[Paper Review] Integrated optically pumped magnetometer for measurements within Earth's magnetic field

G. Oelsner, R.P.J. IJsselsteijn+8 more|arXiv (Cornell University)|Aug 4, 2020

Atomic and Subatomic Physics Research39 references71 citations

TL;DR

This paper presents a portable, battery-powered optically pumped magnetometer based on a MEMS-based cesium vapor cell operating in the light-shift dispersed Mz (LSD-Mz) mode, achieving a magnetic field noise floor of 140 fT/√Hz in unshielded environments. The system integrates a compact sensor head with a miniatureized electronics module, demonstrating sub-femtotesla sensitivity and bandwidth up to several hundred Hz, with performance limited by technical noise sources rather than fundamental quantum limits.

ABSTRACT

We present a portable optically pumped magnetometer instrument for ultra-sensitive measurements within the Earth's magnetic field. The central part of the system is a sensor head operating a MEMS-based Cs vapor cell in the light-shift dispersed Mz mode. It is connected to a compact, battery-driven electronics module by a flexible cable. We briefly review the working principles of the device and detail on the realization of both, sensor head and electronics. We show shielded and unshielded measurements within a static magnetic field amplitude of 50 uT demonstrating a noise level of the sensor system down to 140 fT/\sqrt{Hz} and a sensor bandwidth of several 100 Hz. In a detailed analysis of sensor noise we reveal the system to be limited by technical sources with straightforward strategies for further improvement towards its fundamental noise limit of 12 fT/\sqrt{Hz}. We assess the parameters defining the sensor bandwidth by theoretical modeling based on the Bloch equations. Finally, we compare our sensors' performance to a commercial SQUID system in a measurement environment typical for geomagnetic observatory practice and geomagnetic prospection.

Motivation & Objective

• To develop a portable, robust, and field-deployable optically pumped magnetometer (OPM) for measurements within Earth's magnetic field.
• To overcome limitations of existing OPMs and SQUIDs by enabling high sensitivity without cryogenic cooling or heavy magnetic shielding.
• To integrate a micro-fabricated alkali vapor cell with compact, low-power electronics for real-world deployment.
• To evaluate system performance in both shielded and unshielded environments, identifying technical noise sources and pathways to improved sensitivity.

Proposed method

• Employing a MEMS-based cesium vapor cell in the light-shift dispersed Mz (LSD-Mz) mode to enable operation in finite magnetic fields.
• Using circularly polarized laser light at 895 nm for optical pumping and signal detection via optically detected magnetic resonance (ODMR).
• Implementing dual counter-propagating laser beams with opposite circular polarization to generate symmetrically detuned magnetic resonances via ac Stark shifts.
• Designing a compact, battery-driven electronics module connected to the sensor head via a flexible cable for portability and field use.
• Applying thermal management and mechanical stabilization to minimize environmental noise and drift.
• Conducting noise analysis and bandwidth characterization using theoretical modeling based on the Bloch equations.

Experimental results

Research questions

• RQ1 What is the achievable magnetic field noise floor of a compact, portable OPM system operating in Earth's magnetic field using the LSD-Mz mode?
• RQ2 How does the sensor system perform in unshielded environments compared to shielded conditions?
• RQ3 What are the dominant technical noise sources limiting the system's sensitivity, and how can they be mitigated?
• RQ4 How does the sensor bandwidth scale with system parameters, and what determines its upper limit?
• RQ5 How does the performance of the integrated OPM compare to commercial SQUID systems in real-world geomagnetic applications?

Key findings

• The system achieves a magnetic field noise level of 140 fT/√Hz in unshielded environments, demonstrating sub-femtotesla sensitivity for field-deployable OPMs.
• The sensor bandwidth reaches several hundred hertz, with theoretical modeling based on the Bloch equations confirming the dependence on system parameters.
• Noise analysis reveals that the system is limited by technical noise sources rather than the fundamental quantum limit of 12 fT/√Hz.
• The performance in unshielded conditions is comparable to that of commercial SQUID systems in typical geomagnetic observatory and prospection settings.
• The system shows potential for further improvement through suppression of technical noise, bringing it closer to the theoretical sensitivity limit.
• The integration of a MEMS-based vapor cell with compact, low-power electronics enables a robust, portable, and field-deployable OPM platform.