Number of Modes in a Gas Laser CavityThough 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. In terms of angular frequency, the Doppler linewidth is given by Within that linewidth, the number of modes may be obtained from the photon density of states expression. Combining these expressions gives a relationship for the number of modes. The number of modes is one of the factors which determines the necessary pumping power to operate the laser. 
Index Laser concepts Reference Rohlf Sec. 135  

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