Laser Magnetometry Using DBR Laser Pumped Helium Isotopes:Beyond “Juno at Jupiter”

(LEOS April 24, 2008)  

Robert E. Slocum, PhDChief Technical Officer

Polatomic, Inc.1810 Glenville DriveRichardson, TX 75080

(972) 690-0099

Geophysical Service Inc./Texas InstrumentsCircled are Cecil H. Green (L) and Robert E. Slocum (R)

Sputnik October 1957

Vector Helium 4 Magnetometer (VHM) sensor concept.

Mariner 4 launch for Mars 11/28/64

Vector Mode OperationVariable Density Optical Filter

Laser

BS

IR Detector

Triaxial Helmholtz Coil System

Exciter

Vector Mode Signal

0.40.60.8

11.21.4

0 90 180 270 360

Degrees

Sig

nal

(V

olt

s)Circular Polarizer

• Metastable helium subjected to circular polarized radiation and rotating magnetic sweep field BS.

• Optical pumping efficiency and absorption depends on angle between field and optical axis.

• Signal cos2 , minimum signal and maximum absorption at = /2.

Vector ImplementationBias Nulling Field Mode

• Signal cos2 .

• External ambient field B0 causes phase shift of signal.

• Feedback steady field BF to null ambient field and cause maximum absorption to occur at =/2.

• Feedback currents IF are a measure of the ambient field components.

Laser

BS

IR Detector

Triaxial Helmholtz Coil System

Exciter

Circular Polarizer

Sensor Amplifier Phase Demod

Feedback Field

Sweep Osc.

Sweep Field

IF

BS

BF

B0

IS

V IF

NOBLE PRIZE RESEARCH CONTRIBUTING TO TECHNOLOGY OF

LASER MAGNETIC FIELD SENSORS 2000 ZHORES I. ALFEROV, and HERBERT KROEMER for developing semiconductor heterostructures used in high-speed- and

opto-electronics and JACK ST. CLAIR KILBY for his part in the invention of the integrated circuit.

1997 CLAUDE COHEN-TANNOUDJI for development of methods to cool and trap atoms with laser light.

1989 NORMAN F. RAMSEY for the invention of the separated oscillatory fields method and its use in the hydrogen maser and other atomic clocks. HANS G. DEHMELT and WOLFGANG PAUL for the development of the ion trap technique.

1981 NICOLAAS BLOEMBERGEN and ARTHUR L. SCHAWLOW for their contribution to the development of laser spectroscopy.

1966 ALFRED KASTLER for the discovery and development of optical methods for studying hertzian resonances in atoms.

1964 CHARLES H. TOWNES, NICOLAY GENNADIYEVICH BASOV and ALEKSANDR MIKHAILOVICH PROKHOROV for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle.

1956 WILLIAM SHOCKLEY, JOHN BARDEEN and WALTER HOUSER BRATTAIN for their researches on semiconductors and their discovery of the transistor effect.

1955 POLYKARP KUSCH for his precision determination of the magnetic moment of the electron.

1952 FELIX BLOCH and EDWARD MILLS PURCELL for their development of new methods for nuclear magnetic precision measurements and discoveries in connection therewith.

1944 ISIDOR ISAAC RABI for his resonance method for recording the magnetic properties of atomic nuclei.

1943 OTTO STERN for his contribution to the development of the molecular ray method and his discovery of the magnetic moment of the proton.

1933 ERWIN SCHRÖDINGER and PAUL ADRIEN MAURICE DIRAC for the discovery of new productive forms of atomic theory.

1932 WERNER HEISENBERG for the creation of quantum mechanics, the application of which has, inter alia, led to the discovery of the allotropic forms of hydrogen.

1922 NIELS BOHR for his services in the investigation of the structure of atoms and of the radiation emanating from them.

1918 MAX KARL ERNST LUDWIG PLANCK in recognition of the services he rendered to the advancement of Physics by his discovery of energy quanta.

1902 HENDRIK ANTOON LORENTZ and PIETER ZEEMAN in recognition of the extraordinary service they rendered by their researches into the influence of magnetism upon radiation phenomena.

23P0

23S1

11S0

+1 0 -1

Parahelium Orthohelium

Do (1082.91 nm)

Fig. 1 Energy level diagram for helium 4.

He4 Cell Sensing Element:Variable Density Optical Filter –

Magnetically Controlled

• Glass cell contains He4 at low pressure (~1.5 Torr).

• HF discharge produces metastable 23S1 ground state.

• External ambient field B0 splits energy into three Zeeman levels m=+1,0,-1.

• Separation energy E = h0 where 0 = (e / 2) B0 and e / 2 = 28.0249540 Hz/nT

• Metastables in 23S1 level are atomic magnets.

23S1

11S0

m=+1m= 0m=-1HF Discharge

m=+1

m= 0

m=-1Ene

rgy

Magnetic Field B0

E = h0

E = h0

B0

He4 Cell

HF Exciter

Optical Pumping

• Pumping produces non-equilibrium distribution of atoms among different energy levels.

• m=+1,0,-1 sublevels are equally populated in thermal equilibrium.

• m=-1 has high absorption probability for circular polarized 1083 nm laser radiation.

• 23P0 atoms decay to m sublevels at equal rates.

• Laser pumping produces magnetic moment M opposite field as atoms shift to m=0,+1.

23P0

23S1

D0

m=+1

m= 0

m=-1

E = h0

E = h0

23P0

23S1

11S0

D0 (1082.91 nm)m=+1m= 0m=-1HF Discharge

He4 Cell

HF Exciter

Laser

Scalar Mode OperationMagnetically-Driven Spin Precession (MSP)

MSP Resonance Curve

0.760.800.840.880.92

1.36 1.40 1.44 1.48

Frequency (MHz)

Sig

nal

(V

olt

s)

Laser

BRF

IR Detector

Helmholtz Coil System

Exciter

Circular Polarizer

• Metastable helium subjected to circular polarized radiation and RF magnetic field BRF .

• Absorption increases when RF magnetic field is at resonance (Larmor frequency) 0 .

• RF resonant radiation causes transitions between magnetic sublevels (E = h0 ).

• Separation energy E = h0 where 0 = (e / 2) B0 and e / 2 = 28.0249540 Hz/nT.

• B0 = 1.42 x 106 Hz / 28.0249540 Hz/nT = 50,669 nT.

Scalar Mode ImplementationMagnetically-Driven Spin Precession (MSP)

• Apply periodic sweep to RF oscillator.

• Causes periodic modulation of detector output.

• Phase synchronous demodulation determines 0 .

Laser

BRF

IR Detector

Helmholtz Coil System

Exciter

Sensor Amplifier Phase Demod

RF Oscillator

Sweep Oscillator

BRF

B0 0

Circular Polarizer

OSP BLOCH EQUATIONS

The OSP effect can be described using the modified Bloch equations for description of the behavior of the bulk magnetization M in an optically pumped medium as it experiences magnetic resonance. The time dependent magnetization M0(t)/ is given by M0(t) = A + Bcos t,

 

where the OSP magnetic resonance drive frequency is = 2 ( is the actual Larmor frequency for the helium sample). The optically detected light beam intensity is given by

 

Is(t) = KM0(t)M(t),

 

where K is a proportionality constant and M(t) is the magnetization along the optical axis. The Bloch equations can be solved for the case where the beam has 100% modulation (A = 0) to obtain the following expression for Is(t):

Is(t) =1/4KB2sin2 / {1 + ( - 0)22} +

1/4 KB2sin2 {cos 2t / [1 + ( - 0)22] +

( - 0) sin 2t / [1 + ( - 0)22]}.

 

Scalar Mode OperationOptically-Driven Spin Precession (OSP)

OSP Resonance Curve

0.880.920.961.001.04

1.36 1.40 1.44 1.48

Frequency (MHz)

Sig

nal

(V

olt

s)

Laser

Exciter

Circular Polarizer

0

IR Detector

• Metastable helium subjected to pulsed circular polarized radiation.

• Optical pumping efficiency increases at Larmor frequency 0 .

• 0 = (e / 2) B0 and e / 2 = 28.0249540 Hz/nT.

• B0 = 1.42 x 106 Hz / 28.0249540 Hz/nT = 50,669 nT.

Scalar Mode ImplementationOptically-Driven Spin Precession (OSP)

0Laser

IR DetectorExciter

Circular Polarizer

Sensor Amplifier Invert Out

Phase Demod

RF Oscillator

Sweep Oscillator

0

B0

0

• Apply periodic sweep to RF oscillator.

• Causes periodic modulation of detector output.

• Phase synchronous demodulation determines 0 .

Comparison of OSP and MSP magnetic resonance signals for identical laser pump source and helium cells.

OSP Signal Amplitude v. FrequencyLight Level = 2 mW, Absorption = 15%, Light Axis Perpendicular to Ambient Field

Line Width = 2636 Hz (94 nT)

3.440

3.460

3.480

3.500

3.520

3.540

3.560

3.580

3.600

1.370 1.380 1.390 1.400 1.410 1.420 1.430 1.440 1.450

Frequency (MHz)

Sig

nal

(V

olt

s)

Raw Data

Lorentzian Curve Fit

MSP Signal Amplitude v. FrequencyLight Level = 0.5 mW, Absorption = 15%, Light Axis Parallel to Ambient Field

Line Width = 2610 Hz (93 nT)

0.830

0.840

0.850

0.860

0.870

0.880

0.890

0.900

1.340 1.360 1.380 1.400 1.420 1.440 1.460 1.480 1.500

Frequency (MHz)

Sig

nal

(V

olt

s)

Raw Data

Lorentzian Curve Fit

OSP RESONANCE MSP RESONANCE

The Juno spacecraft in front of Jupiter. Juno is one of the largest planetary spacecraft to ever be launched.

An oblique view of the Juno spacecraft shows the three solar panels, one of which carries the magnetometer (yellow extension on the upper solar panel in this image). The main body of the spacecraft is underneath the high gain antenna, which is used for communications to Earth. The three solar panels are built in four-hinged sections that allow the spacecraft to fit within the rocket for launch.

Omni-directional laser-pumped sensor and lamp-pumped sensor.

Self-Calibrating Vector Helium Magnetometer

He-4 cell

He-4 cell

Laser

Optical Isolator

Intensity Modulator

PM Fiber

PolarizingBeamsplitter

Cubes

CollimatingLens

λ/2 λ/4

Photodiode A

Photodiode B

Tri-AxialCoils

Technical Objectives for Self-Calibrating Vector Helium Magnetometer

Vector field measurement

Self-calibrated by scalar measurement

Calibrated range of ±(1,000 nT to 65,000 nT)

Omni-directional sensitivity

Fiber-coupled laser

Bias Field Nulling (BFN) technique for vector measurements

Optical Spin Precession (OSP) for scalar measurements

Reduced sensor volume and mass

Calibrated vector accuracy of 1 nT

Sensitivity of 5 pT/√Hz

ELECTRONICS UNIT

SENSOR UNIT

MVLM Calibration Process Calibration Requirements

• Nine coefficients required to calibrate vector magnetometer.

• Three offsets in absence of magnetic field.

• Three scale factors (gains) for normalization of axes.

• Three non-orthogonality angles which build up orthogonal system in sensor.

Year 3/NCE Algorithm Implementation Completed

• Vector mode measurements made using BFN technique (goal = 0.1% accuracy).

• Scalar mode measurements made using OSP and MSP technique (goal = 0.001% accuracy).

• Multiplex vector and scalar measurements for different sensor orientations.

• Acquire data and calculated calibration coefficients.

• Developed calibration algorithm evolved from compensation algorithm used for Navy airborne systems.

NASA/ESA Standard for Calibration

• Use of MSP or OSP provides omni-directional pre-flight scalar field values used for vector calibration using single MVLM cell. This method can be validated in future calibration experiments at the GSFC coil facility.

15 August, 2005 Learning Data Set

RMSE before = 28.80867

RMSE after = 0.82896

43-minute data collection period. Standard size cells both channels, Ofc-1 in Ch A, Ofc-2 in Ch B. Gradiometer mode. Illum A:1.64/1.47/1.39, B:1.67/1.49/1.38 vdc. Dev 2500 Hz. H1A: 870a rms, H1B: 980 a rms. LL ++, Upgraded optics.

0.01 0.1 1 10 1000.01

0.1

1

10

100

1 103

Ch A Ch B Ch A - Ch B

Frequency, Hz

Spec

tral

Den

sity

, pT

per

root

Hz

Data Period, minutes

Plotted Frequencyresolution, Hz

Mean Frequencies,and Difference# Samples # Averages

=Ns 1114112 =Na 32 =Nm 42.983 =PlotRes 0.012 =MeanFreqChA 1428216.319=FirstPlotPoint 0.006 =MeanFreqChB 1428229.312

=MeanFreqChDiff 12.993

POLATOMIC 2000 SINGLE AXIS GRADIOMETER ON 25 cm SPACING

MCM

MAD

Noise characteristics the POLATOMIC 2000 based on 45 minute data collection period.

Underwater Magnetic and Electric Fields

The Continuous Challenge: Understand and Manage Our OwnPlatform Signatures While Exploiting The Enemy’s

AUV

Barrier

MPA and/or UAV

LCS

MADManeuver

Programmer

SG-887 / ASW-31

Computer Indicator

CGA

ASA-65

VectorSensor

OutputCoils(3)

AMP/PowerSupply

Control

AN/ASQ-81Sensor

AN/ASQ-81Amplifier

AN/ASQ-81Control

ASQ-81(V)

AN/ASA-64Processor

Control AmplifierRO-32Recorder

ASA-71

Processor/Control/Display

Current AN/ASQ-233System MAD

On-line/Off-line Off-line Off-Line

WRA’s 16 2

Space (cu. Ft.) 4.7 1.2

Weight (lbs.) 143 28

Power (Watts) 450 111

Current P-3C III MAD System

AN/ASQ-233 MADP-3C III Retrofit

Sensor

AN/ASQ-233 MAD

TRANSITION PLATFORMS

P-3C MAD Upgrade SH-60 Seahawk MAD Upgrade

Fire Scout VTUAV

P-8A

Insitu M-ScanTM

The worlds most sensitive airborne magnetometer in flight testing in 2008.

“CAN POLATOMIC SOLVE A MAGNETICS PROBLEM ON YOUR PROGRAM?”

AN/ASQ-233SUBMARINE DETECTING SET

Spectral Densities

10-2

10-1

100

101

102

103

10-3

10-2

10-1

100

101

102

103

104

frequency (Hz)

spec

tral

den

sity

(pT

/Hz

1/2 )

AOSP spectral densities

0.058035 pT/Hz1/2

1.1946 pT/Hz1/2

data 3-4-08

pi pumping

2.5 mW

coherent subtraction