Axial Mode Number of Gas Lasers from Moving-Mirror Experiments

The laser-power-output modulation, obtained by reflection of the laser beam back into the resonator by means of an external moving mirror (EMM), is analyzed in terms of resonator Q modulation. The variation of the amplitude of the output-power modulation as a function of the distance EMM laser is characteristic for the number of excited axial modes. Experiments were performed with various commercially available He-Ne lasers, showing clearly one, two, three, and more modes. A comparison is made with similar experiments which may be made in a Michelson moving-mirror interferometer. An additional feature of EMM modulation is that the saturation of the laser transition in a single-mode laser can easily be demonstrated. The experimental procedure and arrangement are simple, and it is believed that the work presented may have value for demonstration and education.