In this project we seek to develop a high-performance chip-scale atomic magnetometer
(CSAM) that diversifies and enhances the capabilities of the family of CSAM designs at
NIST.
In previous years we developed the CPT CSAM [1], Mx CSAM [2], and studied the potentiality of an Mx CSAM design with a diverging laser beam [3]. When compared to each other, each of these CSAM designs offers advantages, but also drawbacks. The selection of the best CSAM design depends strongly on its real-world application.
Our first CSAM was based on the phenomenon of coherent-population trapping (CPT); it achieved a sensitivity of 50pT/Hz1/2 . Although the CPT CSAM is not the most sensitive CSAM, in principle it offers the capabilities of an all-optical magnetometer and thus is a very appealing device for remote magnetic-sensing applications. However the extraction and conditioning of the magnetometer signal is rather complex in terms of electronics equipment.
Our second CSAM design was implemented by use of the Mx technique. The Mx CSAM is the most sensitive CSAM to date, it achieves a sensitivity of 5 pT/ Hz1/2 in an ambient field of 5.5 pT. However due to the radio-frequency (RF) coils inherent to its operation, the Mx CSAM has some drawbacks. For instance, in array-based magnetometers the RF coils can be a source of systematic errors due to the cross-talk among the magnetometers in the array. In addition, the misalignment of the RF-coils-axis relative to the pumping beam is also a source of systematic errors in the form of spurious phase shifts.
In an attempt to mitigate the drawbacks of the CPT and Mx CSAM, to retain its advantages and produce a simpler magnetometer design we have investigated the performance of the Bell-Bloom (BB) magnetometer with frequency-modulated (FM) light in a microfabricated vapor cell.
The original proposal of an atomic magnetometer with modulated light dates back to 1961 [4]. As in previous CSAMs, in our FM BB magnetometer we use a vertical-cavity surface-emitting laser (VCSEL) as the source for optical pumping and probing and modulate its injection current to generate the FM light; we use 133Cs as the resonant atomic vapor and high pressures of N2 buffer gas confined in a microfabricated cell.
In this work, we demonstrate that the FM BB magnetometer, implemented in a microfabricated miniature vapor cell, can yield sensitivities equivalent to those of the Mx magnetometer. For different atomic densities we optimize the Mx magnetometer by choosing the RF and optical power that yields the best attainable sensitivities. Similarly, we study how parameters such as the frequency modulation waveform, the frequency modulation amplitude, and the waveform duty cycle affect the sensitivity of the BB magnetometer. We select the optimum values of these parameters and observe a FM BB magnetometer whose performance is similar to that obtained
in the optimized Mx magnetometer.
Figure1. Experimental setup used in Mx (a) and frequency-modulated Bell-Bloom (b) magnetometers. Vertical-cavity surface-emitting laser (VCSEL); L- lens; ND- neutral-density filter; λ/4 quarter-wave plate; RF- radiofrequency coils; DC-Helmholtz coils to generate DC magnetic field (Bo); PD- photodetector and transimpedence amplifier.
The best sensitivities achieved with the Mx and FM BB magnetometer correspond to 12 pT/Hz1/2 and 16.7 pT/Hz1/2, respectively, at an ambient field of 14.285 pT. These numbers can be compared to the sensitivity of 5 pT/Hz1/2 Hz reached in the Mx chip-scale magnetometer [2]. The Mx CSAM in [2] used 87Rb instead of 133Cs. The difference between the gyromagnetic ratios of 87Rb and 133Cs, 7 Hz/nT and 3.5 Hz/nT, respectively, can explain the difference in sensitivities reported here and that achieved in [2].
The Bell-Bloom approach offers several attractive features over the Mx approach for chip-scale magnetometer instrumentation.
Difference between sensitivities
Figure2. (a) Magnetometer sensitivity as function of atomic density (optical depth) for BB magnetometer modulated with a sine waveform and a square waveform with 60 % duty cycle, and Mx magnetometer ( Mx). b) Noise in the magnetometer signal as a function of atomic density (optical depth) for BB magnetometer modulated with a sine waveform and a square waveform with 60% duty cycle, and Mx magnetometer (Mx). Solid lines are used to guide the eye. The light power values were optimized for the FM BB and Mx magnetometers and for each optical depth.
References:
[1] P. Schwindt, S.Knappe, V. Shah, L. Hollberg, J. Kitching, L. Liew, and J. Moreland, “Chip-scale atomic magnetometers,” Appl. Phys. Lett., vol. 85, pp. 6409–6411, Dec. 2004.
[2] P. Schwindt, B. Lindseth, S. Knappe, V. Shah, J. Kitching, and L. Liew, “Chip-scale atomic magnetometer with improved sensitivity by use of the Mx technique,” Appl. Phys. Lett., vol. 90, pp. 081102–1–081102–3, Feb. 2007.
[3] E. Hodby, E. Donley, and J. Kitching, “Differential atomic magnetometry based on a diverging laser beam,” Appl. Phys. Lett., vol. 91, pp. 011109–1-011109–3, July 2007.
[4] W. Bell and A. Bloom, “Optically driven spin precession,” Phys. Rev. Lett., vol. 6, pp. 280–281, Mar. 1961.
Acknowledgements:
This work was carried out in collaboration with Geometrics with funding from NIST and the Strategic Environmental Research and Development Program.