Large Mode Area Fiber Optical Transmission Of High-power Amplification Characteristics Of Study

In order to overcome the limitations of nonlinear effects, the optical damage and other physical factors that restrict the power expansion of the fiber lasers, the large-mode-area (LMA) fibers is considered as an effective way to breakthrough these restrictions. However, with the fiber core diameter's increasing, the higher order modes will be excited, and the laser beam quality will be degraded. Meanwhile, they will have an impact on the spectrum and the shape of the broadband pulse. So how to gain high power, high beam quality and high signal noise rate pulse is the key technology of LMA fibers laser system.In this thesis, the impact of transverse modes of the LMA fiber on the near field and the temporal, spectral characteristics of the broadband pulse was studied in theoretical and experimental. The impact of intermodal interference on the FM-AM effects of the phase modulated pulse in the broadband LMA fiber laser was first advanced. We studied the amplification and transmission characteristics of the broadband singal pulse and the transverse modes of the LMA fiber. On this basis, mJ-level quasi-fundamental mode all fiber amplifier system was predicted.We studied the analytical solution and the numerical method of the mode field expressions for the LMA passive fibers. Based on these theoretical models, we derived the mode field expressions are approximated as the eigenmodes of the passive fibers if the gain is lower than 100dB/m for the active fibers. We also derived the theoretical model of the intermodal interference based on the transmission and distribution characteristics of the mode field.The numerical simulation results show that the spectrum transfer functions are different at different positions of the LMA fiber end face owing to the intermodal interference, so there are temporal-special distribution characteristics for the output broadband pulses. We attain the similar results in the experiment when measuring the output pulse with the single mode fiber (SMF) sampling oscilloscope. Whereas there is no amplitude modulation for the output pulse when measured by the bulk detector owing to the orthogonal characteristic of the eigenmodes. For the super LMA fibers, we studied the impact of much more intermodal interference on the phase modulated pulse, and we learned that the more modes there are in the fibers, the more speckle there are for the near field, the less the modulated period of the spectrum transfer functions is, the more complex the pulse shape is. These results are of benefit to the experimental measurement, and they propose more strictly requirements of mode control for the design of all fiber amplifier system with broadband pulse shape fidelity.The pulse shape and the spectrum frame of the phase modulated pulse in the process of amplification were studied based on the rate equations and Ginzburg-Landau equation. Gain saturation is the main factor which lead to pulse distortion, and it can be suppressed by pre-shaping or adopting Gaussian pulse. Whereas, the amplitude modulation depth induced by non-uniform gain is not large, about 3% in the process of amplification. The numerical simulation results show that the mode distortion and self-focusing event will occur when the power of eigenmodes is scaled to several MW, and the self-focusing limit could not be improved by the higher order modes.Gain filtering is a method, simple, robust and reducing pulse distortion, to suppress higher order modes in the LMA fibers for broadband pulse. Gain filtering together with mode field adapters between the two modal mismatch fibers can also suppress intermodal interference and reduce splicing loss. We designed a new cone-shaped coiling device for the coiling finer based on the modes matching.Finally, we designed an mJ-level quasi-fundamental mode all fiber amplifier system based on the theoretical and experimental results of the thesis and validated it with the primary experimental results.