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.

0 0H

d

dtg é ù= ´ë û

MM H

d

dt=

AT

g=M A

0w Magnetic

Field

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

VCSEL Light Source Integration0.5mm VCSEL chip

on 4mm heater glass

Lasing VCSEL

without lens

VCSEL beam after

collimating lens

VCSEL PCB and

collimating lens

VCSELHeater

y = 0.8563x + 26.026

20

30

40

50

60

70

80

90

100

0 20 40 60 80

Heater Power (mW)

Tem

pera

ture

(C)

Cesium Cell Integration

2 mm

8 mm25 mm

Temp

SensorHeater

Cs cell on heater

(bottom view)

Cell heater on PCB

(top view)

Cesium cell epoxied

onto 8mm heater glass

Heater Sensor

-5mA +5mA

+5mA -5mA

-5mA +5mA

+5mA -5mA

Two-layer heater design

minimizes induced

magnetic fields in cell

Heater tested in air (not vacuum)

Z-axis coil integration

10mm

Top coil

Bottom coil

VCSEL PCB

Photodiode

PCB

Cs Cell

Electrical Connections

V-groove clamp

1mm

After soldering (on 0.5mm pitch)

Before

soldering

1mm

9 Twisted Pair Connections34 AWG

(0.16mm)

wires

Pair Label Color Connection

1 V Red/Black VCSEL (red=anode)

2 Hv Yellow/Blue VCSEL heater

3 C1 Yellow/Blue Bottom coil Z1

4 H1 Yellow/Blue Bottom cell heater

5 S1 Red/Black Bottom temp sensor

6 S2 Red/Black Top temp sensor

7 H2 Yellow/Blue Top cell heater

8 C2 Yellow/Blue Top coil Z2

9 D Red/Black Photodiode (red=anode)

Advanced TechniquesSub-pT sensitivities and BW > 10 kHz can be

achieved with Tcell ~ 200 °C

87Rb can give sensitivity near 1 fT/rtHz!

0 100 200 30010

-3

10-2

10-1

100

Spin-exchange broadened

SERF

Se

nsitiv

ity

[pT

/rtH

z]

Temperature [C]

Sensitivity, L = 1 mm

133Cs

87Rb

0 100 200 30010

1

102

103

104

105

106

Spin-exchange broadened

SERF

Bandw

idth

[Hz]

Temperature [C]

Bandwidth, L = 1 mm

133Cs

87Rb

Total Field Sensors for TEM

• Advantages

– Measure B, not dB/dt

– No rotation noise in Earth’s field

– Get DC magnetometer reading as well

• Disadvantages

– Don’t have multi-component measurements

– Sensitivity is not as great at high frequencies

Common TEM WaveformsTransmitter B Field dB/dt (coil voltage)

BUD

AOL2

ALLTEM

Inte

gra

tion

Differentiation

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

?

?

?

?

?

Objective

Background

Atomic magnetometers operate bymeasuring the precession frequencyof a magnetic moment around themagnetic field of interest. Theprecession frequency is proportionalto the magnetic field.

Two processes are necessary to make this procedure work:

1) The magnetic moments of a large number of atoms need to be aligned together,in phase, so they produce a measurable signal,2) The signal needs to be received by some means

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.

Sensor Package

Results

Vapor

cell

Semiconductor

Laser

Photodetector

oB

Preamplifier

Lock-In

Amplifier

Low-frequency

sweep generator

FFT

Spectrum

Analyzer

Laser-frequency

modulation, @ m

Horizontal Input

Vertical

Input

Bell-Bloom Magnetometer

or

or

@/4

Filter

w0=B0/g

The laboratory test setup isshown at the left. In thismeasurement, the magneticfield is actually swept,causing the Cs line to movepast the fixed-frequencylaser. In actual operation,the laser frequency is swept.In the laboratory, we areinvestigating theperformance achievablewith various signalwaveforms to optimize thefinal design.

0.1 1 10 100 100010

1

102

103

104

dBm

in[p

T/r

tHz]

Frequency [Hz]

30 pT / rt Hz

The resulting performance for oneconfiguration is shown at the left. Therandom noise level is 30 pT per root Hz.

Existing commercial sensors used forUXO investigation have a performance of7 pT per root Hz. However, this noiselevel is rarely even approached inpractice, as other sources of magneticfield noise dominate the signals ofinterest from the underground anomaly.

Wide Band SensorsTime domain electromagneticsensors are also widely used in UXOinvestigations. In these systems, atransmitted magnetic field pulse causeseddy currents in underground objects,whose resulting magnetic field may bedetected. Typically, coils of wire areused as the receivers. However, thereare some advantages to using total-fieldmagnetometers as the receiver. To dothat, the bandwidth of the sensor needsto 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

Mx TechniqueThis method uses a coil of wire to apply a signal ofnear 100kHz to the atoms in the cell, to modulatethe population. The electronics are simple, but thestructure requires a wire coil around the sensor.This method has both equatorial and polar deadzones.

Bell-BloomIn the Bell-Bloom technique, the modulation (either offrequency or intensity) is applied to the light itself.Therefore, no coils are needed, making the structuresimpler and there is no equatorial dead zone. Themodulation may be applied at the Larmor frequency, or halfthis frequency.

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