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.

Content © G. T. Purves, Durham University 2005