Optical laser pumping and reflectors 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...
 A: Good question. 
Yes, every photon at the lasing wavelength will potentially eat some of the excited state population through stimulated emission - regardless of the photon's origin. This appears in the rate equations governing the excited state populations and photon number as loss factors.
I believe the key here is that the positive feedback due to the optical resonator will consume the excited state population faster than the random photons will, leaving essentially no gain for them to use.
Note that the Nd:YAG rod will always fluoresce some part of its energy in every direction as 1064nm photons. Having a filter to weed out any stray 1064 nm photons would theoretically lower the losses, but an absorptive filter will most likely only contribute to the amount of heat that needs to be taken away from the system, which in the case of a flash-lamp pumped Nd:YAG is already quite a lot. There is a good reason why diode pumping is more favorable than flash-lamp pumping!
Principles of Lasers by Orazio Svelto is in my opinion one of the best text books on lasers for additional information.
