A method for generating high-quality Laguerre-Gaussian vortex light

Abstract

The present invention discloses a method for generating high-quality Laguerre-Gaussian vortex light. The method comprises: a V-shaped laser resonant cavity based on a concave folding mirror, utilizing the different converging abilities of the concave folding mirror on a light beam in a meridional plane and a sagittal plane to introduce astigmatism, thereby destroying the cylindrical symmetry of the resonant cavity and eliminating the influence of the cylindrical symmetry of the laser resonant cavity on the eigenmode of the cavity, so that the eigenmode is a Hermite-Gaussian mode, and the generation of an elliptical Innes-Gaussian mode is suppressed; an off-axis pumping method is adopted so that the laser resonant cavity generates high-order Hermite-Gaussian mode laser light; a focusing lens focuses the generated high-order Hermite-Gaussian mode laser light so that its beam waist is located at the center of an astigmatic mode converter; and the astigmatic mode converter provides conditions for mode conversion by changing the Gouy phase of the laser mode, thereby achieving high-quality Laguerre-Gaussian vortex light output.

Description

Technical Field

[0001] The present 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: extra-cavity control of the light field and direct excitation in the cavity. Extra-cavity 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 such methods have high requirements for the modulation element or the shaping of the pump light, 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 laser 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, while the maximum intensity of the higher-order Hermite-Gaussian laser is located at the maxima at both ends of its spot. The optical path of the pump light is adjusted to deviate from the optical axis of the resonant cavity. When the overlap of the pump light with a specific mode is better than that with the fundamental mode, a higher-order Hermite-Gaussian mode with an 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 power of up to HG 0,87 Hermite-Gaussian mode output [3] By placing an astigmatic mode converter after 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 continuously changing from 1 to 30. [4] ; High-order Laguerre-Gaussian vortex light has also been realized, 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 converted into a Laguerre-Gaussian beam through an astigmatic 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, if the generated laser mode is further passed through an astigmatic mode converter, it is easier to obtain a helical Innes-Gaussian mode (HIG) laser rather than a high-quality Laguerre-Gaussian mode laser.

[0004] References

[0005] [1] Liu Qiang, Pan Jing, Wan Zhensong, et al. Method for generating 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. The concave folding mirror has different light beam converging abilities on the meridian and sagittal planes, thereby introducing astigmatism into the cavity. This destroys the cylindrical symmetry of the laser resonant cavity and eliminates the influence of cylindrical symmetry on the laser mode. As a result, the Ing-Gaussian mode is no longer an eigenmode of the folded laser resonant cavity. When the pump light deviates from the optical axis, a high-quality high-order Hermite-Gaussian mode is generated. The Hermite-Gaussian laser can be converted into Laguerre-Gaussian vortex light by using an astigmatism mode converter, as described below.

[0012] A method for generating high-quality Laguerre-Gaussian vortex light. 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.

[0013] The pump source emits pump light within 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, causing population inversion and generating laser gain; the laser total reflection mirror and concave folding mirror have high reflectivity for the laser wavelength, and the laser output mirror has partial transmittance for the laser wavelength; the three together constitute a folded V-shaped laser resonant cavity, providing feedback for the laser;

[0014] Because the concave folding mirror has different converging abilities on the beam in the meridian and sagittal planes, astigmatism is introduced. Therefore, the V-shaped laser resonator does not have cylindrical symmetry, and its eigenmode is the Hermite-Gaussian mode, so it does not generate the elliptical Innes-Gaussian mode.

[0015] When the laser gain exceeds the laser resonant cavity loss, the laser reaches the threshold and generates laser output; when the incident position of the pump light deviates from the optical axis of the laser resonant cavity, the laser generates 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 astigmatic 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 used to implement the method is a semiconductor laser, fiber laser, or solid-state laser. The laser mode is a fundamental transverse mode, a super-Gaussian distribution of multiple transverse modes, or a flat-top distribution. The emission wavelength of the pump source used to implement 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 laser output mirror used to implement the method are plane mirrors or concave mirrors. The focusing lens used to implement the method has a focal length sufficient to focus the Hermite-Gaussian laser beam waist at the center of the astigmatic mode converter. The astigmatic mode converter is composed of a set 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 astigmatism introduced by the concave folding mirror to eliminate the cylindrical symmetry of the traditional laser resonant cavity, making it less likely for the laser to generate elliptical Innes-Gaussian modes. As a result, high-quality Hermite-Gaussian mode output can be obtained under off-axis pumping.

[0021] 2. By introducing a V-shaped folded resonant cavity structure and astigmatism, the present invention can generate high-quality, high-order Hermite-Gaussian modes. Furthermore, using an astigmatic mode converter, high-quality Laguerre-Gaussian vortex light can be obtained. The angular and radial indices of the Laguerre-Gaussian vortex light can be easily determined using the Hermite-Gaussian mode order.

[0022] 3. The present invention, through the introduction of a V-shaped folded resonant cavity structure and astigmatism, can obtain high-quality high-order Hermite-Gaussian modes, and further utilize an astigmatic mode converter to obtain high-quality Laguerre-Gaussian vortex light. Compared with methods such as defect points and amplitude templates for selecting high-order Laguerre-Gaussian mode lasers, this method has low loss and high pump utilization. The astigmatism conversion process is a unitary transformation, which is highly efficient and simple.

[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 astigmatic mode converter to obtain high-quality Laguerre-Gaussian vortex light; 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 obtained by the conventional Laguerre-Gaussian mode selection method without net orbital angular momentum p,±l beam. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 A schematic diagram of the optical path for generating high-quality Laguerre-Gaussian vortex lasers;

[0025] Figure 2 A schematic diagram comparing 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 diagram showing 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 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 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 fiber-coupled semiconductor laser 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. 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, the side facing the cavity is coated with a 1064nm laser wavelength high reflection film, and both 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 partially transparent film (transmittance T=5%); the laser total reflection mirror 4, the concave folding mirror 5 ... 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. Figure 1 shown.

[0036] The 878.6nm pump light emitted by the pump source 1 is focused by the pump focusing mirror 2 and then enters the laser gain medium 3Nd:YVO4 crystal with a spot radius of 100μm. 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 with other higher-order modes, and 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 degree of overlap between the pump light and a certain higher-order transverse mode is higher than that with other modes, the laser tends to operate in that 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 focused laser beam waist is located at the center of the astigmatic mode converter 8.

[0037] The laser in this embodiment of the present invention utilizes a V-shaped folded resonant cavity structure. The concave folding mirror 5 converges the light beam differently in the sagittal and meridional planes, eliminating the influence of the resonant cavity's cylindrical symmetry on the laser mode. This results in the laser's eigenmode being a Hermite-Gaussian mode, eliminating the generation of elliptical Ince-Gaussian modes. When the pump light deviates from the resonant cavity's optical axis, a Hermite-Gaussian mode of a certain order, with one of the beam's intensity maxima located near the pump spot, has the highest degree of overlap with the pump light. This mode has the highest gain and the lowest threshold, resulting in the laser operating 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 astigmatism imparts a distinct rectangular symmetry to the resonant cavity, rather than cylindrical symmetry, the laser's mode is a high-quality Hermite-Gaussian mode, rather than an elliptical Ince-Gaussian mode. The Hermite-Gaussian mode output by the laser is converted to correspondingly high-quality Laguerre-Gaussian vortex light after passing through an astigmatic 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 high-quality one-dimensional Hermite-Gaussian laser, which is then converted into high-quality Laguerre-Gaussian vortex light.

[0038] Example 2

[0039] In the above embodiment 1, the laser gain medium 3 can be a neodymium-doped yttrium vanadate crystal, or a crystal or 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, or transition metal ions, as long as it can absorb pump light and provide laser gain. The embodiment of the present invention does not impose any restrictions on this.

[0040] The pump source 1 can be a multimode 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 radius 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 be selected from 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 can be composed of two identical cylindrical lenses, or can 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, curvature radius, transmittance, etc. of the lens, as long as the device can perform the above functions.

[0045] Unless otherwise specified, the embodiments of the present invention do not limit the models of the components. Any component that can perform the above functions may be used.

[0046] Those skilled in the art will understand that the accompanying drawings are 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 principles of the present invention should be included in the scope of protection 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 includes: 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 within 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, causing population inversion and generating laser gain; the laser total reflection mirror and concave folding mirror have high reflectivity for the laser wavelength, and the laser output mirror has partial transmittance for the laser wavelength; the three together constitute a folded V-shaped laser resonant cavity, providing feedback for the laser; The V-shaped laser resonator based on the concave folding mirror destroys the cylindrical symmetry of the resonator. The different focusing abilities of the concave folding mirror on the meridian and sagittal planes are used to introduce astigmatism, eliminating the influence of the cylindrical symmetry of the laser resonator on the eigenmode of the cavity, making the eigenmode a 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 rather than elliptical Innes-Gaussian mode laser when an off-axis pumping method 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 achieving high-quality Laguerre-Gaussian vortex light output.

2. The method for generating high-quality Laguerre-Gaussian vortex light according to claim 1, wherein: 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, wherein: 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, wherein: 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 distribution of multiple transverse modes, or a flat-top distribution.

5. The method for generating high-quality Laguerre-Gaussian vortex light according to claim 1, wherein: 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, wherein: 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 sufficient to focus the Hermite-Gaussian laser beam waist at the center of the astigmatic mode converter.

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

Images