Geometrics, Inc. G-822/G-823 Magnetometer Manual
51
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.
Because Cesium is an alkali metal, the outer most electron shell (orbit) has only one
electron. It is the presence of this single electron that makes the Cesium atom well-suited
for optical pumping and therefore magnetometry.
The Cesium atom has a
net
magnetic dipole moment
. This net dipole moment, termed
F
, is the sum of the
nuclear dipole moment
, called
I
, and the
electron's angular
momentum
, called
J
. In a Cesium atom:
I = 7/2
J = 1/2
and thus
F
can have two values depending on whether the electron's angular momentum
adds to or subtracts from the nuclear dipole moment. Therefore,
F
can have the value of
3
or
4
. These values are called the hyperfine energy levels of the ground state of Cesium.
Normally the net dipole moments are randomly distributed about the direction (vector
sum of the 3 axial components) of the ambient magnetic field (H
0
). Any
misalignment
between the net atomic dipole moment and the ambient field vector causes the Cesium
atom be at a higher energy level than if the vectors were aligned. These small differences
are called
Zeeman splitting
of the base energy level.
The laws of quantum electrodynamics limit the inhabitable atomic magnetic dipole
orientations and therefore the atomic excitation energy to several discreet levels: 9 levels
for the
F=4
state and 7 levels for the
F=3
state.
It is this variation in electron energy
level state that is measured to compute the ambient magnetic field strength.
