Collimated light beam homogenizer based on liquid crystal flat lens pair

Abstract

The invention discloses a collimated light beam homogenizer based on a liquid crystal flat plate lens pair, which comprises two groups of liquid crystal flat plate lenses which are oppositely arranged in parallel, each liquid crystal flat plate lens comprises a glass substrate, a light orientation layer coated on one plane of the glass substrate, and a liquid crystal polymer film coated on the light orientation layer, the first liquid crystal flat plate lens is used for introducing wavefront difference to an incident light beam, geometric phase distribution is annularly changed, and different phase gradients are formed from inside to outside, and the second liquid crystal flat plate lens is used for correcting the wavefront difference of the light beam passing through the first liquid crystal flat plate lens, so that the flat-topped light beam is collimated and emitted. According to the collimated light beam homogenizer based on the liquid crystal flat lens pair, accurate shaping of incident light beams is achieved through the liquid crystal director design, and the collimated light beam homogenizer has the advantages of being low in cost, small in size, high in flexibility and the like and is widely applied to the fields of laser processing, optical measurement, biomedical imaging and the like.

Description

Technical Field

[0001] The invention relates to the technical field of beam shaping, and in particular to a collimated beam homogenizer based on a liquid crystal flat panel lens pair. Background Art

[0002] Beam shaping technology is widely used in the field of industrial and scientific lasers to improve laser processing effects, such as converting a Gaussian beam into a flat-top beam. Existing beam shapers, such as the πShaper series, use multiple lens components to convert the intensity distribution of the beam from a Gaussian distribution to a flat-top distribution or other required intensity distribution. Its structure includes input optical elements, output optical elements, and adjustment components, and each component works together to achieve beam shaping.

[0003] However, traditional lenses and complex optical systems are often large in size, which is not conducive to miniaturization and integration. This will increase system complexity and cost in application scenarios with strict space and weight requirements. In addition, although lenses based on free-form surfaces or diffractive optical elements can achieve complex beam shaping, their processing is complicated and requires high-precision processing equipment and processes, making it difficult to achieve large-scale production. Summary of the invention

[0004] Purpose of the invention: The purpose of the present invention is to provide a collimated beam homogenizer based on a liquid crystal flat lens pair with an overall planar structure and a small volume to achieve beam shaping.

[0005] Technical solution: To achieve the above-mentioned purpose, the collimated beam homogenizer based on a liquid crystal flat lens pair described in the present invention comprises two groups of liquid crystal flat lenses arranged relatively in parallel, wherein the liquid crystal flat lenses comprise a glass substrate, a light orientation layer coated on one plane of the glass substrate, and a liquid crystal polymer film coated on the light orientation layer. The first liquid crystal flat lens is used to introduce a wavefront difference into the incident light beam, and the geometric phase distribution is an annular change with different phase gradients from the inside to the outside. The second liquid crystal flat lens is used to correct the wavefront difference of the light beam passing through the first liquid crystal flat lens, so as to realize the collimated emission of a flat-top light beam.

[0006] Wherein, the liquid crystal polymer films of the two groups of liquid crystal flat-panel lenses are arranged face to face.

[0007] Wherein, the glass substrate is a transparent planar structure.

[0008] Wherein, the alignment layer is written with a geometric phase structure.

[0009] The geometric phase design process written into the first liquid crystal flat lens and the second liquid crystal flat lens is as follows:

[0010] (1) According to the law of conservation of energy, the mapping equation between the incident light field and the outgoing light field is established in polar coordinates;

[0011] (2) obtaining a mapping relationship between the radial position r of the incident light and the radial position R of the exit light based on a mapping equation;

[0012] (3) The mapping relationship between r and R is converted into the geometric phase distribution of the incident surface and the exit surface, that is, the geometric phase distribution Φ1(r) of the first liquid crystal flat lens and the geometric phase distribution Φ2(R) of the second liquid crystal flat lens.

[0013] The mapping equation between the incident light field and the outgoing light field is:

[0014]

[0015] Where r is the radial position on the incident plane, I in (r) is the incident light energy distribution, r in is the maximum radius on the incident surface involved in the energy distribution calculation, R is the radial position on the exit surface, and I out (R) is the incident light energy distribution, R out It is the maximum radius on the exit surface that participates in energy calculation.

[0016] Wherein, under the paraxial approximation, the geometric phase distribution of the incident surface and the exit surface is:

[0017]

[0018] Wherein, k is the wave vector, d is the distance between the incident surface and the exit surface, Φ1(r) and Φ2(R) are the geometric phase distributions of the first and second liquid crystal flat lenses, respectively, R(r) represents the mapping function of the light position R of the exit surface with respect to the light position r of the incident surface, and r(R) represents the mapping function of the light position r of the incident surface with respect to the light position R of the exit surface.

[0019] The liquid crystal director distributions of the first liquid crystal flat lens and the second liquid crystal flat lens are:

[0020]

[0021] Among them, α1(r) and α2(R) are the liquid crystal director distributions in the first and second liquid crystal flat lens respectively.

[0022] Wherein, the design method of the collimated beam homogenizer is:

[0023] (1) Confirm the design parameters of the liquid crystal flat panel lens pair, including the waist radius of the input light beam, the radius of the output light beam, and the spacing between the two liquid crystal flat panel lens pairs;

[0024] (2) calculating the geometric phase distribution of the liquid crystal flat lens pair to obtain the arrangement of the liquid crystal directors of the liquid crystal flat lens pair;

[0025] (3) Preparing a liquid crystal flat lens based on the arrangement of liquid crystal directors;

[0026] (4) Install the prepared liquid crystal flat panel lens pair into the collimated beam homogenizer housing for debugging.

[0027] Beneficial effects: The present invention has the following advantages: 1. The collimated beam homogenizer based on the liquid crystal flat lens pair proposed in the present invention can realize the precise shaping of the incident light beam through the precise liquid crystal director design, and has the advantages of low cost, small size, high flexibility, etc., and is widely used in laser processing, optical measurement, biomedical imaging and other fields; 2. It has a simple structure and a light and thin size, which is conducive to integration into a compact optical system, and has low processing cost and difficulty. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a schematic diagram of the collimated beam homogenizer structure and its working principle;

[0029] Figure 2 Schematic diagram of the light beam shaping process through a collimated beam homogenizer. DETAILED DESCRIPTION

[0030] The technical solution of the present invention is described in detail below in conjunction with the embodiments and drawings.

[0031] The collimated light beam homogenizer based on the liquid crystal flat lens pair of the present invention can realize the functions of light beam homogenization and collimated emission through geometric phase design. The liquid crystal flat lens comprises a glass substrate, a light orientation layer, and a liquid crystal polymer film in sequence, wherein the glass substrate and the light orientation layer do not act on the incident light, the glass substrate is a transparent plane structure used to carry the film, and the light orientation layer is used to control the arrangement of liquid crystal molecules in the liquid crystal polymer film.

[0032] Liquid crystal flat lens is a geometric phase diffraction optical element with a planar structure. It modulates the phase of the incident light beam by changing the arrangement of the liquid crystal director, thereby achieving light beam shaping. Compared with traditional optical lenses, liquid crystal flat lens is a planar structure, and the thickness of the entire film is on the order of a few microns. It has the advantages of small size, light weight, low cost, and is easy to integrate.

[0033] The liquid crystal flat panel lens pair is two groups of liquid crystal flat panel lenses arranged relatively in parallel, and the liquid crystal polymer films of the two groups of liquid crystal flat panel lenses are arranged face to face.

[0034] like Figure 1 As shown, it is a schematic diagram of the collimated beam homogenizer structure and its working principle, wherein the incident beam (beam before shaping) can be a TEM00 single-mode beam or a multi-mode beam with a Gaussian-like intensity distribution.

[0035] Taking the TEM00 single-mode beam as an example, the first liquid crystal flat lens is used to introduce wavefront difference, and its phase distribution is annular, with different phase gradients from the inside to the outside, so that the light in different annular areas is deflected to different positions, thereby changing the energy distribution of the TEM00 mode. Specifically, the light in the central area with higher energy has a high degree of divergence, and the energy is transferred to the periphery. At the same time, the light in the peripheral area with less energy distribution has a small degree of divergence. By designing the appropriate geometric phase of the first liquid crystal flat lens, it is possible to achieve a uniform energy distribution on the cross section at a specific plane after passing through the first liquid crystal flat lens.

[0036] The second liquid crystal flat lens is used to correct the wavefront difference and realize the collimated emission of the flat-top beam. Since the light beams in different radial regions have different convergence or divergence effects after passing through the first liquid crystal flat lens, the flat-top light field can only be realized at a specific position. The role of the second lens is to collimate the light beam at this specific position. By matching the design parameters of the first liquid crystal flat lens, the second liquid crystal flat lens can correct the output direction of light at different positions, so that the output light field maintains the energy of the flat-top beam while being collimated, maintaining good energy uniformity during long-distance propagation.

[0037] Due to the design of the geometric phase of the above-mentioned liquid crystal flat lens, the wavefront of the beam changes, and the intensity distribution of the shaped beam is redistributed to form a uniform flat-top beam output, such as Figure 2 As shown, the fundamental mode Gaussian beam can also be converted into a flat-top beam or other desired intensity distribution.

[0038] This embodiment further provides a design process of the liquid crystal director of the liquid crystal flat lens pair in the collimated beam homogenizer:

[0039] 1. Establish a physical model of incident light and outgoing light energy distribution, where the incident light energy distribution takes Gaussian distribution as an example, and the outgoing light energy distribution is a flat-top beam model, such as super Gaussian distribution, Fermi Dirac distribution, etc.

[0040] 2. According to the law of conservation of energy, that is, the energy of the input light field and the output light field is equal, the mapping equation of the incident light field and the outgoing light field is established. The following equation can be established in polar coordinates:

[0041]

[0042] Where r is the radial position on the incident plane, I in (r) is the incident light energy distribution, r in is the maximum radius on the incident surface involved in the energy distribution calculation, R is the radial position on the exit surface, and I out (R) is the incident light energy distribution, R out It is the maximum radius on the exit surface that participates in energy calculation.

[0043] 3. From the above equation, we can get the mapping relationship between the incident light position r and the outgoing light position R.

[0044] 4. Convert the mapping relationship between r and R into the geometric phase distribution of the incident and exit surfaces. Under the paraxial approximation condition, the formula is as follows:

[0045]

[0046] Wherein, k is the wave vector, d is the distance between the incident surface and the exit surface, Φ1(r) and Φ2(R) are the geometric phase distributions of the first and second liquid crystal flat lenses, respectively; R(r) represents the mapping function of the light position R on the exit surface with respect to the light position r on the incident surface, and r(R) represents the mapping function of the light position r on the incident surface with respect to the light position R on the exit surface.

[0047] 5. Convert the geometric phase into the distribution of the liquid crystal director using the following formula:

[0048]

[0049] Wherein, α1(r) and α2(R) are the arrangements of the liquid crystal directors in the first and second liquid crystal flat lenses, respectively. This embodiment provides a process for preparing the liquid crystal flat lens pair in the collimated beam homogenizer, including:

[0050] 1. Confirm the design parameters: set the waist radius of the input Gaussian beam, the radius of the output flat-top beam, and the distance between the two flat lenses;

[0051] 2. Liquid crystal director design: Calculate the geometric phase distribution of the liquid crystal flat lens pair to obtain the arrangement of the liquid crystal director of the liquid crystal flat lens pair;

[0052] 3. Manufacturing liquid crystal flat lens: According to the distribution of liquid crystal directors, it is prepared by micro-nano processing technology;

[0053] 4. Installation and debugging: Install the liquid crystal flat panel lens pair into the collimated beam homogenizer housing and perform preliminary debugging;

[0054] 5. Beam shaping: Turn on the light source, observe and record the characteristics of the beam after shaping.

[0055] 6. Parameter adjustment: If further optimization is required, adjust the phase distribution of the liquid crystal flat panel lens through the control system and repeat step 5.

[0056] In the above embodiment, when the incident light is non-Gaussian light, the phase matching design can be performed only by knowing the intensity distribution of the incident light, and the homogenization effect can also be achieved.

[0057] Collimated beam homogenizers based on liquid crystal flat lens pairs have application value in many fields. For example, in the field of laser cutting, Gaussian distribution of laser beams can be converted into flat-top distribution, thereby improving cutting accuracy and efficiency. In the field of biomedical imaging, liquid crystal flat lens pairs can convert point light sources into annular beams, which are applied to dark field microscope imaging, effectively enhancing the contrast of sample edges and improving imaging effects.

Claims

1. A collimated beam homogenizer based on a liquid crystal flat lens pair, comprising two sets of liquid crystal flat lenses arranged relatively parallel to each other, wherein the liquid crystal flat lenses comprise a glass substrate, a light alignment layer coated on one plane of the glass substrate, and a liquid crystal polymer film coated on the light alignment layer, characterized in that: The first liquid crystal flat lens is used to introduce wavefront difference into the incident light beam. The geometric phase distribution is annular and presents different phase gradients from inside to outside. The second liquid crystal flat lens is used to correct the wavefront difference of the light beam passing through the first liquid crystal flat lens to realize the collimated emission of the flat-top light beam.

2. The collimated beam homogenizer based on a liquid crystal flat lens pair according to claim 1, characterized in that: The liquid crystal polymer films of the two groups of liquid crystal flat lenses are arranged face to face.

3. The collimated beam homogenizer based on a liquid crystal flat lens pair according to claim 1, characterized in that: The glass substrate is a transparent planar structure.

4. The collimated beam homogenizer based on a liquid crystal flat lens pair according to claim 1, characterized in that: The alignment layer is written with a geometric phase structure.

5. The collimated beam homogenizer based on a liquid crystal flat lens pair according to claim 4, characterized in that: The geometric phase design process written into the first liquid crystal flat lens and the second liquid crystal flat lens is: (1) According to the law of conservation of energy, the mapping equation between the incident light field and the outgoing light field is established in polar coordinates; (2) obtaining a mapping relationship between the radial position r of the incident light and the radial position R of the exit light based on a mapping equation; (3) The mapping relationship between r and R is converted into the geometric phase distribution of the incident surface and the exit surface, that is, the geometric phase distribution Φ1(r) of the first liquid crystal flat lens and the geometric phase distribution Φ2(R) of the second liquid crystal flat lens.

6. The collimated beam homogenizer based on a liquid crystal flat lens pair according to claim 5, characterized in that: The mapping equation between the incident light field and the outgoing light field is: Where r is the radial position on the incident plane, I in (r) is the incident light energy distribution, r in is the maximum radius on the incident surface involved in the energy distribution calculation, R is the radial position on the exit surface, and I out (R) is the incident light energy distribution, R out It is the maximum radius on the exit surface that participates in energy calculation.

7. The collimated beam homogenizer based on a liquid crystal flat lens pair according to claim 6, characterized in that: The geometric phase distribution of the incident surface and the exit surface is: Wherein, k is the wave vector, d is the distance between the incident surface and the exit surface, Φ1(r) and Φ2(R) are the geometric phase distributions of the first and second liquid crystal flat lenses, respectively, R(r) represents the mapping function of the light position R of the exit surface with respect to the light position r of the incident surface, and r(R) represents the mapping function of the light position r of the incident surface with respect to the light position R of the exit surface.

8. The collimated beam homogenizer based on a liquid crystal flat lens pair according to claim 5, characterized in that: The liquid crystal director distributions of the first liquid crystal flat lens and the second liquid crystal flat lens are: Among them, α1(r) and α2(R) are the liquid crystal director distributions in the first and second liquid crystal flat lens respectively.

9. The collimated beam homogenizer based on a liquid crystal flat lens pair according to claim 1, characterized in that: The design method of the collimated beam homogenizer is: (1) Confirm the design parameters of the liquid crystal flat panel lens pair, including the waist radius of the input light beam, the radius of the output light beam, and the spacing between the two liquid crystal flat panel lens pairs; (2) calculating the geometric phase distribution of the liquid crystal flat lens pair to obtain the arrangement of the liquid crystal directors of the liquid crystal flat lens pair; (3) Preparing a liquid crystal flat lens based on the arrangement of liquid crystal directors; (4) Install the prepared liquid crystal flat panel lens pair into the collimated beam homogenizer housing for debugging.

Images