Mode-Locked Thulium-Doped Fiber Lasers Based on Normal Dispersion Active Fiber

We report and compare three configurations of the semiconductor saturable absorber mirror mode-locked, linear cavity femtosecond all-fiber master oscillator power amplifier based on a highly Ge-doped thulium-doped normal dispersion fibers. We have studied the performance of the system and have obtained stable mode-locking in a wide cavity dispersion range ∼ 1.88 μ {m}. In this letter, we focus our attention on three mode-locking regimes: laser operating in the anomalous, nearly zero and normal cavity dispersion regimes without the use of the additional dispersion compensating elements. For the nearly zero and normal cavity dispersion regimes the femtosecond pulses with several nanojoule energy could be obtained. The pulses were further compressed down to 630 fs using a simple fiber compressor consisting of a piece of the conventional telecommunication fiber, making the laser design particularly simple and cost effective.