A variable-reflectance combined output ring mirror for producing a helical ring beam

Abstract

The application discloses a variable reflectivity combined output annular mirror for generating a spiral annular light beam, comprising: an incident end face, an exit end face and a connecting face connected with the two, to form a reflection cavity; the incident end face and the connecting face are total reflection mirrors, and the exit end face is a variable reflectivity combined mirror composed of a series of different reflectivity unit finite elements in an annular distribution; the incident end face is provided with a light inlet hole, an incident laser beam enters the reflection cavity through the light inlet hole at a preset angle, and is reflected back and forth between the incident end face and the exit end face, so that one light beam is divided into a plurality of light beams with the same energy in a period; adjacent divided light beams are distributed in a spiral shape in space due to pulse delay; and a TEM 00 mode laser beam can be converted into a spiral annular light beam with a phase singularity by using an arbitrary pulse collimated light source through the variable reflectivity combined output annular mirror. The application has the advantages of simple overall structure, high conversion efficiency and stable output.

Description

Technical Field

[0001] This invention relates to the field of laser technology, and more specifically to a variable reflectivity combined output ring mirror for generating a spiral ring beam. Background Technology

[0002] A helical ring beam is a beam with zero central or axial intensity in its propagation direction. It features phase singularity, spin, and orbital angular momentum, and has the advantages of non-contact and low-damage operation. It has been successfully applied to the manipulation of living cells and subcellular microparticles. In addition, due to its zero central intensity, the helical ring beam has shown broad application potential in fields such as optical tweezers, optical communication, and super-resolution microscopy, thus attracting widespread attention in the laser field. Therefore, its generation method is also of great interest.

[0003] In recent years, the generation of spiral ring beams and the types of ring beams have attracted close attention from scholars both domestically and internationally, becoming an important research topic. Various methods have been employed to obtain different types of ring beams, including: mode conversion method, geometric optics method, optical holography, computer holography, transverse mode selection method, spiral phase plate method, spatial light modulator method, and hollow wave method. Among these, the spiral phase plate method first requires the fabrication of a π-phase plate, where a precise phase difference exists between the central disk and the adjacent outer rings. Next, a collimated beam is incident on the spiral phase plate and then focused through a lens. Because the beam undergoes destructive interference as it passes through the inner and outer rings of the spiral phase plate, a spiral ring beam with zero intensity is generated in a localized area near the focal point. The spatial light modulator method loads the phase information of two beams onto a single spatial light modulator and outputs a spiral ring beam. The core component of the spatial light modulator is a display similar to a holographic phase sheet, and its working principle is similar to that of the holographic method. First, the phase information of the initial beam and the target beam is input into a computer, which simulates a holographic phase diagram of the two beams. Then, Fourier transform is used to load the information of the hologram onto the display. When the initial beam enters the spatial light modulator that has been loaded with phase information, the output beam is the desired beam.

[0004] Typical spiral ring beam modes can be obtained through the above methods, including: Laguerre-Gaussian (LG) mode, Hermie-Laguerre-Gaussian (HLG) mode, spiral-Ensemble-Gaussian mode, singularity hybrid evolution (SHEN) mode, Bessel mode, Mathieu mode, SU(2) geometric mode, etc. However, most of the above methods for obtaining spiral ring beams require mode conversion of the laser beam, resulting in complex experimental setups, low conversion efficiency, high requirements for components in each part of the experimental setup, high cost, relatively complex operation, and difficulty in precise control of the experimental process. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a variable reflectivity combined output ring mirror for generating a spiral ring beam. It can directly generate a spiral ring beam, has a simple and compact overall structure, low cost, high conversion efficiency, and stable output.

[0006] This invention discloses a variable reflectivity combined output ring mirror for generating a spiral ring beam, characterized in that it includes: an incident end face, an exit end face, and a connecting surface;

[0007] The incident end face and the exit end face are arranged parallel to each other, and the connecting surface connects the incident end face and the exit end face to form a reflecting cavity; wherein, the incident end face and the connecting surface are total reflection mirrors to prevent light from overflowing from the side outside the variable reflectivity combined output ring mirror, and the exit end face is a variable reflectivity combined mirror, which is composed of a series of finite elements with different reflectivity units distributed in a ring;

[0008] An entrance aperture is provided on the incident end face. The incident laser beam of the pulse collimated light source enters the reflection cavity through the entrance aperture at a preset angle. It is reflected back and forth between the incident end face and the exit end face, and a beam of a certain energy is emitted in each unit finite element of the exit end face until it returns to the variable reflectivity combined output ring mirror once. In the last unit finite element, all the remaining energy is emitted, so that beams of the same energy or with energy differences within a preset threshold are emitted from each unit finite element of the exit end face.

[0009] As a further improvement of the present invention, since the co-beam light reflects back and forth between the two mirrors and there is a pulse delay between the emitted beams within a unit finite element of the output end face, the emitted beams should be spatially spirally distributed. Using an arbitrary pulse collimated light source, the output ring mirror through this variable reflectivity combination can transmit the TEM. 00 The mode laser beam is converted into a spiral ring beam with a phase singularity.

[0010] As a further improvement of the present invention, the number of unit finite elements is consistent with the number of beams required in one period, and the refractive index of the unit finite element satisfies:

[0011] R n =1-1 / n

[0012] In the formula, R i Let be the refractive index of the i-th unit finite element, i = 1, 2...n, where n is the number of beams that need to be split within one period;

[0013] The pulse delay time Δt and the spatial separation distance Δd are:

[0014] Δt = 2L / (cosθ·c)

[0015] Δd=2L·θ

[0016] In the formula, L is the spatial separation distance, and θ is the incident angle.

[0017] Specifically:

[0018] The number of unit finite elements corresponds to the number of beams to be split within one cycle. The reflectivity of different unit finite elements on the exit face has a certain functional relationship. Knowing the number of beams to be split within one cycle, the reflectivity within each unit finite element on the exit face can be designed to ensure that the split beams have the same energy and that no laser energy is lost. Assuming the beam needs to be split into n beams within one cycle, the reflectivity of each unit finite element on the exit face should be:

[0019] R n =1-1 / n

[0020] In the formula, R i Let be the refractive index of the i-th unit finite element, i = 1, 2...n;

[0021] The principle of pulse delay beam splitting is as follows: When a pump pulse is incident at a small angle θ close to the incident end face (total reflection mirror HR), after multiple reflections by the two mirrors at the incident and exit ends, the exit end face will output multiple pump light pulses that are separated in time and space. The delay time and spatial separation distance are determined by the distance between the two mirrors and the incident angle θ. For the incident angle θ, the spatial separation distance is L. The relationship between the pulse delay time Δt, the spatial separation distance L, and the incident angle θ is as follows:

[0022] Δt = 2L / (cosθ·c)

[0023] The spatial separation distance Δd between adjacent beams is

[0024] Δd=2L·θ

[0025] By changing the laser incident angle θ and the spatial separation distance L, the pulse delay time Δt and the spatial separation distance Δd can be precisely adjusted according to specific requirements.

[0026] As a further improvement of the present invention, the aperture of the light entrance is larger than the beam diameter when the incident laser beam reaches the incident end face, and the diameter of the incident laser beam should be within a certain range to ensure that all light rays can enter the reflection cavity of the variable reflectivity combined output ring mirror.

[0027] As a further improvement of the present invention, the angle between the incident laser beam and the axis of the incident end face is θ, and the incident angle θ is not greater than 3°.

[0028] As a further improvement of the present invention, the energy difference between different beams after beam splitting is less than 8%, that is, the preset threshold is 8%.

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0030] The variable reflectivity combined output ring mirror of this invention can divide any pulse collimated light source into any number of pulses within one cycle, with each part having equal energy and distributed in a spiral ring shape, resulting in virtually no energy waste during the beam splitting process. Furthermore, the variable reflectivity combined output ring mirror of this invention does not have high requirements for the incident beam, and can directly obtain a spatial spiral ring-shaped beam without a complex shaping system. In experiments, the target size spiral ring beam can be obtained by changing the relative dimensions of the variable reflectivity combined output ring mirror. The overall system does not require high precision and is easy to implement. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the structure of the variable reflectivity combined output ring mirror disclosed in this invention;

[0032] Figure 2 This is an axonometric view of the beam incident end face and the exit end face of the variable reflectivity combined output ring mirror disclosed in this invention.

[0033] Figure 3 The present invention discloses a schematic diagram of the beam splitting principle between the incident end face and the exit end face in the unfolded diagram of the variable reflectivity combined output ring mirror.

[0034] Figure 4 This is a finite element diagram of the reflectivity distribution of the output ring mirror of the variable reflectivity combination under the condition of the number of beam splits within one cycle, as disclosed in one embodiment of the present invention.

[0035] Figure 5 The present invention discloses a design diagram of the reflectivity distribution of the output end face of a variable reflectivity combined output ring mirror;

[0036] Figure 6This is a comparison diagram of the energy of the emitted beam in each unit finite element after beam splitting by a variable reflectivity combined output ring mirror under the condition of the number of beam splits in one cycle disclosed in an embodiment of the present invention.

[0037] Figure 7 This is a ZEMAX ray tracing diagram of the pulse collimated light source in one embodiment of the present invention, showing the round-trip process between the two ends of the variable reflectivity combined output ring mirror.

[0038] In the picture:

[0039] 1. Pulse collimated light source; 2. Variable reflectivity combined output ring mirror; 21. Entrance aperture; 22. Entrance end face; 23. Exit end face; 24. Connection surface. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0041] The present invention will now be described in further detail with reference to the accompanying drawings:

[0042] like Figures 1-2 As shown, this invention provides a variable reflectivity combined output ring mirror for directly generating a spiral ring beam, including a beam incident end face 22, a beam exit end face 23, and a connecting surface 24 connecting the incident end face 22 and the exit end face 23. The connecting surface 24 has a cylindrical structure with openings at both ends. The incident end face 22 and the exit end face 23 are arranged parallel to each other at the openings at both ends of the connecting surface 24 to form a reflecting cavity. The incident end face 22 is a total reflection mirror with a light inlet 21 of a specific aperture. The aperture of the light inlet 21 is related to the beam diameter when the incident beam reaches this surface and should be larger than the beam diameter to ensure that the entire laser beam enters the variable reflectivity combined output ring mirror. The exit end face 23 is a variable reflectivity combined mirror composed of a series of finite element units with different reflectivities arranged in a ring. The number of finite element units is consistent with the number of beams to be split in one cycle, and the reflectivity within each finite element unit is related to the number of beams to be split in one cycle. The connecting surface 24 is a total reflection mirror to prevent the beam from overflowing from the side of the variable reflectivity combined output ring mirror. Furthermore, the non-light-emitting areas at the center of the incident end face 22 and the exit end face 23 can be hollowed out. This is done to make the lenses lighter and save costs, and also to facilitate the removal of the lens shells.

[0043] In use, the light beam emitted by the pulse collimated light source 1 enters the reflective cavity of the variable reflectivity combined output ring mirror through the light entrance hole 21 on the incident end face of the variable reflectivity combined output ring mirror at a certain angle. It forms back and forth reflection on the incident end face 22 and the exit end face 23, and outputs a light beam of a certain energy according to the reflectivity parameter in each unit finite element of the exit end face 23. The reflectivity in each unit finite element of the exit end face 23 should be specifically designed so that the energy emitted in each unit finite element is equal. Due to the pulse delay between adjacent beams, the split pulse light is separated in both space and time, thereby obtaining an output beam that is spirally distributed in a ring within one cycle.

[0044] To more intuitively analyze the beam splitting process, the incident end face 22 and the exit end face 23 of the variable reflectivity combined output ring mirror are unfolded, and the beam splitting principle of the variable reflectivity combined output ring mirror is described in two-dimensional space; such as Figure 3 As shown, the incident end face 22 and the exit end face 23 of the variable reflectivity combined output ring mirror are considered as two parallel planes. The beam splitting principle is as follows: when the incident laser beam is close to HR at a small angle θ, after multiple reflections by the two mirrors, the exit end face will output multiple pump light pulses that are separated in time and space. The delay time and spatial separation distance are determined by the distance between the two mirrors and the incident angle θ. For practical applications, the incident angle θ is very small, generally less than 3°. For a distance of length L between the two mirrors, the relationship between the pulse delay time Δt and the mirror distance L and the incident angle θ is:

[0045] Δt = 2L / (cosθ·c)

[0046] The spatial separation distance Δd between adjacent beams is:

[0047] Δd=2L·θ

[0048] The formula uses trigonometric approximations for small angles:

[0049] sinθ≈tanθ≈θ

[0050] By changing the laser incident angle θ and the mirror spacing L, the pulse delay time Δt and the spatial separation distance Δd can be precisely adjusted according to specific requirements.

[0051] like Figure 4 As shown, given the number of beams required within one cycle, the reflectivity of the output ring mirror within a unit finite element at the exit face can be designed to ensure that the beam energy remains the same after splitting without any loss of laser energy. Assuming the pulsed collimated source needs to be split into n beams within one cycle, the reflectivity within each unit finite element at the exit face should be:

[0052] R n =1-1 / n

[0053] In the formula, R i Let be the refractive index of the i-th unit finite element, i = 1, 2...n.

[0054] Example

[0055] like Figure 5 As shown, if five pulses of light need to be split within one cycle, the reflectivity of each unit finite element of the variable reflectivity combined output ring mirror is R5 = 4 / 5, R4 = 3 / 4, R3 = 2 / 3, R2 = 1 / 2, and R1 = 0. Considering practical applications, we choose several commercially available special reflectivities: R5 = 0.8, R4 = 0.75, R3 = 0.65, R2 = 0.5, and R1 = 0. The energy of the split beam accounts for... Figure 6 As shown.

[0056] from Figure 6 As can be seen, the energy difference between the sub-pulses after beam splitting is very small, less than 8%, and all the laser beam energy is split without energy loss. Using variable reflectivity coating technology, different reflectivity dielectric films can be deposited on the surface of an optical lens material, greatly reducing the pressure of high-precision assembly.

[0057] like Figure 7 As shown, the round-trip trajectory of the laser beam entering the ring mirror was simulated in the optical design software ZEMAX. The variable reflectivity combined output ring mirror has an outer diameter of 25.4 mm, an inner diameter of 12.7 mm, and an overall thickness of 6 mm (1 mm for the incident and exit surfaces and 2 mm for the connecting surface). The aperture of the light source is 4 mm. To facilitate observation of the round-trip trajectory of the light inside the variable reflectivity combined output ring mirror, the incident beam was set to a parallel beam, the analysis beam was set to 1, and the inside of the variable reflectivity combined output ring mirror was entirely composed of total reflection mirrors. This process was only to prove that the laser beam could uniformly round-trip inside the variable reflectivity combined output ring mirror without overflowing outside the mirror. In reality, the reflectivity of the output surface of the variable reflectivity combined output ring mirror varies within a unit finite element method based on the number of beam splits within one period. As can be seen from the ZEMAX simulation diagram, the laser beam travels back and forth uniformly and orderly between the two mirrors of the variable reflectivity combined output ring mirror. Given the number of beams to be split within one cycle and the pulse time between adjacent beams, the corresponding spiral ring beam output can be obtained by designing the width between the mirrors, the reflectivity per unit finite element of the output end face, and the incident angle of the laser beam.

[0058] In summary, this invention discloses a variable reflectivity combined output ring mirror design for directly generating a spiral ring beam. Only one shaping mirror is needed to obtain the spiral ring beam, resulting in a simple overall structure and high conversion efficiency. Compared to existing methods, the variable reflectivity combined output ring mirror not only enhances the system's compactness but also boasts a simple structure, lower cost, higher conversion efficiency, and easier assembly.

[0059] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A variable reflectivity combined output ring mirror for generating a spiral ring beam, characterized in that, include: Incident end face, exit end face, and connecting surface; The incident end face and the exit end face are arranged parallel to each other, and the connecting surface connects the incident end face and the exit end face to form a reflecting cavity; wherein, the incident end face and the connecting surface are total reflection mirrors, and the exit end face is a variable reflectivity composite mirror, and the variable reflectivity composite mirror is composed of a series of finite elements with different reflectivity units distributed in a ring. An incident light aperture is provided on the incident end face. The incident laser beam of the pulse collimated light source enters the reflection cavity through the incident light aperture at a preset angle, reflects back and forth between the incident end face and the exit end face, and emits beams of the same energy or with energy differences within a preset threshold from each unit finite element of the exit end face. The number of unit finite elements is consistent with the number of beams required within one period, and the refractive index of the unit finite element satisfies: R n =1-1 / n In the formula, R i Let be the refractive index of the i-th unit finite element, i = 1, 2...n, where n is the number of beams that need to be split within one period; The pulse delay time Δt and the spatial separation distance Δd are: Δt = 2L / (cosθ·c) Δd=2L·θ In the formula, L is the spatial separation distance, and θ is the incident angle; The angle between the incident laser beam and the axis of the incident end face is θ, and the incident angle θ is not greater than 3°.

2. The variable reflectivity combined output ring mirror as described in claim 1, characterized in that, Because the beam of light reflects back and forth between the two mirrors and there is a pulse delay between the emitted beams within a unit finite element of the emitting end face, the emitted beams should be distributed in a spiral shape in space.

3. The variable reflectivity combined output ring mirror as described in claim 1, characterized in that, The aperture of the light inlet is larger than the beam diameter when the incident laser beam reaches the incident end face.

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