PROCEEDINGS OF SPIESPIEDigitalLibrary.org/conference-proceedings-of-spie

Fully integrated, standalone zero field
optically pumped magnetometer forbiomagnetism

J. Osborne, J. Orton, O. Alem, V. Shah

J. Osborne, J. Orton, O. Alem, V. Shah, "Fully integrated, standalone zero
field optically pumped magnetometer for biomagnetism," Proc. SPIE 10548,
Steep Dispersion Engineering and Opto-Atomic Precision Metrology XI,
105481G (22 February 2018); doi: 10.1117/12.2299197
Event: SPIE OPTO, 2018, San Francisco, California, United States
Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 3/14/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

Invited Paper

Fully Integrated, Standalone Zero Field Optically Pumped
Magnetometer for Biomagnetism
J. Osborne, J. Orton, O. Alem1, V. Shah2
QuSpin, Inc., Louisville, CO, USA 80027
ABSTRACTWe describe the operation and results of our first generation zero field optically pumped magnetometer (OPM)
developed for biomedical applications. The OPM technology is one of the most promising non-cryogenic candidates toreplace superconducting quantum interference device (SQUID) magnetometers in key areas of biomagnetism. The firstgeneration sensors are designed to transition OPM technology from a physics laboratory to researchers in the medical
community. The laser and optical components are tightly integrated inside the sensor package, and the sensor is tethered
to a dedicated electronics signal processing unit that enables automated and standalone operation inside a magneticallyshielded room.
Keywords:
Optically
PumpedMagnetometer,
Magnetoencephalography, and Magnetocardiography.

AtomicMagnetometer,

SQUID,

Biomagnetism,

1. INTRODUCTION
The high sensitivity of superconducting quantum interference device (SQUID) magnetometers has enabled the field of
Biomagnetism since its inception in the 1970s [1]. However, practical and cost constraints originating from cryogenic
nature of the SQUIDs has impeded widescale adoption of the biomagnetic technologies.
A zero-field OPM with nearly 10 fT/√Hz level sensitivity was demonstrated as far back as in 1969 [2]. The subsequentdiscovery of alkali spin-exchange relaxation suppression in near-zero magnetic field environment provided thefundamental breakthrough necessary for miniaturization of zero field OPM technology [3]. Landmark work by Romalisand coworkers demonstrated OPM with sub-femtotesla level sensitivity using spin-exchange suppression in low
magnetic fields [4]. More recently, development of integrated sensor packages [2], [5], [6] and head-to-head comparison
between zero field OPMs and SQUIDs in a clinical setting have highlighted OPMs as a truly viable alternative tocryogenic technologies in many important clinical applications [5]–[9].
Here we describe the development of a fully integrated and automated zero field optically pumped magnetometer(QZFM) that can be used by the medical research community and other non-specialists in this field.

2. OVERVIEW
The basic architecture of QZFM is the same as that outlined in the Ref. [2], [10], [11] , and it utilizes zero-field levelcrossing resonance (Hanle) for operation. Figure 1 shows a schematic representation of sensor head.
A 795 nm, single mode vertical cavity surface emitting laser (VCSEL) resonant with the D1 line of 87Rb is used for
optical pumping and resonance detection. The optical output power of the laser is roughly 200 µW. The VCSEL is
mounted on an electrically heated baseplate, and the temperature of the laser is stabilized at a mK level. The laser
temperature and the laser injection current are controlled to precisely adjust the optical power and wavelength of thelaser using digital feedback loops. The laser light is circularly polarized with a quartz waveplate, and the beam iscollimated with a plano-convex lens. The collimated light beam is reflected 90 degrees using a prism and is then passed
through a 3x3x3 mm3 alkali vapor cell containing 87Rb and a mixture of buffer gases to reduce rubidium relaxation fromwall collisionns. The light trransmitted throough the vaporr cell is then directed by anotther prism towaards a photodeetector to
1 presently at University of Colorado
2vshah@quspin.com

www.quspin.com

Steep Dispersion Engineering and Opto-Atomic Precision Metrology XI, edited bySelim M. Shahriar, Jacob Scheuer, Proc. of SPIE Vol. 10548, 105481G · © 2018
SPIE · CCC code: 0277-786X/18/$18 · doi: 10.1117/12.2299197
Proc. of SPIE Vol. 10548 105481G-1
Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 3/14/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

E0
Figure 1: A laser diode 1 pro
oduces light thatt is tuned to the resonancerfrequeency of the alkalli atoms. The ligght beam is collim
matedand directedd to pass throughh a glass cell 2 coontaining the vappor of alkali atom
ms. The transmitted light beam isi directed onto a
photodiode 3.
3monitor the lightlintensity transmittedtthrrough the vaporr cell.
The vapor ceell is electricallly heated with resistive heateers to around 150 °C to optim
mize rubidium density.
d
The vaapor cellis ovenized to minimize heat loss and redduce the surfacee temperature of the outer sennsor housing. The
T temperatuure of thevapor cell iss actively stabiilized at 10 mK
K level. All thhe componentss inside the seensor housing are chosen to be nonmagnetic to thet greatest exttent possible. AC
A electrical cuurrents are useed to heat the laaser and the vaapor cell.
When the baackground mag
gnetic field is zero,z
the circulaarly polarized laser light spinn polarizes the rubidium atom
ms in thedirection of the light beam
m propagatingg through the vapor cell, makingmrubidiuum atoms larggely transparennt to theincoming ligght. A magnettic field in a direction perppendicular to the light reorrients the spinns increasing thet light
absorption. The
T photodetecctor senses this change in trannsparency as a function of maagnetic field annd produces ann electric
current propoortional to thee light transmittted through thhe vapor cell. In this way, a magnetic siggnal is converted to anelectric signaal forming a zeero-field magneetometer.
In zero fieldd, if a magnetiic field is appplied in a direcction perpendiicular to the liight beam andd the amplitudde of thetransverse appplied field is swept from a positive to a negativenvaluee, the transpareency of the rubbidium atoms changes
such that maximum transpaarency is obserrved when the atoms experiennce a magneticc field that is veryv
close to zeero. The
output of thee photodetecto
or as a functioon of the appliied field has a Lorentzian liine shape. This Lorentzian outputoisreferred to ass the zero-fieldd or Hanle resoonance (see Figgure 2). In QZZFM, the typicaal width (full widthwat half maximumm

Figure 2: Thhe zero-field reso
onance is the ouutput of the photoodetector as the magneticmfield iss scanned from a positive to a neegativevalue in the direction perpen
ndicular to the liight beam througgh the vapor celll. The FWHM off the zero-field resonancer
in QZFM isaround 30 nT.
n The demodullated output of thhe lock-in ampliifier (also referreed to as the errorr signal) has an asymmetrica
Loreenztianline shape.

Proc. of SPIE Vol. 10548 105481G-2
Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 3/14/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

– FWHM) of the zero-field resonance is around 30 nT.
To remove technical 1/f noise and to maximize the slope of the signal around zero field value, phase-sensitive lock-in
detection is utilized. The atoms in the vapor cell are subjected to an oscillating magnetic field at 923 Hz which
modulates the resonance. The modulated signal is synchronously demodulated using a lock-in amplifier producing an
output that has an anti-symmetric Lorentzian line shape as seen in Fig. 2. The output of the lock-in amplifier has amaximum slope at zero field and functions as the raw analog output of the magnetometer.
The sensitive axis of the magnetometer is defined by the direction of the modulation field (projected on a plane
perpendicular to the light beam). To make the magnetometer simultaneously sensitive to two orthogonal axes
(perpendicular to the light beam), we apply two separate modulation fields (same frequency, 90 deg. phase offset) using
separate orthogonal coils. Applying simultaneous modulation fields reduces the sensitivity in both axes by roughly 30%.
The residual magnetic field in a typical magnetically shielded room (MSR) is around 50 nT or lower. Because zero field
OPMs require absolute zero field environment to operate, external field coils are necessary to cancel out any residual
magnetic fields. To simplify the setup of QZFMs, we integrate field cancellation coils with the sensor package. A set offlexible three axis coils centered around the vapor cell is mounted over the sensor housing, and automated field-zeroing
algorithms are used to cancel out any residual fields around the vapor cell. The field zeroing procedure is repeated
whenever the direction or the magnitude of magnetic field changes by more than 1 nT or when the magnetometerorientation changes.
In QZFM, the internal coils are designed to compensate residual magnetic fields up to 50 nT. The primary limitation on
the magnitude of the residual that can be canceled using internal coils comes from the stability and the noise
performance of the internal coil drivers, as well as the inhomogeneity of the internal coils. The width of the zero-fieldresonance in QZFM is 30 nT. Therefore, transverse magnetic field gradients greater than ~10 nT over the length
dimension of the vapor cell (3 mm) can substantially broaden the resonance width and degrade magnetometer
performance.

3. MULTICHANNEL OPERATION AND CROSS-TALK
The QZFM is designed for multi-channel systems with high channel counts. Because the magnetic modulation signal
used for lock-in detection can be picked up by adjacent sensors, we modulate all the sensors with a common drive signal
to minimize interference. An electronics module designated as the ‘Master’ generates the modulation signal anddistributes it to all other electronics modules designated as the ‘Slaves.’ Although deriving the modulation signals from acommon drive resolves aliasing type effects, crosstalk effects are not fully removed by this procedure. Because thesensitive axis is defined by the direction of the modulation field, the modulation fields from adjacent sensors cansuperimpose on a sensor’s primary modulation field and thereby change the direction of the sensitive axis. Results fromour preliminary experiments suggest such effects are at a few percent level when adjacent sensors are separated by >2cm. The sensitivity of the magnetometer also depends on the amplitude of the modulation field. Superimposition ofmodulation fields from adjacent sensors also can also change the net amplitude/phase of modulation field experienced bya sensor, thereby changing the absolute sensitivity/accuracy of the sensor. Lastly, another cross-talk type effect thatarises comes from the DC compensation coils on the sensor used for field zeroing. The internal three-axis fieldcompensation coils used for field zeroing can produce a DC bias field that can be sensed by an adjacent sensor. Thus,running independent internal field zeroing algorithm in one sensor can push an adjacent sensor outside its operating field
range. To alleviate this problem, we simultaneously run field zeroing procedure on all the sensors. Simultaneous fieldzeroing collapses the overall system level DC field configuration in a way that each sensor experiences a zero-fieldenvironment to the greatest extent possible. The downside of this approach is that even if the position of a single sensor
is reconfigured, the automated field zeroing procedure may have to be repeated for all the sensors.

Proc. of SPIE Vol. 10548 105481G-3
Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 3/14/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

ONICS CONNTROLLER
R
4. ELECTRO
The electroniics control uniit houses all thee circuitry for controlling a QZFM
Q
sensor. A schematic overview
oof thee QZFM
electronics controllercis shhown in Figurre 3. A VCSEL driver produuces the low noise current to power the laser. A

1-

Pow

:oil drivers

Laser dri)ver

Power siipply

DSP+

Photodiode Amp.

Ternip. controllers

Microcon troller

Digit

Anal

Figure 3: A schematic overv
view of the QZFM electronics coontroller. The eleectronics controller includes all the necessarys
QZFM sennsor.
componentss for stand-alone operation of a single

transimpedannce amplifier is used to connvert the phottodiode currennt to voltage. The signal frrom the photoodiode isdigitized andd read by a digitaldsignal processorp(DSPP) that stabilizzes the vapor cell and the laser temperatture andgenerates locck-in outputs. The DSP alsoo controls threee low noise cooil drivers to powerp
the DC biasing coils for fieldzeroing. A microcontrollermis used to booot the DSP, runn algorithms foor automated opperation of thee magnetometeer, and tocommunicatee with a host PC over a UAR
RT to USB briddge.
Figure 4 bellow shows a picturepof the QZFM
Q
sensor head along withw its electronnics controllerr. The 6.5-metter cable

FigureF
4: A pictuure of the QZFM
M sensor packagee and the controll electronics.

connecting thhe sensor and thet electronics housing allow
ws the electroniccs to be placedd outside the MSR.
M

5. RESULTS
The typical magneticmfield
d sensitivity off QZFM is 10 fT/√Hz in 1-100 Hz frequenncy band as shhown in Figuree 5. The
sensitivity inn dual axis mode is roughly 30% lower compared with thet sensitivity in single axis mode. Majoritty of thesensor units thatt
we have builtbto date falll in a range beetween 7-13 fT
T/√Hz range inn single axis mode.
m
The typiccal -3 dB
bandwidth off the sensor is 135 Hz, with a response roll--off similar to a first-order low
w-pass filter. In
I zero field, thhe sensor
response is liinear to within 1% when the signal amplitudde is less than 1 nT.
In QZFM, thhe stand-off disstance (see Figuure 6) betweenn the outside off the sensor houusing and the centercof the vaapor cellis minimizedd to 6 mm. Thee small stand-ooff distance alllows the sensoors to be placedd closer to a siignal source coomparedwith SQUID-based systemss where the typpical stand-off distance is 20--30 mm.

Proc. of SPIE Vol. 10548 105481G-4
Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 3/14/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

Sensitivity [pT /tiiHz]óóó,- Z -axis (dual)- Y -axis (dual)- Z -axis (single)- Y -axis (single)

i0 Hz- Laser Noise- Electronics Noi,`.,,,.y`.
.. .i.,..
...

:,.y,,., ..
..........

.

......

..

. .

.

20

40

60

80

Frequency [Hz]Figure 5: A typical noise flo
oor of a QZFM measuredminside a small 3-layer mu-metal magnnetic shields withh an inner ferrite
layer. The noisentraces inclu
ude both single anda simultaneouus dual axis meassurements.

The outside dimension
dof thet sensor is 133x19x110 mm, and the electrronics housingg is 30x110x1770 mm. The eleectronics
unit is separaated from the sensorshead byy a 6.5m cord allowing the electronics
eunitt to be placed outside a magnneticallyshielded roomm if necessaryy. The electronnics unit can alsoabe placed a few feet aw
way from the sensorsinside thhe MSR
without anyy noticeable interference. The
T
total QZFFM power coonsumption(including electronics) is 4..5 W, out of whichw~700 mW
W is used to heaat the vapor
cell and the laser. The surface temperatuure on the subjject facing surrface of ther
by roughlyy 10 degrees C above ambiient. The outsside surface
sensor can risetemperature partlypdependss on the thermaal characteristiccs of the sensorr holder.
Unlike SQU
UID systems, theret
are no consumables
c((for
example, liquid He)
inside QZFM
M. Instead, thee intrinsic lifetime is limited by the lifetim
me rating of
the laser. Thhe laser used in QZFM is rated for sevveral years of continuous
operation, although furtherr long-term tessting is neededd in the contexxt of QZFM
which is higghly sensitive to spectral annd noise charaacteristics of thhe laser. In

-

1e-11

- OPM

.55 (b)
.50
.45
.40
.35

SQUID

.35
.40
.45
.50
.55

.60 (c)
0.5

1.0

1.5

2.0

2.5

3.0

Figure 6: Posittion of the vaporr cellwith respect too the QZFM outeer
housing.

3.55

000

SQiOY

'044400

0 0 0.1 0.2 0.3 0.4 0.5 0.6 0./

Time [Hours]

Time [H ours]

Figure 7: Loow frequency perrformance of thee QZFM. (a) Ouutput of three freee-running QZFM
Ms and three (Ellekta) SQUID
magnetometters inside a three-layer shielded room. (b, c) Synnthetic gradient between two adjjacent SQUID magnetometersmand
QZFMs. (c) A picture showiing placement off the Elekta MEG
G SQUID systemm and the QZFM
Ms inside a MSR
R. The QZFMs shhownin the backgrround are roughly oriented to pooint in the same direction as SQU
UID magnetomeeters. (d) A closee up photo of thee
QZFMs usedd in the measureements reported here.
hProc. of SPIE Vol. 10548 105481G-5
Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 3/14/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

addition, the QZFM is not hermetically sealed at present, and degradation of the surface quality of internal optics mayplace further constraints on the sensor lifetime.
Figure 7 shows the low-frequency performance of QZFM (data courtesy Joonas Iivanainen and Rasmus Zetter, Aalto
University). The data was collected by recording the output of free running QZFMs inside a three-layer MSR. From the
Figure 7(c), an upper bound on the intrinsic drift of QZFM is ~2 pT/hr. Figure 7 (b) and (c) show comparable drift in
SQUID and OPM channels, potentially indicating that the measured drift may be due to changes in the gradient field.
A number of research groups have collected data MCG and MEG data using QZFM validating the technology in aclinical setting [5], [12], [13]. Figure 8 shows MEG recordings collected at the University of Wisconsin Madison(courtesy Prof. R. T. Wakai, University of Wisconsin) using an early version of the QZFM prototypes. A somewhat
lower absolute sensitivity of QZFMs (10 fT/√Hz) compared with SQUIDs (3-5 fT/√Hz) is compensated by the smaller
stand-off distance and proximal placement of the OPMs to the subject, providing equivalent or higher signal-to-noise

Figure 8: (a) A picture showing QZFMs positioned over the auditory cortex. (b) Auditory Evoked Response. Impulse spacing
randomized 2-3 sec, 2 kHz tone, recording length 5 mins, ~175 averages.

ratios compared to SQUID systems.

6. CONCLUSION
We have demonstrated fully integrated, stand-alone zero field OPM sensors and shown that this technology is a truly
viable alternative to SQUIDs sensors in key areas of biomagnetism such as fetal and adult MCG and MEG. The absolutesensitivity of our sensors is comparable to low-temperature SQUID devices. By integrating the laser diode and otheroptical components directly inside the sensor package, the overall robustness of the device is significantly improved
allowing OPMs to be incorporated in routine clinical applications. In addition, integrating the laser diode with the sensor
package has also greatly improved the sensor noise properties especially in the low-frequency domain. Furthermore, theaddition of field zeroing coils inside the sensor has removed the need for external field zeroing infrastructure allowing
the sensors to be used as stand-alone units inside a typical shielded room. We have also addressed many technicalconstraints enabling zero-field OPM operation in the multi-channel configuration for MEG type imaging applications.
The non-cryogenic nature of OPM technology, combined with a highly simplified design is expected to provide a stepchange in technology for future biomagnetism applications.

7. REFERENCES[1]

D. Cohen, E. A. Edelsack, and J. E. Zimmerman, “MAGNETOCARDIOGRAMS TAKEN INSIDE ASHIELDED ROOM WITH A SUPERCONDUCTING POINT #x2010;CONTACTMAGNETOMETER,” Appl. Phys. Lett., vol. 16, no. 7, pp. 278–280, Apr. 1970.

Proc. of SPIE Vol. 10548 105481G-6
Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 3/14/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

[2]J. Dupont-Roc, S. Haroche, and C. Cohen-Tannoudji, “Detection of very weak magnetic fields(10−9gauss) by 87Rb zero-field level crossing resonances,” Phys. Lett. A, vol. 28, no. 9, pp. 638–639,
Feb. 1969.
[3] W. Happer and H. Tang, “Spin-Exchange Shift and Narrowing of Magnetic Resonance Lines in
Optically Pumped Alkali Vapors,” Phys. Rev. Lett., vol. 31, no. 5, p. 273, Jul. 1973.
[4] 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.
[5] V. K. Shah and R. T. Wakai, “A compact, high performance atomic magnetometer for biomedical
applications,” Phys. Med. Biol., vol. 58, no. 22, p. 8153, Nov. 2013.
[6] S. Knappe, T. H. Sander, O. Kosch, F. Wiekhorst, J. Kitching, and L. Trahms, “Cross-validation of
microfabricated atomic magnetometers with superconducting quantum interference devices forbiomagnetic applications,” Appl. Phys. Lett., vol. 97, no. 13, p. 133703, 2010.
[7] O. Alem, A. M. Benison, D. S. Barth, J. Kitching, and S. Knappe, “Magnetoencephalography of
Epilepsy with a Microfabricated Atomic Magnetrode,” J. Neurosci., vol. 34, no. 43, pp. 14324–14327,
Oct. 2014.
[8] C. Johnson, P. D. D. Schwindt, and M. Weisend, “Magnetoencephalography with a two-color pumpprobe, fiber-coupled atomic magnetometer,” Appl. Phys. Lett., vol. 97, no. 24, p. 243703, 2010.
[9] J. Sheng et al., “Magnetoencephalography with a Cs-based high-sensitivity compact atomic
magnetometer,” Rev. Sci. Instrum., vol. 88, no. 9, p. 094304, Sep. 2017.
[10] R. E. Slocum and F. N. Reilly, “Low Field Helium Magnetometer for Space Applications,” IEEE Trans.
Nucl. Sci., vol. 10, no. 1, pp. 165–171, 1963.
[11] V. Shah, S. Knappe, P. D. D. Schwindt, and J. Kitching, “Subpicotesla atomic magnetometry with amicrofabricated vapour cell,” Nat Photon, vol. 1, no. 11, pp. 649–652, Nov. 2007.
[12] E. Boto et al., “A new generation of magnetoencephalography: Room temperature measurements using
optically-pumped magnetometers,” NeuroImage, vol. 149, no. Supplement C, pp. 404–414, Apr. 2017.
[13] H. Eswaran, D. Escalona-Vargas, E. H. Bolin, J. D. Wilson, and C. L. Lowery, “Fetal
magnetocardiography using optically pumped magnetometers: a more adaptable and less expensivealternative?,” Prenat. Diagn., vol. 37, no. 2, pp. 193–196, Feb. 2017.

Proc. of SPIE Vol. 10548 105481G-7
Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 3/14/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use