Threshold characteristics analysis of a forward pumped Nd3+-doped gain-guided and index alternate-guided fiber laser

Gain guided and index alternate-guided (GGIA-G) fiber is suggested by our group recently. In order to grasp more laser performance of the GGIA-G fiber, the threshold characteristics of the fiber laser is analyzed in this work. Firstly, the improved rate equations of the forward pumped GGIA-G fiber laser are established and solved. Secondly, the threshold pump power of the GG-IG fiber, GG-IAG fiber and GGIA-G fiber are compared, and the effects of parameters including Nd3+ concentration N, fiber length L, fiber core radius a, refractive index difference Delta n and output mirror reflectivity R-2 on the threshold pump power Pth are discussed in detail, separately. Finally, the method of designing the fiber parameters for single mode laser operation is given out. When R-1s=1.0, R-2s =0.8, a=50 mu m, N=1.0x1020 cm(3), L-2: L=0.2 and Delta n=-0.005, the allowed fiber length range for single mode laser operation is about 4.1 cm similar to 9.0 cm. The related research results will be helpful to the better application of the GGIA-G fiber.