The Fabry-Perot Etalon:

An integral part of the apparatus was the Fabry-Perot Etalon. This piece of the apparatus allowed a view of more than one Doppler envelope of modes. A Diagram of the Fabry-Perot etalon is shown below:

The laser beam enters the etalon and is reflected repeatedly between two mirrors. A piezoelectric device is attached to one of the mirrors. An AC voltage is applied to the piezoelectric device that causes it to expand and contract. This action effectively moves one of the mirrors forward and backward changing the the cavity length of the etalon. The change in the separation of the mirrors causes a different number of standing waves to be present in the etalon with time.

For transmission through the etalon there must be an integral number of half-wavelengths present in the etalon. So, for a single frequency of light, as the the cavity length of the etalon, l, increases we get transmission only at discrete values of l. (See figure below).

The x-axis can be translated into frequency and in the case of the HeNe laser where there are multiple frequencies of light are being produced, several Doppler envelopes of modes can be seen with a digital oscilloscope if the beam is passed through a Fabry-Perot etalon (See figure below).

The separation between the mirrors of the etalon can be calculated if the free spectral range of the etalon is known. The equation used to calculate the mirror separation is:


where l is the separation between the mirrors in the etalon. The particular HeNe laser used in this experiment has a free spectral range of 2 GHz which means that the separation of the mirrors in the etalon is:

which renders a value for l of 7.5 cm.

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