| With the rapid development of ultrashort pulse technology,femtosecond laser technology has attracted extensive attention in basic fields such as physics,biology and chemistry,as well as in the fields of frequency comb spectroscopy,precision metrology and micromachining.However,practicality has always been the most important development direction of femtosecond laser technology.Nowadays,there is a new practical demand for ultrashort laser pulses,that is,how to make the laser output low repetition frequency,high pulse energy and narrow pulse width pulse.At present,in order to obtain a high-energy ultra-short output pulse,a double-clad gain fiber or a photonic crystal fiber can be added to the laser cavity as a gain fiber to increase the pulse energy.However,compared with the conventional single-mode doped gain fiber,the above two methods have the disadvantages of difficulty in fabrication,poor stability,and complicated influence factors.Therefore,this paper adopts single-mode polarization-maintaining erbium-doped fiber as the gaim fiber,and increases the cavity length of the laser to reduce the repetition frequency and increase the pulse energy.Finally,the laser directly outputs the low-frequency high-energy pulse.This direct output method The complexity of the system is reduced,and the manufacturing cost of the laser is also reduced.Based on this,the dispersive and nonlinear effects caused by increasing the cavity length are theoretically studied,and an all-normal Dispersion(ANDi)passive mode-locked fiber laser based on nonlinear amplifying ring mirror is built.Femtosecond pulses with low repetition rate and high pulse energy are obtained.The main research contents and results of this paper are as follows:1.The background and significance of low-repetition-frequency high-energy mode-locked fiber lasers are briefly described,and the current research progress and research status at home and abroad are summarized.2.The spectral characteristics,energy and structure of the erbium ions,and the principle of luminescence are described,which leads to the advantage of using erbium ions as a gain medium to output laser pulses at 1550 nm.Then,the structure,mode-locking principle,advantages and disadvantages of various mode-locked fiber lasers are analyzed.Therefore,the nonlinear magnifying ring mirror is taken as the advantage of the mode locking mode.3.The influence of dispersion and nonlinear effects on pulse transmission in fiber is analyzed.The basic theoretical derivation of nonlinear amplification environment and the theory of chirped pulse compression are described in detail.The MATLAB simulation of the full positive dispersion passive mode-locked fiber laser is carried out.Therefore,the NALM-based full positive dispersion erbium-doped mode-locked fiber laser designed in this paper is derived.4.In the experiment,a long-cavity passive mode-locked fiber laser with a cavity length of nearly 100m was built,and a laser output pulse with a repetition frequency of 1.96MHz,a stable single pulse energy of 4nJ and a pulse width of 2.5ps was obtained.Thepolarization-maintaining single-mode fiber further compresses the pulse to 660fs,which is close to the conversion limit,and measures the average power fluctuation within 1O0 in about 0.17%. |
