# Different laser modes

As far as I understood, different laser modes correspond to different transversal distributions of the electromagnetic field. Therefore, different modes must have different frequencies.

Why is it, that a laser can produce different modes? I take the HeNe-Laser as an example. The main wavelength stabilized and amplified is at $632.8\ \mathrm{nm}$. This originates simply from the neon emission.

If I would speak of different modes of the HeNe-laser, would that mean, that there occur other wavelenghts next to $632.8\ \mathrm{nm}$ ? How is this possible? I only understand what a mode is in an optical cavity: It is simply an allowed resonance frequency of the cavity, e.g. a standing wave.

• The cavity may support one or more allowed modes at 632.8 nm. Some other modes may occur at other frequencies. But, you must keep separate the allowed transitions (and their width) of the lasing medium from the allowed (propagating) optical modes of the cavity. – Jon Custer Sep 6 '16 at 15:24
• I might have misunderstood this connection pointed out by you. How can I calculate or find out the allowed modes at a certain wavelength? The modes are only a function of the lenght of the cavity. – EpsilonDelta Sep 6 '16 at 15:39
• @EpsilonDelta what do you mean "at a certain wavelength"? The wavelengths will be different, of course, as will the frequencies. Your description of what a mode is is accurate. A laser is an optical cavity with a couple extra bells and whistles. – hobbs Sep 6 '16 at 17:48
• All of the above seems to be related to longitudal modes. But there are also transverse modes. – Jarosław Komar Sep 7 '16 at 15:29