Development of Gain-managed Amplification All-fiber Femtosecond Laser Technology for Multimode Nonlinear Optics Imaging
Multimodal Nonlinear Optical Imaging(NLOI)has revolutionized biological research by enabling high-resolution,three-dimensional fluorescence imaging with minimal cell phototoxicity.Unlike traditional microscopes using ultraviolet or visible light,NLOI employs near-infrared wavelengths(700～1 100 nm),maximizing tissue penetration and minimizing photodamage.NLOI relies on femtosecond lasers to excite fluorescent markers like Green Fluorescent Protein(GFP).Despite their excellent performance,existing high-repetition-rate light sources,often tunable mode-locked Titanium:Sapphire lasers,face limitations in generating high-peak power pulses at low exposure powers.This restricts their capabilities for deep tissue imaging.Fiber lasers,offer a compelling solution.Our novel fiber laser setup,based on Gain-Managed Amplification(GMA),addresses these limitations,generating high-quality,ultrashort femtosecond pulses ideal for NLOI.This compact and cost-effective system boasts outstanding features:a 35 MHz repetition rate,39.5 fs pulse width,and 267.4 mW average power.Significantly broadening the spectrum(～80 nm)and achieving near-Fourier-transform-limited pulses after compression,it surpasses conventional methods in both performance and affordability.Detailed simulations using the Generalized Nonlinear Schrödinger Equation(GNLSE)guided the optimal design of our setup,ensuring precise control over pulse propagation and optimizing pulse compression quality.We demonstrate the success of our approach by constructing an all-fiberized experiment setup encompassing seed source,pre-chirp management,GMA,and pulse compression modules.This innovative fiber laser holds immense potential for advancing NLOI applications,particularly in deep tissue cell imaging.We investigated the effects of different pre-chirp Group-Delay Dispersion(GDD)and seed energy on the output results using Fiberdesk software.The results indicate that input parameters with varying positive and negative pre-chirping GDD lead to a broader spectral broadening compared to unchirped cases.Specifically,conditions with negative pre-chirping GDD result in pulses characterized by smaller pedestals and more effective compression.Additionally,it was observed that within an input pulse energy window of 0.06 to 0.3 nJ(corresponding to an average power of 2 to 10.5 mW),substantial spectral broadening and efficient compression by grating pairs can be achieved.However,further increasing the pulse energy introduces complex higher-order nonlinear phase components,which hinder additional compression by the grating pair.These findings were instrumental in the construction of a gain-managed amplifier.In our experiment,we have measured the spectrum and pulses after GMA when the seed current is 1.5 A,and with the increase of pump current from 731 mW to 950 mW,the spectral width after GMA increased from 20 nm to 80 nm.And the pulse duration notably decreased after post-compression by a 1 000 1/mm grating pair.At the highest pump power of 950 mW,the output power of the pulse after GMA reached 349.4 mW,with a single pulse energy of 9.98 nJ,marking a 20 dB increase from the pre-main amplifier level.The pulse duration after grating pair compression is 39.5 fs,closely approaching the Fourier transform limit.The compressed pulse's output power was measured at 267.4 mW,with a single pulse energy of 7.64 nJ.To facilitate widespread application in NLOI imaging and other non-laboratory environments,we have engineered the entire system for encapsulation,successfully reducing its volume to a compact structure.Additionally,the root mean square error of the measured output power of the laser in three hours was only 0.11％,indicating that the laser not only delivers high-performance output but also maintains long-term stability.
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