Theoretical And Experimental Research On Narrow-linewidth Fiber Amplifiers

The output power from fiber sources has been raised dramatically over the past decade, leading to a wide application of fiber-based devices by right of their outstanding performances, e.g. high brightness and efficiency. In some special application area, such as coherent beam combination, spectral beam combination and frequency transition, the linewidths of fiber sources are restricted (less than 0.1nm), resulting in a profound research on narrow-linewidth fiber amplifiers. SBS is the dominant nonlinear process in continuous-wave narrow-linewidth fiber amplifiers, which limits the maximal output power. As two special narrow-linewidth fiber amplifiers, single-frequency (SF) and multi-single-frequency (MSF) fiber amplifiers are theoretically and experimentally studied systematically in this thesis to explore feasible methods to promote the amplifiers'output power. Generally, the fiber amplifiers with seed linewidths less than or comparable to stimulated Brillouin scattering (SBS) linewidth of fiber (less than 100MHz) are named SF fiber amplifiers. MSF fiber amplifiers are appointed to the fiber amplifiers with several SF lasers as their seeds, which are always ranged to narrow-linewidth fiber amplifiers. In addition, the SBS suppression in single-frequency Raman fiber amplifiers (SF-RFA) is also studied. The main contents are as follows:I. Theoretical and experimental research on SF fiber amplifiersA numerical model, expounding SBS gain process in SF fiber amplifiers, has been established, which is utilized to discuss the influencing factors on SBS in SF fiber amplifiers. Besides, the SBS suppression effects are discussed in SF fiber amplifiers brought in by the inherent and external temperature distribution, by adding temperature and strain distribution to the numerical model. The monitoring and prevention of SBS effect in SF Yb-doped fiber amplifier with all-fiber configuration are investigated to insure the safety and stability of the amplifier. The influences of gain fiber and delivery fiber on the SBS threshold power are experimentally studied and the output power of SF fiber amplifier is boosted to 120W, which is limited by SBS and still the domestic highest power to the best of our knowledge.The two-tone amplification with gain competition can reduce the spectral power density of amplified signal and thereby suppress SBS in SF fiber amplifiers. A simplified model has been built to describe SBS gain process in SF fiber amplifiers utilizing two-tone amplification with gain competition, which is employed to discuss the influencing factors on SF signal amplification and SBS suppression effect. A SF fiber amplifier is carried out, utilizing the two-tone amplification of 1064nm SF and 1040nm broad-linewidth signals. An enhancement factor of about 0.5 on SBS threshold power is obtained in our experiment by optimizing the seed power ratio.II. Theoretical and experimental research on MSF fiber amplifiers The MSF amplification can be used to increase SBS threshold power, which might be a feasible approach to greatly enhance the power of a single chain in coherent beam combination.The numerical model is established to expound SBS gain process in dual- single-frequency (DSF) fiber amplifiers. The influences of seed power ratio and signal linewidths on the SBS threshold power are discussed. The low power two-wavelength DSF and high power single-wavelength DSF amplification experiments have been implemented and the results indicate that SBS threshold powers can be increased by factors of 1.29 and 0.4 respectively, compared to SF amplification. The high power two-wavelength tri-single-frequency and phase modulated MSF amplification are also performed in our experiments and the maximal output power are both about 275W, the highest power of multi-tone amplification to the best of our knowledge. Moreover, the maximal power is limited only by the available pump power, which means that the output power is still potential to be increased.A MSF amplification scheme utilizing multi-tone amplification with gain competition is proposed to further improve SBS threshold power by employing the two sorts of two-tone amplification methods together. The simulation results of the presented numerical model indicate that SBS threshold power could be increased by nearly a factor of 2 in a DSF fiber amplifier utilizing the three-tone amplification with gain competition of two SF and one broad-linewidth signals, compared to SF amplification. The corresponding amplification experiments are executed, resulting in the conformed conclusion with the numerical simulation.III. Analysis and simulation of single-frequency Raman fiber amplifiersThe achievable powers and amplification abilities of SF-RFA are also limited by SBS to a low level. A theoretical model is proposed which describes the SBS limitation of SF-RFA in this thesis. Based on the model, the optimizing measures of SF-RFA are put forward, together with SBS suppression and cascade amplification, aiming at a usable output power.