Double-clad Fiber Lasers By Stimulated Brillouin Scattering And Composite Guidance Effect Of The Theoretical Research

In order to increase the laser output power of double-clad fiber laser, the SBS effect which could restrict laser output power and the gain-guided technology which could increase core diameter are studied in this paper.Based on rate equations and coupling equations, an analytical modal of SBS effect in double-clad fiber laser is developed. Using an algorithm with shooting method and Runge-Kutta way, the relationship between output laser power, stokes power and pump power, as well as the effects of core diameter, fiber length and doped concentration on laser output power limit are obtained. it is found that SBS could be suppressed significantly when core diameter is increased.Gain-guided fiber which can increase core diameter significantly is studied in this paper. Complex-valued refractive and complex-valued normalized frequency being used to describe the structure of mixed-guided fiber is deduced. Then, based on the electromagnetic theoretical modal of conventional index-guided fiber, electromagnetic theoretical modal of gain-guided fiber and mixed-guided fiber is developed. By solving the characteristic formula of mixed-guided fiber through secant method, single mode transmission condition of mixed-guided fiber is obtained, and discussed in some special cases.The mode confinement capability of mixed-guided fiber for the lowest-order mode is investigated. Based on the definition of mode confinement factor and Poynting theorem, the relationship of mode confinement factor and complex-valued normalized frequency for LP01 mode in mixed-guided fiber, gain-guided fiber and index-guided fiber is proposed. Analysis indicated that the mode confinement factor for LP01 mode of gain-guided fiber is smaller than index-guided fiber, but the gain-guided fiber still has strong mode confinement capability for LP01 mode.