Recovering the Backward Traveling Green Beam?
The short answer is: It probably isn’t worth the trouble. While 40 percent or more of the green output may end up wasted, there are several problems with reflecting it back toward the output, especially if a decent quality single beam is the desired result. These are: absorption in the lasing medium (vanadate or YAG), beam walk-off, wavefront matching, and others. • Absorption: I don’t have absorption curves handy but doing an informal experiment with a 1% doped 1 mm piece of vanadate, I’d say 50 to 75 percent of 532 nm light does get through and a good chunk of what doesn’t is not absorbed in the bulk material but is reflected by the two mirror surfaces. So, a substantial amount of green light would survive two passes (up and back). Doing the same experiment with a 3 inch long YAG rod (doped for flashlamp pumping, I don’t know exactly how much), almost no green light gets through, much less than 1%. The obvious solution of putting an HR@532nm coating on the face of the laser crystal n
