A large-aperture two-dimensional continuous polarization modulation element and its preparation method

Abstract

The present invention discloses a large-aperture two-dimensional continuous polarization modulation element and a preparation method. The surface of the element has a two-dimensional lattice internal stress distribution, and each stress point can realize continuous polarization modulation of linearly polarized light-elliptically polarized light-circularly polarized light-elliptically polarized light-circularly polarized light. The preparation of the element includes: selecting optical glass resistant to thermal shock as a substrate; performing heat treatment on a local point of the substrate to introduce thermal residual stress as internal stress, and realizing continuous modulation of the polarization of the local point based on the birefringence effect of the internal stress; and realizing large-aperture two-dimensional continuous polarization modulation by introducing a two-dimensional thermal residual stress array on the surface of the substrate. This method not only greatly simplifies the production process of large-aperture two-dimensional continuous polarization modulation elements and avoids the use of expensive ultra-large-size crystal materials, but also realizes precise modulation of the polarization of the entire large-aperture light beam. It has a high laser damage threshold and is particularly suitable for laser devices with large-aperture light beams.

Description

Technical Field

[0001] The present invention belongs to the field of optical element preparation, and in particular relates to a method for preparing a large-aperture two-dimensional continuous polarization modulation element, and also relates to a large-aperture two-dimensional continuous polarization modulation element. Background Art

[0002] Polarization smoothing technology can effectively reduce the speckle in the target spot of a high-power solid-state laser device and reduce the unevenness of the target surface. It is very effective in suppressing the instability of laser plasma with a fast response time. The beam aperture of a high-power solid-state laser device is close to the meter level. The optical elements currently reported to achieve polarization modulation of such a large-aperture beam are mainly polarization control plates and birefringent wedges (Gao Yanqi et al., Infrared and Laser Engineering, 49(12):20201074,2020). The polarization control plate divides the beam into blocks in the near field and uses crystals or liquid crystals to adjust the polarization direction, thereby reducing the number of interferences between the sub-beams by half. The birefringent wedge scheme utilizes the different refractive indices of the fast and slow light of the uniaxial crystal, and directly divides the incident linearly polarized laser into two beams (o light and e light) with orthogonal polarization directions and a certain angle difference through the potassium dihydrogen phosphate (KDP) wedge plate. The effect of reducing the focal spot speckle is achieved by incoherent superposition of o light and e light on the focal plane. However, it is extremely difficult to prepare crystal polarization modulation optical elements close to the meter level and the cost is extremely high. Although liquid crystal is relatively inexpensive, the transmittance and damage resistance of liquid crystal materials are poor, and they are not suitable for high-throughput, high-power solid-state laser devices.

[0003] In order to solve the engineering and technical problem of economically and efficiently preparing large-aperture polarization modulation elements, people began to try to use relatively economical glass materials to achieve polarization modulation of large-aperture light beams. Rothenberg et al. (Journal of Applied Physics, 87(8):3654-3662, 2000) disclosed a method of fixing eight metal blocks on the four sides of a 420mm×420mm×10mm large-aperture fused quartz glass plate, applying three compressive loads on each metal block, each load reaching 2000 pounds, and polarization modulation of large-aperture light beams by generating highly non-uniform stress birefringence on the fused quartz plate. However, this method of applying external stress requires a very large load to be applied to the edge of the element, and even if the edge is loaded with a very large load, the stress in most areas of the center of the element is still low, making it difficult to effectively polarize the central area of the light beam. Therefore, the method proposed by Rothenberg et al. to introduce stress birefringence in the fused quartz plate by applying external stress is not suitable for effective polarization modulation of the entire large-aperture light beam.

[0004] Therefore, it is urgent to propose a method for preparing a large-aperture polarization modulation element with an optical glass substrate, and to prepare a large-aperture polarization modulation element that can achieve continuous modulation, thereby replacing the edge loading method and avoiding the use of expensive ultra-large-size crystal materials to meet certain specific application scenarios of large-aperture beam laser devices. Summary of the Invention

[0005] To address the above technical issues, the present invention provides a large-aperture two-dimensional continuous polarization modulation preparation method and a large-aperture two-dimensional continuous polarization modulation element. The method and element have the advantages of simplicity and efficiency, good process stability, strong controllability, and high repeatability.

[0006] To achieve the above purpose, the technical solution of the present invention is as follows:

[0007] A method for preparing a large-aperture two-dimensional continuous polarization modulation element comprises the following steps:

[0008] Step S1, selecting thermal shock resistant optical glass as a substrate;

[0009] In step S2, heat treatment is performed on the substrate to generate thermal residual stress as internal stress, thereby achieving stress polarization control.

[0010] Optionally, the thermal shock resistant optical glass is fused quartz glass.

[0011] Optionally, step S2 specifically includes:

[0012] S21 selects a CO2 laser as a point heat source to irradiate the substrate, so that the irradiation point of the substrate is rapidly heated, turns off the CO2 laser, and introduces thermal residual stress at the irradiation point of the substrate after cooling, and makes the maximum optical path difference at the irradiation point of the substrate reach a first target value;

[0013] S22 adjusts the spacing between the CO2 laser irradiation points, completes array irradiation on the substrate, and then introduces a two-dimensional thermal residual stress array so that the maximum optical path difference of the substrate reaches a second target value.

[0014] Optionally, the array irradiation in S22 is point-by-point irradiation or simultaneous multi-point irradiation.

[0015] Optionally, the first target value is 1 / 3 of the wavelength of the regulated laser, and the second target value is 1 / 2 of the wavelength of the regulated laser.

[0016] Optionally, after the array irradiation is completed on the substrate in S22, the maximum fluctuation of the substrate surface profile should be less than or equal to 6 times the wavelength of the controlled laser.

[0017] Optionally, the array irradiation is orthogonal, hexagonal close-packed or other random arrangements.

[0018] Optionally, the large-aperture two-dimensional continuous polarization modulation element is prepared according to the preparation method according to any one of claims 1-7.

[0019] Optionally, each stress point in the two-dimensional residual stress array of the large-aperture two-dimensional continuous polarization modulation element can realize continuous polarization modulation of linear polarized light-elliptically polarized light-circularly polarized light-elliptically polarized light-circularly polarized light-linear polarized light; the large-aperture two-dimensional continuous polarization modulation element area can also realize continuous polarization modulation of linear polarized light → elliptically polarized light → circularly polarized light → elliptically polarized light → circularly polarized light → linearly polarized light; the average depolarization energy ratio of the large-aperture two-dimensional continuous polarization modulation element is 36% to 50%.

[0020] Optionally, the optical path difference distribution of each stress point in the two-dimensional residual stress array of the large-aperture two-dimensional continuous polarization modulation element gradually increases radially from the center of the stress point, and the optical path difference at the edge of the stress point is the largest.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] The large-aperture two-dimensional continuous polarization modulation element and the preparation method thereof provided by the present invention are novel and greatly simplify the production process of the large-aperture polarization modulation element, avoid the use of expensive oversized crystal materials, and avoid loading external loads around the element by adopting internal stress. While being simple and efficient, it achieves precise modulation of the polarization of the entire large-aperture light beam. The manufactured large-aperture two-dimensional continuous polarization modulation element has a smooth surface and a high laser damage threshold, and is particularly suitable for applications in strong light environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The pattern of the two-dimensional thermal residual stress lattice in a partial area of a large-aperture two-dimensional continuous polarization modulation element prepared when the transmission directions of the polarizer and the analyzer are parallel;

[0024] Figure 2 The pattern of the two-dimensional thermal residual stress lattice in a partial area of a large-aperture two-dimensional continuous polarization modulation element prepared when the transmission directions of the polarizer and the analyzer are orthogonal;

[0025] Figure 3 The two-dimensional distribution of optical path difference caused by residual stress birefringence in 1 / 4 of the square lattice unit with a side length of 13 mm in the prepared large-aperture two-dimensional continuous polarization modulation element. DETAILED DESCRIPTION

[0026] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. 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 any creative effort. The present invention is described in detail below with reference to the drawings.

[0027] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below with reference to specific implementation methods and drawings.

[0028] Example 1

[0029] A large-aperture two-dimensional continuous polarization modulation element and a method for preparing the same are carried out according to the following steps:

[0030] S1: A polished 430 mm × 430 mm × 10 mm large-aperture fused silica plane element is selected as the substrate for the large-aperture two-dimensional continuous polarization modulation element.

[0031] S2: Set the spot size of the RF-excited CO2 laser to 30 mm, the power to 250 W, and the light output time to 30 s. After single-point irradiation on the surface of the fused quartz substrate, cool it down to introduce thermal residual stress.

[0032] S3: Using a photoelastic stress meter to measure the thermal residual stress distribution of a fused quartz substrate after a single-point CO2 laser exposure, the measurement results show that the optical path difference caused by stress birefringence is distributed in a circular patch pattern, with the optical path difference being 0 at the center and increasing radially outward. In this embodiment, the maximum optical path difference at a single point caused by stress birefringence is 130 nm, and the diameter of the maximum optical path difference is 10 mm. The maximum optical path difference in this step can be adjusted based on actual needs by optimizing the CO2 laser spot size, power, and light emission time. Generally, the maximum optical path difference at a single point is 1 / 3 of the target wavelength to be adjusted.

[0033] S4: Use a step profiler to measure the surface profile of the fused silica substrate after a single-point CO2 laser exposure. The maximum fluctuation of the substrate surface profile should be controlled within 6 times the wavelength of the controlled laser. The measurement results of this embodiment show that the maximum fluctuation of the surface profile is 2μm, which meets the requirements.

[0034] S5: Setting the spacing between CO2 laser irradiation points. In this embodiment, the irradiation point spacing is 13 mm. CO2 laser irradiation is performed point-by-point on the surface of a 430 mm x 430 mm large-aperture fused silica substrate using a square grid arrangement. A two-dimensional translation stage is used to move the elements to form a multi-row, multi-column CO2 laser processing point array. In this embodiment, 28 rows and 28 columns of CO2 laser processing points are used. The spacing between CO2 laser irradiation points can also be adjusted. By adjusting the spacing and arrangement of the irradiation points, the optical path differences between multiple irradiation points can be superimposed, thereby achieving continuous polarization modulation of the entire substrate and ensuring that the maximum optical path difference of the entire CO2 laser processing point array is less than a second target value. The second target value is set according to actual needs and is generally 1 / 2 of the controlled laser wavelength. The square grid arrangement can also be replaced with hexagonal close packing or other random arrangements besides square grids and hexagonal close packing. Hexagonal close packing means that the irradiation points are arranged according to the vertex positions of a regular hexagon.

[0035] S6: Using a photoelastic stress meter to detect the pattern of two-dimensional thermal residual stress lattices formed after CO2 laser irradiation of fused quartz substrate point by point, see Figure 1 and Figure 2 , Figure 1 This is the pattern of part of the two-dimensional thermal residual stress lattice when the transmission directions of the polarizer and analyzer are parallel. Figure 2 The pattern of part of the two-dimensional thermal residual stress lattice when the transmission directions of the polarizer and analyzer are orthogonal. Figure 1 and Figure 2 It can be observed that the incident single linearly polarized light becomes a beam with two-dimensional continuously changing polarization after passing through the prepared large-aperture two-dimensional continuous polarization modulation element on the fused quartz substrate.

[0036] S7: The photoelastic stress meter is used to detect the two-dimensional distribution of the optical path difference caused by the residual stress birefringence of 1 / 4 of the square grid unit with a side length of 13 mm in the large-aperture two-dimensional continuous polarization modulation element prepared on the fused silica substrate, such as Figure 3 As shown, the continuously varying stress optical path difference clearly indicates the continuously varying polarization state, with a maximum stress optical path difference of 176 nm. This means that for a 351 nm laser, the maximum continuously varying phase difference reaches π, achieving continuous polarization modulation from linearly polarized light to elliptically polarized light to circularly polarized light to elliptically polarized light to circularly polarized light to linearly polarized light (polarization direction rotated 90°). Calculations indicate that the average depolarization energy ratio for a linearly polarized 351 nm target laser from a high-power solid-state laser device reaches 36%. The fabricated large-aperture two-dimensional continuous polarization modulation element exhibits effective two-dimensional continuous polarization modulation capability. By controlling the two-dimensional thermal residual stress lattice, the average depolarization energy ratio can reach a maximum of 50%.

Claims

1. A method for preparing a large-aperture two-dimensional continuous polarization modulation element, characterized in that: The preparation method comprises the following steps: Step S1, selecting thermal shock resistant optical glass as a substrate; Step S2, performing heat treatment on the substrate to generate thermal residual stress as internal stress, thereby achieving stress polarization control; Step S2 specifically includes: S21 selects a CO2 laser as a point heat source to irradiate the substrate, so that the irradiation point of the substrate is rapidly heated, turns off the CO2 laser, and introduces thermal residual stress at the irradiation point of the substrate after cooling, and makes the maximum optical path difference at the irradiation point of the substrate reach a first target value; S22 adjusts the spacing between the CO2 laser irradiation points, completes array irradiation on the substrate, and then introduces a two-dimensional thermal residual stress array so that the maximum optical path difference of the substrate reaches a second target value.

2. The method for preparing a large-aperture two-dimensional continuous polarization modulation element according to claim 1, wherein: The thermal shock resistant optical glass is fused quartz glass.

3. The method for preparing a large-aperture two-dimensional continuous polarization modulation element according to claim 1, wherein: The array irradiation in S22 is point-by-point irradiation or multi-point simultaneous irradiation.

4. The method for preparing a large-aperture two-dimensional continuous polarization modulation element according to claim 1, wherein: The first target value is 1 / 3 of the wavelength of the regulated laser, and the second target value is 1 / 2 of the wavelength of the regulated laser.

5. The method for preparing a large-aperture two-dimensional continuous polarization modulation element according to claim 1, wherein: After the array irradiation is completed on the substrate in S22, the maximum fluctuation of the substrate surface profile should be less than or equal to 6 times the wavelength of the controlled laser.

6. The method for preparing a large-aperture two-dimensional continuous polarization modulation element according to claim 1, wherein: Array irradiation can be orthogonal, hexagonal close-packed or other random arrangements.

7. A large-aperture two-dimensional continuous polarization modulation element, characterized in that: The large-aperture two-dimensional continuous polarization modulation element is prepared according to the preparation method according to any one of claims 1-6.

8. The large-aperture two-dimensional continuous polarization modulation element according to claim 7, wherein: Each stress point in the two-dimensional residual stress array of the large-aperture two-dimensional continuous polarization modulation element can realize continuous polarization modulation of linear polarized light-elliptically polarized light-circularly polarized light-elliptically polarized light-circularly polarized light-linear polarized light; the overall array area of the large-aperture two-dimensional continuous polarization modulation element can also realize continuous polarization modulation of linear polarized light → elliptically polarized light → circularly polarized light → elliptically polarized light → circularly polarized light → linearly polarized light; the average depolarization energy ratio of the large-aperture two-dimensional continuous polarization modulation element is 36% to 50%.

9. The large-aperture two-dimensional continuous polarization modulation element according to claim 7, wherein: The optical path difference distribution of each stress point in the two-dimensional residual stress array of the large-aperture two-dimensional continuous polarization modulation element gradually increases radially from the center of the stress point, and the optical path difference at the edge of the stress point is the largest.

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