  A self-oscillating magnetometer based on nonlinear magneto-optical rotation
using amplitude-modulated pump light and unmodulated probe light (AM-NMOR) in
87Rb has been constructed and tested towards a goal of airborne detection of
magnetic anomalies. In AM-NMOR, stroboscopic optical pumping via amplitude
modulation of the pump beam creates alignment of the ground electronic state of
the rubidium atoms. The Larmor precession causes an ac rotation of the
polarization of a separate probe beam; the polarization rotation frequency
provides a measure of the magnetic field. An anti-relaxation coating on the
walls of the atomic vapor cell results in a long lifetime of 56 ms for the
alignment, which enables precise measurement of the precession frequency. Light
is delivered to the magnetometer by polarization-maintaining optical fibers.
Tests of the sensitivity include directly measuring the beat frequency between
the magnetometer and a commercial instrument and measurements of Earth's field
under magnetically quiet conditions, indicating a sensitivity of at least 5
pT/\sqrt{Hz}. Rotating the sensor indicates a heading error of less than 1 nT,
limited in part by residual magnetism of the sensor.
