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Applied Physics Letters

Optically pumped magnetometers based on pseudo-random-bit-sequence phase modulation

Ning Ma, Lihong Duan, Xiujie Fang, Bozheng Xing, Danyue Ma, Jixi Lu, Bangcheng Han

Published Aug 14, 2023• 12 citations
Publisher

Abstract

A high-sensitivity atomic magnetometer based on broad linewidth laser pumping is demonstrated. The broad linewidth laser was obtained by broadening a narrow linewidth laser utilizing an external phase modulator driven by a pseudo-random-bit-sequence (PRBS) waveform. This ingenious optical pumping scheme can significantly enhance the signal response and suppress light shift noise, as well as can reduce the gradient of electron spin polarization and improve the stability of magnetometers. Compared with a narrow linewidth laser pumping scheme, the response of the magnetometer is increased by 56%, experimentally. The relative polarization gradient is reduced from 9.8%/cm to better than 4%/cm by setting PRBS with a bit rate of 16 GHz. This scheme can also be applied to many other fields such as noninvasive bio-magnetic imaging and atomic spin gyroscopes.

DOI
10.1063/5.0164896

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A high-sensitivity atomic magnetometer based on broad linewidth laser pumping is demonstrated. The broad linewidth laser was obtained by broadening a narrow linewidth laser utilizing an external phase modulator driven by a pseudo-random-bit-sequence (PRBS) waveform. This ingenious optical pumping scheme can significantly enhance the signal response and suppress light shift noise, as well as can reduce the gradient of electron spin polarization and improve the stability of magnetometers. Compared with a narrow linewidth laser pumping scheme, the response of the magnetometer is increased by 56%, experimentally. The relative polarization gradient is reduced from 9.8%/cm to better than 4%/cm by setting PRBS with a bit rate of 16 GHz. This scheme can also be applied to many other fields such as noninvasive bio-magnetic imaging and atomic spin gyroscopes.
An optical pumping scheme is proposed for reducing the gradient of electron spin polarization and suppressing light source noise in a spin-exchange relaxation-free magnetometer. This is achieved by modulating only the phase of a narrow-linewidth pump light field with external Gaussian noise. Compared to the absence of phase modulation, the uniformity of electron spin polarization was improved by over 40%, and the light-frequency noise suppression ratio of the magnetometer was enhanced by 4.3 times. Additionally, the response of the magnetometer was increased by 54%, resulting in a sensitivity of 0.34 fT/Hz 1/2 at 30 Hz. The applicability of this scheme can extend to other optical pumping experiments involving large atom ensembles requiring uniform electron spin polarization distribution, which is beneficial for developing ultra-high sensitivity and high stability magnetometers essential for magneto-cardiography and magneto-encephalography research applications.
A hybrid optical pumping method employing a broad linewidth laser is proposed to enhance spin coherence and suppress virtual magnetic field noise in atomic magnetometers. The central frequency of broad linewidth laser is tuned to the resonance peak of the alkali-metal atoms D1 line. Its multiple frequency components suppress AC-Stark shift fluctuations caused by pumping laser frequency variations, thereby reducing virtual magnetic field noise. In addition, these components decrease the average optical absorption cross-section, improving the uniformity of atomic spin polarization, further enhancing spin coherence and extending transverse relaxation time. Experimental results show that, compared with narrow linewidth pumping, the proposed method suppresses virtual magnetic field noise by 17.95% to 34.50%, and increases the magnetometer response by 1.62% to 30.31% at 5 Hz. The average sensitivity within the frequency range of 1-20 Hz is improved to 0.158 fT/Hz 1/2 , representing a significant enhancement of 30.70% compared to narrow linewidth pumping.
This work proposes an optical pumping strategy based on a broadband laser to overcome the problem of atomic spin polarization gradients and sensitivity to optical frequency noise in NMR gyroscopes. A band-limited Gaussian white noise radiofrequency signal is applied to a lithium niobate electro-optic phase modulator to do this. A narrow linewidth laser maintains its frequency stability while uniformly broadening its spectrum to the hundreds of MHz range. Compared to unmodulated pumping, the proposed scheme enhances the uniformity of rubidium spin polarization by approximately 10%, reaching a measured value of 92.8%. Additionally, the sensitivity of the inert gas precession signal to optical frequency noise is reduced by half. This scheme can be extended to other atomic sensors requiring higher uniform spin polarization distributions and provides a promising way to achieve high-stability, low-noise NMR gyroscope systems.
A high-sensitivity optically pumped atomic magnetometer working in the geomagnetic range utilizing the light-narrowing effect and in-phase excitation is described. The setup is based on a simple pump–probe arrangement built around a Cs vapor cell whose active volume is 64 mm3. The transverse oscillating field is applied parallel to the probe beam to drive Zeeman resonance, and the in-phase component of the resonance signal is measured to determine the field. The sensitivity of the magnetometer is improved by pumping most atoms into the stretched state. Consequently, spin-exchange relaxation is suppressed, and a sensitivity of 0.1 pT/Hz1/2 in the range of 10 μT is achieved. This magnetometer has the advantages of large dynamic range, high performance of low-frequency stabilization, high response speed, and compact size. It can be used for many cutting-edge applications such as detection of magnetic anomalies.
The ultrasensitive magnetometer has a vital importance in fundamental research and applications. Currently, the spin-exchange relaxation-free (SERF) atomic magnetometer has been reported with a sensitivity around the level of fT/Hz 1/2 . To enhance the sensitivity, a gradiometer configuration has usually been introduced to cancel the common-mode noise between two separate channels. However, the signal and response from different channels are not the same due to the attenuation of the pump beam. Here, we proposed a counter-propagating optical sideband pumping method to polarize the atoms, using the electro-optic modulator to modulate the single-pump beam, generating two symmetrically red- and blue-detuned sidebands of frequency. This scheme leads to a significant reduction of undesirable effects coming along with the optical pumping, such as light shifts and spatial inhomogeneity in atomic spin polarization. With the help of this pumping scheme, the two channels have the same magnetic response, and we have built a gradiometer atomic magnetometer with a sensitivity of 0.5 fT/Hz 1/2 ranging from 5 to 40 Hz. Our results propose the possibility of creating larger arrays of atomic magnetometers (AMs) with high sensitivity and spatial resolution based on single-vapor cells for magnetocardiography and magnetoencephalography imaging or searching for exotic spin-dependent interactions.
We demonstrate an all-optical single beam pulsed atomic magnetometer assisted by laser mode-hopping in a distributed Bragg reflector (DBR) laser. We implement a temporal sequence of the laser current, with sinusoidal current modulation including the laser mode-hop current for synchronous optical pumping, and a following constant current for paramagnetic Faraday rotation measurements, to probe the free induction decay (FID) of transverse 87^{87}87Rb spin polarization. Repetitive sudden frequency shifts of 20 GHz around the pressure-broadened 87^{87}87Rb spectra, originating from laser mode-hopping, enable discontinuous optical pumping modulation with a large depth, which enhances transverse spin polarization. We achieved a sensitivity of 0.6 pT/Hz1/2^{1/2}1/2 in a magnetic field of 27 μμμT, mainly limited by the photon-shot-noise and the magnetic field noise induced by the current noise in the current supply for driving the bias magnetic field coil. The Cramer-Rao lower bound (CRLB) of the sensitivity due to the non-magnetic noise such as photon shot-noise is 131 fT/Hz1/2^{1/2}1/2. Our approach based on laser mode-hopping can be applied for the miniaturization of all-optical atomic magnetometers with sub-pT/Hz1/2^{1/2}1/2 sensitivities.