A reflection type volume grating uniformity improving device and method

Abstract

The application discloses a reflection type volume grating uniformity improving device and method, which adopts a double-system cooperative structure of axial exposure dose compensation and double-beam interference exposure. The axial exposure dose compensation system compensates the non-uniform axial exposure dose distribution caused by energy attenuation through laser irradiation of a photosensitive wave band of a recording medium and multi-axis rotation of the recording medium. The double-beam interference exposure system generates two collimated coherent lights with adjustable polarization states and powers by using laser of a high-transmittance wave band of the recording medium and a polarized beam splitter, and accurately controls the interference angle by adjusting the mirror posture, so that the high-contrast grating structure is written in the recording medium subjected to the axial exposure dose compensation. The application can effectively solve the non-uniform exposure dose distribution problem caused by laser energy attenuation in the traditional preparation process, thereby guaranteeing the consistency of the axial performance of the volume grating, and effectively promoting the development of the fields of high-performance laser beam combination, pulse compression and optical communication filter which need high-uniformity volume grating.

Description

Technical Field

[0001] This invention relates to volume gratings and related fields, and in particular to a device and method for improving the uniformity of a reflective volume grating. Background Technology

[0002] In fields such as laser spectral beam combining, pulse broadening and compression, optical communication filters, and near-eye displays, reflective volume gratings have gradually become key optical components due to their superior laser tolerance, environmental stability, compact structure, and cost-effectiveness. Therefore, the performance of reflective volume gratings directly affects the performance of related devices in terms of output power, dispersion control, beam quality, and filtering accuracy. However, in traditional fabrication processes, the uneven distribution of exposure dose along the laser interference irradiation direction severely restricts the axial uniformity of the volume grating, leading to diffraction wavefront distortion, decreased beam quality, and inconsistent overall diffraction efficiency distribution.

[0003] Currently, there are various existing methods for fabricating holographic reflective volume Bragg gratings, with related technologies covering improvements in multiple aspects such as optical path design, material systems, and fabrication processes. For example, the device proposed by Hua Lu et al. aims to achieve simultaneous adjustment of all optical components in one go after a change in beam angle [Prior Technology 1: CN115793117A]; while Song Renjie et al. focus on simplifying the exposure optical path structure of the holographic grating [Prior Technology 2: CN221977145U]. In terms of materials and processes, Jin Yunxia et al. elucidated the complete fabrication process of reflective volume gratings based on photothermal refractive (PTR) glass [Prior Technology 3: CN110879433B]; Hu Kunxing et al. fabricated volume gratings by using polymer-dispersed liquid crystals as recording materials, aiming to enhance their transparency and improve the effect of light entering the human eye in real-world environments, thereby improving the performance of optical waveguide displays [Prior Technology 4: CN118244531A]; Jin Shuai et al. proposed a method to directly form interference fringes inside a single piece of PTR glass with a wedge angle using a single collimated beam [Prior Technology 5: CN119805873A]; in addition, Jin Shuai et al. addressed the problem of complex fabrication processes for dichromate gelatin wavelength combiners by proposing a volume Bragg grating wavelength combiner based on PTR glass and its fabrication method [Prior Technology 6: CN114779382A]. However, apart from Guan Peiwen et al. using lasers to locally heat the volume grating and improve the crystallization degree of the glass in the target area to compensate for its diffraction efficiency, thereby improving the overall uniformity of the volume Bragg grating [Prior Art 7: CN117192668B], no subsequent reports have proposed a technical solution to further solve the uniformity (especially axial uniformity) of reflective volume gratings.

[0004] Based on the above analysis, it is necessary to propose a device and method for improving the uniformity of reflective volume gratings, so as to provide a new approach for the fabrication of high-uniformity volume gratings. Summary of the Invention

[0005] The purpose of this invention is to provide a device and method for improving the uniformity of reflective volume gratings, thereby overcoming the problems existing in the prior art. To achieve this objective, this invention is implemented through the following technical solution:

[0006] On the one hand, a reflective volume grating uniformity enhancement device is proposed, comprising an axial exposure dose compensation system and a dual-beam interference exposure grating structure writing system used in sequence, with the two systems sharing a multi-axis rotating sample stage for loading recording media.

[0007] The axial exposure dose compensation system, during the fabrication of a reflective volume grating, compensates for the non-uniformity of exposure dose distribution caused by laser energy attenuation as the laser penetrates the recording medium along its laser transmission axis. This improves the uniformity of the reflective volume grating. The system includes a first laser source with a wavelength within the photosensitive band of the recording medium, a first electrically controlled aperture, a laser power regulator, and a first beam expander and collimator unit placed sequentially along the laser propagation direction. The emitted light from the first beam expander and collimator unit ultimately acts on a multi-axis rotating sample stage containing the recording medium.

[0008] The dual-beam interference exposure grating structure writing system is used to write a grating structure into the recording medium after using the axial exposure dose compensation system. The system includes a second laser source with a wavelength within a high transmittance band of the recording medium, and an interference optical path for splitting, collimating, and forming two interference beams from the second laser source. In the interference optical path, the beam from the second laser source passes sequentially through a second electrically controlled aperture and a first laser polarization controller before being incident on a polarization beam splitter. The reflected optical path output from the polarization beam splitter is sequentially provided with a first reflecting mirror and a second beam expanding and collimating unit, while its transmitted optical path is sequentially provided with a second laser polarization controller, a second reflecting mirror, and a third beam expanding and collimating unit. The two collimated beams emitted from the second and third beam expanding and collimating units are angled... Interference occurs at the multi-axis rotating sample stage.

[0009] Furthermore, adjusting the orientation of the first and / or second reflector can change the interference angle of the two collimated beams emitted from the second and third beam expanding and collimating units. .

[0010] Furthermore, by adjusting the first laser polarization controller and / or the second laser polarization controller, the polarization state consistency and power ratio of the two beams after being split by the polarization beam splitter are controlled to optimize the interference contrast and exposure effect.

[0011] Furthermore, the multi-axis rotating sample stage can drive the recording medium to achieve continuous rotation or step-by-step rotation to a series of predetermined angles, thus changing the orientation of the recording medium during the irradiation phase of the axial exposure dose compensation system and the dual-beam interference exposure grating structure writing system. Specifically, the sample stage can cause the recording medium to rotate around at least two mutually perpendicular axes: a rotation axis defined as the z-axis, orthogonal to the optical axis of the first laser source and parallel to the length direction of the recording medium; and a rotation axis defined as the y-axis, which is orthogonal to both the z-axis and the optical axis of the first laser source. During the irradiation phase of the axial exposure dose compensation system and the dual-beam interference exposure grating structure writing system, the sample stage can be controlled to drive the recording medium to continuously rotate around the z-axis and / or y-axis, or to rotate step-by-step to a series of predetermined angles, thereby achieving uniform distribution of exposure dose within the recording medium and high-contrast grating structure writing.

[0012] On the other hand, a method for improving the uniformity of a reflective volume grating using the aforementioned reflective volume grating uniformity improvement device is proposed, comprising the following steps:

[0013] S1. Parameter Determination: Based on the usage requirements of the reflective volume grating and the photosensitivity characteristics of the recording medium, the axial exposure dose compensation value E1 and the two-beam interference exposure dose E2 are determined, and the grating period distribution of the reflective volume grating to be fabricated is determined; then, based on the grating period distribution, the interference angle of the two collimated beams of the two-beam interference exposure grating structure writing system at the multi-axis rotating sample stage is determined. The possible values ​​of ;

[0014] S2. Constructing the axial exposure dose compensation system and its optical path calibration: According to the composition and connection relationship of the axial exposure dose compensation system in the aforementioned device, construct the system, measure the power P1 acting on the recording medium, and calculate the corresponding exposure time t1 = E1 / P1.

[0015] S3. Constructing the dual-beam interference exposure grating structure writing system and calibrating its optical path: Turn off the axial exposure dose compensation system, and construct the system according to the composition and connection relationship of the dual-beam interference exposure grating structure writing system in the aforementioned device; calibrate the included angle of the two interference beams in the system to the interference angle described in step S1. The values ​​of are determined, and the polarization states of the two interfering beams are adjusted to be consistent and their power ratio is adjusted; the power P2 of the system acting on the recording medium is measured, and the corresponding exposure time t2 = E2 / P2 is calculated;

[0016] S4. Perform axial exposure dose compensation: Place the recording medium on the multi-axis rotating sample stage and turn on the axial exposure dose compensation system for exposure; during this process, in order to change the penetration path of the light beam in the recording medium to achieve uniform axial compensation, control the multi-axis rotating sample stage to make the recording medium continuously rotate around its z-axis and / or y-axis, or based on multiple angles pre-selected around the z-axis and / or y-axis, control the multi-axis rotating sample stage stepwise to make the recording medium rotate to a predetermined angle; when the exposure time reaches the control time t1, that is, when the axial exposure dose of the recording medium reaches the compensation value E1, turn off the axial exposure dose compensation system;

[0017] S5. Restore initial posture: Restore the posture of the recording medium to the initial posture before the start of step S4;

[0018] S6. Perform dual-beam interference exposure: Turn on the dual-beam interference exposure grating structure writing system to perform interference exposure; during this process, in order to write a high-contrast uniform grating structure in the recording medium, control the multi-axis rotating sample stage to make the recording medium continuously rotate around its z-axis, or based on multiple pre-selected angles around the z-axis, control the multi-axis rotating sample stage step by step to make the recording medium rotate to the predetermined angle. When the exposure time reaches the control time t2, that is, when the dual-beam interference exposure dose of the recording medium reaches E2, turn off the dual-beam interference exposure grating structure writing system.

[0019] S7. Post-processing: Axial exposure dose compensation and dual-beam thermal treatment of the exposed recording medium to form a reflective volume grating with improved uniformity.

[0020] Further, in step S2, constructing the axial exposure dose compensation system specifically involves: turning on a first laser source with a wavelength within the photosensitive band of the recording medium, and sequentially placing a first electrically controlled aperture, a laser power regulator, a first beam expander and collimator, and a multi-axis rotating sample stage loaded with the recording medium along the propagation direction of the first laser source; adjusting the position of the multi-axis rotating sample stage loaded with the recording medium so that a collimated beam emitted from the first beam expander and collimator is incident directly onto the multi-axis rotating sample stage.

[0021] Further, in step S3, the construction of the dual-beam interference exposure grating structure writing system specifically involves: turning on a second laser source with a wavelength located in the high transmittance band of the recording medium, and sequentially placing a second electrically controlled aperture, a first laser polarization controller, and a polarization beam splitter along the propagation direction of the second laser source; at the reflecting end of the polarization beam splitter, sequentially placing a first reflecting mirror and a second beam expanding and collimating unit along the laser propagation direction; at the transmitting end of the polarization beam splitter, sequentially placing a second laser polarization controller, a second reflecting mirror, and a third beam expanding and collimating unit along the laser propagation direction; adjusting the angle between the two collimated beams emitted from the second and third beam expanding and collimating units at the multi-axis rotating sample stage, so that the angle is equal to the interference angle described in step S1. The value of the polarization state and the power ratio are adjusted by controlling the polarization state of the two beams after they have been split by the polarization beam splitter to be consistent with the polarization state and have a power ratio of 0.8-1.2. In a preferred embodiment, the power ratio of the two beams is 1.

[0022] The apparatus and method provided by this invention are universal. Based on the above-mentioned combination of "axial exposure dose compensation" and "dual-beam interference exposure", it is not limited to the preparation of uniformity-enhanced periodic reflective volume gratings. The apparatus and method are also applicable to the preparation of uniformity-enhanced chirped volume gratings and other reflective volume gratings with non-uniform periodic distribution.

[0023] Furthermore, the aforementioned apparatus and method are widely adaptable to recording medium materials. Because the axial exposure dose compensation system is universally applicable to laser sources with wavelengths within the photosensitive band of the recording medium, and the dual-beam interference exposure system is universally applicable to laser sources with wavelengths within the high transmittance band of the recording medium, it is suitable for various recording medium materials, such as silver halide materials, dichromate gelatin, photopolymers, photothermal refractive glass, and calcium fluoride.

[0024] Compared with the prior art, the technical effects of the present invention are as follows:

[0025] 1. This invention first utilizes the absorption characteristics of the recording medium in the photosensitive band for axial uniform pre-compensation exposure, and then switches to its high transmittance band to perform interferometric exposure. This reverse process of "compensation first, then interference" avoids the problem of decreased uniformity caused by axial attenuation of light intensity in traditional interferometric exposure methods, thereby effectively improving the axial uniformity of the fabricated bulk grating in terms of refractive index modulation and diffraction efficiency.

[0026] 2. This invention integrates two functionally independent optical systems—axial compensation exposure and dual-beam interference exposure—and allows them to share a single multi-axis rotating sample stage. This technical solution eliminates the cumulative errors introduced by repeated disassembly and repositioning of the recording medium, improves process stability and repeatability, and makes large-scale production possible. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the optical path of a reflective volume grating uniformity enhancement device provided by the present invention.

[0028] Figure 2 This is a graph showing the dependence of transmittance on wavelength for the recording medium (PTR glass) used in the embodiments of the present invention.

[0029] In the figure, 1-recording medium, 2-multi-axis rotating sample stage, 3-first laser source, 4-first electrically controlled aperture, 5-laser power regulator, 6-first beam expander and collimator unit, 7-second laser source, 8-second electrically controlled aperture, 9-first laser polarization controller, 10-polarization beam splitter, 11-second laser polarization controller, 12-first reflector, 13-second beam expander and collimator unit, 14-second reflector, 15-third beam expander and collimator unit, 16-laser transmission axis. Detailed Implementation

[0030] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments, but this should not be construed as limiting the scope of protection of the present invention.

[0031] Example:

[0032] based on Figure 1 The diagram shows the optical path of a reflective volume grating uniformity enhancement device, designed for use with PTR glass as the recording medium. The device includes an axial exposure dose compensation system and a dual-beam interference exposure grating structure writing system, both sharing a multi-axis rotating sample stage 2 that holds the recording medium 1.

[0033] The axial exposure dose compensation system includes: a first laser source 3 with a wavelength located in the photosensitive band of the recording medium, and a first electrically controlled aperture 4, a laser power regulator 5, and a first beam expander and collimator unit 6 arranged sequentially along the laser transmission axis 16 of the first laser source 3. The emitted light from this system is incident directly onto the multi-axis rotating sample stage 2.

[0034] The dual-beam interference exposure grating structure writing system includes: a second laser source 7 with a wavelength in the high transmittance band of the recording medium 1, and a second electrically controlled aperture 8, a first laser polarization controller 9, and a polarization beam splitter 10 arranged sequentially along its optical path. The polarization beam splitter 10 splits the beam from the second laser source 7 into two beams: the reflected light is expanded and collimated sequentially by a first reflecting mirror 12 and a second beam expanding and collimating unit 13; the transmitted light is expanded and collimated sequentially by a second laser polarization controller 11, a second reflecting mirror 14, and a third beam expanding and collimating unit 15. The two collimated beams are aligned at an angle... An interference field is formed at the multi-axis rotating sample stage 2 (within the recording medium 1).

[0035] Set recording medium 1 to PTR glass, and according to Figure 2 The transmittance versus wavelength curve of the PTR glass is shown. The wavelength of the first laser source 3, located within the photosensitive band of the PTR glass, is set to 325 nm. The wavelength of the second laser source 7, located within the high transmittance band of the PTR glass, is set to 405 nm. Based on the requirements of the reflective volume grating, its center wavelength is set to 1064 nm, and the grating period of the reflective volume grating is set to 358.15 nm. The interference angle of the two collimated beams at the PTR glass is... The value is 26.98. .

[0036] Subsequently, the apparatus and method for improving the uniformity of reflective volume gratings based on PTR glass are implemented in the following order:

[0037] (A) Parameter determination: Based on the aforementioned principles, the specific values ​​of the axial exposure dose compensation value E1 and the dual-beam interference exposure dose E2 required for the PTR glass in this embodiment are determined.

[0038] (B) Constructing an axial exposure dose compensation system: combining Figure 1 An axial exposure dose compensation system was constructed. A first laser source 3 with a wavelength of 325 nm was turned on, and a first electrically controlled aperture 4, a laser power regulator 5, a first beam expander and collimator 6, and a multi-axis rotating sample stage 2 loading the recording medium were placed sequentially along the propagation direction of the first laser source 3. The position of the multi-axis rotating sample stage 2 was adjusted so that a collimated beam formed by the first beam expander and collimator 6 was incident directly onto the multi-axis rotating sample stage 2.

[0039] (C) Power measurement and irradiation time calculation of the axial exposure dose compensation system: The power P1 at the multi-axis rotating sample stage 2 in the axial exposure dose compensation system is measured using a power meter, and the time t1=E1 / P1 is calculated for a collimated beam formed by the first beam expanding and collimating unit 6 to irradiate the PTR glass.

[0040] (D) Constructing a dual-beam interference exposure grating structure writing system: The first electrically controlled aperture 4 is turned off, and a dual-beam interference exposure grating structure writing system is constructed. A second laser source 7 with a wavelength of 405 nm is turned on, and a second electrically controlled aperture 8, a first laser polarization controller 9, and a polarization beam splitter 10 are placed sequentially along the propagation direction of the second laser source 7. At the reflecting end of the polarization beam splitter 10, a first reflecting mirror 12 and a second beam expanding and collimating unit 13 are placed sequentially along the laser propagation direction. At the transmitting end of the polarization beam splitter 10, a second laser polarization controller 11, a second reflecting mirror 14, and a third beam expanding and collimating unit 15 are placed sequentially along the laser propagation direction. The angle between the two collimated beams formed by the second beam expanding and collimating unit 13 and the third beam expanding and collimating unit 15 at the multi-axis rotating sample stage 2 is adjusted so that the angle is equal to the interference angle. The value is 26.98. .

[0041] (E) Calibration of the polarization state and power of the dual-beam interference exposure grating structure writing system: The ratio of S-polarization to P-polarization of the laser generated by the second laser source 7 is adjusted by the first laser polarization controller 9. The second laser polarization controller 11 is adjusted so that the polarization states of the two beams separated by the polarization beam splitter 10 are consistent. The use of the first laser polarization controller 9 and the second laser polarization controller 11 ensures that the power ratio of the two beams separated by the polarization beam splitter 10 is 1. Through the above adjustments, the contrast of the two collimated interference beams is optimized.

[0042] (F) Power measurement and irradiation time calculation of the dual-beam interference exposure grating structure writing system: The power P2 at the multi-axis rotating sample stage 2 in the dual-beam interference exposure grating structure writing system is measured using a power meter, and the irradiation time t2=E2 / P2 of the two collimated beams formed by the second beam expanding and collimating unit 13 and the third beam expanding and collimating unit 15 at the PTR glass is calculated.

[0043] (G) Performing Axial Exposure Dose Compensation: The second electrically controlled aperture 8 is closed. A PTR glass is placed on the multi-axis rotating sample stage 2 containing the recording medium. The rotation mode of the PTR glass on the multi-axis rotating sample stage 2 containing the recording medium 1 during the irradiation stage of the axial exposure dose compensation system is determined. Specifically, a collimated beam formed by the first beam expanding and collimating unit 6 is directed to the PTR glass. Simultaneously, the multi-axis rotating sample stage 2 containing the recording medium 1 is adjusted so that the PTR glass continuously rotates around the z-axis or y-axis. It should be noted that this embodiment uses continuous rotation of the recording medium (PTR glass) 1 to uniformly irradiate it, thereby achieving axial exposure dose compensation for the PTR glass. Those skilled in the art will understand that by controlling the multi-axis rotating sample stage 2 to rotate the recording medium (PTR glass) 1 stepwise to multiple specific angles and expose it sequentially, axial exposure dose compensation can also be achieved when the sum of the accumulated exposure doses at each angle reaches E1. Subsequently, the first electrically controlled aperture 4 is activated, allowing a collimated beam formed by the first beam expanding and collimating unit 6 to irradiate the PTR glass. Simultaneously, the multi-axis rotating sample stage 2 is adjusted to continuously rotate the PTR glass around the z-axis or y-axis. When the first electrically controlled aperture 4 is activated for a duration of t1, i.e., the axial exposure dose of the PTR glass reaches the compensation value E1, the first electrically controlled aperture 4 is closed, and the orientation of the PTR glass is restored to its initial orientation before irradiation by the axial exposure dose compensation system. The z-axis is orthogonal to the laser transmission axis 16 of the first laser source with a wavelength of 325 nm in the axial exposure dose compensation system and parallel to the length direction of the PTR glass. The y-axis is perpendicular to both the z-axis and the laser transmission axis 16.

[0044] (H) Performing dual-beam interferometry exposure: Determine the rotation mode of the PTR glass at the multi-axis rotating sample stage 2 during the irradiation stage of the dual-beam interferometry exposure grating structure writing system. Specifically, the two collimated beams formed by the second beam expanding and collimating unit 13 and the third beam expanding and collimating unit 15 interfere at the PTR glass, while adjusting the multi-axis rotating sample stage so that the PTR glass rotates continuously only around the z-axis. Similarly, in the dual-beam interferometry exposure stage, this embodiment uses a continuous rotation of the recording medium (PTR glass) 1 to ensure the uniformity of grating structure writing. By rotating stepwise to multiple angles and performing interferometry exposure sequentially, uniform writing of the grating structure can also be achieved when the cumulative dose reaches E2. Subsequently, the second electrically controlled aperture 8 is opened so that the two collimated beams formed by the second beam expanding and collimating unit 13 and the third beam expanding and collimating unit 15 interfere at the PTR glass, while adjusting the multi-axis rotating sample stage 2 so that the PTR glass rotates continuously only around the z-axis. When the second electronically controlled aperture 8 is open for a duration of t2, that is, when the double-beam interference exposure dose of the PTR glass reaches E2, the second electronically controlled aperture 8 is closed.

[0045] (I) Post-processing: The PTR glass that has undergone axial exposure dose compensation and dual-beam interference exposure is heat-treated to finally obtain a reflective volume grating with improved uniformity using PTR glass as the recording medium.

[0046] The above embodiments are used to explain and illustrate the present invention, but not to limit the present invention. Any modifications and changes made to the present invention within the spirit and scope of the claims shall fall within the protection scope of the present invention.

Claims

1. A reflective volume grating uniformity enhancement device, characterized by, It includes an axial exposure dose compensation system and a dual-beam interference exposure grating structure writing system used sequentially, and the two systems share a multi-axis rotating sample stage (2) that loads the recording medium (1). The axial exposure dose compensation system includes a first laser source (3) with a wavelength located in the photosensitive band of the recording medium (1), and a first electronically controlled aperture (4), a laser power regulator (5) and a first beam expansion and collimation unit (6) arranged sequentially along the optical path of the first laser source (3). The emitted light from the first beam expansion and collimation unit (6) acts on the multi-axis rotating sample stage (2). The dual-beam interference exposure grating structure writing system includes a second laser source (7) with a wavelength located in the high transmittance band of the recording medium (1), and an interference optical path for splitting, collimating, and forming two interference beams from the second laser source (7). In the interference optical path, the beam from the second laser source (7) passes through a second electrically controlled aperture (8) and a first laser polarization controller (9) in sequence before being incident on a polarization beam splitter (10). The reflected optical path output by the polarization beam splitter (10) is provided with a first reflector (12) and a second beam expansion and collimation unit (13) in sequence, while the transmitted optical path is provided with a second laser polarization controller (11), a second reflector (14), and a third beam expansion and collimation unit (15) in sequence. The two collimated beams emitted from the second and third beam expansion and collimation units (13, 15) are angled. Interference occurs at the multi-axis rotating sample stage (2).

2. The reflective volume grating uniformity improvement device according to claim 1, characterized in that, Adjusting the orientation of the first reflector (12) and / or the second reflector (14) can change the interference angle of the two collimated beams emitted from the second beam expanding and collimating unit (13) and the third beam expanding and collimating unit (15). .

3. The reflective volume grating uniformity improvement device according to claim 1, characterized in that, By adjusting the first laser polarization controller (9) and / or the second laser polarization controller (11), the polarization state consistency and power ratio of the two beams after being split by the polarization beam splitter (10) are controlled.

4. The reflective volume grating uniformity improvement device according to claim 1, characterized in that, The multi-axis rotating sample stage (2) can drive the recording medium (1) to rotate continuously or in stages to a series of predetermined angles, so as to change the orientation of the recording medium (1) during the irradiation stage of the axial exposure dose compensation system and the dual-beam interference exposure grating structure writing system.

5. The reflective volume grating uniformity improvement device according to claim 4, characterized in that, The multi-axis rotating sample stage (2) can drive the recording medium (1) to rotate around its z-axis and / or y-axis; the z-axis is orthogonal to the laser transmission axis (16) of the first laser source (3) and parallel to the length direction of the recording medium (1); the y-axis is perpendicular to both the z-axis and the laser transmission axis (16).

6. A method for improving the uniformity of a reflective volume grating, characterized in that, Using the apparatus of claim 5, and comprising the following steps: S1. Parameter Determination: Based on the usage requirements of the reflective volume grating and the photosensitivity of the recording medium (1), the axial exposure dose compensation value E1 and the dual-beam interference exposure dose E2 are determined, and the grating period distribution of the reflective volume grating to be prepared is determined; then, based on the grating period distribution, the interference angle of the two collimated beams of the dual-beam interference exposure grating structure writing system at the multi-axis rotating sample stage (2) is determined. The value of ; S2. Constructing an axial exposure dose compensation system and calibrating its optical path: Construct the system according to the composition and connection relationship of the axial exposure dose compensation system described in claim 1, and measure the power P1 acting on the recording medium to calculate the corresponding exposure time t1 = E1 / P1; S3. Constructing a dual-beam interference exposure grating structure writing system and calibrating its optical path: Turn off the axial exposure dose compensation system, and construct the system according to the composition and connection relationship of the dual-beam interference exposure grating structure writing system as described in claim 1; calibrate the included angle between the two interference beams in the system to the interference angle described in step S1. The value of is determined, and the polarization states of the two interfering beams are adjusted to be consistent and their power ratio is adjusted; the power P2 of the system acting on the recording medium is measured, and the corresponding exposure time t2 = E2 / P2 is calculated; S4. Perform axial exposure dose compensation: Place the recording medium (1) on the multi-axis rotating sample stage (2) and turn on the axial exposure dose compensation system for exposure; during this process, in order to change the penetration path of the light beam in the recording medium (1) to achieve axial uniform compensation, control the multi-axis rotating sample stage (2) to make the recording medium (1) continuously rotate around its z-axis and / or y-axis, or based on multiple angles around the z-axis and / or y-axis selected in advance, control the multi-axis rotating sample stage (2) stepwise to make the recording medium (1) rotate to the predetermined angle. When the exposure time reaches the exposure time t1, that is, when the axial exposure dose of the recording medium (1) reaches the compensation value E1, turn off the axial exposure dose compensation system; S5. Restore initial posture: Restore the posture of the recording medium (1) to the initial posture before the start of step S4; S6. Perform dual-beam interference exposure: Turn on the dual-beam interference exposure grating structure writing system to perform interference exposure; during this process, in order to write a high-contrast uniform grating structure in the recording medium, control the multi-axis rotating sample stage (2) to make the recording medium (1) rotate continuously around its z-axis, or based on multiple angles around the z-axis selected in advance, control the multi-axis rotating sample stage (2) step by step to make the recording medium (1) rotate to the predetermined angle. When the exposure time reaches the exposure time t2, that is, when the dual-beam interference exposure dose of the recording medium (1) reaches E2, turn off the dual-beam interference exposure grating structure writing system. S7. Post-processing: The recording medium (1) after axial exposure dose compensation and dual-beam interference exposure is heat-treated to form a reflective volume grating with improved uniformity.

7. The method for improving the uniformity of a reflective volume grating according to claim 6, characterized in that, In step S2, the construction of the axial exposure dose compensation system specifically involves: turning on the first laser source (3) with a wavelength located in the photosensitive band of the recording medium (1), and sequentially placing the first electronically controlled aperture (4), laser power regulator (5), first beam expansion and collimation unit (6), and multi-axis rotating sample stage (2) loaded with the recording medium along the propagation direction of the first laser source (3); adjusting the position of the multi-axis rotating sample stage (2) loaded with the recording medium so that a collimated beam emitted through the first beam expansion and collimation unit (6) is incident directly onto the multi-axis rotating sample stage (2).

8. The method for improving the uniformity of a reflective volume grating according to claim 6, characterized in that, In step S3, the construction of the dual-beam interference exposure grating structure writing system specifically involves: turning on the second laser source (7) with a wavelength located in the high transmittance band of the recording medium (1), and sequentially placing the second electronically controlled aperture (8), the first laser polarization controller (9), and the polarization beam splitter (10) along the propagation direction of the second laser source (7); at the reflecting end of the polarization beam splitter (10), sequentially placing the first reflecting mirror (12) and the second beam expanding and collimating unit (13) along the laser propagation direction; at the transmitting end of the polarization beam splitter (10), sequentially placing the second laser polarization controller (11), the second reflecting mirror (14), and the third beam expanding and collimating unit (15) along the laser propagation direction; adjusting the angle between the two collimated beams emitted from the second beam expanding and collimating unit (13) and the third beam expanding and collimating unit (15) at the multi-axis rotating sample stage (2), so that the angle is equal to the interference angle described in step S1. The value of the value is as follows: The adjustment of the polarization state of the two interfering beams and their power ratio is specifically as follows: the polarization state of the two beams after being split by the polarization beam splitter (10) is controlled to be consistent and their power ratio is 0.8-1.2 by adjusting the first laser polarization controller (9) and / or the second laser polarization controller (11).

9. The method for improving the uniformity of a reflective volume grating according to claim 8, characterized in that, The power ratio of the two beams is 1.

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