阅读说明：本技术 全立体空间相位匹配提升宽带opcpa性能的方法 (Method for improving broadband OPCPA performance by full-three-dimensional spatial phase matching ) 是由 陈丽明 於亮红 梁晓燕 李进峰 王傲天 徐至展 于 2021-02-02 设计创作，主要内容包括：本发明涉及一种全立体空间相位匹配提升宽带OPCPA性能的方法,其特征在于通过在双轴非线性晶体全立体空间内求解最优相位匹配角度,获得非线性光参量啁啾脉冲放大(OPCPA)超宽的增益带宽和较高的有效非线性系数,实现最优化的相位匹配及OPCPA过程,同时该方法也拓展了非线性晶体OPCPA支持的波段范围。本发明适用于高重频OPCPA激光装置,也适用于数拍瓦级激光装置的主放大器。(The invention relates to a method for improving broadband OPCPA performance by full-three-dimensional spatial phase matching, which is characterized in that an optimal phase matching angle is solved in a double-shaft nonlinear crystal full-three-dimensional space to obtain ultra-wide gain bandwidth and high effective nonlinear coefficient of nonlinear Optical Parametric Chirped Pulse Amplification (OPCPA), so that the optimized phase matching and OPCPA processes are realized, and meanwhile, the method also expands the band range supported by the nonlinear crystal OPCPA. The invention is suitable for high repetition frequency OPCPA laser devices and is also suitable for the main amplifier of a laser device with several beat wattage.)

1. A method for improving broadband OPCPA performance by full-stereo spatial phase matching is characterized by comprising the following steps:

step 1, setting a preset non-collinear angle alpha;

step 2, pumping light wavelength lambda_pCentral wavelength lambda of signal light_s0And idler center wavelength λ_i0The condition of wave vector matching is satisfied in the nonlinear crystal:wherein wave vector k_p＝2πn_p/λ_pLetter ofWave vector k of central wavelength of light_s0＝2πn_s0/λ_s0Idler light center wavelength wave vector k_i0＝2πn_i0/λ_i0，n_pIs the refractive index of the pump light, n_s0Is the central wavelength λ of the signal light_s0Corresponding refractive index, n_i0For the idler center wavelength λ_i0Corresponding refractive index satisfying 1/lambda_p＝1/λ_s0+1/λ_i0Calculating the pump light λ_pCentral wavelength lambda of signal light_s0And idler center wavelength λ_i0Phase matching angle group (theta)_i,φ_i) 1., where i is a group of phase matching angles (θ)_i,φ_i) Denotes the wave vector k of the pump light_pIn the spatial direction inside the crystal, θ represents the pump light wave vector k_pIn the direction of the Z axis of the crystal, phi represents the pumping light wave vector k_pThe projection on the XOY plane forms an included angle with the X axis;

step 3, according to the effective nonlinear coefficient d of the nonlinear crystal_effAnd phase matching angle group (theta)_i,φ_i) Calculating an effective nonlinear coefficient d_effWith phase matching the angular group (theta)_i,φ_i) The variation curve and the value of (a);

step 4, according to wave vector mismatch quantityCalculating the phase matching angle group (theta) in the full spectrum range of the signal light_i,φ_i) Wherein the broadband signal light wave-vector k_s＝2πn_s/λ_sBroadband idler wavevector k_i＝2πn_i/λ_iSatisfy 1/lambda_p＝1/λ_s+1/λ_iWherein λ is_sAnd n_sRespectively, the broadband wavelength of the signal light and the corresponding refractive index, lambda_iAnd n_iBroadband wavelength of idler frequency light and corresponding refractive index, when the central wavelength of signal light is near, from lambda_i1To lambda_i2Within a wavelength band of (lambda)_i2≥λ_i1) When all the wavelengths satisfy | Δ k | ≦ pi, Δ λ ═ λ_i2-λ_i1Is thatGain bandwidth, calculating the angle group (theta) of the signal light along with the phase matching under the condition that the crystal length is L_i,φ_i) A varying gain bandwidth Δ λ;

step 5, comparing the effective nonlinear coefficient d obtained in the step 3_effAnd step 4, judging whether the gain bandwidth delta lambda meets the OPCPA requirement or not:

when the gain bandwidth delta lambda does not meet the bandwidth required by the OPCPA, returning to the step 1) to reselect the initial non-collinear angle alpha;

when the gain bandwidth delta lambda meets the bandwidth required by the OPCPA, the step 6) is carried out;

step 6, judging the effective nonlinear coefficient d_effAnd whether the gain bandwidth Δ λ is an optimal value:

when effective nonlinear coefficient d_effWhen the sum gain bandwidth Delta lambda is the optimal value, the non-collinear angle alpha at the moment is selected₀And phase matching angle (theta)₀,φ₀) For an optimum non-collinear angle alpha₀And an optimum phase matching angle (theta)₀,φ₀) (ii) a Otherwise, returning to the step 1).

2. The method for improving broadband OPCPA performance through full-stereo spatial phase matching according to claim 1, wherein the non-collinear angle α in step 1) is an included angle between the pump light and the signal light inside the crystal, and is 0 ° α or more and 5 ° or less.

3. The method for improving broadband OPCPA performance by full stereo spatial phase matching according to claim 1, wherein the phase matching angle group (θ) is calculated in step 2)_i,φ_i) The method comprises the following specific steps:

step 2.1, setting a preset non-collinear angle alpha and a pumping light wavelength lambda_pCentral wavelength lambda of signal light_s0And idler center wavelength λ_i0The condition of wave vector matching is satisfied in the nonlinear crystal:and satisfies 1/lambda_p＝1/λ_s0+1/λ_i0；

Step 2.2, according to the refractive index equation of the biaxial nonlinear crystal Where j is (x, y, z) and represents three principal axes, λ is the wavelength, A, B, C, D, E is the coefficient, and for a general biaxial crystal, the wavelength λ of the pump light can be found in the open literature_pCentral wavelength lambda of signal light_s0And idler center wavelength λ_i0Refractive index n of nonlinear crystal in three principal axes, X axis_xRefractive index n of Y-axis_yZ-axis refractive index n_z；

Step 2.3, according to the formula, the refractive index of the main shaftWherein The refractive index n of the pump light can be obtained_pRefractive index n of signal light_s0And idler refractive index n_i0；

Step 2.4, according to the wave vector matching condition:and satisfies 1/lambda_p＝1/λ_s0+1/λ_i0And pump light refractive index n_pRefractive index n of signal light_s0And idler refractive index n_i0Determining a phase matching angle group (theta)_i,φ_i)。

4. The all-dimensional spatial phase-matching lifting wideband of claim 1Method for OPCPA performance, characterized in that it further comprises a step 7) of determining the optimal non-collinear angle α₀And an optimum phase matching angle (theta)₀,φ₀) Cutting the crystal, specifically:

establishing a rectangular coordinate system by taking the central point of the crystal as an original point O and the crystal axis as a Z axis, and finding an included angle phi between the central point and the X axis on the XOY plane of the crystal₀And a plane P1 perpendicular to the XOY plane, and a through-origin and an angle theta with the Z axis are found from the plane P1₀The straight line L1 of (1) is a plane P2 perpendicular to the plane P1 with the straight line L1, and the plane P2 is a cut plane.

Technical Field

The invention relates to the technical field of ultrashort pulses, in particular to a method for improving broadband OPCPA performance by full-three-dimensional spatial phase matching.

Background

The nonlinear Optical Parametric Chirped Pulse Amplification (OPCPA) technology is to realize a beat (PW: 10)¹⁵W) level or even hundred PW level amplification. With the development of laser technology and laser material technology, the output power of ultrashort laser becomes higher and higher, and more extreme conditions can be provided for laser physical experiments.

Because the OPCPA technology can not generate parasitic oscillation effect in the amplification process, and has the performances of high signal-to-noise ratio, low system B integral, wide gain bandwidth, high one-way gain and the like, the large-caliber high-energy OPCPA is expected to break through hundred PW (watt) and even Egwatt (EW: 10)¹⁸W) amplification. In 2015, Shanghai optical precision mechanical research institute achieved high-energy OPCPA amplified output of 1PW based on LBO crystals of 100mm x 100 mm; in 2017, the eight departments of the middle courtyard realizes the broadband amplification output of 4.9PW based on two cascaded LBO crystals, and the OPCPA technology is verified to realize the super-strong ultrashort laser pulse amplification of beat-watt or even several beat-watt level.

In current OPCPA systems, the vast majority of OPAs employ principal plane (XOZ, YOZ, XOY plane) class I phase matching modes. For example, in an 800nm broadband OPCPA amplification process based on an LBO crystal, the phase matching angle is 90 degrees, 13.85 degrees, and the phase matching is performed on an XOY plane; the YCOB crystal-based OPCPA phase matching angle is 26.5 °, 180 ° or 0 °, and the phase matching is on the XOZ plane. The phase matching mode in the main plane has the advantages of simple calculation and crystal cutting, low experimental adjustment requirement, small walking angle per unit length and the like, but for some biaxial crystals, the main plane matching is not necessarily the only matching mode. Due to the limitation of a special angle (theta or phi is 0 degrees, 90 degrees and 180 degrees), the supportable gain bandwidth and the effective nonlinear coefficient are not at the maximum, and the optimal phase matching can be searched in the whole stereo space range of the non-principal plane so as to obtain the maximum bandwidth and the maximum effective nonlinear coefficient supported by the nonlinear crystal and realize the optimal phase matching mode.

The invention relates to a special corner cut calcium oxide borate rare earth salt laser frequency doubling crystal [ patent number: 99112260.7, describes that the best frequency doubling phase matching is realized by YCOB crystal under special angle (theta is 65.9 degrees plus or minus 5 degrees, phi is 36.5 degrees plus or minus 5 degrees or 66.3 degrees plus or minus 5 degrees, phi is 143.5 degrees plus or minus 5 degrees) in the three-dimensional space, and solves the problem of low frequency doubling conversion efficiency at that time. However, the stereo space angle given by the patent is only suitable for the frequency doubling angle of the YCOB crystal from single wavelength (1064nm) to 532nm, and cannot solve the problem of solving the optimal phase matching angle of the OPCPA in the stereo space of the biaxial crystal.

Disclosure of Invention

The invention aims to break through the limit of phase matching of a traditional nonlinear crystal on a main plane to a broadband OPCPA (optical phase locked amplification), improve the output performance of the broadband OPCPA and provide a method for improving the performance of the broadband OPCPA by phase matching in a full three-dimensional space.

In order to achieve the purpose of the invention, the technical solution of the invention is as follows:

the optimal phase matching angle is solved in the full three-dimensional space of the double-shaft nonlinear crystal, the ultra-wide gain bandwidth and the high effective nonlinear coefficient of the nonlinear optical parameter chirped pulse amplification are obtained, the optimized phase matching and OPCPA process are realized, and the wave band range supported by the nonlinear crystal OPCPA is expanded. A method for improving broadband OPCPA performance by full-three-dimensional spatial phase matching comprises the following specific steps:

step 1, setting a pre-set non-collinear angle alpha, wherein the non-collinear angle alpha is an included angle of pump light and signal light in a crystal, and is generally more than or equal to 0 degree and less than or equal to 5 degrees;

step 2, pumping light wavelength lambda_pCentral wavelength lambda of signal light_s0And idler center wavelength λ_i0The condition of wave vector matching is satisfied in the nonlinear crystal:wherein wave vector k_p＝2πn_p/λ_p，k_s0＝2πn_s0/λ_s0,k_i0＝2πn_i0/λ_i0，n_pIs the refractive index of the pump light, n_s0Is the central wavelength λ of the signal light_s0Corresponding refractive index, n_i0For the idler center wavelength λ_i0Corresponding refractive index satisfying 1/lambda_p＝1/λ_s0+1/λ_i0Calculating the pumping light lambda_pCentral wavelength lambda of signal light_s0And idler center wavelength λ_i0Phase matching angle group (theta)_i,φ_i) 1., where i is a group of phase matching angles (θ)_i,φ_i) Denotes the wave vector k of the pump light_pIn the spatial direction inside the crystal, θ represents the pump light wave vector k_pIn the direction of the Z axis of the crystal, phi represents the pumping light wave vector k_pThe projection on the XOY plane forms an included angle with the X axis;

step 3, according to the effective nonlinear coefficient d of the nonlinear crystal_effAnd phase matching angle group (theta)_i,φ_i) The relationship (d) can be found in the open literature for a general biaxial crystal, and the effective nonlinear coefficient d can be calculated_effWith phase matching the angular group (theta)_i,φ_i) The variation curve and the numerical distribution of (2); wherein the effective nonlinear coefficient d_effIs (theta)_i,φ_i) The functions of (A) and (B) are different from each other, and the functional relationship of different crystals can be found from the crystal handbook (handbook of nonlinear optical crystals, German Risper et al, Wang-cong Yangyi, higher education Press).

Step 4, according to wave vector mismatch quantityCalculating the phase matching angle group (theta) in the full spectrum range of the signal light_i,φ_i) Wave vector mismatch of (a), wherein the wave vector k_s＝2πn_s/λ_s，k_i＝2πn_i/λ_iSatisfy 1/lambda_p＝1/λ_s+1/λ_iWherein λ is_sAnd n_sRespectively, the broadband wavelength of the signal light and the corresponding refractive index, lambda_iAnd n_iBroadband wavelength of idler frequency light and corresponding refractive index, when the central wavelength of signal light is near, from lambda_i1To lambda_i2Within a wavelength band of (lambda)_i2≥λ_i1) When all the wavelengths satisfy | Δ k | ≦ pi, Δ λ ═ λ_i2-λ_i1That is, the gain bandwidth, the angle group (θ) of the signal light along with the phase matching under the length L of the crystal can be calculated_i,φ_i) A varying gain bandwidth Δ λ;

step 5, comparing the effective nonlinear coefficient d obtained in the step 3_effAnd step 4, judging whether the gain bandwidth delta lambda meets the OPCPA requirement or not:

when the gain bandwidth delta lambda does not meet the bandwidth required by the OPCPA, returning to the step 1) to reselect the initial non-collinear angle alpha;

when the gain bandwidth delta lambda meets the bandwidth required by the OPCPA, the step 6) is carried out;

step 6, judging the effective nonlinear coefficient d_effAnd whether the gain bandwidth Δ λ is an optimal value:

when effective nonlinear coefficient d_effWhen the sum gain bandwidth Delta lambda is the optimal value, the non-collinear angle alpha at the moment is selected₀And phase matching angle (theta)₀,φ₀) For an optimum non-collinear angle alpha₀And an optimum phase matching angle (theta)₀,φ₀) (ii) a Otherwise, returning to the step 1).

Further, the non-collinear angle alpha in the step 1) is an included angle between the pump light and the signal light in the crystal, and is more than or equal to 0 degree and less than or equal to 5 degrees.

Further, the phase matching angle group (θ) is calculated in step 2)_i,φ_i) The method comprises the following specific steps:

step 2.1, setting a preset non-collinear angle alpha and a pumping light wavelength lambda_pCentral wavelength lambda of signal light_s0And idler center wavelength λ_i0The condition of wave vector matching is satisfied in the nonlinear crystal:and satisfies 1/lambda_p＝1/λ_s0+1/λ_i0；

Step 2.2, according to the refractive index equation of the biaxial nonlinear crystal Where j is (x, y, z) and represents three principal axes, λ is the wavelength, C, D, E, F, G is the coefficient, and for a general biaxial crystal, it can be found in the open literature that the pump light wavelength λ can be found_pCentral wavelength lambda of signal light_s0And idler center wavelength λ_i0Refractive index n of nonlinear crystal in three principal axes, X axis_xRefractive index n of Y-axis_yZ-axis refractive index n_z；

Step 2.3, according to the formula, the refractive index of the main shaftWherein The refractive index n of the pump light can be obtained_pRefractive index n of signal light_s0And idler refractive index n_i0；

Step 2.4, according to the wave vector matching condition:and satisfies 1/lambda_p＝1/λ_s0+1/λ_i0And pump light refractive index n_pRefractive index n of signal light_s0And idler refractive index n_i0The phase matching angle group (theta) can be obtained_i,φ_i)。

The invention also comprises a step 7) of determining the optimal non-collinear angle alpha₀And an optimum phase matching angle (theta)₀,φ₀) Cutting the crystal, specifically:

establishing a rectangular coordinate system by taking the central point of the crystal as an original point O and the crystal axis as a Z axis, and finding an included angle phi between the central point and the X axis on the XOY plane of the crystal₀And a plane P1 perpendicular to the XOY plane, and a through-origin and an angle theta with the Z axis are found from the plane P1₀The straight line L1 of (1) is a plane P2 perpendicular to the plane P1 with the straight line L1, and the plane P2 is a cut plane.

Compared with the prior art, the invention has the following technical effects:

1) the gain bandwidth of the broadband OPCPA is improved, compared with the gain bandwidth of 130nm obtained by phase matching of the main plane, the gain bandwidth of 167nm can be obtained by phase matching of the full three-dimensional space, and the band range supported by the nonlinear crystal OPCPA is expanded;

2) compared with the phase matching effective nonlinear coefficient of a main plane, the effective nonlinear coefficient of the broadband OPCPA is 1.1pm/V, and the effective nonlinear coefficient obtained by the phase matching of the full three-dimensional space is 1.708 pm/V;

3) the main amplifier is suitable for a high repetition frequency OPCPA amplifying laser device and a main amplifier of a laser device with several watt levels.

Drawings

FIG. 1 is a flow chart of a method for improving the performance of a broadband OPCPA by stereo spatial phase matching according to the present invention;

FIG. 2 is a diagram of the relationship between the nonlinear crystal non-principal plane phase matching support bandwidth, effective nonlinear coefficient and phase matching angle;

Detailed Description

The invention is described in detail below with reference to the drawings and specific examples.

The invention breaks through the limitation of the traditional nonlinear crystal on the phase matching broadband OPCPA of the main plane, improves the output performance of the broadband OPCPA, and provides a method for improving the performance of the broadband OPCPA by full three-dimensional space phase matching, which comprises the following specific steps:

step S1, based on the YCOB crystal, selecting an included angle alpha between the pump light and the signal light in the crystal, wherein the selected range is generally between 0 degrees and 5 degrees;

step S2, according to the wavelength lambda of the pump light_p527nm, center wavelength λ of signal light_s0800nm and idler center wavelength λ_i01603nm satisfies the condition of wave vector matching within the nonlinear crystal YCOB:and satisfies 1/lambda_p＝1/λ_s0+1/λ_i0The pump light lambda can be calculated_pCentral wavelength lambda of signal light_s0And idler center wavelength λ_i0Phase matching angle group (theta)_i,φ_i) Wherein i 1.;

step S3, according to the effective nonlinear coefficient d of the nonlinear crystal_effAnd phase matching angle group (theta)_i,φ_i) Wherein the effective nonlinear coefficient d can be calculated for a general biaxial crystal which can be found in the open literature_effWith phase matching the angular group (theta)_i,φ_i) The variation curve and the numerical distribution of (2);

step S4, according to wave vector mismatch quantityCalculating the phase matching angle group (theta) in the full spectrum range of the signal light_i,φ_i) And when the wavelength of the signal light satisfies | Δ k | ≦ pi, the angle group (θ) along with the phase matching of the signal light with the crystal length L can be calculated_i,φ_i) A varying gain bandwidth Δ λ;

step S5, comparing the effective nonlinear coefficient d obtained in step 3_effAnd step 4, judging the effective nonlinear coefficient d under the condition that the gain bandwidth delta lambda meets the OPCPA requirement_effAnd whether the gain bandwidth delta lambda is an optimal value;

step S6, if the gain bandwidth Delta lambda does not meet the bandwidth required by the OPCPA, reselecting the initial non-collinear angle alpha, and repeating the steps 1-4;

step S7, when the gain bandwidth Delta lambda meets the OPCPA requirement, the effective nonlinear coefficient d_effWhen the sum-and-break gain bandwidth DeltaLambda is an optimal value, selecting the non-sum at the momentLine angle alpha₀And phase matching angle (theta)₀,φ₀) For an optimum non-collinear angle alpha₀And an optimum phase matching angle (theta)₀,φ₀)；

The invention can judge the optimal phase matching angle (theta) from figure 2₀,φ₀) When the phase matching angle is (66.15 degrees and 137.9 degrees), the maximum effective nonlinear coefficient is 1.753pm/V, the gain bandwidth is 167nm, but the angle tolerance is very small, so that the phase matching angle (63.15 degrees and 137.8 degrees) with larger angle tolerance is selected as the optimal phase matching angle, the effective nonlinear coefficient is 1.708pm/V, and the gain bandwidth is 161 nm;

after the optimal phase matching angle (63.15 degrees and 137.8 degrees) is obtained, as can be seen from fig. 2, the effective nonlinear coefficient is 1.708pm/V and is 1.1pm/V larger than the maximum effective nonlinear coefficient of the phase matching of the main plane; the gain bandwidth is 161nm, which is larger than the maximum gain bandwidth 130nm of the planar phase matching, and the band range supported by the nonlinear crystal OPCPA is expanded.

Finally, it should be noted that the above embodiments are only used for illustrating the technical solutions of the present invention and are not limited. Although the present invention has been described in detail with reference to the embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the spirit and scope of the invention as defined in the appended claims.