Fiber lasers operating at the wavelength of 2.05 μm (corresponding to the absorption peak of CO₂ and the atmospheric transmission window) have attracted intense interest owing to their applications in coherent Doppler lidars, free space communication, etc. To date, the highest output power of a 2.05 μm fiber laser has been scaled to the kilowatt level; it is obtained through a master oscillator power amplifier (MOPA) configuration combined with large mode area fiber. However, studying high-power single-mode fiber lasers is also important because they are cost-effective and more resistant to environmental disturbance. To date, the output power of single-mode thulium-doped fiber lasers has been limited to the multiwatt level owing to the diminishing emission cross section of thulium at 2.05 μm and the rising background loss of silica fiber at wavelengths above 2 μm. This work develops a rate equation model to determine the optimal incident signal power and thulium-doped fiber length, based on which a high power, single-transverse-mode 2.05 μm fiber laser with a MOPA configuration is presented.

The schematic of the thulium-doped fiber laser MOPA, which contains an oscillator and two stages of amplifiers, is given in Fig.1. The seed laser is generated from a homemade ring-cavity thulium-doped fiber laser, after which a filter-type wavelength division multiplexer (FWDM) is inserted to improve the optical signal noise ratio (OSNR). In the amplifiers, the gain fiber is 10 μm/130 μm thulium-doped fiber and is forward-pumped by a 793 nm laser diode. In the power-amplifier, the gain fiber is coiled on a water-cooled plate for effective heat dissipation. A rate equation model is developed to determine the optimal incident signal power and thulium-doped fiber length for high-efficiency laser generation.

The simulation results are given in Fig.2; they indicate an optimal fiber length of 3.7 m and incident signal power of 4.4 W. In the experiment, the output power of the 2.05 μm ring-cavity seed is 1.01 W and decreased to 0.88 W after the FWDM, whereas the OSNR increases from 58.6 dB to 62.8 dB. The 3 dB spectral linewidth is 0.07 nm (Fig.3). In the pre-amplifier, the 2.05 μm laser is boosted to 8 W under the 793 nm diode pump power of 17 W, with a slope efficiency of 41.8% [Fig.4(a)]. The OSNR at 4.4 W output power still reaches 59.9 dB [Fig.4(b)] despite the increased amplified spontaneous emission (ASE). In the power-amplifier, a maximum output power of 57 W at 2048.7 nm with an OSNR of 58.8 dB is obtained when the 793 nm diode pump power is 102.6 W, corresponding to a slope efficiency of 52.6% (Fig.5). The root-mean-square (RMS) fluctuation of output power is below 2% within 30 min. The beam quality factor (Mx2) in the horizontal direction and the beam quality factor (My2) in the vertical direction are 1.08 and 1.11 under 57 W output power at 2.05 μm, demonstrating a near diffraction-limited beam quality. Further power scaling is only limited by the available pump power. The experimental results show that a high-efficiency, high-OSNR 2.05 μm laser with an output power of at least tens of watts can be achieved based on a single-mode thulium-doped gain fiber through system parameter optimization combined with efficient water-cooled heat dissipation.

This work demonstrates a high-power all-fiber MOPA at 2.05 μm based on a commercial single-mode thulium-doped silica fiber. A rate equation model is developed to optimize the incident signal power and thulium-doped fiber length of the power-amplifier. In an experiment, a maximum output power of 57 W at 2048.7 nm is obtained under a 793 nm diode pump power of 102.6 W, corresponding to a slope efficiency of 52.6%. The linewidth and OSNR are measured as 0.08 nm and 58.8 dB, respectively.