Latest developments in high power, tunable, CW, narrow line thulium fiber laser for deployment to the ISTEF

We have designed and developed a grating based thulium (Tm) doped fiber laser with ~150 nm tuning range which is used as the master oscillator in a master oscillator power amplifier (MOPA) thulium fiber laser system. Due to thermal instability in the grating used for tuning, the MO could produce a power up to 4.5 W, beyond which the oscillator became unstable. Injecting the seed laser into a bidirectionally pumped large mode area (LMA) Tm fiber amplifier, a stable, tunable, narrow linewidth high beam quality amplified signal of >100 W was achieved. In the absence of stable and sufficiently high power from the seed laser, the amplifier could not be tested to its full potential. The amplifier was also, converted into an oscillator to investigate its power handling capability. An excellent beam quality and ~200 W of power were achieved by running the power amplifier as an oscillator. Operation stability of the oscillator was measured to be more than one hour with a minimum power fluctuation of 0.5%. Currently efforts are underway to increase the seed laser power to ~10 W, large enough to reduce ASE and mitigate feedback to the master oscillator to demonstrate a 200 W, tunable (150 nm) and narrow linewidth (0.15 nm) MOPA system.