This paper uses SeeFiberLaser simulation software to simulate and optimize a new type of double-ended output fiber laser. Using the controlled variable method, the reflectance range of the high reflectivity fiber grating is set to 50%-95%, with 5% Step, change the reflectivity of the low reflectivity fiber grating, step by 2%, respectively simulate the laser output power of different reflectivity combinations. The simulation results show that the double-ended laser output and the power can be controlled by changing the reflectivity of the fiber grating. When the reflectivity of the high-reflectivity fiber grating is 50%, and the reflectivity of the low-reflectivity fiber grating is 5%, The total output power of the forward and backward directions is the largest; when the reflectivity of the two fiber gratings are both 50%, the output power of the front and rear ends of the laser is the closest; no matter which direction the fiber grating reflectivity is increased, the output power in this direction will become lower, the output power in the other direction becomes higher and the total output power becomes lower.
In this work, we reported a 2 kW monolithic master oscillator-power amplifier (MOPA) configuration based on a novel constant-cladding tapered-core (CCTC) Yb-doped fiber. The CCTC fiber has a constant cladding diameter of ~400 μm and a varying core diameter along the fiber, with a ~24 μm at both ends and ~31 μm in the middle. This special fiber design can not only effectively suppress the stimulated Raman scattering, but also smooth the thermal load in the fiber. The output performance of this fiber was carefully investigated in a bidirectional pump MOPA configuration with respective co-pump and counter-pump scheme, especially on the aspects of the SRS and TMI. As a result, the TMI threshold is measured at ~870 W and ~1980 W in the co-pump and counter-pump scheme, respectively. The maximum output power of 2023 W is achieved with no sign of SRS in the counter-pump scheme. Before the TMI threshold, the beam quality (M2 factor) remains ~1.4 without any mode distortion, and the M2 factor is measured to be ~1.65 at the maximum output power. These results indicate that the CCTC fiber has great potential to simultaneously mitigate the SRS and TMI effect in the high power operation.
At present, fiber amplifiers are generally pumped by fiber-coupled semiconductor laser with a wavelength of 915nm or 976nm. With fast development of application field, the requirement of pump injection power is becoming higher and higher, single-band pumping may not meet practical requirements in high-power fiber laser applications. The total power of the pump source can be greatly increased by combining the 915nm and 976nm pump sources with spectral bundling technology. Based on this, the gain fiber length of hybrid wavelength pumped fiber amplifiers at 915nm and 976nm was optimized under different pump power ratios. By usingSeeFiberLaser software, we investigated the relationship between pump power ratio and output power, efficiency, residual pump power, Raman power and the length of active fiber. This paper has some guiding significance for choosing the pump power ratio and gain fiber length of mixed wavelength pump fiber amplifier.
With the increase of fiber laser output power, stimulated Raman scattering (SRS) and transverse mode instability (TMI) effects have become the main factors limiting the power boost of high-power, high-brightness fiber lasers. In this work, the TMI effect in different core diameter fiber laser oscillators are studied experimentally. In the 25/400YDF laser oscillator, the TMI threshold is around 1780W, while the TMI threshold of the 30/400YDF is around 1070W. A new type gain fiber (25-30-25YDF) by precisely splicing 25/400YDF with 30/400YDF is employed in the fiber oscillator. When the launched pump power is ~2044W, the maximum average power of the 25-30-25YDF laser oscillator is up to ~1573W without any sign of the TMI. The experimental results show that the larger the efficient core diameter of the gain fiber, the lower TMI threshold of the fiber laser oscillator under the identical experimental structure. By optimizing the length of different core diameter fibers in the cavity and improving the fusion quality of each splice point of the laser, the output average power and beam quality of the fiber laser can be further improved.