Nonlinear Crystal Optimization Method for Ultra-High Power Laser Frequency Doubling

Abstract

An optimization method for the cutting scheme of nonlinear crystals based on the theory of crystal frequency doubling, which relates to the field of ultra-high power laser frequency doubling. According to the theory of crystal frequency doubling, this method adjusts the cutting angle of the frequency doubling crystal, while ensuring the efficient frequency doubling output of the crystal, appropriately increasing the crystal thickness to reduce the crystal processing difficulty. This method can obtain a scheme that not only meets the efficient frequency doubling but also conforms to the existing processing situation, and finally achieves the efficient frequency doubling output of ultra-high power.

Description

Technical Field

[0001] The present invention relates to the technical field of crystal frequency doubling, and in particular to a method for optimizing a non-linear crystal for ultra-high power laser frequency doubling. Background Art

[0002] With the development of chirped pulse amplification (CPA) technology and the improvement of the output performance of ultra-high power lasers, the requirements for the output power and output efficiency of laser frequency doubling are gradually increasing. For ultra-high power laser frequency doubling, due to the need to ensure high-efficiency output, the requirements for non-linear crystals are very strict. While meeting the large aperture, there are also strict requirements for the thickness of the non-linear crystal, and the required thickness is often on the order of a few millimeters, and the existing processes cannot meet the processing and installation conditions. Summary of the Invention

[0003] To solve the above problems, the present invention proposes a method for optimizing a non-linear crystal for ultra-high power laser frequency doubling, which can achieve a high-efficiency frequency doubling scheme under the condition of reducing the engineering difficulty.

[0004] The technical solution of the present invention is as follows:

[0005] A method for optimizing a non-linear crystal for ultra-high power laser frequency doubling, characterized in that it comprises the following steps:

[0006] S1. Calculate the thickness of the non-linear crystal when it reaches the best frequency doubling efficiency under specific conditions through theoretical calculation, and obtain the frequency doubling conversion efficiency of the non-linear crystal at this time;

[0007] When the non-linear crystal is in the case of type-I second harmonic generation of KDP crystal, there is the following coupled wave equation:

[0008]

[0009]

[0010]

[0011] Where represents the change of the optical electric field E 1 ′ along the z direction of the crystal, n 1 n 2 n 3 represents the refractive index of the medium corresponding to the respective frequencies, Δk is the phase mismatch factor, α represents the absorption coefficient, ω 1 is the angular frequency of the fundamental frequency light, c is the speed of light in vacuum, ε 0 is the vacuum permittivity, θ m is the phase matching angle of the crystal, d 36 is the effective second harmonic generation coefficient of the KDP crystal, is the crystal cutting angle, and K is the second harmonic conversion efficiency coefficient. By solving the coupled wave equation, the theoretically optimal thickness required for the crystal to achieve the highest efficiency in ultra-high power second harmonic generation can be obtained.

[0012] S2. By changing the cutting angle of the nonlinear crystal, the relationship between the second harmonic conversion efficiency and the thickness of the nonlinear crystal under different cutting angles is obtained; after changing the cutting angle of the crystal, the highest second harmonic efficiency obtained will be lower than the conversion efficiency mentioned in S1, but the crystal thickness will also increase, thus greatly reducing the processing and installation difficulty of the crystal.

[0013] S3. Compare the crystal thickness and the highest second harmonic conversion efficiency obtained in step S1 with the thickness and second harmonic conversion efficiency of the nonlinear crystal at different cutting angles obtained in step S2, and select a suitable crystal cutting angle, that is, the optimal solution for the increase in the thickness of the nonlinear crystal and the decrease in the second harmonic conversion efficiency, and finally select an angle that can not only meet the high-efficiency output but also meet the existing processing and installation conditions.

[0014] Compared with the prior art, the technical effect of the present invention is:

[0015] While ensuring the high-efficiency second harmonic generation of ultra-high power lasers, the processing and installation difficulty of the crystal is reduced. Description of the Drawings

[0016] Figure 1 It is the thickness required to reach the highest conversion efficiency when the cutting angle is 68° under theoretical analysis, which is 3.5 mm.

[0017] Figure 2 It is that when the cutting angle is changed from 45° to 68° under theoretical analysis, the crystal thickness at the best conversion efficiency changes from 3.5 mm to 5 mm.

[0018] Figure 3 It is the comparison chart of the conversion efficiency and the theoretical analysis during offline experimental verification. Among them, (a) is the comparison chart when the cutting angle is 45° and the thickness is 8 mm. (b) is the comparison chart when the cutting angle is 68° and the thickness is 5.9 mm. Specific Embodiments

[0019] The present invention will be further described and explained below in conjunction with examples and drawings.

[0020] Example:

[0021] A picosecond laser with a fundamental frequency energy of 1600 joules, a pulse width of 15 ps, and a spot diameter of 350 mm is used for simulation. The nonlinear crystal uses a KDP crystal. Theoretical calculation shows that a 3.5-mm-thick KDP crystal can meet the high-efficiency conversion (simulation results are as Figure 1As shown, the cutting angle is 68° at this time. To meet the 350 mm diameter light spot, the KDP crystal size is 410 mm * 410 mm * 3.5 mm, and the processing and installation are difficult.

[0022] Changing the cutting angle from 45° to 68° reduces the nonlinear coefficient, and the crystal thickness increases from 3.5 mm to 5 mm. (The efficiency drops by 6.1%). Finally, the conversion efficiency that can be obtained is ~62%, which can still ensure the high-efficiency frequency doubling of picosecond pulsed lasers (as Figure 2 shown).

[0023] The off-line experiment was verified. The picosecond laser has an energy of 45 mJ and a diameter of 7 mm. The two crystals are 8 mm thick with a cutting angle of 45° and 5.9 mm thick with a cutting angle of 68° respectively. The off-line experiment energy is 4 mJ and the diameter is 3 mm. Finally, when the cutting angle is 45°, the efficiency is 45%, and when the cutting angle is 68°, the efficiency is 20% (as Figure 3 shown), which meets the experimental expectations.

Claims

1. A method for optimizing a nonlinear crystal for high-power laser frequency doubling, characterized in that, it includes the following steps: S1. Calculate the thickness of the nonlinear crystal when the nonlinear crystal reaches the best frequency doubling efficiency under specific conditions through theoretical calculation, and obtain the frequency doubling conversion efficiency of the nonlinear crystal at this time; S2. By changing the cutting angle of the nonlinear crystal, obtain the relationship between the frequency doubling conversion efficiency and the thickness of the nonlinear crystal under different cutting angle conditions; S3. Compare the crystal thickness and the highest frequency doubling conversion efficiency obtained in step S1 when the best frequency doubling efficiency is obtained with the thickness and frequency doubling conversion efficiency of the nonlinear crystal at different cutting angles obtained in step S2, and select a suitable crystal cutting angle, that is, the optimal scheme for the increase in the thickness of the nonlinear crystal and the decrease in the frequency doubling conversion efficiency; In the case of the coupled wave equation of the first-order second harmonic generation of the KDP crystal in step S1, there is the following: Among them represents the variation of the optical electric field E 1 ′ along the z - direction of the crystal, n 1 n 2 n 3 represents the refractive index of the medium at the corresponding frequency, Δk is the phase - mismatch factor, α represents the absorption coefficient, ω 1 is the angular frequency of the fundamental - frequency light, c is the speed of light in vacuum, ε 0 is the vacuum permittivity, θ m is the phase - matching angle of the crystal, d 36 is the effective second - harmonic generation coefficient of the KDP crystal, is the crystal cutting angle, and K is the second - harmonic conversion efficiency coefficient.

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