A HeNe (Helium-Neon) laser generates an output of red light primarily because the red wavelength is the most dominant and efficient transition in the atomic energy levels of the helium-neon gas mixture used in the laser.
Inside a HeNe laser, there is a discharge tube filled with a mixture of helium and neon gases. When an electric current is applied to the gas mixture, it becomes excited, and electrons within the gas atoms are promoted to higher energy levels. As these excited electrons return to their lower energy levels, they release energy in the form of light.
The specific color of light emitted depends on the energy difference between the excited and lower energy levels involved in the electron transitions. In the case of a HeNe laser, the dominant transition that occurs is between the 2s and 3p energy levels of neon atoms, which results in the emission of red light at a wavelength of 632.8 nanometers (nm).
Although other transitions within the helium-neon gas mixture are possible, they are less likely to occur compared to the 2s to 3p transition of neon. The energy differences for these other transitions correspond to wavelengths in the yellow, green, and orange regions of the spectrum. However, due to their lower probability of occurrence, the red wavelength is the most prominent output of a HeNe laser.
It's worth noting that HeNe lasers can exhibit minor spectral variations and some weak emission lines in addition to the dominant red wavelength. However, the red light is the most intense and commonly observed output from these lasers, making them well-suited for applications that require a reliable and stable source of red laser light.