Measuring Magnetic Fields with Light
All-optical magnetometer gains sensitivity by using dual-beam system.
Researchers at the University of California, Berkeley, have demonstrated that a cloud of atoms, a diode laser and a couple of detectors can measure a magnetic field with a sensitivity of better than 3 nG for a measurement time of 1 s. Potentially, the technique could be orders of magnitude more sensitive, with performance equal to that of magnetometers based on superconducting quantum interference devices. Unlike these devices, however, the researchers’ implementation is entirely optical, can be miniaturized and does not require cryogenic cooling.
“It is fundamentally very low-power, which is important for field applications, embedded use and space magnetometry, among others,” said research team member James M. Higbie.
He added that the lack of radio-frequency coils in the design offers another potential advantage. Arrays of magnetometers can be produced without interchannel crosstalk.
The group is not the only one producing optical magnetometers. Such devices exploit the fact that an atomic vapor cloud consists of individual atoms, each its own magnet. These tiny magnets precess around an external magnetic field much like a top does when its spin axis is off center. The atomic spin precession changes the optical properties of the cloud, which provides a measure of the magnetic field.
For the process to work, two light sources are needed, although they can be derived from a single laser. One beam acts as a pump that puts the cloud of atoms in a long-lived alignment. This requires that the wavelength of the pump match the material and be modulated at a frequency that matches the magnetic field. In their device, the investigators made use of self-oscillation of the cloud by pumping it at a frequency derived from the signal.
The second light source is a probe that reports on the optical properties of the atomic cloud. Typically, detection is performed by monitoring the polarization rotation of the probe.
Although optical magnetometry has been around for decades, the recent advent of reliable, small, inexpensive and easily tunable diode lasers is starting to make field instruments based on such schemes practical for high-sensitivity measurements.
In their implementation, the Berkeley researchers made use of vertical-cavity surface-emitting lasers (VCSELs) from Ulm photonics GmbH of Germany, distributed feedback lasers from eagleyard Photonics GmbH of Berlin and conventional edge-emitting lasers from Sharp. VCSELs are better suited for portable applications, and, Higbie noted, with narrower linewidths, would help instrument performance. “Technical developments, such as higher-power VCSELs or fiber-coupled VCSELs at the wavelengths of interest, can make a big difference,” he said.
Likewise, he noted, improvements in detectors would help. In particular, the speed of the photodetector is important because a faster detector helps minimize the signal’s phase shift and thereby helps enhance the long-term stability of the magnetometer.
Since reporting its results, the group has been optimizing the device. Currently, sensitivities are in the 30-pG range, but Higbie noted that it is hard to verify this performance because of the difficulty in producing a field of the required stability. As for applications, he noted that the instrument could be used for detection of buried objects, whether a treasure or a bomb. Another use that the group hopes to demonstrate soon is magnetocardiography and, possibly, magnetoencephalography, thereby covering both head and heart.
Nature Physics, April 2007, pp. 227-234, and Review of Scientific Instruments, November 2006, Vol. 77, 113106.
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