LIMITING  MODES

      Large apeture gain media as well as certain cavity configurations often give rise to high-order modes such as TEM₁₀ or higher. Physically speaking, high order modes consume a larger volume of the gain medium than do low-order modes. For this

Figure 1.1. Use of an aperture to limit modes.

reason, many small-bore lasers often operate exclusively in TEM₀₀ mode. This effect can be exploited to prevent a laser from oscillating in higher-order modes by placing an aperture of the proper size inside the cavity so that only the TEM₀₀ mode will fit through it, as illustrated in Figure 1.1. Higher-order modes will be extinguished because the loss imposed on them by the aperture will be greater than the gain provided by the active lasing medium. For example, many large argon lab lasers have an apeture disk or mechanically adjustable iris installed between the laser tube and the output coupler, allowing the user to select the size of hole inserted into the cavity. At a large apeture, the TEM₁₀ donut mode frequently shows up, but insertion of a smaller apeture forces the laser to operate strictly in the TEM₀₀ mode, accompanied by a drastic loss in power! From the figure it is evident that the TEM₀₁ mode occupies a larger volume in the gain medium than the TEM₀₀ mode does, so the higher-order mode can therefore interact with more of the excited laser medium and hence extract more power from the laser. Lasers oscillating in high order modes usually produce more power than do similar lasers limited to the TEM₀₀ mode. Still, the beam purity of the TEM₀₀ mode is often sought after, especially for applications where minimum beam divergence is required.