Axial Modes of the HeNe Laser:
In any type of laser, to maintain lasing one must enclose a volume of excited material in some sort of optical cavity, for example, between two mirrors. This can be accomplished by reflecting photons through the cavity multiple times in order to promote the possibility of stimulated emission. The HeNe laser consists of gas confined to a glass tube which has a highly reflective mirror at either end. In this case, light having a wavelength which satisfies the condition,
creates a standing wave pattern. Here L is the cavity length and m is the integer number of nodes in the standing wave pattern in the cavity. Based on the superposition principle, the standing waves will add together amplifying the light. For the laser, the standing waves that are produced are known as "longitudinal" or "axial" modes.(See Figure 1)
The presence of more than one mode can be explained by the Doppler effect in the laer cavity. Because of the random motion of the neon atoms in the cavity, a Doppler effect is produced which allows the laser to emit light waves of varying wavelength represented by the Doppler envelope (See Figure 2).
Any change in the wavelength of the laser light results in a slight increase or decrease in the number of half-wavelengths in the cavity normally produced by the 632.8 nm light of the HeNe laser. The result is multiple longtiudinal modes. For the HeNe laser that I used, there were either three or four modes per envelope depending on the cavity length of the laser.
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