Digital alkali spin maser for measurement of geophysical fields

The creation of a self-oscillating alkali metal spin maser by radio-frequency feedback has found longstanding use for optically pumped magnetometry, and offers high dynamic range and bandwidth. These features are of particular value in real-world unshielded applications, in which the magnetometer must operate in the presence of rapidly changing geophysical fields. The key to creating the alkali spin maser system is phase-matching to achieve coherent feedback and amplification of the radio-frequency resonance. We have demonstrated a system in which this condition for resonant spin precession is met by digital signal processing integrated into the spin maser feedback loop. The magnetometer sensor head is built using a chip-scale vertical-cavity surface-emitting laser (VCSEL) and a microfabricated dual-pass caesium vapour cell. The digital feedback loop utilises matched dual finite-impulse response (FIR) filters to achieve a real-time Hilbert transform and generation of phase-coherent feedback to the alkali spin system. This system is optimised for and demonstrated in a 50 μT bias field, achieving sensor bandwidth of 10 kHz and Cramér-Rao lower bound-limited resolution of 50 fT at 1 s sampling cadence.