Miniature Total Field MagnetometersDr. Mark D. Prouty, Geometrics, Inc

Dr. John Kitching, (National Institute of Standards and Technology)Dr. Darwin Serkland (Sandia National Laboratories)

Funded by SERDP under projects MM-1512 and MM-1568

(markp@geometrics.comCurrent generation total field magnetometersare extremely large, bulky, and they consumea lot of power. This furthers requires large,heavy batteries. Funded by DARPA, recenttechnologies have been developed towardsminiaturizing atomic clocks. Since atomicclock technology is closely related to total-field magnetometer techniques, we areapplying those techniques towards total fieldmagnetometers.

MagneticField

Light source

Photodetector

Perturbationcoils

Vaporcell

• Example: Cesium D1 (895nm) spectroscopy

• Tuning VCSEL to absorption lines

– Drive current (fine, fast control)

– Temperature (micro heater)

VCSEL chip on micro-heater

Tuning across cesium lines

Lasing VCSEL

Cesium transmission

VCSEL current

0.5mm

L. Liew, et al., Appl. Phys. Lett., 2004PyrexTM

300 oC

V

+

-

PyrexTM

300 oC

V

+

-300 oC

V

+

-

300 oC

V

+

-

• Etch hole in Si

• Anodically bond

Pyrex to Si

• Fill cell with Cs

and buffer gas

under vacuum

• Bond top

Pyrex to Si

Goals• Develop efficient method of fabricating and filling a MEMs cell with the correct

mixture of gases. Determine stability of the mixture.

• Develop methodology for coating cell walls to reduce signal loss es due to wall

collisions

Physics Package

• No magnetic materials

– PCBs: FR4 + Cu,Au

– Heaters: Ti,Pt on pyrex

– Delrin spacers

– Nylon screws

– Glass optics

Cs cell

VCSEL

Photodiode

95°C

Coil z1

Coil z2

QWPPol

Lens

Optics

Cell

Heaters

Light Source CellFabrication

InterrogationMethods

PhysicsPackage

ElectronicsDesign

LaboratoryTesting

FieldTesting

These photos show the difficulties in fielding existing total field magnetometers. Inorder to best identify the object underground, a high density of readings is desirable.This requires several sensors, which creates large, bulky platforms.

The sensor on the right is a prototypedevice created at National Institute ofStandards and Technolgy (NIST) usingMicro-Electro-Mechanical Systems(MEMS) technology to miniaturize thesensor elements.

Miniaturizing the total-field sensor consistsof the following tasks:

Replace the high power Cs dischargelamp with a laser diode light source

Miniaturize the hand-blown Cs cell toreduce the power required to heat the cell

Apply recent advances in signalinterrogation techniques to recover thesensitivity lost in reducing the size of thecell

Design a mechanical structure with therequired elements for the sensor head(“physics package”)

Implement the electronics design for lowpower and small size

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Objective

BackgroundAtomic magnetometers operate by measuring the precession frequency of a magneticmoment around the magnetic field of interest. The precession frequency isproportional to the magnetic field.

Optical pumping is used to produce a netmagnetic moment in the Cs cell. Light ofthe correct wavelength and polarization willde-populate some states thereby creating anet magnetic moment.

The precessing magnetic moments maybe interrogated optically as well. As thespins rotate, they modulate the intensity ofthe light beam passing through the cell.The frequency of this modulation will beproportional to the magnetic field beingmeasured.

Electronics Development

Wide Band Sensors

ApplicationsTime domain electromagnetic sensors are also widely used in UXOinvestigations. In these systems, a transmitted magnetic field pulse causes eddycurrents in underground objects, whose resulting magnetic field may be detected.Typically, coils of wire are used as the receivers. However, there are someadvantages to using total-field magnetometers as the receiver. To do that, thebandwidth of the sensor needs to be increased to 10 kHz or so.

(TEM)

The device at the left is 1 cm across.This is an actual atomic clockdeveloped by Symmetricom withDARPA funding. Ultimately, this can bethe size of our magnetometer sensor

Dead ZonesThere are certain angles or sensororientations where the magnetometergives no reading. These occur whenthe magnetic field is near the polaraxis of the sensor, or near theequatorial axis, or both.

Heading ErrorWhile the sensor is supposed to givereadings independent of orientation,asymmetries in the energy levels giverise to readings that do change a bitwith orientation. This effect isundesirable, and must be minimized.

SensitivityObviously, sensitivity is of importance ina magnetometer. The small size ofthese sensors tends to reduce theirsensitivity. However, improvedtechniques in extracting the signal fromthe device will maximize performance

Z

X Y

Z

X Y

We have built a test structure to measure the performance of the variouscomponents of the sensor. This device is much larger than the final sensorpackage will need to be. The interior components themselves are only a few mmacross, while the entire structure is 1 inch across.

Laser Diode Development Cesium Cell Development

1. Signal buffers

2. External counter

assembly

3. H1 synthesizer

4. Photo preamplifier5. DAC board

6. Processor board

7. Regulators and

protection

8. Heater power amplifiers9. Logging system

We have just finished building a testplatform to allow us to take measurementsin the field, ensuring the system is tolerantof spatial and temporal gradients.

Data taken while waving an object in thevicinity of the sensor is shown below.

41720

41730

41740

41750

41760

41770

41780

41790

41800

41810

41820

0 50 100 150 200 250

MagneticField

Time (seconds)

On-resonanceLow Tcell

High Tcell

Off-resonance

Low Tcell

High Tcell

Interfere beams

Reduced power

Absorption method

Phase shift method

Vapor

cell

VCSEL

pump

Analyzer

VCSEL

probe VCSELs

heater

QWP

Neutral filters

Photode-

tector

-200 -100 0 100 200

100

120

140

160

180

200

Tra

nsm

itte

dO

pti

ca

l

Po

we

r(m

W)

Transverse Magnetic Feild, B0

(nT)

0 100 200

101

102

103

Se

nsitiv

ity

(fT

/Hz

1/2

)

Frequency [Hz]

AB

• Magnetometer bandwidth given by

magnetic resonance linewidth (T2 relaxation)Low atomic densities:

- diffusion through buffer gas and wall

collisions.

High atomic densities:

- Spin-exchange relaxation.

• Magnetometer sensitivityIf signal-to-noise (S/N) ratio is limited by

atom shot-noise:

min

2

11

at

B

NT

dg

=

At high atomic densities( Spin-exchange

regime) for Cs

Assume

V = 0.2cm3

Extreme SensitivitiesAt low magnetic fields, the Cs cell may beoperated in the Spin-Exchange RelaxationFree (SERF) regime. This occurs when therate at which atoms collide in the Cs cell ismuch faster than the Larmor frequency. Underthese conditions, sensitivities of better than 0.1femto-Tesla may be achieved.

There are two challenges in using thisregime: 1) the method of operating the sensorresults in a vector magnetometer, and 2) theEarth’s field must be subtracted with nullingcoils.

Later ResultsBy using an off-resonancepolarization rotationinterrogation scheme, wehave achieved the projectgoals of 10 pT sensitivity and10kHz bandwidth.

Initial ResultsIn laboratory experimentswith 1 mm cells, we foundthat at the requiredtemperature, the cellsbecome nearly opaque tothe laser beams. The loss ofsignal lowers theperformance of the devices

HypothesisBy increasing celltemperature, the sensitivityimproves due to the greaternumber of Cs atoms, andthe bandwidth goes up dueto the increased rate ofcollisions. We win on bothavenues....

1 2 3 4

-0.5

0.5

1.0

H1Drive

Physics Package

Photo-Diode

1 2 3 4 5 6

-1.0

-0.5

0.5

1.0

1 2 3 4 5 6

0.2

0.4

0.6

0.8

1.0

The electronics performs manyoperations to extract the proper signal fromthe physics package:

The Cs cell must be kept at the propertemperature

The laser must be brought to the correctwavelength

The correct modulation signals must beapplied to the laser or H1 coil

The photodiode signal must be amplifiedand processed

This process must be done minimizingany cross talk or coupling from interferingwith the signal of interest. The couplingincludes the creation of tiny magnetic fieldsnear the sensor, which alter the magneticfield being measured.

The modulation scheme used tointerrogate the resonance inside the cellmust consider all the nearby signals, theirharmonics, and any non-linear modulationbetween them which creates low frequencysignals in the band of interest.