What are the modes in a gas laser and what do the various specifications mean?

The Modes or Mode Spacing of a laser beam refer to the quality of the laser beam. The Transverse Electromagnetic Mode (or TEM) describes the pattern that the laser spot makes. A TEM₀₀ means a single laser spot (or beam cross-section) and it is typically used to specify how much of the laser output is in this mode. Due to its symmetrical beam intensity, this mode is typically selected. The total spectral bandwidth (sometimes referred to as the linewidth) is determined by multiplying the mode spacing value by (the number of modes minus one). For example, a He-Ne laser with a longitudinal mode spacing value of 730 MHz with 3 modes will have a total spectral bandwidth of 1.460 GHz. This defines the frequency range that most of the energy is distributed over.