Spectrally narrow outputs of about 10 MHz (full width at half‐maximum) are obtained from an optically pumped high‐power (100 kW) D2O laser system by using the oscillator‐amplifier configuration, in which a low‐power single‐mode output from a compact D2O laser with a short cavity length is amplified by a large D2O laser amplifier. In order to obtain spectrally narrow outputs from the amplifier, the following three conditions need to be satisfied: (1) the D2O gas pressure of the amplifier is adjusted in the range from 4 to 6 Torr; (2) the oscillation frequency of the compact laser is tuned in the range about 15–35 MHz lower than the Raman resonance frequency; and (3) the intensity of the compact laser beam injected into the amplifier is larger than about 50 W/cm2. ©1995 American Institute of Physics.