| The high-power,high-energy femtosecond lasers with narrow pulse width and high peak power are widely used in precision micromachining of materials,clinical medicine,ultrafast biomedical photonics,defense and military applications and other fields.From the perspective of development trend,femtosecond lasers are developing in the direction of integration and miniaturization.Although the traditional solid-state lasers have excellent performance,they have high environmental sensitivity,large volume and poor mobility.In contrast,ultrafast fiber lasers have gradually become a research hotspot,due to its many advantages such as good stability,high integration and strong anti-interference ability.There are many problems appearing with the output power of ultrafast fiber lasers gradually increasing.Firstly,due to the small core diameter of the fiber,various nonlinear effects may appear when the pulses in the fiber are directly amplified,which limits the pulse peak power;secondly,the optics devices are easily damaged with the gradual increase of the peak power.To increase the peak power while reducing the nonlinear effects,the following two technical means are usually adopted:in time domain,the chirped pulse amplification technology is used to reduce the peak power;in space domain,photonic crystal fiber and double-cladding fiber with large mode field area are used as the gain medium of the main amplifier.Combining the above two technical means is a more favorable technical mean to reduce the nonlinear effects and increase the peak power.Simultaneously,it is also the current standard solution for building high-power femtosecond fiber lasers.In addition,it has great significance to achieve high-quality pulse when building high-power and high-energy femtosecond lasers.In this dissertation,we are dedicated to the exploration and development of high-power and high-energy femtosecond laser systems in the 1 μm band.Starting from the theoretical research,according to the actual needs of the laser system,a variety of high-stability modelocked oscillators and grating-based pulse stretchers are experimentally realized.A high repetition frequency and high pulse energy FCPA laser system with a center wavelength of 1035 nm was constructed based on a rod-type photonic crystal fiber.In order to improve the average power and single pulse energy of the laser,a series of research work has been carried out.In addition,the research on high repetition rate(～MHz)and single pulse energy greater than 50μJ femtosecond lasers with a center wavelength of 1064 nm are still in the blank.In this dissertation,a femtosecond laser with a center wavelength of 1064 nm,a repetition rate of 500 kHz,a single pulse energy of 55 μJ is realized for the first time,filling this gap.Finally,a series of research work on nonlinear frequency conversion has been carried out based on the high repetition rate and high pulse energy FCPA laser system in the 1 μm band.The specific research contents of this dissertation are as follows:1.The free-space coupling technology of Gaussian beam between optical fibers,the stimulated Raman effect in fibers,B-integration,etc.were theoretically analyzed,and the design of the cost-effective Martinez-type pulse stretcher was discussed.The above work has laid a theoretical foundation for the design and construction of femtosecond fiber laser systems.2.Several mode-locking oscillators were built by using Yb-doped fiber as the gain medium based on nonlinear polarization-evolution mode-locking technology.Firstly,an all normal dispersion mode-locking oscillator was built with a chip filter with a center wavelength of 1064 nm and a bandwidth of 10 nm as the wavelength tuning element,and realized the wavelength tuning range of 1049.7 nm to 1064.5 nm and 1054.3 nm to 1064.5 nm.Secondly,a dispersionmanaged mode-locking oscillator with a center wavelength of 1030 nm was built by using the grating pair as dispersion control element.The characteristics of the spectral evolution,pulse width variation,etc.were studied.In order to reduce the impact of environmental disturbance on the stability of the mode-locking oscillator,a prototype oscillator was designed and constructed.3.Using the nonlinear amplify loop mirror mode-locking technology,the characteristics of the mode-locking fiber laser with the center wavelength of 1064 nm were studied.The stable mode-locking pulses were obtained by optimizing the mode-locking device in the cavity.Under the optimized conditions,the laser with a pulse width of 1.52 ps,a spectral bandwidth of 14.5 nm and a repetition rate of 51.5 MHz was obtained.4.A high-average-power and high-pulse-energy laser system with a center wavelength of 1035 nm was built based on the chirped pulse amplification technology.The system consists of a mode-locking oscillator based on NPE technology,a pulse menu,a delay unit,a pulse stretcher and compressor based on gratings with high damage threshold and high diffraction efficiency.multi-stage fiber preamplifiers,and a main amplifier based on rod-type photonic crystal fiber.Using an acousto-optic modulator to reduce the repetition rate of the laser system,a high-pulsequality femtosecond laser with a single pulse energy of 61.5 μJ was obtained when the pulse repetition rate was 1 MHz;a laser system with a single-pulse energy of 103 μJ was obtained when the pulse repetition rate was 500 kHz.The integrated and miniaturized design of the tabletop laser system was carried out,and the water path,optical path,circuit and other aspects of the laser were improved and optimized.According to the three-dimensional model of the laser,the construction of the laser system prototype was completed.5.Using a NPE mode-locking oscillator with a spectral bandwidth of 9.5 nm as the seed source,the high-average-power and high-pulse-energy FCPA laser system with the center wavelength of 1064 nm was built.In the main amplifier stage of the laser system,a rod-type photonic crystal fiber with the high gain for 1030 nm was used to conduct power amplification in the center wavelength of 1064 nm.Using a high-power fiber-coupled-output semiconductor laser with a center wavelength of 976 nm as the pump source of the main amplifier stage,the laser systems with single-pass and double-pass amplifier structures can obtained the highest average power output of 7.9 W and 29.3 W,respectively.After compression,the femtosecond laser with a single pulse energy of 45.6 μJ was obtained.Due to the accumulated nonlinear phase shift in the laser system,there was still uncompensated high-order dispersion after the compressor.The base of the pulse was high,and the pulse quality needs to be further optimized.6.Using the NALM mode-locking fiber laser with a spectral bandwidth of～15 nm as the seed source,the high-average-power and high-pulse-energy FCPA laser system with the center wavelength of 1064 nm was built.The narrow-band filter in the laser system was replaced with a large-band of 17 nm fiber filter.The output parameters of the fiber pre-amplification stage were optimized to improve the spectral quality.By finely adjusting the position and incident angle of the grating,the high-order dispersion in the system was fully compensated,and highquality laser with a repetition frequency of 500 kHz,a single pulse energy of 55 μJ,and a shortest pulse width of 336 fs was obtained.The shortest pulse width of 204 fs was obtained under the condition that the dispersion of the laser system was not sufficiently compensated.Due to the limitation of the grating size in the compressor,the spectral components at the edge position do not pass through the grating,and the corresponding compression efficiency is 79.7%.To calculate the actual compression efficiency,the ASE power separated by the first grating of the compressor was taken into account,corresponding to an actual compression efficiency of 82.8%.At the highest output power,the pulse beam quality factor in the horizontal and vertical directions measured by the M2 meter was 1.04 and 1.09,respectively.7.A FCPA system with a central wavelength of 1035 nm and an average power of 50 W was used as the fundamental frequency light source,the BBO crystal was used as the nonlinear frequency conversion medium for the frequency-doubling and frequency-tripling.In the frequency-doubling experiment,when the average power of the fundamental frequency light source was 55 W,a green laser with the repetition frequency of 1MHz,the center wavelength of 517 nm,the maximum average power of 21.1 W,corresponding to the highest frequencydoubling conversion efficiency of 43.3%(the average power of fundamental frequency light source is about 40 W)was obtained.In the frequency-tripling experiment,when the average power of fundamental frequency light source was 55 W,a UV laser output with a repetition frequency of 1 MHz,a center wavelength of 344 nm,a maximum average power of 4.7 W,corresponding to a maximum frequency triple conversion efficiency of 8.5%was obtained. |
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