Abstract: The cavity modes of the HeNe laser may be investigated using a Fabry-Perot etalon and a digital storage oscilloscope. By examining the graphs of the relative intensity of the laser light versus its frequency, students can calculate the cavity length of the laser. By introducing a strand of human hair or a very thin glass slide into the laser cavity, transverse modes can be produced. Students may render theoretical representations of these modes by using a general software system for mathematical applications and compare them to the modes generated experimentally. Students may use a photo diode to measure the intensity of the laser beam as the length of the cavity is increased to investigate laser cavity stability. In addition, the diode may be scanned across a transverse mode in order to measure the beam intensity at different positions in the mode.

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