Magnetometers and electromagnetic (EM) devices are two of the most commonly used instruments for searching for unexploded ordnance (UXO). Research in this area has progressed from the problem of detecting possible UXO items to that of discriminating between hazardous and non-hazardous underground objects. Accurate identification of underground items requires that as much information as possible be gathered about the anomaly. This includes taking a dense array of magnetometer readings as well as EM readings.
The magnetometer sensor of choice is typically an atomic vapor total field magnetometer. These devices have extremely high sensitivity and give readings that are independent of orientation. For EM systems, the sensor of choice has been a wire coil. A coil generates a voltage that is proportional to the time rate of change of the average magnetic field through the coil. An ideal sensor would combine the advantages of both types of measurement by making a magnetic field reading from DC to about 10 kHz – covering the range of frequencies of both magnetometry and EM systems.
Building on the results of SERDP project MR-1512, the objective of this project was to develop small, wide bandwidth, extremely sensitive total-field magnetic sensors.
The project team developed a magnetic field sensor that achieves a high sensitivity simultaneously with high bandwidth, small size, and low power. The small size and low power operation was enabled by the use of technology used in micro electro mechanical systems (MEMS). The project team developed chip-scale atomic magnetometers by examining the physics issues related to future sensor needs of SERDP. The high bandwidth is achieved by operating the alkali vapor cell at a high temperature, so that the alkali density is large and therefore the spin-exchange broadening is large. At a temperature of 130°C, the alkali atom vapor pressure is roughly 10-3 Torr, corresponding to an alkali atom number density of 3×1013/cm3, and the spin-exchange linewidth is roughly 50 kHz. In the spin-exchange limit, the sensitivity remains roughly 100 fT/√Hz.
In the first year of this project, the project team fabricated the initial devices to be used in further testing for the fundamental performance. This included fabricating a range of small cells with various buffer gas pressures to study the performance of miniature magnetometers at high cell temperatures.
Measurements of the linewidth of the Larmor resonance confirmed that in Cs, a magnetometer bandwidth of 10 kHz can be achieved at a cell temperature of 135°C. This was achieved in a cell containing Cs and approximately 350 Torr N2. Measurements also confirmed the increased bandwidth that can be achieved at higher alkali atom densities.
Measurements as a function of cell temperature confirmed that at a cell temperature of 135°C, the linewidth is dominated by spin-exchange collisions. This implies that the magnetometer is indeed operating in the appropriate high-density regime. Also measured was the magnetometer signal size as a function of the cell temperature. The project team found that at temperatures above approximately 100°C, the signal does not continue to increase proportionally with the alkali density, but begins to saturate. This saturation is consistent with the effects of optical thickness in the cell, and was not unexpected.
In the first year of the project, the team also measured the sensitivity and linewidth attainable with a single laser beam. Sensitivity dropping was noticed due to the optical thickness of the cells, and did not yet reach the program goals. It was determined a more complicated dual-beam method was required to pump the cell and probe it with different wavelengths.
In the second year, the project team investigated novel schemes of probing the cell in a manner circumventing the difficulty due to the optical thickness. A dual-laser configuration was used, in which one laser is used to pump the atoms, while a second probe laser, linearly polarized and tuned off resonance, measures the atomic polarization via magneto-optical polarization rotation. Since the polarization rotation can be generated by pure phase shifts of the optical field, the probe can be strongly detuned from resonance, leading to reduced absorption and a corresponding reduction of the loss of sensitivity due to the cell optical thickness. The results indicate that even extremely small sensors may simultaneously achieve high sensitivity and wide bandwidth.
In addition to the progress in evaluating the fundamental limits to the performance of the devices at high cell temperatures, several MEMS designs for devices were investigated. In addition, MEMS-based alkali vapor cells have been developed with internal surfaces which reflect light by approximately 90° from the vertical direction into the horizontal. High-reflection optical coatings have also been demonstrated, so that optical losses incurred in the propagation of the light through the cell are minimized. These cells appear suitable for magnetometers in which the pump and probe fields must propagate in perpendicular directions.
The project team also demonstrated magnetometry in the spin-exchange relaxation free (SERF) regime in a millimeter-scale alkali vapor cell. This measurement resulted in a sensitivity of 70 fT/√Hz with a bandwidth of 290 Hz using a very simple excitation/detection scheme based on the optical absorption of a single laser beam. In this experiment, the cell temperature was 152°C with further increase in the cell temperature limited by the optical thickness effect. If the polarization-rotation solution can be applied in this situation, it seems likely that the bandwidth of the sensor could be increased significantly by operating at even higher cell temperatures with little loss in sensitivity.
The project team further built a prototype sensor exploring avenues for the fabrication of miniature physics packages and designed and prototyped electronics circuitry for operating the systems at wide bandwidth.
Though the project team had originally intended to build a complete system that could be tested in the field with an electromagnetic source, difficulties were encountered while manufacturing the prototype sensors (primarily with the heating elements) which prevented fabrication of a complete prototype sensor package. In addition, the electronics system proved to require a more complicated digital control system than possible within project constraints.
This work has laid a solid foundation for future designs of both narrow and wide-band sensors. The simple optical designs that were prototyped place overly stringent requirements on the electronics design. This has pointed the researchers towards new possibilities of interrogation structures that reduce the complexities of the electronics design and could lead to many new generations of sensors.