A method and device for generating a cylindrical symmetric vector beam

Abstract

The application relates to a cylindrical symmetry vector beam generation method and device. A cylindrical symmetry vector beam is generated by using a laser, a polarizer, a beam expander, a polarization direction rotator, a double-free-surface beam shaping system, a phase control device, and a Mach-Zehnder interferometer optical path structure composed of a beam splitter, a beam rotator, two mirrors, a phase compensation device and a beam combiner. The light beam output by the laser is linearly polarized light after passing through the polarizer, the polarization direction is adjusted after passing through the beam expander, then the light beam is shaped into a TEM10 mode Gaussian light beam after passing through the beam shaping system and the phase control device is used to make the polarization directions of the two light spots opposite, that is, the phase difference of the two light spots is pi, then the light beam is split and respectively controlled by using the Mach-Zehnder interferometer optical path structure, one branch becomes a TEM01 mode Gaussian light beam, and the phase of the light beam of the other branch is adjusted by using the phase compensation device to make the two light beams in phase, and finally the two light beams are combined into radial or angular polarization light.

Description

Technical Field

[0001] This invention belongs to the field of laser beam shaping, specifically relating to a method and apparatus for shaping a laser beam based on a freeform surface beam shaping system, and for controlling the polarization state and phase of the laser beam using polarization and phase modulation devices to generate a cylindrically symmetric vector beam. More particularly, it relates to a method and apparatus for generating a cylindrically symmetric vector beam. Background Technology

[0002] A cylindrical symmetric vector beam is a beam with cylindrical symmetry in intensity and polarization distribution, and its polarization direction forms an angle with the beam's radial direction. And for a known cylindrical symmetric vector beam, The value remains constant at any point on the cross-section of the beam, when When the beam polarization direction is parallel to the radial direction, it is a radially polarized beam; when... When the beam polarization direction is perpendicular to the radial direction, it is an angularly polarized beam. Therefore, it can be seen that radially polarized beams and angularly polarized beams are special modes of cylindrically symmetric vector beams.

[0003] Cylindrical symmetric vector beams have important applications in laser processing, super-resolution imaging, optical capture, and other fields, and therefore have received widespread attention from academia and industry. The generation of cylindrical symmetric vector beams is generally divided into intracavity methods and extracavity methods. Intracavity methods, as the name suggests, directly generate the desired cylindrical symmetric vector beam within the laser cavity. The advantage is high conversion efficiency, but the disadvantages include poor application flexibility and the fact that each laser can only generate one type of cylindrical symmetric vector beam. Extracavity methods mainly generate cylindrical symmetric vector beams by designing specific diffraction or interference optical paths outside the laser cavity to control the optical field. Different types of cylindrical symmetric vector beams can be generated by changing the optical path or the components in the optical path according to the application requirements. A common method is to use the optical path structure of a Mach-Zehnder interferometer and diffractive optical elements to shape the fundamental mode Gaussian beam into TEM10 and TEM01 mode Gaussian beams and combine them to generate a cylindrical symmetric vector beam. However, the disadvantage is that the damage threshold of the diffractive optical elements is relatively low, and their performance is unstable or even damaged in high-power laser applications. The double freeform surface beam shaping system used in this invention, especially the off-axis reflection double freeform surface beam shaping system, has a high damage threshold and is suitable for high-power laser applications. Summary of the Invention

[0004] To address the shortcomings of existing methods, this invention proposes a method and apparatus for generating cylindrical symmetric vector beams, which can be used to modulate high-power laser beams into cylindrical symmetric vector beams. This apparatus is highly flexible, compact, has a high damage threshold, and allows for adjustments to the components in the optical path as needed to obtain cylindrical symmetric vector beams such as radially / angularly polarized light.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A method for generating a cylindrical symmetric vector beam, comprising: a laser beam emitted from a laser sequentially passing through a polarizer, a beam expander, a polarization direction rotator, a double freeform surface beam shaping system, and a phase adjustment device; then entering a Mach-Zehnder interferometer optical path structure consisting of a beam splitter, a beam rotator, two mirrors, a phase compensation device, and a beam combiner; wherein the polarizer modulates the laser beam into linearly polarized light, the beam expander expands the laser beam, the polarization direction rotator adjusts the polarization direction of the laser beam, the double freeform surface beam shaping system shapes the laser beam into a TEM10 mode Gaussian beam, and the phase adjustment device... The device reverses the polarization directions of the two lobes of the TEM10 mode Gaussian beam. The Mach-Zehnder interferometer optical path structure splits the laser beam into equal-intensity beams, modulates them separately, and combines them into a cylindrical symmetric vector beam. One branch uses a beam rotator to rotate the laser beam into a TEM01 mode Gaussian beam, while the other branch adjusts the phase of the beams to make the beams of the two branches co-phase. The TEM10 mode and TEM01 mode Gaussian beams are combined at the beam combiner within the Mach-Zehnder interferometer optical path structure. Depending on the polarization state adjustment by the polarization direction rotator and the phase adjustment device, the combined beams are of different types of cylindrical symmetric vector beams.

[0007] Furthermore, the polarizer is a polarization adjustment element used to modulate the laser beam emitted by the laser into linearly polarized light.

[0008] Furthermore, the polarization direction rotator is used to adjust the polarization direction of the linearly polarized laser beam according to the need to generate different types of cylindrical symmetric vector beams.

[0009] Furthermore, the dual freeform surface beam shaping system consists of two freeform surface mirrors. The first mirror adjusts the irradiance distribution of the beam, and the second mirror adjusts the beam phase to collimate the beam. This beam shaping system can shape the outgoing laser beam into a TEM10 mode Gaussian beam. When both mirrors are reflectors, combined with the surface coating characteristics of the mirrors, it is suitable for broadband beams.

[0010] Furthermore, the phase modulation device is used to apply different phase delays to different parts of the laser beam, so that the polarization directions of the two lobes of the TEM10 mode Gaussian beam are opposite, that is, the phase difference between the two lobes is... .

[0011] Furthermore, the beam rotator in one branch of the Mach-Zehnder interferometer is used to rotate the laser beam in that branch by 90° around the optical axis, and is orthogonal to the energy distribution of the beam in the other branch, that is, the two branches are TEM10 mode and TEM01 mode Gaussian beams respectively.

[0012] Furthermore, the phase compensation device is used to adjust the phase delay in one of the branches, so that the laser beams of the two branches are in phase so that they can be linearly and coherently superimposed at the beam combiner of the Mach-Zehnder interferometer optical path structure to generate a cylindrically symmetric vector beam, which is radially polarized or angularly polarized.

[0013] The present invention also proposes a cylindrical symmetric vector beam generating device used in the above-mentioned method. The device includes: a laser, a polarizer, a beam expander, a polarization direction rotator, a double freeform surface beam shaping system, a beam rotator, a beam splitter, a beam combiner, a phase adjustment device, a reflector, and a phase compensation device. The beam splitter, beam combiner, two reflectors, beam rotator, and phase compensation device constitute the optical path structure of a Mach-Zehnder interferometer.

[0014] The beneficial effects of this invention also include the following aspects:

[0015] (1) The dual freeform surface beam shaping system used in this invention is an off-axis reflective type with a flexible optical path structure, suitable for a wide spectral range, and the reflective freeform surface mirror has high reflection efficiency and high damage threshold, making it suitable for application scenarios that generate high-power cylindrical symmetric vector beams.

[0016] (2) The freeform beam shaping system used in this invention can also be a coaxial transmission type, which can be replaced according to the optical path structure and application requirements.

[0017] (3) In this invention, a freeform beam shaping system is used to shape the incident laser beam into a TEM10 mode Gaussian beam. The purpose is to avoid the disadvantage that the damage threshold of diffractive optical elements is low and they are not suitable for high-power laser beam shaping applications.

[0018] (4) In this invention, the polarization direction rotator can be adjusted as needed to change the polarization direction of the laser beam, thereby generating polarization directions at different angles to the radial direction of the light spot. A cylindrically symmetric vector beam.

[0019] (5) In this invention, a beam rotator is used to rotate the TEM10 mode Gaussian beam to obtain the TEM01 mode Gaussian beam, without the need to shape the laser beam in two separate branches to obtain the TEM10 mode and TEM01 mode Gaussian beam. Attached Figure Description

[0020] Figure 1 A schematic diagram of the optical path structure of a cylindrical symmetric vector beam generating device;

[0021] Figure 2(a) is a schematic diagram of the optical path simulation of the beam shaping system, and Figure 2(b) is a irradiance distribution diagram of the fundamental mode Gaussian beam after being shaped into a TEM10 mode Gaussian beam by the beam shaping system.

[0022] Figure 3 A schematic diagram of the phase modulation device structure and its modulation effect on the TEM10 mode Gaussian beam;

[0023] Figure 4(a) is a schematic diagram of the irradiance distribution and polarization direction of the TEM10 mode Gaussian beam in the transmission branch in Example 1; Figure 4(b) is a schematic diagram of the irradiance distribution and polarization direction of the TEM01 mode Gaussian beam after the laser beam passes through the Dowell prism in the reflection branch; Figure 4(c) is the simulation result of radially annular polarized light obtained by simulation software in this example.

[0024] Figure 5(a) shows the irradiance distribution and polarization direction of the TEM10 mode Gaussian beam in the transmission branch; Figure 5(b) shows the irradiance distribution and polarization direction of the TEM01 mode Gaussian beam after the laser beam passes through the Dowell prism in the reflection branch; Figure 5(c) shows the angularly polarized light generated after the laser beams from the two branches are combined. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described below in conjunction with the accompanying drawings.

[0026] Example 1

[0027] refer to Figure 1 This invention employs a Gaussian beam with an output wavelength of 532 nm and a cutoff radius of 1 mm. The beam is modulated into a linearly polarized beam by a polarizer, and then expanded by a beam expander with a magnification of 10. A half-wave plate adjusts the polarization direction of the incident beam to be parallel to the x-axis. An off-axis reflection double freeform surface beam shaping system shapes the incident Gaussian beam into a TEM10 mode Gaussian beam, with its two lobes symmetrical about the y-axis. The TEM10 mode Gaussian beam passes through a phase-adjusting device, causing the polarization directions of the left and right lobes to be opposite, i.e., the phase difference between the two lobes is [missing value]. The Mach-Zehnder interferometer's optical path structure splits and combines the laser beam. A beam splitter divides the TEM10 mode laser beam, with its two lobes having opposite polarization directions, into two equal beams. The transmitted laser beam is reflected by mirror 2 and then passes through a phase compensation device composed of two glass plates. This branch's laser beam is adjusted to be in phase with the other branch's laser beam. The reflected laser beam passes through a Dowell prism, where it rotates 90° around the x-axis to become a TEM01 mode Gaussian beam with its polarization direction parallel to the y-axis and the upper and lower beams having opposite polarization directions. The rotated laser beam passes through mirror 1, and the TEM10 and TEM01 mode Gaussian beams from both branches are combined at the beam combiner in the Mach-Zehnder interferometer's optical path, linearly and coherently superimposed to form radially polarized light. Finally, a CCD camera is placed on the target surface to detect the intensity distribution of the radially polarized light.

[0028] In this embodiment, other devices within the corresponding operating wavelength range can be replaced according to the different wavelengths emitted by the laser. Therefore, using lasers of different wavelengths through the optical path can generate radially polarized light of different wavelengths.

[0029] The off-axis reflective dual freeform surface beam shaping system in this embodiment consists of two freeform surface mirrors. The first freeform surface mirror can redistribute the irradiance distribution of the laser beam cross-section, and the second freeform surface mirror adjusts the phase of the beam emitted from the first freeform surface mirror to collimate the beam. This off-axis reflective dual freeform surface beam shaping system can shape a fundamental mode Gaussian beam into a TEM10 mode Gaussian beam. The optical path diagram obtained using TracePro simulation software is shown in Figure 2(a), and the irradiance distribution diagram is shown in Figure 2(b).

[0030] The phase modulation device in this embodiment is designed as follows:

[0031] The phases on the left and right sides of the phase modulation device are 0 and 0, respectively. The two lobes of the beam are of equal area and correspond one-to-one with the two lobes of the TEM10 mode Gaussian beam, with the right lobe having a phase delay relative to the left lobe. A schematic diagram of the phase modulation device and its modulation effect on the TEM10 mode Gaussian beam is shown below. Figure 3 As shown. The right side of the phase modulation device has a step difference relative to the left side, and its height... for:

[0032] (1)

[0033] In the formula, The wavelength of the laser beam The refractive index of the special polymer material. The composite transmittance function of the phase modulation device. for:

[0034] (2)

[0035] In the formula, The angle of phase delay, These are the coordinates of the symmetry axis of the two lobes of the TEM10 mode Gaussian beam, when for The polarization state of the TEM10 mode Gaussian beam after passing through the phase modulation device is shown in Figure 4(a).

[0036] In this embodiment, the beam splitter and beam combiner both have a splitting ratio of 50:50. The Mach-Zehnder interferometer optical path structure composed of the beam splitter, beam combiner, and two mirrors is rectangular to ensure that the beams from the two branches are linearly and coherently superimposed at the beam combiner.

[0037] In this embodiment, the function of the Dowell prism is to use its own optical rotation characteristics to rotate the laser beam in the reflection branch. When the Dowell prism rotates 45°, the beam rotates 90° around the optical axis. At this time, the TEM10 mode Gaussian beam becomes the TEM01 mode Gaussian beam. The beam spot and polarization direction are orthogonal to the laser beam in the transmission branch. The shape of the beam spot and the polarization state are shown in Figure 4(b).

[0038] In this embodiment, the phase compensation device in the optical path of the Mach-Zehnder interferometer consists of two glass plates. By adjusting the angle between the two glass plates, the optical path in the branch where the device is located can be changed, that is, the phase of the laser beam in the branch can be adjusted to be in phase with the laser beam in the other branch.

[0039] The simulation results of radially polarized light obtained by simulation software in this embodiment are shown in Figure 4(c). The irradiance distribution of the light spot is a ring beam with a central light intensity of 0, and the polarization direction at any position is along the radial direction of that position.

[0040] Example 2

[0041] Using the same devices and optical path structure as in Example 1, the polarization direction of the laser beam is adjusted to be parallel to the y-axis by rotating the half-wave plate. The simulation results of the laser beams of the transmission and reflection branches are shown in Figures 5(a) and 5(b). After the laser beams of the two branches are combined, angularly polarized light is generated, as shown in Figure 5(c). The irradiance distribution of the light spot is a ring beam with a central light intensity of 0, and the polarization direction at any position is perpendicular to the radial direction at that position.

[0042] The above description represents a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All design schemes that fall within the scope of the present invention's concept are within the scope of protection of the present invention.

Claims

1. A method for generating a cylindrical symmetric vector beam, characterized in that, The method is as follows: The laser beam emitted from the laser sequentially passes through a polarizer, a beam expander, a polarization direction rotator, a double freeform surface beam shaping system, and a phase adjustment device. It then enters a Mach-Zehnder interferometer optical path structure consisting of a beam splitter, a beam rotator, two mirrors, a phase compensation device, and a beam combiner. The polarizer modulates the laser beam into linearly polarized light, the beam expander expands the laser beam, the polarization direction rotator adjusts the polarization direction of the laser beam, the double freeform surface beam shaping system shapes the laser beam into a TEM10 mode Gaussian beam, and the phase adjustment device makes the TEM10 mode Gaussian beam... The two lobes of the 0-mode Gaussian beam have opposite polarization directions. The optical path structure of the Mach-Zehnder interferometer splits the laser beam, modulates and combines them into a cylindrical symmetric vector beam. One branch uses a beam rotator to rotate the laser beam into a TEM01-mode Gaussian beam, while the other branch adjusts the phase of the beam to make the two beams co-phase. The TEM10-mode and TEM01-mode Gaussian beams are combined at the beam combiner within the Mach-Zehnder interferometer optical path structure. Depending on the polarization state adjustment by the polarization direction rotator and phase control device, the combined beams are of different types of cylindrical symmetric vector beams.

2. The method for generating a cylindrical symmetric vector beam according to claim 1, characterized in that, The polarizer is a polarization adjustment element used to modulate the laser beam emitted from the laser into linearly polarized light.

3. The method for generating a cylindrical symmetric vector beam according to claim 1, characterized in that, The polarization direction rotator is used to adjust the polarization direction of the linearly polarized laser beam according to the need to generate different types of cylindrical symmetric vector beams.

4. The method for generating a cylindrical symmetric vector beam according to claim 1, characterized in that, The dual freeform surface beam shaping system consists of two freeform surface mirrors. The first mirror adjusts the irradiance distribution of the beam, and the second mirror adjusts the beam phase to collimate the beam. This beam shaping system can shape the outgoing laser beam into a TEM10 mode Gaussian beam. When both mirrors are reflectors, combined with the surface coating characteristics of the mirrors, it is suitable for broadband beams.

5. The method for generating a cylindrical symmetric vector beam according to claim 1, characterized in that, The phase modulation device is used to apply different phase delays to different parts of the laser beam, so that the polarization directions of the two lobes of the TEM10 mode Gaussian beam are opposite, that is, the phase difference between the two lobes is... .

6. The method for generating a cylindrical symmetric vector beam according to claim 1, characterized in that, The beam rotator in one branch of the Mach-Zehnder interferometer is used to rotate the laser beam in that branch by 90° around the optical axis, and is orthogonal to the energy distribution of the beam in the other branch, that is, the two branches are Gaussian beams of TEM10 mode and TEM01 mode, respectively.

7. The method for generating a cylindrical symmetric vector beam according to claim 1, characterized in that, The phase compensation device is used to adjust the phase delay in one of the branches, so that the laser beams of the two branches can be in phase so that they can be linearly and coherently superimposed at the beam combiner of the Mach-Zehnder interferometer optical path structure to generate a cylindrically symmetric vector beam, which is radially or angularly polarized.

8. A cylindrically symmetric vector beam generating apparatus used in the method according to any one of claims 1-7, characterized in that, The device includes: a laser, a polarizer, a beam expander, a polarization direction rotator, a double freeform surface beam shaping system, a beam rotator, a beam splitter, a beam combiner, a phase adjustment device, a reflector, and a phase compensation device. The beam splitter, beam combiner, two reflectors, beam rotator, and phase compensation device constitute the optical path structure of a Mach-Zehnder interferometer.

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