Geometrics, Inc. G-824A Magnetometer Manual
52
APPENDIX A4. CESIUM-VAPOR SENSOR THEORY
Note: The following section is provided for information purposes only. Understanding
this theoretical discussion is not required for proper operation of the magnetometer.
For purposes of this discussion, the ambient magnetic field or earth's magnetic field is
called H
0
. A separate magnetic field generated by an AC signal applied to a coil inside
the sensor is called H1. This coil is shown cross-section along with the other sensor
components in Figure A11.
To initiate operation of the sensor, the lamp oscillator's RF power increases until the lamp
strikes (plasma ignites and fluoresces). The lamp oscillator then reduces its power to
produce the regulated amount of light. The heater warms the absorption cell until a
Cesium vapor is formed. A lens bends the light from the lamp to parallel rays. The lamp
produces many spectral lines but only one line in the infrared region is employed. All of
the other light is blocked by a high grade optical filter.
The infrared line of interest is then passed through a split-circular polarizer. On one side
of the polarizer the transmitted light has an electrostatic vector that advances with a
right-handed rotation. For conceptual purposes, it can be said that all of the photons in
this light have the same right-hand spin direction. The light transmitted through the other
side of the split-circular polarizer produces light in which the vector advances with a left-
handed rotation, therefore having the opposite spin. Both circular polarized light beams
pass through the absorption cell. Because there is a buffer gas in the cell, the single cell
can be considered as two separate cells, each having the opposite sense polarized light
passed through it. Both light beams exit the cell and pass to a second lens. This lens
focuses the light onto an infrared photo detector.
Figure A11. G-824A cesium-vapor
Lamp
Lens
Filter
Polarizer
Photocell
Lens
H1 Coil
Absorption
Cell
