A nonlinear near-field filtering method

Abstract

The application discloses a nonlinear near-field filtering method, which is characterized in that: a single nonlinear filtering crystal or multiple cascaded nonlinear filtering crystals are arranged in a laser filtering light path, frequency conversion of near-field high-frequency components of different solid angles in a to-be-filtered light beam is carried out by using the nonlinear filtering crystal, and the near-field high-frequency components are filtered out in a subsequent amplification transmission process, so that spatial filtering is realized, and the problem that an original spatial filter needs to be equipped with a long light beam transmission pipeline and needs to be vacuumized under high-power conditions, thereby causing high cost, is effectively solved.

Description

Technical Field

[0001] This invention relates to the field of beam filtering technology, and more particularly to a nonlinear near-field filtering method. Background Technology

[0002] A single high-power laser driver can generate tens of thousands of joules of energy within a few nanoseconds timescale, serving as a driver for inertial confinement fusion and providing a driving source for interaction processes under extreme conditions such as high-energy-density physics.

[0003] In high-power lasers, spatial filters are commonly used to filter high-frequency spatial components of the beam to ensure beam transmission quality and suppress B-integral growth. A spatial filter consists of two lenses (front and rear) and a small aperture placed at the focal length. Parallel light, after being focused by the front lens, exhibits a near-field Fourier transform (spatial spectrum) at the focal point. The high-frequency components are filtered out by the small aperture of a specific size placed at the focal point. The filtered-out light then passes through the rear lens for beam expansion or conversion into parallel light. This filtering method requires a long beam transmission channel, occupies a large area, and requires vacuum evacuation of the optical channel under high-power conditions to prevent the focused light from breaking down the air. Therefore, it is expensive and accounts for a significant portion of the overall cost of high-power laser devices. Summary of the Invention

[0004] In view of this, the present invention provides a nonlinear near-field filtering method, which sets one or more cascaded nonlinear filtering crystals in the laser filtering optical path, uses the nonlinear filtering crystals to perform frequency conversion on the near-field high-frequency components of different solid angles in the beam to be filtered, and filters them out in the subsequent amplification and transmission process, thereby realizing spatial filtering. This effectively solves the problem that the original spatial filter requires a very long beam transmission pipe and requires vacuuming of the optical pipe under high power conditions, resulting in high cost.

[0005] A nonlinear near-field filtering method specifically includes the following steps:

[0006] S1. Determine the placement position of the nonlinear filter crystal in the optical path based on the B integral during the amplification process of the laser filter optical path.

[0007] Determine the spatial frequency range of the beam that has the most severe impact on the self-focusing effect of the optical path;

[0008] S2, the nonlinear filter crystal is designed by calculating the optimal solution of incident light phase matching and nonlinear filter crystal thickness to maximize the frequency conversion efficiency of near-field high-frequency components in the spatial frequency range of the beam;

[0009] S3, a nonlinear filter crystal fabricated and designed;

[0010] S4. Place the nonlinear filtering crystal in the optical path to filter the near-field high-frequency components in the beam to be filtered. Adjust the angle between the nonlinear filtering crystal and the optical axis according to the processing error of the nonlinear filtering crystal. During the adjustment process, the filtering effect of the near-field high-frequency components is detected simultaneously. When the filtering effect of the near-field high-frequency components is optimal, stop adjusting the angle between the nonlinear filtering crystal and the optical axis.

[0011] Preferably, the specific steps in step S1 for determining the placement position of the nonlinear filtering crystal in its optical path based on the B integral during the laser filtering optical path amplification process are as follows:

[0012] The nonlinear coefficient γ(z) of the amplification medium and the distribution of energy flux I(z) along the beam propagation direction during laser amplification are determined based on the laser filter amplification configuration.

[0013] The B integral during the amplification process of the laser filtering optical path is calculated based on the nonlinear coefficient γ(z) of the amplification medium, the distribution of energy flux I(z) along the beam propagation direction during laser amplification, and the center wavelength λ of the beam to be filtered. The position in the optical path corresponding to the B integral value of 1.8 rad (or other critical B integral empirical value) is taken as the critical position.

[0014] The nonlinear filter crystal is placed in the laser optical path before the critical position.

[0015] Preferably, in step S2, the incident light phase matching method includes a birefringence phase matching method, a temperature phase matching method, an adiabatic phase matching method, and a quasi-phase matching method;

[0016] The nonlinear process of frequency conversion of near-field high-frequency components within the spatial frequency range of a beam includes frequency doubling conversion, third-order and higher-order frequency conversion, and nonlinear processes of changes in the spectrum, phase, and wave vector direction of higher-order spatial frequency components.

[0017] Preferably, the nonlinear filter crystal is designed using a frequency doubling process and a birefringence phase matching method, including the following steps:

[0018] The nonlinear filtering crystal is cut in a specific direction so that the refractive index of the beam to be filtered is equal to that of the frequency-doubled light to achieve phase matching of the incident light. The frequency-doubled light refers to the light obtained by frequency conversion of the near-field high-frequency components in the beam to be filtered within the spatial frequency range of the beam by the birefringent crystal.

[0019] The optimal solution for the thickness of the birefringent crystal was calculated by simulation based on the incident light phase mismatch, the nonlinear coefficient of the birefringent crystal, and the incident light intensity.

[0020] Preferably, in step S4, a single or cascaded nonlinear filtering crystal is placed in the optical path to filter the near-field high-frequency components in the beam to be filtered.

[0021] Preferably, in step S4, the filtering effect of near-field high-frequency components is detected by using a filter combined with a lens for far-field imaging. The filtering effect of near-field high-frequency components is optimal when the far-field focal point is minimized.

[0022] Preferably, the nonlinear filter crystal includes potassium dihydrogen phosphate (KDP), potassium dideuterium phosphate (DKDP), lithium niobate (LN), periodically polarized lithium niobate (PPLN), barium metaphosphate (BBO), and lithium triborate (LBO).

[0023] The beneficial effects of this invention are:

[0024] 1. This invention sets up one or more cascaded nonlinear filtering crystals in the laser filtering optical path. The nonlinear filtering crystals perform frequency conversion on the near-field high-frequency components of different solid angles in the beam to be filtered, and filter them out during subsequent amplification and transmission, thereby realizing spatial filtering. This effectively solves the problem that the original spatial filter requires a very long beam transmission pipe and requires vacuuming of the optical pipe under high power conditions, resulting in high cost.

[0025] 2. This invention uses single or cascaded planar optical elements to replace the lens elements that are indispensable in traditional space filters. At the same time, the beam transmission channel can filter out near-field high-frequency components without vacuuming, which greatly reduces the number of required components and lowers the processing difficulty and cost.

[0026] 3. Traditional spatial filter 4f system plus vacuum system near-field filtering method has a large footprint, and its spatial filter is more than ten meters long. In contrast, the nonlinear filtering method of this application uses flat plate element, which greatly reduces the space occupied. Moreover, the flat plate element only needs to be matched with a two-dimensional adjustment mechanism, and there is no need to set up optical pipes and vacuum equipment, which greatly reduces the cost. Attached Figure Description

[0027] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 This is a schematic diagram illustrating the near-field filtering principle of a nonlinear filtering crystal.

[0029] Figure 2 This is a schematic diagram of the arrangement of nonlinear near-field filters in the amplification optical path.

[0030] Figure 3 This is a flowchart of the nonlinear near-field filtering method of the present invention. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this invention clearer, the invention is described below with reference to specific embodiments shown in the accompanying drawings. However, it should be understood that these descriptions are merely exemplary and not intended to limit the scope of the invention. Furthermore, descriptions of well-known structures and technologies are omitted in the following description to avoid unnecessarily obscuring the concept of the invention.

[0032] The terminology used in this disclosure is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. The singular forms “a,” “the,” and “the” as used in this disclosure and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the term “and / or” as used herein refers to and includes any and all possible combinations of one or more of the associated listed items.

[0033] This invention provides a nonlinear near-field filtering method, which specifically includes the following steps:

[0034] S1. Determine the placement position of the nonlinear filter crystal in the optical path based on the B integral during the amplification process of the laser filter optical path, and determine the spatial frequency range of the beam that has the most serious impact on the self-focusing effect of the optical path.

[0035] The specific steps for determining the placement of the nonlinear filtering crystal in the laser filtering optical path are as follows:

[0036] First, the nonlinear coefficient γ(z) of the amplification medium and the distribution of energy flux I(z) along the beam propagation direction during laser amplification are determined based on the laser filter amplification configuration.

[0037] Then, based on the nonlinear coefficient γ(z) of the amplification medium, the distribution of energy flux I(z) along the beam propagation direction during laser amplification, and the center wavelength λ of the beam to be filtered, the B integral during the amplification process of the laser filtering optical path is calculated, and the position in the optical path corresponding to the B integral value of 1.8 rad (or other critical B integral empirical value) is taken as the critical position. The nonlinear filtering crystal is placed in the laser optical path before the critical position.

[0038] The formula for calculating the B integral during the laser filtering optical path amplification process is as follows:

[0039]

[0040] The angle between the transmission direction of the near-field high-frequency components within the spatial frequency range of the beam and the optical axis is α1~α2, and this part of the near-field high-frequency components in the beam to be filtered needs to be filtered out.

[0041] like Figure 1 As shown, Figure 1This is a schematic diagram of the near-field filtering principle of a nonlinear filtering crystal. In the diagram, 1 is the incident light (i.e. the beam to be filtered), and 2-4 are different frequency components in the incident beam. Among them, 2 is the near-field low-frequency component in the beam, and its transmission direction is the optical axis direction. 3 and 4 are the near-field high-frequency components in the beam that need to be filtered out, and the angle between the transmission direction and the optical axis is in the range of α1 to α2.

[0042] S2, the nonlinear filter crystal is designed by calculating the optimal solution of incident light phase matching and nonlinear filter crystal thickness to maximize the frequency conversion efficiency of near-field high-frequency components in the spatial frequency range of the beam.

[0043] Incident light phase matching methods include, but are not limited to, birefringence phase matching method, temperature phase matching method, adiabatic phase matching method, and quasi-phase matching method.

[0044] The nonlinear process of frequency conversion of near-field high-frequency components within the spatial frequency range of a beam includes frequency doubling conversion, third-order and higher-order frequency conversion, and nonlinear processes of changes in the spectrum, phase, and wave vector direction of higher-order spatial frequency components.

[0045] S3 is a nonlinear filter crystal that has been fabricated and designed.

[0046] The nonlinear filter crystal can be made of any nonlinear material such as potassium dihydrogen phosphate (KDP), potassium dideuterium phosphate (DKDP), lithium niobate (LN), periodically polarized lithium niobate (PPLN), barium metaphosphate (BBO), or lithium triborate (LBO).

[0047] S4. Place the nonlinear filtering crystal in the optical path to filter the near-field high-frequency components in the beam to be filtered. Adjust the angle between the nonlinear filtering crystal and the optical axis according to the processing error of the nonlinear filtering crystal. During the adjustment process, the filtering effect of the near-field high-frequency components is detected simultaneously. When the filtering effect of the near-field high-frequency components is optimal, stop adjusting the angle between the nonlinear filtering crystal and the optical axis.

[0048] In this embodiment, a single or cascaded nonlinear filtering crystal is placed in the optical path to filter the near-field high-frequency components in the beam to be filtered, thereby further suppressing the growth of the B integral.

[0049] In this embodiment, the filtering effect of near-field high-frequency components can be detected by using a filter combined with a lens for far-field imaging. The filtering effect of near-field high-frequency components is optimal when the far-field focal point is minimized.

[0050] To better understand the technical solution of the present invention, the present invention will be described in detail below with reference to specific examples and accompanying drawings.

[0051] Taking a neodymium glass laser as an example, the overall B-integral of its entire laser optical path should be controlled below 3.5 rad, while the B-integral between its two spatial filtering devices should be controlled below 1.8 rad. Therefore, the placement position of the nonlinear filtering crystal in the optical path is determined based on the B-integral value between the two spatial filtering devices. The B-integral value of the placement position of the nonlinear filtering crystal in the optical path should not exceed 1.8 rad, preferably 1.5 rad. Simultaneously, the spatial frequency range of the beam that has the most severe impact on the self-focusing effect is determined, i.e., the angle range α1 to α2 corresponding to the optical axis direction.

[0052] Then, using the frequency doubling process and birefringence phase matching method, the nonlinear filtering crystal is designed based on the center wavelength λ, polarization state, and near-field high-frequency components to be filtered (frequency components in the beam whose transmission direction and optical axis are within the range of α1 to α2) of the beam to be filtered, so that the nonlinear process efficiency of the near-field high-frequency components in the angle range of α1 to α2 of the beam to be filtered is the highest, while the nonlinear process efficiency of the low-frequency components corresponding to other angles is 0; the nonlinear process efficiency refers to the frequency conversion efficiency.

[0053] Since the wave vector of the beam to be filtered with wavelength λ is k1=2πn ω / λ, where n ω Let be the refractive index of the beam to be filtered. To achieve efficient frequency conversion, energy and momentum conservation must be satisfied. Therefore, the wavelength of the frequency-doubled light generated after the nonlinear filtering crystal performs frequency conversion on the high-frequency components of the incident light is λ / 2, and the wave vector of the frequency-doubled light should satisfy k² = 2 × 2πn. 2ω / λ=2k1, where n 2ω The refractive index is the same as that of the frequency-doubled light, meaning the refractive index of the beam to be filtered should be the same as that of the frequency-doubled light. However, in a medium, due to dispersion, the refractive index varies with wavelength. In a birefringent crystal, the refractive indices of ordinary and extraordinary light are different. Therefore, the phase matching of incident light at a specific angle can be achieved by designing the nonlinear crystal cutting direction. The optimal solution for the crystal thickness d can be obtained through simulation calculations, thus completing the nonlinear crystal design.

[0054] Therefore, when designing a nonlinear filtering crystal using the frequency doubling process and birefringence phase matching method, firstly, the nonlinear filtering crystal is cut in a specific direction so that the refractive index of the beam to be filtered is equal to the refractive index of the frequency-doubled light to achieve phase matching of the incident light. After being cut in a specific direction, the nonlinear filtering crystal becomes a birefringent crystal. The frequency-doubled light refers to the light obtained by frequency conversion of the near-field high-frequency components in the beam to be filtered, which are located within the spatial frequency range α1 to α2 of the beam, by the birefringent crystal. Figure 1 As shown, Figure 1In section 5, a nonlinear crystal is used for near-field filtering. After the incident beam 1 passes through the nonlinear crystal 5, it becomes beam 1′. The low-frequency component 2′ propagating along the optical axis remains unchanged, but the high-frequency components (3, 4) with angles to the optical axis in the range of α1 to α2 are converted into frequency-doubled light (3′, 4′). The wavelength of the frequency-doubled light (3′, 4′) is half the wavelength of the beam 1 to be filtered. Then, based on the incident light phase mismatch, the nonlinear coefficient of the birefringent crystal, and the incident light intensity, the optimal solution for the thickness d of the birefringent crystal is calculated, thus completing the design of the nonlinear crystal.

[0055] Then, the designed nonlinear crystal is fabricated. After fabrication, one or more cascaded nonlinear crystals are placed in the optical path. The B-integral value of the nonlinear crystal's placement position in the optical path should not exceed 1.8 rad, preferably 1.5 rad. Figure 2 As shown, Figure 2 This diagram illustrates the arrangement of a nonlinear near-field filter in an amplifying optical path. In the diagram, 6 represents the optical axis direction of the beam propagation, 7-10 represent the gain medium, and 11 is the nonlinear crystal that provides the nonlinear near-field filtering effect. The nonlinear crystal 11 is placed between the gain mediums 9 and 10. Figure 2 The gain medium in this example is a sheet-shaped gain medium, but the gain medium is not limited to rod-shaped gain medium, thin sheet gain medium, slab gain medium, etc.

[0056] Because there are processing errors in the cutting angle of nonlinear crystals, it is necessary to adjust the angle between the nonlinear crystal and the optical axis to compensate for the processing errors. At the same time, the filtering effect of high-frequency components is detected by using a filter combined with a lens for far-field imaging, so that the filtering effect can be optimized.

[0057] It should be understood that the described embodiments are merely some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A nonlinear near-field filtering method, characterized in that, Specifically, the following steps are included: S1, determine the placement position of the nonlinear filter crystal in the optical path based on the B integral during the laser filter optical path amplification process; Determine the spatial frequency range of the beam that has the most severe impact on the self-focusing effect of the optical path; S2, by determining the cutting direction of the nonlinear filter crystal and optimizing the thickness of the nonlinear filter crystal through the incident light phase matching condition, so as to maximize the frequency conversion efficiency of the near-field high-frequency components in the spatial frequency range of the beam, thereby completing the design of the nonlinear filter crystal; S3, fabrication and design of nonlinear filter crystals; S4, the nonlinear filtering crystal is placed in the optical path to filter the near-field high-frequency components in the beam to be filtered. The angle between the nonlinear filtering crystal and the optical axis is adjusted according to the processing error of the nonlinear filtering crystal. During the adjustment process, the filtering effect of the near-field high-frequency components is detected simultaneously. When the filtering effect of the near-field high-frequency components is optimal, the adjustment of the angle between the nonlinear filtering crystal and the optical axis is stopped.

2. The nonlinear near-field filtering method according to claim 1, characterized in that, The specific steps in step S1 for determining the placement position of the nonlinear filtering crystal in its optical path based on the B integral during the laser filtering optical path amplification process are as follows: The nonlinear coefficient of the amplification medium and the distribution of energy flux along the beam propagation direction during laser amplification are determined based on the laser filter amplification configuration. The B integral during the laser filtering optical path amplification process is calculated based on the nonlinear coefficient of the amplification medium, the distribution of energy flux along the beam propagation direction during laser amplification, and the center wavelength of the beam to be filtered. The position in the optical path corresponding to the B integral value being equal to the set critical value is taken as the critical position. The nonlinear filter crystal is placed in the laser optical path before the critical position.

3. The nonlinear near-field filtering method according to claim 1, characterized in that, In step S2, the incident light phase matching method includes any one of the following: birefringence phase matching method, temperature phase matching method, adiabatic phase matching method, and quasi-phase matching method; The nonlinear process of frequency conversion of near-field high-frequency components within the spatial frequency range of a beam includes frequency doubling conversion, third-order and higher-order frequency conversion, and nonlinear processes of changes in the spectrum, phase, and wave vector direction of higher-order spatial frequency components.

4. The nonlinear near-field filtering method according to claim 3, characterized in that, The design of a nonlinear filter crystal using a frequency doubling process and birefringence phase matching method includes the following steps: The nonlinear filtering crystal is cut in a specific direction so that the refractive index of the beam to be filtered is equal to that of the frequency-doubled light to achieve phase matching of the incident light. The frequency-doubled light refers to the light obtained by frequency conversion of the near-field high-frequency components in the beam to be filtered within the spatial frequency range of the beam by the birefringent crystal. The optimal solution for the thickness of the birefringent crystal was calculated by simulation based on the incident light phase mismatch, the nonlinear coefficient of the birefringent crystal, and the incident light intensity.

5. The nonlinear near-field filtering method according to claim 1, characterized in that, In step S4, a single or cascaded nonlinear filtering crystal is placed in the optical path to filter the near-field high-frequency components in the beam to be filtered.

6. The nonlinear near-field filtering method according to claim 1, characterized in that, In step S4, the filtering effect of near-field high-frequency components is detected by using a filter combined with a lens for far-field imaging. The filtering effect of near-field high-frequency components is optimal when the far-field focal point is minimized.

7. The nonlinear near-field filtering method according to claim 1, characterized in that, The nonlinear filter crystal includes any one of potassium dihydrogen phosphate (KDP), potassium dideuterium phosphate (DKDP), lithium niobate (LN), periodically polarized lithium niobate (PPLN), barium metaphosphate (BBO), and lithium triborate (LBO).

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