Also it should be pointed out that when u have external mirrors like some older HeNes, all the modes have the same polarization... so if you put a polarizer in the beam, rotate the transmission axis of the polarizer and look at the spectrum of the laser light as a function of polarization u can extinguish or bring up all of the lines by changing the axis of the polarizer. So there is enough gain for several frequencies to oscillate.
Whereas HeNe's with internal mirrors, u can actually get rid of some lines and keep the others by putting the polarizer outside the tube because they have different polarization.
Whereas HeNe's with internal mirrors, u can actually get rid of some lines and keep the others by putting the polarizer outside the tube because they have different polarization.