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We describe an optically pumped Cs magnetometer containing a 1.5 mm thick microfabricated vapor cell with nitrogen buffer gas operating in a free-induction-decay (FID) configuration. This allows us to monitor the free Larmor precession of the spin coherent Cs atoms by separating the pump and probe phases in the time domain. A single light pulse can sufficiently polarize the atomic sample however, synchronous modulation of the light field actively drives the precession and maximizes the induced spin coherence. Both amplitude and frequency modulation have been implemented with noise floors of 3 pT/ √ Hz and 16 pT/ √ Hz respectively within the Nyquist limited bandwidth of 500 Hz.

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We demonstrated a scalar atomic magnetometer using a micro-fabricated Cs vapor cell. The atomic spin precession is driven by an amplitude-modulated circularly-polarized pump laser resonant on D1 transition of Cs atoms and detected by an off-resonant linearly-polarized probe laser using a balanced polarimeter setup. Under a magnetic field with amplitude in the Earth's magnetic field range, the magnetometer in the gradiometer mode can reach sensitivities below 150 fT/√Hz, which shows that the magnetometer by itself can achieve sub-100 fT/√Hz sensitivities. In addition to its high sensitivity, the magnetometer has a bandwidth close to 1 kHz due to the broad magnetic resonance inside the small vapor cell. Our experiment suggests the feasibility of a portable, low-power and high-performance magnetometer which can be operated in the Earth's magnetic field. Such a device will greatly reduce the restrictions on the operating environment and expand the range of applications for atomic magnetometers, such as detection of nuclear magnetic resonance (NMR) in low magnetic fields.