High-order two-dimensional Hermitian-Gaussian mode laser

Abstract

The invention discloses a high-order two-dimensional Hermitian-Gaussian mode laser, which comprises a pumping source, a pumping focus lens, a laser gain medium, a laser total reflective mirror, a concave folding mirror and a laser output mirror, and is characterized in that the pumping source emits pumping light in an absorption band of the laser gain medium; pump light is focused by the pump focus lens and enters the laser gain medium. The laser gain medium absorbs the pump light, population inversion is formed, and laser gain is generated; the laser total reflective mirror and the concave folding mirror have high reflectivity for laser wavelength, the laser output mirror has partial transmittance for laser wavelength, and the laser total reflective mirror, the concave folding mirror and the laser output mirror jointly form a folded laser resonant cavity to provide feedback for laser.

Description

Technical Field

[0001] The present invention relates to the field of lasers, and in particular to a high-order two-dimensional Hermite-Gaussian mode laser. Background Art

[0002] High-order two-dimensional Hermite-Gaussian modes have unique rectangular lattice-shaped light intensity and phase distribution, and have important applications in processing, lighting and other fields; in particular, high-order Hermite-Gaussian mode lasers can be transformed into high-order Laguerre-Gaussian mode lasers with orbital angular momentum through devices such as cylindrical lenses, providing important tools for cutting-edge applications such as quantum optics, large-capacity optical communications, and fluorescence microscopy. After transformation, a one-dimensional Hermite-Gaussian beam corresponds to a Laguerre-Gaussian beam with an angular index of zero, while a two-dimensional Hermite-Gaussian beam can be transformed to obtain a multi-ring non-zero radial index Laguerre-Gaussian beam that is generally difficult to obtain. The smaller value of the mode order in the two dimensions is the radial index of the Laguerre-Gaussian beam after transformation. Therefore, high-order two-dimensional Hermite-Gaussian mode laser light sources have received widespread attention.

[0003] The generation of two-dimensional Hermite-Gaussian beams is quite difficult. Off-axis pumping is the most commonly used method to generate one-dimensional Hermite-Gaussian modes: the centers of the fundamental mode and higher-order modes of the resonant cavity coincide with the optical axis of the resonant cavity, the maximum intensity of the fundamental mode is located in the center, the maximum intensity of the one-dimensional Hermite-Gaussian mode is located at the maximum points at both ends, and the maximum intensity of the two-dimensional Hermite-Gaussian mode is located at the maximum points at the four corners of the rectangle; when the optical path of the pump light deviates from the optical axis of the resonant cavity, the pump light and the higher-order Hermite-Gaussian mode have better overlap, which can excite higher-order Hermite-Gaussian modes whose outermost maximum coincides with the pump light. For example, H.Laabs et al. in Germany first proposed the method of vertical axis pumping, which achieved a power of up to HG 0,87The output of a one-dimensional Hermite-Gaussian mode of the order. Furthermore, it is easy to intuitively think that if the pump light path is off-axis in both the x-axis and y-axis (with the direction of the resonant cavity optical axis as the z-axis), a two-dimensional rectangular Hermite-Gaussian mode can be generated in which the light intensity maximum at the corner coincides with the pump light. However, this method is often not feasible in actual operation: for a general laser resonant cavity, even if the pump light path is off-axis in both the x-axis (horizontal) and y-axis (vertical) dimensions, since the tilted one-dimensional Hermite-Gaussian mode with the outermost maximum located near the pump light path is also an eigenmode of the resonant cavity, its energy concentration is much stronger than that of the two-dimensional Hermite-Gaussian mode with the light intensity maximum at the corner located near the pump light, the tilted one-dimensional Hermite-Gaussian mode has a higher overlap with the pump light, reaches the laser threshold oscillation first, and wins in the mode competition, resulting in two-dimensional off-axis pumping. It is difficult to directly generate a two-dimensional Hermite-Gaussian mode. Due to this factor, the two-dimensional Hermite-Gaussian mode solid-state lasers reported so far are still based on methods such as using a digital micromirror array (DMD) to shape the pump light into a dot matrix or using a spatial light modulator (SLM) in the cavity. The device cost is high and the damage threshold is low, which limits related applications.

[0004] References

[0005] [1]H.Laabs and B.Ozygus,Excitation of Hermite Gaussian modes in end-pumped solid-state lasers via off-axis pumping,Optics&Laser Technology 28213-214,1996.

[0006] [2] Liu Qiang, Pan Jing, Wan Zhensong, Shen Yijie, Zhang Hengkang, Fu Xing, Gong Mali, Generation method of complex vortex structure light field, Chinese Journal of Lasers, 47(5), 0500006(2020).

[0007] [3] F. Schepers, T. Bexter, T. Hellwig, and C. Fallnich, Selective Hermite–Gaussian mode excitation in a laser cavity by external pump beam shaping, Appl. Phys. B 125 75, 2019.

[0008] [4] S. Ngcobo, I. Litvin, L. Burger, and A. Forbes, A digital laser for on-demand laser modes, Nat. Commun. 4, 2289, 2013. Summary of the invention

[0009] The present invention provides a high-order two-dimensional Hermite-Gaussian mode laser. The present invention constructs a folded laser resonant cavity with a concave reflector as a folding mirror. When an incident light beam is at a certain angle with the concave mirror, the concave mirror has different converging effects on the light beam on the meridian plane and the sagittal plane, that is, astigmatism is introduced. Under the effect of the astigmatism, the tilted one-dimensional Hermite-Gaussian mode is no longer an eigenmode of the resonant cavity, while the two-dimensional Hermite-Gaussian mode is still an eigenmode of the resonant cavity. Therefore, when the pump light is off-axis in both the x-axis and y-axis directions, the required two-dimensional Hermite-Gaussian mode no longer faces competition with the tilted one-dimensional Hermite-Gaussian mode with more concentrated energy. The two-dimensional mode order whose light intensity maximum at the corner coincides with the pump light has a higher pump overlap than other two-dimensional mode orders, that is, a lower laser threshold, and the laser works in the two-dimensional Hermite-Gaussian mode of this order, as described below:

[0010] A high-order two-dimensional Hermite-Gaussian mode laser, comprising: a pump source, a pump focusing mirror, a laser gain medium, a laser total reflection mirror, a concave folding mirror, and a laser output mirror.

[0011] Among them, 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 laser resonant cavity to provide feedback for the laser.

[0012] Among them, when the laser gain exceeds the loss of the resonant cavity, the laser reaches the threshold and generates laser output; when the incident position of the pump light deviates from the optical axis of the resonant cavity, the overlap of the pump light with the higher-order mode is higher than the overlap with the fundamental transverse mode, and the laser operates in a higher-order Hermite-Gaussian transverse mode.

[0013] Among them, when the pump light deviates from the optical axis of the resonant cavity in both the x-axis and y-axis directions, the high-order two-dimensional Hermite-Gaussian mode at one of the four corners where the maximum light intensity coincides with the position of the pump light has the highest pump overlap compared to other orders of two-dimensional Hermite-Gaussian modes.

[0014] The laser gain medium is a bulk crystal, glass or ceramic gain medium 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.

[0015] Wherein, the pump source 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.

[0016] The emission wavelength of the laser 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.

[0017] Furthermore, the laser total reflection mirror and the laser output mirror are plane mirrors or concave mirrors.

[0018] The laser constructs a folded laser resonant cavity with a concave reflector as a folding mirror. When an incident light beam is at a certain angle to the concave mirror, the concave mirror has different converging effects on the light beam in the meridian plane and the sagittal plane, i.e., astigmatism is introduced. Under the effect of the astigmatism, the tilted one-dimensional Hermite-Gaussian mode is no longer an eigenmode of the resonant cavity, while the two-dimensional Hermite-Gaussian mode is still an eigenmode of the resonant cavity. Therefore, when the pump light is off-axis in both the x-axis and y-axis directions, the required two-dimensional Hermite-Gaussian mode no longer faces competition with the tilted one-dimensional Hermite-Gaussian mode with more concentrated energy. The two-dimensional mode order whose light intensity maximum at the corner coincides with the pump light has a higher pump overlap than other two-dimensional mode orders, i.e., a lower laser threshold, and the laser works in the two-dimensional Hermite-Gaussian mode of this order.

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

[0020] 1. The present invention adopts a folded cavity structure and utilizes the astigmatism of a concave folding mirror to effectively suppress the competition between a one-dimensional Hermite-Gaussian mode and a two-dimensional Hermite-Gaussian mode, thereby obtaining a two-dimensional Hermite-Gaussian mode laser output with good mode purity;

[0021] 2. The present invention introduces astigmatism to force the directions of the two order principal axes of the two-dimensional Hermite-Gaussian mode to be consistent with the meridian plane and sagittal plane of the resonant cavity, respectively. In this way, the light intensity distribution characteristics of the two principal axis directions are easy to determine respectively, and the relationship between the mode order of the two-dimensional Hermite-Gaussian and the off-axis amount of the pump in the x-direction and the y-direction can be directly determined;

[0022] 3. The present invention introduces astigmatism to make the resonant cavity have obvious rectangular symmetry, destroy the circular / elliptical symmetry of the resonant cavity, avoid the conversion of Hermite-Gauss to Ins-Gauss caused by circular / elliptical symmetry, and eliminate the ellipticity of high-order laser modes; therefore, whether it is a one-dimensional Hermite-Gaussian mode generated by only one direction off-axis or a two-dimensional Hermite-Gaussian mode generated by two directions off-axis at the same time, both are Hermite-Gaussian mode outputs with higher purity, and it is not easy to generate an elliptical Ins-Gaussian mode. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 A schematic diagram of the optical path of a high-order two-dimensional Hermite-Gaussian mode laser;

[0024] Figure 2 A schematic diagram of the overlap of the pump light on the optical axis and the off-axis pump light at the laser crystal with the one-dimensional tilted Hermite-Gaussian mode and the two-dimensional Hermite-Gaussian mode in a high-order two-dimensional Hermite-Gaussian mode laser;

[0025] Figure 3 Schematic diagram of a high-order two-dimensional Hermite-Gaussian mode spot obtained in a high-order two-dimensional Hermite-Gaussian mode laser experiment.

[0026] In the accompanying drawings, the components represented by the reference numerals are listed as follows:

[0027] 1: Pump source; 2: Pump focusing mirror;

[0028] 3: Laser gain medium; 4: Laser total reflection mirror;

[0029] 5: Concave folding mirror; 6: Laser output mirror. DETAILED DESCRIPTION

[0030] 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.

[0031] Example 1

[0032] The embodiment of the present invention provides a high-order two-dimensional Hermite-Gaussian mode 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, and a laser output mirror 6;

[0033] The pump source 1 is a semiconductor laser with fiber-coupled output, with a wavelength of 808 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 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×5mm; 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 808nm pump wavelength anti-reflection film; the concave folding mirror 5 is a plano-concave mirror with a concave curvature radius of 300mm, 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 folding mirror 5, and the laser output mirror 6 together constitute 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, such as Figure 1 shown.

[0034] The 808nm 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 100μm. 3+ The ions absorb the 808nm pump light and are excited to the upper laser energy level, generating laser gain. Figure 2 The solid black frame represents the end face area of ​​the Nd:YVO4 crystal, and the solid gray dots represent the pumping area. 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, that is, when the laser is pumped off-axis, the pump light has a higher overlap with a certain higher-order transverse mode with the maximum intensity close to the pump light position, and the laser tends to operate in a higher-order transverse mode. When the pump light deviates from the optical axis in both the x and y directions, such as Figure 2 The solid gray dot in the upper left center shows the pump light and the tilted one-dimensional higher-order Hermite-Gaussian mode ( Figure 2 The one-dimensional dot pattern shown in the dashed line) and the two-dimensional high-order Hermite-Gaussian pattern ( Figure 2 The two-dimensional dot lattice spot shown in the figure has obvious overlap, but because the energy concentration of the one-dimensional Hermite-Gaussian mode is higher than that of the two-dimensional Hermite-Gaussian mode, its pump overlap is higher and it is easier to oscillate; therefore, it is difficult to produce a two-dimensional Hermite-Gaussian mode without taking measures to distinguish between the two.

[0035] In the embodiment of the present invention, the laser adopts a folded cavity structure. The concave folding mirror 5 has different refractive effects on the light beam in the meridian plane (x=0 plane) and the sagittal plane (y=0 plane). The symmetry of the resonant cavity is broken, and the line from the crystal center to the off-axis pump light position is no longer the symmetry axis of the resonant cavity cross section. Therefore, Figure 2The tilted one-dimensional high-order Hermite-Gaussian mode is no longer the eigenmode of the resonant cavity and cannot oscillate, while the two-dimensional Hermite-Gaussian mode in the figure is still the eigenmode of the folded astigmatic resonant cavity. This eliminates the competition between the tilted one-dimensional high-order Hermite-Gaussian mode and the two-dimensional high-order mode. When the pump light is off-axis in both the x and y directions, one of the four corners has the highest pump overlap with a certain order of the two-dimensional Hermite-Gaussian mode that is closest to the pump light position, forming a laser oscillation output. By adjusting the off-axis displacement of the pump light in the x and y directions, the order of the two-dimensional Hermite-Gaussian mode can be selected. Under the experimental parameters given in the embodiment of the present invention, HG was achieved at an 808nm pump power of 1W in the experiment. 0,0 To Gundam HG 63,63 High-contrast 2D high-order Hermite-Gaussian pattern output, such as Figure 3 This result is higher than the highest order 2D HG mode (HG 8,8 ), the highest order 2D HG mode achieved by modulated pumping (HG 25,27 ), the highest order two-dimensional HG mode achieved by extracavity modulation (HG 25,25 ) several times.

[0036] Example 2

[0037] 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.

[0038] 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.

[0039] The curvature radius of the laser total reflection mirror 4, the concave folding mirror 5, the laser output mirror 6, and the distance between them can select the parameters in Example 1, or other parameters, as long as the resonant cavity is within the stable zone and the concave folding mirror 5 can introduce astigmatism to achieve the destruction of the symmetry of the resonant cavity.

[0040] 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 the type and parameters of gain, the material, curvature radius, transmittance, etc. of the lens, as long as the device can complete the above functions.

[0041] 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.

[0042] 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.

[0043] 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 high-order two-dimensional Hermite-Gaussian mode laser, characterized in that: The laser comprises: a pump source, a pump focusing mirror, a laser gain medium, a laser total reflection mirror, a concave folding mirror, and a laser output mirror. Among them, 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 laser resonant cavity to provide feedback for the laser.

2. A high-order two-dimensional Hermite-Gaussian mode laser according to claim 1, characterized in that: When the laser gain exceeds the resonant cavity loss, the laser reaches the threshold and produces laser output; when the incident position of the pump light deviates from the optical axis of the resonant cavity, the overlap of the pump light with the higher-order mode is higher than the overlap with the fundamental transverse mode, and the laser operates in a higher-order Hermite-Gaussian transverse mode.

3. A high-order two-dimensional Hermite-Gaussian mode laser according to claim 1, characterized in that: When the pump light deviates from the optical axis of the resonant cavity in both the x-axis and y-axis directions, the high-order two-dimensional Hermite-Gaussian mode at one of the four corners where the maximum light intensity coincides with the position of the pump light has the highest pump overlap compared to other orders of two-dimensional Hermite-Gaussian modes.

4. A high-order two-dimensional Hermite-Gaussian mode laser according to claim 1, characterized in that: The laser gain medium is a bulk crystal, glass or ceramic gain medium 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.

5. A high-order two-dimensional Hermite-Gaussian mode laser according to claim 1, characterized in that: The pump source 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.

6. A high-order two-dimensional Hermite-Gaussian mode laser according to claim 1, characterized in that: The emission wavelength of the laser 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.

7. A high-order two-dimensional Hermite-Gaussian mode laser according to claim 1, characterized in that: The laser total reflection mirror and the laser output mirror are plane mirrors or concave mirrors.

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