Reverse double-vortex laser with adjustable mode order

Abstract

The invention provides a reverse double-vortex laser with adjustable mode orders. The reverse double-vortex laser generates Hermite-Gaussian (HG) mode laser with different orders by using multiple beams of pump light based on an off-axis pump mode; astigmatism is introduced into a laser resonant cavity, and the direction of an intrinsic mode of the laser is limited, so that off-axis pump light in two mutually orthogonal directions can respectively excite corresponding modes in the two directions, gain competition between different modes is avoided, and the purity and the mode quality of generated Here-Gaussian mode laser are improved; furthermore, the output HG mode laser is converted by utilizing a cylindrical lens to obtain dual-mode LG vortex laser with opposite chirality, and the order of the HG mode laser can be adjusted by controlling the position of the pump light, so that the order of the LG vortex light is adjusted and controlled.

Description

Technical Field

[0001] The present invention relates to the field of lasers, and in particular, to an inverse double-vortex laser with adjustable mode order. Background Art

[0002] Vortex beams have a helical wavefront, and the wavefront phase changes by an integer multiple of 2π around the center for one week. Each photon in the beam carries orbital angular momentum. For the most typical vortex beam, the Laguerre-Gaussian (LG) beam, l is its angular index; due to the presence of a phase singularity at the center of the vortex beam, its light intensity is distributed in a hollow ring shape. These related characteristics make vortex beams have important applications in optical tweezers, quantum communication, micro-nano manufacturing, etc. The methods for generating vortex beams include two major categories: passive and active. The passive method modulates and transforms the existing Gaussian beam or Hermite-Gaussian beam outside the cavity to obtain vortex light. The active method controls the gain and loss of the laser mode in the resonator by pump light shaping and the design and processing of the size and pattern of the adjuster / defect point, so as to achieve mode selection and generate vortex light oscillation output. [1] .

[0003] In applications such as quantum entanglement and free-space optical communication, it is often necessary to use a vortex light source that simultaneously contains multiple modes for multiplexing to improve system performance. However, it is often difficult to control the mode purity and power ratio of multimode beams, and mode competition will also cause fluctuations in light intensity. Therefore, a method for generating controllable multi-order vortex beams is very important. Among the currently reported methods for generating multi-mode vortex beams, in reference [2], it is proposed to etch multiple concentric rings with different sizes on the laser resonator mirror to control the losses of each order mode with different sizes, so as to achieve the output of dual-mode vortex light. However, etching a complex structure pattern on the cavity mirror has high requirements for the processing technology, and the device preparation is very difficult; on the other hand, after the device is prepared, its corresponding mode order is determined accordingly, and it is very difficult to achieve flexible independent adjustment of the mode order; furthermore, the defect point is easily further damaged by the high-intensity laser in the cavity, causing the laser mode to change and even unable to continue working. Therefore, it is very difficult for the prior art to achieve the output of dual-mode vortex light with adjustable order, nor can it achieve the control of chirality to realize dual-mode vortex lasers with opposite chirality.

[0004] References

[0005] [1] A. Forbes, “Structured light from lasers,” Laser Photonics Rev. 13(11), 1900140(2019).

[0006] [2]Method for directly generating multi-vortex beams in a cavity, Chinese invention patent, authorization number CN 109031674 B Summary of the Invention

[0007] The present invention provides a reverse double-vortex laser with adjustable mode order, which uses multiple pump beams to generate Hermite-Gaussian (HG) mode lasers with different orders based on an off-axis pumping method; by introducing astigmatism into the laser resonator to define the direction of the intrinsic mode of the laser, so that the off-axis pump beams in two mutually orthogonal directions can respectively excite the modes in the corresponding two directions, in order to avoid gain competition between different modes and improve the purity and mode quality of the generated Hermite-Gaussian mode laser; further, a cylindrical lens is used to convert the output HG mode laser to obtain a double-mode LG vortex laser with opposite chirality. By controlling the position of the pump beam, the order of the HG mode laser can be adjusted, and then the order of the LG vortex light can be regulated. See the following description for details:

[0008] A reverse double-vortex laser with adjustable mode order, the laser comprising:

[0009] A first pump source and a second pump source, capable of emitting pump light within the absorption band of the laser gain medium; a laser gain medium, coated with an antireflection film system for the pump light wavelength and an antireflection film system for the laser wavelength; a pump light reflecting mirror, coated with a high-reflection film system for the pump light wavelength and an antireflection film system for the laser wavelength; a laser high-reflection mirror, coated with an antireflection film system for the pump light wavelength and a high-reflection film system for the laser wavelength; a concave laser folding mirror, coated with a high-reflection film system for the laser wavelength; a laser output mirror, coated with a partial output film system for the laser wavelength; a focusing lens, a first cylindrical lens, and a second cylindrical lens, coated with an antireflection film system for the laser wavelength.

[0010] When the pump light deviates from the axis of the laser resonator, the laser will operate in a higher-order HG mode with a higher overlap with the pump light. The laser resonator is composed of a laser high-reflection mirror, a concave laser folding mirror, and a laser output mirror. Since there is a certain angle between the incident direction of the light beam and the normal of the concave laser folding mirror, the effective focal lengths of the concave folding mirror in the meridional plane (vertical direction) and the sagittal plane (horizontal direction) are different, that is, astigmatism is introduced, so that the intrinsic mode of the laser resonator is limited to the HG mode along the x and y directions. The first pump source and the second pump source each emit a pump beam, which respectively deviate from the optical axis of the resonator in the x and y directions, and will respectively excite the higher-order modes along the x and y directions. The gain regions and optical field distributions of the two overlap very little, avoiding the gain competition between the two and also avoiding the laser operating in other modes. By respectively controlling the offsets Δx and Δy of the two pump beams relative to the axis of the resonator, a dual-mode HG laser output with adjustable mode order can be obtained. After adjusting the Rayleigh length of the dual-mode output from the laser resonator through a focusing lens, and through the astigmatic transformation of the first cylindrical lens and the second cylindrical lens, a dual-mode vortex light is obtained.

[0011] Among them, the focused size of the pump light in the laser gain medium should be relatively small, preferably smaller than the laser spot size of the oscillating fundamental mode, so as to improve the overlap between the pump light and the corresponding higher-order HG modes and enhance the mode discrimination ability of the laser.

[0012] Preferably, the concave laser folding mirror should be selected with a relatively small radius of curvature, such as below 300 mm; the angle between the optical path and the normal of the concave laser folding mirror should be selected to be relatively large, such as above 10°, so as to strengthen the limiting effect of astigmatism on the resonant cavity modes.

[0013] Preferably, a pump source with higher brightness should be selected for the laser pump source to avoid the divergence of the pump light in the laser gain medium, which makes it difficult to control its overlap with different modes; correspondingly, a laser gain medium with a shorter length should be selected to reduce the divergence of the pump light therein, and its doping concentration should be selected to be relatively high to enhance pump absorption.

[0014] Preferably, a plane mirror should be selected for the laser output mirror, so that the beam waists of different orders of HG modes are always at the output mirror, which is conducive to the astigmatic transformation from HG mode laser to LG mode vortex light.

[0015] The laser gain medium is: Nd:YVO 4 , Nd:YAG, Ti:Sa, or common solid laser gain media such as Nd-doped, Yb-doped, and Er-doped laser glasses and laser ceramics.

[0016] The beneficial effects of the technical solution provided by the present invention are:

[0017] 1) Based on two off-axis pump lights respectively exciting higher-order HG modes, the present invention can obtain dual-mode vortex lights with opposite chirality, and can flexibly and independently control the orders of the dual-mode vortex lights respectively;

[0018] 2) By introducing astigmatism into the laser resonant cavity to limit the eigenmodes of the laser resonant cavity, the present invention enables the off-axis pump lights along two orthogonal directions to respectively excite higher-order HG mode lasers along their respective directions, avoiding mode competition and the generation of other modes.

[0019] 3) The dual-mode HG modes of the present invention are generated in the same resonant cavity and have a common axis, and the concentricity of the dual-mode vortex lights obtained after transformation is high. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a schematic optical path diagram of a reverse dual-vortex laser with adjustable mode order provided by the present invention;

[0021] Figure 2 is a schematic diagram of the deviation of two pump lights of the reverse dual-vortex laser with adjustable mode order provided by the present invention from the optical axis.

[0022] Attached Figure 1 In the following, the list of components represented by each reference numeral is as follows:

[0023] 1-1: First pump source; 1-2: Second pump source;

[0024] 2: Laser highly reflective mirror; 3: Laser gain medium;

[0025] 4: Pump light reflecting mirror; 5: Concave laser folding mirror;

[0026] 6: Laser output mirror; 7: Focusing lens;

[0027] 8-1: First cylindrical lens; 8-2: Second cylindrical lens. Detailed implementation manners

[0028] To make the objectives, technical solutions and advantages of the present invention clearer, the implementation manners of the present invention will be further described in detail below.

[0029] Embodiment 1

[0030] A reverse double-vortex laser with adjustable mode order, referring to Figure 1 , the laser includes: a first pump source 1-1, a second pump source 1-2, a laser highly reflective mirror 2, a laser gain medium 3, a pump light reflecting mirror 4, a concave laser folding mirror 5, a laser output mirror 6, a laser output mirror 7, a first cylindrical lens 8-1, and a second cylindrical lens 8-2.

[0031] Among them, the first pump source 1-1 and the second pump source 1-2 emit pump light within the absorption band of the laser gain medium 3. The pump light emitted by the first pump source 1-1 passes through the laser highly reflective mirror 2 and is incident on the laser gain medium 3. The pump light emitted by the second pump source 1-2 is reflected by the pump light reflecting mirror 4 and is incident on the laser gain medium 3 from the other end. The laser gain medium 3 absorbs the pump light to generate laser gain. The laser highly reflective mirror 2 is coated with a film system with high reflectivity at the laser wavelength and high transmittance at the pump wavelength. The concave laser folding mirror 5 is coated with a high-reflection film system at the laser wavelength, and the laser output mirror 6 is coated with a partially transmissive film system at the laser wavelength. The three form a folded laser resonator. The focusing lens 7, the first cylindrical lens 8-1, and the second cylindrical lens 8-2 are coated with a film system with high transmittance at the laser wavelength. When the laser gain exceeds the threshold, laser oscillation is formed in the resonator and is output through the laser output mirror 6. Since there is a certain angle between the laser optical path in the cavity and the normal of the concave laser folding mirror 5, its effective focal lengths in the meridional plane and the sagittal plane are different, which are f T = Rcosθ / 2 and f S= R / (2cosθ), where R and θ are the radius of curvature and the incident angle respectively, i.e., astigmatism is introduced, and the astigmatism confines the eigenmodes of the resonator to the HG modes along the meridional plane and the sagittal plane. Therefore, when the two pump lights emitted by the first pump source 1-1 and the second pump source 1-2 deviate from the axis of the resonator along the x-axis (horizontal) and the y-axis (vertical) respectively, high-order HG modes with non-zero mode orders that have the maximum overlap with the two pump lights in the horizontal and vertical directions respectively will be excited, oscillate in the laser resonator, and output through the laser output mirror 6. After the two HG mode lasers are focused by the focusing lens 7, they are transformed into double-vortex LG beams through the first cylindrical lens 8-1 and the second cylindrical lens 8-2. Since the directions of the two HG mode lasers are orthogonal to each other, the chiralities of the transformed LG beams are opposite. Since the overlap between the pump light and the cavity mode determines the order of the HG mode oscillating in the cavity, by controlling the positions of the two pump lights on the laser gain medium 3 respectively, the order of the generated HG mode can be controlled respectively, realizing the adjustment of the order of the double-vortex LG beam.

[0032] Preferably, when generating vortex light with a higher order, the clear aperture of the laser gain medium 3 should be larger than the spot size of the corresponding oscillating high-order HG mode.

[0033] Preferably, the radius of curvature of the concave laser folding mirror 5 should be selected to be a smaller radius of curvature, and the angle between the optical path and the normal of the concave laser folding mirror should be selected to be a relatively large angle to strengthen the confinement effect of astigmatism on the resonator mode.

[0034] Preferably, the pump sources 1-1 and 1-2 should be pump sources with higher brightness to avoid the divergence of the pump light in the laser gain medium 3 making it difficult to control its overlap with different modes; correspondingly, the laser gain medium 3 should be selected to have a shorter length to reduce the divergence of the pump light therein, and its doping concentration should be selected to be a higher concentration to enhance pump absorption.

[0035] Preferably, the laser output mirror 6 should be a plane mirror, so that the beam waists of HG modes with different orders are always at the output mirror, which is convenient for the astigmatic transformation from HG mode laser to LG mode vortex light.

[0036] In summary, in the embodiment of the present invention, the eigenmodes of the laser resonator are confined by introducing astigmatism, two HG mode lasers in two directions are excited by two pump lights respectively, and then double-vortex lasers with opposite chiralities and adjustable orders are obtained through transformation by cylindrical lenses, meeting various needs in practical applications.

[0037] Embodiment 2

[0038] An embodiment of the present invention provides a multi-mode vortex laser with adjustable order. The laser includes: a first pump source 1-1, a second pump source 1-2, a laser high reflector 2, a laser gain medium 3, a pump light reflector 4, a concave laser folding mirror 5, a laser output mirror 6, a laser output mirror 7, a first cylindrical lens 8-1, and a second cylindrical lens 8-2.

[0039] Among them, both the first pump source 1-1 and the second pump source 1-2 are 808nm semiconductor lasers, with a fiber core diameter of 50μm and a numerical aperture of 0.14. The laser gain medium 3 is an a-cut Nd:YVO 4 crystal, 5×5×2mm 3 , with a doping concentration of 2.0at.%, coated with an antireflection coating system for 808nm pump light and 1064nm laser. The pump light reflector 4 is coated with an 808nm 45° high reflection coating system. The beam waist radius of the pump light in the laser gain medium 3 is 70μm. The laser high reflector 2 is a plane mirror, coated with an antireflection coating for 808nm and a high reflection coating for 1064nm; the concave laser folding mirror 5 has a curvature radius of 200mm and is coated with a 1064nm high reflection coating system. The laser output mirror 6 is a plane mirror, coated with a 1064nm transmittance of 2% coating system. The distance between the laser high reflector 2 and the concave laser folding mirror 5 is 130mm, the distance between the concave laser folding mirror 5 and the laser output mirror 6 is 170mm, and the half angle of the folding angle is 15°. The 808nm pump light emitted by the first pump source 1-1 and the second pump source 1-2 respectively enters the Nd:YVO 4 crystal of the laser gain medium 3 from both ends. The Nd:YVO 4 crystal absorbs the pump light to generate laser gain. When the gain is greater than the loss, a laser oscillating in the laser resonant cavity composed of the laser high reflector 2, the concave laser folding mirror 5, and the laser output mirror 6 is formed, and the laser is output through the laser output mirror 6. The focal length of the focusing lens 7 is 100mm, and the focal lengths of the first cylindrical lens 8-1 and the second cylindrical lens 8-2 are both 100mm, and the distance between them is 141mm.

[0040] When the positions of the pump light in the laser gain medium 3 deviate from the axis of the resonant cavity, the higher-order HG mode with the highest overlap with the pump light obtains higher gain. Therefore, by respectively adjusting the focusing positions of the first pump source 1-1 and the second pump source 1-2 in the laser gain medium 3 in the horizontal and vertical directions so that they deviate from the axis of the resonant cavity, the HGm,0 and HG0,n modes along the x direction and the y direction can be respectively excited. By changing the deviation amounts Δx and Δy relative to the axis, the mode order of the HG mode can be controlled. The mutually orthogonal dual-mode HG laser output passes through the focal length of the focusing lens 7 and is transformed by the first cylindrical lens 8-1 and the second cylindrical lens 8-2 to obtain dual-mode vortex light with opposite chirality.

[0041] In the above embodiments, the laser gain medium may be a laser crystal such as Nd:YVO4, Nd:YAG (neodymium-doped yttrium aluminum garnet), Ti:Sa (titanium-doped sapphire), etc., or may also be a common laser gain medium such as laser glass or laser ceramic doped with Nd, Yb (ytterbium), Er (erbium), or other luminescent ions. The corresponding pump source wavelength and coating wavelength only need to correspond to the absorption peak and emission peak of the laser gain medium, and the embodiments of the present invention do not limit this.

[0042] The embodiments of the present invention do not specifically limit the resonator length, the curvature of the concave mirror, and the folding angle, as long as the resonator is within the stable region.

[0043] In summary, the purpose of the embodiments of the present invention is to use two pump lights to excite two different HG modes, and combine the limitation of the astigmatism on the eigenmode of the resonator to make the modes generated by the two off-axis pump lights orthogonal to each other in direction. Then, through the transformation by a cylindrical lens, a dual-mode vortex light with opposite chirality is obtained. By adjusting the position of the off-axis pump light, the convenient adjustment of the order of the vortex light is realized.

[0044] Except for those with special descriptions, the embodiments of the present invention do not limit the models of each device, and any device that can complete the above functions can be used.

[0045] Those skilled in the art can understand that the drawings are only schematic diagrams of a preferred embodiment, and the serial numbers of the above embodiments of the present invention are only for description and do not represent the advantages or disadvantages of the embodiments.

[0046] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A reverse double vortex laser with adjustable mode order, characterized in that: The laser comprises: The first pump source and the second pump source can emit pump light within the absorption band of the laser gain medium; the laser gain medium is coated with a film system with pump light wavelength transmittance enhancement and laser wavelength transmittance enhancement; the pump light reflector is coated with a film system with high reflectance at the pump light wavelength and laser wavelength transmittance enhancement; the laser high reflector is coated with a film system with pump light wavelength transmittance enhancement and laser wavelength high reflectance; the concave laser folding mirror is coated with a film system with high reflectance at the laser wavelength; the laser output mirror is coated with a film system for partial output of the laser wavelength; the focusing lens, the first column lens and the second column lens are coated with a laser wavelength transmittance enhancement film system. Under the excitation of two off-axis pump lights, two mutually orthogonal high-order Hermite-Gaussian mode laser outputs are generated in the resonant cavity, and after transformation by a cylindrical lens, dual-mode vortex light with opposite chirality is obtained.

2. The reverse double vortex laser with adjustable mode order according to claim 1, characterized in that: The laser resonant cavity is a folded cavity including a concave laser folding mirror, and the astigmatism of the folding mirror limits the eigenmode of the resonant cavity to be a Hermite-Gaussian mode along the horizontal direction and the vertical direction.

3. The reverse double vortex laser with adjustable mode order according to claim 1, characterized in that: The first pump source and the second pump source each emit a beam of pump light, which is incident from both ends of the laser gain medium, respectively along two mutually orthogonal deviated resonant cavity axes to excite two mutually orthogonal high-order Hermite-Gaussian mode oscillation outputs, thereby obtaining dual-mode vortex light with opposite chirality.

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