FR2668862A1 - METHOD FOR DETERMINING AMPLITUDE OF A MAGNETIC FIELD BY OPTICAL PUMPING BY LASER AND MAGNETOMETER FOR IMPLEMENTING THE METHOD. - Google Patents

Abstract

Optical pumping is carried out by a laser (5) of a gas consisting of 4 He atoms of a cell (1) which is subjected to the field to be determined and to an electric discharge (4) and the polarization of the incident pumping light to cause a resonant phenomenon in the optical and electrical properties of the cell (1). The magnetometer comprises, in addition to the laser (5), the source (4) of electric discharge and the modulator (8), means (11 - 15) for determining by resonance the modulation frequency and therefore the field. The invention makes it possible in particular to detect magnetic anomalies.

Description

1 - The present invention relates first of all to a method of

  determining the amplitude of a magnetic field by optical laser pumping of a cell of a gas subjected to the field to be determined, with precession of the magnetic moment of the gas at a frequency f The method in which, by modulation of a parameter defining the environment of the atoms of the gas of the cell, at a frequency fm = nf L, N being at least equal to 1, a resonant phenomenon is generated in the properties

optics of the cell.

  Let us consider a 4 He isotopic helium cell whose atoms are at the fundamental quantum electronic level symbolized 1 SO Under the action of a soft electric discharge, there is a population of the next higher level metastable 3 Si The transition l So-3 If it is not an electrical dipolar transition, in that there can be no absorption of photons from the fundamental level to the higher level nor, conversely, spontaneous emission of photons from the higher level to the fundamental level, it is On the other hand, if a 4 He cell at level 351 is subjected to optical pumping by a suitably polarized light source of appropriate wavelength, there will be even higher levels of in particular levels 3 P 0,1 or 2 connected at level 351 by electrical dipole transitions of wavelength substantially pumping

from 1.08 to 1 m.

  After fluorescence, the atomic gas at level 35 is endowed with a macroscopic electronic magnetization defined by a moment M. It has long been proposed to apply the phenomena of optical pumping to magnetometry and in particular to the determination of the amplitude of a magnetic field B,

  for example for the detection of magnetic anomalies.

  Under the action of the magnetic field to be measured, this moment M precesses at the frequency f L called Larmo ', characteristic of the atomic system implemented and proportional to B. this precession can be forced in the same way as one can force the oscillation of a pendulum and bring out the resonance phenomenon when the excitation frequency is equal to that characteristic

of the pendulum.

  Several methods allow this forced excitation.

  The first method consists of subjecting the atoms simultaneously to the static field to be measured B and to an alternating magnetic field Bm of frequency fm.

  adjustable This field is called the radio frequency field.

  At equality f M = f There is resonance that we can

detect optically.

  the light signal creating the optical pumping is detected after passing through the cell containing the helium atoms subjected to the soft discharge The resonance is manifested by a sudden drop in the output signal of a photodetector receiving the light transmitted by the cell Knowing f , and therefore f IBI can be deduced from it, as perfectly explained in the document "Resonance Magnetometers, F Hartmann, IEEE Trans

  Magnetics, Flight Mag-8, No. 1, March 1972 ".

  Schematically, and with reference to FIG. 1, the magnetometer thus included a measuring cell 21, subjected to the action of a discharge source 22, of a pump light source 23, through a polarizer 24, and a magnetic modulation source 25, a photodetector 26 and, at the output of the photodetector, a

processing module 27.

  Resonance can also be detected by frequency analysis of the intensity of the light transmitted by

the cell.

  Resonant variations of the electrical properties of the discharge can still be detected; it's here

opto-galvanic detection.

  For the magnetometric application of optical pumping, the problem of the emission of pumping light at a wavelength of 1.08 / m still had to be solved.

for helium.

  we began by proposing, as a source of pumping, a

helium lamp.

  But the three lines associated with the three levels 3 P previously mentioned are very close to each other, without the possibility of filtering, so that with a helium lamp, the three levels 3 P are simultaneously excited This has the result of reducing the efficiency of the measurement In addition, the lamp must be very close to the cell, which leads to a large assembly Finally, the life of helium lamps is not the best These disadvantages of the helium lamp have well remembered in the patent

US-4,780,672.

  4 - It is then that it has been proposed in patent FR-2,598,518 to substitute a helium lamp with a laser. US Pat. No. 4,780,672 also relates to a laser-pumped helium magnetometer. The advantage of the laser lies in its ability to be tuned to one of the three lines, and therefore in the choice of the most efficient of the three, with a gain of 500 to 1000 compared to the helium lamp, in the possibility to use an optical fiber and to deport the source of pumping with respect to the cell, thus in the miniaturization of the assembly, and in its lifetime, and

  therefore in the improvement of the maintenance conditions.

  It should be noted that the adoption of the laser as a source of pumping of a helium cell at a wavelength of 1.08 m was mainly allowed by the synthesis of lanthanum-neodymium aluminate crystals LNA and the development of the laser diode pumped single-mode LNA laser, as described, for example, in the article "JJ Aubert

et al., Opt Comm 69, 299, 1989 ".

  The accuracy of the magnetic resonance resonance determination is altered by: broadening and displacement of the resonance curves by radio frequency; significant spatial anisotropy effects; resonance position displacement as a function of

  luminous flux and polarization.

  The second method, which makes it possible to force the precession of the moment M, consists, as proposed in the document IW.E Bell and AL Bloom, Phys Rev Lett 6, 280 (1961) 1, to modulate, not the magnetic field, by superposition. However, by this method, the broadening of the lines due to the radiofrequency but not of the broadening due to the intensity of the radiation is avoided. optical pumping source, this method having the same disadvantages as the magnetic resonance method with respect to spatial anisotropy and the offsets of the

resonance position.

  In short, for twenty years, we seek to define an optical pump magnetometer satisfactory in precision and sensitivity and, also, which is suitably isotropic, which was none of those proposed so far, at least with a signal / noise ratio perfectly exploitable according to any

  what orientation of the magnetometer.

  The present invention, for this purpose, relates first of all to a method for determining the amplitude of a magnetic field of the type defined above, characterized in that the resonant phenomenon is caused by modulating the polarization of the light of incidental pumping. Preferably, the gas of the cell is also subjected to an electric discharge, the resonant phenomenon being caused in the optical and electrical properties of the cell.

the cell.

  In this case, and more preferably, the gas of the

  cell is made up of 4 He atoms.

  It will be noted that the magnetic field to which the gas atoms are subjected is exclusively the static field whose amplitude is to be determined and that the intensity of the pumping light is strictly constant. Only the direction of the electric field characterizing this light is subject to variations of direction This is

  this modulation that causes resonance.

  -6- The inventive step involved in the process of

  the invention must be recognized from the outset.

  This method solves a problem of twenty years ago that had been solved so far, but not fully satisfactory, either by magnetic resonance or by modulation of the intensity of the light pompagne, while, at the time when this problem arose for the first time, we already knew the principle of the modulation of

  polarization of optical pumping light.

  However, this polarization modulation had not been applied at all to the magnetometry, and could not be applied to it, for a modulation by rotation of a polarizer by means of an engine, therefore at

speeds much too slow.

  The plaintiff's merit has therefore been to disregard the solutions proposed so far for a long time, to take up the problem entirely and to inquire whether an old principle which could have been originally exploited, but which was not, could not be adapted to the particular needs of the application of the invention for which this principle was ultimately not

planned.

  The advantage of the method of the invention, induced by its isotropy, is further noted, according to which, in the absence of gas in the cell, the light transmitted by the cell remains constant in intensity, which is not true in the case of modulating the intensity of light from

incidental pumping.

  In the preferred implementation of the method of the invention, a frequency analysis of the intensity of the light transmitted by the cell is carried out, after being detected, as a function of the frequency of the transmission.

modulation of the polarization.

  More preferably, this analysis is performed after frequency filtering at twice the polarization modulation frequency and at this modulation frequency. This preferred implementation of the method of the invention is based on an experimental finding, confirmed by the theory, that the applicant and who

  finally gives the invention its full interest.

  Let us consider the two cases where the field B is respectively parallel B // and perpendicular Bj_ to

pump laser beam.

  p B 11 In this case, we observe the resonance for two values

the modulation frequency f.

  The most intense resonance arises for fm = 2 f L We can then detect a modulation on the received signal

  by a photodetector at frequencies fm and 2 fm.

b.the

  We then observe only one resonance, when f.

  f L, and this resonance gives rise to modulated signals

at fm and 2 fm.

  To set orders of magnitude, in a magnetic field close to 0.5 gauss, in the case B //, the resonance can be detected on signals modulated at 4 f L, -8 - about 5.6 M Hz. the case of B ,, we can

  detect resonance on modulated signals at 2.8 M Hz.

  Advantageously, it is possible to implement a decision logic to consider only the best of the

  detections from the point of view of the signal-to-noise ratio.

  Finally, the method of the invention makes it possible to determine the amplitude of the field B regardless of the relative orientation of this field and the measuring means; even if the signal-to-noise ratio is not constant in all the directions of space, the fact remains that the method of the invention in its preferred implementation, for its part, is suitably isotropic. additionally, by average effect, offsets

  frequency due to pumping are reduced.

  The invention also relates to a magnetometer for determining the amplitude of a magnetic field by the method of the invention, comprising a cell of a gas intended to be subjected to the magnetic field, a laser for emitting light. pumping the gas from the cell, means for polarizing the light emitted by the laser, means, adjustable frequency, modulating a parameter defining the environment of the atoms of the gas of the cell at a frequency fm = nf L , N being at least equal to 1, to precase the magnetic moment of the gas at a frequency f L and thus cause a resonant phenomenon in the optical properties of the cell, and resonance determination means of the modulation frequency and therefore of the field, magnetometer characterized in that said modulation means are arranged to modulate the

  polarization of the incident pumping light.

  Preferably, the magnetometer also comprises an electric discharge source to which the gas of the cell is intended to be subjected, the gas of the cell consists of 4 He atoms and the modulation means comprise an electro-optical modulator. . More preferably, the electro-optical modulator is controlled by a DC voltage generator, setting its operating point, and a synthesizer of a sinusoidal voltage of variable frequency modulation.

  superimposed on the DC voltage of the generator.

  Advantageously, the synthesizer of the modulation voltage is controlled by frequency tuned means for detecting the output signal of a photodiode for detecting the light transmitted by the cell in order to control the frequency of the synthesizer on

a central value of resonance.

  In this case, the control means of the synthesizer may comprise two synchronous detectors with the synthesizer, respectively tuned to the modulation frequency and on the double thereof, and connected, by their outputs, to a circuit, controlling the synthesizer, selecting the best of the measures

synchronous detectors.

  The invention will be better understood by means of the

  following description of a preferred embodiment

  magnetometer of the invention, with reference to the accompanying drawings, in which FIG. 1 is a diagrammatic representation of a magnetometer of the prior art and FIG. 2 is a diagrammatic representation of the

magnetometer of the invention.

  The magnetometer shown in FIG. 2, intended to determine the amplitude of a magnetic field B, comprises a cell 1, here in pyrex, spherical, in this case 4 cm in diameter, filled with helium atoms 4 He under pressure, here again, about 100 Pa Electrodes 2, 3, painted conductive lacquer directly on the surface of the cell, are connected to an oscillator HF 4, providing a few tens of milliwatts, to create an electric field HF and, therefore, a gentle discharge and bring the helium atoms to the metastable excited level 351 An optical pumping beam is emitted by a tunable solid laser and sent on the cell 1 through, successively, an optical fiber 6, a polarizer 7 and an electro-optical modulator 8 The laser 5 is a LNA laser pumped by a diode emitting at 800 nm wavelength under a continuous power of 500 mw and tuned to the line

  DOl 3 S 3 P O l of 4 He wavelength 1 =, 0829 / m.

  The wavelength is slaved by means of a photodiode close to the cell and a piezoelectric wedge in the laser. The Pockels cell type modulator 8 behaves like a birefringent blade whose phase shift along the axes neutral

  ordinary and extraordinary, respectively, is modulated.

  The intensity of the pumping beam remains constant but its polarization is modulated It is also quite complex It changes at the frequency of the modulation and, periodically, from rectilinear it becomes elliptical and circular In other words, the geometric parameters of the polarization ellipse

  vary at the modulation frequency.

  The modulator 8 is preceded by the polarizer 7 which is a rectilinear polarizer whose polarization direction, after adjustment, is here inclined by 45 degrees on the

Neutral axes of the modulator.

  The modulator 8 is electrically controlled by a DC voltage generator 9, fixing its operating point, and a synthesizer 10 delivering a sinusoidal modulation voltage, superimposed on the DC voltage and of variable frequency fm The pump light transmitted by the cell 1 is received on a photodetector 11, in this case a germanium photodiode, whose output current is converted into voltage in an amplifier 12 The modulated component of the signal detected by the diode 11, after conversion, is analyzed in two synchronous detectors 13 , 14 controlled by the synthesizer 10 and tuned, one 13, on the frequency f and the other 14, on the frequency 2 fm A signal processing circuit 15 is connected, by its inputs, to the outputs of the detectors 13 , 14 and, by its output, to a control input of the synthesizer, to select the best of the two measurements of the intensity of the light transmitted by the cell 1 effect by the detectors 13, 14, in the direction of the field B to be determined, and thus to slave the frequency of the modulation signal of the synthesizer 10 to

a central value of resonance.

  A magnetometer has been described with a cell filled with 4 He helium atoms but the cell could also be filled with atoms of an alkaline gas, but with instead of the oscillator 4, thermal regulation means . Instead of optically determining the field, as described above, one could proceed to an optogalvanic method. In this case, the current of the varying discharge oscillator is analyzed.

quickly to resonance.

  There is described a magnetometer with a polarizer 7 between the laser 5 and the modulator 8, more precisely between the

optical fiber 6 and the modulator 8.

  If a polarization-maintaining optical fiber is used, the modulator between the laser and the fiber could also be arranged. It would also be possible, also in the case of a polarization-maintaining optical fiber, to adopt polarization modulation means. incident pumping light, means acting directly on the fiber, in particular by

interaction of an acoustic wave.

-13-

Claims (15)

  1. Method for determining the amplitude of a magnetic field by optical laser pumping of a cell (1) of a gas subjected to the field to be determined, with precession of the magnetic moment of the gas at a frequency f in which, by modulation of a parameter defining the environment of the atoms of the gas of the cell, at a frequency fm = nf L, N being at least equal to 1, a resonant phenomenon is generated in the optical properties of the cell, characterized by causing the resonant phenomenon by modulating (9, 10) the polarization of the light of incidental pumping.   2. Method according to claim 1, wherein the gas of the cell is also subjected to an electric discharge (4), the resonant phenomenon being caused in the optical and electrical properties of the cell (1).   3. The process according to claim 2, wherein the gas   of the cell (1) consists of 4 He atoms.   4. Method according to one of claims 1 to 3, in   which is carried out, after detecting it, with a frequency analysis (13, 14) of the intensity of the light transmitted by the cell (1) as a function of the frequency modulation of the polarization.   Method according to Claim 4, in which a double analysis is carried out after frequency filtering (14, 13) at twice the modulation frequency of the   polarization and at this modulation frequency.   The method of claim 5, wherein decision logic (15) is used to select the best of two analyzes from the point of view of signal-to-noise ratio.   7. Magnetometer for determining the amplitude of a magnetic field by the method of claim 1, comprising a cell (1) of a gas to be subjected to the magnetic field, a laser (5) for transmitting a light for pumping the gas from the cell, means (7) for polarizing the light emitted by the laser (5), means (8-10) of adjustable frequency, modulation of a parameter defining the environment of the atoms of the gas of the cell at a frequency f = nf L, N being at least equal to 1, to precase the magnetic moment of the gas at a frequency f L and thus cause a resonant phenomenon in the optical properties of the cell (1 ), and means (11-15) for determining by resonance the modulation frequency and therefore the field, said magnetometer characterized in that said modulation means (8) are arranged to modulate the   polarization of the incident pumping light.   8. Magnetometer according to claim 7, wherein there is provided a source (4) of electric discharge to which the gas of the cell (1) is intended to be subjected. 9. The magnetometer according to claim 8, wherein   the gas of the cell (1) consists of 4 He atoms. -15-   10. Magnetometer according to one of claims 7 to 9,   wherein the modulation means (8) comprises a electro-optical modulator.   A magnetometer according to claim 10, wherein the electro-optical modulator (8) is controlled by a DC voltage generator (9), setting its operating point, and a synthesizer (10) of a sinusoidal modulation voltage of variable frequency   superimposed on the DC voltage of the generator (9).   A magnetometer according to claim 11, wherein the modulation voltage synthesizer (10) is controlled by frequency tuned means (13, 14) for detecting the output signal of a detection photodiode (11). of light transmitted by the cell (1) to slave the frequency of the synthesizer (10) to a central resonance value.   13. Magnetometer according to claim 12, wherein the control means of the synthesizer (10) comprise two detectors (13, 14) synchronous with the synthesizer (10), tuned respectively to the frequency fm and the frequency 2 fm '14. Magnetometer according to Claim 13, in which the two synchronous detectors (13, 14) are connected by their outputs to a circuit (15) controlling the synthesizer (10) for selecting the best   measurements of the synchronous detectors (13, 14).   15. The magnetometer according to claim 7, wherein it is provided between the laser (5) and the gas cell. (1), an optical fiber (6).   16. Magnetometer according to claim 15, wherein there is provided, between the optical fiber (6) and an electro-optical modulator (8), a rectilinear polarizer (7) whose polarization direction is inclined by 45 °.   degrees on the neutral axes of the modulator (8).   Magnetometer according to claim 15, wherein   the optical fiber is polarization-maintaining.   A magnetometer according to claim 17, wherein a modulator between the laser and the laser is provided. optical fiber.   19. A magnetometer according to claim 17, wherein the means for modulating the polarization of the incident pumping light are arranged to act. directly on the optical fiber.   20. Magnetometer according to claim 19, in which the modulation means act on the optical fiber.   by interaction of an acoustic wave.