Parasitic Oscillations

Internal reflections at the boundaries of the active medium can drastically enhance the onset of ASE, particularly if these reflections lead to a closed path, that is, a ray that is reflected upon itself. In this case, we have a strong feedback mechanism and as soon as the gain in the laser medium exceeds the reflection losses parasitic oscillations will set in. For example, in rods with polished walls there can exist internal modes completely contained by total internal reflection. These are particularly troublesome.
One type of internal mode, the whisper mode, propagates circumferentially and has no longitudinal component. This mode can penetrate into the rod as far as r₀/n₀, where n₀ is the index of refraction and r₀ is the rod radius. Another type of internal mode, the light-pipe mode, propagates down the rod in a zigzag fashion. The gain limitations due to internal radial modes are considerably improved in immersed rods particularly in the case of an index matching fluid, or rods with rough surfaces, grooves, or antireflection coatings. Longitudinal modes can be suppressed if the faces of the rod are either wedged or antireflection coated. Parasitic modes in laser disks have already.