Optimal output coupler grating reflectivity for Er/Yb fiber lasers

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% at ~1546 nm. 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. With a flat cleaved broadband output coupler, the efficiency is ~42%, which is the highest reported efficiency of an Er/Yb fully integrated fiber laser.