High-Power Spatiotemporal Mode-Locked Fiber Laser Based on Nonlinear Regulation
Objective Fiber lasers have been widely used in fields such as optical communication,radar,and signal processing,and have an irreplaceable role.Single-mode fiber(SMF)lasers typically operate at a low power and generate pulses by locking multiple longitudinal modes that are the fundamental transverse modes.However,the small mode-field area of single-mode fibers can easily lead to strong nonlinear effects,thereby limiting the performance of single-mode lasers.Owing to the limitations of the soliton area theorem and spectral sideband effect,the maximum energy of a single soliton pulse can only reach the order of 0.1 nJ.This is because nonlinearity and anomalous dispersion often lead to pulse splitting.In a dissipative soliton laser with a positive dispersion cavity,large nonlinearity can induce multiple pulses.Compared with SMFs,graded-index multimode fibers(GIMFs)can support hundreds of lateral modes,have a higher transmission capacity,carry more energy,and have complex spot characteristics.In this study,we propose a nonlinear regulation technique and implement a high-power spatiotemporal mode-locked(STML)fiber laser using nonlinear polarization rotation(NPR).The number of modes and nonlinearity in the cavity are controlled by changing the fusion misalignment between the gain and multimode fibers.By adjusting the misalignment amount to 8 μm,the maximum average spatial light power output by the laser can reach 690 mW,and the single pulse energy can reach 39 nJ at a repetition rate of 17.67 MHz.This is a suitable solution for generating high-power laser pulses.Methods In the current laser configuration,we first set the fusion misalignment amount between the gain fiber and GIMF to 0.In other words,there is no misalignment fusion,enabling the fiber cavity to achieve STML operation in the few-mode state.In this case,the pump light can increase from 1.32 W to 1.92 W for maintaining a single-pulse output with a maximum spatial output power of 264 mW and intracavity single-pulse energy of 15.1 nJ.An additional increase of the pump power will result in excessive nonlinearity and pulse splitting.To further increase the single-pulse energy,we offset the nonlinearity caused by the excessive power by increasing the fusion misalignment amount.When producing dissipative solitons in a positive dispersion cavity,it is necessary to balance the nonlinearity to achieve stable dissipative solitons.In the experiment,the nonlinear coefficient in the cavity increases sharply with increasing pump power in the cavity,which affects the stability of the single pulse.As the misalignment amount increases,the number of transverse modes in the cavity increases,and the corresponding high energy in the cavity is transferred to higher-order modes.Therefore,increasing the amount of fusion misalignment based on nonlinear regulation can counteract excess nonlinearity.In addition,the misalignment values are set to 4 μm and 8 μm in sequence,and the output energy and spot characteristics are analyzed.Results and Discussions First,a few-mode STML fiber laser is studied.Increasing the pump power may lead to excessive nonlinearity and pulse splitting,making it impossible for a single pulse to carry a high power.In this experiment,the number of transverse modes is increased by adjusting the fusion misalignment amount to counteract the nonlinearity caused by the excessive pump power,thereby increasing the single-pulse energy.As the fusion misalignment amount increases,the number of transverse modes also increases.Therefore,when the nonlinearity caused by the increase in pump power leads to pulse splitting,the number of transverse modes offsets the excessive nonlinearity.When the misalignment amount increases from 4 μm to 8 μm,the spectrum exhibits a blue shift trend,as shown in Fig.4(a).This is because an increase in fusion misalignment amount leads to an increase in the intracavity loss,and the laser wavelengths shift towards shorter wavelengths with a higher gain.Stable STML operation can be achieved by adjusting the pump power to 1.9 W at 8 μm misalignment amount.Figure 4(b)records the time-domain pulse sequence at 8 μm misalignment amount with a pulse interval of 56.6 ns and a repetition frequency of 17.67 MHz.Pulse splitting is observed only when the pump power is continuously increased to 3.75 W,and the average output spatial light power is 690 mW.The maximum single-pulse energy is 39 nJ and peak power is 161.8 W.Conclusions In this study,a nonlinear regulation technique is proposed to realize a high-power spatiotemporal mode-locked fiber laser with a spatial structure.The changes in the output power and spot profile under different fusion misalignment amounts are investigated.In the experiment,a large number of high-order mode excitations under 8 μm fusion misalignment amount between gain fiber and graded-index multimode fiber are used to balance the high nonlinearity brought by high power,so that the single pulse can carry higher energy.The average power of the generated space light can reach 690 mW,and the single pulse energy is 39 nJ.This type of high-power single-pulse laser has application value in the fields of medical treatment and optical communication.
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