My new 5 mw laser pointer

Hey guys, i recently bought a 5 mw laser pointer and will be getting a 100 mw very soon. I hvae realized that the power output of my 5 mw is not constant. It starts of powerfull for a second, then diminishes power. The warm up time is less then 2-3 minutes, which i dont understand what it means. I am using duracell rechargeable 1000 mAh AAA batteries thinking they last long and deliver alot of power. Can anyone tell me what batteries i should use and why am i getting a low output after one second. Thanks!

First, welcome to the forum! and may i ask where you bought the laser from? some lasers do that, and there is nothing you can do about it, but if it is a good quality one, it shouldn't!