Optimization of Erbium-doped Fiber Length based on Variable Gain Coefficient in Femtosecond Pulse Amplification System

In order to solve the problem that the length of gain fiber is difficult to be determined, and implement the femtosecond pulse features of high average power and narrow pulse width in optical measurement, the effect of erbium-doped fiber length on the pulse features is studied. In the simulation analysis, combined the rate equation of two-level system with the nonlinear Schrodinger equation describing the ultrafast pulse propagation, the variation trend of average power and pulse width with erbium-doped fiber length is numerically studied. According to the two-level model, a variable gain coefficient curve with the fiber length is obtained and applied to ultrafast pulse propagation model by polynomial curve-fitting method to improve the simulation accuracy. In the experiment, a 980nm pump source is used to build a single-stage forward amplification system, and the fiber truncation method is employed to verify the numerical simulation. The results show that the simulation is consistent with the experimental data. When the average power of femtosecond pulse signal light is 50mW and the pump light is 1000mW, the optimum fiber length is 55 similar to 100cm. Compared with traditional simulation method of using fixed gain coefficient, the introduction of variable gain coefficient has an impact on simulation parameters p and d, which present the gain effect and dispersion effect in transmission model.