Broadly Tunable and Low-Threshold Cr4+:YAG Crystal Fiber Laser

We demonstrate a Cr⁴⁺:YAG crystal fiber laser tunable from 1353 to 1509 nm using a birefringent filter and with a threshold pump power of 70 mW operating at room temperature. It was found that the coupling loss between the crystal fiber and the collimator due to lens aberration is critical to the fiber laser performance and can be reduced by choosing an aspheric lens with a short focal length. The crystal fiber gain was optimized by aligning the pump polarization to the crystal axis with the highest gain. The advantages of the crystal fiber compared to the bulk crystals include reduced pump threshold due to light confinement and better thermal dissipation due to high surface area-to-volume ratio. The temperature rise of the crystal fiber with a 400-mW incident peak pump power with a duty cycle of 5% was only 1.7 °C using passive cooling. This low-threshold crystal fiber laser is suitable for the application as a tunable seed laser for the power amplifier chain.