Study On Long Cavity Passively Mode-locked Fiber Lasers With High Energy

Ultrashort pulse laser with high pulse energy and low repetition rate is widely used in micro-mechanical processing, biological sample testing and eye surgery. And it is one of the hotspots of the recent domestic and foreign scientific research [1]. At present, there are two ways to obtain high energy ultrashort pulse with low repetition rate. The traditional approach is based on outer cavity CPA (Chirped Pulse Amplification). Another way is obtaining high-energy pulse directly from the fiber laser, generally by increasing the cavity lengyth to reduce the repetition rate and increase the pulse energy. In contrast to the traditional approach, the second method of increasing the cavity length reduces the complexity of the system and improves the utilization of energy to a large extent. All these advantages enable a wide range of studies on it. But there are still relatively few theories about this method. While increasing the cavity will introduce too much nonlinear effects and dispersion effects. Nonlinear and dispersion will cause the pulse to be difficult to compress. The pulse width that can be compressed at present is several times the theoretical transform limit value, or even more.In this paper, we build our simulation model based on MATLAB. The characteristics of the output pulse and the problems of pulse compression in long cavity passive mode-locked laser with cavity length of 200m?400m are simulated and analyzed. What's more, we proposed a solution to compress the output pulse of the long cavity passively mode-locking laser to the theoretical limit valve by adding a filter outside the cavity.The main research work and achievements of this paper are as follows:1. We established the simulation model of long cavity passively mode-locking laser based on saturable absorber based on MATLAB2. The process of solving the generalize NLSE (GNLSE) by using the Runge-Kutta method under the interaction representation is introduced in detail.3. A long cavity passive mode-locked laser with cavity length of 500m is built up, and the mode-locked pulse output with repetition rate of 390.118kHz and pulse energy greater than 100nJ is obtained.4. It is verified that the high-order dispersion is not the decisive factor that cause the pulse can not be compressed to the theoretical transformation limit value.5. A scheme is proposed to filter out the non-linear part of the pulse'phase differential by adding a filter outside the cavity so that the output pulse of the long cavity mode-locking laser can be compressed to the theoretical transformation limit value...