Measurement of weak magnetic fields using zero-field parametric resonance in optically pumped He<sup>4</sup>

Slocum, Robert E.; Marton, B.

doi:10.1109/tmag.1973.1067647

Cited by 35 publications

(18 citation statements)

References 12 publications

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“…Parametric modulation measures the magnetometer signal with lock-in detection. 21,26 In our parametric modulation, a 100 nT oscillating magnetic field (roughly an order of magnitude larger than the dynamic range of the sensor) transverse to the laser beam is applied at ω m ¼ 1:77 kHz, generated by the lock-in amplifier driving Helmholtz coils in series with a 1.2 kΩ resistor. The signal from the photodiode is phase-sensitively demodulated.…”

Section: Experimental Methodsmentioning

confidence: 99%

“…In the following discussion, we define a right-handed coordinate system with the direction of the laser beam propagation alonĝ x, and the external magnetic field to be alongẑ. If the atoms are exposed to a magnetic field transverse to the laser beam, which consists of a DC component B 0 ¼ γ À1 ω Lẑ and an oscillating component B m ¼ γ À1 ω 1 cos(ω m t)ẑ, the atomic polarization along the laser beam (x) can be approximated by 26,30…”

Section: A Noise Measurementsmentioning

confidence: 99%

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Characterization of noise sources in a microfabricated single-beam zero-field optically-pumped magnetometer

Krzyzewski

Perry

Gerginov

et al. 2019

Journal of Applied Physics

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We present an experimental noise characterization of a miniature single-beam absorption-based optically-pumped magnetometer with a noise floor of 7 fT/Hz 1/2 operating in the spin-exchange relaxation-free regime. We experimentally evaluate noise arising from the laser intensity, laser frequency, laser polarization, cell temperature, and magnetic field coils used for the phase-sensitive detection of the magnetometer signal. We find that noise in the range between DC and 30 Hz is a result of noise sources coupling to the atoms in a manner similar to a magnetic field, while the noise at frequencies above 30 Hz is mainly due to laser intensity noise. Our results place an upper limit on the noise sources for our system that matches well with the noise spectrum of the magnetometer at frequencies above 5 Hz.

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Section: Experimental Methodsmentioning

confidence: 99%

Section: A Noise Measurementsmentioning

confidence: 99%

Characterization of noise sources in a microfabricated single-beam zero-field optically-pumped magnetometer

Krzyzewski

Perry

Gerginov

et al. 2019

Journal of Applied Physics

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“…If magnetic-field modulation of the form cos( ωt ) is applied along z, the dispersion curve that serves as the OPM output is obtained by demodulating at sin( ωt ). The output is proportional to P x at sin( ωt ), given by (assuming that the background magnetic field is nulled; Cohen-Tannoudji et al, 1970; Slocum and Marton, 1973; Shah and Romalis, 2009) Px=P0J0true(ω1ωtrue)J1true(ω1ωtrue)ωzτ1+(ωzτfalse)2, where P 0 ' is related to the steady-state polarization P 0 ' = P 0 T 1 /( T 1 + T p ), J n is an n th-order Bessel function of the first kind, ω 1 = γB 1 is the Larmor frequency of the modulation field B 1 , ω is the frequency of the modulation field, ω z is the Larmor frequency of the field B z that is detected and τ is the atomic spin coherence time given by 1τ=1Tp+1T1, ιwhere 1/ T p is the optical pumping rate and T 1 is the relaxation time of the atoms.When transverse magnetic field components ( ω x and ω y ) are present, the output of the magnetometer is given by the series (Cohen-Tannoudji et al, 1970) Px=P0J0true(ω1ωtrue)J1true(ω1ωtrue)true[ωzτ1+ωz2τ2−J02t...…”

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confidence: 99%

On-scalp MEG system utilizing an actively shielded array of optically-pumped magnetometers

et al. 2019

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The spatial resolution of magnetoencephalography (MEG) can be increased from that of conventional SQUID-based systems by employing on-scalp sensor arrays of e.g. optically-pumped magnetometers (OPMs). However, OPMs reach sufficient sensitivity for neuromagnetic measurements only when operated in a very low absolute magnetic field of few nanoteslas or less, usually not reached in a typical magnetically shielded room constructed for SQUID-based MEG. Moreover, field drifts affect the calibration of OPMs. Static and dynamic suppression of interfering fields is thus necessary for good-quality neuromagnetic measurements with OPMs. Here, we describe an on-scalp MEG system that utilizes OPMs and external compensation coils that provide static and dynamic shielding against ambient fields. In a conventional two-layer magnetically shielded room, our coil system reduced the maximum remanent DC-field component within an 8-channel OPM array from 70 to less than 1 nT, enabling the sensors to operate in the sensitive spin exchange relaxation-free regime. When compensating field drifts below 4 Hz, a low-frequency shielding factor of 22 dB was achieved, which reduced the peak-to-peak drift from 1.3 to 0.4 nT and thereby the standard deviation of the sensor calibration from 1.7% to 0.5%. Without band-limiting the field that was compensated, a low-frequency shielding factor of 43 dB was achieved. We validated the system by measuring brain responses to electric stimulation of the median nerve. With dynamic shielding and digital interference suppression methods, single-trial somatosensory evoked responses could be detected. Our results advance the deployment of OPM-based on-scalp MEG in lighter magnetic shields.

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“…Assuming that the background magnetic field is nulled, the output of an optically-pumped magnetometer utilizing field modulation in z-direction (i.e. it is sensitive in that direction) and probing atoms with a laser beam in x-direction is proportional to the spin-polarization in xdirection (Cohen-Tannoudji et al, 1970;Slocum and Marton, 1973;Shah and Romalis, 2009)…”

Section: Conflict Of Interestmentioning

confidence: 99%

On-scalp MEG system utilizing an actively shielded array of optically-pumped magnetometers

Iivanainen¹,

Zetter²,

Groen³

et al. 2018

Preprint

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The spatial resolution of magnetoencephalography (MEG) can be increased from that of conventional SQUID-based systems by employing on-scalp sensor arrays of e.g. optically-pumped magnetometers (OPMs). However, OPMs reach sufficient sensitivity for neuromagnetic measurements only when operated in a very low absolute magnetic field of few nanoteslas or less, usually not reached in a typical magnetically shielded room constructed for SQUID-based MEG. Moreover, field drifts affect the calibration of OPMs. Static and dynamic control of the ambient field is thus necessary for good-quality neuromagnetic measurements with OPMs. Here, we describe an on-scalp MEG system that utilizes OPMs and external compensation coils that provide static and dynamic shielding against ambient fields.In a conventional two-layer magnetically shielded room, our coil system reduced the maximum remanent DC-field component within an 8-channel OPM array from 70 to less than 1 nT, enabling the sensors to operate in the sensitive spin exchange relaxation-free regime. When compensating field drifts below 4 Hz, a low-frequency shielding factor of 22 dB was achieved, which reduced the peak-to-peak drift from 1.3 to 0.4 nT and thereby the standard deviation of the sensor calibration from 1.6% to 0.4%. Without band-limiting the field that is compensated, a low-frequency shielding factor of 43 dB was achieved. We validated the system by measuring brain responses to electric stimulation of the median nerve. With dynamic shielding and digital interference suppression methods, single-trial somatosensory evoked responses could be detected. Our results advance the deployment of OPM-based on-scalp MEG in lighter magnetic shields.

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Measurement of weak magnetic fields using zero-field parametric resonance in optically pumped He⁴

Cited by 35 publications

References 12 publications

Characterization of noise sources in a microfabricated single-beam zero-field optically-pumped magnetometer

Characterization of noise sources in a microfabricated single-beam zero-field optically-pumped magnetometer

On-scalp MEG system utilizing an actively shielded array of optically-pumped magnetometers

On-scalp MEG system utilizing an actively shielded array of optically-pumped magnetometers

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Measurement of weak magnetic fields using zero-field parametric resonance in optically pumped He4

Cited by 35 publications

References 12 publications

Characterization of noise sources in a microfabricated single-beam zero-field optically-pumped magnetometer

Characterization of noise sources in a microfabricated single-beam zero-field optically-pumped magnetometer

On-scalp MEG system utilizing an actively shielded array of optically-pumped magnetometers

On-scalp MEG system utilizing an actively shielded array of optically-pumped magnetometers

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Product

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Measurement of weak magnetic fields using zero-field parametric resonance in optically pumped He⁴