Frequency Dithering
Instead of simply measuring the transmission of a weak probe beam through a
medium it is desirable to measure the dispersion of the medium. Dispersion
type spectra can be recorded by "dithering" the frequency of
the laser and using a lock-in amplifier (locked to the frequency of the
dither). The disadvantage of such a method is that a certain amount of
broadening of features is inevitable due to the "dithering" of the laser
frequency. This is actually measuring a derivative signal, though in the
region of the centre of the resonance it is similar to the dispersion
signal.
It would be highly desireable to measure a dispersion signal directly,
without introducing a dither and the concomitant broadening.
Deflection Measurements
Sagnac Interferometer
The sagnac interferometer beneifits from the fact that both of the beam
paths are coincident. This means that the interferometer is more stable to
vibrations than other types of innterferoometer, such as a Michelson. The
signal that we look for out of the interferometer is the diffference
between the two output ports. In order for the difference signal to be
non-zero, there must be an asymmetry in the paths followed by the two
counter-propagating arms. In the case of the interferometer used for
taking Saturation and Hypefine Pumping measurements, seen here, the
assymmetry is provided by the neutral density filter on one side of the
Rubidium cell.
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