Generation And Noise Characteristics Of High-average-power Femtosecond Fiber Optical Frequency Comb

High-power optical fiber frequency comb?OFC?based on femtosecond mode-locked pulses and high-power fiber amplifier,is an alternative light source to replace Ti:sapphire OFC,and plays a significant role in many applicaitions,such as precision spectral measurement,time-frequency metrology and attosecond ultrafast optics.At present,there are some bottlenecks urgently needed to be broke through listed below in the development of high-power fiber OFC:?i?Find more stabilized mode-locked technology to reduce noises of laser pulses and improve the stability of fiber laser sources;?ii?Explore advanced technology of pulse amplification to overcome the limitations of gain narrowing,nonlinear effect and mode instability and reduce the nonlinear noises accumulated in amplification process;?iii?Develop high-power time-frequency-domain locking technology to control femtosecond laser pulses and realize higher average power fiber OFC.In this paper,we study the generation and the noise suppression of high-average-power femtoscond fiber OFC.First,we achieve an ultra-low-noise high-repetition-rate femtosecond pulse train through optimization of pulse mode locking and noise suppression in fiber cavity.Second,using large-mode-area high-gain Yb-doped fiber,we separately realize a chirped-pulse amplifier?CPA?and a self-similar amplifier?SSA?,and the additive noises introduced in high-power amplification process are reduced.Finally,we build a time-frequency-domain locking system to control high-power femtosecond pulses,and obtain two low-noise high-power fiber OFC.The details are summarized as follows:1.We build an ultra-low-noise integrated fiber laser based on nonlinear polarization rotation?NPR?.In detail,through cavity designing,intracavity dispersion management and pumping optimization,we achieve a 50-fs laser pulse train with a spectral half width of 50 nm and a repetition rate of 500MHz.Then,utilizing harmonic NPR mode-locking technology,we obtain a 2nd-order-harmonic mode-locked pulse train with a repetition rate of 1 GHz.Moreover,we sharply reduce the phase and intensity noise by the comprehensive inhibition of pump noise,dispersion noise and environmental noise.Finally,we realize ultra-low-noise femtosecond pulses with integrated phase and intensity noise?Integrating range:1Hz to 10MHz?of 1.6 mrad and 0.085%,which is the lowest free-running noises in current reported 1-GHz femtosecond mode-locked fiber lasers.2.We study the time-frequency-domain pulse evolution in a high-power chirped-pulse fiber amplifier though theoretical calculation and designed experiments.Specifically,some effects,such as pulse broadening,gain narrowing,self-phase modulation?SPM?and stimulated Raman scattering?SRS?,are calculated to simulate the influence on output pulses of a fiber CPA system.Then,through the suppression of amplification noise and the optimization of high-power higher-order-dispersion grism compressor,we develop a high-power low-noise CPA system,and obtain a250-MHZ,132-W,180-fs Fourier-transform pulse train.To further compress the pulse width of high power optical frequency comb,we develop a high-average-power low-noise SSA system,in which the output spectrum overcomes the limitation of gain narrowing and covers from 1000 to 1100 nm via SPM-based spectral broaden in nonlinear amplification process.Finally,the high-power SSA-based pulses are compressed to 42fs,which is the shortest pulse duration in current reported high-repetition-rate hectowatt-level fiber laser.3.A high-power femtosecond pulse time-frequency-domain control system is realized to lock repetition frequency?f_r?and carrier envelope phase offset frequency?f₀?of high-average-power femtosecond pulse.Using low-noise detection and phase-locking loop technology,we separately lock the f_r and f₀ of SSA-based high-power ultrashort pulses,and the frequency standard deviations of the locked f_r and f₀ decline to 1.32 and 0.94 mHz.Finally,we achieve a 109W,42 fs broadband fiber OFC with a long-term stability of 8.6×10^-13 and 9.7×10^-13 for f_r and f₀,respectively,which is the first reported SSA-based hectowatt-level fiber OFC.4.We conduct an experiment on nonlinear spectral expanding based on high-power near-infrared femtosecond pilses.Based on high power near-infrared femtosecond pulses,an experiment on nonlinear spectral expanding is carried out.Based on the broadband femtosecond OFC generated by SSA,a⁹ W tunable OFC covering from the 505 to 555 nm is achieved using the nonlinear second harmonic generation.Moreover,using the nonlinear four harmonic generation,a 1W tunable ultraviolet OFC covering from 253.6 to 275.0 nm is realized.