Question

when a side-pumped Dye laser is built, something that looks like a laser beam is seen before the end mirror is put in place, What is the beam?

Answer 1

Side pumping is a technique of optically pumping a laser gain medium (usually of a solid-state laser), where the pump light is injected from the side, i.e. in a direction which is roughly perpendicular to that of the laser beam.

The key advantage of side pumping is that it allows the use of pump sources with very low spatial coherence, such as arc lamps, flash lamps or high-power diode bars. Also, side pumping makes it easy to combine multiple pump sources. It is possible e.g. to pump a rod laser with several diode bars (side-by-side) which have fast axis collimation only; the total width of the emitting region can approach the rod length. Another advantage is that the absorbed pump power can be smoothly distributed in the longitudinal direction. For such reasons, side pumping is often used for high-power solid-state lasers.