Four-beam-splitting optical device

Abstract

The invention relates to the technical field of optical four-wave shearing interference, in particular to a four-beam-splitting optical device. The four-beam-splitting optical device comprises a first modulation unit, a second modulation unit and a third modulation unit which are sequentially arranged at intervals; an incident light beam forms a first light beam after passing through the first modulation unit, and the first light beam forms a second light beam with the polarization state changing along with space after passing through the second modulation unit. The second light beam forms a third light beam after passing through a polarization analyzer, and the third light beam is equivalent to being formed by coherence superposition of four light beams with different propagation directions. The problem that extra diffraction orders are generated due to wavelength changes is avoided. Compared with the prior art, the invention has good modulation performance and does not generate the problem of extra diffraction order.

Description

Technical Field

[0001] The present application relates to the technical field of optical four-wave shearing interference, and in particular to a four-beam splitting optical device. Background Art

[0002] A wavefront detector is a precision instrument specifically used to measure the wavefront distribution of light waves. It plays a key role in many fields such as astronomy, adaptive optics, microscopic imaging, laser systems and biomedicine.

[0003] In recent years, wavefront sensors based on four-wave shearing interferometry have attracted widespread attention due to their excellent performance. The core component of this sensor is a modified Hartmann mask (MHM), which consists of two main parts: one is a square light-blocking plate with a spacing of d and an opening size of a (the ratio is a:d=2:3), and the other is a checkerboard phase grating with a period of 2d, which introduces a phase difference of π between two steps. This diffraction element can copy the incident light wave into four light waves with different propagation directions (corresponding to four diffraction orders), which are displaced after a certain distance and produce interference patterns. By analyzing these interference patterns, the information of the original wavefront can be reconstructed. The design of the checkerboard phase grating effectively eliminates the zero-order light and the even-order diffracted light, while the square light-blocking plate eliminates the integer multiple of three diffracted light. However, the diffracted light of the fifth, seventh, eleventh and other higher orders still exists, which will affect the measurement results. In addition, since the checkerboard phase grating with a phase difference of π is only applicable to light wave modulation of a specific wavelength, any change in the wavelength of the incident light may induce additional diffraction orders, especially the zero-order diffraction light. The generation of additional diffraction orders will cause the interference pattern to deviate from the mathematical model of the ideal four-wave shearing interferometer, thereby affecting the accuracy of the wavefront measurement. Therefore, in order to improve the accuracy of the measurement, it is necessary to optimize the four-beam optical device used in four-wave shearing interferometer to reduce the influence of these non-ideal factors. Summary of the invention

[0004] The present application provides a four-beam splitting optical device to solve one or more technical problems existing in the prior art, and at least provide a beneficial choice or create conditions.

[0005] Other features and advantages of the present application will become apparent from the following detailed description, or may be learned in part by the practice of the present application.

[0006] According to one aspect of an embodiment of the present application, a four-beam splitting optical device is provided, comprising a first modulation unit, a second modulation unit and a third modulation unit which are arranged in sequence and spaced apart; the first modulation unit is a polarizer which only allows a polarization component with a first preset polarization direction in an incident light beam to pass through, and the second modulation unit is a half-wave plate whose optical axis varies with spatial distribution; the incident light beam forms a first light beam after passing through the first modulation unit, and the first light beam forms a second light beam whose polarization state varies with space after passing through the second modulation unit; the third modulation unit is an analyzer which only allows a polarization component with a second preset polarization direction in the second light beam to pass through, and the second light beam forms a third light beam after passing through the analyzer, and the third light beam is equivalent to being formed by coherent superposition of four light beams with different propagation directions.

[0007] In one embodiment of the present application, based on the above-mentioned solution, the first preset polarization direction and the second preset polarization direction are perpendicular to each other.

[0008] In one embodiment of the present application, based on the above solution, the angle θ of each optical axis distributed in the second modulation unit satisfies the following function:

[0009]

[0010] X=xcos(α)-ysin(α)

[0011] Y=ycos(α)+xsin(α)

[0012] In the formula, θ is the angle between the optical axis of any target point in the second modulation unit and the first preset polarization direction, x is the horizontal coordinate of the target point, y is the vertical coordinate of the target point, and the two-dimensional coordinate plane formed by x and y is parallel to the plane where the surface of the second modulation unit is located; α is the preset first design parameter, and f is the preset second design parameter.

[0013] The beneficial effects of the present application are as follows: an incident light beam is received through a first modulation unit, the incident light beam is also the wavefront to be measured, and the incident light beam is modulated into a first light beam having only a first preset polarization direction, the first light beam passes through a second modulation unit, that is, through a half-wave plate optical axis structure that varies with space, to form a second light beam whose polarization state varies with space, and then passes through a third modulation unit to form a third light beam, wherein the third light beam is equivalent to four light beams with different propagation directions formed by coherent superposition. Through the four-beam splitting optical device of the present application, the wavefront to be measured can be split, and only four diffraction orders required for the four-wave shearing interference technology are generated, and at the same time, there will be no problem of generating additional diffraction orders due to changes in wavelength. Therefore, compared with the optical diffraction device in the prior art, the four-beam splitting optical device of the present application has good modulation performance and will not have the problem of generating additional diffraction orders. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following is a brief description of the drawings required for the description of the embodiments. Obviously, the drawings described are only part of the embodiments of the present application, not all of the embodiments, and those skilled in the art can also obtain other design solutions and drawings based on these drawings without creative work.

[0015] Figure 1 is a diagram showing the overall structural position distribution of a four-beam splitting optical device according to an embodiment of the present application;

[0016] Figure 2 is a schematic diagram of a conventional optical diffraction device according to an embodiment of the present application;

[0017] Figure 3 A schematic diagram of modulation logic of an incident light beam passing through a four-beam splitting optical device according to an embodiment of the present application;

[0018] Figure 4 This is a complete material production diagram of a four-beam optical device according to an embodiment of the present application. DETAILED DESCRIPTION

[0019] Example embodiments will now be described more fully with reference to the accompanying drawings. However, example embodiments can be implemented in a variety of forms and should not be construed as limited to the examples set forth herein; rather, these embodiments are provided so that this application will be more comprehensive and complete and fully convey the concept of the example embodiments to those skilled in the art.

[0020] In addition, described feature, structure or characteristic can be combined in one or more embodiments in any suitable manner. In the following description, many specific details are provided to provide a full understanding of the embodiments of the present application. However, those skilled in the art will appreciate that the technical scheme of the present application can be put into practice without one or more of the specific details, or other methods, components, devices, steps, etc. can be adopted. In other cases, known methods, devices, realizations or operations are not shown or described in detail to avoid blurring the various aspects of the application.

[0021] The block diagrams shown in the accompanying drawings are merely functional entities and do not necessarily correspond to physically independent entities. That is, these functional entities may be implemented in software form, or in one or more hardware modules or integrated circuits, or in different networks and / or processor devices and / or micro-control node devices.

[0022] The flowcharts shown in the accompanying drawings are only exemplary and do not necessarily include all the contents and operations / steps, nor must they be executed in the order described. For example, some operations / steps can be decomposed, and some operations / steps can be combined or partially combined, so the actual execution order may change according to actual conditions.

[0023] It should be noted that the "multiple" mentioned in this article refers to two or more. "And / or" describes the association relationship of the associated objects, indicating that there can be three relationships. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone. The character " / " generally indicates that the associated objects before and after are in an "or" relationship.

[0024] The following is a detailed introduction to the background technology of the embodiments of the present application:

[0025] like Figure 2 As shown, Figure 2 The existing optical diffraction device is composed of a square light shielding plate with a spacing of d and an opening size of a (the ratio is a:d=2:3, Figure 2 The black light-blocking plate on the left in the middle), and a checkerboard phase grating with a period of 2d ( Figure 2 The phase grating on the right in the figure shows a phase grating, where the phase difference introduced to the light between the two steps of the phase grating is π. The incident light wave is replicated into four waves by the diffraction element, and these waves overlap and produce interference patterns after propagating a certain distance. By analyzing these interference patterns, the information of the original wavefront can be reconstructed.

[0026] Among them, the design of the checkerboard phase grating effectively eliminates the zero-order light and the even-order diffracted light, while the checkered light-blocking plate eliminates the integer multiple of three diffracted light. However, the fifth, seventh, eleventh and other higher-order diffracted light still exist, which will affect the measurement results. In addition, since the checkerboard phase grating with a phase difference of π is only applicable to the modulation of light waves of a specific wavelength, any change in the wavelength of the incident light beam may induce additional diffraction orders, especially the zero-order diffracted light. These factors may cause the interference pattern to deviate from the mathematical model of ideal four-wave shearing interference, thereby affecting the accuracy of the wavefront measurement.

[0027] Therefore, in order to improve the measurement accuracy, the core of the embodiment of the present application is to carefully optimize the four-beam optical device used in the four-wave shearing interferometer technology, aiming to reduce the influence of non-ideal factors. The main purpose of this embodiment is to perform a four-beam processing on the wavefront to be measured, and then use the four-wave shearing interferometer method to perform an in-depth interference pattern analysis to restore the information of the original wavefront. Specifically, when a beam of light passes through the object to be measured, a specific wavefront is formed, the so-called wavefront to be measured. The embodiment of the present application regards the wavefront to be measured as an incident light beam emitted by a light source, and by performing a four-beam splitting on this incident light beam, a solid foundation is laid for the subsequent interference pattern analysis, ensuring the reliability and accuracy of the analysis results.

[0028] The following is a detailed introduction to the embodiments of the present application:

[0029] like Figure 1 As shown, Figure 1 This is a schematic diagram of a four-beam splitting optical device proposed in an embodiment of the present application, wherein serial number ① is a first modulation unit, that is, a polarizer, which can be specifically a polarizer with a first preset polarization direction, serial number ② is a second modulation unit, which is specifically a half-wave plate whose optical axis changes with spatial distribution; serial number ③ is a third modulation unit, that is, an analyzer that only allows polarization components with a second preset polarization direction to pass through. The first modulation unit is a polarizer, and its function is to only allow polarization components with a first preset polarization direction in the incident light beam to pass through to form a first light beam. The second modulation unit is a half-wave plate whose optical axis changes with spatial distribution, which is used to adjust the polarization direction of each point after the first light beam passes through the unit, and generate a second light beam with polarization changing with spatial distribution. The third modulation unit is an analyzer, and its function is to only allow polarization components with a second preset polarization direction in the second light beam to pass through to form a third light beam. The third light beam is equivalent to the result generated by four light beams with different propagation directions through the coherent superposition effect.

[0030] Wherein, the first preset polarization direction and the second preset polarization direction are perpendicular to each other.

[0031] In order to further explain the device, a three-dimensional coordinate system is constructed in the embodiment of the present application for easy understanding, such as Figure 3As shown. In this coordinate system, the x-axis is along the first preset polarization direction, the y-axis is along the second preset polarization direction, and the z-axis is perpendicular to the xy plane and points to the emission direction of the third light beam. The origin of the coordinate system is set at the location of the optical element in the second modulation unit. The xy plane coincides with the surface plane of the second modulation unit. At this time, the optical axis angle θ at any point in the second modulation unit satisfies the following function:

[0032]

[0033] X=xcos(α)-ysin(α)

[0034] Y=ycos(α)+xsin(α)

[0035] Wherein, θ is the angle between the optical axis direction of any target point in the second modulation unit and the first preset polarization direction (i.e., the x-axis). The coordinate system formed by (X, Y) is the coordinate in another rectangular coordinate system obtained by rotating the coordinate system where (x, y) is located counterclockwise around the z-axis by an angle of α. Among the four beams, two are symmetrically distributed along the z-axis in the Xz plane, and the other two are symmetrically distributed along the z-axis in the Yz plane. The angle between the four beams and the z-axis is β=arcsin(1 / fλ), where λ is the wavelength of the incident beam (wavefront to be measured).

[0036] The working principle of the device is further explained below.

[0037] The first modulation unit is a polarizer, and its Jones matrix is:

[0038]

[0039] The second modulation unit is a half-wave plate whose optical axis is distributed in space, and its Jones matrix is:

[0040]

[0041] Where θ is a function of the two-dimensional space (x, y).

[0042] The Jones matrix of the third modulation unit is:

[0043]

[0044] After the light beam passes through the first modulation unit, the second modulation unit and the third modulation unit in sequence, the light modulation effect of the entire system can be described by the following Jones matrix:

[0045]

[0046] Assume that the Jones vector of the input light to be measured is:

[0047]

[0048] Then the Jones vector of the output light is:

[0049]

[0050] Substituting the expression of the optical axis angle θ, we can get:

[0051]

[0052] It can be seen that the output light only has a component E in the second preset polarization direction. 2y . Use Euler's formula to calculate the 2y To expand,

[0053]

[0054] It can be seen from the above formula that the polarization direction of the output light beam is the component of the second preset polarization direction, which is actually the component with the first preset polarization direction in the input light beam equivalently copied into four light beams with different propagation directions and formed by coherent superposition. Therefore, the output light beam can be regarded as the component of the first preset polarization direction being divided into four beams.

[0055] In one embodiment of the present application, based on the above solution, if the wavelength of the incident light is different from the design wavelength of the second modulation unit, the Jones matrix M2 of the second modulation unit is:

[0056]

[0057] In the formula, is the phase error caused by the wavelength of the incident light not being within the design wavelength of the second modulation unit. At this time, the component of the output light along the second preset polarization direction becomes:

[0058]

[0059] It can be seen from the above formula that the component with the second preset polarization direction in the output beam is obtained by multiplying the component with the first preset polarization direction in the input beam by a constant factor Then it is equivalently copied into four light beams with different propagation directions, which are finally formed by coherent superposition. Therefore, the output light beam can still be regarded as the component of the first preset polarization direction of the input light beam is divided into four beams. It can be seen that even if the wavelength of the incident light deviates from the design wavelength, four beams can still be achieved. Therefore, compared with the optical diffraction device in the prior art, the four-beam optical device of the present application has good modulation performance, and only four required diffraction orders will be generated, and the change of the wavelength of the incident light will not introduce additional diffraction orders.

[0060] The core goal of the embodiment of the present application is to achieve accurate modulation of incident light beams in different wavelength ranges, ensuring that after they are continuously modulated by the system (including the first modulation unit, the second modulation unit and the third modulation unit), the output light beam (i.e., the third light beam) can simulate the effect formed by the coherent superposition of four light beams with different propagation directions. Since the four-beam splitting optical device of the present application only produces the four diffraction orders required by the four-wave shearing interference technology, it avoids the problem of additional diffraction orders caused by wavelength changes. This design enables the four-beam splitting optical device of the present application to adapt to incident light beams of various wavelengths while ensuring that no additional diffraction orders are generated, thereby improving the applicability and measurement accuracy of the four-wave shearing interference system.

[0061] It can be seen that the four-beam splitting optical device proposed in the embodiment of the present application does not produce additional diffraction orders compared to the traditional MHM (Modified Hartmann Mask). It has better performance than the traditional MHM. The purpose of the present application is to split the incident light beam into four beams, that is, the incident light beam (wavefront to be measured) is copied into four identical copies. The four-beam splitting optical device proposed in the present application can perfectly split the light beam into four beams and can be used as the core device in the four-wave shearing interferometer.

[0062] Therefore, compared with the optical diffraction device in the prior art, the four-beam optical device of the present application has good modulation performance and will not have the problem of generating additional diffraction orders due to deviation of the design wavelength.

[0063] In one embodiment of the present application, Figure 4 What is shown is a complete four-beam optical device, the materials of which from bottom to top are: glass (blue), polarizing film (black), photo-orientation layer (orange), liquid crystal polymer (yellow), polarizing film (black), glass (blue). A layer of polarizing film (the black polarizing film at the bottom) is pasted on the glass, and the direction of the polarizing film (the black polarizing film at the bottom) is horizontal. Then, a layer of photo-orientation agent is spin-coated on the polarizing film (the black polarizing film at the bottom), and the polarization direction of the exposure light beam on each pixel is controlled by a patterned exposure process (i.e., a half-wave plate). After the exposure is completed, a layer of liquid crystal polymer is spin-coated, and the thickness of the liquid crystal polymer can introduce a half-wave phase shift to the light of the designed wavelength. Subsequently, another layer of polarizing film (the black polarizing film at the top) is pasted, and the direction of the polarizing film is vertical, and a protective glass is added for protection.

[0064] In summary, the embodiment of the present application can restore the wavefront distribution of the incident light beam (wavefront to be measured) by splitting the emitted four light beams, without the problem of generating additional diffraction orders due to the change in the wavelength of the incident light beam. Therefore, compared with the optical diffraction device in the prior art, the four-beam splitting optical device of the present application has good modulation performance and does not have the problem of additional diffraction orders.

[0065] It should be understood that the present application is not limited to the precise structures that have been described above and shown in the drawings, and that various modifications and changes may be performed without departing from the scope thereof. The scope of the present application is limited only by the appended claims.

Claims

1. A four-beam splitting optical device, characterized in that: It comprises a first modulation unit, a second modulation unit and a third modulation unit which are arranged in sequence at intervals; the first modulation unit is a polarizer which only allows the polarization component with a first preset polarization direction in the incident light beam to pass through, and the second modulation unit is a half-wave plate whose optical axis varies with spatial distribution; the incident light beam forms a first light beam after passing through the first modulation unit, and the first light beam forms a second light beam whose polarization state varies with space after passing through the second modulation unit; the third modulation unit is an analyzer which only allows the polarization component with a second preset polarization direction in the second light beam to pass through, and the second light beam forms a third light beam after passing through the analyzer, and the third light beam is equivalent to being formed by coherent superposition of four light beams with different propagation directions.

2. The four-beam splitting optical device according to claim 1, characterized in that: The first preset polarization direction and the second preset polarization direction are perpendicular to each other.

3. The four-beam splitting optical device according to claim 1, characterized in that: The angle θ of each optical axis distributed in the second modulation unit satisfies the following function: X=xcos(α)-ysin(α) Y=ycos(α)+xsin(α) In the formula, θ is the angle between the optical axis of any target point in the second modulation unit and the first preset polarization direction, x is the horizontal coordinate of the target point, y is the vertical coordinate of the target point, and the two-dimensional coordinate plane formed by x and y is parallel to the plane where the surface of the second modulation unit is located; α is the preset first design parameter, and f is the preset second design parameter.

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