Square-law Theory Of Gain-guided Optical Fiber Research In High-power Double-clad Fiber Laser

Double-clad fiber laser is the most important representative of fiber lasers, it has an advantage that the pump energy may be coupled into the large cladding field instead of into a small core field, and one can make a multi-mode high power laser as its pump energy. Because of this advantage, double-clad fiber laser was focused on widely. Many investigations show that the output power of the laser is most limited because of having a very small core which be made as gain medium. In order to improve the output of a fiber laser, a large size of fiber core is necessary. In this paper, a new method about enlarging the size of single-mode optical fiber is introduced, and the single-mode parabolic gain-guiding optical fiber is focused on. The article may be divided into three parts as following:In the first section, the theory of double-clad fiber laser and gain-guiding in fiber are introduced simply, while the theory of gain-guided in step-profile fiber is introduced as a focus.In the second one, the theory of gain-guided in parabolic-profile single-mode fiber is introduced. This section is most important among this article, and some new ideas are discussed in this section. On the shoulder of the theory of step-profile gain-guided single mode fiber, the similar method is used in the parabolic-profile gain-guided fiber while only the lowest mode (LP₀₁ mode) will be propagated. By a series of analysis and computing, one can obtained the range of complex-value dimensionless parameter v while only basic mode should be propagated.In the last section, the capability of optical fiber confining the power carried by the modes is also discussed, it can be expressed by a parameter called confinement factor. By the lower of Poynting and a series of math deducing, one can obtain a formula of confinement factor, and using this formula one can also obtain the confinement of every modes which propagating in parabolic-profile gain-guided fiber. One can find that the confinement capability of parabolic gain-guided fiber is stronger than the one of step gain-guided fiber.