rare earth oxides doped multicomponent glass fibers for laser generation and amplification , including a core and a cladding , the core comprising at least 2 weight percent glass network modifier selected from bao , cao , mgo , zno , pbo , k 2 o , na 2 o , li 2 o , y 2 o 3 , or combinations ; wherein the mode of the core is guided with step index difference between the core and the cladding , a numerical aperture of the fiber is between 0 . 01 and 0 . 04 ; core diameter is from 25 to 120 micron , and a length of the gain fiber is shorter than 60 cm .