A broadband Nd3+-doped fiber laser has been operated in an external ca
vity by pumping with an Ar+-ion laser. The emission spectrum of the fi
ber laser shows high sensitivity to intracavity absorption. The equiva
lent absorber path length is 15 km; with a loss-reduced setup that inc
ludes pumping by a diode laser this path length is 150 km. The spectru
m of atmospheric absorption in the range from 1.065 to 1.145 mum has b
een recorded by stepwise tuning of the 0.3-nm-wide laser emission band
by an intracavity prism.