Method and system for measuring optical axis deviation angle of electro-optical crystal

Abstract

The invention discloses a measuring method and a measuring system for an optical axis deviation angle of an electro-optical crystal. The measuring method comprises the following steps: enabling polarized laser to enter a light passing surface of the electro-optical crystal to be measured; enabling the electro-optical crystal to be measured to continuously rotate for N times around the rotating shaft along a preset direction; at each rotation position, the first imaging device is used for imaging the first reflected light, and coordinates of the calibration points are obtained; the polarized laser is focused and then enters the electro-optical crystal to be measured again, and first refracted light and second refracted light are obtained; enabling the first refracted light and the second refracted light to interfere with each other, imaging the interference light at each rotation position by using a second imaging device to obtain an interference image which comprises an optical axis exposure point, and acquiring coordinates of the optical axis exposure point in the interference image; and correcting the coordinates of the optical axis exposure points of the rotation positions by using the coordinates of the calibration points of the rotation positions, and obtaining the optical axis deviation angle of the electro-optical crystal according to all the corrected coordinates of the optical axis exposure points.

Description

Technical Field

[0001] The present invention relates to the field of measurement technology, and in particular to a method and a system for measuring an optical axis deviation angle of an electro-optical crystal. Background Art

[0002] The Pockels cell is one of the key components for achieving high-energy, short-pulse laser output. The electro-optical crystal inside the device is the main body of the Pockels effect. During the processing of the crystal, it is usually cut in a direction perpendicular to its optical axis. The design requires that the direction of its optical axis must be parallel to the normal direction of the light-passing surface, so the determination of its optical axis direction is particularly important. The axis error determines the cutting error. As the cutting error increases, the light energy loss of the electro-optical crystal will increase rapidly during use, and the conversion efficiency will also become lower.

[0003] To reduce the angular error of electro-optical crystals, accurate measurements are first required, which requires professional measuring instruments. Conventional X-ray crystal axis determination detection equipment is based on the principle of Bragg diffraction, uses a standard steel plate as the reference for the light-transmitting surface, and requires the crystal to be tightly adsorbed on the standard steel plate. Scratches are easily generated when the crystal surface contacts the standard steel plate. In high-power laser systems, scratches can easily cause energy concentration and lead to damage to the electro-optical crystal. Therefore, the X-ray diffraction method is not suitable for optical axis determination detection of electro-optical crystals that have high requirements for surface defects. In addition, existing X-ray diffraction orientation instruments cannot achieve optical axis orientation of large-aperture crystal elements.

[0004] The direction of the optical axis of the crystal can also be determined by using the polarization interference pattern of the crystal. The optical axis emergence point (i.e., the black cross intersection) is found in the interference pattern, and the direction of the optical axis is measured according to the position of the optical axis emergence point relative to the center of the field of view. Polarized light interference of crystals is commonly achieved by using a polarizing microscope. The distance between the optical axis emergence point and the field of view center is measured by using the eyepiece graticule and visual inspection. The optical axis deviation angle can be calculated by combining the numerical aperture of the microscope, but this method has errors. Summary of the invention

[0005] In view of this, the present invention provides a method and a system for measuring the optical axis deviation angle of an electro-optical crystal.

[0006] As a first aspect of the present invention, the measuring method comprises:

[0007] The polarized laser is incident on the light-transmitting surface of the electro-optical crystal to be tested, wherein the polarized laser is a collimated laser;

[0008] The electro-optical crystal to be tested is made to rotate continuously N times around a rotation axis in a preset direction, and the electro-optical crystal to be tested can return to a starting state after rotating N times, and the rotation axis is a straight line where the polarized laser is located;

[0009] At each rotation position, the first reflected light is imaged by a first imaging device, and the coordinates of the image of the first reflected light in the first imaging device, i.e., the coordinates of the calibration point, are obtained; the rotation position is the position reached by the electro-optical crystal to be tested after each rotation, and the first reflected light is the polarized laser reflected by the electro-optical crystal to be tested;

[0010] focusing the polarized laser light and then incident it again on the electro-optical crystal to be tested, wherein the focused polarized laser light undergoes birefringence in the electro-optical crystal to be tested to obtain a first refracted light and a second refracted light;

[0011] The first refracted light and the second refracted light are interfered with each other, and at each rotation position, the interference light is imaged by a second imaging device to obtain the interference image, wherein the interference image includes an optical axis exposure point, and the coordinates of the optical axis exposure point in the interference image are obtained;

[0012] The coordinates of the optical axis exposure points at each rotation position are corrected using the coordinates of the calibration points at the rotation position, and the optical axis deviation angle of the electro-optical crystal is obtained according to the coordinates of all corrected optical axis exposure points.

[0013] According to an embodiment of the present invention, using the coordinates of the calibration point of each rotation position to correct the coordinates of the optical axis exposure point of the rotation position includes:

[0014] The coordinates of the optical axis exposure point at each rotational position are corrected according to the offset of the coordinates of the calibration point at each rotational position relative to the coordinates of the calibration point.

[0015] According to an embodiment of the present invention, the method for obtaining the calibration points includes:

[0016] The polarized laser light is made to pass through the position where the electro-optical crystal to be measured is located, and is incident on the plane surface of a plano-convex lens, wherein the plane of the plano-convex lens is parallel to the plane where the electro-optical crystal to be measured is located;

[0017] The second reflected light is imaged by the first imaging device to obtain the coordinates of the image of the second reflected light in the first imaging device, namely the coordinates of the calibration point, wherein the second reflected light is the polarized laser reflected by the plane surface.

[0018] According to an embodiment of the present invention, causing the first refracted light and the second refracted light to interfere includes:

[0019] The target polarization component of the first refracted light and the target polarization component of the second refracted light are caused to interfere with each other, wherein the polarization directions of the target polarization component of the first refracted light and the target polarization component of the second refracted light are the same.

[0020] According to an embodiment of the present invention, the polarization directions of the target polarization component of the first refracted light and the target polarization component of the second refracted light are both opposite to the polarization direction of the polarized laser light.

[0021] According to an embodiment of the present invention, obtaining the optical axis deviation angle of the electro-optical crystal to be tested according to the coordinates of all corrected optical axis exposure points includes:

[0022] Perform circle fitting on the coordinates of all corrected optical axis exposure points to obtain the radius of the fitting circle;

[0023] The optical axis deviation angle of the electro-optical crystal to be tested is obtained according to the radius of the fitting circle.

[0024] As a second aspect of the present invention, a system for measuring the optical axis deviation angle of an electro-optical crystal is further provided, which is used to implement the measurement method as described above, and the measurement system comprises:

[0025] A laser assembly adapted to emit polarized laser light;

[0026] A sample fixing assembly, adapted to cause the electro-optical crystal to be tested to rotate N times continuously around a rotation axis in a preset direction, wherein the rotation axis is a straight line where the polarized laser light is located;

[0027] A first imaging component, adapted to image the first reflected light at each rotational position to obtain the coordinates of the calibration point;

[0028] The first lens is used to focus the polarized laser light and then make it incident on the electro-optical crystal to be tested again, so that the focused polarized laser light is birefred in the electro-optical crystal to be tested to obtain the first refracted light and the second refracted light;

[0029] An interference component, adapted to obtain interference light according to the first refracted light and the second refracted light;

[0030] A second imaging component is adapted to image the interference light at each rotation position to obtain an interference image, wherein the interference image includes an optical axis exposure point;

[0031] The processing component is suitable for correcting the coordinates of the optical axis exposure point at each rotation position using the coordinates of the calibration point at the rotation position, and is suitable for obtaining the optical axis deviation angle of the electro-optical crystal based on the coordinates of all corrected optical axis exposure points.

[0032] According to an embodiment of the present invention, the laser assembly comprises:

[0033] A laser, adapted to emit an initial laser;

[0034] A second lens, adapted to focus the initial laser;

[0035] A filtering unit, adapted to filter the focused initial laser to obtain a filtered laser;

[0036] A third lens, adapted to convert the filtered laser light into parallel laser light;

[0037] A polarizer is used to convert the parallel laser light into the polarized laser light.

[0038] According to an embodiment of the present invention, the above measurement system further includes:

[0039] A beam splitter, adapted to reflect the first reflected light;

[0040] The fourth lens is arranged between the beam splitter and the first imaging component, and is suitable for focusing the reflected first reflected light onto the first imaging component.

[0041] According to an embodiment of the present invention, the measurement system further includes:

[0042] An electric aperture, disposed between the first lens and the electro-optical crystal to be tested, and adapted to control the intensity of the polarized laser light incident on the electro-optical crystal to be tested after being focused;

[0043] a fifth lens, adapted to focus the first refracted light and the second refracted light;

[0044] The interference component is an analyzer, and the analyzer and the polarizer form an orthogonal polarization system, which is suitable for obtaining a target polarization component of the first refracted light according to the first refracted light, and obtaining a target polarization component of the second refracted light according to the second refracted light, so that the target polarization component of the first refracted light and the target polarization component of the second refracted light interfere with each other to obtain the interference light;

[0045] The sixth lens is adapted to focus the interference light onto the second imaging component.

[0046] According to an embodiment of the present invention, the rotational error is traced by utilizing the first image formed by the first reflected light to obtain the tracing result of the rotational error of the electro-optical crystal to be tested at each rotational position. The position of the optical axis leakage point located at the same rotational position is corrected by utilizing the tracing result. The coordinates of the optical axis leakage point are corrected to the position of the optical axis exposure point corresponding to the time when the sample surface is parallel to the system optical axis in the rotational position, thereby realizing the accurate measurement of the optical axis deviation angle of the electro-optical crystal. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The above and other objects, features and advantages of the present invention will become more apparent through the following description of the embodiments of the present invention with reference to the accompanying drawings, in which:

[0048] Figure 1A flow chart of a method for measuring an optical axis deviation angle of an electro-optical crystal provided according to an embodiment of the present invention is shown.

[0049] Figure 2 A system for measuring the optical axis deviation angle of an electro-optical crystal provided according to an embodiment of the present invention is shown.

[0050] Figure 3 The figure shows the images formed by the first reflected light at two different rotation positions of the electro-optical crystal to be tested according to an embodiment of the present invention.

[0051] Figure 4 The figure shows a first image obtained when the electro-optical crystal to be tested is at a certain rotation position according to an embodiment of the present invention.

[0052] Figure 5 The coordinates of the N=4 time-scaled fixed points and the coordinates of the calibration points provided according to an embodiment of the present invention are shown.

[0053] Figure 6 The interference image obtained when the electro-optical crystal to be tested is at a certain rotation position according to an embodiment of the present invention is shown.

[0054] Figure 7 Shows Figure 6 The identification result of the cross intersection in the interference image.

[0055] Figure 8 The figure shows the fitting results of the cross intersections of all interference images when N=4 according to an embodiment of the present invention.

[0056] Fig. 9 yes Figure 8 A partial enlarged view of .

[0057] Fig.10 The figure shows the leakage points and correction vectors before and after correction according to the embodiment of the present invention.

[0058] Fig.11 The figure shows the coordinate fitting results of a leakage point of a certain optical axis before and after correction when N=4 provided by an embodiment of the present invention.

[0059] Fig.12 The optical path diagram of a light beam after passing through a single refractive surface according to an embodiment of the present invention is shown.

[0060] Fig.13 The figure shows the propagation states of the central light and the edge light in each lens and the sample being tested. Finally, the sixth lens shrinks the light to an appropriate size and the camera collects the interference pattern.

[0061] Fig.14 for Fig.13The local three-dimensional diagram shows the situation where the interference pattern is collected by the camera after the sixth lens reduces the beam to an appropriate size.

[0062] Reference numerals

[0063] 1 laser assembly; 2 sample fixing assembly; 3 first imaging assembly; 4 first lens;

[0064] 5 eighth lens; 6 second imaging component; 7 processing component; 8 electro-optical crystal to be tested;

[0065] 9 fourth lens; 10 electric aperture; 11 fifth lens; 12 analyzer;

[0066] 13: sixth lens; 14: plano-convex lens; 15: beam splitter;

[0067] 1-1 laser; 1-2 second lens; 1-3 filter unit; 1-4 third lens

[0068] 1-5 polarizer; DETAILED DESCRIPTION

[0069] In the process of implementing the present invention, it is found that there are two main methods for calculating the deviation angle of the electro-optical crystal. The first method relies on the image matching algorithm to determine the position of the optical axis exposure point, and combines the system parameters (such as lens focal length, beam aperture and number of detector pixels) to calculate the deviation angle. However, this method has significant disadvantages when positioning and testing samples: it requires frequent movement of the focusing lens in and out, and the sample turntable needs to rotate the sample around the system optical axis, and observe the interference fringes on the split light path to determine whether the sample surface normal is parallel to the test system optical axis. This operation not only causes the interference pattern to be offset as a whole due to the movement of the lens, introducing errors, but also the interference fringes are difficult to intuitively and quickly indicate the specific direction and angle of the sample surface normal deviating from the system optical axis, thereby increasing the difficulty of manually correcting the sample posture and complicating the code writing for automatically correcting the sample posture. In addition, this method does not fully consider the influence of sample thickness on the imaging beam.

[0070] Another method uses a digital photoelectric internal focusing autocollimator to calibrate the optical axis direction, and calculates the deviation angle by combining the optical axis exposure point position of the interference fringes with the crystal surface normal position. Although this method performs well in ensuring measurement accuracy and repeatability, it requires replacing lenses with different F numbers to adapt the measurement when facing electro-optical crystal samples of different thicknesses and refractive indices, which obviously increases costs and time in practical applications.

[0071] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

[0072] The terms used herein are only for describing specific embodiments and are not intended to limit the present invention. The terms "comprise", "include", etc. used herein indicate the existence of the features, steps, operations and / or components, but do not exclude the existence or addition of one or more other features, steps, operations or components.

[0073] All terms used herein, including technical and scientific terms, have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein should be interpreted as having a meaning consistent with the context of this specification, and should not be interpreted in an idealized or overly rigid manner.

[0074] When using expressions such as "at least one of A, B, and C, etc.", it should generally be interpreted as the meaning of the expression generally understood by those skilled in the art. For example, "a system having at least one of A, B, and C" should include but not be limited to a system having A alone, B alone, C alone, A and B, A and C, B and C, and / or A, B, C, etc. When using expressions such as "at least one of A, B, or C, etc.", it should generally be interpreted as the meaning of the expression generally understood by those skilled in the art. For example, "a system having at least one of A, B, or C" should include but not be limited to a system having A alone, B alone, C alone, A and B, A and C, B and C, and / or A, B, C, etc.

[0075] It should also be noted that the directional terms mentioned in the embodiments, such as "upper", "lower", "front", "back", "left", "right", etc., are only reference directions of the drawings and are not intended to limit the scope of protection of the present invention. Throughout the drawings, the same elements are represented by the same or similar reference numerals. Conventional structures or configurations will be omitted when they may cause confusion in the understanding of the present invention.

[0076] Figure 1 A flow chart of a method for measuring an optical axis deviation angle of an electro-optical crystal provided according to an embodiment of the present invention is shown.

[0077] like Figure 1 As shown, the measuring method includes operations S1 to S6.

[0078] In operation S1 , a polarized laser is incident on a light-transmitting surface of an electro-optical crystal to be tested.

[0079] In operation S2, the electro-optical crystal to be tested is rotated N times continuously around the rotation axis in a preset direction. After rotating N times, the electro-optical crystal to be tested can return to the initial state, and the rotation axis is the straight line where the polarized laser is located.

[0080] In operation S3, at each rotation position, the first reflected light is imaged by using a first imaging device, and the coordinates of the image of the first reflected light in the first imaging device, i.e., the coordinates of the calibration point, are obtained; the rotation position is the position reached by the electro-optical crystal to be tested after each rotation, and the first reflected light is the polarized laser reflected by the electro-optical crystal to be tested.

[0081] In operation S4, the polarized laser light is focused and incident again on the electro-optical crystal to be tested, wherein the focused polarized laser light undergoes birefringence in the electro-optical crystal to be tested, thereby obtaining a first refracted light and a second refracted light;

[0082] In operation S5, the first refracted light and the second refracted light are interfered with each other, and at each rotation position, the interference light is imaged by a second imaging device to obtain an interference image, the interference image includes an optical axis exposure point, and the coordinates of the optical axis exposure point in the interference image (i.e., the coordinates of the cross intersection point) are obtained;

[0083] In operation S6, the coordinates of the optical axis exposure point at each rotation position are corrected using the coordinates of the calibration point at the rotation position, and the optical axis deviation angle of the electro-optical crystal is obtained according to the coordinates of all corrected optical axis exposure points.

[0084] According to an embodiment of the present invention, the optical axis deviation angle of the electro-optical crystal is the angle between the optical axis of the electro-optical crystal and the normal line of its light-transmitting surface.

[0085] According to an embodiment of the present invention, the image of the first reflected light is used to trace the rotational error, and the tracing results of the rotational error of the electro-optical crystal 8 to be tested in each rotational position are obtained. The position of the optical axis leakage point located at the same rotational position is corrected using the tracing results, and the coordinates of the optical axis leakage point are corrected to the position of the optical axis exposure point corresponding to the sample surface and the system optical axis in the rotational position, thereby achieving accurate measurement of the optical axis deviation angle of the electro-optical crystal.

[0086] According to an embodiment of the present invention, in operation S5, the first refracted light and the second refracted light are interfered, including interfering the target polarization component of the first refracted light and the target polarization component of the second refracted light, wherein the polarization directions of the target polarization component of the first refracted light and the target polarization component of the second refracted light are the same.

[0087] According to the embodiment of the present invention, the polarization directions of the target polarization component of the first refracted light and the target polarization component of the second refracted light are opposite to the polarization direction of the polarized laser, which can effectively filter out stray light and reduce interference factors in the measurement process, thereby improving the precision and accuracy of the measurement. By accurately controlling the polarization state of light, the reliability and repeatability of the measurement results can be ensured.

[0088] According to an embodiment of the present invention, in operation S6, the coordinates of the optical axis exposure point of each rotation position are corrected using the coordinates of the calibration point of that rotation position, including: correcting the coordinates of the optical axis exposure point of that rotation position according to the offset of the coordinates of the calibration point of each rotation position relative to the coordinates of the calibration point.

[0089] According to an embodiment of the present invention, the method for acquiring the calibration points includes operations S61 and S62.

[0090] In operation S61, a polarized laser is made to pass through the position where the electro-optical crystal to be tested is located, and is incident on the plane surface of the plano-convex lens, wherein the plane of the plano-convex lens is parallel to the plane where the electro-optical crystal to be tested is located.

[0091] In operation S62, the second reflected light is imaged by using the first imaging device to obtain coordinates of the image of the second reflected light in the first imaging device, ie, coordinates of the calibration point. The second reflected light is polarized laser light reflected by a plane surface.

[0092] According to an embodiment of the present invention, obtaining the optical axis deviation angle of the electro-optical crystal to be tested according to the coordinates of all corrected optical axis exposure points includes operations S63 to S64.

[0093] In operation S63, circle fitting is performed on the coordinates of all corrected optical axis exposure points to obtain a fitting circle radius.

[0094] In operation S64, the optical axis deviation angle of the electro-optical crystal to be tested is obtained according to the radius of the fitting circle.

[0095] According to an embodiment of the present invention, as a second aspect of the present invention, a system for measuring the optical axis deviation angle of an electro-optical crystal is provided, which is used to implement the above-mentioned measurement method.

[0096] Figure 2 A system for measuring the optical axis deviation angle of an electro-optical crystal provided according to an embodiment of the present invention is shown.

[0097] like Figure 2 As shown, the measurement system includes: a laser component 1, a sample fixing component 2, a first imaging component 3, a first lens 4, an interference component, a second imaging component 6 and a processing component 7.

[0098] According to an embodiment of the present invention, the laser assembly 1 is suitable for emitting polarized laser light. The sample fixing assembly 2 is suitable for causing the electro-optical crystal 8 to be tested to rotate continuously N times along a preset direction around a rotation axis, and the rotation axis is a straight line where the polarized laser light is located. The first imaging assembly 3 is suitable for imaging the first reflected light at each rotation position to obtain the coordinates of the calibration point. The first lens 4 is suitable for focusing the polarized laser light and then incident it on the electro-optical crystal to be tested again, so that the focused polarized laser light undergoes birefringence in the electro-optical crystal to be tested, and obtains the first refracted light and the second refracted light. The interference assembly is suitable for causing the first refracted light and the second refracted light to interfere. The second imaging assembly 6 is suitable for imaging the interference light at each rotation position, and the interference image includes an optical axis exposure point. The processing assembly 7 is suitable for correcting the coordinates of the optical axis exposure point at each rotation position using the coordinates of the calibration point at the rotation position, and is suitable for obtaining the optical axis deviation angle of the electro-optical crystal according to the coordinates of all corrected optical axis exposure points.

[0099] According to an embodiment of the present invention, the first reflected light is imaged by a first imaging component to obtain an image of the first reflected light, the interference image is obtained by a second imaging component, the rotational error of the electro-optical crystal 8 to be tested at each rotational position is obtained according to the image of the first reflected light by a processing component, and the position of the optical axis leakage point in the interference image at the same rotational position is corrected by the rotational error, and the coordinates of the optical axis leakage point are corrected to the position of the optical axis exposure point corresponding to the sample surface being parallel to the system optical axis in the rotational orientation, thereby realizing accurate measurement of the optical axis deviation angle of the electro-optical crystal.

[0100] According to an embodiment of the present invention, the sample fixing component 2 may be, for example, a large six-dimensional adjustment frame, the first imaging component may be a first charge coupled device (CCD1), and the second imaging component may be, for example, a second charge coupled device (CCD2).

[0101] According to an embodiment of the present invention, the laser assembly 1 comprises: a laser 1-1, a second lens 1-2, a filtering unit 1-3, a third lens 1-4 and a polarizer 1-5.

[0102] The laser 1-1 is adapted to emit an initial laser. The second lens 1-2 is adapted to focus the initial laser. The filter unit 1-3 is adapted to filter the focused initial laser to obtain a filtered laser. The third lens 1-4 is adapted to convert the filtered laser into a parallel laser. The polarizer 1-5 is adapted to convert the parallel laser into a polarized laser.

[0103] According to an embodiment of the present invention, the measurement system further includes: a beam splitter 15 and a fourth lens 9. The beam splitter 15 is used to reflect the first reflected light. The fourth lens 9 is arranged between the beam splitter 8 and the first imaging component 3, and is suitable for focusing the reflected first reflected light onto the first imaging component 3. The beam splitter 15 is also suitable for transmitting the polarized laser so that the polarized laser can be incident on the light-transmitting surface of the electro-optical crystal 8 to be measured.

[0104] According to an embodiment of the present invention, the above-mentioned measurement system further includes: an electric aperture 10, a fifth lens 11, an analyzer 12, and a sixth lens 13. The electric aperture 10 is arranged between the first lens 4 and the electro-optical crystal 8 to be measured, and is suitable for controlling the light intensity incident on the electro-optical crystal to be measured after the polarized laser is focused. The fifth lens 11 is suitable for focusing the first refracted light and the second refracted light. The analyzer 12, together with the polarizers 1-5, forms an orthogonal polarization system, and is suitable for obtaining the target polarization component of the first refracted light according to the first refracted light, and obtaining the target polarization component of the second refracted light according to the second refracted light, so that the target polarization component of the first refracted light and the target polarization component of the second refracted light interfere with each other to obtain interference light. The sixth lens 13 is suitable for focusing the interference light onto the second imaging component 6.

[0105] Acquire an image formed by the second reflected light in the first imaging device, namely the calibration point, wherein the second reflected light is the polarized laser reflected by the plane surface

[0106] According to an embodiment of the present invention, when the first reflected light is imaged using the first imaging component, the first lens 4 can be removed from the system optical path first, and when the interference image is imaged using the second imaging device, the first lens 4 can be moved into the system optical path so that the polarized laser can be focused and incident on the light-transmitting surface of the electro-optical crystal 8 to be tested.

[0107] According to an embodiment of the present invention, the measurement system further includes a plano-convex lens (also called the seventh lens) 14, which is located between the sample fixing assembly 2 and the fifth lens 11. When obtaining the calibration point, the sample needs to be removed from the measurement system so that the polarized laser passes through the position where the electro-optical crystal to be measured is located and is incident on the plane surface of the plano-convex lens 14. The polarized laser is reflected on the plane surface of the plano-convex lens 14 to obtain a second reflected light. The image formed by the second reflected light in the first imaging device is the calibration point. According to an embodiment of the present invention, the measurement system further includes an eighth lens 5 disposed between the fifth lens 11 and the analyzer 12, which is used to collimate the focused first refracted light and the second refracted light output by the fifth lens, and transmit the collimated light to the analyzer 12.

[0108] The following is a specific embodiment, and based on the above-mentioned measurement system, the measurement process of the optical axis deviation angle is as follows.

[0109] Step A: After the electro-optical crystal 8 to be tested is clamped on the large six-dimensional adjustment frame, the first lens 4 is moved out of the measurement system through the one-dimensional precision electric translation stage, so that the collimated polarized laser beam is incident on the light-transmitting surface of the electro-optical crystal 8 to be tested, and then is reflected by the electro-optical crystal 8 to be tested, split by the beam splitter 15, and focused by the fourth lens 9 to form an image on the first imaging component 3.

[0110] Figure 3 The figure shows the images formed by the first reflected light at two different rotation positions of the electro-optical crystal to be tested according to an embodiment of the present invention.

[0111] like Figure 3 As shown, after the collimated polarized laser beam is incident on the light-transmitting surface of the electro-optical crystal 8 to be measured, it is imaged on the first imaging component 3 after being reflected by the electro-optical crystal 8 at two different rotation positions, split by the beam splitter 15, and focused by the fourth lens 9. That is, the image formed on the first imaging component is the calibration point, which is represented by P1 and P2.

[0112] Figure 4 The figure shows an image formed by the first reflected light when the electro-optical crystal to be tested is in a certain rotation position according to an embodiment of the present invention.

[0113] like Figure 4 As shown, the image obtained when the electro-optical crystal 8 to be tested is at a certain rotation position is a light spot.

[0114] Step B: Use a large six-dimensional adjustment frame to rotate the electro-optical crystal 8 sample to be tested once, i.e., 360°, and evenly rotate N times (rotation interval 360° / N) to collect the image formed by the first reflected light.

[0115] Taking N=4 as an example, due to the rotation error of the large six-dimensional adjustment frame, the surface normal of the electro-optical crystal 8 to be measured will deviate from the system optical axis after each rotation. At this time, the offset of the image of the first reflected light on CCD1 (i.e., the calibration point) at any rotation position relative to the calibration point and the angle of the surface normal of the sample at the rotation position deviating from the system optical axis satisfy the relationship in the following formula (1).

[0116] θ = arcsin (S * d / f) (1)

[0117] Among them, θ represents the angle at which the surface normal of the electro-optical crystal 8 to be tested deviates from the optical axis of the system at any rotational position, S represents the offset of the calibration point at the rotational position relative to the calibration point, d represents the length of a single pixel in CCD1, and f represents the focal length of the fourth lens 9. After the electro-optical crystal 8 to be tested completes one rotation, the image of the first reflected light at each rotational position (i.e., the calibration point at each rotational position) is obtained, and the coordinates of the image of the first reflected light on CCD1 (i.e., the coordinates of the calibration point) are obtained by using the binarization, median filtering, contour recognition, and contour center of gravity calculation in the image processing algorithm. These are saved and recorded together with the coordinates of the calibration points, and the vector of each calibration point pointing to the calibration point will be used for the subsequent cross leakage point correction calculation. The coordinates of the image of the first reflected light on CCD1 (i.e., the coordinates of the calibration point) and the coordinates of the calibration point obtained are as follows: Figure 5 As shown. Figure 5 The mid-calibration points are denoted as P3~P6, and the calibration point is denoted as Q.

[0118] Step C: Reset the first lens 4 to the optical path of the measurement system through a one-dimensional precision electric translation stage. At this time, a conoscopic interference pattern can be observed at CCD2.

[0119] Figure 6 The interference image obtained when the electro-optical crystal to be tested is at a certain rotation position according to an embodiment of the present invention is shown.

[0120] like Figure 6 The figure shows the interference image of the cone light with deviation angle. At this time, a large six-dimensional adjustment frame is used to rotate the sample with the system optical axis as the rotation axis. The interference image is collected by N rotations evenly (the rotation position collected each time corresponds to the rotation position of the sample when the first reflected light is collected).

[0121] At this time, the computer system stores N calibration points and N conoscopic interference images. The rotation positions corresponding to the N calibration points correspond to the rotation positions of the N conoscopic interference patterns. The processing component (such as a computer) uses the method of identifying the edges of the four contours in the interference ring to find the coordinates of the cross intersection points (optical axis emergence points) of the N conoscopic interference images and record them. The recognition results are as follows: Figure 7 shown.

[0122] At this point, the data stored by the processing component include: the coordinates of the images formed by the N first reflected lights on CCD1 (N calibration point coordinates) and the coordinates of the black cross intersections (optical axis leakage points) of the N conoscopic interference images. Due to the influence of the rotation error of the large six-dimensional adjustment frame, directly performing circle fitting on the coordinates of the cross intersections (optical axis leakage points) of the N conoscopic interference images will find that each coordinate has a certain deviation (deviation from the fitting circle), and is not strictly a circular motion trajectory.

[0123] Figure 8 The figure shows the fitting results of the cross intersections of all interference images when N=4 according to an embodiment of the present invention.

[0124] exist Figure 8 The cross intersection points of the interference image in are represented as M1, M2, M3 and M4 respectively.

[0125] Fig. 9 yes Figure 8 A partial enlarged view of .

[0126] like Figure 8~Figure 9 As shown, the fitting result of the optical axis leakage point corresponding to the surface normal of the electro-optical crystal 8 to be tested deviates from the system optical axis by an angle of ≈11 mrad (M=2) is locally enlarged. It can be seen that the cross leakage point does not strictly fall on the circular trajectory. This is caused by the inevitable rotation error when the electro-optical crystal 8 to be tested is rotated on the large six-dimensional adjustment frame, resulting in a small angle deviation of the surface of the electro-optical crystal 8 to be tested.

[0127] Step D, using the coordinates of the N calibration points and the standard points to calculate the correction vector, using these correction vectors to correct the coordinates of the optical axis leakage points in different rotational orientations obtained by N image processing one by one, so as to obtain the N corrected optical axis leakage points of the sample when the surface normal is perpendicular to the system optical axis and rotates around the system optical axis. The coordinate comparison of a certain optical axis leakage point before and after correction is as follows: Fig.10 shown.

[0128] Fig.11 The figure shows the coordinate fitting results of all optical axis leakage points before and after correction when N=4 according to an embodiment of the present invention.

[0129] like Fig.11 As shown, the solid line is the fitted circle trajectory after correction, and the dotted line is the fitted circle trajectory before correction. According to the corrected fitting circle radius r0, the optical axis deviation angle of the electro-optical crystal 8 to be tested can be obtained, which is specifically expressed as formula (2).

[0130] α = r0 ÷ R0 × u'' (2)

[0131] Wherein, α represents the optical axis deviation angle of the electro-optical crystal 8 to be tested, r represents the corrected fitting circle radius, R represents the beam half width of the interference image, and u'' represents the image side aperture angle after the interference light is emitted from the sixth lens.

[0132] According to an embodiment of the present invention, the following content explains in detail the derivation method of u''.

[0133] Because the initial laser light emitted by the laser 1-1 needs to pass through six lenses in sequence before finally forming an image on the second imaging device, namely, the second lens 1-2, the third lens 1-4, the first lens 4, the plano-convex lens 14, the fifth lens 11, and the sixth lens 13. The calculation method of the image-side aperture angle of the light beam after passing through the refractive surface of the lens is as follows.

[0134] Fig.12 The optical path diagram of a light beam after passing through a single refractive surface according to an embodiment of the present invention is shown.

[0135] like Fig.12 As shown, the light beam starts from point A and enters the medium with a refractive index of n at an incident angle of I. At the refraction point E, the light beam is deflected and enters the medium with a refractive index of n' at a refraction angle of T'. The refractive surface has a curvature radius r. The light beam is incident on the refractive surface from point O at an object side aperture angle of -U, and the incident height is h. After refraction, the light beam continues to move forward and propagates in the direction of point A' at an image side aperture angle of U'. The directions of the light beams before and after refraction are marked as U and U', respectively. The propagation distances of the light beam on the refractive surface are the object side intercept -L and the image side intercept L', respectively. This figure schematically shows the path change of the light beam after passing through the refractive surface, providing a theoretical basis for related optical design. In this figure, the lens refractive surface OE is the interface between two media with refractive indices of n and n', C is the center of the lens, and OC is the radius of curvature of the spherical surface, represented by the letter r. The straight line through the center of the sphere is the optical axis, and its intersection with the spherical surface is called the vertex, represented by the letter O. The distance from vertex O to the intersection point A of the light ray and the optical axis is represented by L and is called the object intercept. The angle ∠EAO between the incident light ray and the optical axis is represented by U and is called the object aperture angle. After the light AE is refracted by the refractive surface, it intersects the optical axis at point A'. The image intercept L' = OA', and the image aperture angle U' = ∠EOA'. I is the angle of incidence, l' is the angle of refraction, and clockwise is positive and counterclockwise is negative.

[0136] According to the triangle theorem, in triangle AEC,

[0137] (3)

[0138] From the law of refraction, we can get:

[0139] (4)

[0140] As can be seen in the figure,

[0141] ϕ=U′+I′(5)

[0142] I = −U + ϕ (6)

[0143] The image side aperture angle is expressed as formula (5),

[0144] (7)

[0145] Applying the sine law to triangle A'EC, the square intercept is expressed as (8)

[0146] (8)

[0147] Among them, U, U′ and are the object-side and image-side aperture angles and the object-side and image-side intercepts of the light, and d is the distance between the two refractive surfaces.

[0148] When there are multiple lenses in the system, the above formula needs to be used to calculate the image-side aperture angle of the light emitted from the rear surface of the sixth lens one by one.

[0149] Fig.13 The propagation state of the light beam in each lens and the sample under test is shown. Finally, the sixth lens shrinks the beam to an appropriate size and the camera completes the collection of the interference pattern.

[0150] Fig.14 for Fig.13 The local three-dimensional diagram shows the situation where the interference pattern is collected by the camera after the sixth lens reduces the beam to an appropriate size.

[0151] Combination Figure 13-14 The light path diagram in , as well as the parameters in equation (8) and Table 1, can be calculated lens by lens to obtain u'' of the light emitted from the rear surface of the sixth lens. Calculation of each lens requires transferring the calculation result of the previous refractive surface to the next refractive surface, and the surface rotation formula can be used to transfer the calculation result. The surface rotation formula involves the parameter changes of light propagating from one surface to another in the optical system. The surface rotation formula is expressed as equation (9) to equation (10).

[0152] u m+1 =u m '(9)

[0153] L m+1 =L m ′−d(10)

[0154] The surface rotation formula reflects how the light parameters are transferred from the mth surface to the m+1th surface under certain conditions. From the surface rotation formula, it can be seen that after the light passes through a refractive surface (or curved surface), its direction will not change before reaching the next surface position. The image side aperture angle u of the previous refractive surface m ′ is the object aperture angle u of the next surface m+1. That is, in the propagation between the two surfaces, the direction of the light remains unchanged. This formula can also express that when the light propagates from the first surface to the second surface, its height (intercept) will change due to the propagation distance d. Through this formula, the object intercept of the light from the first refractive surface on the second surface can be calculated. The above formula is usually used in the linear approximation transfer step in the ray tracing algorithm. Through the above formula, the state of the light can be gradually updated and its behavior in the multi-optical system can be analyzed.

[0155] The parameters of the various devices in the embodiment of the present invention are shown in Table 1. u'' can be calculated according to the above formula and the parameters in Table 1.

[0156] Table 1

[0157]

[0158] The measurement method provided according to the embodiment of the present invention avoids the back-and-forth translation and resetting of the first lens, adopts the method of identifying the rotation error and correcting the coordinates of the leakage point of the cross optical axis, and the measurement process is simple and achieves a high degree of automation. The method of calculating the deviation angle takes into account the influence of the thickness and refractive index of different samples (electro-optical crystals to be measured), so that the measurement system can adapt to more specifications and types of electro-optical crystal samples to be measured (such as lithium niobate, BBO, etc.) without changing the lens.

[0159] The embodiments of the present invention are described above. However, these embodiments are only for the purpose of illustration, and are not intended to limit the scope of the present invention. Although each embodiment is described above, it does not mean that the measures in each embodiment cannot be used in combination. The scope of the present invention is defined by the attached claims and their equivalents. Without departing from the scope of the present invention, those skilled in the art may make various substitutions and modifications, which should all fall within the scope of the present invention.

Claims

1. A method for measuring the optical axis deviation angle of an electro-optical crystal, comprising: The polarized laser is incident on the light-transmitting surface of the electro-optical crystal to be tested, wherein the polarized laser is a collimated laser; The electro-optical crystal to be tested is made to rotate continuously N times around a rotation axis in a preset direction, and the electro-optical crystal to be tested can return to a starting state after rotating N times, and the rotation axis is a straight line where the polarized laser is located; At each rotation position, the first reflected light is imaged by a first imaging device, and the coordinates of the image of the first reflected light in the first imaging device, i.e., the coordinates of the calibration point, are obtained; the rotation position is the position reached by the electro-optical crystal to be tested after each rotation, and the first reflected light is the polarized laser reflected by the electro-optical crystal to be tested; focusing the polarized laser light and then incident it again on the electro-optical crystal to be tested, wherein the focused polarized laser light undergoes birefringence in the electro-optical crystal to be tested to obtain a first refracted light and a second refracted light; The first refracted light and the second refracted light are interfered with each other, and at each rotation position, the interference light is imaged by a second imaging device to obtain the interference image, wherein the interference image includes an optical axis exposure point, and the coordinates of the optical axis exposure point in the interference image are obtained; The coordinates of the optical axis exposure points at each rotation position are corrected using the coordinates of the calibration points at the rotation position, and the optical axis deviation angle of the electro-optical crystal is obtained according to the coordinates of all corrected optical axis exposure points.

2. The measuring method according to claim 1, wherein: Correcting the coordinates of the optical axis exposure point at each rotation position by using the coordinates of the calibration point at the rotation position includes: The coordinates of the optical axis exposure point at each rotational position are corrected according to the offset of the coordinates of the calibration point at each rotational position relative to the coordinates of the calibration point.

3. The measuring method according to claim 2, wherein: The method for obtaining the calibration point includes: The polarized laser light is made to pass through the position where the electro-optical crystal to be measured is located, and is incident on the plane surface of a plano-convex lens, wherein the plane of the plano-convex lens is parallel to the plane where the electro-optical crystal to be measured is located; The second reflected light is imaged by the first imaging device to obtain the coordinates of the image of the second reflected light in the first imaging device, namely the coordinates of the calibration point, wherein the second reflected light is the polarized laser reflected by the plane surface.

4. The measuring method according to claim 1, wherein: The first refracted light and the second refracted light are caused to interfere with each other, comprising: The target polarization component of the first refracted light and the target polarization component of the second refracted light are caused to interfere with each other, wherein the polarization directions of the target polarization component of the first refracted light and the target polarization component of the second refracted light are the same.

5. The measuring method according to claim 4, wherein: The polarization directions of the target polarization component of the first refracted light and the target polarization component of the second refracted light are both opposite to the polarization direction of the polarized laser light.

6. The measuring method according to claim 1, wherein: Obtaining the optical axis deviation angle of the electro-optical crystal to be tested according to the coordinates of all corrected optical axis exposure points includes: Perform circle fitting on the coordinates of all corrected optical axis exposure points to obtain the radius of the fitting circle; The optical axis deviation angle of the electro-optical crystal to be tested is obtained according to the radius of the fitting circle.

7. A system for measuring the optical axis deviation angle of an electro-optical crystal, used to implement the measurement method according to any one of claims 1 to 6, the measurement system comprising: A laser assembly adapted to emit polarized laser light; A sample fixing assembly, adapted to cause the electro-optical crystal to be tested to rotate N times continuously around a rotation axis in a preset direction, wherein the rotation axis is a straight line where the polarized laser light is located; A first imaging component, adapted to image the first reflected light at each rotational position to obtain the coordinates of the calibration point; The first lens is used to focus the polarized laser light and then make it incident on the electro-optical crystal to be tested again, so that the focused polarized laser light is birefred in the electro-optical crystal to be tested to obtain the first refracted light and the second refracted light; An interference component, adapted to obtain interference light according to the first refracted light and the second refracted light; A second imaging component is adapted to image the interference light at each rotation position to obtain an interference image, wherein the interference image includes an optical axis exposure point; The processing component is suitable for correcting the coordinates of the optical axis exposure point at each rotation position using the coordinates of the calibration point at the rotation position, and is suitable for obtaining the optical axis deviation angle of the electro-optical crystal based on the coordinates of all corrected optical axis exposure points.

8. The measurement system according to claim 7, wherein: The laser assembly comprises: A laser, adapted to emit an initial laser; A second lens, adapted to focus the initial laser; A filtering unit, adapted to filter the focused initial laser to obtain a filtered laser; A third lens, adapted to convert the filtered laser light into parallel laser light; A polarizer is used to convert the parallel laser light into the polarized laser light.

9. The measurement system according to claim 7, further comprising: A beam splitter, adapted to reflect the first reflected light; The fourth lens is arranged between the beam splitter and the first imaging component, and is suitable for focusing the reflected first reflected light onto the first imaging component.

10. The measurement system according to claim 7, wherein: The measuring system further comprises: An electric aperture, disposed between the first lens and the electro-optical crystal to be tested, and adapted to control the intensity of the polarized laser light incident on the electro-optical crystal to be tested after being focused; a fifth lens, adapted to focus the first refracted light and the second refracted light; The interference component is an analyzer, and the analyzer and the polarizer form an orthogonal polarization system, which is suitable for obtaining a target polarization component of the first refracted light according to the first refracted light, and obtaining a target polarization component of the second refracted light according to the second refracted light, so that the target polarization component of the first refracted light and the target polarization component of the second refracted light interfere with each other to obtain the interference light; The sixth lens is adapted to focus the interference light onto the second imaging component.

Images