We present the development and field validation of a nanotesla-precision 3-axis vector optically pumped magnetometer (OPM) utilizing a commercial rubidium ampoule and a vertical-cavity surface-emitting laser (VCSEL). While OPMs achieve ultra-high sensitivity, maintaining long-term stability in unshielded, dynamic geomagnetic environments remains a significant challenge [1]. To address this, our system employs a robust single-beam architecture with orthogonal magnetic field modulation [2]. Furthermore, to effectively suppress the long-term thermal drift of the VCSEL, we implemented an active frequency stabilization mechanism based on a direct absorption curve (Fig. 1a). This direct absorption spectroscopy lock provides excellent wavelength stability without the optical complexity associated with saturated absorption setups. The long-term stability and practical viability of the sensor were validated through continuous geomagnetic field observations. Operating in an unshielded laboratory environment, our system successfully tracked dynamic geomagnetic phenomena over a multi-day period in July 2026. Specifically, the continuous vector measurement of the 3-axis geomagnetic field components showed excellent agreement with provisional reference data from the Kakioka Magnetic Observatory (Fig. 1b). This successful tracking of natural geomagnetic variations demonstrates the robust suppression of slow drift components in our compact, cost-effective OPM platform.
