IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 11, NOVEMBER 2014 4006903

Measurements of Magnetic Field Distributions With an OpticallyPumped K-Rb Hybrid Atomic Magnetometer

Yosuke Ito1, Daichi Sato1, Keigo Kamada1,2, and Tetsuo Kobayashi1

1Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan2Japan Society for the Promotion of Science, Tokyo 102-0083, Japan

We have developed a K-Rb hybrid optically pumped atomic magnetometer (OPAM) for biomagnetic measurements. With thishybrid OPAM, we used a linear photodiode array and a charge-coupled device (CCD) sensor to instantaneously measure 1-D and 2-Dmagnetic field distributions generated by a test coil, respectively. The measured distributions were compared with those calculatedusing the Biot–Savart law and showed good agreement for the linear photodiode array; however, in the case of the CCD sensor inthe area away from the test coil, the intensity of the detected magnetic field was slightly different from the calculated one, possiblybecause of the diffusion of the spin-polarized atoms as well as smear and blooming in the CCD sensor. The sensitivity of the OPAMwas 5–6 pT/Hz1/2 using the linear photodiode array and ∼10 pT/Hz1/2 using the CCD sensor. In future experiments, we plan tofabricate a sensor cell with a high density of probe atoms and reduce the noise generated in the electronic circuitry of the detectorsin order to increase sensitivity.

Index Terms— Atomic magnetometer, biomagnetics, CCD sensor, linear photodiode array, optical pumping, spin exchange.

I. INTRODUCTION

OPTICALLY pumped atomic magnetometers (OPAMs)have extremely high sensitivity (theoretically,

0.01 fT/Hz1/2) under spin-exchange-relaxation-free (SERF)conditions [1]–[3], and as such are presumably goodalternatives to superconducting quantum interference devicesfor biomagnetic measurements, such as magnetoencephalo-graphy (MEG) and magnetocardiography (MCG). In fact,some researchers now use OPAMs for biomagnetic applicat-ions [4]–[7]. We have also developed OPAMs for biomagneticmeasurements [8]–[10]. In general, MEG and MCGmeasurements are made with multiple sensors in order toobtain magnetic field distributions. However, the sensingarea of the OPAMs where the two perpendicular laser beams(pump and probe beams) intersect is considerably small.Therefore, most previous studies conducted biomagneticmeasurements using multi-cell OPAMs, although it is difficultto fabricate cells with identical responses.

In a different approach, Gusarov et al. [11] widened thelaser beam cross-sectional areas to achieve 3-D magnetic fieldmeasurements using a large-volume cell. However, if thiscross-sectional area is greatly widened, the distribution of lightcan be inhomogeneous because of attenuation within the pumpbeams, which in turn affects magnetic field measurements.

For our approach, we developed OPAMs with hybrid cellsof K and Rb atoms (hybrid OPAMs), which have the advantageof high spatial homogeneity of spin polarization in the cell,because the pumped and probed atoms are different from eachother. In previous studies, we measured the spatial homogene-ity typical single-alkali metal OPAMs, and we demonstratedthat hybrid OPAMs have a more homogeneous spin polariza-tion over a wide sensing area in a single cell [12], [13].

Manuscript received March 7, 2014; accepted May 30, 2014. Date ofcurrent version November 18, 2014. Corresponding author: Y. Ito (e-mail:yito@kuee.kyoto-u.ac.jp).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TMAG.2014.2329856

In this paper, we used our newly developed hybrid OPAMfor magnetic field distribution measurements with a viewtoward developing novel MCG and MEG systems. A linearphotodiode array and a charge-coupled device (CCD) sen-sor were used as the detectors of the probe beams in thehybrid OPAM, so that we could obtain 1-D and 2-D datainstantaneously. The performance of the hybrid OPAM withtwo detectors was evaluated by measuring magnetic fieldsgenerated by a test coil.

II. METHODS

Fig. 1 shows the schematic of our basic SERF magnetometerconfiguration. The details of the measurement process weresimilar to those presented in our previous research [12]. Thesensor head of the hybrid OPAM was a 5 cm × 5 cm × 5 cmcubic glass cell which enclosed K and Rb atoms with He andN2 buffer gases at a ratio of 10:1 and a total pressure of150 kPa at room temperature. The cell was kept at 453 K dur-ing operation of the OPAM, and the densities of gaseous K andRb measured by absorption spectroscopy at this temperaturewere 3.4 × 1012 cm−3 and 4.5 × 1014 cm−3, respectively. Weintroduced a slight amount of the K source in the cell, so thatthe density of the K was less than the saturated vapor density.

The K atoms’ spins were polarized by a circularly polarizedpump beam whose wavelength was 770.11 nm, and spin polar-ization was transferred to the Rb atoms by spin exchange colli-sions, as shown in Fig. 2. This spin polarization rotated arounda test coil-generated external magnetic field orthogonal to boththe pump and probe beams. The linearly polarized probe beam,whose wavelength was 794.42 nm, passed through the cell andwas detected with a linear photodiode array and a CCD sensor.The pump and probe laser beams were widened, and only thepump beam was narrowed down with a 4 mm slit, as shownin Fig. 1. We scanned 2 cm in the x-direction by moving theslit in 5 mm increments.

The linear photodiode array had 46 pixels, each of size4.4 mm × 0.9 mm, and the CCD sensor had 1024 × 256

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4006903 IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 11, NOVEMBER 2014

Fig. 1. Experimental setup of our hybrid OPAM with a linear photodiodearray and a CCD sensor.

Fig. 2. Principle of K-Rb hybrid OPAMs.

pixels, each of size 26 μm × 26 μm. In the case of the linearphotodiode array, we used five pixels at intervals of 4 mm.On the other hand, in the case of the CCD sensor, we integrated64 × 32 pixels for each channel in order to enlarge the sensingvolume per channel. The CCD sensor used a global shutter,so the signals taken from every pixel were synchronized.

The 10 mm diameter test coil was 40 mm above the sensingarea for the linear photodiode array and 34 mm above the topof the pixels for the CCD sensor. To the test coil, we applieda sinusoidal wave of 0.5 mA and 100 Hz for measurementswith the linear photodiode array and 1 mA and 40 Hz formeasurements with the CCD sensor. This was because theframes per second of the CCD sensor was 174.52 Hz, andthe CCD sensor was less sensitive than the linear photodiodearray in this system.

The measured magnetic field distributions were comparedwith those calculated from the Biot–Savart law

δB = μ0

4π

Iδs × r|r|3 (1)

Fig. 3. (a) Magnetic field distributions measured with our hybrid OPAMusing the linear photodiode array and (b) calculated by Biot–Savart law.

where μ0 is the permeability of vacuum, Iδs is the currentelement of the test coil, and r is the position vector.

III. RESULTS AND DISCUSSION

Fig. 3(a) and (b) shows the magnetic field distributionsmeasured with our hybrid OPAM using the linear photodiodearray and calculated from the Biot–Savart law, respectively.We obtained five sets of 1-D data by shifting the 4 mm slitacross the pump beam in steps of 5 mm and then mergingthe data in order to acquire the entire distribution. Notethat the center of the test coil was at (z, x) = (0, 0).Results show that the measured distribution shows good agree-ment with the calculated one. However, at locations wherex = −1.0 cm, the signal responses were quite weak, whichmay be attributed to the spatial inhomogeneity of the intensityof the widened pump beam and not to inhomogeneity in thespin polarization, because the same result was observed whenwe pumped and probed Rb atoms. The sensitivity of the hybridOPAM was 5–6 pT/Hz1/2 at 100 Hz, which is smaller than thevalue for polarimeter detection [13]. This may be due to theinhomogeneity of the pump and probe beam profiles and/ora higher noise floor because of noise in the circuitry for thelinear photodiode array.

The top row in Fig. 4 shows the magnetic field distributionmeasured with our hybrid OPAM using the CCD sensor,and the bottom row shows the magnetic field distributioncalculated from the Biot–Savart law. Although the measureddistribution can be fit relatively well by Biot–Savart law, themeasured intensities in the area away from the test coil wereslightly larger than expected. We believe this is due to the dif-fusion of the spin-polarized atoms. The spin-polarized atomsoutside of the area of the crossed pump-probe beams flow intothe sensing area, which would limit spatial resolution even ifwe could make the detector pixels infinitely small. Therefore,we could improve spatial resolution using a high-pressuresensor cell in order to prevent diffusion of the sensing atoms.

Furthermore, smear and/or blooming can occur with CCDsensors, and our probe beam was initially too intense forour CCD sensor, because of which we attenuated the beamusing neutral density filters. However, the beam was still toointense, which led to interference between photodiodes in theCCD sensor. A possible solution is to use 2-D photodiodearrays with low electrical noise. The other might be to useCMOS sensors, which do not suffer smear and blooming;however, commonly used CMOS sensors have rollingshutters, i.e., there is a delay between signals from each pixel.

ITO et al.: MEASUREMENTS OF MAGNETIC FIELD DISTRIBUTIONS 4006903

Fig. 4. Magnetic field distributions measured with our hybrid OPAN using the CCD sensor (top) and calculated by Biot–Savart law (bottom).

Global shutter CMOS sensors would be suitable forsimultaneous sensing at multiple locations and would notsuffer from smear and blooming.

In the case of the CCD sensor, the sensitivity of eachchannel was ∼10 pT/Hz1/2 at 40 Hz. This was worse than thesensitivity for the linear photodiode array and is not sensitiveenough for MEG measurements. The sensing area of a channelof the CCD sensor was about three times smaller than that ofthe linear photodiode array. The smaller sensing area leads tothe low signal intensity. The primary noise was magnetic noisedue to the low signal intensity. To enhance the signal intensity,we need to increase the density of the probed atoms.

IV. CONCLUSION

We demonstrated instantaneous 1-D and 2-D magneticfield measurements using a linear photodiode array and aCCD sensor with a hybrid OPAM. The measured magneticfield distributions generated from a test coil showed goodagreement with calculations. The sensitivities of the hybridOPAM were 5–6 pT/Hz1/2 using the linear photodiode arrayand ∼10 pT/Hz1/2 using the CCD sensor. Although thiswas not low enough for MEG measurements, we believe thesensitivity was limited by electrical noise from the circuitry ofthe detectors. Therefore, in future studies, we plan to improvethe circuitry and apply a 2-D photodiode array in order toreduce noise as well as fabricate a cell with a high density ofprobe atoms to boost the signal.

ACKNOWLEDGMENT

This work was supported in part by the Innovative Techno-Hub for Integrated Medical Bio-Imaging of the Project forDeveloping Innovation Systems, Grant-in-Aid for ScientificResearch (A), under Grant 24240081, and in part by theGrant-in-Aid for Challenging Exploratory Research underGrant 24650221 and Grant 25610171, through the Ministry ofEducation, Culture, Sports, Science, and Technology, Japan.

REFERENCES

[1] J. C. Allred, R. N. Lyman, T. W. Kornack, and M. V. Romalis,“High-sensitivity atomic magnetometer unaffected by spin-exchangerelaxation,” Phys. Rev. Lett., vol. 89, no. 13, p. 130801, Sep. 2002.

[2] I. K. Kominis, T. W. Kornack, J. C. Allred, and M. V. Romalis,“A subfemtotesla multichannel atomic magnetometer,” Nature, vol. 422,no. 6932, pp. 596–599, Apr. 2003.

[3] D. Budker and M. V. Romalis, “Optical magnetometry,” Nature Phys.,vol. 3, no. 4, pp. 227–234, 2007.

[4] G. Bison, R. Wynands, and A. Weis, “A laser-pumped magnetometer forthe mapping of human cardiomagnetic fields,” Appl. Phys. B, vol. 76,no. 3, pp. 325–328, Mar. 2003.

[5] J. Belfi, G. Bevilacqua, V. Biancalana, S. Cartaleva, Y. Dancheva,and L. Moi, “Cesium coherent population trapping magnetometer forcardiosignal detection in an unshielded environment,” J. Opt. Soc. Amer.B, vol. 24, no. 9, pp. 2357–2362, Sep. 2007.

[6] S. Knappe, T. H. Sander, O. Kosch, F. Wiekhorst, J. Kitching, andL. Trahms, “Cross-validation of microfabricated atomic magnetometerswith superconducting quantum interference devices for biomagneticapplications,” Appl. Phys. Lett., vol. 97, no. 3, p. 133703, Sep. 2010.

[7] V. K. Shah and R. T. Wakai, “A compact, high performance atomicmagnetometer for biomedical applications,” Phys. Med. Biol., vol. 58,no. 22, pp. 8153–8161, Nov. 2013.

[8] S. Taue, Y. Sugihara, T. Kobayashi, S. Ichihara, K. Ishikawa, andN. Mizutani, “Development of a highly sensitive optically pumpedatomic magnetometer for biomagnetic field measurements: A phan-tom study,” IEEE Trans. Magn., vol. 46, no. 9, pp. 3635–3638,Sep. 2011.

[9] S. Taue, Y. Sugihara, T. Kobayashi, K. Ishikawa, and K. Kamada,“Magnetic field mapping and biaxial vector operation for biomagneticapplications using high-sensitivity optically pumped atomic magnetome-ters,” Jpn. J. Appl. Phys., vol. 50, no. 11, pp. 116604-1–116604-6, 2011.

[10] K. Kamada, Y. Ito, and T. Kobayashi, “Human MCG measurementswith a high-sensitivity potassium atomic magnetometer,” Physiol. Meas.,vol. 33, no. 6, pp. 1063–1071, May 2012.

[11] A. Gusarov, D. Levron, E. Paperno, R. Shuker, and A. B.-A. Baranga,“Three-dimensional magnetic field measurements in a single SERFatomic-magnetometer cell,” IEEE Trans. Magn., vol. 45, no. 10,pp. 4478–4481, Oct. 2009.

[12] Y. Ito, H. Ohnishi, K. Kamada, and T. Kobayashi, “Sensitivityimprovement of spin-exchange relaxation free atomic magnetometersby hybrid optical pumping of potassium and rubidium,” IEEE Trans.Magn., vol. 47, no. 10, pp. 3550–3553, Oct. 2011.

[13] Y. Ito, H. Ohnishi, K. Kamada, and T. Kobayashi, “Effect of spatialhomogeneity of spin polarization on magnetic field response of anoptically pumped atomic magnetometer using a hybrid cell of K and Rbatoms,” IEEE Trans. Magn., vol. 48, no. 11, pp. 3715–3718, Nov. 2012.