Integrated Optically Pumped Magnetometer for Measurements within Earth’s Magnetic Field

We present a portable optically pumped magnetometer instrument for ultrasensitive 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 ${M}_{z}$ 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 the realization of both the sensor head and electronics. We show shielded and unshielded measurements within a static magnetic field amplitude of $50\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{T}$, demonstrating a noise level of the sensor system down to $140\phantom{\rule{0.2em}{0ex}}\mathrm{fT}/\sqrt{\mathrm{Hz}}$ 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 toward its fundamental noise limit of $12\phantom{\rule{0.2em}{0ex}}\mathrm{fT}/\sqrt{\mathrm{Hz}}$. 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 superconducting quantum interference device system in a measurement environment typical for geomagnetic observatory practice and geomagnetic prospection.