Introduction

Optically pumped helium (He) magnetometers have provided magnetic field data for military, aeromagnetic survey, space exploration and geophysical laboratory applications for over five decades. The characteristics of He magnetometers that have made them instruments of choice for these varied applications include high sensitivity, high accuracy, simplicity of the resonance line, small heading errors due to light shifts, temperature independence of resonance cells, linear relationship between the magnetic field and the resonance frequency, excellent stability for gradiometer operation and robustness for field and space use. Scalar He magnetometers can easily be configured for omnidirectional operation with no moving parts to provide full sensitivity on all headings relative to the magnetic field direction.

Helium magnetometers have two types of optical pumping radiation sources. All He magnetometers manufactured from 1960 to 1990 utilized an RF electrodeless discharge He-4 lamp as an optical pumping source of 1083 nm resonance radiation which is composed of three closely spaced He-4 resonance lines D0, D1, and D2. In the 1980s, the development efforts for a single-line pump source for both He magnetometers and basic research on He isotopes resulted in both high-efficiency semiconductor lasers and optical fiber lasers at 1083 nm. Laser-pumped He magnetometers are characterized by sensitivities up to two orders of magnitude better than lamp-pumped He magnetometers and are more accurate, smaller, and very stable for use in magnetic gradiometers. L. D. Schearer provided a comprehensive review of the beginning science of He-4 magnetometers [1] and a review of the first 25 years of progress in optically pumped He magnetometers [2].