This paper presented a near diffraction limit output of YDFL employing 20/400 μm gain fiber. By coiling the fiber at different radius, different beam quality output was obtained. Combined with the BPM analysis, including the impact of fiber bending and the eccentric core loss induced by fusion splicing on beam quality, the bending radius was optimized to be 4.5 cm. Then, two optimized bending fiber parts were added in our FL system. Accordingly, an excellent beam quality output of M2 = 1.06 under the output power of 50 W was achieved successfully.
A theoretical research of fiber laser in single mode operation was presented to evaluate laser targeting characteristics, namely, the directed-energy application. The beam propagation efficiency (BPE) was chosen as the factor to assess the performance of the lasers. Many parameters were studied about their impacts on BPE, including the lens parameters and the transmitting distance. As a result, it was found that the defocus quantity (DQ) valve had a huge impact on directed-energy performance. Given a proper DQ value, the BPE can reach a near limit value. By comparing the BPE performances under different transmission distances, we concluded that when target radius was set to be 5 cm, the ideal state (BPE＞90%) of directed-energy transmission can be achieved within a certain distance of 7 km.