Polarization diagnosis method based on wedge-shaped crystal

Abstract

The invention relates to the technical field of optical measurement, in particular to a wedge-shaped crystal-based polarization diagnosis method, which comprises the following steps: placing a wedge-shaped crystal in a scattered light transmission path, transmitting the wedge-shaped crystal to a scatter plate through an optical system, recording the light intensity distribution of the scatter plate through camera imaging, and diagnosing the polarization direction. The method can diagnose the polarization direction of the polarized scattered light. The method can be directly used for polarization direction measurement on a large laser device, and compared with a traditional polarization beam splitting method, other elements do not need to be placed in a light path, the light path is not shielded, so that the polarization distribution of the whole backscattered light can be obtained, interference to the energy diagnosis of the scattered light at the rear end is avoided, and the measurement accuracy is improved. The defects of a current large laser device polarization diagnosis method are overcome.

Description

Technical Field

[0001] The invention relates to the technical field of optical measurement, and in particular to a polarization diagnosis method based on a wedge crystal, which is used for polarization diagnosis of scattered light of a large laser device. Background Art

[0002] In large-scale laser devices, high-power density lasers will heat the target to produce plasma. The laser and plasma will produce nonlinear interactions, and backscattered light will be generated through processes such as stimulated Brillouin scattering and stimulated Raman scattering. On the one hand, since the scattered light carries part of the energy and cannot be effectively utilized, the energy coupling efficiency between the laser and the target is greatly reduced. On the other hand, the scattered light transmitted in the reverse direction along the laser optical path may cause irreversible damage to the laser device.

[0003] On the one hand, in order to study the characteristics of scattered light and understand the physical mechanism of the process, it is necessary to diagnose the scattered light in multiple dimensions such as polarization, energy and spectrum. For scattered light polarization diagnosis, a Wollaston prism is usually used. After the horizontal polarization component and the vertical polarization component pass through the prism, the transmission directions are slightly different. By measuring the intensity of the two polarization components, the polarization direction of the scattered light can be obtained.

[0004] On the other hand, in order to suppress the backscattered light generated by high-power density lasers, it is usually necessary to smooth the beam. Polarization smoothing is one of the beam smoothing technologies. It uses a wedge-shaped birefringent crystal. In the near-field cross section, the polarization direction of the laser gradually changes with the lateral distance. After being focused by the target lens, the polarization state of the scattered light at the focus is disordered, which can effectively reduce the energy of the backscattered light.

[0005] The full-aperture backscattered light of a large laser device is usually large in lateral dimensions. The side length of a single beamlet can reach tens of centimeters, which is comparable to the channel size of the driver beam transmission. Polarization diagnosis has the following shortcomings:

[0006] 1) Polarization diagnosis adopts a local sampling method, which can neither give the overall polarization state of the scattered light nor obtain the spatial distribution of the polarization state;

[0007] 2) The polarizing prism needs to be placed in the scattered light transmission path. The bracket holding the prism will block the light path and interfere with the energy diagnosis at the rear end of the light path.

[0008] 3) If the sampling beam is split to perform polarization component diagnosis, the required optical element is too large, the cost is high, and the actual spatial position is limited, the flexibility is poor, and the feasibility is low. Summary of the invention

[0009] In view of the above-mentioned deficiencies in the prior art, the present invention proposes a polarization diagnosis method based on a wedge-shaped crystal. A wedge-shaped crystal is placed in the light transmission path of the scattered light. After being transmitted through the optical system, the light reaches the scattering plate. The light intensity distribution of the scattering plate is recorded by camera imaging, and the polarization direction can be diagnosed.

[0010] A polarization diagnosis method based on a wedge crystal comprises the following steps:

[0011] S1. sequentially placing a wedge-shaped crystal, an optical transmission system and a scattering plate in a light transmission optical path of light scattering, and recording through an imaging lens and a camera;

[0012] S2, the scattered light first passes through the wedge-shaped crystal, and then passes through the optical transmission system; thereafter, the scattered light modulated by the optical transmission system reaches the scattering plate, and the scattering plate makes the angular distribution of the diffuse scattered light obey the Lambertian distribution; finally, the light intensity distribution on the scattering plate is recorded through the imaging lens and the camera;

[0013] S3. Calculate the polarization direction or polarization component of the scattered light by modulating the distribution of the scattered light.

[0014] Preferably, the wedge-shaped crystal is a birefringent crystal, and the crystal axis direction does not coincide with the light beam transmission direction; the optical transmission system is a transmission type or a reflection type, and the transmission efficiency of horizontally polarized light and vertically polarized light is different.

[0015] Preferably, when the scattered light is linearly polarized light, the light intensity distribution on the scattering plate obeys:

[0016]

[0017] Among them, η p and η s are the light intensity transmittance of p-polarized light and s-polarized light passing through the specified optical system, I0 is the average light intensity, k is the lateral spatial modulation wave number introduced by the wedge crystal, is the initial phase, and θ is the angle between the polarization direction of the scattered light and the p-polarization.

[0018] Preferably, the scattered light distribution is divided into a plurality of two-dimensional sub-regions according to the period of the scattered light spatial distribution, and the scale of the smallest region in the polarization modulation direction is not less than the modulation period of a wedge-shaped crystal to the space; the polarization direction of the scattered light in the local region is calculated as follows:

[0019] When the transmittance of the optical system is known, that is, η p and η s It is known that the polarization direction can be solved by the maximum and minimum values ​​of the light intensity distribution through the scattering plate:

[0020]

[0021] Wherein, max(I) or min(I) represents the maximum light intensity or the minimum light intensity of the light to be measured.

[0022] As a preferred embodiment, when the transmittance of the optical system is unknown, that is, η p and η s Unknown, the distribution of the entire system - wedge crystal, optical transmission system, and scattering plate is calibrated by p-polarized light, and then the polarization direction of the scattered light is calculated by the extreme value of the light intensity distribution of the scattering plate:

[0023]

[0024] Among them, I p The light intensity distribution of the scattering plate representing the timing of the p-polarized cursor, max(I p ) or min(I p ) represents the maximum light intensity or minimum light intensity in the p-polarized light measurement, and max(I) or min(I) represents the maximum light intensity or minimum light intensity of the light to be measured;

[0025] Based on this, the polarization direction of the scattered light in each sub-region can be obtained, and combined with the two-dimensional sub-region division, the two-dimensional distribution of the polarization direction of the scattered light can be finally given.

[0026] The beneficial effects of the present invention are:

[0027] The present invention proposes a new diagnostic method based on wedge crystals, which can diagnose the polarization direction of polarized scattered light. This method can be directly used for polarization direction measurement on large laser devices. Compared with the traditional polarization beam splitting method, no other components need to be placed in the optical path, and there is no obstruction to the optical path. Therefore, the polarization distribution of the entire backscattered light can be obtained, and there is no interference with the scattered light energy diagnosis at the rear end, which makes up for the shortcomings of the current polarization diagnostic method of large laser devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0029] Figure 1 It is a schematic diagram of the technical solution of the present invention;

[0030] 1-wedge crystal, 2-optical transmission system, 3-scattering plate, 4-imaging lens and camera. DETAILED DESCRIPTION

[0031] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations.

[0032] In the description of the embodiments of the present invention, it should be noted that if the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc. appear, the orientation or position relationship indicated is based on the orientation or position relationship shown in the drawings, or the orientation or position relationship in which the invented product is usually placed when used. It is only for the convenience of describing the present invention and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation, and therefore cannot be understood as a limitation on the present invention. In addition, the terms "first", "second", "third", etc. are only used to distinguish the description, and cannot be understood as indicating or implying relative importance.

[0033] As shown in the figure, a polarization diagnosis method based on a wedge crystal is shown. A wedge crystal 1 is placed in the scattered light transmission path. The transmission efficiency of horizontally polarized and vertically polarized light in the optical system is different. After being transmitted through the optical transmission system 2, the light reaches the scattering plate 3. The light intensity distribution on the scattering plate will be modulated. The light intensity distribution of the scattering plate 3 is recorded by the imaging lens and the camera 4, and the polarization direction can be diagnosed.

[0034] The wedge-shaped crystal 1 adopts a birefringent crystal, the crystal axis direction does not coincide with the light beam transmission direction, and the ratio of e-light and o-light after passing through the crystal is roughly the same. The scattered light first passes through the wedge-shaped crystal 1, and then passes through the optical transmission system 2. The optical transmission system 2 subsystem has no strict requirements on the specific composition. It can be either transmission or reflection, but the overall transmission efficiency of horizontal polarized light and vertical polarized light must be different. After that, the modulated scattered light reaches the scattering plate 3, which can make the angular distribution of the diffuse scattered light basically obey the Lambertian distribution, that is, isotropic scattering is maintained in all directions in space. Finally, the light intensity distribution on the scattering plate 3 is recorded by the imaging lens and the camera 4. The polarization direction or polarization component of the scattered light can be calculated by the modulation degree of the scattered light distribution.

[0035] When the scattered light is linearly polarized light, the light intensity distribution on the scattering plate obeys:

[0036]

[0037] Among them, η p and η sare the light intensity transmittance of p-polarized light and s-polarized light passing through the specified optical system, I0 is the average light intensity, and is the lateral spatial modulation wave number introduced by the wedge crystal. is the initial phase, θ is the angle between the polarization direction of the scattered light and the p-polarization. Specifically, when θ=0, the coefficient of formula (1) is positive; when θ=π / 2, the coefficient of formula (1) is negative.

[0038] If the transmission efficiency of the optical system for p-light and s-light is different, that is, η p ≠η s ,The scattered light intensity distribution is modulated with the space existence, and the modulation amplitude is related to the polarization direction and the p / s light transmission efficiency difference.

[0039] When used specifically, the scattered light distribution is divided into multiple two-dimensional sub-areas according to the period of the scattered light spatial distribution. In the polarization modulation direction, the size of the smallest area is not less than the modulation period of a wedge-shaped crystal to space. There are two methods for calculating the polarization direction of scattered light in a local area, as follows.

[0040] 1) Method 1

[0041] When the transmittance of the optical system (η p and η s ) is known, the polarization direction can be directly solved through the maximum and minimum values ​​of the light intensity distribution of the scattering plate:

[0042]

[0043] Wherein, max(I) or min(I) represents the maximum light intensity or the minimum light intensity of the light to be measured.

[0044] 2) Method 2

[0045] When the transmittance of the optical system (η p and η s ) is unknown, the distribution of the entire optical system (including wedge-shaped crystal, optical transmission system, and scattering plate) can be calibrated by p-polarized light, and then the polarization direction of the scattered light can be calculated by the extreme value of the light intensity distribution of the scattering plate:

[0046]

[0047] Among them, I p The light intensity distribution of the scattering plate representing the timing of the p-polarized cursor, max(I p ) or min(I p ) represents the maximum light intensity or minimum light intensity in the p-polarized light measurement, and max(I) or min(I) represents the maximum light intensity or minimum light intensity of the light to be measured.

[0048] Based on this, the polarization direction of the scattered light in each sub-region can be obtained. Combined with the spatial grid division given above, the two-dimensional distribution of the polarization direction of the scattered light can be finally given.

[0049] The method of the present invention can diagnose the polarization direction of polarized scattered light, and provides the overall polarization state of the scattered light and the spatial distribution of the polarization state; there is no obstruction or interference from other components in the optical path; and the method can be used for polarization direction measurement on large laser devices, with high measurement flexibility and reduced costs.

[0050] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention shall be included in the protection scope of the present invention.

Claims

1. A polarization diagnosis method based on a wedge crystal, characterized in that: The following steps are involved: S1, sequentially arranging a wedge-shaped crystal (1), an optical transmission system (2) and a scattering plate (3) in a scattered light transmission optical path, and recording through an imaging lens and a camera (4); S2, the scattered light first passes through the wedge-shaped crystal (1), and then passes through the optical transmission system (2); thereafter, the scattered light modulated by the optical transmission system (2) reaches the scattering plate (3), and the scattering plate (3) makes the angular distribution of the diffusely scattered light obey the Lambertian distribution; finally, the light intensity distribution on the scattering plate (3) is recorded through the imaging lens and the camera (4); S3. Calculate the polarization direction or polarization component of the scattered light by modulating the distribution of the scattered light.

2. The polarization diagnosis method based on a wedge crystal according to claim 1, characterized in that: The wedge-shaped crystal (1) is a birefringent crystal, and the crystal axis direction does not coincide with the light beam transmission direction; the optical transmission system (2) is a transmission type or a reflection type, and the transmission efficiencies of horizontally polarized light and vertically polarized light are different.

3. The polarization diagnosis method based on a wedge crystal according to claim 2, characterized in that: When the scattered light is linearly polarized light, the light intensity distribution on the scattering plate (3) obeys: Among them, η p and η s are the light intensity transmittance of p-polarized light and s-polarized light passing through the specified optical system, I0 is the average light intensity, is the transverse spatial modulation wave number introduced by the wedge crystal (1), is the initial phase, and θ is the angle between the polarization direction of the scattered light and the p-polarization.

4. The polarization diagnosis method based on a wedge crystal according to claim 3, characterized in that: According to the period of the scattered light spatial distribution, the scattered light distribution is divided into a plurality of two-dimensional sub-regions. In the polarization modulation direction, the size of the smallest region is not less than the modulation period of a wedge-shaped crystal (1) to space. The polarization direction of the scattered light in the local region is calculated as follows: When the transmittance of the optical system is known, that is, η p and η s It is known that the polarization direction can be solved by the maximum and minimum values ​​of the light intensity distribution of the scattering plate (3): Wherein, max(I) or min(I) represents the maximum light intensity or the minimum light intensity of the light to be measured.

5. The polarization diagnosis method based on a wedge crystal according to claim 4, characterized in that: When the transmittance of the optical system is unknown, that is, η p and η s Unknown, the distribution of the entire system - wedge crystal (1), optical transmission system (2), and scattering plate (3) is calibrated by p-polarized light, and then the polarization direction of the scattered light is calculated by the extreme value of the light intensity distribution of the scattering plate (3): Among them, I p The light intensity distribution of the scattering plate (3) representing the timing of the p-polarized cursor, max(I p ) or min(I p ) represents the maximum light intensity or minimum light intensity in the p-polarized light measurement, and max(I) or min(I) represents the maximum light intensity or minimum light intensity of the light to be measured; Based on this, the polarization direction of the scattered light in each sub-region can be obtained, and combined with the two-dimensional sub-region division, the two-dimensional distribution of the polarization direction of the scattered light can be finally given.

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