Researches On The New Technology Of The Ultra-short Pulse Fiber-based Amplification System

The ultrashort pulse laser which can be generated by the mode-locking method has very short time duration.Compared with the traditional continuous-wave laser,the interacted processes with materials are ultrafast.Therefore,the pulse leaser is extensively applied to many scientific field such as ultrafast optics,biomedical field,industrial processing and national defense and military.In recent decades,varies techniques about ultrashort pulse laser and ultrashort pulse amplifier have been researched.In many pulse-laser systems,which use the rare-earth doped fiber as the gain medium,are more suitable for many application.The fiber-based pulse laser can make the system more compact and more stabilized.In addition,the beam quality is much better than the solid laser.In this paper,we have researched on some new technologies about the ultrashort pulse fiber-based amplification system which are different from the traditional technology based on the chirp pulse amplification.These technologies include three part: the high power,nanosecond pulse amplification system based on rod-type fiber;the divided pulse amplification system based on coherent beam combine;the nonlinear pulse amplification system.In these systems,two kinds of the rare-earth doped gain fibers,which are ytterbium doped gain fiber and erbium doped gain fiber,are used for the amplification.Through the researches,we learned how to use the rod-type fiber,and studied the new theory about pulse amplification.All these systems are applied in the related productions which have been utilized for the scientific education,radar,distance measurement,laser processing,supercontinuum generation and optical pulse combs.The mainly contents and the novelty are arranged as follows:1.We studied the high power,nanosecond pulse amplification system.The seed laser is a single-longitudinal-mode directly modulated laser diode.The two-stage amplifier have a large-mode-are gain fiber and the new rod-type photonic-crystal gain fiber.Through the adjustment of the pump power supplied for the gain fibers, the noise about amplified spontaneous emission(ASE)and the stimulated Raman scattering(SRS)has been restrained.The characteristics about the output pulse of this system are arranged as follows: the repetition rate is 10 k Hz,the output average power is 10 W,the pulse energy is 1 m J,the pulse duration is 2.2 ns,the peak power is more than 450 k W,the noise about ASE is 25 d B lower than the signal and the noise about SRS is 27 d B lower than the signal.This the first report about the nanosecond pulse amplification system based on the rod-type fiber in China and the millijoule-level pulse energy is the highest one of the nanosecond fiber-based pulse laser in our nation.2.We studied the divided pulse amplification system.This system has a master oscillator power amplifier based on chirp pulse amplification technology.It also has an all fiber-based pulse divider,a single-mode pulse amplifier and the spatial pulse combiner.In order to stabilize the combined pulse,the HC phase detector and the error-signal feed-back system are used.Through the sensitive phase detector and the efficient feed-back system,the divided and amplified pulse is coherently recombined.The system efficiency is higher than 95% and the degree of linear polarization of the recombined pulse is higher than 93%.The chirp characteristics and spectrum of the recombined pulse are as good as the seed laser.The recombined pulse can also be compressed into 1 ps.The beam quality is good and the M2 factor is 1.23×1.27.In addition,the error signal detected both in the situations when the system is locked and unlocked shows that the error of the optical path difference can be controlled in the range of 5 nm.The usage of all fiber-based divider and the fixed spatial combiner in divided pulse amplification is first reported by us.And this is the first studies about the divided ultra-short pulse amplification and ultra-short pulse cohenet combination in China.3.We studied the nonlinear pulse amplification system.Through the theoretical analysis,the characteristics of dispersion and nolinearity are studied.Through the theoretical the numerical simulation,an all polarization-maintained(PM)singlemode femtosecond pulse nonlinear amplification system is designed and constructed.This system has an all PM fiber-based mode-lock pulse laser whichis used as seed laser.The amplifier has two-stages PM single-mode erbium-doped gain fiber.The dispersion and the nonlinearity of each parts are analyzed carefully.The spectrum of the pulse is broaden and the dispersion of the pulse is compensated.Therefore,the seed pulse is nonlinear amplified.The characteristics about the output pulse of this system are arranged as follows: the repetition rate is 200 MHz,the average power is 320 m W,the pulse energy is 1.6 n J,the pulse duration is 34 fs,the peak power is 23.5 k W.In addition,this pulse can be utilized for the supercontinuum generation.In this work an octavespanning supercontinuum is generated.The short wavelength of the supercontinuum is shorter than 1.01?m and the long wavelength is longer than 2.02?m.This system use the gain fiber as the pulse pre-shaper which is different from the the traditional pulse nonlinear amplification system.The grating-pairs which used as compressor are replaced by the single-mode fiber.All this realized the realy all-fiber system and the tiny system.The 34 fs pulse duration is the shortest one in the field of the PM single-mode fiber pulse amplification system.