2008

Nuclear-Spin Gyroscope Based on an Atomic Co-Magnetometer

Sensitivity to magnetic fields is eliminated.

An experimental nuclear-spin gyroscope is based on an alkali-metal/noblegas co-magnetometer, which automatically cancels the effects of magnetic fields. Whereas the performances of prior nuclear-spin gyroscopes are limited by sensitivity to magnetic fields, this gyroscope is insensitive to magnetic fields and to other external perturbations. In addition, relative to prior nuclear-spin gyroscopes, this one exhibits greater sensitivity to rotation. There is commercial interest in development of small, highly sensitive gyroscopes. The present experimental device could be a prototype for development of nuclear-spin gyroscopes suitable for navigation. In comparison with fiberoptic gyroscopes, these gyroscopes would draw less power and would be smaller, lighter, more sensitive, and less costly.

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This Atomic Co-Magnetometer generates an output signal proportional to the rate of rotation aboutthe y axis.
The co-magnetometer (see figure) includes a spherical aluminosilicate glass cell containing potassium vapor, several atmospheres of helium-3, and a small quantity of nitrogen (which serves as a buffer gas). The cell resides in a small oven, which is used to maintain the cell contents at a temperature of 170 °C. The oven is located within a housing that includes several layers of magnetic shielding.

Potassium atoms are polarized by optical pumping, and the polarization is transferred to the helium by spin-exchange collisions. A high-power diode laser generates the pump beam, which passes through holes in the magnetic-shielding layers and oven and through the cell along the z axis of an xyz Cartesian coordinate system. Another, lower-power diode laser generates a linearly polarized probe beam, which similarly passes through the cell along the z axis. The probe beam is used to measure the direction of polarization of the electrons in the potassium atoms, which is coupled to the nuclear polarization of the helium due to the imaginary part of the spin-exchange crosssection.

For sufficiently high buffer-gas pressure in a spherical cell, this coupling can be represented by an effective magnetic field that each spin species (K or He) experiences from the average magnetization of the other.

It has been shown that the relationships among the electron polarization of the potassium atoms, the nuclear polarization of the helium atoms, the magnetic fields, and the mechanical rotation of the magnetometer are described by a system of coupled Bloch equations. The equations have been solved to obtain an equation for (1) a compensating magnetic field, automatically generated in the magnetometer, that exactly cancels other magnetic fields; and (2) a gyroscope output signal that is proportional to the rate of mechanical rotation about the y axis and independent of magnetic fields. In experiments, the gyroscope-output equation has been verified to within a calibration error of 3 percent, and the expected insensitivity to rotation about the x and z axes was confirmed. In a future version, sensitivity could be increased by substituting 21Ne for 3He as the noble gas and increasing the sensitivity of optical measurement of rotation of polarization at low frequencies to approach the spin-projection noise.

This work was done by Michael Romalis, Tom Kornack, and Rajat Ghosh of Princeton University for Glenn Research Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-17942-1.

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