Compact arbitrary transverse mode laser

Abstract

The invention discloses a compact arbitrary transverse mode laser. The laser comprises a pumping source, a pumping focusing optical system, a laser total reflective mirror, a laser gain medium, a laser output mirror and an astigmatism mode converter, the pumping source emits pumping light in a gain medium absorption band; the pump light is focused and then enters the gain medium; the gain medium absorbs the pump light and generates laser gain; the total reflective mirror highly reflecting laser wavelength and the output mirror partially transmitting the laser wavelength jointly form a simple linear resonant cavity. When the laser gain medium is inclined by a preset angle relative to the optical axis of the resonant cavity, meridian light and sagittal light pass through different optical paths of the gain medium to introduce astigmatism, an HG mode facing the directions of two astigmatism symmetry axes can be reproduced, and modes facing other directions cannot be reproduced due to the astigmatism effect. Therefore, when the pump light deviates from the axis of the resonant cavity in the directions of the two astigmatic symmetry axes, any high-order HG mode with mode ordinal numbers not being zero in the two directions can be excited, and any LG mode laser can be generated through astigmatic mode conversion.

Description

Technical Field

[0001] This invention relates to the field of lasers, and more particularly to a compact arbitrary transverse mode laser. Background Technology

[0002] Higher-order Hermitian-Gaussian (HG) and Laguerre-Gaussian (LG) transverse mode lasers hold immense potential for applications in cutting-edge fields such as high-capacity communications, quantum optics, gravitational wave detection, super-resolution fluorescence microscopy, and particle manipulation. In particular, the discovery of the orbital angular momentum (OAM) characteristic of LG beams has greatly advanced research into the generation and application of higher-order transverse mode lasers. [1] How to controllably generate HG m,n model( m and n (These are the mode ordinal numbers in the two orthogonal directions of the HG mode and LG, respectively) p,l model( p and l The mode ordinal numbers (radial and angular, respectively) of the LG mode become particularly important. m,n Model and LG p,l Modes can be converted to each other through astigmatism transformation, and the transformation process follows the principle of astigmatism transformation. p =min( m , n ), l =| m - n The correspondence between | means that as long as any HG can be generated. m,n Mode or any LG p,l This pattern allows for the further realization of lasers in arbitrary higher-order transverse modes through astigmatism transformation. [2] .

[0003] As intrinsic modes of optical resonators, both HG and LG modes can be directly excited from the laser using appropriate mode selection methods. Common methods include providing selective gain to the target mode through pump shaping, selectively inducing loss on modes other than the target mode by inserting amplitude templates into the resonator, and using aberration-inducing devices in conjunction with resonator design to distinguish the optical paths of different modes. Related literature reports the realization of HG modes exceeding 200 orders. 0,n LG with more than 300 levels 0,l model [3] However, achieving simultaneous control of the ordinal numbers of both modes in HG or LG has been a long-standing challenge, making it difficult to generate arbitrary transverse mode control.

[0004] Off-axis pumping is one of the most important methods for generating higher-order HG modes: the overlap between the pump light and the modes of each resonator determines the threshold of each mode; when a smaller pump spot deviates from the optical axis of the laser resonator, the higher-order HG mode will obtain higher gain and win in mode competition. Therefore, by adjusting the position of the pump light, the desired higher-order HG transverse mode can be selected. [4] Off-axis pumping eliminates the need for pump light shaping or the introduction of additional modulation devices within the cavity, thus offering advantages such as simple structure, low loss, and high efficiency. However, existing reports indicate that most off-axis pumping methods cannot simultaneously tune the mode ordinal numbers of both HG modes; typically, only a one-dimensional HG with a mode ordinal number of 0 can be generated. m,0 or HG 0,n Even if the pump light is deviated from the axis of the resonant cavity in two orthogonal directions, due to the cylindrical symmetry (rotational symmetry) of the resonant cavity, the HG mode along any angle direction is an eigenmode of the resonant cavity. Accordingly, only a more concentrated one-dimensional HG with higher overlap with the pump light can be generated. m,0 model [5] However, it cannot be achieved. m and n HG that are all non-zero m,n Since it is a mode laser, it is impossible to further generate arbitrary HG and LG mode lasers.

[0005] References [1] Liu Qiang, Pan Jing, Wan Zhensong, et al. Methods for generating optical fields of complex vortex structures [J]. Chinese Journal of Lasers, 2020, 47(5): 0500006. [2]Beijersbergen MW, Allen L, Van der Veen H, et al. Astigmaticlaser mode converters and transfer of orbital angular momentum[J]. OpticsCommunications, 1993, 96(1-3): 123-132. [3]Sheng Q, Wang A, Geng J, Fu S, Ma Y, Shi W, Yao J, Omatsu T, Spence D, Ultra-high-order Laguerre–Gaussian field generated directly from alaser cavity with spherical aberration[J], Laser&Photonics Reviews, 2023, 17(6), 2300369. [4]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. [5]Zhang Y, Zhang S, Qi Y, Lü J, Liu Z, Li J, Lin X, and Lu Z, On-demand spatial modes and vortex beams from a visible Pr:YLF orbital Poincarélaser with a hybrid pumping scheme[J], Opt. Express, 2025, 33(2), 1952-1961. Summary of the Invention

[0006] This invention provides a compact arbitrary transverse mode laser. The laser crystal is deflected relative to the resonant cavity optical axis by a small angle, creating a difference in optical path length between the meridional and sagittal planes, thus disrupting the cylindrical symmetry of the resonant cavity. Under these conditions, the eigenmodes of the resonant cavity are limited to HG modes along the rectangular symmetry axis of the resonant cavity. The tilted one-dimensional HG mode is no longer an eigenmode of the resonant cavity. The deflection of the pump light along two orthogonal directions respectively affects the ordinal numbers of the two modes. m and n The regulatory function means that arbitrary HG can be generated by adjusting the pump position. m,n Mode, higher-order HG output outside the cavity m,n By performing astigmatism transformation on the pattern beam, arbitrary LG can be obtained. p,l Pattern beams, which generate arbitrary pattern lasers as needed, are described below: A compact arbitrary transverse mode laser, the laser comprising: a pump source, a pump focusing optical system, a laser total reflection mirror, a laser gain medium, a laser output mirror, and an astigmatic mode converter.

[0007] The pump source emits pump light within the absorption band of the laser gain medium. The pump light is focused by the pump focusing lens optical system and enters the laser gain medium. The laser gain medium absorbs the pump light, forming population inversion and generating laser gain. The laser total reflection mirror has high reflectivity to the laser wavelength, and the laser output mirror has partial transmittance to the laser wavelength. Together, they form a linear laser resonant cavity to provide feedback for the laser. When the laser gain medium is tilted at a certain angle to the optical axis of the resonant cavity, the meridional and sagittal rays experience different optical paths when passing through the laser crystal, which introduces astigmatism. The cylindrical symmetry of the resonant cavity is destroyed. At this time, only the HG mode with the direction along the two astigmatic symmetry axes can be self-reproduced; while the modes with other directions cannot be self-reproduced after going back and forth in the cavity under the action of astigmatism.

[0008] When the laser gain exceeds the laser resonator loss, the laser reaches a threshold and produces laser output. When the pump light incident position deviates from the optical axis of the laser resonator, the laser produces a higher-order HG mode laser. Laser output can be generated when the pump light is offset relative to the resonator optical axis in both the crystal tilt direction and the direction orthogonal to it. m and n Two-dimensional HG with all non-zero values m,n The mode, by controlling the offset of the pump light in two directions, can controllably generate any HG within the allowable aperture range of the optical element. m,n model.

[0009] Furthermore, the astigmatic mode converter is used to directly output HG from the laser resonant cavity. m,n By transforming the pattern, any... The pattern, the relationship between the two follows p =min( m , n ), .

[0010] Among them, the laser gain medium of the laser 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.

[0011] Furthermore, the pump source of the laser is a semiconductor laser, a fiber laser, or a solid-state laser, and the laser mode is a fundamental transverse mode, a super-Gaussian or flat-top distribution of multiple transverse modes.

[0012] The laser's pump source emission wavelength is located within the absorption band of the laser gain medium or the transmission band and phase matching range of the nonlinear gain medium, thereby generating gain in the laser gain medium.

[0013] Among them, the laser total reflection mirror and the laser output mirror of the laser are either plane mirrors or concave mirrors, but they cannot both be plane mirrors at the same time. One of them must be a concave mirror so as to define the optical axis of the resonant cavity, that is, the position of the fundamental mode spot.

[0014] Preferably, the laser gain medium should be a device with a relatively large aperture to avoid limiting higher-order modes; correspondingly, the pump light should have a small spot size at the laser gain medium, for example, significantly smaller than the size of the fundamental mode spot of the resonant cavity at that location, in order to improve the distinction between adjacent modes and improve the purity of the output higher-order modes. Preferably, the laser output mirror is selected to have a relatively low output transmittance to the laser (e.g., below 5%), in order to reduce the impact of gain non-uniformity on mode quality.

[0015] The beneficial effects of the technical solution provided by this invention are: 1. This invention introduces astigmatism to disrupt the cylindrical symmetry of the laser resonator, thus limiting the eigenmodes of the resonator to HG modes along the astigmatism symmetry axis, suppressing competition from tilted HG modes, and enabling arbitrary HG modes to be obtained through simple two-dimensional off-axis pumping. m,n The pattern is then used to obtain any LG through astigmatic transformation. p,l The mode allows for the acquisition of laser output in any intrinsic mode. 2. This invention introduces astigmatism by tilting the laser gain medium, eliminating the need for any additional astigmatic optical elements. The structure is very simple and compact, with low loss and high efficiency. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the optical path of a compact arbitrary transverse mode laser. Figure 2 To obtain typical HG in the experiment m,n The pattern and the LG obtained after astigmatic transformation p,l Schematic diagram of the light spot pattern; Figure 3 This is a schematic diagram of another embodiment of a compact arbitrary transverse mode laser.

[0017] The attached diagram lists the components represented by each number as follows: 1: Pump source; 2: Pump focusing optical system; 3: Laser total reflection mirror; 4: Laser gain medium; 5: Laser output mirror; 6: Astigmatism mode converter. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below.

[0019] Example 1 This invention provides a compact arbitrary transverse mode laser, such as... Figure 1 As shown, the laser includes: a pump source 1, a pump focusing optical system 2, a laser total reflection mirror 3, a laser gain medium 4, a laser output mirror 5, and an astigmatic mode converter 6. The system comprises: a pump source 1, a fiber-coupled semiconductor laser with a wavelength of 808 nm and a fiber core diameter of 62.5 μm; a pump focusing optical system 2, consisting of two plano-convex lenses, each with a focal length of 40 mm; a laser total reflection mirror 3, a plane mirror with one side facing the cavity coated with a 1064 nm laser wavelength high-reflection film and an 808 nm pump wavelength antireflection film, and the other side coated with the same film; and a laser gain medium 4, a neodymium-doped yttrium vanadate (Nd:YVO4) crystal cut along the a-axis, with an emission peak at 1064 nm and Nd... 3+ The doping concentration is 1 at.%, and the dimensions are 5mm × 5mm × 3mm; the laser output mirror 5 is a concave mirror with a radius of curvature of 100mm, the concave surface is coated with a film system with a transmittance of 2% for a 1064nm laser wavelength, and the flat surface is coated with an antireflection film for a 1064nm laser wavelength; the laser resonant cavity composed of the laser total reflection mirror 3 and the laser output mirror 5 has a length of 95mm; the horizontal yaw direction of the laser gain medium 4 Nd:YVO4 crystal is tilted at a 5° angle to the optical axis of the laser resonant cavity; the astigmatic mode converter 6 is composed of a spherical lens with a focal length of 100mm and two plano-convex cylindrical lenses with a focal length of 50mm, such as... Figure 1 As shown.

[0020] The 808 nm pump light emitted from pump source 1 is focused by pump focusing optical system 2 and incident on the laser gain medium 4 Nd:YVO4 crystal with a spot radius of 66 μm. The Nd in the Nd:YVO4 crystal... 3+Ions absorb the 808 nm pump light and are excited to the upper energy level of the laser, generating laser gain. When the pump light spot is located at the center of the crystal and coincides with the optical axis of the laser resonator formed by the laser total reflection mirror 3 and the laser output mirror 5, 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. Adjusting the transverse position of the pump light spot, i.e., off-axis pumping of the laser, 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 that higher-order Hermitian-Gaussian mode. The higher-order Hermitian-Gaussian laser output from the laser output mirror 5 is transformed into a Laguerre-Gaussian beam after passing through the astigmatic mode converter 6. Since the laser crystal is placed at a certain tilt angle relative to the optical axis of the resonator, considering the birefringence of the crystal, the optical path length experienced by the light rays in the meridional plane (horizontal plane) and the sagittal plane (vertical plane) is different when passing through the crystal, and their transmission matrices are as follows: Meridian noodles: (1) Sagittal plane: (2) in, n o and n e These represent the refractive indices of the o-ray and e-ray of the laser gain medium 4 (e.g., Nd:YVO4 crystal), respectively. d The length of the laser gain medium 4 (e.g., Nd:YVO4 crystal) is represented. This astigmatism gives the resonant cavity a distinct rectangular symmetry, rather than cylindrical symmetry; in this case, only HG modes symmetrical along the horizontal and vertical directions can self-reproduce within the resonant cavity, while HG modes with symmetry axes along other directions are affected by astigmatism when traveling back and forth within the resonant cavity and are no longer propagation-invariant eigenmodes. Therefore, by adjusting the position of the pump light in the horizontal and vertical directions, the mode numbers of the two HG mode laser outputs can be controlled separately. m and n Generates HG in any mode m,n The pattern laser is further transformed by an astigmatic mode converter 6 to obtain arbitrary LG. p,l Pattern laser.

[0021] Figure 2 Give a typical two-dimensional HG image recorded by a CCD camera in the experiment. m,n The pattern and the LG obtained after astigmatic transformation p,l The mode laser spot. The offset Δ of the pump light relative to the resonant cavity optical axis (i.e., the position of the pump light that causes the laser to operate in fundamental mode) in the horizontal and vertical directions is adjusted independently. x and Δ y When Δ x = 0.306 mm, Δy When = 0, we get HG 10,0 The pattern, after transformation, yields LG. 0,10 Pattern; when Δ x = 0.306 mm, Δ y When =0.216 mm, HG is obtained. 10,5 The pattern, after transformation, yields LG. 5,5 Pattern; when Δ x =0.306 mm, Δ y When =0.306 mm, HG is obtained. 10,10 The pattern, after transformation, yields LG. 10,0 A pattern, that is, an HG capable of implementing any pattern ordinal number. m,n and LG p,l Transverse-mode laser. In contrast, when the laser crystal is not tilted, in the aforementioned Δ... x =0.306 mm, Δ y =0.216 mm and Δ x =0.306 mm, Δ y The results obtained at a thickness of 0.306 mm are one-dimensional HG values ​​tilted at different angles. m,0 The mode cannot obtain lasers of any mode.

[0022] Example 2 This invention provides a compact arbitrary transverse mode laser, such as... Figure 3 As shown, the laser includes: a pump source 1, a pump focusing optical system 2, a laser total reflection mirror 3, a laser gain medium 4, a laser output mirror 5, and an astigmatic mode converter 6. The difference from Embodiment 1 is that the laser gain medium 4 in this embodiment of the invention is cut into a trapezoidal shape instead of a conventional cuboid shape, meaning that the two end faces in the light transmission direction are not parallel. In this case, even if one end face of the crystal is perpendicular to the pump light path, the other tilted end face will introduce astigmatism, disrupting the symmetry of the resonant cavity and causing the laser to... x and Δ y When all are non-zero, use two-dimensional HG m,n The pattern operates, and further, arbitrary LG is obtained through astigmatic transformation. p,l Modular laser. Compared with Example 1, the present invention uses a trapezoidal-cut crystal, making laser optical path collimation operation simpler.

[0023] In the above embodiments, the laser gain medium 4 can be a neodymium-doped yttrium vanadate crystal, or a crystal or glass / ceramic matrix doped with rare earth ions such as ytterbium, thulium, erbium, titanium, iron, holmium, or transition metal ions such as yttrium aluminum garnet, potassium gadolinium tungstate, lithium yttrium fluoride, and zinc selenide. As long as it can absorb pump light and provide laser gain, it is acceptable. The embodiments of the present invention do not limit this.

[0024] Pump source 1 can be a multimode semiconductor laser, 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 4. This embodiment of the invention does not impose any restrictions on this.

[0025] The doping concentration and size of the laser gain medium 4, the radius of curvature of the laser total reflection mirror 3 and the laser output mirror 5, and the distance between them can be selected from the parameters in the above embodiments, or other parameters, as long as the resonant cavity is within the stable region and the tilted laser gain medium 4 can introduce astigmatism to destroy the cylindrical symmetry of the resonant cavity.

[0026] Unless otherwise specified, the model numbers of the various devices in this embodiment of the invention are not specifically limited, including the type and parameters of gain, the material, radius of curvature, and transmittance of the lens, etc. Any device that can perform the above functions is acceptable.

[0027] Those skilled in the art will understand that the accompanying drawings are merely schematic diagrams of a preferred embodiment, and the sequence numbers of the above embodiments of the present invention are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0028] 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 within the protection scope of the present invention.

Claims

1. A compact arbitrary transverse mode laser, characterized in that, The laser includes: a pump source, a pump focusing optical system, a laser total reflection mirror, a laser gain medium, a laser output mirror, and an astigmatic mode converter; The pump source emits pump light within the absorption band of the laser gain medium. After being focused by the pump focusing optical system, the pump light enters the laser gain medium. The laser gain medium absorbs the pump light, forming population inversion and generating laser gain. The laser total reflection mirror has high reflectivity for the laser wavelength, and the laser output mirror has partial transmittance for the laser wavelength. Together, they form a linear laser resonant cavity. The high-order Hermitian-Gaussian laser output from the laser output mirror is transformed into a Laguerre-Gaussian beam after being converted by an astigmatic mode converter. When the laser gain medium is tilted at a preset angle to the optical axis of the laser resonant cavity, the different optical paths experienced by the meridional and sagittal rays when passing through the laser crystal introduce astigmatism. Only the HG mode along the two astigmatism symmetry axes can be self-reproduced, while the modes along other directions cannot be self-reproduced after going back and forth in the cavity under the action of astigmatism.

2. A compact arbitrary transverse mode laser according to claim 1, characterized in that, When the pump light incident position deviates from the optical axis of the laser resonator, the laser generates a higher-order HG mode laser. When the pump light is offset relative to the optical axis of the resonant cavity in both the crystal tilt direction and the direction orthogonal to it, a two-dimensional HG with non-zero m and n is generated. m,n The mode, by controlling the offset of the pump light in two directions, produces arbitrary HG within the allowable aperture range of the optical element. m,n model.

3. A compact arbitrary transverse mode laser according to claim 1, characterized in that, The astigmatic mode converter is used to directly output HG from the laser resonant cavity. m,n The pattern is transformed to obtain any The pattern, the relationship between the two follows p=min(m,n). .

4. A compact arbitrary transverse mode laser according to claim 1, characterized in that, Laser gain media are bulk crystals, glasses, or ceramics doped with rare earth ions or transition metal ions, or solidified dyes or encapsulated gas or liquid gain media, or gain media that provide nonlinear gain through nonlinear processes such as optical parametric oscillation or stimulated Raman scattering.

5. A compact arbitrary transverse mode laser according to claim 1, characterized in that, The pump source is a semiconductor laser, fiber laser, or solid-state laser, and the laser mode is a fundamental transverse mode, a super-Gaussian or flat-top distribution of multiple transverse modes.

6. A compact arbitrary transverse mode laser according to claim 1, characterized in that, Laser total reflection mirrors and laser output mirrors are either plane mirrors or concave mirrors; they cannot both be plane mirrors at the same time.

7. A compact arbitrary transverse mode laser according to claim 1, characterized in that, The laser output mirror is selected to have a relatively low output transmittance for the laser in order to reduce the impact of gain non-uniformity on mode quality.

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