The invention relates to a device and a method making it possible notably to characterize a pulsed laser beam of high energy or else 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.
It also applies when it is desired to measure the magnetization generated within an active medium exhibiting an Inverse Cotton-Mouton effect.
In general, the power of high-power lasers is measured by total or partial absorption of the laser beam. This leads to consumption of the energy of the laser at the level of the target used to execute the measurement. This is manifested by a loss of energy of the beam and presents the drawback of not being able to use the laser beam, simultaneously with the measurement of its energy or of its power.
To ascertain the temporal shape, it is known to extract a part of the beam and to send it to another apparatus based on fast photodiodes. To determine the polarization of the laser beam it is necessary to possess polarizers specially designed for high powers and to perform the measurement of the energy of the pulse as a function of the polarization.
The publication by Zon. B. A entitled “observation of inverse Cotton-Mouton effect in the magnetically ordered crystal (Lu, Bi)3(FE, Ga)5O12, published at the JEPT Let. 45 (1987) 5, pages 272-275 describes the inverse Cotton-Mouton effect. This publication describes the measurement of the magnetization induced by a pulsed laser beam in a magnetic film which is situated in a static exterior magnetic field oriented parallel to the direction of propagation of the pulsed laser beam.
The publication by Marmo S. I entitled “Electric field induced magnetization and inverse Cotton-Mouton effect in atomic gases”, published in Physics letters A, 202 (1995), pages 201-205 discloses the prediction of the inverse Cotton-Mouton effect in atomic and molecular systems.
In the subsequent description the expression “active medium” designates a material, a crystal, a glass, a gas, a liquid which, when it is subjected to a magnetic field, will exhibit an inverse Cotton-Mouton effect.
The term “characterize or characterization” of a laser beam will be used to refer to a measurement of instantaneous power of the beam, a measurement of power or else the determination of the polarization of the beam.
The invention relates to a device making it possible to measure a magnetization generated within an active medium or to characterize a linearly polarized electromagnetic wave when said active medium exhibits an Inverse Cotton-Mouton effect, characterized in that it comprises in combination at least the following elements:
In the case of an electromagnetic wave, passing through an active medium, the measurement 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 else the polarization of the wave.
According to one embodiment, the electromagnetic wave is a pulsed laser beam and the measurement device characterizes the pulsed laser beam 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.
According to another embodiment, the electromagnetic wave is a continuous laser beam and the measurement 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, the magnetic field being variable over time.
Said active medium is, for example, subjected to a static exterior magnetic field Bext which is variable or constant over time.
The device for measuring the signal comprises, for example, at least one coil of pickup type.
The device for measuring the signal can comprise at least two coils of pickup type placed on either side of the active medium, the normal to their surface, being oriented substantially parallel to the magnetic field Bext.
The electronic device for measuring the signal manifesting the instantaneous energy value of the electromagnetic wave, or the value of the power of said electromagnetic wave comprises the following elements:
The device comprises, for example, a rotating mount in which are disposed the active medium, the means for producing the magnetic field.
The device can comprise an optical adaptation system.
Said active medium is a crystal of TGG or Terbium Gallium Garnet.
The invention also relates to a method for measuring a magnetization generated within an active medium, when said active medium exhibits an Inverse Cotton-Mouton effect, the method being implemented within a device exhibiting one of the aforementioned characteristics, the method comprising at least the following steps:
Other characteristics and advantages of the present invention will be more apparent on reading the description of one or more embodiments given by way of wholly nonlimiting illustration, together with the figures which represent:
In a general manner, the device according to the invention makes it possible to measure a magnetization generated within an active medium when the active medium exhibits an Inverse Cotton-Mouton effect. The examples illustrated in the figures relate to an application to the characterization of a pulsed or continuous laser beam, but can without departing from the scope of the invention apply in the case of a linearly polarized electromagnetic wave.
In this
In the case of an active medium of ferromagnetic type, it is not necessary to use an exterior magnetic field Bext to obtain the inverse Cotton-Mouton effect, the magnetic field Bt 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 Bt, or the magnetic field Bext used, is a time-variable magnetic field, whose law of temporal variation is known. In this case, it is possible to measure the value of a constant or substantially constant power of the continuous laser beam.
In the case of an electromagnetic wave, passing through an active medium, the same device applies. The measurement 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 else 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 pickup coils of planar type. It would also be conceivable to use coils having a curved surface which best follows the field lines of the exterior magnetic field.
The example given refers to two coils, but without departing from the scope of the invention, it would be possible to perform the characterization of the laser beam or of an electromagnetic wave by using a single pickup coil or a number of coils greater than 2, depending on the application. Likewise, as will be presented in
The active medium 10 is a medium which exhibits an inverse Cotton-Mouton effect when it is subjected to an exterior magnetic field Bext or else to the intrinsic magnetic field Bt in the case of a ferromagnetic medium or of other media which do not need any exterior enticement. It is thus possible to use crystal or glass. Liquid or gaseous active media can also be envisaged. The dimensions and the nature of the active medium will be chosen as a function of the desired application. For example, for a use within the framework of very intense lasers, 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. It will also be possible for the nature of the active medium to be chosen as a function of the wavelength of the laser.
The assembly comprising the active medium 10, the magnets 11, 12 and the coils 14, 15 can be positioned inside a rotating mount 30 which makes it possible to rotate with respect 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 Bt and the polarization of the laser on which the value of the inverse Cotton-Mouton 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.
In this example, the two pickup coils 14, 15 of
This electronic circuit can without departing from the scope of the invention be implemented for application to a measurement of magnetization generated within an active medium or else in order to characterize a continuous laser beam.
The operating principle of the device according to the invention is, for example, as follows: the device for characterizing a laser beam or an electromagnetic wave according to the invention is positioned in the optical path of the laser beam or of the electromagnetic wave to be characterized (measurement of the instantaneous power, of the total power and/or of the polarization). The optical adaptation system when it is present is optimized in such a way that the laser beam or the wave to be characterized preserves the characteristics at its desired prime use after passage 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 by taking account of the characteristics of this electromagnetic wave.
A way of proceeding when it is desired to ascertain the direction of the polarization of a linearly polarized pulsed or continuous laser beam is to use the rotating mount and to maximize the value displayed on the device. In this case, the mount is rotated until a maximum signal is read off at the level of the display device. The mount being graduated, its position gives the direction of the polarization of the beam. It is also possible to use this scheme to ascertain the polarization of a linearly polarized electromagnetic wave.
The example which follows was obtained in the case of a pulsed laser, by using a TGG or terbium gallium garnet crystal as active medium. The laser source used is a Nd:YAG laser (lambda=1064 nm) generating light pulses of a duration of 10 ns and an energy of about 0.5 J/pulse. The example is illustrated in
Changes in the magnetization of the crystal were measured using a device comprising a dual-pickup coil such as that described in
In
In
The pulsed laser beam is controlled by extracting a small part of the beam injected into the crystal with a beam splitter. A fast diode is used to control the laser pulse. The photodiode was calibrated with respect to a device measuring the pulsed energy reaching the crystal.
The ICME magnetization in a TGG crystal can be defined as follows:
M=C
ICM
P
d
B
ext( 1)
where CICM designates the constant of the Inverse Cotton-Mouton effect specific to the active medium, Pd the power density of the light beam and Bext the external magnetic field. The relation remains valid for a magnetic field intrinsic to the material.
This magnetization can be measured with the aid of a pickup coil if it varies over time. Indeed, the variation of the magnetization M(t) induces a potential difference V(t) across the terminals of the measurement coil in accordance with the relation
where g is the gain of the amplifier of the measurement coil. Ae=10 mm2 is the effectively calibrated zone of the signal coil and Bp is the density of the magnetic flux through the surface of the measurement coil produced by the magnetization M of the crystal.
It is then noted that the temporal variation of Bp(t) can be achieved in two ways (see relation (1)):
a) by varying the power density Pd of the beam (pulsed or modulated laser),
b) by varying the external magnetic field.
In case a), the variation of Bp(t) may be written:
and the ICME signal V(t) may then be written:
where Pd is the density of the laser beam, Bext 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.
Thus, the ICME signal is proportional to the time derivative of the intensity of the pulsed laser as is represented in
For a magnetization M of 1 A/m, a magnetic field density of about 4×10−8 T was found. By using a conversion factor f to convert between the value of the density of the flux Bp and the magnetization M of the crystal of about 2.5×107 (A/m) T−1.
In case b), by varying the external magnetic field the variation of Bp(t) may be written:
The ICME signal V(t) may then be written:
This time the beam is continuous and it is the magnetic field which is pulsed.
The invention offers notably the following advantages:
The device according to the invention can combine three functionalities which, in the prior art apparatuses known to the Applicant are in general separate.
Another advantage afforded by the device and the method according to the invention is the ability to perform the measurements described previously, without needing to extract or to attenuate a part of the beam. The device presented can be inserted into an existing optical circuit without modifying it. It therefore makes it possible to view the laser pulse and to measure its characteristics during the actual use of the beam.
According to an exemplary use, the magnetic field/pickup coil assembly can be disposed around a laser crystal, in order to measure the temporal evolution of the power in the crystal. Another possibility is to integrate the system into a Faraday isolator becoming at one and the same time a standard isolator and a power measurement apparatus.
Number | Date | Country | Kind |
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1057007 | Sep 2010 | FR | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2011/065227 | 9/2/2011 | WO | 00 | 6/27/2013 |