Method for generating high-quality Laguerre-Gaussian vortex light

Abstract

The invention discloses a method for generating high-quality Laguerre-Gaussian vortex light, which comprises the following steps: based on a V-shaped laser resonant cavity of a concave folded mirror, astigmatism is introduced by utilizing different convergence capacities of the concave folded mirror on a meridian plane and a sagittal plane of a light beam, so that the cylindrical symmetry of the resonant cavity is destroyed, and the high-quality Laguerre-Gaussian vortex light is generated; the influence of the cylindrical symmetry of the laser resonant cavity on the intrinsic mode of the cavity is eliminated, so that the intrinsic mode of the cavity is a Hermite-Gaussian mode, and the generation of an ellipse due-Gaussian mode is inhibited; an off-axis pumping method is adopted, and the laser resonant cavity generates high-order Hermite-Gaussian mode laser; the focusing lens focuses the generated high-order Hermite-Gaussian mode laser, so that the beam waist of the high-order Hermite-Gaussian mode laser is positioned in the center of the astigmatism mode converter; the astigmatism mode converter provides conditions for mode conversion by changing the Gouy phase of the laser mode, and high-quality Laguerre-Gaussian vortex light output is achieved.

Description

Technical Field

[0001] The invention relates to the field of lasers, and in particular to a method for generating high-quality Laguerre-Gaussian vortex light. Background Art

[0002] Laguerre-Gaussian vortex light carrying orbital angular momentum has a unique annular light intensity and spiral phase distribution, and has important applications in optical imaging, optical manipulation, quantum communication and other fields. [1] . The methods for generating Laguerre-Gaussian vortex light are mainly divided into two categories: extracavity control of the light field and direct excitation in the cavity. Extracavity control of the light field refers to the use of spatial light modulators or by superimposing different combinations of the eigenmodes of the resonant cavity to achieve light field control. However, such methods usually face problems such as high cost and low integration. Direct excitation in the cavity refers to adding a modulation element to the cavity to modulate a laser mode with orbital angular momentum, or by shaping the pump light to excite Laguerre-Gaussian vortex light, but this type of method has high requirements for the modulation element or pump light shaping, and the flexibility of mode order control is low. Another method to achieve Laguerre-Gaussian vortex light by direct excitation in the cavity is to generate Hermite-Gaussian lasers through off-axis pumping, and then use an astigmatic mode converter to convert the Hermite-Gaussian laser into Laguerre-Gaussian vortex light. This method is simple to operate, low in cost, and controllable; especially in recent years, various researchers have made great breakthroughs in the field of achieving high-order Hermite-Gaussian lasers. [2] , which further improves the feasibility and practicality of obtaining Laguerre-Gaussian mode vortex light from Hermite-Gaussian mode laser.

[0003] Off-axis pumping is the most commonly used method to generate Hermite-Gaussian lasers: the centers of the fundamental mode and higher-order modes of the resonant cavity generally coincide with the optical axis of the resonant cavity, the maximum intensity of the fundamental mode is located at the center, and the maximum intensity of the higher-order Hermite-Gaussian laser is located at the maxima at both ends of its spot. Adjust the optical path of the pump light to deviate from the optical axis of the resonant cavity. When the overlap of the pump light with a specific mode is better than the overlap with the fundamental mode, a higher-order Hermite-Gaussian mode with the outermost maximum very close to the pump spot can be excited. For example, H.Laabs et al. in Germany first proposed the off-axis pumping method and achieved a high-frequency pulse width of HG 0,87 Hermite-Gaussian mode output [3] . By placing an astigmatic mode converter behind the generated Hermite-Gaussian laser and satisfying the mode conversion conditions, the corresponding Laguerre-Gaussian vortex light can be obtained. Using this principle, many research groups have conducted in-depth research on the generation of Laguerre-Gaussian vortex light. For example, domestic researchers have experimentally generated Laguerre-Gaussian vortex light with a topological charge number that continuously changes from 1 to 30. [4] ; also realized high-order Laguerre-Gaussian vortex light, such as: LG 0,80 LG0,102 wait [5] However, the LG p,l The mode quality is not ideal, especially its light intensity distribution is usually elliptical, rhombic, or even irregular ring-shaped, rather than a standard perfect circular ring. The fundamental reason is that the quality of the Hermite-Gaussian beam itself is not good before it is transformed into a Laguerre-Gaussian beam through an astigmatism mode converter. This is because traditional laser resonators generally have cylindrical symmetry. Affected by the cylindrical symmetry, the laser mode generated by off-axis pumping is often an Innes-Gaussian mode with elliptical symmetry, which is a continuous transition between the Hermite-Gaussian mode and the Laguerre-Gaussian mode. [6] Due to this factor, it is easier to obtain a helical Innes-Gaussian mode (HIG) laser rather than a high-quality Laguerre-Gaussian mode laser by further passing the generated laser mode through an astigmatic mode converter.

[0004] References

[0005] [1] Liu Qiang, Pan Jing, Wan Zhensong, et al. Generation method of complex vortex structure light field[J]. Chinese Journal of Lasers, 2020, 47(5): 0500006.

[0006] [2]Beijersbergen MW,Allen L,Van der Veen H,et al.Astigmatic lasermode converters and transfer of orbital angular momentum[J].OpticsCommunications,1993,96(1-3):123-132.

[0007] [3]Laabs H,Ozygus B.Excitation of Hermite Gaussian modes in end-pumped solid-state lasers via off-axis pumping[J].Optics&laser technology,1996,28(3):213-214.

[0008] [4]Liu H, Yan L, Chen H, et al. High-order femtosecond vortices up to the30th order generated from a powerful mode-locked Hermite-Gaussian laser[J]. Light: Science&Applications, 2023, 12(1):207.

[0009] [5]Li N, Xu B, Cui S, et al. High-order vortex generation from CW andpassively Q-switchedPr:YLF visible lasers[J]. IEEE Photonics Technology Letters, 2019, 31(17):1457-1460.

[0010] [6]Bandres MA, Gutiérrez-Vega J C.Ince–gaussian beams[J].Opticsletters,2004,29(2):144-146. Summary of the invention

[0011] The present invention provides a method for generating high-quality Laguerre-Gaussian vortex light. The present invention constructs a V-shaped folded resonant cavity with a concave reflector as a folding mirror, and introduces astigmatism into the cavity by utilizing the different converging abilities of the concave folding mirror on the meridian plane and the sagittal plane on the light beam, thereby destroying the cylindrical symmetry of the laser resonant cavity and eliminating the influence of the cylindrical symmetry on the laser mode, so that the Innes-Gaussian mode is no longer the intrinsic mode of the folded laser resonant cavity; when the pump light deviates from the optical axis, a high-quality high-order Hermite-Gaussian mode will be generated; and the Hermite-Gaussian laser can be converted into Laguerre-Gaussian vortex light by an astigmatism mode converter, as described below:

[0012] A method for generating high-quality Laguerre-Gaussian vortex light, wherein a laser for implementing the method comprises: a pump source, a pump focusing mirror, a laser gain medium, a laser total reflection mirror, a concave folding mirror, a laser output mirror, a focusing lens, and an astigmatism mode converter.

[0013] The pump source emits pump light in the absorption band of the laser gain medium, and the pump light is focused by the pump focusing mirror into the laser gain medium; the laser gain medium absorbs the pump light, forms a population inversion, and generates laser gain; the laser total reflection mirror and the concave folding mirror have high reflectivity to the laser wavelength, and the laser output mirror has partial transmittance to the laser wavelength, and the three together constitute a folded V-shaped laser resonant cavity to provide feedback for the laser;

[0014] Based on the different convergence capabilities of the concave folding mirror on the meridian plane and the sagittal plane, astigmatism is introduced. Therefore, the V-shaped laser resonator does not have cylindrical symmetry, and its eigenmode is the Hermite-Gaussian mode, so the elliptical Innes-Gaussian mode will not be generated.

[0015] When the laser gain exceeds the laser resonant cavity loss, the laser reaches the threshold and generates laser output; when the pump light incident position deviates from the optical axis of the laser resonant cavity, the laser generates a high-order Hermite-Gaussian mode laser; the focusing lens focuses the generated high-order Hermite-Gaussian mode laser so that its beam waist is located at the center of the astigmatism mode converter composed of two cylindrical lenses with the same focal length, and the focal length of the cylindrical lens is 1 / 2 of the distance from the cylindrical lens to the laser beam waist. The astigmatic mode converter makes the Gouy phase difference of the Hermite-Gaussian mode laser on the x-axis and the y-axis equal to π / 2, and converts the Hermite-Gaussian mode laser into Laguerre-Gaussian mode vortex light.

[0016] The laser gain medium for implementing the method is a bulk crystal, glass or ceramic doped with rare earth ions or transition metal ions, or a solidified dye or encapsulated gas or liquid gain medium, or a gain medium that provides nonlinear gain through nonlinear processes such as optical parametric oscillation or stimulated Raman scattering.

[0017] The pump source for implementing the method is a semiconductor laser, a fiber laser, or a solid laser, and the laser mode is a fundamental transverse mode, a super-Gaussian or flat-top distribution of multiple transverse modes. The emission wavelength of the pump source for implementing the method is within the absorption band of the laser gain medium or the light transmission band and phase matching range of the nonlinear gain medium, so that the laser gain medium generates gain.

[0018] The laser total reflection mirror and the laser output mirror for implementing the method are plane mirrors or concave mirrors. The focusing lens for implementing the method has a focal length that satisfies the need to focus the Hermite-Gaussian laser beam waist at the center of the astigmatism mode converter. The astigmatism mode converter is composed of a group of identical cylindrical lenses, or a cylindrical lens and a focusing lens.

[0019] The beneficial effects of the technical solution provided by the present invention are:

[0020] 1. The present invention adopts a V-shaped folded resonant cavity structure and utilizes the characteristics of the concave folding mirror to introduce astigmatism into the light beam to eliminate the cylindrical symmetry of the traditional laser resonant cavity, so that the laser is not easy to generate an elliptical Innes-Gaussian mode, thereby being able to obtain a high-quality Hermite-Gaussian mode output under off-axis pumping;

[0021] 2. The present invention can obtain high-quality high-order Hermite-Gaussian modes by introducing a V-shaped folded resonant cavity structure and astigmatism, and then obtain Laguerre-Gaussian vortex light with high mode quality by using an astigmatism mode converter; the angular index and radial index of the Laguerre-Gaussian vortex light can be easily determined by the Hermite-Gaussian mode order;

[0022] 3. The present invention can obtain high-quality high-order Hermite-Gaussian modes by introducing a V-shaped folded resonant cavity structure and astigmatism, and then use an astigmatism mode converter to obtain Laguerre-Gaussian vortex light with high mode quality; compared with methods such as defect points and amplitude templates for selecting high-order Laguerre-Gaussian mode lasers, the present invention has low loss and high pump utilization rate; the astigmatism conversion process is a unitary conversion, which is highly efficient and simple to convert;

[0023] 4. The present invention can obtain high-quality high-order Hermite-Gaussian modes by introducing a V-shaped folded resonant cavity structure and astigmatism, and then use an astigmatism mode converter to obtain Laguerre-Gaussian vortex light with high mode quality; since the resonant cavity has no cylindrical symmetry, the obtained Laguerre-Gaussian vortex light is an LG with net orbital angular momentum. p,l beam, rather than the LG without net orbital angular momentum obtained by the conventional Laguerre-Gaussian mode selection method p,±l beam. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 A schematic diagram of an optical path for producing high-quality Laguerre-Gaussian vortex laser;

[0025] Figure 2 A schematic diagram showing the comparison between the Ince-Gaussian mode generated by a conventional resonant cavity laser through simulation and experiment, and the spiral-Ince-Gaussian mode converted through an astigmatic mode converter;

[0026] Figure 3 A comparative schematic diagram of the one-dimensional Hermite-Gaussian mode generated by an astigmatic resonant cavity laser obtained through simulation and experiment, and the Laguerre-Gaussian mode converted through an astigmatic mode converter.

[0027] In the accompanying drawings, the components represented by the reference numerals are listed as follows:

[0028] 1: Pump source; 2: Pump focusing mirror;

[0029] 3: Laser total reflection mirror; 4: Laser gain medium;

[0030] 5: Concave folding mirror; 6: Laser output mirror;

[0031] 7: Focusing lens; 8: Astigmatism mode converter. DETAILED DESCRIPTION

[0032] In order to make the objectives, technical solutions and advantages of the present invention more clear, the embodiments of the present invention are described in further detail below.

[0033] Example 1

[0034] The embodiment of the present invention provides a high-quality Laguerre-Gaussian vortex optical laser, which includes: a pump source 1, a pump focusing mirror 2, a laser gain medium 3, a laser total reflection mirror 4, a concave folding mirror 5, a laser output mirror 6, a focusing lens 7, and an astigmatism mode converter 8;

[0035] The pump source 1 is a semiconductor laser with fiber-coupled output, with a wavelength of 878.6 nm and a fiber core diameter of 200 μm; the pump focusing lens 2 is composed of two plano-convex lenses with a focal length of 40 mm; the laser gain medium 3 is a neodymium-doped yttrium vanadate (Nd:YVO4) crystal cut along the a-axis, with an emission peak of 1064 nm, Nd 3+ The doping concentration is 0.5-at.%, and the size is 5mm×5mm×8mm; the laser total reflection mirror 4 is a plane mirror, and the side facing the cavity is coated with a 1064nm laser wavelength high reflection film, and the double sides are coated with an 878.6nm pump wavelength anti-reflection film; the concave folding mirror 5 is a plano-concave mirror with a concave curvature radius of 200mm, and the concave surface is coated with a 1064nm laser wavelength high reflection film; the laser output mirror 6 is a plane mirror, coated with a 1064nm laser wavelength partial transmission film (transmittance T=5%); the laser total reflection mirror 4, the concave The folding mirror 5 and the laser output mirror 6 together form a "V"-shaped 1064nm laser folding resonant cavity. The distance between the laser total reflection mirror 4 and the concave folding mirror 5 is 130mm, the distance between the laser output mirror 6 and the concave folding mirror 5 is 170mm, and the folding angle is 20°. The laser gain medium 3Nd:YVO4 crystal is placed close to the laser total reflection mirror 4. The focusing lens 7 is a convex lens with a focal length of 110mm. The astigmatism mode converter 8 is composed of two plano-convex cylindrical lenses with a focal length of 100mm each. Figure 1 shown.

[0036] The 878.6 nm pump light emitted by pump source 1 is focused by pump focusing mirror 2 and enters laser gain medium 3 Nd:YVO4 crystal with a spot radius of 100 μm. 3+The ions absorb the 878.6nm pump light and are excited to the upper laser energy level, generating laser gain. When the pump light spot is located at the center of the crystal and coincides with the optical axis of the resonant cavity, the overlap between the pump light and the fundamental transverse mode Gaussian beam is higher than the overlap with other higher-order modes. At this time, the laser operates in the fundamental mode. When the lateral position of the pump light spot is adjusted, the laser is pumped off-axis. When the overlap between the pump light and a certain higher-order transverse mode is higher than that of other modes, the laser tends to operate in this higher-order Hermite-Gaussian mode. The high-order Hermite-Gaussian laser output from the laser output mirror 6 is focused by the focusing lens 7, and the waist of the focused laser beam is located in the center of the astigmatic mode converter 8.

[0037] The laser in the embodiment of the present invention adopts a V-shaped folded resonant cavity structure. The concave folding mirror 5 has different converging effects on the light beam on the sagittal plane and the meridian plane, eliminating the influence of the cylindrical symmetry of the resonant cavity on the laser mode, so that the intrinsic mode of the laser is the Hermite-Gaussian mode, and no elliptical Ins-Gaussian mode is generated. When the pump light deviates from the optical axis of the resonant cavity, a certain order of Hermite-Gaussian mode with one of the maximum values ​​of the light intensity at both ends of the light beam located near the pump light spot has the highest degree of overlap with the pump light. The gain of this mode is the highest and the threshold is the lowest, so the laser operates in this Hermite-Gaussian mode. By adjusting the offset of the pump light relative to the optical axis, Hermite-Gaussian modes of different orders can be selected. Because the astigmatism makes the resonant cavity have obvious rectangular symmetry rather than cylindrical symmetry, the mode of the laser is a high-quality Hermite-Gaussian mode rather than an elliptical Ins-Gaussian mode. The Hermite-Gaussian mode laser output by the laser obtains corresponding high-quality Laguerre-Gaussian vortex light after the astigmatism mode converter 8. Figure 2 The Ince-Gaussian mode IG obtained by the influence of the cylindrical symmetry of the resonant cavity when the laser adopts a general cylindrical symmetric resonant cavity is given. o (16,16,35) and the spiral-Ince-Gaussian mode HIG obtained after the astigmatism mode converter (0,16,35) Simulation and experimental results. Figure 3 The one-dimensional Hermite-Gaussian mode HG obtained by using the V-shaped folded resonant cavity structure in the embodiment of the present invention is given. 63,0 and the Laguerre-Gaussian vortex light LG obtained after the astigmatism mode converter 0,63 The simulation and experimental results show that the V-shaped folded resonant cavity structure can obtain a one-dimensional Hermite-Gaussian laser with good quality, which can be converted into high-quality Laguerre-Gaussian vortex light.

[0038] Example 2

[0039] In the above-mentioned embodiment 1, the laser gain medium 3 can be a neodymium-doped yttrium vanadate crystal, or a crystal or a glass or ceramic matrix of yttrium aluminum garnet, zinc selenide, lithium yttrium fluoride, etc. doped with rare earth ions such as ytterbium, thulium, erbium, titanium, iron, holmium, etc., or transition metal ions, as long as it can absorb pump light and provide laser gain, and the embodiment of the present invention does not impose any limitation on this.

[0040] The pump source 1 may be a multi-mode semiconductor laser, or a single transverse mode semiconductor laser or other types of lasers. The pump wavelength only needs to correspond to the pump absorption band of the laser gain medium 3, and the embodiment of the present invention does not impose any limitation on this.

[0041] The curvature radii of the laser total reflection mirror 4, the concave folding mirror 5, the laser output mirror 6, and the focusing lens 7, as well as the distances therebetween can select the parameters in Example 1, or other parameters, as long as the resonant cavity is within the stable region, the concave folding mirror 5 can introduce astigmatism to achieve the destruction of the cylindrical symmetry of the resonant cavity, and the focusing lens 7 can focus the Hermite-Gaussian laser beam waist at the center of the astigmatism mode converter 8.

[0042] The astigmatism mode converter 8 may be composed of two identical cylindrical lenses, or may be composed of a cylindrical lens and a focusing lens, as long as the light beam satisfies the mode conversion condition after passing through the astigmatism mode converter.

[0043] The resonant cavity can be a three-mirror V-shaped cavity, a four-mirror Z-shaped cavity, or a folded cavity with more mirrors and more arms, as long as at least one of the folded mirrors is a concave reflector that can introduce astigmatism.

[0044] Except for special instructions on the models of various components in the embodiments of the present invention, there are no specific restrictions on the models and specifications of other components, including: type and parameters of gain, material, radius of curvature, transmittance, etc. of the lens, as long as the device can complete the above functions.

[0045] Unless otherwise specified, the models of the components in the embodiments of the present invention are not limited, and any device that can perform the above functions may be used.

[0046] Those skilled in the art will appreciate that the accompanying drawing is only a schematic diagram of a preferred embodiment, and the serial numbers of the embodiments of the present invention are only for description and do not represent the advantages or disadvantages of the embodiments.

[0047] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for generating high-quality Laguerre-Gaussian vortex light, characterized in that: The laser for implementing the method comprises: a pump source, a pump focusing mirror, a laser gain medium, a laser total reflection mirror, a concave folding mirror, a laser output mirror, a focusing lens, and an astigmatism mode converter. The pump source emits pump light in the absorption band of the laser gain medium, and the pump light is focused by the pump focusing mirror into the laser gain medium; the laser gain medium absorbs the pump light, forms a population inversion, and generates laser gain; the laser total reflection mirror and the concave folding mirror have high reflectivity to the laser wavelength, and the laser output mirror has partial transmittance to the laser wavelength, and the three together constitute a folded V-shaped laser resonant cavity to provide feedback for the laser; The V-shaped laser resonant cavity based on the concave folding mirror destroys the cylindrical symmetry of the resonant cavity. The different converging abilities of the concave folding mirror on the meridian plane and the sagittal plane are used to introduce astigmatism, eliminate the influence of the cylindrical symmetry of the laser resonant cavity on the eigenmode of the cavity, and make its eigenmode the Hermite-Gaussian mode. Since the cylindrical symmetry of the laser resonant cavity is eliminated, the laser resonant cavity generates high-order Hermite-Gaussian mode laser instead of elliptical Innes-Gaussian mode laser when off-axis pumping is adopted; the focusing lens focuses the generated high-order Hermite-Gaussian mode laser so that its beam waist is located at the center of the astigmatic mode converter; the astigmatic mode converter provides conditions for mode conversion by changing the Gouy phase of the laser mode, thereby realizing high-quality Laguerre-Gaussian vortex light output.

2. The method for generating high-quality Laguerre-Gaussian vortex light according to claim 1, characterized in that: When the laser gain exceeds the loss of the V-shaped laser resonant cavity, the laser reaches the threshold and produces laser output; when the incident position of the pump light deviates from the optical axis of the laser resonant cavity, the laser operates in this high-order Hermite-Gaussian transverse mode.

3. The method for generating high-quality Laguerre-Gaussian vortex light according to claim 1, characterized in that: The laser gain medium for implementing the method is a bulk crystal, glass or ceramic doped with rare earth ions or transition metal ions, or a solidified dye or encapsulated gas or liquid, or a nonlinear gain medium of an optical parametric oscillator or Raman laser.

4. The method for generating high-quality Laguerre-Gaussian vortex light according to claim 1, characterized in that: The pump source for implementing the method is a semiconductor laser, a fiber laser, or a solid laser, and the laser mode is a fundamental transverse mode, a super-Gaussian or flat-top distribution of multiple transverse modes.

5. The method for generating high-quality Laguerre-Gaussian vortex light according to claim 1, characterized in that: The emission wavelength of the laser implementing the method is within the absorption band of the laser gain medium or the light transmission band and phase matching range of the nonlinear gain medium, so that the laser gain medium generates gain.

6. The method for generating high-quality Laguerre-Gaussian vortex light according to claim 1, characterized in that: The laser total reflection mirror and laser output mirror for implementing the method are plane mirrors or concave mirrors.

7. The method for generating high-quality Laguerre-Gaussian vortex light according to claim 1, characterized in that: The focusing lens for implementing the method has a focal length that satisfies the requirement of focusing the Hermite-Gaussian laser beam waist at the center of the astigmatism mode converter.

8. The method for generating high-quality Laguerre-Gaussian vortex light according to claim 1, characterized in that: The astigmatism mode converter is composed of a group of identical cylindrical lenses, or a cylindrical lens and a focusing lens.

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