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to high-prole interplanetary missions. Present instru-ments of this type require signicant resources, and theirsize, mass, and power impose signicant constraints ontheir use in space. Recent breakthroughs in microfabri-cated atomic devices have demonstrated reductions ofresources by one to two orders of magnitude over con-ventional instruments by incorporating micromachinedelements into the devices and by using lasers rather thanlamps as the excitation source. In a joint research proj-ect, APL, the JHU Department of Electrical and Com-puter Engineering, and the Atomic Devices and Instru-ments Group at NIST are developing this technology foruse in space and expect to gain corresponding advan-tages over existing space-based instruments.

The fundamental technology is a miniature rubidium(Rb) vapor cell of chip-scale dimensions fabricated byusing microelectromechanical systems (MEMS) tech-niques developed by NIST. These vapor cells have beenused as frequency references in clocks, 1,2 but using suit-able atomic transitions, they also function as magne-tometers. 35 The Rb vapor cell is enclosed by a ceramicheater (Fig. 1) and nally fabricated into a magnetic eldsensor (Fig. 2), all optical components of which have beenevaluated for radiation tolerance to at least 50 krad(Si)of both proton and gamma radiation. In the assembledsensor, the microfabricated Rb vapor cell is illuminatedwith circular-polarized light from a vertical-cavity sur-face-emitting laser (VCSEL), and the resonant responseof the atoms is detected by a photodiode. Ultimately,

the size of the sensor may be signicantly reduced, and asensor package that measures

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netic eld strength is detected by searching for the reso-nance between the atomic spin precession and the RFmagnetic eld applied via the Helmholtz coils. Whenthe RF magnetic eld oscillates at the Larmor frequency,both an amplitude change in the photocurrent and adifference in phase shift of 90 between the applied RFmagnetic eld and the resonant response are observed,and these features will be identied and tracked by acontrol loop implemented in the instruments eld-programmable gate array. Figure 4 shows the photocur-rent amplitude (upper) and phase shift (lower) as func-tion of the applied Helmholtz coil frequency, where thethree colors represent different ambient magnetic eldstrengths ranging from 8880 nT to 9680 nT appliedvia the solenoid. The Larmor frequency is found at theinection point (sign change in second derivative) of thephase shift curve, which is found at approximately 46.5,52.0, and 57.5 kHz for the blue, green, and red curves,respectively. The difference between these frequenciesof ~5.5 kHz, corresponding to 786 nT (gyromagneticratio is 7 nT/Hz), agrees with the 800 nT steps in theapplied solenoid eld to better than 5%. For an appliedsolenoid eld of 8080 nT, the sensor measures a Larmorfrequency of 52.0 kHz, which corresponds to an absolutemagnetic eld strength of ~7400 nT. The 600 nT DCoffset is likely caused by the residual magnetic eld gen-erated by the heater.

Figure 2. Magnetometer sensor optical assembly.

Figure 3. Rb atomic line detection using laser temperature tocontrol the VCSEL wavelength.

Figure 4. Demonstration of magnetic sensitivity by sweeping thedrive frequency of the Helmholtz coil RF magnetic eld for threedifferent ambient magnetic eld strengths..

rent, and either quantity can be used for tuning. Figure3 shows the variation of the photocurrent as function

of the difference in the wavelength due to changes intemperature. The D 1 line width of ~0.05 nm full widthat half maximum (FWHM) is consistent with the linewidth of 18 GHz cited by Schwindt et al. 5

With the VCSEL wavelength tuned to the D 1 atomictransition, the Larmor frequency identifying the mag-

Photodiode

Rb vaporcell

1/4 Plate Polarizer VCSEL

P h o

t o d i o d e c u r r e n

t ( A )

Delta wavelength (nm)

10 46.5

6

5.5

5

0.326 0.160 0.003 0.166 0.329

4.5

4

3.5

3

2.5

2

Heater Configuration 1Heater Configuration 2

P h a s e

( d e g r e e s

)

Frequency (kHz)

0.025

0.02

0.015

0.01

0.0050

30 35 40 45 50 55 60 65 70 75

30 35 40 45 50 55 60 65 70 75

50

0

50

100

150200

Frequency (kHz)

M a g n

i t u

d e

( A B U )

DC Field = 8.08 uTDC Field = 8.88 uTDC Field = 9.68 uT

For further information on the work reported here, see the references below or contact haje.korth@jhuapl.edu. 1Kitching, J., Knappe, S., and Hollberg, L., Miniature Vapor-Cell Atomic-Frequency References, Appl. Phys. Lett. 81, 553555 (2002). 2Knappe, S., Shah, V., Schwindt, P. D. D., Hollberg, L., Kitching, J., Liew, L. A., and Moreland, J., A Microfabricated Atomic Clock, Appl. Phys. Lett. 85, 14601462 (2004).

3Schwindt, P. D. D., Hollberg, L., and Kitching, J., Self-Oscillating Rubidium Magnetometer Using Nonlinear Magneto-Optical Rota-tion, Rev. Sci. Instrum. 76, 126103, doi: 10.1063/1.2136885 (2005).

4Schwindt, P. D. D., Knappe, S., Shah, V., Hollberg, L., Kitching, J., Liew, L. A., and Moreland, J., Chip-Scale Atomic Magnetometer, Appl. Phys. Lett. 85, 64096411 (2004).

5Schwindt, P. D. D., Lindseth, B., Knappe, S., Shah, V., Kitching, J., and Liew, L. A., Chip-Scale Atomic Magnetometer with ImprovedSensitivity by Use of the Mx Technique, Appl. Phys. Lett. 90, 081102, doi: 10.1063/1.2709532 (2007).