Objective Miniature Total Field Magnetometers Wide Band...
1
Miniature Total Field Magnetometers Dr. 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 ([email protected] Current generation total field magnetometers are extremely large, bulky, and they consume a lot of power. This furthers requires large, heavy batteries. Funded by DARPA, recent technologies have been developed towards miniaturizing atomic clocks. Since atomic clock technology is closely related to total- field magnetometer techniques, we are applying those techniques towards total field magnetometers. Magnetic Field Light source Photodetector Perturbation coils Vapor cell • 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., 2004 Pyrex TM 300 o C V + - Pyrex TM 300 o C V + - 300 o C V + - 300 o C 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 QWP Pol Lens Optics Cell Heaters Light Source Cell Fabrication Interrogation Methods Physics Package Electronics Design Laboratory Testing Field Testing These photos show the difficulties in fielding existing total field magnetometers. In order 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 prototype device created at National Institute of Standards and Technolgy (NIST) using Micro-Electro-Mechanical Systems (MEMS) technology to miniaturize the sensor elements. Miniaturizing the total-field sensor consists of the following tasks: Replace the high power Cs discharge lamp with a laser diode light source Miniaturize the hand-blown Cs cell to reduce the power required to heat the cell Apply recent advances in signal interrogation techniques to recover the sensitivity lost in reducing the size of the cell Design a mechanical structure with the required elements for the sensor head (“physics package”) Implement the electronics design for low power and small size ? ? ? ? ? Objective Background Atomic magnetometers operate by measuring the precession frequency of a magnetic moment around the magnetic field of interest. The precession frequency is proportional to the magnetic field. Optical pumping is used to produce a net magnetic moment in the Cs cell. Light of the correct wavelength and polarization will de-populate some states thereby creating a net magnetic moment. The precessing magnetic moments may be interrogated optically as well. As the spins rotate, they modulate the intensity of the light beam passing through the cell. The frequency of this modulation will be proportional to the magnetic field being measured. Electronics Development Wide Band Sensors Applications Time domain electromagnetic sensors are also widely used in UXO investigations. In these systems, a transmitted magnetic field pulse causes eddy currents in underground objects, whose resulting magnetic field may be detected. Typically, coils of wire are used as the receivers. However, there are some advantages to using total-field magnetometers as the receiver. To do that, the bandwidth 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 clock developed by Symmetricom with DARPA funding. Ultimately, this can be the size of our magnetometer sensor Dead Zones There are certain angles or sensor orientations where the magnetometer gives no reading. These occur when the magnetic field is near the polar axis of the sensor, or near the equatorial axis, or both. Heading Error While the sensor is supposed to give readings independent of orientation, asymmetries in the energy levels give rise to readings that do change a bit with orientation. This effect is undesirable, and must be minimized. Sensitivity Obviously, sensitivity is of importance in a magnetometer. The small size of these sensors tends to reduce their sensitivity. However, improved techniques in extracting the signal from the device will maximize performance Z X Y Z X Y We have built a test structure to measure the performance of the various components of the sensor. This device is much larger than the final sensor package will need to be. The interior components themselves are only a few mm across, 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 preamplifier 5. DAC board 6. Processor board 7. Regulators and protection 8. Heater power amplifiers 9. Logging system We have just finished building a test platform to allow us to take measurements in the field, ensuring the system is tolerant of spatial and temporal gradients. Data taken while waving an object in the vicinity of the sensor is shown below. 41720 41730 41740 41750 41760 41770 41780 41790 41800 41810 41820 0 50 100 150 200 250 M a g n e t i c F i e l d Time (seconds) On-resonance Low T cell High T cell Off-resonance Low T cell High T cell 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 Transmitted Optical Power ( m W) Transverse Magnetic Feild, B 0 (nT) 0 100 200 10 1 10 2 10 3 Sensitivity (fT/Hz 1/2 ) Frequency [Hz] A B • Magnetometer bandwidth given by magnetic resonance linewidth (T 2 relaxation) Low atomic densities: - diffusion through buffer gas and wall collisions. High atomic densities: - Spin-exchange relaxation. • Magnetometer sensitivity If signal-to-noise (S/N) ratio is limited by atom shot-noise: min 2 11 at B NT d g = At high atomic densities( Spin-exchange regime) for Cs Assume V = 0.2cm 3 Extreme Sensitivities At low magnetic fields, the Cs cell may be operated in the Spin-Exchange Relaxation Free (SERF) regime. This occurs when the rate at which atoms collide in the Cs cell is much faster than the Larmor frequency. Under these conditions, sensitivities of better than 0.1 femto-Tesla may be achieved. There are two challenges in using this regime: 1) the method of operating the sensor results in a vector magnetometer, and 2) the Earth’s field must be subtracted with nulling coils. Later Results By using an off-resonance polarization rotation interrogation scheme, we have achieved the project goals of 10 pT sensitivity and 10kHz bandwidth. Initial Results In laboratory experiments with 1 mm cells, we found that at the required temperature, the cells become nearly opaque to the laser beams. The loss of signal lowers the performance of the devices Hypothesis By increasing cell temperature, the sensitivity improves due to the greater number of Cs atoms, and the bandwidth goes up due to the increased rate of collisions. We win on both avenues.... 1 2 3 4 -0.5 0.5 1.0 H1 Drive 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 many operations to extract the proper signal from the physics package: The Cs cell must be kept at the proper temperature The laser must be brought to the correct wavelength The correct modulation signals must be applied to the laser or H1 coil The photodiode signal must be amplified and processed This process must be done minimizing any cross talk or coupling from interfering with the signal of interest. The coupling includes the creation of tiny magnetic fields near the sensor, which alter the magnetic field being measured. The modulation scheme used to interrogate the resonance inside the cell must consider all the nearby signals, their harmonics, and any non-linear modulation between them which creates low frequency signals in the band of interest.
Transcript of Objective Miniature Total Field Magnetometers Wide Band...
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
([email protected] 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
?
?
?
?
?
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.
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