GB964027A - Optical magnetometers - Google Patents

Abstract

964,027. Gyromagnetic resonance testing. VARIAN ASSOCIATES. Oct. 6, 1961 [Oct. 13, 1960], No. 36076/61. Heading G1N. In an optical magnetometer oscillator of the type described in Specification 881,424 comprising an absorption cell containing an assemblage of atoms or other quantum systems having a longlived ground or metastable state split into a plurality of magnetic sub-levels by the field to be measured, wherein a monitoring optical radiation with such spectral characteristics that it is more strongly absorbed by quantum systems in one set of sub levels than by those in the other set is passed through the cell and intercepted by an optical detector to produce as a result of its intensity modulation an h.f. signal which is amplified and fed back to the cell in the form of a resonance frequency alternating magnetic field with the proper phase to sustain the beam modulation by forced precession of the systems, and in which the oscillation frequency provides a continuous indication of the field intensity; difficulties arising from orientation dependence in airborne equipment namely hemisphere effect and dead zone effect are overcome respectively by using a pair of cells 1, 2, Fig. 1, the output signals i, i, of which are cross coupled through amplifiers 12, and by using a plurality of optical magnetometers la-c, 2a-c, Fig. 3 with their optical axes disposed at an acute angle for example 45 degrees. The light source comprises an electrodeless discharge lamp comprising a bulb of rubidium vapour mixed with Krypton for starting purposes energized by an external coil 4 from an RF source 5, a coaxial feeder 6 being used to reduce magnetic distortion. The beams are filtered in filters 7, to select the desired wavelength, and passes through collimating lenses 8 and a circular polariser 9 to the cells, the cell light output focused on photo-cells 11 which each comprise a mosaic of silicon cells, producing a signal for application to coaxial coils 1<1>, 2<1> which are oppositely wound and maintain precession. The cells contain a rubidium vapour isotopically enriched with rubidium 85, or rubidium 87 mixed with a buffer such as nion to reduce wall collisions. The fed back signal is mixed with the output of a crystal controlled oscillator 14 for frequency measurement which is recorded on an instrument 15. Separate lamps providing parallel beams in the same or opposite directions may alternatively be used, and according to the arrangement the beams may be similarly or oppositely polarised to produce inverse distributions of sub level absorption. The use of two magnetometers to measure field gradient is referred to, the second being connected to replace the crystal oscillator 14 so that instrument 15 indicates gradient.