laser source in free space, the amplitudes of the amplified pulses will be less sensitive to variations in the external environment (mechanical, thermal variations, etc.) and therefore more stable from measurement to measurement. other. Thus, when the single-mode amplifying optical fiber operates in saturated gain mode, a variation in intensity or a variation in the injection of the secondary laser pulses into the fiber will not modify the spatial superposition of the pulses nor the intensity of the pulses of the group pulses at the output of the single mode amplifying optical fiber.
    As illustrated in FIG. 2, the amplification module 26 can comprise a laser diode 266 for carrying out the population inversion in said single-mode amplifying fiber 262. For this a pumping light beam is emitted by the diode 266 and injected into the optical fiber using transmitting elements such as lenses 265 and 266 and a mirror 264. The pumping light beam can be injected into the single-mode amplifying optical fiber counterpropagatively, that is to say at the output of the 262 ”optical fiber or copropagatively, ie at the input of the 262 'optical fiber.
    The single-mode amplifying optical fiber is for example a double-clad fiber with a core doped with active ions such as rare earths (for example Erbium, Neodymium, Ytterbium, Thulium, Praseodymium, Holmium); the single-mode amplifying optical fiber can be a doped fiber with a large modal area or a fiber with distributed modal filtering ensuring a single-mode behavior with a core diameter ranging for example up to 80 μm, or else a wide-pitch rod fiber (LPF) . The single mode amplifying optical fiber supports a single mode and advantageously has a length sufficient to produce a total gain of between 20 and 30 dB.
    In the case of a single-mode amplifying optical fiber with a core doped with ytterbium ions operating at the wavelength of lpm, pumping diodes emitting at 915 or 976 nm can for example be used.
    In FIG. 2, the secondary laser pulses 3, 4 at the output of the optical fiber are then collected using for example a lens 263. The spatial energy distributions (3 ′) of the secondary laser pulses 3 and 4 are now collinear and spatially superimposed as illustrated in the inset on the right of FIG. 2. Furthermore, due to the saturated gain regime of the example in FIG. 2, the intensities of the secondary laser pulses 3, 4 at the output of the optical fiber are substantially identical.
    FIG. 3 shows another example of a laser source according to the present description. The laser source 30 of FIG.3 comprises for example a primary laser source 22, a pulse stretcher 32, one or more interferometer (s) (in FIG.3 a single interferometer 24 is shown), a module amplification 26 comprising a single-mode amplifying optical fiber and a pulse compressor 36.
    The example of FIG. 3 also has the advantage of forming a group of high intensity pulses while reducing the risks of damage to the fiber or generation of non-linear effects which can cause degradation of the fiber output pulses.
    The primary laser source 22 is suitable for the emission of at least one primary laser pulse of subpicosecond duration. The primary laser pulse is sent to a pulse stretcher 32 to temporally stretch the primary laser pulse so that the primary laser pulse has a pulse duration greater than 50 ps. The temporal stretching of the primary pulse will decrease its peak power by a factor ranging from 50 to more than 1000. The stretcher can be placed upstream of the single-mode amplifying optical fiber. In the example of FIG. 3, the stretcher 23 is placed upstream of the interferometer 24. The primary stretched laser pulse then follows the same path to the output of the single-mode amplifying optical fiber as that of the primary laser pulse of FIG. 2.
    Downstream of the single mode amplifying optical fiber, the spatially indistinguishable secondary laser pulses are sent to a pulse compressor 36. The pulse compressor makes it possible to compress the secondary laser pulses in time so that the durations of the secondary laser pulses are subpicoseconds as output from the primary source. The temporal compression of the amplified secondary laser pulses generates a strong rise in peak power of the secondary laser pulses superimposed spatially.
    The stretcher is for example of the massive lattice type. It can be a normal dispersion optical fiber or a Bragg grating with variable pitch photo-registered on optical fiber or in a solid glass.
    The compressor can comprise, as illustrated in FIG. 3, a half-wave plate 361 for modifying the orientation of the linear polarization of the amplified secondary pulses, a polarization splitter cube 362 for reflecting the secondary laser pulses towards a volume Bragg grating at variable pitch 364 passing through a quarter-wave plate to transform the linear polarization into circular polarization. Secondary laser pulses which are fully reflected by the variable pitch Bragg grating see their durations decrease to a sub-picosecond value close to the duration of the primary pulse. The compressed secondary pulses are sent to the output of the laser source by the combined actions of the quarter-wave plate and the polarization splitter to emit spatially superimposed secondary laser pulses with a pulse duration of less than 500 fs and a power peak greater than 2 MW. Other types of compressor can be used as a pair of diffraction gratings.
    FIG. 4 shows an example of a laser source according to an example of the present description in which the laser source is fiberized. In this example, a stretcher 32 and a compressor 36 have also been shown, but these elements are optional. The stretcher 32 is disposed upstream of a fiber interferometer 44 and the compressor 36 is disposed downstream of the amplification module 26. In this example, the path of the pulses between the primary laser source 22 and the amplification module 26 is fully fiber. Likewise, the interferometer 44 is fully fiberized. In this case, the interferometer may comprise at least one motorized delay line 463 which can be placed on a fiber system and making it possible to obtain a delay greater than 500 ps. Elements 461 and 467 are fiber optic couplers. The primary pulse is separated into two pulses by the single-mode fiber coupler 461. After propagation in the two arms of the interferometer, the latter are recombined in the coupler 467.
    In the case of a fiber laser source with a stretcher, it may be advantageous to have the stretcher 32 upstream of the interferometer 44 in order to limit the distortions of the pulses during their propagation in the fiber interferometer by the effects non-linear.
    In the example of FIG. 4, there are also represented spectral or temporal shaping means 462. The spectral shaping system can consist of a spectral filter (dielectric band pass filter or Bragg grating with not fixed). The temporal shaping can be carried out for example with a Bragg grating with variable pitch. Of course, such means can be integrated into any arm of an interferometer (including non-fiber) in order to be able, for example, to generate asymmetric secondary laser pulses in duration and in power. This configuration has the advantage of improving the sensitivity of LIBS measurements. For example in the case where two asymmetrical pulses are emitted by the laser source, the first pulse of strong powerful peak and of short duration is used to ablate the material to be analyzed and the second pulse strongly drawn and of less peak power is used to heat the plasma formed by the group's first impulse.
    Note that one or more all-fiber preamplifiers can be used upstream of the single-mode amplifier optical fiber to ensure its operation in saturated gain mode. Furthermore, the coupling of the secondary pulses in the single-mode amplifying optical fiber can be carried out by splicing (by welding), making it possible to give the system a monolithic architecture from the primary source to the compression stage. For example, in the embodiment of FIG. 4, it is possible to have a preamplifier upstream of the single-mode amplifying optical fiber in order to adjust the power level to the saturation of the gain in the fiber.
    FIG. 5, FIG. 6 and FIG. 7 present other examples of embodiment in which the source can emit a group of more than two pulses such as a triplet or even a quadruplet. For this, the laser source can comprise several interferometers arranged in series to generate even groupings (FIG.5, FIG.6 FIG.7) or nested one inside the other to generate odd groupings.
    In FIG. 5, the laser source according to an example of the present description comprises two interferometers in series. At the output of the first interferometer 24, two secondary laser pulses 1, 2 are generated. The two secondary laser pulses 1, 2 are sent to a second interferometer 54, for example of the Mach-Zehnder type. The second interferometer 54 can include, like the first interferometer 24, two fully reflecting mirrors 543, 544 and two semi-reflecting mirrors 541 and 542. Using a semi-reflecting mirror 541, the two secondary laser pulses from the first interferometer are divided into two packets of two secondary laser pulses 11, 12 and 13, 14 which travel two different optical paths. Then using the two reflecting mirrors 543, 544, the optical path of the secondary laser pulses 13 and 14 is lengthened relative to the optical path of the secondary laser pulses 11 and 12. The semi-reflecting mirror 542 makes it possible to combine the four pulses secondary laser to form a group of pulses composed of the four secondary laser pulses. The spatial distribution 11 ', 12', 13 'and 14' of the energy of the pulses is shown in the box at the bottom left of FIG. 5. The secondary laser pulses 11, 12 on the one hand and 13, 14 on the other hand, are separated by a delay Δί calibrated by the first interferometer 24. The packet of pulses 11, 12 is time-shifted relative to the packet of pulse 13, 14 of a delay Δί 'calibrated by the second interferometer 54. The four secondary laser pulses are then sent to the amplification module 26 comprising the single-mode amplifying optical fiber to form a group of four spatially superimposed pulses 15, 16 , 17, 18 as illustrated in the insert at the bottom right of FIG. 5.
    Obviously, depending on requirements, it is possible to insert more than two interferometers into the laser source according to the present description.
    FIG. 6 represents an example according to the present description of a fiber laser source for the emission of a group of four pulses. In this example, we find the elements of FIG. 4: the primary laser source 22, the first fiber interferometer 44 and the amplifier module 26 comprising the single-mode amplifier fiber; this example illustrates the case of two fiber interferometers 44, 64, arranged in series, to form a group of 4 pulses. The operation is similar to that of the example in FIG. 5; similarly the fiber elements are similar to those of FIG. 4. In fact, the entire optical path from the primary laser source to the input of the single-mode amplifying optical fiber is fiber.
    According to one or more embodiments, one or more fiber or non-fiber preamplifiers can be used upstream of the single mode amplifying optical fiber to ensure its operation in saturated gain regime.
    FIG. 7 represents an example according to the present description of a diagram of a fiber laser source for the emission of a group of four pulses further comprising a stretcher 32 placed after the primary sources 22, before a first interferometer 74 and a compressor 36 placed downstream of the amplification module 26 comprising the single-mode amplifying optical fiber. The operation is identical to that of FIG. 3. There may also be a second interferometer 76, as illustrated in the figure.
    The laser source according to the present description is perfectly suited for LIBS measurements. FIG. 8 illustrates an example of a LIBS spectroscopic analysis system according to the present description. The LIBS spectroscopic analysis system comprises a laser source 80 according to the present description for the emission of a group of pulses. The pulse group is focused on a sample 83 to be analyzed using for example a lens 81. The interaction between the pulse group and the sample 83 will initially generate a plasma 82 then in secondly a light radiation. A fraction of the emitted radiation is collected by an optical system 84 such as for example a pair of lenses 841, 842. The beam is then detected by a spectrometer 85 making it possible, from the spectral analysis of the collected radiation, to obtain the LIBS spectrum. of the object to be analyzed.
    According to one or more examples of the present description, the laser source emits trains of pulse groups, it is thus possible to perform LIBS measurements averaged over several shots.
    FIGS. 9A and FIG. 9B present spectra obtained with a LIBS spectroscopic analysis system according to the present description. The analysis system in this example includes a laser source for the emission of pulse doublets, as shown in FIG. 2. The primary laser source includes a mode-locked fiber laser emitting pulses of 500 fs duration at a rate of 18 MHz at a wavelength of 1040nm. It is followed by an acousto-optic modulator to divide the cadence by a factor of more than 40 and a fiber preamplifier to ensure the saturation of the power amplifier. A Mach Zehnder interferometer based on mirrors as in FIG. 2 is used to generate two separate secondary pulses with a delay between 50 ps and 10 ns. The amplifying optical fiber is a microstructured fiber of the bar type whose core of 80 μm in diameter is doped with ytterbium ions. The fiber is pumped backpropagated by a laser beam centered at 976 nm from a fiber laser diode. The spectra 92 and 96 correspond to the emission spectra of the ablation plasma of an aluminum sample in the spectral range 375-405 nm obtained in double pulse mode for a delay of 50 ps for FIG. 9A and 750 ps for FIG. 9B. These spectra are compared to spectra 94, 98 (identical) obtained in single-pulse regime. The single-pulse regime consists in making a single pulse interact with the sample to be analyzed by blocking one of the arms of the interferometer. It should be noted that the total energy sent to the sample to be analyzed in these experiments is kept constant from one experiment to another. In particular, it is set at 2 pj per pulse in the case of mono-pulse LIBS and 1 μJ per pulse in the case of DP-LIBS, which corresponds to a total of 2pJ as well. The spectra measured bring out the two main lines known for aluminum and which are centered at 394.40 nm and 396.15 nm. Compared to single-pulse LIBS measurements, the detection sensitivity of these lines increases by a factor of DP-LIBS for an inter-pulse delay of 50 ps and by a factor of 20 for a delay of 750 ps.
    Although described through a number of exemplary embodiments, the laser source for the emission of a group of pulses, the Plasma Spectroscopic Analysis System
    Induced by Laser and the Method for generating a group of pulses include different variants, modifications and improvements which will be obvious to those skilled in the art, it being understood that these different variants, modifications and improvements are part of the scope of the invention as defined by the claims which follow.
    权利要求:
    Claims (15)
    [1" id="c-fr-0001]
    1. Laser source for the emission of a group of pulses comprising:
    - a primary laser source (22) adapted for the emission of at least one primary laser pulse,
    - one or more interferometers (24, 44, 54, 64, 74, 76) adapted for the formation, from said primary laser pulse, of a plurality of secondary laser pulses (1, 2, 11, 12, 13 , 14), each interferometer comprising at least one delay line making it possible to temporally separate two secondary laser pulses with a delay (Δί, Δί ') between 50 ps and 10 ns,
    - a single mode amplifying optical fiber (262) intended to receive the said plurality of secondary laser pulses, to form at the output a group of pulses (3, 4, 15, 16, 17, 18) spatially superimposed.
    [2" id="c-fr-0002]
    2. Laser source according to any one of the preceding claims, in which at least one of said interferometers comprises means for controlling the relative optical power of the secondary laser pulses.
    [3" id="c-fr-0003]
    3. Laser source according to any one of the preceding claims, further comprising:
    a stretcher (32) disposed upstream of the single-mode amplifying optical fiber so that the primary laser pulse is stretched at a pulse duration greater than 50 ps, and
    - a compressor (36) downstream of the single-mode amplifying optical fiber to temporally compress the pulses of the pulse group so that the pulses of the pulse group have a pulse duration of less than 500 fs.
    [4" id="c-fr-0004]
    4. Laser source according to any one of the preceding claims, further comprising:
    at least one optical preamplifier disposed upstream of the amplifying optical fiber.
    [5" id="c-fr-0005]
    5. Laser source according to any one of the preceding claims, comprising:
    a laser diode (266) for optically pumping the single mode amplifier optical fiber (262).
    [6" id="c-fr-0006]
    6. Laser source according to any one of the preceding claims, in which the single-mode amplifying optical fiber is intended to operate in a saturated gain regime.
    [7" id="c-fr-0007]
    7. Laser source according to any one of the preceding claims, in which the path of said at least one primary laser pulse and secondary laser pulses between the primary laser source and the input of the single-mode amplifying optical fiber is fiber.
    [8" id="c-fr-0008]
    8. Laser source according to any one of the preceding claims, in which the primary laser source (22) is adapted to the emission of a train of primary laser pulses temporally separated by a delay of at least 2 ps.
    [9" id="c-fr-0009]
    9. Laser source according to any one of the preceding claims, in which the said interferometer (s) (24, 44, 54, 64, 74, 76) comprises at least one interferometer of the Mach-Zehnder type, the delay line being formed by an interferometer arm.
    [10" id="c-fr-0010]
    10. Laser Induced Plasma Spectroscopic Analysis System (LIBS) comprising:
    - a laser source (80) for the emission of a group of pulses according to any one of the preceding claims,
    - a collector (84) for collecting a beam resulting from the interaction between an object (83) to be analyzed and the group of pulses emitted by the laser source; and
    - a spectrometer (85) making it possible, from the spectral analysis of the collected beam, to obtain the LIBS spectrum of the object to be analyzed.
    [11" id="c-fr-0011]
    11. Method for generating a group of pulses comprising the following steps:
    emission of at least one primary laser pulse, generation from said primary pulse of a plurality of secondary laser pulses (1, 2, 11, 12, 13, 14) temporally separated by a delay of between 50 ps and 10 ns, using one or more interferometers (24, 44, 54, 64, 74, 76), injection of the secondary laser pulses into a single mode amplifying optical fiber (262) to form a group of pulses (3, 4 , 15, 16, 17, 18) spatially superimposed.
    [12" id="c-fr-0012]
    The method for generating a group of pulses according to claim 11, wherein the step of generating a plurality of secondary laser pulses further comprises the step of spectral and / or temporal shaping of at least one of the secondary laser pulses.
    [13" id="c-fr-0013]
    13. Method for generating a group of pulses according to any one of claims 11 to 12, further comprising the following steps:
    temporal stretching of the primary laser pulse such that the primary laser pulse is stretched to a pulse duration greater than 50 ps, and temporal compression of the pulses of the group of pulses superimposed spatially so that the pulses of the group of spatially superimposed pulses have a pulse duration of less than 500 fs.
    [14" id="c-fr-0014]
    14. A method of generating a group of pulses according to any one of claims 11 to 13, further comprising saturation of the gain of the single mode amplifying optical fiber.
    [15" id="c-fr-0015]
    15. A method of generating a group of pulses according to any one of claims 11 to 14, further comprising adjusting the delay between the secondary laser pulses.
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    JP2020511634A|2020-04-16|
    US10879667B2|2020-12-29|
    WO2018158261A1|2018-09-07|
    US20200028316A1|2020-01-23|
    EP3590159A1|2020-01-08|
    EP3590159B1|2021-04-21|
    引用文献:
    公开号 | 申请日 | 公开日 | 申请人 | 专利标题
    US20090080467A1|2007-09-24|2009-03-26|Andrei Starodoumov|Pulse repetition frequency-multipler for fiber lasers|
    US20090246413A1|2008-03-27|2009-10-01|Imra America, Inc.|Method for fabricating thin films|WO2021059003A1|2019-09-26|2021-04-01|Uab „Ekspla“|Method for generating gigahertz bursts of pulses and laser apparatus thereof|
    WO2021185967A3|2020-03-18|2021-11-18|Thermo Fisher ScientificSarl|Double-pulse laser system|US7190705B2|2000-05-23|2007-03-13|Imra America. Inc.|Pulsed laser sources|
    US7006230B2|2003-05-16|2006-02-28|Lucent Technologies Inc.|Interferometric method and apparatus for the characterization of optical pulses|
    EP2672318B1|2011-01-20|2017-08-23|Nippon Telegraph And Telephone Corporation|Optical amplifier|
    GB2510130B|2013-01-24|2015-05-13|Toshiba Res Europ Ltd|Modulation Unit|
    US9876580B2|2014-04-22|2018-01-23|Kabushiki Kaisha Toshiba|Optical device|DE102017121147A1|2017-09-04|2019-03-07|Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.|Generation of multiple laser pulses by superposition of laser radiation|
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    优先权:
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    FR1751653A|FR3063395B1|2017-02-28|2017-02-28|LASER SOURCE FOR EMISSION OF A PULSE GROUP|
    FR1751653|2017-02-28|FR1751653A| FR3063395B1|2017-02-28|2017-02-28|LASER SOURCE FOR EMISSION OF A PULSE GROUP|
    PCT/EP2018/054829| WO2018158261A1|2017-02-28|2018-02-27|Laser source for emitting a group of pulses|
    US16/487,792| US10879667B2|2017-02-28|2018-02-27|Laser source for emitting a group of pulses|
    EP18708954.5A| EP3590159B1|2017-02-28|2018-02-27|Laser source for emitting a group of pulses|
    JP2019546172A| JP2020511634A|2017-02-28|2018-02-27|Laser light source for emitting pulses|
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