Three-dimensional geometric mode structured light field laser

Abstract

The invention discloses a three-dimensional geometric mode structured light field laser, which is characterized in that a pumping source emits pumping light in an absorption band of a laser gain medium, and the pumping light enters the laser gain medium after being focused by a pumping focus lens; the laser gain medium absorbs the pump light, population inversion is formed, and laser gain is generated; the laser total reflection mirror and the concave surface folding mirror have high reflectivity for laser wavelength, the laser output mirror has partial transmittance for laser wavelength, the laser total reflection mirror, the concave surface folding mirror and the laser output mirror form a folded laser resonant cavity, the laser resonant cavity provides feedback for laser, and the laser output mirror outputs laser to the concave surface folding mirror. And the laser gain medium is arranged close to the laser total reflective mirror. The astigmatic folded cavity structure is adopted, the stable areas of the two arms of the folded cavity correspond to the whole eigen-frequency spectrum, the mode degeneracy state can be adjusted by adjusting the length of one arm which is not occupied by the laser crystal, the whole eigen-frequency spectrum, namely the whole degeneracy state, can be covered easily, and therefore light field output of any three-dimensional geometric mode is achieved.

Description

Technical Field

[0001] This invention relates to the field of lasers, and more particularly to a three-dimensional geometric pattern structured light field laser. Background Technology

[0002] When frequency degeneracy occurs between transverse and longitudinal modes of different mode ordinal numbers in a laser resonator, an off-axis pumped laser will produce laser oscillations in a wave-track geometric mode. In this case, the energy of the optical field is no longer distributed according to the resonator's eigenmodes such as Laguerre-Gaussian and Hermi-Gaussian, but is concentrated on a set of self-consistent ray trajectories, hence the name geometric mode. When there is a difference in the spacing between transverse modes in two orthogonal directions within the resonator, and both are frequency degenerate with the longitudinal modes, the geometric mode evolves from a two-dimensional distribution into a three-dimensional Lissajous trajectory optical field that varies periodically along the propagation direction. Combined with astigmatic transformation, a cocycloid optical field can be further obtained. Geometric modes embody the coherent superposition state of eigenmodes and are considered a bridge between quantum and classical theories, providing a visual projection of many concepts in quantum mechanics and fractal mathematics. [1] It has become an important tool in the field of laser physics research.

[0003] Three-dimensional geometric mode optical fields are generally generated by off-axis pumped solid-state lasers. In a 2006 report by Chen et al., a Lissajous field was realized using an end-pumped Nd:YVO4 laser. [2,3 The birefringence of the Nd:YVO4 laser crystal introduces astigmatism in two orthogonal directions, causing a difference in transverse mode spacing in these two orthogonal directions, resulting in different TEM frequencies for different mode numbers. m,n,q Frequency degeneracy can occur between modes (m, n are the transverse mode numbers in two orthogonal directions, and q is the longitudinal mode number). Subsequent researchers have largely followed this method, constructing a short linear resonant cavity with a plano-concave or biconcave structure using a cavity mirror with a small radius of curvature. Astigmatism is generated by the birefringence effect of the laser crystal to distinguish the transverse mode spacing in the two orthogonal directions. When the cavity length is adjusted throughout the stable region, its normalized intrinsic frequency spectrum can cover all degenerate states, achieving different three-dimensional geometric mode laser outputs. Based on this structure, researchers have successively reported three-dimensional geometric mode lasers with watt-level output power and Q-switched pulse operation. [4,5]However, this commonly used method has several drawbacks: First, the degeneracy state of the resonant cavity is adjusted by changing the cavity length. As the cavity length changes throughout its stable region, the degeneracy state changes accordingly across the complete normalized degeneracy spectrum. However, since the astigmatism introduced by the birefringence of the laser crystal is very limited, the difference in cavity parameters and longitudinal mode spacing in the two orthogonal directions can only be observed when the cavity length is very short. At this point, the small length adjustment range corresponds to all degeneracy states, making the cavity length differences corresponding to different degeneracy states very small, and it is difficult to achieve effective differentiation in experiments. Second, because the cavity length is very short and the laser crystal occupies a certain physical length, the cavity length adjustment process cannot actually cover the entire stable region, thus failing to achieve all degeneracy states and generate arbitrary three-dimensional Lissajous geometric modes. These problems limit the range of achievable structured light fields. Even with a longer birefringent crystal, the astigmatism is still insufficient to support a longer resonant cavity, and the crystal occupies more space, further limiting the adjustment range.

[0004] References

[0005] [1]Y.Shen,Rays,waves,SU(2)symmetry and geometry:toolkits for structured light,Journal of Optics,23,124004,2021.

[0006] [2] YFChen, et al., Devil's Staircase in Three-Dimensional CoherentWaves Localized on Lissajous Parametric Surfaces, Physical Review Letters, 96, 213902, 2006.

[0007] [3] Liu Qiang, et al. Methods for generating optical fields of complex vortex structures. Chinese Journal of Lasers, 47(5), 0500006(2020).

[0008] [4] PHTuan, et al., Generating high-power Lissajous structured modes and trochoidal vortex beams by an off-axis end-pumped Nd:YVO4 laser with astigmatic transformation, Optics Express, 29, 22957, 2021.

[0009] [5]PHTuan,et al.,Stable pulsed operation of Lissajous structuredbeams byNd:YVO4 / Cr 4+ :YAG laser in a concave–convex resonator, Optics Letters, 48, 2245, 2023. Summary of the Invention

[0010] This invention provides a three-dimensional geometric mode structured light field laser. The invention constructs a folded laser resonator using a concave mirror as the folding mirror. Astigmatism is introduced by the concave mirror, and the magnitude of the astigmatism can be controlled by the folding angle and the radius of curvature of the concave mirror. On the one hand, by enhancing astigmatism, the distinguishability of different three-dimensional geometric modes corresponding to different degeneracy states is improved. On the other hand, the stable regions of the two arms of this folded cavity correspond to the entire intrinsic frequency spectrum of the resonator, and the adjustment of the resonator parameters is not limited by other devices, easily covering all degeneracy states in the entire intrinsic frequency spectrum. This enables the realization of a richer three-dimensional Lissajous geometric mode structured light field, as detailed below:

[0011] A three-dimensional geometric mode structured light field laser, the laser comprising: a pump source, a pump focusing mirror, a laser gain medium, a laser total reflection mirror, a concave folding mirror, a laser output mirror, and a displacement stage.

[0012] 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 and enters the laser gain medium. The laser gain medium absorbs the pump light, forming population inversion and generating laser gain.

[0013] The laser total reflection mirror and the concave folded mirror have high reflectivity for the laser wavelength, and the laser output mirror has partial transmittance for the laser wavelength. The laser total reflection mirror, the concave folded mirror, and the laser output mirror constitute a folded laser resonant cavity, which provides feedback for the laser. The laser gain medium is placed close to the laser total reflection mirror.

[0014] In this process, the laser beam inside the resonant cavity is obliquely incident on the concave folding mirror at a non-zero angle. The effective focal length of the beam on the concave folding mirror is different on the meridional plane and the sagittal plane, which introduces astigmatism. This causes the laser to have different oscillation mode frequencies in the two orthogonal directions.

[0015] The laser output mirror is mounted on the displacement stage. By adjusting the displacement stage, the distance between the concave folding mirror and the laser output mirror is controlled, thereby changing the length of the laser resonant cavity and adjusting the longitudinal and transverse mode frequencies of the laser resonant cavity.

[0016] Specifically, when the laser gain exceeds the loss of the laser resonator, the laser reaches a threshold and generates laser output; when the incident position of the pump light deviates from the optical axis of the laser resonator, it provides gain for higher-order transverse modes.

[0017] When the laser is operating in a higher transverse mode, the frequency degeneracy state of the laser resonant cavity is controlled by adjusting the displacement stage.

[0018] When the laser operates at a degeneracy point on its intrinsic frequency spectrum, it will generate a three-dimensional Lissajous geometric mode structured light field. By adjusting the length of the laser resonator, the laser can operate at different degeneracy points, thereby realizing different three-dimensional geometric mode structured light fields.

[0019] Wherein, the stroke of the displacement stage is greater than the stable range of the distance between the concave folding mirror and the laser output mirror: that is, the adjustment range of the laser resonant cavity length covers the entire stable range, realizing the entire degenerate state on the entire intrinsic frequency spectrum.

[0020] The laser gain medium is a bulk crystal, glass, or ceramic doped with rare earth ions or transition metal ions, or a cured dye or encapsulated gas or liquid, or a nonlinear gain medium of an optical parametric oscillator or Raman laser.

[0021] The pump source 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.

[0022] The laser's 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.

[0023] The beneficial effects of the technical solution provided by this invention are:

[0024] 1. This invention employs an astigmatic folded cavity structure, utilizing a concave folded mirror to introduce significant astigmatism, thereby enhancing the distinguishability of the degeneracy state of the resonant cavity and making it easier to generate three-dimensional Lissajous geometric modes;

[0025] 2. The present invention adopts a folded cavity structure, which has a larger stable region of resonant cavity length and greater differences in cavity parameters corresponding to different degeneracy states, thus enabling the realization of richer three-dimensional geometric mode structured light fields;

[0026] 3. The present invention adopts a folded cavity structure. The stable regions of the two arms of the folded cavity each correspond to the entire intrinsic frequency spectrum. The mode degeneracy state can be adjusted by adjusting the length of the arm not occupied by the laser crystal, which makes it easy to achieve coverage of the entire intrinsic frequency spectrum, that is, all degeneracy states. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the optical path of a three-dimensional geometric mode structured light field laser.

[0028] Figure 2 This refers to the stable region of the second arm in an embodiment of a three-dimensional geometric pattern structured light field laser.

[0029] Figure 3 This is the intrinsic frequency spectrum of the resonant cavity when the length of the second arm changes in an embodiment of a three-dimensional geometric mode structured light field laser.

[0030] Figure 4 This is a typical three-dimensional Lissajous geometric pattern light spot obtained experimentally in an embodiment of a three-dimensional geometric pattern structured light field laser.

[0031] The attached diagram lists the components represented by each number as follows:

[0032] 1: Pump source; 2: Pump focusing lens;

[0033] 3: Laser total reflection mirror; 4: Laser gain medium;

[0034] 5: Concave folding mirror; 6: Laser output mirror;

[0035] 7: Displacement stage. Detailed Implementation

[0036] 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.

[0037] Example 1

[0038] This invention provides a three-dimensional geometric mode structured light field laser, which includes: a pump source 1, a pump focusing mirror 2, a laser total reflection mirror 3, a laser gain medium 4, a concave folding mirror 5, a laser output mirror 6, and a displacement stage 7.

[0039] The system comprises: pump source 1, an optical fiber coupled semiconductor laser with a wavelength of 808 nm and an optical fiber core diameter of 100 μm; pump focusing mirror 2, consisting of two plano-convex lenses, each with a focal length of 40 mm; laser total reflection mirror 3, a plane mirror with a 1064 nm laser wavelength high-reflection coating on the side facing the cavity and an 808 nm pump wavelength antireflection coating on both sides; and laser gain medium 4, a neodymium-doped yttrium vanadate (Nd:YVO4) crystal cut along the a-axis with an emission peak at 1064 nm. 3+The doping concentration is 0.5-at.%, and the dimensions are 5mm×5mm×3mm. The concave folding mirror 5 is a concave reflecting mirror with a concave curvature radius of 100mm, and the concave surface is coated with a high-reflectivity film for 1064nm laser wavelength. The laser output mirror 6 is a plane mirror coated with a partially transparent film for 1064nm laser wavelength (transmittance T=2%). The laser output mirror 6 is mounted on a displacement stage 7 with a travel of 100mm. The laser total reflection mirror 3, the concave folding mirror 5, and the laser output mirror 6 together form a "V"-shaped 1064nm laser folding resonant cavity. The length of the first arm (denoted as d1) between the laser total reflection mirror 3 and the concave folding mirror 5 is 78mm, and the length of the second arm (denoted as d2) between the laser output mirror 6 and the concave folding mirror 5 is adjustable within the range of 35-135mm. The folding angle half-angle is 8°. The laser gain medium, 4Nd:YVO4 crystal, is placed close to the laser total reflection mirror 3. Figure 1 As shown.

[0040] The 808nm pump light emitted from pump source 1 is focused by pump focusing mirror 2 and then incident on the laser gain medium 4Nd:YVO4 crystal with a spot radius of ~50μm. The Nd in the Nd:YVO4 crystal... 3+ Ions absorb the 808nm pump light and are excited to the upper energy level of the laser, generating laser gain. When the pump light path deviates from the optical axis of the resonant cavity, the higher-order modes overlap significantly with the pump light, and the laser operates in a higher-order transverse mode.

[0041] Calculations using the ABCD matrix (a well-known technical term in the art) show that when d1 = 78 mm and considering the astigmatism of the concave folding mirror 5, the stable range of the second arm length (d2) of the folded resonant cavity is ~52-130 mm (both the meridional and sagittal planes satisfy -1 < (A+D) / 2 < 1, where A and D are the first and fourth terms in the ABCD matrix, respectively). Within the coverage area of ​​the displacement stage 7, such as... Figure 2 As shown. Calculate the eigenfrequency spectrum of the resonant cavity when the folding angle is 8°, as shown. Figure 3 As shown. The degeneracy points on the intrinsic frequency spectrum change from rhombuses (where there is no astigmatism) to polygons, and new degeneracy fractals appear near the degeneracy centers. When the length of the second arm (d2) of the laser resonator varies throughout the entire stable region, all degeneracy states in the entire intrinsic frequency spectrum can be achieved. Therefore, by adjusting the length (d2) of the second arm of the laser resonator through the displacement stage 7, the laser can operate in different degeneracy states, thus enabling various three-dimensional geometric mode laser outputs. The typical three-dimensional geometric mode optical field output obtained in the experiment is shown below. Figure 4As shown in the figure, the numbers in the figure represent the corresponding lengths (d2) of the second arm of the resonant cavity. The lengths of the second arm can be: 111.315mm; 112.13mm; 112.67mm; 120.19mm; 121.1mm; 121.71mm. In practice, the lengths can be adjusted to other lengths by means of the displacement stage 7. This embodiment of the invention will not elaborate on this aspect.

[0042] In this embodiment of the invention, the stable region of the second arm length (d2) of the folded resonant cavity is ~52-130mm, that is, the stable region is ~78mm, which means that the different degenerate states in the intrinsic spectrum are dispersed within the range of ~78mm. In contrast, the stable region of the prior art based on short cavity and crystal birefringence is generally no more than 30mm. Therefore, the different degenerate states are more distinguishable in this embodiment of the invention, and it is easier to achieve more geometric mode structured light field output in actual operation.

[0043] In this embodiment of the invention, the degeneracy state of the laser resonator is changed by adjusting the length (d2) of the second arm. Since there are no other optical elements in the second arm of the laser resonator, it is allowed to be adjusted throughout the entire stable region, thus enabling arbitrary states in the intrinsic frequency spectrum. In contrast, in the prior art based on short cavity and crystal birefringence, the length of the laser resonator cannot be arbitrarily adjusted throughout the entire stable region because the laser crystal occupies a certain space, thus preventing the realization of arbitrary degeneracy states.

[0044] In this embodiment of the invention, astigmatism is introduced by an inclined concave folding mirror 5. The astigmatism is larger than that of the prior art based on short cavity and crystal birefringence, and its size can be adjusted by the folding angle. It can be seen from the calculation of the intrinsic frequency spectrum that when the folding angle is 2°, the fractal degree of the intrinsic frequency spectrum is similar to that of the prior art based on short cavity and crystal birefringence. Therefore, the distinction between different degeneracy states is higher, and it is easier to achieve more geometric mode structured light field output in actual operation.

[0045] Example 2

[0046] In this embodiment of the invention, the laser resonator still adopts the V-shaped folded cavity design of Embodiment 1. The concave folding mirror 5 is changed from a concave mirror with a curvature radius of 100mm in Embodiment 1 to a concave mirror with a curvature radius of 50mm, and the folding angle half-angle is 5°. Other components remain unchanged. Calculations using the ABCD matrix show that (the specific calculation process is well known to those skilled in the art, and will not be elaborated upon in this embodiment), when the length of d1 is 42mm, the stable region of d2 is 26-58mm. That is, by moving the position of the laser output mirror 6 using the displacement stage 7, and changing d2 within the range of 26-58mm, a fully degenerate state can be achieved, realizing the corresponding arbitrary three-dimensional geometric mode optical field output. Compared with Embodiment 1, the stable region of d2 in this embodiment of the invention is smaller, meaning that changing the resonator length within a relatively small adjustment range can obtain a fully degenerate geometric mode optical field output.

[0047] In practical work, the length of d1 and the radius of curvature of the concave folding mirror 5 can be changed as needed to adjust the stable region range of d2: for example, if it is desired that the distinction between different degenerate states is more obvious, a cavity parameter combination with a larger stable region range of d2 can be selected; for example, if more states need to be achieved within a smaller cavity length adjustment range, a cavity parameter combination with a smaller stable region range of d2 can be selected. See the numerical range of the stable region of d2 in Examples 1 and 2. This embodiment of the invention does not limit this.

[0048] In the above embodiments, the laser gain medium 4 can be a neodymium-doped yttrium vanadate crystal, or a crystal or glass or ceramic matrix doped with rare earth ions such as ytterbium, thulium, erbium, titanium, iron, holmium, or transition metal ions such as yttrium aluminum garnet, zinc selenide, lithium yttrium fluoride, etc., as long as it can absorb pump light and provide laser gain. The embodiments of the present invention do not limit this.

[0049] 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 3. This embodiment of the invention does not impose any restrictions on this.

[0050] The curvature radii of the laser total reflection mirror 3, the concave folding mirror 5, and the laser output mirror 6, as well as the distance and folding angle between them, can be selected from the parameters in Embodiment 1, or other parameters, as long as the laser resonator is within the stable region and the concave folding mirror 5 can introduce astigmatism to distinguish the mode frequencies in two orthogonal directions. This embodiment of the invention does not limit this.

[0051] The displacement stage 7 can be electrically or manually adjusted. As long as its stroke can cover the stable region of the laser resonator, it can realize the three-dimensional geometric mode structured light field output corresponding to any degenerate state. This embodiment of the invention does not limit this.

[0052] 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.

[0053] 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.

[0054] 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 three-dimensional geometric pattern structured light field laser, characterized in that, The laser 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, and a displacement stage. 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 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 and the concave folded mirror have high reflectivity for the laser wavelength, and the laser output mirror has partial transmittance for the laser wavelength. The laser total reflection mirror, the concave folded mirror, and the laser output mirror constitute a folded laser resonant cavity, which provides feedback for the laser. The laser gain medium is placed close to the laser total reflection mirror.

2. A three-dimensional geometric pattern structured light field laser according to claim 1, characterized in that, The laser beam inside the resonant cavity is obliquely incident on the concave folding mirror at a non-zero angle. The effective focal length of the beam on the concave folding mirror is different on the meridional plane and the sagittal plane, which introduces astigmatism. This causes the laser to have different oscillation mode frequencies in the two orthogonal directions.

3. A three-dimensional geometric pattern structured light field laser according to claim 1, characterized in that, The laser output mirror is mounted on the displacement stage. By adjusting the displacement stage, the distance between the concave folding mirror and the laser output mirror is controlled, thereby changing the length of the laser resonant cavity and adjusting the longitudinal and transverse mode frequencies of the laser resonant cavity.

4. A three-dimensional geometric pattern structured light field laser according to claim 3, characterized in that, When the laser gain exceeds the loss of the laser resonator, 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 resonator, it will provide gain for higher-order transverse modes.

5. A three-dimensional geometric pattern structured light field laser according to claim 3, characterized in that, When the laser is operating in a higher transverse mode, the frequency degeneracy state of the laser resonant cavity can be controlled by adjusting the displacement stage. When the laser operates at a degenerate point on its intrinsic frequency spectrum, it will generate a three-dimensional Lissajous geometric mode structured light field. By adjusting the length of the laser resonator, the laser can operate at different degeneracy points, thereby achieving different three-dimensional geometric mode structured light fields.

6. A three-dimensional geometric pattern structured light field laser according to claim 1, characterized in that, The travel of the displacement stage is greater than the stable range of the distance between the concave folding mirror and the laser output mirror: that is, the adjustment range of the laser resonator length covers the entire stable range, realizing the entire degenerate state on the entire intrinsic frequency spectrum, thereby generating arbitrary three-dimensional geometric mode optical field output.

7. A three-dimensional geometric pattern structured light field laser according to claim 1, characterized in that, The laser gain medium is a bulk crystal, glass, or ceramic doped with rare earth ions or transition metal ions, or a cured dye or encapsulated gas or liquid, or a nonlinear gain medium for optical parametric oscillators or Raman lasers.

8. A three-dimensional geometric pattern structured light field 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.

9. A three-dimensional geometric pattern structured light field laser according to claim 1, characterized in that, The emission wavelength of the laser 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.

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