| Intense ultrashort laser gives rise to femtochemistry,femtosecond frequency comb,femtosecond laser processing,ultrafast spectroscopy,strong-field physics,etc and boosts the development of physics,chemistry,biology and material science,etc.Currently,the available intense ultrashort laser is mainly at near-infrared wavelength of 0.8-1.5?m,which proves to be a powerful tool for scientific research.The research of mid-infrared intense ultrashort laser can further broaden the application of intense ultrashort laser in areas of spectroscopy,high energy X-ray,attosecond physics and so on.Thus intense ultrashort laser technology in mid-infrared becomes an active research topic.There are three high transparency atmospheric windows in mid-infrared region.Most organic and gas molecules have strong fundamental absorption lines in mid-infrared region.In the strong-field physics long wavelength ultrashort laser is superior than short wavelength ultrashort laser.Besides,mid-infrared ultrashort laser has many ap-plications in spectroscopy,medical science,national defense,strong-field physics and so on.However,laser technology of mid-infrared lags behind that of near-infrared.In-novative mid-infrared laser technology is required to improve the present mid-infrared laser technology.The research of this dissertation is mainly about nonlinear generation and amplifi-cation of mid-infrared ultrashort pulse laser.Systematic work regarding new physical mechanism,new design of mid-infrared laser and experimental research is studied.The main innovation points of this dissertation are as follows:?1?We demonstrate,for the first time to our knowledge,dissipative soliton can be formed in ultrashort pulse optical parametric oscillator?OPO?and systematically study the formation mechanism of dissipative soliton.The parametric conditions for formation of dissipative soliton in OPO are concluded.Only if gain,dispersion and third-order nonlinearity satisfy certain conditions can signal pulse evolve into linearly-chirped dissipative soliton.The balance between gain and loss plays an important role for dissipative soliton formation in OPO.The parametric conversion from pump to sig-nal contributes to signal gain.Because the gain of signal is time dependent,the gain is limited in a certain temporal and spectral width.With accumulation of nonlinear phase-shift signal spectrum is broadened continuously with larger third-order nonlinearity in OPO cavity.When signal spectrum broadened by nonlinear phase-shift exceeds the spectral width of gain,gain filtering effect occurs,which leads to signal pulse energy dissipation.The formation of dissipative soliton requires gain and loss that governed by nonlinearity and dispersion get balanced in OPO.Therefore,linearly-chirped signal pulse with broad bandwidth can be generated by setting special gain,loss,nonlinearity and dispersion in OPO cavity.We call this kind of optical parametric oscillator the dissipative OPO.?2?We confirm that ultrashort pulses,even pulses with few-cycle duration at mid-infrared wavelengths,can be generated from high power and long pulse pumped dissi-pative OPO,which provides a new technology for improving the peak power of ultra-short pulse mid-infrared OPO.Signal pulse in OPO cavity can be evovle into linearly-chirped pulse with broad bandwidth in dissipative soliton OPO under certain para-metric conditions.Chirped signal pulse can be compressed to ultrashort femtosecond regime through dechirping outside cavity.Moreover,the output idler pulse from dissi-pative OPO is also linearly-chirped and can be compressed to femtosecond regime.The parametric conditions of dissipative OPO,characteristic of soliton and compression property are systematically studied by numerical simulation based on the governing equations of OPO.Dissipative soliton can be formed in OPO with positive or nega-tive dispersion so long as the certain parametric conditions are satisfied.We find both Kerr nonlinearity of material and cascade second-order nonlinearity can provide the required third-order nonlinear phase-shift for dissipative OPO.Using cascade second-order nonlinearity in nonlinear crystal can simplify the design of dissipative OPO.?3?We experimentally confirm,for the first time,that dissipative soliton can be generated in ultrashort pulse OPO and high-ratio pulse compression is achieved.Mid-infrared gaintly-chirped dissipative soliton is generated in OPO cavity with adopting high-power picosecond pumping system and inserting ZnSe with high nonlinearity into cavity as third-order nonlinear material.Dechirped outside cavity a mid-infrared pulse as short as?126 fs is realized.We also experimentally confirm that dissipative soli-ton can be formed in OPO with cascaded nonlinearity providing nonlinear phase-shift.Under the cascaded nonlinearity of second harmonic generation of signal in PPLN,mid-infrared dissipative soliton is generated with pump duration of 2.5 ps.By compen-sating second-order dispersion outside OPO cavity,a mid-infrared pulse with duration of 208 fs is obtained.Our simulation results are agree well with experimental results,which proves the reliability of our dissipative soliton OPO theory.?4?Based on the available experimental situation we design 100-TW-level mid-infrared laser system,which theoretically proves the practicability of building 100-TW-level mid-infrared laser system.The 100-TW-level mid-infrared laser system is based on two-stage optical parametric chirped pulse amplifier?OPCPA?.Mid-infrared seed pulse is generated by femtosecond optical parametric amplifier?OPA?.100-TW-level mid-infrared laser centering at 2.2?m is expected to be generated in two-stage OPCPA based on LiNbO3crystal.The choice of signal wavelength,design of nonlinear crystal,optimization of amplified signal bandwith,tolerance of time jitter and pump intensity fluctuation are systemically considered in the OPCPA system.The theoretical scheme based on the OPCPA in this dissertation paves the way for building a 100-TW-level mid-infrared laser system. |
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