He–Ne laser cavity has a spacing of 15 cm between the mirrors, and the optical
mode in the cavity has a diameter of 3 mm (a) Determine the frequency differ
ence between adjacent laser modes. (b) Determine the frequency difference be
tween all possible cavity modes contained within the laser cavity volume. (c) The
He–Ne gas mixture provides optical gain over a frequency width of 1.5 GHz.
Compare the number of laser modes that are within this width to the total number
of cavity modes within this width.