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We report on an all-optical magnetometric technique based on nonlinear magneto-optical rotation with amplitude-modulated light. The method enables sensitive magnetic field measurements in a broad dynamic range. We demonstrate the sensitivity of 4.3×10−9G∕Hz at 10mG and the magnetic field tracking in a range of 40mG. The fundamental limits of the method sensitivity and factors determining current performance of the magnetometer are discussed.

Recent work investigating resonant nonlinear magneto-optical rotation (NMOR) related to longlived (τ rel ∼ 1 s) ground-state atomic coherences has demonstrated potential magnetometric sensitivities exceeding 10 −11 G/ √ Hz for small ( < ∼ 1 µG) magnetic fields. In the present work, NMOR using frequency-modulated light (FM NMOR) is studied in the regime where the longitudinal magnetic field is in the geophysical range (∼ 500 mG), of particular interest for many applications. In this regime a splitting of the FM NMOR resonance due to the nonlinear Zeeman effect is observed. At sufficiently high light intensities, there is also a splitting of the FM NMOR resonances due to ac Stark shifts induced by the optical field, as well as evidence of alignment-to-orientation conversion type processes. The consequences of these effects for FM-NMOR-based atomic magnetometry in the geophysical field range are considered.

The technique of nonlinear magneto-optical rotation with amplitude modulated light is developed. The technique is an alternative to its counterpart with frequency modulated light and can be applied to sensitive measurements of magnetic fields ranging from microgauss to the Earth-field level. The rotation signals exhibit nontrivial features such as narrowed non-Lorentzian line shapes and multicomponent resonances.