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IEEE International Magnetics Conference

Chip Scale Atomic Magnetometers

Peter D. D. Schwindt, B. J. Lindseth, Svenja A. Knappe, Vishal Shah, Li‐Anne Liew, John Moreland, Leo W. Hollberg, John E. Kitching

Published May 1, 2006• 15 citations
Publisher

Abstract

An optically pumped magnetometer was drastically miniaturized, by taking advantage of MEMS techniques, producing the chip-scale atomic magnetometers (CSAM) physics package. The key component of the package is an alkali vapor cell. To probe the magnetic field experienced by the atoms, the injection current to the VCSEL was modulated at 3.4 GHz near half the hyperfine frequency of87Rb. Lock-in detection is used to determine the center of the resonance, and the magnetic field is determined by counting the modulation frequency, which is related to the Larmor precession frequency under these locked conditions. The ITO heaters create a large magnetic field gradient, which broadens the resonance and reduces the sensitivity.

DOI
10.1109/intmag.2006.376110

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An optically pumped magnetometer was drastically miniaturized, by taking advantage of MEMS techniques, producing the chip-scale atomic magnetometers (CSAM) physics package. The key component of the package is an alkali vapor cell. To probe the magnetic field experienced by the atoms, the injection current to the VCSEL was modulated at 3.4 GHz near half the hyperfine frequency of87Rb. Lock-in detection is used to determine the center of the resonance, and the magnetic field is determined by counting the modulation frequency, which is related to the Larmor precession frequency under these locked conditions. The ITO heaters create a large magnetic field gradient, which broadens the resonance and reduces the sensitivity.
The fabrication and performance of a miniature optically pumped atomic magnetometer constructed with microfabricated components are discussed. This device measures the spin precession frequency of Rb87 atoms to determine the magnetic field by use of the Mx technique. It has a demonstrated sensitivity to magnetic fields of 5pT∕Hz1∕2 for a bandwidth from 1to100Hz, nearly an order of magnitude improvement over our previous chip-scale magnetometer. The 3dB bandwidth has also been increased to 1kHz by reconfiguring the miniature vapor cell heater.
A wafer-level optical package solution for chip-scale atomic magnetometers is presented, which is based on microelectronic fabrication process on a 4 inch wafer. A module is fabricated following the process. By measuring the intensity of the light, it is indicated that the quality of parallelism and polarizability of the laser beam from a Vertical Cavity Surface Emitting Laser (VCSEL) in the module are acceptable. The frequency of the VCSEL is locked to D1 line of Rubidium through proper adjustment of temperature and driving current. The module is employed in an atomic magnetometer which operates in Mz configuration for the detection the strength of magnetic fields. Results show the feasibility of the package in a magnetometer system.
A scalar magnetic field sensor based on a millimeter-size Rb87 vapor cell is described. The magnetometer uses nearly copropagating pump and probe laser beams, amplitude modulation of the pump beam, and detection through monitoring the polarization rotation of the detuned probe beam. The circularly polarized pump laser resonantly drives a spin precession in the alkali atoms at the Larmor frequency. A modulation signal on the probe laser polarization is detected with a lock-in amplifier. Since the Larmor precession is driven all-optically, potential cross talk between sensors is minimized. And since the pump light is turned off during most of the precession cycle, large offsets of the resonance, typically present in a single-beam Bell–Bloom scheme, are avoided. At the same time, relatively high sensitivities can be reached even in millimeter-size vapor cells: The magnetometer achieves a sensitivity of 1 pT/Hz1/2 in a sensitive volume of 16 mm3, limited by environmental noise. When a gradiometer configuration is used to cancel the environmental noise, the magnetometer sensitivity reaches 300 fT/Hz1/2. We systematically study the dependence of the magnetometer performance on the optical duty cycles of the pump light and find that better performance is achieved with shorter duty cycles, with the highest values measured at 1.25% duty cycle.
Highly sensitive optical magnetometers present high performance in detection of biological magnetic fields from the heart or the brain and have impact on other areas as diverse as magnetic resonance imaging, space science and remote monitoring. A chip-scale SERF-regime optical magnetometer based on a MEMS process is fabricated instead of a large-chamber one and the optical module and the vapor cell are fabricated based on the MEMS process. Industrialization, low cost and high systematic integration level of magnetic sensors can also be achieved based on the wafer-level packaging. Besides, a 200KHz AC electronic current was applied to the heaters for reducing magnetic noises generated by DC currents from heating resistors. The experimental setup based on a SERF-regime magnetometer to test magnetic fields is shown and results indicate that magnetic fields are tested successfully.
Introduction Shortly after the inception of atomic magnetometry, alkali-vapor magnetometers were being used to measure the Earth's magnetic field to unprecedented precision. During the same era, Bell and Bloom first demonstrated all-optical atomic magnetometry through synchronous optical pumping [1] (see Chapters 1 and 6). In this approach, optical-pumping light is frequency- or amplitude-modulated at harmonics of the Larmor frequency ω L to generate a precessing spin polarization within an alkali vapor at finite magnetic field [2, 3]. Although this technique received considerable attention from the atomic physics community for its applicability to optical pumping experiments, Earth's-field alkali-vapor atomic magnetometers continued to rely on radiofrequency (RF) field excitation for several decades (see Chapter 4). Upon the advent of diode lasers addressing alkali and metastable helium transitions, synchronously pumped magnetometers experienced a revival beginning in the late 1980s. In recent years, such magnetometers have found applications in nuclear magnetic resonance detection [4] (see also Chapter 14), quantum control experiments [5], and chip-scale devices intended for spacecraft use [6] (see also Chapters 7 and 15). All-optical magnetometers possess several advantages over devices that employ RF coils. RF-driven magnetometers can suffer from cross-talk if two sensors are placed in close proximity, since the AC magnetic field driving resonance in one vapor cell can adversely affect the other. All-optical magnetometers are free from such interference. When operated in self-oscillating mode [7], RF-driven magnetometers require an added ±90° electronic phase shift in the feedback loop to counter the intrinsic phase shift between the RF field and the probe-beam modulation.
The physics package of a chip-scale atomic clock (CSAC) has been successfully realized by integrating vertical cavity surface emitting laser (VCSEL), neutral density (ND) filter, λ/4 wave plate, 87 Rb vapor cell, photodiode (PD), and magnetic coil into a cuboid metal package with a volume of about 2.8 cm 3 . In this physics package, the critical component, 87 Rb vapor cell, is batch-fabricated based on MEMS technology and in-situ chemical reaction method. Pt heater and thermistors are integrated in the physics package. A PTFE pillar is used to support the optical elements in the physics package, in order to reduce the power dissipation. The optical absorption spectrum of 87 Rb D1 line and the microwave frequency correction signal are successfully observed while connecting the package with the servo circuit system. Using the above mentioned packaging solution, a CSAC with short-term frequency stability of about 7 × 10 −10 τ −1/2 has been successfully achieved, which demonstrates that this physics package would become one promising solution for the CSAC.