We report a low-threshold, high-conversion-efficiency erbium-doped fib
er laser in a 100-parts-in-10(6) fiber pumped at 1.48 mum. The thresho
ld and efficiency are found to deteriorate noticeably as the erbium co
ncentration is increased. We propose that this is due mostly to rapid
cross relaxation between ion pairs or clusters, and through modeling w
e show that the cluster content increases with concentration.