Rami Arieli: "The Laser Adventure" Chapter 6, Helium-Neon Lasers, page 2a
Absorption and Amplification in He-Ne Laser
As light moves through the active medium, two different processes act on the radiation: absorption and amplification. In a standard He-Ne laser, the amplification by the active medium is about 2%. During one pass through the active medium (from one mirror to the other) the amount of radiation inside increases by 1.02. Thus, to get amplification of light, all the losses, including collisions of the excited atoms with the walls of the gas tube, absorption by other molecules, etc. should be less than 2%.

He-Ne laser is a 4 level laser, so the lifetime of the lower laser energy level needs to be very short. In a Neon gas, which is the active lasing gas, the transition (decay) from the lower laser level is not fast enough, but it is accelerated by collisions with the tube walls. Because the number of collisions with the tube walls increase as the tube becomes narrow, the laser gain is inversely proportional to the tube radius. So, the tube diameter of a He-Ne laser must be as small as possible.

The low gain of the active medium in a He-Ne laser limits the output power to low power. In laboratory prototypes an output power of the order of 100 [mW] was achieved, but commercial lasers are available only in the output range of 0.5-50 milliwatts [mW].

The output coupler of He-Ne laser is a mirror with coating that transmits about 1% of the radiation to the output. This means that the power inside the optical cavity is a 100 times more than the emitted power.