Research On High Power Ultrafast Fiber Laser And Pulse Measurement

The ultrafast fiber laser with high power is one of the most important topics in the field of ultrafast laser.Compared with conventional continuous fiber lasers,it has higher peak power,narrower pulse width output,and wider spectral range.Compared with the all-solid-state laser,it has better thermal performance,more compact packaging design,lower manufacturing cost,and better beam quality.So it has a wide range of applications in precision processing,optical comb,attosecond science,medical detection,military and defense.In this paper,a series of researches are carried out on the construction of a high-power ultrafast fiber laser system and the control of the nonlinear effect of the fiber,which further improves the power output of the ultrafast fiber laser,reduces the pulse width and improves the pulse energy.On the other hand,the design and development of the ultrafast pulse measuring device has the advantage of wavelength insensitivity.At the same time,the experimental research on the generation of mid-infrared ultrabroad spectrum is carried out,which has the characteristics of carrier envelop phase passively locked.The main research contents and innovative results of this paper are as follows:1.Firstly,the advantages and disadvantages of several fiber optic mode-locking technologies were discussed in principle,and then a set of spatial ytterbium-doped fiber mode-locking oscillator based on nonlinear polarization rotation technology was built.By adjusting the important parameters such as the distance and angle of the diffraction grating in the resonant cavity,the total dispersion in the cavity was controlled,and the parameters of each element in the cavity,including the length of gain fiber and the bandwidth of WDM,were optimized.The average output power of the pulse was 35 mW.The repetition frequency was 49.5 MHz.The central wavelength was 1035 nm.The spectral half-height and width were 20 nm.The pulse width was 4 ps without external compression.The optical fiber oscillator could stably mode-lock for more than six months,showing excellent anti-interference ability and high tolerance to environmental temperature,humidity and other external influences,which was very suitable for the seed source of high power ytterbium doped fiber amplifier.Based on the nonlinear polarization rotation technique,a spatial erbium-doped fiber mode-locking oscillator was constructed.There was no dispersion control element in the oscillation chamber.The mode-locking pulse was of soliton type,and the spectrum has obvious Kelly sideband structure,with a repetition frequency of 41.8 MHz.The oscillator was compact and has self-start mode-locking function.At the pump power of 260 mW,the average output power of the space was 15 mW,and the output power of the 20% fiber coupler was 10 mW.Without external cavity compression,the measured pulse width was 305 fs.At present,it is used for the passive synchronization experiment of ytterbium and erbium oscillator,and can be used for the subsequent experimental study of the mid-infrared differential frequency of the carrier envelope phase locking with the dual-optical comb system.2.In order to obtain high power pulse output,the self-built ytterbium-doped fiber oscillator was used as the seed source to carry out the study of fiber chirped pulse amplification.A 20 m passive polarization-maintaining optical fiber was used as a stretcher.The pulse from 4 ps was broaden to 21 ps.By using single mode fiber amplifier and double cladding fiber amplifier,the pulse power was amplified to an average of 6 W.By using a pair of diffraction grating of 1000 lines/mm,pulse was compressed to 110 fs with average power of 5 W.To further pursue shorter pulse output and higher power,pre-chirp management amplification technology was used for subsequent amplification.Firstly,a pair of diffraction gratings before the main amplification were reserved to adjust the dispersion.Combined with the large mode field rod-type photonic crystal fiber,the pulse spectrum was broadened while the power was amplified.The power was amplified to 49 W.Then,the pulse was compressed to 61 fs using a 1000 line /mm transmission grating pair.In order to improve the compression efficiency and pulse beam quality,a scheme was proposed to replace the grating pair with a highly dispersive chirped mirror.Finally,the pulse was compressed to 55 fs under 6 pieces of chirped mirrors.In order to further improve the pulse energy of the fiber laser and avoid the self-focusing effect of the fiber,the nonlinear intensity in the fiber is reduced to 2/3 by circular polarization amplification technology.After pumping was increased,the pulse power was further increased.Finally,under the condition of 6-piece chirped mirror compression,the output result of average power of 85 W and pulse width of 42 fs was obtained.The corresponding pulse energy exceeded 1.5 μJ,and the pulse compression efficiency was greater than 99%.3.Firstly,single autocorrelation measurement was described in principle.Then,a single autocorrelation measurement device was designed and developed independently.At present,preliminary test results have been obtained,and the construction and improvement of commercial prototype are still under further study.On the other hand,a frequency resolved optical gating measuring device based on transient grating was built.The measuring device was a simplified upgrade of the traditional optical path,which could be used for ultrafast laser pulse measurement of ultrabroad spectrum pulses in different bands below 10 fs.And the information of pulse width in time domain and phase in frequency domain can be obtained simultaneously.In order to verify its practicability,the device had been used to measure the pulse width of Ti:sapphire laser amplifier,second harmonic generation,and supercontinuum spectrum,and compared with commercial pulse diagnostic instrument SPIDER and interference autocorrelator.4.Based on the supercontinuum experiment of solid thin plate generation,the study of midinfrared wide spectrum pulse output generated by deviation frequency was carried out for the first time in the world.The near-octave mid-infrared spectrum covering 1300~2500 nm was successfully obtained.The laser pulse can be used as a seed source for optical parametric chirped pulse amplification and further exploration of high energy single attosecond.At the same time,the results of spectral evolution and efficiency change of frequency transformation process were calculated numerically,which can be used to guide the experiment.