High Energy Femtosecond Fiber Chirped Pulse Amplification And Transmission Technology For Industrial Processing Applications

Femtosecond laser has the characteristics of ultrashort pulse width,high peak power,lower thermal effect and nearly no microcracks around the processing holes when interaction with the materials,so the femtosecond laser plays an important role in industrial micro-processing field.As the core component of the micro-prosessing equipment,the research and technology development of high power and high energy femtosecond laser has already attracted much attention of the researchers from domestic and overseas,and becomes a focused research topic.In this dissertation,the research work is focused on exploring and developing the high energy femtosecond laser source and the related optical component which are suitable for micro-machining application.The most important work is the research of high energy femtosecond laser amplification technology,including the large mode area photonic crystal fiber amplifier and the single crystal fiber amplifier.Meanwhile,for the urgent needs of ultrafast laser micromachining application,the delivery properties of high energy ultrashort pulse laser through the Kagome type hollow core fiber is studied.We also utilized the nonlinear spectrum broadening when the high energy femtosecond laser delivered in the hollow core fiber to make the exploration of nonlinear ultrashort pulse compression.For the needs of the micromachining application,a high repetition rate ultrashort laser control technology based on electro-optic modulation effect is developed,so the high speed control of laser on and off,even the pulse trains selection is achieved.Through these researches of the technology,we understand the new amplification principle and mastered the amplification technology,some technologies have already been converted to some related products,such as the high energy high power femtosecond laser,high energy femtosecond laser delivery system,high speed laser shutter and high repetition rate pulse trains selection module,and some product is already used in the microprocessing fields.The main content and the innovations are as follows:1.A fiber chirped pulse amplification system based on Yb3+doped fibers is demonstrated.The oscillator is a mode-locked fiber laser by using semiconductor saturable absorber.The chirped pulse amplification technique is applied to stretch the1030 nm pulse to several hundred picosecond to be amplified.The single mode picosecond pulses with average power of 50 W and repetition rates of 211 kHz are achieved by using multistage pre-amplifier consisted with Yb³⁺doped single mode fiber amplifier and double cladding fiber amplifier and the main amplifier with a large mode area rod-typed photonic crystal fiber as the laser gain material.Properly controlled the gain and the nonlinearity in every step of the amplification system,the influence of nonlinear effects are effectively suppressed on the pulse of time domain in high energy amplification.Pulse duration of 887 fs are obtained by using a reflective grating-pair compressor,the single pulse energy of 124?J and peak power of 139.8 MW are achieved.This experimental result of hundred micro joules level femtosecond laser based on fiber structure is firstly reported in China.2.By using the technology of splicing large mode area double cladding fiber with photonic crystal fiber,the high energy ultrashort laser output is achieved.The variations of the spectra and pulse widths as functions of the amplified output power have also been studied.We achieved strictly all-fiber high energy femtosecond laser output.Pulse energy as high as 50?J,pulse width of 933 fs and average power of 10watts are obtained.To our best knowledge,this is the highest energy extracted from the strictly all-fiber chirped pulse amplification system.Due to the all-fiber structure,the laser is environmentally stable with high integrity,which has a good prospect to make a standard laser product.3.The hybrid amplifier technology of fiber-single crystal fiber was developed.By using chirped pulse amplification technology,under the condition of 1.4W@100kHz of the signal power from the pre-amplifier,the amplification output of14.6w is obtained through the one-stage polarization controlled dual-pass amplification structure.The amplification gain is more than 10 times,and the high-beam quality ofM_x²?28?.1233,M_y²?28?.1239is obtained,After compression with reflective grating pairs,pulses with the duration of 859 fs and energy as high as63.5?J are achieved,on the basis of the one stage of single crystal fiber amplifier,we build one more single crystal fiber amplifier.The maximum power of 44 W@100kHz is obtained.To guarantee the beam quality M² is better than 1.3,compressed pulse width of 715 fs with the pulse energy of 155.7?J is obtained,and the average power output was 15.57 W.To our best of knowledge,this is the highest average power of femtosecond laser based on single crystal fiber at hundred micro-Joules energy level.A picosecond laser master oscillator pulse amplification?MOPA?system based on hybrid fiber-single crystal fiber is also experimentally studied,and achieved output average power of 41.7 W@250 kHz,pulse duration of 16.95ps with good beam quality.4.The experiment of high energy ultrashort pulse laser delivery is implemented.we mastered the coupling technology and can make the coupling efficiency more than80%.With different pulse energy input,the output beam quality and pulse width is studied.The nonlinear effect during the delivery is studied,the spectrum evolution is detected.The spectrum is effectively broadened from 10nm to more than 60nm by using the nonlinear effect of the ultrashort laser delivered in the HC-PCF,by using GTI mirrors to compress the pulse,the initial pulse duration of 279fs has been shortened to 36 fs with 20?J of pulse energy,which corresponds to about 556 MW of peak power.To our best knowledge,it is the highest peak power obtained from the solid-state laser at the wavelength around 1?m by using Kagome-type HC-PCF without extra gas fill.For precised dispersion compensation and higher energy input to broadening the spectrum,shorter pulse width and higher peak power can be obtained.We also did a high energy laser delivery experiment,the laser pulse width is234 fs,pulse energy is 100?J and average power is 20 watts,during the delivery,we use the vacuum pump to lower the nonlinear effect,the output beam profile and pulse width nearly has no change,and the system is also successfully used in the micromachining.5.A fiber chirped-pulse amplification system with pulse energy of 105?J,pulse duration of 808 fs at 200-kHz repetition rate is used as the laser source.By using two KTP crystals as the pocels cell,the two crystals are the same and aligned along the laser propagation axes,furthermore,one of the KTP crystals is rotated 90deg relative to the other one.On the one hand,this can compensate the natural birefringence introduced by the KTP crystal,on the other hand,it lowered the half-wave voltage from the 4600 V to 2300 V,which lowered the requirement of the high voltage driver.So we achieved nanosecond level high speed switch control of high energy femtosecond laser at the high repetition rate of 500 kHz,and the efficiency reached more than 96%,this function is indispensible in ultrafast laser micromachining system based on galvanometer scanning.Meanwhile,combining the programmable high voltage with narrow pulse width and the polarization splitting method,we can generate pulse trains with controllable pulse number and pulse splitting without changing the pulse energy,this function is important for exploration on laser processing technology of different materials.