Switching method of vortex beam, flat-top beam and gaussian beam and solid-state laser

Abstract

The application provides a vortex beam, flat-top beam and Gaussian beam switching method, which is realized based on a conical refraction annular pumped solid-state laser; the change of a light spot structure between a conical refraction annular plane and a Raman point plane is utilized by means of biaxial crystal conical refraction, the optical path between the biaxial crystal and a laser gain medium is adjusted, and selective output of first-order vortex light, flat-top light and Gaussian light beams is realized. Based on the above principle, the application further provides a plurality of solid-state lasers for vortex beam, flat-top beam and Gaussian beam switching. The application effectively utilizes the advantages of vortex beam, flat-top beam and Gaussian beam, and can simultaneously utilize the three light sources in the same optical path experiment without changing the original optical path structure, and further expansion is realized on the basis of a Laguerre Gaussian beam solid-state laser based on conical refraction annular light pumping.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of laser, in particular to a vortex beam, flat-top beam and Gaussian beam switching method and solid-state laser. BACKGROUND

[0002] Since the advent of laser, it has been widely used in laser processing, high-precision material detection, biomedical research, optical data storage and lithography process, etc. due to its good monochromaticity, directionality and high coherence. However, for single-mode laser, its energy is in non-uniform Gaussian distribution. Therefore, in practical application, the non-uniformity of the beam energy causes the edge effect in the low-energy area outside the waist radius of the Gaussian beam, thereby limiting the application of laser in laser processing, laser welding, uniform illumination and optical communication, etc. Therefore, it is necessary to convert the non-uniform illumination Gaussian beam into a flat-top beam with uniform illumination to obtain better processing, illumination, etc.

[0003] Compared with Gaussian beam, flat-top beam is a kind of beam with flat and uniform energy distribution, which allows more efficient energy transfer and smaller heat-affected zone. This makes flat-top beam more suitable for applications with constant light intensity in a given area, such as semiconductor wafer processing, high-power level nonlinear frequency conversion and material processing, fluorescence microscopy, holography and interferometry. At present, the methods for obtaining flat-top beam mainly include: using special resonant cavity, laser medium gain saturation effect method, beam synthesis and beam shaping technology, etc., among which the simplest and most common one is beam shaping method.

[0004] Vortex beam is a kind of beam with spiral distribution of equal phase surface and circular ring distribution of light intensity, which carries orbital angular momentum. Due to the dynamic and quantum properties of the orbital angular momentum beam, the rotating beam with hollow light intensity distribution is generated by the phase singularity of the spiral phase wavefront, which has important potential value and application in many fields such as particle manipulation, information coding, high-capacity high-speed large-scale communication and super-resolution microscopic imaging.

[0005] The common methods for generating vortex light can be divided into active method and passive method. Passive method mainly uses various optical elements to shape the laser outside the cavity, including spiral phase plate, spatial light modulator, astigmatic mode converter, etc.; this method has poor beam quality and low peak power of the beam. Active method refers to directly obtaining LG mode laser output in a cylindrical symmetric laser resonant cavity by mode selection method; this method has high power output and high electro-optical conversion efficiency.

[0006] In order to better utilize the advantages of the above-mentioned beams, it is a technical problem to be solved in the field that the three kinds of light sources can be used simultaneously in the same optical path experiment without changing the original optical path structure. SUMMARY

[0007] The application aims to provide a switching method and a solid laser for realizing selective output of first-order vortex light, flat-top light and Gaussian light beams.

[0008] To achieve the above-mentioned purpose, the application provides a switching method for vortex light beams, flat-top light beams and Gaussian light beams, which is realized based on a solid laser of a conical refraction annular pumping; the variation of a light spot structure between a conical refraction annular plane and a Raman point plane is utilized to adjust an optical path between a biaxial crystal and a laser gain medium, so that selective output of first-order vortex light, flat-top light and Gaussian light beams is realized.

[0009] The application further provides a solid laser of a conical refraction annular pumping, which comprises a rotating disc on which different length parallel plates are installed; the rotating disc is arranged between the biaxial crystal and the laser gain medium.

[0010] Further, the number of the plates is at least three, the plates are transparent parallel plates or transparent uniaxial crystals, and a pump light anti-reflection film is coated on the front surface of the plates.

[0011] Further, the displacement of the focal point between the longest plate and the intermediate length plate is equal to the longitudinal displacement caused by the conical refraction of the biaxial crystal.

[0012] Further, the biaxial crystal is cut along any biaxial crystal optical axis direction, and the cutting direction is parallel to the output direction of the pump light.

[0013] Further, the biaxial crystal and the rotating disc with different length plates are located within one focal length of the focusing lens.

[0014] The application further provides a solid laser of a conical refraction annular pumping, which comprises a pump source outputting a circularly polarized light or non-polarized light, a focusing lens, a biaxial crystal, a laser gain medium and a laser output coupling arranged in sequence along the output light direction of the pump source and on the same optical axis; the laser gain medium and the laser output coupling mirror are integrally packaged into a resonant cavity and arranged behind the biaxial crystal.

[0015] The application further provides a solid laser of a conical refraction annular pumping, which comprises a pump source outputting a circularly polarized light or non-polarized light, a focusing lens, a biaxial crystal, an imaging lens, a laser gain medium and a laser output coupling arranged in sequence along the output light direction of the pump source and on the same optical axis;

[0016] The imaging lens is provided with a pump light anti-reflection film on the front and rear surfaces, and the imaging lens is moved forward and backward between the biaxial crystal and the laser gain medium.

[0017] Compared with the prior art, the advantages of the present application are that: the present application effectively utilizes the advantages of vortex beams, flat-top beams and Gaussian beams, and can simultaneously utilize the three kinds of light sources in the same optical path experiment without changing the original optical path structure, and further expands on the basis of the Laguerre-Gaussian beam solid-state laser pumped by the conical refraction annular light.

[0018] The present application also proposes a variety of solid-state lasers capable of switchable output of first-order vortex light, flat-top light and Gaussian beam based on biaxial crystal conical refraction pumping. The variation of the spot structure between the conical refraction annular plane and the Raman point plane is utilized, and the optical path between the biaxial crystal and the laser gain medium is adjusted, so that the selective output of the first-order vortex light, the flat-top light and the Gaussian beam is realized. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 Figure 1 is a structural schematic diagram of a solid-state laser in the embodiment 1 of the present application;

[0020] Figure 2 Figure 2 is a side structural schematic diagram of a turntable in the embodiment of the present application;

[0021] Figure 3 Figure 3 is a schematic diagram of spot structures at different propagation distances of conical refraction;

[0022] Figure 4 Figure 4 is a structural schematic diagram of a solid-state laser with a quarter-wave plate added

[0023] Figure 5 Figure 5 is a structural schematic diagram of a solid-state laser in the embodiment 2 of the present application;

[0024] Figure 6 Figure 6 is a structural schematic diagram of a solid-state laser in the embodiment 3 of the present application. DETAILED DESCRIPTION

[0025] In order to make the purpose, technical scheme and advantages of the present application more clear, the technical scheme of the present application will be further described below.

[0026] In the following embodiments, some terms are explained as follows: conical refraction: focused circularly polarized light is incident along a certain optical axis of a biaxial crystal, and a hollow refraction cone is formed inside the crystal, which evolves into a hollow cylinder after exiting the crystal, and the cross-sectional intensity distribution is two concentric bright rings divided by a Poggendorff dark ring.

[0027] Annular pumping: the pump light is shaped into an annular intensity distribution, so that its spatial distribution is overlapped with the oscillation mode in the laser gain medium, thereby being beneficial to high-quality and high-power output of the laser mode.

[0028] Embodiment 1:

[0029] AsFigure 1 As shown in the figure, a switchable output vortex beam, flat-top beam and Gaussian beam laser based on conical refraction annular pumping, the composition includes a pump source 1 outputting circularly polarized or unpolarized light, a focusing lens 3, a biaxial crystal 4, a rotating disc 5 with different length parallel plates, a laser gain medium 6 and a laser output coupling 7 placed in turn along the output direction of the pump source and on the same optical axis.

[0030] The pump source is a laser or laser system outputting circularly polarized or unpolarized light. The biaxial crystal is a biaxial crystal cut along any one of its biaxial crystal optical axis directions, and the cutting direction is parallel to the output direction of the pump light. The difference between the focal point displacement generated by the longest plate and the intermediate length plate in the rotating disc with different length plates is equal to the longitudinal displacement generated by the biaxial crystal conical refraction, and the front and back surfaces of the plate are coated with pump light anti-reflection film. As Figure 2 shown, the number of parallel plates in the rotating disc with different length plates is not less than 3. The plates in the rotating disc with different length plates are transparent parallel plates or transparent uniaxial crystals, and the front surface of the plate is coated with pump light anti-reflection film. The biaxial crystal and the rotating disc with different length plates are located within one focal length of the focusing lens. The front surface of the gain medium is coated with a laser high reflection film and a pump light anti-reflection film, and the back surface is coated with a laser anti-reflection film. The high reflection film serves as a laser input coupling mirror and a laser output coupling mirror to form a laser resonant cavity. The laser gain medium is a laser crystal, a laser ceramic or a laser glass. The laser coupling mirror is a plane mirror or a concave mirror that partially transmits and partially reflects laser light.

[0031] Experimental principle:

[0032] When the focused circularly polarized or unpolarized light beam is incident along a certain optical axis of the biaxial crystal, two concentric bright rings are observed on the focal plane of the incident light beam, separated by a Poggendorff dark ring. The contrast of the inner and outer rings is the highest in this plane, the intensity of the outer ring is much higher than that of the inner ring, and the image plane is the clearest, which is called the conical refraction ring plane (Z=0 plane); Z=0 plane is a symmetric plane, and the planes equidistant on both sides within a certain distance have the same ring intensity distribution. Moving away from the Z=0 plane, the double ring becomes wider; the outer ring becomes dim, the inner ring becomes bright and obtains an axial spot structure in the form of fine inner stripes. Further moving away, the outer ring disappears and the inner ring collapses into a bright central light spot surrounded by fine circular diffraction fringes. When the axial spot reaches the maximum intensity, it is called the Raman point plane (Z=ZF plane). In the far zone (Z>ZF), the intensity distribution of the conical refraction beam cannot be visually distinguished from that of the incident beam, which indicates that the two distributions are asymptotically equal. The optical path diagram is shown in Figure 3 ;

[0033] Figure 3 (a) is a conical refraction diagram;Figure 3 (b) is a schematic diagram of the spot structure on the left and right sides of the conical refraction ring plane, the ring plane (Z=0 plane) is the symmetry plane, and the spot structures of the left and right sides of the equal distance surface correspond to each other.

[0034] Therefore, the pump light of the double-ring structure can be obtained in the conical refraction ring plane (Z=0 plane); between the Z=0 plane and the Z=ZF plane, there is a range in which the spot structure is a flat-top beam; and in the far zone (Z>ZF), the pump light of a Gaussian beam can be obtained.

[0035] According to the parallel-plate imaging characteristics, the direction of the light is unchanged after passing through the parallel plate, and the image of the object is not magnified or reduced, but only lateral and axial displacements are generated. Therefore, a turntable is added in front of the gain medium, and only by rotating the turntable can different lengths of parallel plates be placed in the light path to change the spot structure obtained by the front surface of the gain medium, which is simple to operate and has good system stability.

[0036] When the turntable is rotated and the longest parallel plate is placed in the light path, the front surface of the gain medium obtains a conical refraction ring plane pattern (i.e., a Z=0 plane pattern), and an LG01 mode is output after the gain of the resonant cavity; when the middle-length parallel plate is placed in the light path, the front surface of the gain medium obtains a flat-top beam spot pattern, and a flat-top beam is output after the gain of the resonant cavity; and when the shortest parallel plate is placed in the light path, the front surface of the gain medium obtains a Gaussian spot pattern, and a Gaussian beam is output after the gain of the resonant cavity.

[0037] The laser gain medium and the rear laser output coupling mirror constitute a solid-state laser pumped by a ring-shaped light. By rotating the turntable, different lengths of parallel plates are placed in the light path, so that different spot structures are obtained at the front surface of the gain medium. After the pump light reaches the threshold to generate laser output, the power is increased, and the first-order vortex light, flat-top light, and Gaussian beam can be selectively output.

[0038] Detailed description of the scheme:

[0039] If the biaxial crystal length is large or R0>>ω0 (ω0 is the spot radius of the pump light, and R0 is the radius of the conical refraction ring, which is proportional to the length of the biaxial crystal), the ring radius and the longitudinal displacement of the biaxial crystal are large, and the conical refraction phenomenon is more easily directly observed in experiments, so the requirement for the focal length of the focusing lens is not high, and a lens with too large focal length can be avoided. Because the incident spot after focusing by a long-focus lens is large, the fine structure of the conical refraction spot is not so clear. At least three parallel plates with different lengths are arranged in the turntable, and to reduce the loss of light energy of the pump light, the shortest plate has a length of 0, i.e., no parallel plate is installed.

[0040] If the biaxial crystal is short and the condition R0»ω0 is not met, a short-focus lens is used to facilitate the observation and switching of the conical refraction spot structure. At this time, the longitudinal shift caused by the biaxial crystal conical refraction is very small. In order to make the front surface of the gain medium better receive different spot structures, the parallel plate is required to have a refractive index n 板 板 The parallel plate has a refractive index n1 and a length l1, and the biaxial crystal has a refractive index n2 along the optical axis.

[0041] The intensity of the conical refraction ring obtained by the conical refraction of circularly polarized light or non-polarized light incident on the biaxial crystal is uniformly distributed, while the intensity of the conical refraction ring obtained by the conical refraction of linearly polarized light or partially polarized light is not uniformly distributed. If the pump source is a laser or a laser system that outputs linearly polarized light or partially polarized light, in order to obtain a conical refraction pattern with uniform intensity, a quarter-wave plate 2 is inserted between the pump source and the focusing lens to adjust the pump light to circularly polarized light or non-polarized light. The experimental device is as shown in Figure 4 .

[0042] The principle of the quarter-wave plate is that linearly polarized pump light becomes circularly polarized light after passing through the quarter-wave plate; partially polarized pump light can be adjusted to non-polarized light by adjusting the fast-axis direction of the quarter-wave plate. Then the circularly polarized or non-polarized light beam is incident along an optical axis of the biaxial crystal to produce conical refraction. The other devices and their principles are consistent with the first technical solution.

[0043] Example 2:

[0044] As shown in Figure 5 , directly moving the gain medium forward and backward to control the spot structure of the pump light will change the cavity length of the resonant cavity, thereby changing the stable state of the resonant cavity and affecting the stable output of the laser. However, the laser gain medium and the output coupling mirror can be packaged into a resonant cavity as a whole and placed behind the biaxial crystal, without the need for a turntable or an imaging lens. The entire resonant cavity can be directly moved forward and backward to control the output of the laser.

[0045] The other devices and their principles are consistent with example 1. This solution reduces one optical element and reduces the loss of optical energy.

[0046] Example 3:

[0047] As shown in Figure 6 , the turntable 5 in technical solution 1 is replaced by an imaging lens 8, and the position of the laser gain medium is fixed and remains unchanged.

[0048] ​The distance between the imaging lens and the crystal is changed by moving the imaging lens back and forth, so that the conical refraction patterns at different distances behind the crystal are imaged on the front surface of the laser gain medium. To reduce the loss of light energy, the front and back surfaces of the imaging lens 8 are coated with a pump light anti-reflection film. Other devices and principles are consistent with embodiment 1. This scheme avoids the determination and installation of the length of each parallel plate, and reduces the cost.

[0049] The above are only preferred embodiments of the present application, and do not have any limiting effect on the present application. Any person skilled in the art can make any form of equivalent replacement, modification or change of the technical solutions and technical contents disclosed by the present application without departing from the scope of the technical solutions of the present application, which still belongs to the protection scope of the present application.

Claims

1. A conical refraction ring pumped solid-state laser, based on a conical refraction ring pumped solid-state laser; utilizing the change in the spot structure between the conical refraction ring plane and the Raman point plane caused by the conical refraction of the biaxial crystal, adjusting the optical path between the biaxial crystal and the laser gain medium, and realizing the selective output of first-order vortex light, flat-top light, and Gaussian beam, characterized in that: The invention comprises a turntable on which parallel plates of different lengths are mounted; the turntable is placed between the biaxial crystal and the laser gain medium; There are three flat plates, each of which is a transparent parallel plate or a transparent uniaxial crystal, and the front surface of each flat plate is coated with a pump light anti-reflection film; The difference in focal shift between the longest plate and the intermediate length plate among the plates is equal to the longitudinal shift caused by the conical refraction of the biaxial crystal.

2. The conical refractive ring pumped solid-state laser according to claim 1, characterized in that: The biaxial crystal is a biaxial crystal cut along any one of its optical axes, and the cutting direction is parallel to the output direction of the pump light.

3. The conical refractive ring pumped solid-state laser according to claim 2, characterized in that: The biaxial crystal and the turntable equipped with plates of different lengths are located within one focal length of the focusing lens.

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