| Among the rare-earth-doped fiber, Ytterbium (Yb)-doped fiber amplifiers are ideal for high-power amplification because of a broader gain bandwidth, a broader range of possible pump wavelengths, higher pump efficiency, high saturation output power and excellent beam quality. In practical application, the optical pulse generating system of high energy laser facility also require high energy output, good beam quality, high signal-to-noise ratio (SNR). They were determined by the Yb3+ doped fiber amplifier. However, the amplified spontaneous emission (ASE), nonlinear effect and saturation distortion became serious especially when the output power of amplifier was quite high, which had bad effect on the SNR, pulse waveform, signal spectral and signal gain. The introduction of large-mode-area (LMA) fiber in fiber amplifier is a promising method to increase the nonlinear threshold. As the numerical aperture (NA) of fiber cannot unlimited decrease, more than one mode could exist in LMA fiber. Meanwhile, there is not a widely accepted effective method for ASE suppression in high power fiber amplifier, to the best of the author's knowledge. In this paper, the theoretical study on high power LMA Yb3+ fiber amplifier is presented. The main work is as follows:1. Pulsed pump is proposed to increase the pump energy efficiency, suppress the ASE and correct the waveform deformation of the amplified signal. The basic principle of pulsed pump is presented. Based on the transient propagation-rate equation of fiber amplifier, the correlation between ASE built-up time and pump power is studied; ASE suppression under pulsed pump is obviously observed and the waveform deformation is corrected. Key parameters are optimized such as the width of pump, the energy of pump, and the delay time of signal injection after the pump. Compared with CW pump, pulsed pump can increase the energy efficiency and suppress ASE obviously. The SNR can be optimized by adjusting the delay time of signal injection. Under certain condition, we got 12 dB suppression of ASE, 18 dB improvements of SNR and significant waveform correction. For the cascade amplifiers, when the amplified signal from the front amplifier took with noise, the gain in the next amplifier decrease quite a lot under CW pump while it is almost remain the same under pulsed pump. 2. Transverse mode distribution and filling factor are introduced in propagation-rate equations. An approximate numerical algorithm by multilayer method is presented to calculate the power of amplified signal and ASE in multimode fiber amplifiers. Numerical results of the propagation-rate equations before and after the improvement of classical model are compared. It shows that the power of amplified signal is more or less the same with two models, yet the ASE powers differed more as propagating along the fiber. Power filling factor is the key indication to describe the transverse mode competition behavior.3. The large-mode-area active fibers in different dopant profiles are studied respectively and its effect on transverse multimode competition is presented. Power filling factor is the key indication to describe the transverse mode competition behavior. Meanwhile, the SNR under different flat doping confinement is also plotted in a curve. The results showed that 60%-70% confinement of flat doping is more helpful in high-order mode suppression. ASE significantly dropped when the ions are doped more concentrated in the fiber core and the SNR got highest when the confinement of flat doping on 40%. |
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