EP2612158A1 - Device and method for characterizing a laser beam - Google Patents

Device and method for characterizing a laser beam

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Publication number
EP2612158A1
EP2612158A1 EP11749458.3A EP11749458A EP2612158A1 EP 2612158 A1 EP2612158 A1 EP 2612158A1 EP 11749458 A EP11749458 A EP 11749458A EP 2612158 A1 EP2612158 A1 EP 2612158A1
Authority
EP
European Patent Office
Prior art keywords
active medium
electromagnetic wave
laser beam
magnetic field
signal
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Withdrawn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
EP11749458.3A
Other languages
German (de)
French (fr)
Inventor
Geert Rikken
Rémi BATTESTI
Andrei Ben-Amar Baranga
Mathilde Fouche
Carlo Rizzo
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.)
Centre National de la Recherche Scientifique CNRS
Original Assignee
Centre National de la Recherche Scientifique CNRS
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 Centre National de la Recherche Scientifique CNRS filed Critical Centre National de la Recherche Scientifique CNRS
Publication of EP2612158A1 publication Critical patent/EP2612158A1/en
Withdrawn legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/12—Measuring magnetic properties of articles or specimens of solids or fluids
    • G01R33/1215—Measuring magnetisation; Particular magnetometers therefor
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J1/00—Photometry, e.g. photographic exposure meter
    • G01J1/42—Photometry, e.g. photographic exposure meter using electric radiation detectors
    • G01J1/4257—Photometry, e.g. photographic exposure meter using electric radiation detectors applied to monitoring the characteristics of a beam, e.g. laser beam, headlamp beam
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J4/00—Measuring polarisation of light
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/02—Measuring direction or magnitude of magnetic fields or magnetic flux
    • G01R33/032—Measuring direction or magnitude of magnetic fields or magnetic flux using magneto-optic devices, e.g. Faraday or Cotton-Mouton effect
    • 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

Definitions

  • the invention relates to a device and a method that makes it possible in particular to characterize a pulsed laser beam of high energy or a continuous laser beam.
  • the present invention is used to measure the instantaneous power of a laser beam, or the total energy of a laser pulse and / or the polarization of the beam.
  • the power of high power lasers is measured by total or partial absorption of the laser beam. This leads to consuming the laser energy at the target used to perform the measurement. This results in a loss of energy of the beam and has the disadvantage of not being able to use the laser beam, simultaneously with the measurement of its energy or its power.
  • FIG. 1 shows the inverse Cotton-Sheep or ICME effect produced in a medium 1 by a laser beam propagating in the medium in the presence of a magnetic field transverse to the direction of a light beam.
  • active medium designates a material, a crystal, a glass, a gas, a liquid which, when subjected to a magnetic field, will have a reverse Cotton-Sheep effect. .
  • a laser beam will be used to mention an instantaneous power measurement of the beam, a power measurement or the determination of the polarization of the beam.
  • the invention relates to a device for measuring a magnetization generated within an active medium or to characterize a linearly polarized electromagnetic wave when said active medium exhibits an inverse Cotton-Sheep effect, characterized in that it comprises in combination at least the following elements:
  • ⁇ means for producing a transverse magnetic field B t with respect to the direction of propagation of said electromagnetic wave
  • the measuring device makes it possible to characterize the electromagnetic wave by at least one of the following parameters: the instantaneous power of the electromagnetic wave, the integral power or the polarization of the electromagnetic wave. wave.
  • the electromagnetic wave is a pulsed laser beam and the measuring device characterizes the laser beam pulsed by at least one of the following parameters: the instantaneous power of a pulse of said laser beam, the integral power of a pulse of said laser beam, the polarization of said laser beam.
  • the electromagnetic wave is a continuous laser beam and the measuring device characterizes the laser beam by at least one of the following parameters: the instantaneous power of said laser beam, the integral power of said laser beam, the polarization of said laser beam; laser beam, the magnetic field being variable in time.
  • Said active medium is, for example, subjected to a static external magnetic field B ex t variable or constant over time.
  • the signal measuring device comprises, for example, at least one pickup type coil.
  • the signal measuring device may comprise at least two pickup type coils placed on either side of the active medium, the normal to their surface being oriented substantially parallel to the magnetic field B ex t.
  • the electronic signal measuring device translating the instantaneous energy value of the electromagnetic wave, or the value of the power of said electromagnetic wave comprises the following elements:
  • ⁇ a summing and low noise amplifier operable to remove extraneous noise not corresponding to the signal associated with the inverse Cotton-Sheep effect, the summed and amplified signal being transmitted to
  • ⁇ a high pass filter, then to an integrator before being transmitted to a display device and / or a storage memory.
  • the device comprises, for example, a rotating mount in which are disposed the active medium, the means for producing the magnetic field.
  • the device may include an optical matching system.
  • Said active medium is a TGG or Terbium Gallium crystal
  • the invention also relates to a method for measuring a magnetization generated within an active medium, when said active medium exhibits a Cotton-Sheep Inverse effect, the method being implemented within a device having one of the characteristics above, the method comprising at least the following steps:
  • ⁇ measure the electrical signal translating the magnetization generated within said active medium by said electromagnetic wave.
  • the device according to the invention makes it possible to measure a magnetization generated within an active medium when the active medium has an inverse Cotton-Sheep effect.
  • the examples illustrated in the figures relate to an application to the characterization of a pulsed or continuous laser beam, but without departing from the scope of the invention can be applied in the case of a linearly polarized electromagnetic wave.
  • FIG. 2A schematizes an example of a given device in order to illustrate the elements of the device 1 according to the invention in the case of an application to the characterization of a pulsed laser beam.
  • an active medium 10 is disposed between two permanent magnets 1 1, 12 which provide a transverse static magnetic field B t oriented perpendicularly to the direction of propagation D
  • the pickup coils 14, 15 are, for example, placed on either side of the active medium 10.
  • the normal to their surface Au, Ai 5 is oriented parallel to the magnetic field B t .
  • the electrical signals Su, Sis generated at each of the coils are transmitted to an electronic measuring circuit, an exemplary embodiment of which is given in FIG. 2B.
  • the magnetic field B t is intrinsic to the material.
  • the device can also be used to characterize a continuous laser beam.
  • the transverse magnetic field intrinsic to the material B t or the magnetic field B ex t used, is a time-varying magnetic field whose temporal variation law is known. In this case, it is possible to measure the value of a constant or substantially constant power of the continuous laser beam.
  • the measuring device makes it possible to characterize the electromagnetic wave by at least one of the following parameters: the instantaneous power of the electromagnetic wave, the integral power or the polarization of the wave.
  • the shape and the number of turns of the coils are chosen as a function, for example, of the magnetic flux variation. It is possible to use planar-type pickup coils. It would also be possible to use coils having a curved surface which best follows the field lines of the external magnetic field.
  • the characterization of the laser beam or of an electromagnetic wave could be carried out using a single pickup coil or a number of coils greater than 2 depending on the application. .
  • the pickup coils can be connected to compensation coils making it possible to limit or even cancel parasitic effects.
  • the active medium 10 is a medium which exhibits an inverse Cotton-Sheep effect when it is subjected to an external magnetic field B ex t or to the intrinsic magnetic field B t in the case of a ferromagnetic medium or other media which do not need external solicitation. It is thus possible to use crystal or glass. Active liquid or gaseous media can also be envisaged.
  • the dimensions and the nature of the active medium will be chosen according to the desired application. For example, for use in the context of very intense lasers, it It is possible to choose an active medium coupled to an optical adaptation system of dimensions such that the energy density of the beam remains below the damage threshold of the active medium. The nature of the active medium may also be chosen according to the wavelength of the laser.
  • FIG. 2A shows an optical matching system 20, represented by an input lens 201 and an exit lens 202.
  • This system advantageously makes it possible to adapt the size of the pulsed or continuous laser beam to be characterized to the dimensions of the apparatus .
  • this adaptation system notably makes it possible to adapt the size of the laser beam to be characterized to the optical systems existing in the system.
  • the assembly comprising the active medium 1 0, the magnets 1 1, 1 2 and the coils 1 4, 1 5 can be positioned inside a rotating mount 30 which allows rotation relative to an axis AR parallel to the direction of propagation of the laser beam so as to adjust the angle ⁇ between the direction of the transverse magnetic field B T and the polarization of the laser, the value of the inverse Cotton-Sheep effect depends.
  • the ICME signal By measuring the ICME signal as a function of the angle ⁇ , it is possible to determine the laser polarization state, and in particular its ellipticity.
  • FIG. 2B represents an example of an electronic circuit 2 associated with the device for characterizing the pulsed laser beam.
  • the two pickup coils 1 4, 1 5 of FIG. 2A are connected to an adder and amplifier 40 of low noise whose function is to eliminate the parasitic noise that does not correspond to the signal associated with the Cotton-Sheep effect. reverse.
  • the summed and amplified signal S t is transmitted to a high-pass filter 41, then to an integrator 42 before being transmitted to a display device 43 and / or a storage memory 44.
  • This electronic circuit can without departing from the scope of the invention be implemented for application to a magnetization measurement generated within an active medium or to characterize a continuous laser beam.
  • the operating principle of the device according to the invention is, for example, the following: the device for characterizing a laser beam or an electromagnetic wave according to the invention is positioned on the optical path of the laser beam or the electromagnetic wave to be characterized (measurement of instantaneous power, total power and / or polarization).
  • the optical adaptation system when it is present is optimized so that the laser beam or the wave to be characterized keeps the characteristics at its desired first use after passing through the device in the active medium.
  • the optical adaptation system will be defined taking into account the characteristics of this electromagnetic wave.
  • One way of proceeding when one wishes to know the direction of the polarization of a linearly polarized pulsed or continuous laser beam is to use the rotating mount and maximize the value displayed on the device.
  • the mount is rotated until a maximum signal is displayed at the display.
  • the mount being graduated, its position gives the direction of the polarization of the beam. It is also possible to use this method to know the polarization of a linearly polarized electromagnetic wave.
  • the following example was obtained in the case of a pulsed laser, using as active medium a TGG crystal or terbium gallium garnet.
  • the example is illustrated in Figures 3A and 3B.
  • FIG. 3A represents a part of the measurement device constituting the measurement zone.
  • the signal coil 51 is brought into contact with the crystal 10 to be characterized, whereas the coil compensation 50 is arranged at a distance.
  • the characteristics and the shape of this double coil (signal and compensation) are chosen in such a way that any signal which does not result from the crystal is canceled.
  • the distance between the centers of each of the coils is, for example, 5 mm.
  • Each pickup coil is calibrated by measuring the signal obtained in a known modulated magnetic field.
  • the output signal of the coils is amplified by a fast low noise amplifier and filtered through a high pass filter. Two identical montages are used, on both sides of the crystal.
  • the laser beam passes through 2 polarizers 60, 61.
  • the second polarizer 61 fixes the polarization of the beam while the first polarizer 60 is used to change the power of the laser delivered to the TGG crystal by rotating its axis of rotation relative to the direction of polarization given by the first polarizer.
  • a waveguide 62 is placed after the polarizers to rotate the laser polarization as necessary.
  • follow-up mirrors 63 and a lens make it possible to deliver and focus the laser beam a few centimeters behind the TGG crystal.
  • the size of the crystal is 2 * 2 * 2 mm.
  • the shape of the crystal is a cube immersed in a magnetic field parallel to the direction [0, 0, 1].
  • the field value was in the range [0-2.5 T].
  • the vector k of light in this application is parallel to the direction [0, 0, 1] and perpendicular to the external magnetic field, that is to say parallel to the direction [0, 1, 0].
  • indicates a measured quantity for a polarization of the light parallel to the magnetic field and a siglel a quantity measured with a polarization of light perpendicular to the external field.
  • a typical laser pulse is represented with the corresponding signal detected by one of the two coil signals. Both signals are recorded on an oscilloscope with 1 GS / s.
  • the pulsed laser beam is controlled by extracting a small portion of the beam injected into the crystal with a beam splitter.
  • a diode fast is used to control the laser pulse.
  • the photodiode has been calibrated with respect to a device measuring the pulsed energy reaching the crystal.
  • the magnetism ICME in a TGG crystal can be defined as follows:
  • CICM denotes the constant of the Cotton Sheep Inverse effect specific to the active medium, Pd the power density of the light beam and B ex t the external magnetic field. The relation remains valid for a magnetic field intrinsic to the material.
  • This magnetization can be measured using a pick up coil if it varies over time. Indeed, the variation of the magnetization M (t) induces a potential difference V (t) at the terminals of the measuring coil according to the reference
  • g is the gain of the amplifier of the measuring coil.
  • P d is the density of the laser beam
  • B ex t is the transverse static magnetic field
  • b is a proportionality factor characterizing the ICME value. This factor depends on the properties of the medium which is illuminated by the laser beam and thus magnetized.
  • the ICME signal is proportional to the time derivative of the pulsed laser intensity as shown in FIG. 4A showing in a time axis diagram the value of the ICME signal and the value of the laser intensity.
  • FIG. 4B shows the magnetic flux density for a 2.5 T magnetic field value modifying the value of the pulsing energy of 0 to 0.250 J.
  • the data were obtained in two configurations of the laser polarization: one parallel to the magnetic field corresponding to the measured magnetic flux density Bp
  • the diameter of the laser spot in the crystal was 1.2 mm, corresponding to a laser energy density Pd in the range 0 - 2.2 x 10 13 W / m 2 .
  • Figure 4B shows that the magnetic flux density linearly depends on the laser power density.
  • V (t) -gxA xbx P. x- ⁇ * 1
  • the device according to the invention can combine three functionalities which, in the apparatus of the prior art known to the Applicant, are generally separated.
  • Another advantage provided by the device and the method according to the invention is to perform the measurements described above, without the need to extract or attenuate a portion of the beam.
  • the presented device can be inserted into an existing optical circuit without modifying it. It thus makes it possible to visualize the laser pulse and to measure its characteristics during the very use of the beam.
  • the set magnetic field and pickup coil can be arranged around a laser crystal, in order to measure temporal evolution of the power in the crystal.
  • Another possibility is to integrate the system into a Faraday isolator that becomes both a standard insulator and a power meter.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • Engineering & Computer Science (AREA)
  • Electromagnetism (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • Power Engineering (AREA)
  • Plasma & Fusion (AREA)
  • Measuring Magnetic Variables (AREA)
  • Photometry And Measurement Of Optical Pulse Characteristics (AREA)
  • Investigating Or Analysing Materials By Optical Means (AREA)

Abstract

The invention relates to a device (1) and a method for measuring a magnetization generated within an active medium (10) or characterizing a linearly polarized electromagnetic wave when said active medium exhibits an inverse Cotton-Mouton effect, characterized in that it includes, in combination, at least one of the following elements: an active medium (10) in which a linearly polarized electromagnetic wave propagates; a means (11, 12) for producing a transverse magnetic field Bt relative to the propagation direction D1 of said electromagnetic wave; and a device (2) for measuring the electrical signal appropriate for translating the magnetization generated within said active medium (10) by said electromagnetic wave.

Description

DISPOSITIF ET PROCEDE POUR CARACTERISER UN FAISCEAU  DEVICE AND METHOD FOR CHARACTERIZING A BEAM
LASER  LASER
L'invention concerne un dispositif et un procédé permettant notamment de caractériser un faisceau laser puisé de haute énergie ou encore un faisceau laser continu. Ainsi, la présente invention est utilisée pour mesurer la puissance instantanée d'un faisceau laser, ou l'énergie totale d'une impulsion laser et/ou la polarisation du faisceau. The invention relates to a device and a method that makes it possible in particular to characterize a pulsed laser beam of high energy or a continuous laser beam. Thus, the present invention is used to measure the instantaneous power of a laser beam, or the total energy of a laser pulse and / or the polarization of the beam.
Elle s'applique aussi lorsque l'on souhaite mesurer l'aimantation générée au sein d'un milieu actif présentant un effet Cotton-Mouton Inverse.  It is also applicable when it is desired to measure the magnetization generated within an active medium exhibiting an inverse Cotton-Sheep effect.
En général, la puissance des lasers de haute puissance est mesurée par absorption totale ou partielle du faisceau laser. Ceci conduit à consommer l'énergie du laser au niveau de la cible utilisée pour exécuter la mesure. Ceci se traduit par une perte d'énergie du faisceau et présente comme inconvénient de ne pas pouvoir utiliser le faisceau laser, simultanément à la mesure de son énergie ou de sa puissance. In general, the power of high power lasers is measured by total or partial absorption of the laser beam. This leads to consuming the laser energy at the target used to perform the measurement. This results in a loss of energy of the beam and has the disadvantage of not being able to use the laser beam, simultaneously with the measurement of its energy or its power.
Pour connaître la forme temporelle, il est connu d'extraire une partie du faisceau et de l'envoyer sur un autre appareil basé sur des photodiodes rapides. Pour déterminer la polarisation du faisceau laser il est nécessaire de posséder des polariseurs spécialement conçus pour les hautes puissances et d'effectuer la mesure de l'énergie de l'impulsion en fonction de la polarisation.  To know the temporal shape, it is known to extract a part of the beam and send it to another device based on fast photodiodes. To determine the polarization of the laser beam it is necessary to have polarizers specially designed for high power and to measure the energy of the pulse according to the polarization.
La figure 1 présente l'effet Cotton-Mouton inverse ou ICME produit dans un milieu 1 par un faisceau laser se propageant dans le milieu en présence d'un champ magnétique transverse à la direction d'un faisceau lumineux.  FIG. 1 shows the inverse Cotton-Sheep or ICME effect produced in a medium 1 by a laser beam propagating in the medium in the presence of a magnetic field transverse to the direction of a light beam.
La publication de Zon.B.A intitulée « observation of inverse Cotton-Mouton effect in the magnetically ordered crystal (Lu, Bi)3(FE, Ga)5Oi2, publié au JEPT Let. 45(1987)5, pages 272-275 décrit l'effet de Cotton-Mouton inverse. Cette publication décrit la mesure de l'aimantation induite par un faisceau laser puisé dans un film magnétique qui se trouve dans un champ magnétique extérieur statique orienté parallèlement à la direction de propagation du faisceau laser puisé. The Zon.BA publication titled "Observation of Reverse Cotton-Sheep Effect in the magnetically ordered crystal (Lu, Bi) 3 (FE, Ga) 5 O12, published in JEPT Let. 45 (1987) 5, pages 272-275 describes the inverse Cotton-Sheep effect. This publication describes the measurement of magnetization induced by a pulsed laser beam in a magnetic film which is in a static external magnetic field oriented parallel to the direction of propagation of the pulsed laser beam.
La publication de Marmo S.l intitulée « Electric field induced magnetization and inverse Cotton-Mouton effect in atomic gases », publiée dans Physics letters A, 202 (1995), pages 201 -205 divulgue la prédiction de l'effet Cotton-Mouton inverse dans les systèmes atomiques et moléculaires. Définitions des termes utilisés  The Marmo Sl publication entitled "Electric field induced magnetization and inverse Cotton-Sheep effect in atomic gases", published in Physics Letters A, 202 (1995), pages 201 -205 discloses the prediction of the Cotton-Sheep inverse effect in atomic and molecular systems. Definitions of terms used
Dans la suite de la description, l'expression « milieu actif >> désigne un matériau, un cristal, un verre, un gaz, un liquide qui, lorsqu'il est soumis à un champ magnétique, va présenter un effet Cotton-Mouton inverse.  In the remainder of the description, the expression "active medium" designates a material, a crystal, a glass, a gas, a liquid which, when subjected to a magnetic field, will have a reverse Cotton-Sheep effect. .
Le terme « caractériser ou caractérisât! on >> d'un faisceau laser sera utilisé pour mentionner une mesure de puissance instantanée du faisceau, une mesure de puissance ou encore la détermination de la polarisation du faisceau.  The term "characterize or characterize! a laser beam will be used to mention an instantaneous power measurement of the beam, a power measurement or the determination of the polarization of the beam.
L'invention concerne un dispositif permettant de mesurer une aimantation générée au sein d'un milieu actif ou caractériser une onde électromagnétique polarisée linéairement lorsque ledit milieu actif présente un effet Cotton-Mouton Inverse, caractérisé en ce qu'il comporte en combinaison au moins les éléments suivants : The invention relates to a device for measuring a magnetization generated within an active medium or to characterize a linearly polarized electromagnetic wave when said active medium exhibits an inverse Cotton-Sheep effect, characterized in that it comprises in combination at least the following elements:
■ un milieu actif dans lequel se propage une onde électromagnétique polarisée linéairement, ■ an active medium in which propagates an electromagnetic wave linearly polarized,
■ un moyen de production d'un champ magnétique transverse Bt, par rapport à la direction de propagation de ladite onde électromagnétique, ■ means for producing a transverse magnetic field B t with respect to the direction of propagation of said electromagnetic wave,
■ un dispositif de mesure du signal électrique traduisant l'aimantation générée au sein dudit milieu actif traversé par ladite onde électromagnétique. Dans le cas d'une onde électromagnétique, traversant un milieu actif, le dispositif de mesure permet de caractériser l'onde électromagnétique par au moins un des paramètres suivants : la puissance instantanée de l'onde électromagnétique, la puissance intégrale ou encore la polarisation de l'onde. ■ a device for measuring the electric signal expressing the magnetization generated within said active medium through said electromagnetic wave. In the case of an electromagnetic wave, passing through an active medium, the measuring device makes it possible to characterize the electromagnetic wave by at least one of the following parameters: the instantaneous power of the electromagnetic wave, the integral power or the polarization of the electromagnetic wave. wave.
Selon un mode de réalisation, l'onde électromagnétique est un faisceau laser puisé et le dispositif de mesure caractérise le faisceau laser puisé par au moins un des paramètres suivants : la puissance instantanée d'une impulsion dudit faisceau laser, la puissance intégrale d'une impulsion dudit faisceau laser, la polarisation dudit faisceau laser.  According to one embodiment, the electromagnetic wave is a pulsed laser beam and the measuring device characterizes the laser beam pulsed by at least one of the following parameters: the instantaneous power of a pulse of said laser beam, the integral power of a pulse of said laser beam, the polarization of said laser beam.
Selon un autre mode de réalisation, l'onde électromagnétique est un faisceau laser continu et le dispositif de mesure caractérise le faisceau laser par au moins un des paramètres suivants : la puissance instantanée dudit faisceau laser, la puissance intégrale dudit faisceau laser, la polarisation dudit faisceau laser, le champ magnétique étant variable dans le temps.  According to another embodiment, the electromagnetic wave is a continuous laser beam and the measuring device characterizes the laser beam by at least one of the following parameters: the instantaneous power of said laser beam, the integral power of said laser beam, the polarization of said laser beam; laser beam, the magnetic field being variable in time.
Ledit milieu actif est, par exemple, soumis à un champ magnétique extérieur statique Bext variable ou constant dans le temps. Said active medium is, for example, subjected to a static external magnetic field B ex t variable or constant over time.
Le dispositif de mesure du signal comporte, par exemple, au moins une bobine de type pickup.  The signal measuring device comprises, for example, at least one pickup type coil.
Le dispositif de mesures du signal peut comporter au moins deux bobines de type pickup placées de part et d'autre du milieu actif, la normale à leur surface, étant orientée sensiblement parallèlement au champ magnétique Bext. The signal measuring device may comprise at least two pickup type coils placed on either side of the active medium, the normal to their surface being oriented substantially parallel to the magnetic field B ex t.
Le dispositif de mesure électronique du signal traduisant la valeur d'énergie instantanée de l'onde électromagnétique, ou la valeur de la puissance de ladite onde électromagnétique comporte les éléments suivants :  The electronic signal measuring device translating the instantaneous energy value of the electromagnetic wave, or the value of the power of said electromagnetic wave comprises the following elements:
■ un sommateur et amplificateur de bas bruit ayant pour fonction d'éliminer le bruit parasite ne correspondant pas au signal associé à l'effet Cotton-Mouton inverse, le signal sommé et amplifié étant transmis à ■ a summing and low noise amplifier operable to remove extraneous noise not corresponding to the signal associated with the inverse Cotton-Sheep effect, the summed and amplified signal being transmitted to
■ un filtre passe haut, puis à un intégrateur avant d'être transmis à un dispositif d'affichage et/ou à une mémoire de stockage. ■ a high pass filter, then to an integrator before being transmitted to a display device and / or a storage memory.
Le dispositif comporte, par exemple, une monture tournante dans lequel sont disposés le milieu actif, les moyens de production du champ magnétique.  The device comprises, for example, a rotating mount in which are disposed the active medium, the means for producing the magnetic field.
Le dispositif peut comporter un système d'adaptation optique. Ledit milieu actif est un cristal de TGG ou Terbium Gallium The device may include an optical matching system. Said active medium is a TGG or Terbium Gallium crystal
Garnet. Garnet.
L'invention concerne aussi un procédé pour mesurer une aimantation générée au sein d'un milieu actif, lorsque ledit milieu actif présente un effet Cotton-Mouton Inverse, le procédé étant mis en œuvre au sein d'un dispositif présentant l'une des caractéristiques précitées, le procédé comportant au moins les étapes suivantes :  The invention also relates to a method for measuring a magnetization generated within an active medium, when said active medium exhibits a Cotton-Sheep Inverse effect, the method being implemented within a device having one of the characteristics above, the method comprising at least the following steps:
■ transmettre dans un milieu actif une onde électromagnétique polarisée linéairement, ■ transmit in an active medium a linearly polarized electromagnetic wave,
générer un champ magnétique transverse Bt par rapport à la direction de propagation D| de ladite onde électromagnétique, generating a transverse magnetic field B t with respect to the propagation direction D | of said electromagnetic wave,
■ mesurer le signal électrique traduisant l'aimantation générée au sein dudit milieu actif par ladite onde électromagnétique. ■ measure the electrical signal translating the magnetization generated within said active medium by said electromagnetic wave.
D'autres caractéristiques et avantages de la présente invention apparaîtront mieux à la lecture de la description d'un ou de plusieurs modes de réalisation donnés à titre illustratif et nullement limitatif annexés des figures qui représentent : Other features and advantages of the present invention will appear better on reading the description of one or more embodiments given by way of illustration and in no way limitatively appended to the figures which represent:
■ la figure 1 , une représentation de l'effet inverse de Cotton-Mouton, ■ Figure 1, a representation of the inverse effect of Cotton-Mouton,
■ la figure 2A, un exemple de dispositif de mesure selon l'invention, et la figure 2B, un exemple de circuit électronique associé, ■ Figure 2A, an example of measurement device according to the invention, and Figure 2B, an example of associated electronic circuit,
■ les figures 3A et 3B, une variante de réalisation du dispositif de la figure 2A, et ■ les figures 4A, 4B les résultats obtenus en utilisant un cristal de terbium gallium garnet. ■ Figures 3A and 3B, an alternative embodiment of the device of Figure 2A, and ■ Figures 4A, 4B the results obtained using a terbium gallium garnet crystal.
De manière générale, le dispositif selon l'invention permet de mesurer une aimantation générée au sein d'un milieu actif lorsque le milieu actif présente un effet Cotton-Mouton Inverse. Les exemples illustrés sur les figures concernent une application à la caractérisation d'un faisceau laser puisé ou continu, mais peuvent sans sortir du cadre de l'invention s'appliquer dans le cas d'une onde électromagnétique polarisée linéairement. In general, the device according to the invention makes it possible to measure a magnetization generated within an active medium when the active medium has an inverse Cotton-Sheep effect. The examples illustrated in the figures relate to an application to the characterization of a pulsed or continuous laser beam, but without departing from the scope of the invention can be applied in the case of a linearly polarized electromagnetic wave.
La figure 2A schématise un exemple de dispositif donné afin d'illustrer les éléments du dispositif 1 selon l'invention dans le cas d'une application à la caractérisation d'un faisceau laser puisé.  FIG. 2A schematizes an example of a given device in order to illustrate the elements of the device 1 according to the invention in the case of an application to the characterization of a pulsed laser beam.
Sur cette figure 2A un milieu actif 10, est disposé entre deux aimants permanents 1 1 , 12 qui fournissent un champ magnétique statique transverse Bt orienté perpendiculairement à la direction de propagation D| du faisceau laser 13 à caractériser, sous l'effet d'un champ magnétique extérieur. Deux bobines pickup 14, 15, par exemple, détectent le signal d'aimantation dû à la propagation du faisceau laser dans le milieu actif et à la présence du champ magnétique Bt. Ce signal d'aimantation est variable en fonction du temps. Les bobines pickup 14, 15 sont, par exemple, placées de part et d'autre du milieu actif 10. La normale à leur surface Au, Ai5, est orientée parallèlement au champ magnétique Bt. Les signaux électriques Su, Sis générés au niveau de chacune des bobines sont transmis à un circuit électronique de mesure dont un exemple de réalisation est donné à la figure 2B. In this FIG. 2A, an active medium 10 is disposed between two permanent magnets 1 1, 12 which provide a transverse static magnetic field B t oriented perpendicularly to the direction of propagation D | of the laser beam 13 to be characterized, under the effect of an external magnetic field. Two pickup coils 14, 15, for example, detect the magnetization signal due to the propagation of the laser beam in the active medium and the presence of the magnetic field B t . This magnetization signal is variable as a function of time. The pickup coils 14, 15 are, for example, placed on either side of the active medium 10. The normal to their surface Au, Ai 5 , is oriented parallel to the magnetic field B t . The electrical signals Su, Sis generated at each of the coils are transmitted to an electronic measuring circuit, an exemplary embodiment of which is given in FIG. 2B.
Dans le cas d'un milieu actif de type ferromagnétique, il n'est pas nécessaire d'utiliser un champ magnétique extérieur Bext pour obtenir l'effet inverse de Cotton-Mouton, le champ magnétique Bt est intrinsèque au matériau. Le dispositif peut aussi être utilisé pour caractériser un faisceau laser continu. Dans ce cas, le champ magnétique transverse intrinsèque au matériau Bt, ou le champ magnétique Bext utilisé, est un champ magnétique variable dans le temps, dont la loi de variation temporelle est connue. Dans ce cas, il est possible de mesurer la valeur d'une puissance constante ou sensiblement constante du faisceau laser continu. In the case of an active medium of ferromagnetic type, it is not necessary to use an external magnetic field B ex t to obtain the opposite effect of Cotton-Sheep, the magnetic field B t is intrinsic to the material. The device can also be used to characterize a continuous laser beam. In this case, the transverse magnetic field intrinsic to the material B t , or the magnetic field B ex t used, is a time-varying magnetic field whose temporal variation law is known. In this case, it is possible to measure the value of a constant or substantially constant power of the continuous laser beam.
Dans le cas d'une onde électromagnétique, traversant un milieu actif, le même dispositif s'applique. Le dispositif de mesure permet de caractériser l'onde électromagnétique par au moins un des paramètres suivants : la puissance instantanée de l'onde électromagnétique, la puissance intégrale ou encore la polarisation de l'onde.  In the case of an electromagnetic wave passing through an active medium, the same device applies. The measuring device makes it possible to characterize the electromagnetic wave by at least one of the following parameters: the instantaneous power of the electromagnetic wave, the integral power or the polarization of the wave.
La forme et le nombre de spires des bobines sont choisis en fonction par exemple de la variation de flux magnétique. Il est possible d'utiliser des bobines pickup de type planaire. Il serait aussi envisageable d'utiliser des bobines ayant une surface courbe qui suit au mieux les lignes de champ du champ magnétique extérieur.  The shape and the number of turns of the coils are chosen as a function, for example, of the magnetic flux variation. It is possible to use planar-type pickup coils. It would also be possible to use coils having a curved surface which best follows the field lines of the external magnetic field.
L'exemple donné mentionne deux bobines, mais sans sortir du cadre de l'invention, on pourrait effectuer la caractérisation du faisceau laser ou d'une onde électromagnétique en utilisant une seule bobine pickup ou un nombre de bobines supérieur à 2 selon l'application. De même, comme il sera présenté en figure 3A, les bobines pickup peuvent être reliées à des bobines de compensation permettant de limiter, voire annuler des effets parasites.  The example given mentions two coils, but without departing from the scope of the invention, the characterization of the laser beam or of an electromagnetic wave could be carried out using a single pickup coil or a number of coils greater than 2 depending on the application. . Likewise, as will be presented in FIG. 3A, the pickup coils can be connected to compensation coils making it possible to limit or even cancel parasitic effects.
Le milieu actif 10 est un milieu qui présente un effet Cotton- Mouton inverse lorsqu'il est soumis à un champ magnétique extérieur Bext ou bien au champ magnétique intrinsèque Bt dans le cas de milieu ferromagnétique ou d'autres milieux qui n'ont pas besoin de sollicitation extérieure. Il est ainsi possible d'utiliser du cristal ou du verre. Des milieux actifs liquides ou gazeux peuvent aussi être envisagés. Les dimensions et la nature du milieu actif seront choisies en fonction de l'application souhaitée. Par exemple, pour une utilisation dans le cadre des lasers très intenses, il est possible de choisir un milieu actif couplé à système d'adaptation optique de dimensions telles que la densité d'énergie du faisceau reste en dessous du seuil d'endommagement du milieu actif. La nature du milieu actif pourra aussi être choisie en fonction de la longueur d'onde du laser. The active medium 10 is a medium which exhibits an inverse Cotton-Sheep effect when it is subjected to an external magnetic field B ex t or to the intrinsic magnetic field B t in the case of a ferromagnetic medium or other media which do not need external solicitation. It is thus possible to use crystal or glass. Active liquid or gaseous media can also be envisaged. The dimensions and the nature of the active medium will be chosen according to the desired application. For example, for use in the context of very intense lasers, it It is possible to choose an active medium coupled to an optical adaptation system of dimensions such that the energy density of the beam remains below the damage threshold of the active medium. The nature of the active medium may also be chosen according to the wavelength of the laser.
La figure 2A montre un système optique d'adaptation 20, représenté par une lentille d'entrée 201 et une lentille de sortie 202. Ce système permet avantageusement d'adapter la taille du faisceau laser puisé ou continu à caractériser aux dimensions de l'appareil. Lorsque le dispositif de caractérisation selon l'invention est positionné dans un système existant, ce système d'adaptation permet notamment d'adapter la taille du faisceau laser à caractériser aux systèmes optiques existants dans le système.  FIG. 2A shows an optical matching system 20, represented by an input lens 201 and an exit lens 202. This system advantageously makes it possible to adapt the size of the pulsed or continuous laser beam to be characterized to the dimensions of the apparatus . When the characterization device according to the invention is positioned in an existing system, this adaptation system notably makes it possible to adapt the size of the laser beam to be characterized to the optical systems existing in the system.
L'ensemble comprenant le milieu actif 1 0, les aimants 1 1 , 1 2 et les bobines 1 4, 1 5 peuvent être positionnés à l'intérieur d'une monture 30 tournante qui permet de tourner par rapport à un axe AR parallèle à la direction de propagation du faisceau laser de façon à régler l'angle δ entre la direction du champ magnétique transverse BT et la polarisation du laser dont la valeur de l'effet Cotton-Mouton inverse dépend. En mesurant le signal ICME en fonction de l'angle δ, on peut déterminer l'état de polarisation de laser, et en particulier son ellipticité. The assembly comprising the active medium 1 0, the magnets 1 1, 1 2 and the coils 1 4, 1 5 can be positioned inside a rotating mount 30 which allows rotation relative to an axis AR parallel to the direction of propagation of the laser beam so as to adjust the angle δ between the direction of the transverse magnetic field B T and the polarization of the laser, the value of the inverse Cotton-Sheep effect depends. By measuring the ICME signal as a function of the angle δ, it is possible to determine the laser polarization state, and in particular its ellipticity.
La figure 2B représente un exemple de circuit électronique 2 associé au dispositif de caractérisation du faisceau laser puisé.  FIG. 2B represents an example of an electronic circuit 2 associated with the device for characterizing the pulsed laser beam.
Dans cet exemple, les deux bobines pickup 1 4, 1 5 de la figure 2A sont reliées à un sommateur et amplificateur 40 de bas bruit ayant pour fonction d'éliminer le bruit parasite ne correspondant pas au signal associé à l'effet Cotton-Mouton inverse. Le signal sommé et amplifié St est transmis à un filtre passe haut 41 , puis à un intégrateur 42 avant d'être transmis à un dispositif d'affichage 43 et/ou à une mémoire 44 de stockage. In this example, the two pickup coils 1 4, 1 5 of FIG. 2A are connected to an adder and amplifier 40 of low noise whose function is to eliminate the parasitic noise that does not correspond to the signal associated with the Cotton-Sheep effect. reverse. The summed and amplified signal S t is transmitted to a high-pass filter 41, then to an integrator 42 before being transmitted to a display device 43 and / or a storage memory 44.
Ce circuit électronique peut sans sortir du cadre de l'invention être mis en œuvre pour l'application à une mesure d'aimantation générée au sein d'un milieu actif ou encore pour caractériser un faisceau laser continu. Le principe de fonctionnement du dispositif selon l'invention est, par exemple, le suivant : le dispositif de caractérisation d'un faisceau laser ou d'une onde électromagnétique selon l'invention est positionné sur le trajet optique du faisceau laser ou de l'onde électromagnétique à caractériser (mesure de la puissance instantanée, de la puissance totale et/ou de la polarisation). Le système optique d'adaptation lorsqu'il est présent est optimisé de façon telle que le faisceau laser ou l'onde à caractériser conserve les caractéristiques à son utilisation première souhaitée après passage dans le dispositif, dans le milieu actif. Dans le cas de la mesure d'une aimantation (onde électromagnétique), le système optique d'adaptation sera défini en tenant compte des caractéristiques de cette onde électromagnétique. This electronic circuit can without departing from the scope of the invention be implemented for application to a magnetization measurement generated within an active medium or to characterize a continuous laser beam. The operating principle of the device according to the invention is, for example, the following: the device for characterizing a laser beam or an electromagnetic wave according to the invention is positioned on the optical path of the laser beam or the electromagnetic wave to be characterized (measurement of instantaneous power, total power and / or polarization). The optical adaptation system when it is present is optimized so that the laser beam or the wave to be characterized keeps the characteristics at its desired first use after passing through the device in the active medium. In the case of the measurement of a magnetization (electromagnetic wave), the optical adaptation system will be defined taking into account the characteristics of this electromagnetic wave.
Une manière de procéder lorsque l'on souhaite connaître la direction de la polarisation d'un faisceau laser puisé ou continu polarisé linéairement est d'utiliser la monture tournante et de maximiser la valeur affichée sur le dispositif. Dans ce cas, on tourne la monture jusqu'à faire apparaître un signal maximum relevé au niveau du dispositif d'affichage. La monture étant graduée, sa position donne la direction de la polarisation du faisceau. Il est aussi possible d'utiliser cette méthode pour connaître la polarisation d'une onde électromagnétique polarisée linéairement.  One way of proceeding when one wishes to know the direction of the polarization of a linearly polarized pulsed or continuous laser beam is to use the rotating mount and maximize the value displayed on the device. In this case, the mount is rotated until a maximum signal is displayed at the display. The mount being graduated, its position gives the direction of the polarization of the beam. It is also possible to use this method to know the polarization of a linearly polarized electromagnetic wave.
L'exemple qui suit a été obtenu dans le cas d'un laser puisé, en utilisant comme milieu actif un cristal de TGG ou terbium gallium garnet. La source laser utilisée est un laser Nd :YAG (lambda = 1064 nm) générant des puises de lumière d'une durée de 10ns et une énergie d'environ 0.5 J/pulse. L'exemple est illustré aux figures 3A et 3B.  The following example was obtained in the case of a pulsed laser, using as active medium a TGG crystal or terbium gallium garnet. The laser source used is an Nd: YAG laser (lambda = 1064 nm) generating light pulses with a duration of 10 ns and an energy of approximately 0.5 J / pulse. The example is illustrated in Figures 3A and 3B.
Des changements dans la magnétisation du cristal ont été mesurés en utilisant un dispositif comprenant une bobine double pickup telle que celle décrite à la figure 2A, mais cette fois-ci constituée d'une bobine de compensation 50 et d'une bobine de mesure 51 . La figure 3A représente une partie du dispositif de mesure constituant la zone de mesure. La bobine signal 51 est mise au contact du cristal 10 à caractériser, alors que la bobine de compensation 50 est disposée à une certaine distance. Les caractéristiques et la forme de cette bobine double (signal et compensation) sont choisies de façon telle que tout signal qui ne résulte pas du cristal soit annulé. La distance entre les centres de chacune des bobines est, par exemple, de 5 mm. Chaque bobine pickup est calibrée en mesurant le signal obtenu dans un champ magnétique modulé connu. Le signal de sortie des bobines est amplifié par un amplificateur rapide faible bruit et filtré au travers d'un filtre passe haut. Deux montages identiques sont utilisés, de part et d'autre du cristal. Changes in the magnetization of the crystal have been measured using a device comprising a double pickup coil such as that described in FIG. 2A, but this time consisting of a compensation coil 50 and a measuring coil 51. FIG. 3A represents a part of the measurement device constituting the measurement zone. The signal coil 51 is brought into contact with the crystal 10 to be characterized, whereas the coil compensation 50 is arranged at a distance. The characteristics and the shape of this double coil (signal and compensation) are chosen in such a way that any signal which does not result from the crystal is canceled. The distance between the centers of each of the coils is, for example, 5 mm. Each pickup coil is calibrated by measuring the signal obtained in a known modulated magnetic field. The output signal of the coils is amplified by a fast low noise amplifier and filtered through a high pass filter. Two identical montages are used, on both sides of the crystal.
Sur la figure 3B, le faisceau laser passe à travers 2 polariseurs 60, 61 . Le second polariseur 61 fixe la polarisation du faisceau alors que le premier polariseur 60 est utilisé pour changer la puissance du laser délivrée au cristal TGG en faisant tourner son axe de rotation par rapport à la direction de polarisation donnée par le premier polariseur. Une lame demi- onde 62 est placée après les polariseurs pour faire tourner la polarisation laser si nécessaire. Des miroirs suiveurs 63 et une lentille permettent de délivrer et de focaliser le faisceau laser quelques centimètres derrière le cristal TGG. La taille du cristal est de 2*2*2 mm. Dans cet exemple, la forme du cristal est un cube immergé dans un champ magnétique parallèle à la direction [0, 0, 1 ]. La valeur du champ était comprise dans l'intervalle [0-2,5 T]. Le vecteur k de la lumière dans cette application est parallèle à la direction [0, 0, 1 ] et perpendiculaire au champ magnétique externe, c'est-à- dire parallèle à la direction [0, 1 , 0]. Dans la suite le sigle || indique une quantité mesurée pour une polarisation de la lumière parallèle au champ magnétique et un siglel une quantité mesurée avec une polarisation de lumière perpendiculaire au champ externe. In FIG. 3B, the laser beam passes through 2 polarizers 60, 61. The second polarizer 61 fixes the polarization of the beam while the first polarizer 60 is used to change the power of the laser delivered to the TGG crystal by rotating its axis of rotation relative to the direction of polarization given by the first polarizer. A waveguide 62 is placed after the polarizers to rotate the laser polarization as necessary. Follow-up mirrors 63 and a lens make it possible to deliver and focus the laser beam a few centimeters behind the TGG crystal. The size of the crystal is 2 * 2 * 2 mm. In this example, the shape of the crystal is a cube immersed in a magnetic field parallel to the direction [0, 0, 1]. The field value was in the range [0-2.5 T]. The vector k of light in this application is parallel to the direction [0, 0, 1] and perpendicular to the external magnetic field, that is to say parallel to the direction [0, 1, 0]. In the following the acronym || indicates a measured quantity for a polarization of the light parallel to the magnetic field and a siglel a quantity measured with a polarization of light perpendicular to the external field.
Dans la figure 4A, sont représentés une impulsion laser typique avec le signal correspondant détecté par un des deux signaux bobine. Les deux signaux sont enregistrés sur un oscilloscope avec 1 GS/s.  In FIG. 4A, a typical laser pulse is represented with the corresponding signal detected by one of the two coil signals. Both signals are recorded on an oscilloscope with 1 GS / s.
Le faisceau laser puisé est contrôlé en extrayant une faible partie du faisceau injecté dans le cristal avec un séparateur de faisceau. Une diode rapide est utilisée pour contrôler l'impulsion laser. La photodiode a été calibrée par rapport à un dispositif mesurant l'énergie puisée atteignant le cristal. The pulsed laser beam is controlled by extracting a small portion of the beam injected into the crystal with a beam splitter. A diode fast is used to control the laser pulse. The photodiode has been calibrated with respect to a device measuring the pulsed energy reaching the crystal.
La magnétisation ICME dans un cristal TGG peut être définie comme suit :  The magnetism ICME in a TGG crystal can be defined as follows:
où CICM désigne la constante de l'effet Cotton Mouton Inverse propre au milieu actif, Pd la densité de puissance du faisceau lumineux et Bext le champ magnétique externe. La relation reste valable pour un champ magnétique intrinsèque au matériau. where CICM denotes the constant of the Cotton Sheep Inverse effect specific to the active medium, Pd the power density of the light beam and B ex t the external magnetic field. The relation remains valid for a magnetic field intrinsic to the material.
Cette magnétisation peut être mesurée à l'aide d'une bobine pick up si elle varie dans le temps. En effet, la variation de la magnétisation M(t) induit une différence de potentiel V(t) aux bornes de la bobine de mesure suivant la r lation This magnetization can be measured using a pick up coil if it varies over time. Indeed, the variation of the magnetization M (t) induces a potential difference V (t) at the terminals of the measuring coil according to the reference
où g est le gain de l'amplificateur de la bobine de mesure. Ae= 1 0 mm2 est la zone effectivement calibrée de la bobine signal et Bp est la densité du flux magnétique au travers de la surface de la bobine de mesure produite par l'aimantation M du cristal. where g is the gain of the amplifier of the measuring coil. A e = 10 mm 2 is the effectively calibrated area of the signal coil and B p is the density of the magnetic flux across the surface of the measuring coil produced by the magnetization M of the crystal.
On remarque alors que la variation temporelle de Bp(t) peut être réalisée de deux manières (voir relation (1 )) : We notice then that the temporal variation of B p (t) can be realized in two ways (see relation (1)):
- a) en faisant varier la densité de puissance Pd du faisceau (laser puisé ou modulé),  a) by varying the power density Pd of the beam (pulsed or modulated laser),
- b) en faisant varier le champ magnétique externe.  - b) by varying the external magnetic field.
Dans le cas a), la variation de Bp(t) s'écrit :In case a), the variation of B p (t) is written:
dBp (t)/dt = bBext (3) dB p (t) / dt = bBext (3)
dt et le signal ICME V(t) s'écrit alors : dt and the signal ICME V (t) is written then:
où Pd est la densité du faisceau laser, Bext est le champ magnétique statique transverse et b est un facteur de proportionnalité caractérisant la valeur ICME. Ce facteur dépend des propriétés du milieu qui est illuminé par le faisceau laser et ainsi magnétisé. where P d is the density of the laser beam, B ex t is the transverse static magnetic field and b is a proportionality factor characterizing the ICME value. This factor depends on the properties of the medium which is illuminated by the laser beam and thus magnetized.
Ainsi, le signal ICME est proportionnel à la dérivée temporelle de l'intensité du laser puisé comme il est représenté à la figure 4A montrant dans un diagramme d'axe temporel la valeur du signal ICME et la valeur de l'intensité laser.  Thus, the ICME signal is proportional to the time derivative of the pulsed laser intensity as shown in FIG. 4A showing in a time axis diagram the value of the ICME signal and the value of the laser intensity.
La figure 4B représente la densité de flux magnétique pour une valeur de champ magnétique 2.5 T modifiant la valeur de l'énergie de puise de 0 à 0.250 J. les données ont été obtenues dans deux configurations de la polarisation laser : l'une parallèle au champ magnétique correspondant à la densité de flux magnétique mesuré Bp||, l'autre perpendiculaire au champ magnétique correspondant à Bpl. Le diamètre du spot laser dans le cristal était de 1 .2 mm, correspondant à une densité d'énergie laser Pd comprise dans la gamme 0 - 2.2 x 1013 W/m2. La figure 4B montre que la densité de flux magnétique dépend linéairement de la densité de puissance laser. FIG. 4B shows the magnetic flux density for a 2.5 T magnetic field value modifying the value of the pulsing energy of 0 to 0.250 J. The data were obtained in two configurations of the laser polarization: one parallel to the magnetic field corresponding to the measured magnetic flux density Bp ||, the other perpendicular to the magnetic field corresponding to Bpl. The diameter of the laser spot in the crystal was 1.2 mm, corresponding to a laser energy density Pd in the range 0 - 2.2 x 10 13 W / m 2 . Figure 4B shows that the magnetic flux density linearly depends on the laser power density.
Pour une magnétisation M de 1 A/m, il a été trouvé une densité de champ magnétique d'environ 4x10"8 T. En utilisant un facteur de conversion f entre la valeur de la densité du flux Bp et la magnétisation M du cristal d'environ 2.5 x 107 (A/m)T1. For magnetization M 1 A / m, it has been found a magnetic field density of approximately 4x10 "8 T. Using a conversion factor f between the density value of Bp flux and the magnetization M of the crystal about 2.5 x 10 7 (A / m) T 1 .
Dans le cas b), en faisant varier le champ magnétique externe la variation de Bp(t) s'écrit : In case b), by varying the external magnetic field, the variation of B p (t) is written:
dB .  dB.
dB (t)/dt = bxP . x— ^ dB (t) / dt = bxP. x- ^
Pw cl dt P w cl dt
Le signal ICME V(t) s'écrit alors : dB . The ICME signal V (t) is then written: dB.
V(t) = -gxA xbx P . x— ^*1 V (t) = -gxA xbx P. x- ^ * 1
w a e d dt w a ed dt
Cette fois le faisceau est continu et c'est le champ magnétique qui est puisé.  This time the beam is continuous and it is the magnetic field that is drawn.
L'invention offre notamment les avantages suivants : The invention notably offers the following advantages:
• mesurer à la fois la puissance instantanée d'un faisceau laser puisé de haute puissance avec une réponse temporelle inférieure à la nanoseconde, l'énergie totale de l'impulsion laser et la polarisation du faisceau ;  • measure both the instantaneous power of a high power pulsed laser beam with a time response of less than a nanosecond, the total energy of the laser pulse and the polarization of the beam;
• mesurer la puissance d'un laser continu si on applique un champ magnétique variable ;  • measure the power of a continuous laser if a variable magnetic field is applied;
• réaliser une mesure d'aimantation au sein d'un matériau qui présente un effet Cotton-Mouton Inverse.  • perform a magnetization measurement in a material that has a Cotton-Sheep Inverse effect.
Le dispositif selon l'invention peut combiner trois fonctionnalités qui, dans les appareils de l'art antérieur connus du Demandeur sont en général séparées.  The device according to the invention can combine three functionalities which, in the apparatus of the prior art known to the Applicant, are generally separated.
Un autre avantage procuré par le dispositif et le procédé selon l'invention est de pouvoir effectuer les mesures décrites précédemment, sans avoir besoin d'extraire ou d'atténuer une partie du faisceau. Le dispositif présenté peut être inséré dans un circuit optique existant sans le modifier. Il permet donc de visualiser l'impulsion laser et de mesurer ses caractéristiques pendant l'utilisation même du faisceau.  Another advantage provided by the device and the method according to the invention is to perform the measurements described above, without the need to extract or attenuate a portion of the beam. The presented device can be inserted into an existing optical circuit without modifying it. It thus makes it possible to visualize the laser pulse and to measure its characteristics during the very use of the beam.
Selon un exemple d'utilisation, l'ensemble champ magnétique et bobine pickup peut être disposé autour d'un cristal laser, afin de mesurer évolution temporelle de la puissance dans le cristal. Une autre possibilité est d'intégrer le système dans un isolateur Faraday devenant à la fois un isolateur standard et un appareil de mesures de puissance.  According to an example of use, the set magnetic field and pickup coil can be arranged around a laser crystal, in order to measure temporal evolution of the power in the crystal. Another possibility is to integrate the system into a Faraday isolator that becomes both a standard insulator and a power meter.

Claims

REVENDICATIONS
1 - Dispositif (1 ) permettant de mesurer une aimantation générée au sein d'un milieu actif (10) ou caractériser une onde électromagnétique polarisée linéairement lorsque ledit milieu actif présente un effet Cotton-Mouton Inverse, caractérisé en ce qu'il comporte en combinaison au moins les éléments suivants : 1 - Device (1) for measuring a magnetization generated within an active medium (10) or to characterize a linearly polarized electromagnetic wave when said active medium exhibits an inverse Cotton-Sheep effect, characterized in that it comprises in combination at least the following:
■ un milieu actif (10) dans lequel se propage une onde électromagnétique polarisée linéairement, ■ an active medium (10) in which propagates an electromagnetic wave linearly polarized,
■ un moyen de production (1 1 , 12) d'un champ magnétique transverse Bt par rapport à la direction de propagation D| de ladite onde électromagnétique, ■ producing means (1 1, 12) a transverse magnetic field B t with respect to the propagation direction D | of said electromagnetic wave,
■ un dispositif de mesure (2) du signal électrique adapté à traduire l'aimantation générée au sein dudit milieu actif (1 0) par ladite onde électromagnétique. ■ a measuring device (2) of the electrical signal adapted to translate the magnetization generated within said active medium (1 0) by said electromagnetic wave.
2 - Dispositif selon la revendication 1 caractérisé en ce que l'onde électromagnétique est un faisceau laser puisé, ledit dispositif de mesure étant adapté à caractériser ledit faisceau laser puisé par au moins un des paramètres suivants : la puissance instantanée d'une impulsion dudit faisceau laser, la puissance intégrale d'une impulsion dudit faisceau laser, la polarisation dudit faisceau laser. 2 - Device according to claim 1 characterized in that the electromagnetic wave is a pulsed laser beam, said measuring device being adapted to characterize said pulsed laser beam by at least one of the following parameters: the instantaneous power of a pulse of said beam laser, the integral power of a pulse of said laser beam, the polarization of said laser beam.
3 - Dispositif selon la revendication 1 caractérisé en ce que l'onde électromagnétique est un faisceau laser continu, le champ magnétique transverse étant variable dans le temps et en ce que ledit dispositif de mesure étant adapté à caractériser ledit faisceau laser continu par au moins un des paramètres suivants : la puissance instantanée dudit faisceau laser, la puissance intégrale, la polarisation dudit faisceau laser. 4 - Dispositif selon la revendication 1 caractérisé en ce que ledit milieu actif (10) est soumis à un champ magnétique extérieur statique Bext ayant une valeur constante ou variable dans le temps 3 - Device according to claim 1 characterized in that the electromagnetic wave is a continuous laser beam, the transverse magnetic field being variable in time and in that said measuring device being adapted to characterize said continuous laser beam by at least one the following parameters: the instantaneous power of said laser beam, the integral power, the polarization of said laser beam. 4 - Device according to claim 1 characterized in that said active medium (10) is subjected to a static external magnetic field B ex t having a constant value or variable in time
5 - Dispositif selon la revendication 1 caractérisé en ce que le dispositif de mesure du signal (2) comporte au moins une bobine de type pickup (14, 15). 5 - Device according to claim 1 characterized in that the signal measuring device (2) comprises at least one pickup type coil (14, 15).
6 - Dispositif selon la revendication 5 caractérisé en ce que le dispositif de mesure du signal (2) comporte au moins deux bobines de type pickup (14, 15) placées de part et d'autre du milieu actif (10), la normale à leur surface (A 4, A 5), étant orientée sensiblement parallèlement au champ magnétique6 - Device according to claim 5 characterized in that the signal measuring device (2) comprises at least two pickup type coils (14, 15) placed on either side of the active medium (10), the normal to their surface (A 4 , A 5 ) being oriented substantially parallel to the magnetic field
Bext- Bext-
7 - Dispositif selon la revendication 1 caractérisé en ce que ledit dispositif de mesure électronique (2) du signal adapté à traduire la valeur d'énergie instantanée de l'onde électromagnétique, ou la valeur de la puissance de l'onde électromagnétique comporte les éléments suivants : 7 - Device according to claim 1 characterized in that said electronic measuring device (2) of the signal adapted to translate the instantaneous energy value of the electromagnetic wave, or the value of the power of the electromagnetic wave comprises the elements following:
■ un sommateur et amplificateur (40) de bas bruit ayant pour fonction d'éliminer le bruit parasite ne correspondant pas au signal associé à l'effet Cotton-Mouton inverse, le signal sommé et amplifié étant transmis à, ■ and a summing amplifier (40) of low noise operable to remove extraneous noise not corresponding to the signal associated with the Cotton-Mouton opposite effect, the summed and amplified signal being transmitted,
■ un filtre passe haut (41 ), puis à un intégrateur (42) avant d'être transmis à un dispositif d'affichage (43) et/ou à une mémoire (44) de stockage. ■ a high pass filter (41), then an integrator (42) before being transmitted to a display device (43) and / or a storage memory (44).
8 - Dispositif selon la revendication 1 caractérisé en ce qu'il comporte une monture tournante dans lequel sont disposés le milieu actif (10), les moyens de production du champ magnétique. 8 - Device according to claim 1 characterized in that it comprises a rotating mount in which are disposed the active medium (10), the means for producing the magnetic field.
9- Dispositif selon l'une des revendications 2 ou 3 caractérisé en ce qu'il comporte un système d'adaptation optique (201 , 202). 10 - Dispositif selon la revendication 1 caractérisé en ce que ledit milieu actif (10) est un cristal de TGG ou Terbium Gallium Garnet. 9- Device according to one of claims 2 or 3 characterized in that it comprises an optical matching system (201, 202). 10 - Device according to claim 1 characterized in that said active medium (10) is a TGG crystal or Terbium Gallium Garnet.
1 1 - Procédé pour mesurer une aimantation générée au sein d'un milieu actif (10) lorsque ledit milieu actif présente un effet Cotton-Mouton Inverse, en utilisant le dispositif selon l'une des revendications 1 à 10 caractérisé en ce qu'il comporte en combinaison au moins les étapes suivantes : 1 1 - Method for measuring a magnetization generated within an active medium (10) when said active medium exhibits an inverse Cotton-Sheep effect, using the device according to one of Claims 1 to 10, characterized in that comprises in combination at least the following steps:
■ transmettre dans un milieu actif (10) une onde électromagnétique polarisée linéairement, ■ transmitted in an active medium (10) an electromagnetic wave linearly polarized,
générer un champ magnétique transverse Bt par rapport à la direction de propagation D| de ladite onde électromagnétique, generating a transverse magnetic field B t with respect to the propagation direction D | of said electromagnetic wave,
■ mesurer (2) le signal électrique traduisant l'aimantation générée au sein dudit milieu actif (10) par ladite onde électromagnétique. ■ measure (2) the electrical signal translating the magnetization generated within said active medium (10) by said electromagnetic wave.
EP11749458.3A 2010-09-03 2011-09-02 Device and method for characterizing a laser beam Withdrawn EP2612158A1 (en)

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FR1057007A FR2964504B1 (en) 2010-09-03 2010-09-03 DEVICE AND METHOD FOR CHARACTERIZING A PULSE BEAM
PCT/EP2011/065227 WO2012028726A1 (en) 2010-09-03 2011-09-02 Device and method for characterizing a laser beam

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US8000767B2 (en) * 2004-01-20 2011-08-16 Board Of Trustees Of The University Of Illinois Magneto-optical apparatus and method for the spatially-resolved detection of weak magnetic fields
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