Let's say we are building Nd:YAG laser. It is optically pumped by some linear xenon flash lamps, it absorbs light around 750nm and 800nm, and emitted light is at 1064nm.
The question is why doesn't 1064nm emission from the flash lamps interfere with laser operation?
Why doesn't 1064nm photons emitted in the Nd:YAG at 'wrong' directions (not coaxially to the resonator path) and reflected back and forth from the cylindrical/oval reflector (for the flashlights) interfere with the laser emission?
As far as I see it, both these factors should consume precious atoms in excited state, and probably require some 1064nm filter around the Nd:YAG rod...