4.3 GHz fundamental repetition rate passively mode-locked fiber laser using a silicate-clad heavily Tm-doped germanate core multimaterial fiber.

We report a silicate-clad heavily Tm-doped germanate core multimaterial fiber that is successfully drawn by using a rod-in-tube method. This new fiber has a high gain per unit length of 6.11 dB/cm at 1.95 µm, which is, to the best of the authors' knowledge, the highest gain per unit length reported so far for Tm-doped glass fibers. By virtue of this high-gain glass fiber, an all-fiber-integrated passively mode-locked fiber laser with a fundamental repetition rate up to 4.3 GHz is demonstrated. Remarkably, the generated pulse operating at 1968 nm exhibits a signal-to-noise ratio of >76 dB in the radio-frequency domain. These results suggest that the silicate-clad heavily Tm-doped germanate core multimaterial fiber can act as a key building block for high repetition rate mode-locked fiber lasers at 2 µm.