Study And Design Of High-Efficiency Phase-Matching In Nonlinear Frequency Conversions

Manipulation of the crucial phase-matching?PM?condition is the core for efficient nonlinear frequency conversions.However,the physics effects including temperature and dispersion in a nonlinear crystal and the beam divergence will restrict ideal PM condition in practical nonlinear frequency conversions.Particularly,the influences of these effects on the PM condition and conversion efficiency become more and more remarkable with the improvement of the average-power and spectral bandwidth of lasers.This has posed great challenges for high-efficiency frequency conversion.As a result,it is necessary to explore new PM scheme and develop new PM technology for efficient conversion in the case of high average-power and wide spectral bandwidth.Aiming at this goal,we have theoretically and experimentally studied the following four aspects in this dissertation:1.PM optimization in second-harmonic generation with focused vortex beamsVortex beams have different properties of intensity distribution and beam diver-gence compared to an ordinary Gaussian beam.This determines that vortex beams will present new traits in SHG.In Chapter 2,the PM optimization of SHG with fo-cused vortex beams is theoretically studied.We find that,in the noncritical PM condi-tion,the optimal focusing parameter for maximizing SHG efficiency of vortex beams is exactly the same with that predicted by the Boyd and Kleinman?BK?criteria of a Gauss-beam SHG.In contrast,in critical PM condition,the BK criteria is no longer applicable for vortex beams.Owing to the role of the beam divergence,the optimal focusing parameter will decrease with the vortex order for a vortex-beam SHG.Meanwhile,it is necessary to introduce the on-axis phase-mismatch to maximize the SHG efficiency.In CPM condition,the required phase-mismatch should decrease with the vortex order.Moreover,we also study the vortex traits of second-harmonic beam in different PM conditions.2.Temperature-insensitive PM in SHGSo far,high-efficiency SHG in the high average power regime remains a bottleneck problem in the field of laser technology.The reason is:in high average power SHG,the absorption of the interacting waves by the impurity defects results in inhomoge-neous temperature distribution?temperature distortion?in SHG crystals and thus de-stroy the key PM?phase-mismatch distortion?.To improve the SHG efficiency in high average power regime,it is necessary to increase the temperature?acceptance?band-width of SHG first.To solve this problem,in Chapter 3 we innovatively proposed to employ the noncollinear configuration to achieve temperature insensitive PM,which significantly improved the temperature bandwidth of SHG.In the proof-of-principle experiment with a LiB₃O₅?LBO?crystal,the temperature bandwidth of SHG was proved to be as large as 50 K×cm^1/2 in the temperature insensitive PM,which was more than 13 times compared with that in traditional collinear PM.We also studied the performances of the temperature insensitive PM in high average power regime.It is shown that,compared to traditional collinear PM,the temperature insensitive PM is advantageous in both the SHG efficiency and beam quality.3.Temperature and wavelength insensitive PM in optical parametric chirped-pulse amplification Ultrashort intense lasers with peak power exceeding 1 petawatt have be obtained by optical parametric chirped-pulse amplification?OPCPA?.However,in high average power regime,the thermal load caused by absorption of impurities in nonlinear crys-tals will destroy the PM condition of the amplification process,which weakens the ability of the amplifier.To solve this problem,in Chapter 4,we manipulated the PM with the two degrees of freedom of noncollinear configuration and angular dispersion and innovatively proposed a new PM that is insensitive to both temperature and wavelength.In traditional noncollinear OPCPA,the noncollinear configuration was employed for obtaining wavelength insensitive PM.In contrast,in this dissertation,the noncollinear configuration was used to achieve temperature insensitive PM.Be-sides,simultaneous wavelength insensitive PM can be achieved by imposing the seed signal with the angular dispersion.We theoretically and experimentally proved the feasibility of the new PM scheme.In the proof-of-principle experiment based on a LBO crystal,the temperature bandwidth of the OPCPA amplifier can be improved by a factor of 6.In addition,the PM scheme is able to support broadband amplification,which can be used to generate ultrashort pulses with durations less than 20 fs in the near-infrared wavelength range.4.Theoretical design of high average power OPCPA based on temperature and wavelength insensitive PMThe noncollinear configuration is an effective means in manipulating the PM.Pre-viously,the noncollinear configuration was employed for wavelength insensitive PM in OPCPA.We find that noncollinear configuration can also be utilized for tempera-ture insensitive PM.However,generally the noncollinear angles for this two kinds of PM are different,thus it is difficult to achieve temperature and wavelength insensitive PM?TWPM?only by employing noncollinear configuration.To solve this problem,in Chapter 5,the PM can be made simultaneously insensitive to temperature and wave-length only by noncollinear configuration with the overall design of PM parameters such as crystal temperature and signal wavelength.We have achieved“multi parame-ters”design of TWPM for the first time.We theoretically studied the performances of the proposed TWPM scheme in high average power OPCPA by coupling OPCPA with the heat transfer process.It is shown that,compared to traditional wavelength insensi-tive PM,TWPM exhibits significant advantages in conversion efficiency and pulse characteristics.