Double Cladding Ytterbium-doped Fiber Pulse Amplifier

Fiber lasers and amplifiers have become the development focus of laser field because of their advantages of absolutely ideal beam quality, high conversion efficiency, completely exempting the maintenance, high stability, small size and so on. Pulse lasers can achieve the aims which the general laser techniques or tools can not complete. A number of significant discoveries and important research achievements have been produced in the information, energy, environment, medical science, defense and basic science research fields. The major duty of this thesis is high power pulse fiber laser amplifier made of large mode area ytterbium-doped fibers. Devote to construct an all-fiber pulse power amplified system which could generate pulses with high peak power and high energy.This paper analyzes the mechanism of ytterbium doped fiber amplifiers, including the absorption and emission characteristics of ytterbium ions, the transmission equation of pulses in optical fibers, the nonlinear effects in fibers, and the stimulated Raman scattering effects are discussed especially.Pulse lasers need to support higher peak power, so in order to improve the power margin of fibers, some large mode area fibers used in high power fiber amplifiers are studied. The signal quality in fiber core depends on the details of fiber structure with the refractive index profile, doped concentration and shape of cladding. By simulation to bending loss to various kinds of modes with several common refractive index profile, the result shows that compared with the fundamental mode, the bending loss of high order modes can be much larger by bending the fibers at a suitable bend radius, thereby the single-mode transmission can be achieved.The concrete structure of a multistage ytterbium-doped fiber pulse amplifier is designed. Mid-amplifier and post-amplifier are composed with large mode area fibers to decrease the optical power density in the core effectively, so that nonlinear effects can be avoided. Causes of nonlinear effects and methods of how to overcome them are researched. And system performance is simulated with the pulse width of1ns-10ns and pulse repetition frequency of10kHz-100kHz.