Geometrics G-822A, Руководство по эксплуатации

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Geometrics, Inc. G-822/G-823 Magnetometer Manual
52
When a photon of the infrared light strikes a Cesium atom in the absorption cell, it may
be captured and drive the atom from its present energy level to a higher energy level. To
be absorbed the photon must not only have the exact energy of the Cesium band gap
(therefore the narrow IR line) but must also have the correct spin orientation for that
atom.
There is a high probability that the atom will immediately decay back to the initial energy
level but its original orientation to the ambient field is lost and it assumes a random
orientation. An atom that returns to the base level aligned such that it can absorb another
photon, will be driven back to the higher state. Alternately, if the atom returns to the base
level with an orientation that does not allow it to absorb an incoming photon, then it will
remain at that level and in that orientation. Atoms will be repeatedly driven to the higher
state until they happen to fall into the orientation that cannot absorb a photon.
Consequently, the circularly polarized light will depopulate either the aligned or inverse
aligned energy states depending on the orientation (spin) of light polarization. Remember
that one side of the cell is right-hand polarized and the other left-hand polarized to
minimize sensor rotational light shifts and subsequent heading errors.
Once most of the Cesium atoms have absorbed photons and are in a state that does not
allow them to absorb another photon, the light absorption of the cell is greatly reduced,
i.e., more light hits the photo detector. If an oscillating electromagnetic field of the
correct radio frequency is introduced into the cell, the atoms will be driven back
(depopulating the energy level) into an orientation that will allow them to absorb photons
again. This frequency is called the Larmor frequency and is exactly proportional to the
energy difference caused by the Zeeman splitting mentioned previously. This energy
splitting is in turn directly proportional to the ambient magnetic field strength. The
relationship between frequency and energy is given by:
E = f

Where:
E is the Zeeman energy difference
f is the frequency of the Larmor

is Planck's constant
In Cesium this Larmor frequency is exactly 3.498572 times the ambient field measured in
nano-teslas (gammas). In the G-822A this radio frequency field is generated by a coil,
called the H1 coil that is wound around the tube holding the optical components. When
the R.F. field is present the total light passing through the cell is reduced because atoms
are in an energy state in which they can again absorb the infrared light.
There is a small variation in the atomic light absorption at the frequency of the applied
H1 depopulation signal. This variation in light intensity appears on the photo-detector as
a small AC signal (micro-volts). If this AC signal is amplified and shifted to the correct
phase, it can be fed back to the H1 coil to produce a self sustaining oscillation. In