Study On Ultra-Long Cavity Mode-Locked Erbium-Doped Fiber Lasers

Ultra-long cavity passive mode-locked fiber laser has many advantages.It can not only achieve ultra-low pulse repetition rate operation but also achieve higher single pulse energy with the same average power.Ultra-short pulse with low repetition frequency(tens of k Hz)and high energy is widely used in the fields of micromachining,eye surgery,biomedical diagnosis,lidar,etc.which is one of the hotspots of research at home and abroad.Fiber loss is reduced with the increasing sophistication of fiber manufacturing technology,and ultra-long cavity fiber laser has become a reality.In this paper,the experimental research and theoretical analysis of long-cavity and ultra-long-cavity Erbium-doped fiber lasers based on nonlinear polarization rotation(NPR)mode-locking are carried out.The specific contents are as follows:1)The discovery and development of laser and laser machines are described generally.The ultra-long cavity fiber lasers are simply classified according to the cavity structure,gain medium,doping element,and mode-locking mechanism.The current development of ultra-long cavity Raman fiber lasers and ultra-long cavity doped fiber lasers is introduced in detail.2)A long-cavity erbium-doped fiber laser based on NPR mode-locked was experimentally built to study the operation mode of multi-soliton in the cavity.By keeping the polarization state in the cavity unchanged but changing and controlling the pump power to adjust the polarization state in the cavity,the switch from single pulse to multiple pulses was realized.In addition,the mode-locked dual-wavelength operation was also achieved.3)The ring erbium-doped fiber lasers with different cavity lengths were built experimentally to study the influence of cavity length and pump power on NLP’s maximum spectrum width,respectively.The results show that the maximum spectral width of NLP decreases with the increase of the cavity length for a given pump power.For a fixed cavity length,the maximum spectral width of NLP increases with the increase of pump power.4)An ultra-long cavity erbium-doped fiber laser was built based on ultra-long single-mode fiber(SMF)to study the ultra-low repetition rate NLP and its supercontinuum spectrum.By increasing the length of the SMF,an ultra-low repetition frequency NLP with a minimum of 10.1 k Hz was obtained.After amplification by an erbium-doped fiber amplifier(EDFA),the final output of single pulse energy can reach4.44 m J.In addition,the NLP supercontinuum spectrum with a 10 d B spectral width of 160 nm and spectral coverage from 1525 nm to 1685 nm was obtained.