Transverse Laser Cavity Modes

In this project, I will try to show the mode patterns for transverse laser cavity modes.


To show the different transverse laser cavity mode patterns experimentally.


He-Ne Laser: M/n 05-2PM-379-1 Input:120VAC mA:5.5


Transverse modes are stable modes of oscillation involving light waves that are not propagating parallel to the laser cavity axis. Such modes arise from the use of cavity mirrors with a spherical rather than plane contour and from the effects of diffraction. They are condiderably more difficult to analyse mathematically than are longitudinal modes and resonators are usually designed with the intention of ensuring that high order transverse modes do not occur. Except in the case of the simplest mode, their effect on the distribution of intensity over a cross section of the output beam is to give rise to two or more regions of high intensity separated by regions of lower intensity. Examples of such intensity distributions are illustrated below.

Transverse electromagnetic modes are designated by the letters "TEM" and the numerical subscripts refer to the number of nodes, or low intensity regions, in the horizontal and vertical directions respectively. A laser can oscillate in several transverse modes simultaneously, but the aim is usually to restrict oscillation to the lowest order '00' mode. Ideally, this mode has a Gaussian intensity profile and shows lower divergence than higher order transverse modes.

The characteristics of a Gaussian intensity profile are indicated in the diagram above. The most intense part of the beam is on the central axis and the intensity falls away on either side. By convention, w is regarded as a measure of the half width of the beam.


Turn on the laser, rotate the curve mirror carefully, oberve the pattern on the wall.

The picture below is one of the patterns I observed in the experiment.


I cannot put more photos here because the pattern are blured when they are shown on the photos though they can be observed clearly on the wall.