Research On Nonlinear Pulse Dynamics And Characteristics Of Saturated Absorber Mode-Locked Fiber Lasers

With the advancement of the Internet of Things and information technology,the demand for artificial intelligence is growing rapidly.The photonic neural network,which combines the benefits of neuron and photonic technology,is currently a research hotspot because of its ultrahigh speed and low energy consumption.Passively mode-locked fiber lasers based on saturable absorbers possess an all-optical structure,good beam quality,and ultra-short pulse width,which enable them suitable as the core components of photonic neurons for generating photonic neural pulse signals and constructing photonic neural network systems.To better design a mode-locked fiber laser for generating photonic neural pulse signals,this paper focuses on the saturable absorption mechanism in mode-locked fiber lasers,the simulation efficiency of modelocked fiber lasers,and the potential neuron-like characteristics in modelocked fiber lasers.In this paper,a series of theoretical analyses and simulation research on these three points had been presented.The main contents are as follows:1.A simulation model of a passively mode-locked fiber laser was established to study the influence of the saturable absorber in the modelocked laser on the output pulse.A physical model of saturable absorber was established,and the simulation analysis was carried out for its modulation depth,unsaturated loss,and recovery time.This study revealed that the modulation depth is inversely proportional to the pulse width in the time domain.When the modulation depth increases from 0.1 to 0.6,the pulse width decreases from 26.80 ps to 8.78 ps.The non-saturation loss affects the pulse energy.When the unsaturation loss increases from 0.05 to 0.6,the pulse energy decreases from 2.65 nJ to 1.27 nJ.The effects of fast and slow saturable absorbers on pulse formation and evolution in fiber lasers were studied.In the fast recovery region,when the recovery time increases from 0.1 ps to 0.2 ps,the pulse width decreases from 14.39 ps to 11.41 ps,but continues to increase after 0.2 ps,and the pulse width continues to increase and reaches 17.90 ps at the recovery time of 17 ps;In the slow recovery zone,the pulse width fluctuates around 18 ps.According to the evolution results,the shaping of pulses under different saturation recovery times was analyzed,enriching the theoretical basis of nonlinear pulse dynamics.2.A high-efficiency calculation method was proposed for the computational efficiency of passively mode-locked fiber lasers.The method consists of the symmetric split-step Fourier method(SSFM)and the global-error-local-energy(GELE)method for solving the propagation equation.This improved method can limit the local energy increment related to the global error within a certain range to control the step size,and has the advantage of an automatic step size adjustment mechanism.Simulation results showed that such improved method did not introduce additional errors in the simulation process.At the same time,to achieve the same degree of calculation accuracy,the calculation time of the improved method was 255 seconds,while the small constant step size SSFM method needed to calculate 3855 seconds.The computation time of the method proposed in this paper is one or two orders of magnitude less than conventional algorithms.It is proved that the method is effective and efficient.3.Neuron-like properties were explored based on mode-locked fiber lasers.Two pump structures that could generate neuron-like light pulses were designed based on the definition of neuron-like excitability in spiking neural networks.In the first structure,980 nm pump source had been adjusted to output 6 mW,which enables the mode-locked fiber laser to work below the mode-locking threshold,resulting in low-energy noise pulses in the cavity.The 1480 nm pump source was then adjusted to output a square wave disturbance to the laser system,providing 54 mW and exciting a pulse during the disturbance.In the second structure,980 nm pump source had been adjusted to output 13 mW for generating lowintensity mode-locked pulses.When the input disturbance at 1480 nm pump source is 476 mW,high-intensity mode-locked pulses can be excited.The output pulses in both cases exhibit the excitability and refractory period characteristics of neurons.It is demonstrating that the mode-locking technology has the potential to serve as a core device in the neuron system and providing a new idea for the development of photonic neural networks.