Number of Modes in a Gas Laser Cavity

Though a laser is said to be monochromatic and may involve only one atomic transition to produce the laser light, there are nevertheless a number of cavity modes within the width of that spectral line. Under the operating conditions of gas lasers, the Doppler broadening of the spectral line is the main source of linewidth.

For cavity dimension L and reflection coefficient a, the time to make n reflections is

After n reflections, the radiation intensity is

Using the series approximation

leads to

This cavity lifetime is one of the parameters which determines the pumping power with must be used to maintain a given level of radiation from a laser.