All-fiber passively mode-locked thulium/holmium laser with two center wavelengths

We have demonstrated a self-starting, passively mode-locked Tm/Ho codoped fiber laser that lases at one of two center wavelengths. An amplified 1.56 μm distributed feedback laser pumps a ring laser cavity which contains 1 m of Tm/Ho codoped silica fiber. Mode locking is obtained via nonlinear polarization rotation using a c-band polarization sensitive isolator with two polarization controllers. The laser is able to pulse separately at either 1.97 or 2.04 μm by altering the intracavity polarization during the initiation of mode locking. The codoped fiber permits pulsing at one of two wavelengths, where the shorter is due to the Tm(3+) emission and the longer due to the Ho(3+) emission. The laser produces a stable pulse train at 28.4 MHz with 25 mW average power, and a pulse duration of 966 fs with 9 nm bandwidth.