  We measure the output power of an Er/Yb fiber laser with twelve different
SMF-28 narrowband output couplers and demonstrate experimentally that the
optimal reflectivity is ~ 1 %. The fiber laser efficiency with the optimal
output coupler is ~ 38 %. In addition, we successfully inscribe a similar
output coupler in-situ during laser operation with 800 nm femtosecond pulses
and the phase mask technique. An output power very close to the optimal was
obtained with the in-situ inscribed output coupler.
