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EP-4735905-A1 - OPTICALLY PUMPED MAGNETOMETER

EP4735905A1EP 4735905 A1EP4735905 A1EP 4735905A1EP-4735905-A1

Abstract

A magnetic field strength measurement system including: a vapor chamber holding alkali metal vapor; a light source unit configured to supply circularly polarized laser light and positioned to feed a first pass beam of the circularly polarized light into the vapor chamber; a retroflection unit including least one optical element and configured to reverse a direction, while maintaining polarization, of the first pass beam upon exit from the vapor chamber to provide a second pass beam to the vapor chamber, the second pass beam having an opposite direction, and same circular polarization as the first pass beam; a detector positioned to measure light emitted from the vapor chamber; and circuitry configured to: provide a modulating signal to the source, the modulating signal configured to modulate the first pass beam; receive a measurement signal from the detector; demodulate the measurement signal using the modulating signal to provide a demodulated measurement signal; and determine a resonance frequency from one or both of the modulating signal and the demodulated measurement signal.

Inventors

  • ROSENZWEIG, Yossi
  • SHCHERBACK, IGOR
  • TOKAR, Dima

Assignees

  • Elta Systems Ltd.

Dates

Publication Date
20260506
Application Date
20240624

Claims (20)

  1. 1. A magnetic field strength measurement system comprising: a vapor chamber holding alkali metal vapor; a light source unit configured to supply circularly polarized laser light and positioned to feed a first pass beam of said circularly polarized light into said vapor chamber; a retroflection unit comprising least one optical element and configured to reverse a direction, while maintaining polarization, of said first pass beam upon exit from said vapor chamber to provide a second pass beam to said vapor chamber, said second pass beam having an opposite direction, and same circular polarization as said first pass beam; a detector positioned to measure light emitted from said vapor chamber; and circuitry configured to: provide a modulating signal to said source, said modulating signal configured to modulate said first pass beam; receive a measurement signal from said detector; demodulate said measurement signal using said modulating signal to provide a demodulated measurement signal; and determine a resonance frequency from one or both of said modulating signal and said demodulated measurement signal.
  2. 2. The system according to claim 1, wherein said circuitry is configured to provide said modulation signal at a plurality of modulation frequencies, wherein said demodulated measurement signal comprises a plurality of amplitudes each amplitude associated with a frequency of said plurality of modulation frequencies.
  3. 3. The system according to claim 1, wherein said circuitry is configured to extract a resonance frequency from said demodulated measurement signal.
  4. 4. The system according to claim 3, wherein said circuitry comprises a lock- in amplifier configured to receive said measurement signal and said modulating signal and outputting said demodulated measurement signal.
  5. 5. The system according to claim 3, wherein said demodulated measurement signal comprises a quadrature plot and where said circuitry is configured to extract a resonance frequency as a zero-crossing of a said quadrature plot.
  6. 6. The system according to claim 1, wherein said circuitry is configured to use said demodulated measurement signal as feedback to said modulation signal to maintain said demodulated measurement signal at a set value, where said resonance frequency is extracted as a steady state frequency of said modulation signal.
  7. 7. The system according to any one of claims 1-6, wherein said light source unit comprises a laser source of linearly polarized light and one or more optical element configured to convert the linearly polarized light into circularly polarized light.
  8. 8. The system according to any one of claims 1-7, wherein said first pass beam is fed to said vapor chamber in a first direction and where said retroflector is positioned to reverse a direction of said first pass beam to provide said second pass beam.
  9. 9. The system according to any one of claims 1-8, comprising a loss compensation unit configured to increase a resonance light signal produced by said vapor chamber and associated with said second pass beam.
  10. 10. The system according to claim 9, wherein said compensation unit comprises at least one heater configured to heat at least a portion of said alkali metal vapor through which said second pass beam passes.
  11. 11. The system according to any one of claims 9-10, wherein said loss compensation unit comprises one or more temperature controller configured to control temperature of one or more portion of said vapor chamber.
  12. 12. The system according to any one of claims 9-11, wherein said loss compensation unit comprises one or more beam broadening element in a path of said second pass beam prior to entry of said second pass beam to said vapor chamber as said second pass beam.
  13. 13. The system according to any one of claims 1-12, wherein said detector comprises a photodiode.
  14. 14. The system according to any one of claims 1-13, wherein said retroflector unit comprises at least two light diverting elements.
  15. 15. The system according to claim 14, wherein each of said at least two light diverting element comprises a reflecting prism or a mirror.
  16. 16. The system according to any one of claims 14-15, wherein said at least two light diverting elements comprises three light diverting elements.
  17. 17. A method of magnetic field strength measurement comprising: supplying a first beam of circularly polarized light and a second beam of circularly polarized light to a pumped vapor chamber holding alkali metal vapor; measuring light emitted from said vapor chamber to provide one or more measurement signal; controlling one or more of temperature of one or more portion of said alkali metal vapor and beam cross sectional area of one or more of said first beam and said second beam.
  18. 18. The method according to claim 17, comprising determining a magnetic resonance frequency from said measurement signal.
  19. 19. The method according to claim 18, comprising determining the magnetic field strength using said magnetic resonance frequency.
  20. 20. A system of magnetic field strength measurement system comprising: a vapor chamber holding alkali metal vapor; a source of laser light; one or more optical element configured to receive said laser light and provide a first circularly polarized beam of light and a second circularly polarized beam of light to said vapor chamber; one or more sensor configured to measure one or more of: said first circularly polarized beam of light; said second circularly polarized beam of light; light emitted from said vapor chamber; a compensation unit comprising one or more temperature controller and one or more beam area controller; a controller configured to receive one or more measurement from said one or more sensor and to provide control signals to said compensation unit.

Description

OPTICALLY PUMPED MAGNETOMETER TECHNOLOGICAL FIELD The present disclosure, in some embodiments, thereof, relates to magnetometers and, more particularly, but not exclusively, to optically pumped magnetometers (OPMs). BACKGROUND ART Page 11 of reference [12] as listed below, states: “The Cesium alkali vapor magnetometer offers good sensitivity and bandwidth (about 10 pT i Hz) but has a few disadvantages, including heading errors. As described previously, Cesium ’s spectral lines are wide, meaning that the electron energy levels associated with the Zeeman effect vary widely in magnitude over a population of Cesium atoms. In static conditions (i.e. the orientation of the magnetic field is stationary), a peak becomes apparent, and the system will self oscillate at this peak. When the magnetic field direction changes, however, the position of this peak will change because the spectral line amplitudes change. As a result, the self-oscillating frequency will shift, producing a heading error. Since Cesium ’s spectral lines are spread over some 20 nT, the heading error could be that severe. ” “In static conditions (i.e. the orientation of the magnetic field is stationary), a peak becomes apparent, and the system will self-oscillate at this peak. When the magnetic field direction changes, however, the position of this peak will change because the spectral line amplitudes change. As a result, the self-oscillating frequency will shift, producing a heading error. ” Reference [13] as listed below, states “The frequency shifts of a self-oscillating magnetometer with cesium vapor are studied quantitatively by solving rate equations for cesium atoms undergoing the action of a circularly polarized pumping light beam. Particular attention is given to the frequency shifts due to the line asymmetry of the rf spectrum in the light transmitted through the vapor. The line asymmetry varies with the orientation of the light beam relative to the field to be measured, the intensity of the light, and the temperature of the absorption cell. As an effect of a buffer gas, two extreme cases are considered, i.e., the optically excited state is in the complete mixing state and in the nonmixing state. The results show the fact that the frequency shifts depend largely on the collisional mixing in the excited state and on the sense of the circular polarization of the light. In order to reduce the frequency shifts, a new type of magnetometer, called the symmetrized spectrum magnetometer, is discussed, in which two circularly polarized light beams with opposite sense are simultaneously applied to a partitioned absorption cell. ” Background art, where each art is incorporated in its entirety by reference, includes the below list. In the following document these arts are referred to by number e.g. using the relevant reference number/s in square brackets: [number]. [1] J. Allred, R. Lyman, T. Kornack, and M. V. Romalis, “High-sensitivity atomic magnetometer unaffected by spin-exchange relaxation,” Physical review letters, vol. 89, no. 13, p. 130801, 2002. [2] H. Dang, A. C. Maloof, and M. V. Romalis, “Ultrahigh sensitivity magnetic field and magnetization measurements with an atomic magnetometer,” Applied Physics Letters, vol. 97, no. 15, p. 151110, 2010. [3] D. Sheng, S. Li, N. Dural, and M. V. Romalis, “Subfemtotesla scalar atomic magnetometry using multipass cells,” Physical review letters, vol. 110, no. 16, p. 160802, 2013. [4] E. J. Pratt, M. Ledbetter, R. Jimenez-Martinez, B. Shapiro, A. Solon, G. Z. Iwata, S. Garber, J. Gormley, D. Decker, D. Delgadillo, et al., “Kernel flux: a wholehead 432-magnetometer optically-pumped magnetoencephalography (op-meg) system for brain activity imaging during natural human experiences,” in Optical and Quantum Sensing and Precision Metrology, vol. 11700, pp. 162-179, SPIE, 2021. [5] V. K. Shah and R. T. Wakai, “A compact, high performance atomic magnetometer for biomedical applications,” Physics in Medicine & Biology, vol. 58, no. 22, p. 8153, 2013. [6] T. Wang, D. F. J. Kimball, A. O. Sushkov, D. Aybas, J. W. Blanchard, G. Centers, S. R. O’Kelley, A. Wickenbrock, J. Fang, and D. Budker, “Application of spinexchange relaxation-free magnetometry to the cosmic axion spin precession experiment,” Physics of the dark universe, vol. 19, pp. 27-35, 2018. [7] W. Li, X. Peng, S. Li, C. Liu, H. Guo, P. Lin, and W. Zhang, “Unshielded scalar magnetometer based on nonlinear magneto-optical rotation with amplitude modulated light,” in 2016 IEEE International Frequency Control Symposium (IFCS), pp. 1-4, IEEE, 2016. [8] V. G. Lucivero, P. Anielski, W. Gawlik, and M. W. Mitchell, “Shot-noiselimited magnetometer with subpicotesla sensitivity at room temperature,” Review of Scientific Instruments, vol. 85, no. 11, p. 113108, 2014. [9] V. Acosta, M. Ledbetter, S. Rochester, D. Budker, D. J. Kimball, D. Hovde, W. Gawlik, S. Pustelny, J. Zachorowski, and V. Yashchuk, “Nonlinear magneto-optical rotation with frequency-modulated l