Research On Single-mode Pumped All Normal Dispersion Ytterbium Doped Femtosecond Fiber Lasers

In recent years, femtosecond fiber lasers with center wavelength around 976 nm have attracted the attention of some scholars because of the potential use as seed sources in fiber amplification systems and solid-state lasers. Furthermore, via frequency doubling, 976 nm laser can be further converted to 488 nm, which makes it possible to replace inefficient argon ion lasers in areas such as communications and underwater exploration of marine resources. These applications,makes fiber lasers centered at 976nm playing a big role in modern society.The main content of this paper is to investigate how to make Yb-doped fiber laser mode locked at 976 nm. To do so, this paper is divided into the following several parts:The first part explains the theoretical basis of the ytterbium doped fiber lasers at 976 nm. By analyzing the absorption and emission cross section of ytterbium ions,two key issues are obtained to achieve laser oscillation at 976 nm. One is to suppress ytterbium ion four-level oscillation, the other is to avoid the reabsorption of 976 nm by ytterbium ions to be transformed into 1030nm laser. The mode-locking mechanism of passive mode locking fiber lasers is then introduced. Based on Nonlinear Schrodinger Equation (NLSE), the law of pulse propagation infiber is also analyzed, which lays the foundation of numerical analysis for the next part.The second part further proves ytterbium doped fiber lasers can achieve mode-locking at 976 nm by numerical simulation. A semiconductor saturable absorber ytterbium-doped fiber laser is simulated as an example by soling Nonlinear Schrodinger Equation.The modeling results suggest ytterbium-doped fiber can achieve mode locking around 976 nm. During simulation, we also analyzed the amplitude modulation effect of the spectrum filter on pulse shaping.In third part, we conduct the experimental research on core pumped ytterbium-doped femtosecond fiber laser around 976 nm.The optimal length of gain fiber for ytterbium-doped fiber is achieved and mode-locking at 976 nm is obtained. The repetition rate is measured to be 44.3 MHz, the pulse energy is approximately 1 nJ,and the pulse duration is 250 fs after compression. The laser achieves stable mode locking andcanselfstart. The experimental results demonstrate ytterbium doped fibers can get mode locking near 976 nm by replacing the pumped wavelength from the traditional 976 nm to 915 nm.In fourth part, the experiment set up is further improved. The dispersion control device is added into the cavity, then final output spectrum is analyzed by adjusting the net dispersion. It is found that the final output exhibits a different mode-locked spectrum when the net dispersion in the cavity varies. It is shown that dispersion plays a significant role in mode locking at 976 nm and suppressing pulse conversion from 976 nm to 1030 nm.