Apparatus and method for generating a vortex air laser

Abstract

A vortex air laser generation device and method. In the device, femtosecond pump laser is focused to form a plasma optical filament and excite the nitrogen molecule ion inside. At the same time, the vortex seed pulse is collinearly focused to the middle of the optical filament to induce the nitrogen molecule ion to produce vortex amplified air laser. However, due to the uneven distribution of the plasma space, the vortex phase of the air laser will be destroyed in the further transmission process, so that the finally generated air laser is difficult to maintain the perfect vortex phase and spot shape of the seed light. The device is designed for the gas cavity, and the method of gradient air pressure is adopted to cut off the rear end of the plasma optical filament. The vortex seed light is amplified in the uniform plasma, thereby generating a vortex air laser with stable wavefront, complete phase and topological charge greater than 1. The acquisition of the vortex air laser not only can obtain the amplification of the ultrafast vortex light with the plasma as the gain medium, but also can promote the important application of the air laser in the fields of nonlinear spectral detection, long-distance optical communication and super-resolution imaging. The present application has the advantages of simple operation, high stability, strong universality and expansibility.

Description

Technical Field

[0001] This invention relates to the field of ultrafast optics and light field manipulation, specifically to a device and method for generating vortex air lasers. Background Technology

[0002] Vortex light is a special type of structured light field with a spatial spiral phase structure and intrinsic photon orbital angular momentum (OAM). As a frontier in optical field manipulation research, it has broad application prospects in high-capacity optical communication, high-density optical storage, and high-resolution imaging. Traditional vortex light is usually generated using spiral phase plates, spatial modulators, etc. Due to the limitations of the damage threshold of these components, researchers have begun to explore the amplification behavior of vortex light in order to generate high-intensity vortex light [see J. Qian et al., Photonics Res. 8, 421-425 (2020); H. Tong et al., Opt. Lett. 45, 989-992 (2020)]. Air lasers are coherent lasing generated by femtosecond lasers exciting air plasma, representing cavity-free amplification behavior with plasma as the gain medium [see P. Polynkin and Y. Cheng, Air Lasing, Springer International Publishing, Switzerland, 2018; Yao Jinping and Cheng Ya, Chinese Journal of Lasers 47, 0500005 (2020)]. In 2023, researchers introduced vortex light into air laser research, successfully generating vortex air lasers [see Y. Hu et al., Optica 10, 682-687 (2023); J. Gao et al., Commun. Phys. 6, 97 (2023)]. Research has found that vortex pump light can directly generate vortex air lasers in plasma, and can also amplify externally applied Gaussian seed pulses into vortex air lasers [see J. Gao et al., Commun. Phys. 6, 97 (2023)]. Similarly, weak seed pulses carrying OAM (l=±1) can be significantly amplified in nitrogen plasma excited by femtosecond Gaussian pump light, and the output air laser essentially inherits the vortex phase of the seed pulse [see H. Mei et al., Opt. Express 31, 31912-31921 (2023)]. The generation of vortex air lasers not only provides a new perspective for vortex light amplification research but also holds significant application potential in nonlinear spectral detection, long-distance optical communication, and microscopic imaging. However, due to the non-uniform plasma distribution, currently generated vortex air lasers struggle to inherit the perfect vortex phase structure and spatial distribution characteristics of the pump or seed light. Furthermore, the generation of higher-order vortex air lasers with a topological charge greater than 1 faces significant challenges, severely limiting their practical applications. Simultaneously, vortex air lasers are a novel type of ultrafast structured light, and no technological inventions have yet been developed to improve their quality; moreover, higher-order vortex light amplification in plasma has not been reported. Therefore, the present invention aims to obtain a vortex air laser with wavefront stability, phase integrity, and topological charge greater than 1, so as to realize a wider range of vortex air laser applications. Summary of the Invention

[0003] To further promote the practical application of vortex air lasers, this invention aims to provide a device and method for generating vortex air lasers. It optimizes the quality of the vortex air laser by truncating the plasma filament, offering advantages such as simple operation, high stability, versatility, and strong scalability. Simultaneously, the vortex air laser achieves further amplification and transmission of the vortex beam, which has significant implications and potential application value in fields such as free-space optical communication, remote sensing, and quantum repeaters.

[0004] The technical solution of the present invention is as follows:

[0005] A device for generating a vortex air laser is characterized by comprising a focusing lens, a gas cavity I, a steel sheet, a gas cavity II, and a collimating lens arranged along the beam transmission direction; the gas cavity I is filled with a fixed concentration of pure N2, the gas cavity II is in a vacuum state, and a side viewing window is provided above it; the steel sheet is used to separate the interiors of the gas cavity I and the gas cavity II, so that a gradient gas pressure is formed in the gas cavity I and the gas cavity II.

[0006] When the pump pulse and the vortex seed pulse enter the air cavity I through the focusing lens, the pump pulse is focused to form a plasma filament, and a circular pinhole is etched in the center of the steel sheet. The vortex seed pulse is focused in the middle of the filament. By observing through the side window, it is ensured that the steel sheet cuts off the rear end of the plasma filament. The resulting vortex air laser is amplified in the middle of the plasma filament and output with complete phase information and uniform distribution of light spots on the ring.

[0007] Both the focusing lens and the collimating lens are placed on an electric displacement stage to adjust the position of the plasma wire relative to the steel sheet, thereby changing the wire cutting position; the side window is used to observe and determine the wire cutting position.

[0008] Furthermore, it also includes an infrared femtosecond laser, a beam splitter, a first reflecting mirror, a BBO crystal, a first dichroic mirror, a first bandpass filter, a first adjustable attenuator, a second reflecting mirror, a spiral phase plate, a retroreflector, a beam combiner, a second dichroic mirror, a second adjustable attenuator, a second bandpass filter, a cylindrical lens, and a CCD camera.

[0009] The near-infrared femtosecond laser outputs a femtosecond laser with a center wavelength of 800nm, which is split into two beams by a beam splitter. One beam is reflected by a first mirror and frequency doubled by a BBO crystal to generate a seed pulse with a center wavelength of 400nm. After being filtered by a first dichroic mirror, a first bandpass filter, and a first adjustable attenuator, a seed light with suitable intensity and a center wavelength of 391nm is obtained. Then, after passing through a second mirror and a spiral phase plate, it becomes a vortex seed light. The other beam serves as the pump light and, after passing through a retroreflector, is combined with the vortex seed light pulse at a beam combiner.

[0010] Furthermore, the vortex air laser is collimated by a collimating lens, filtered by a second dichroic mirror, a second adjustable attenuator, and a second bandpass filter, and then focused onto a CCD camera by a cylindrical lens.

[0011] Furthermore, the first and second dichroic mirrors have high reflectivity in the 800nm ​​band and high transmittance in the 400nm band, used to filter out residual pumping; the first and second bandpass filters have high transmittance in the 391nm band, used to obtain pure vortex seed or vortex air laser; the first and second adjustable attenuators are used to adjust the pulse energy.

[0012] Furthermore, the spiral phase plate transforms the Gaussian seed pulse into a vortex seed pulse with a spatial spiral phase structure and intrinsic photon orbital angular momentum.

[0013] Furthermore, the retroreflector is placed on an electric displacement stage to adjust the time delay between the pump and the seed pulse.

[0014] Furthermore, the cylindrical lens is used to measure the topological charge of the vortex beam.

[0015] On the other hand, the present invention also provides a method for generating a vortex air laser, characterized by comprising the following steps:

[0016] S1. A steel sheet is placed close to the air outlet of air chamber I, separating the interior of air chamber I and air chamber II, thus constructing air chamber I and air chamber II with a pressure difference;

[0017] S2. When the pump pulse and the vortex seed pulse enter the air cavity I through the focusing lens, the pump pulse is focused to form a plasma filament, and a circular pinhole is etched in the center of the steel sheet. The vortex seed pulse is focused in the middle of the filament. It is observed through the side window, and the position of the focusing lens is moved so that the steel sheet cuts off the rear end of the plasma filament. The resulting vortex air laser is amplified in the middle of the plasma filament and output. The phase information is complete and the light spot on the ring is evenly distributed.

[0018] Furthermore, S1. constructs air chamber I and air chamber II with pressure difference, specifically as follows:

[0019] When pure N2 is continuously filled into the air chamber I, the pressure gauge reading remains unchanged and the pressure is constant; when air is continuously evacuated from the air chamber II, the pressure gauge reading is very small, close to a vacuum state.

[0020] Compared with the prior art, the technical effects of the present invention are as follows:

[0021] 1) By adopting a dual-cavity structure combined with gradient gas pressure, the rear end of the plasma filament is cut off, while the front end is difficult to generate significant plasma amplification. This allows only the uniform plasma portion in the middle to be effectively utilized, which has the advantages of simple experimental setup, convenient experimental operation, wide applicability, and strong versatility.

[0022] 2) Vortex air laser amplification was performed using the middle of a highly cylindrically symmetric plasma filament, avoiding the destruction of its vortex characteristics by the non-uniform plasma filament rear end. This resulted in a vortex air laser with a stable wavefront structure, complete vortex phase, and annular spatial distribution. This provides a simple and effective method and device for improving the quality of vortex air lasers, generating higher-order vortex air lasers with a topological charge greater than 1, and promoting the practical application of vortex air lasers. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of Embodiment 1 of the vortex air laser generation device.

[0024] Figure 2 This is a schematic diagram of plasma filament truncation.

[0025] In the figure, 1 is a near-infrared femtosecond laser, 2 is a beam splitter, 3 is a first reflecting mirror, 4 is a BBO crystal, 5 is a first dichroic mirror, 6 is a first bandpass filter, 7 is a first adjustable attenuator, 8 is a second reflecting mirror, 9 is a spiral phase plate, 10 is a retroreflector, 11 is a beam combiner, 12 is a focusing lens, 13 is gas cavity I, 14 is a barometer, 15 is a steel sheet, 16 is gas cavity II, 17 is a side window, 18 is a barometer, 19 is a collimating lens, 20 is a second dichroic mirror, 21 is a second adjustable attenuator, 22 is a second bandpass filter, 23 is a cylindrical lens, 24 is a CCD camera, and 25 is a plasma filament.

[0026] Figure 3 This is a schematic diagram of the experimental results obtained by cutting the plasma wire.

[0027] Figure 4 This is a schematic diagram of the experimental results obtained when the plasma filament was not cut. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments, but this should not be construed as limiting the scope of protection of this invention.

[0029] Please see Figure 1 , Figure 1This is a schematic diagram of the vortex air laser generation device of the present invention. As shown in the figure, it includes a near-infrared femtosecond laser 1, a beam splitter 2, a first reflecting mirror 3, a BBO crystal 4, a first dichroic mirror 5, a first bandpass filter 6, a first adjustable attenuator 7, a second reflecting mirror 8, a spiral phase plate 9, a retroreflector 10, a beam combiner 11, a focusing lens 12, an air cavity I 13, a barometer 14, a steel plate 15, an air cavity II 16, a side window 17, a barometer 18, a collimating lens 19, a second dichroic mirror 20, a second adjustable attenuator 21, a second bandpass filter 22, a cylindrical lens 23, and a CCD camera 24.

[0030] The spiral phase plate 9 transforms the Gaussian seed pulse into a vortex seed pulse. The steel sheet 15 is 0.5 mm thick, facilitating plasma etching of small holes. The focusing lens 12 and collimating lens 19 are placed on an electric displacement stage to adjust the position of the plasma filament relative to the steel sheet 15. The side window 17 is used to observe and determine the filament cutting position. The cylindrical lens 23 is used to measure the topological charge of the vortex beam. Under the strong astigmatism of the cylindrical lens, the focal field spot of the vortex beam exhibits an elliptical intensity distribution, and one or more separate dark nuclei appear at the center of the light field. The number of dark nuclei corresponds to the topological charge.

[0031] Please see Figure 2 , Figure 2 This is a schematic diagram of plasma filament cutting. It includes a steel sheet 15 and a plasma filament 25. A gradient gas pressure method is used in the gas cavity to achieve the effect of cutting off the rear end of the plasma filament.

[0032] Please see Figure 3 , Figure 3 This is a schematic diagram of the experimental results obtained by cutting off the plasma filament. By moving the position of the focusing lens 12, the steel sheet cuts off the rear end of the plasma filament. The resulting vortex air laser is magnified and output in the middle of the plasma filament. The spot information is recorded using a CCD camera 24, and it is observed that the spot distribution on the ring is uniform.

[0033] Please see Figure 4 , Figure 4 This is a schematic diagram of the experimental results obtained with the plasma filament intact. By moving the focusing lens 12 so that the steel sheet is positioned at the rear end of the entire plasma filament, the resulting vortex air laser is amplified and output within the intact plasma filament. The spot information is recorded using a CCD camera 24. (Comparison) Figure 3 , Figure 4 The light spot distribution on the ring is uneven.

[0034] The principle of obtaining vortex air laser in this invention is as follows:

[0035] The femtosecond pump laser is focused into gas cavity I, ionizing nitrogen molecules and generating plasma. When the self-focusing caused by the Kerr effect and the self-defocusing caused by the plasma reach equilibrium, a plasma filament is formed. Generally, the middle part of the plasma filament exhibits a uniform columnar distribution, while the plasma density at both ends is lower. The non-uniform distribution of plasma along the femtosecond light propagation direction disrupts the wavefront structure stability, vortex phase integrity, and uniformity of the spot distribution on the ring of the vortex beam. In other words, this plasma not only makes it difficult to propagate the vortex seed light, but also makes it difficult for the vortex seed light to maintain complete vortex characteristics during plasma amplification. Therefore, vortex air lasers generated by plasma amplification are usually of poor quality, and higher-order vortex air lasers with a topological charge greater than 1 present even greater technical challenges.

[0036] In this device, gas cavity II is kept in a near-vacuum state, where the extremely low gas density makes it difficult to generate plasma, thus cutting off the air laser generation process. Simultaneously, the plasma filament, typically on the order of hundreds of micrometers in diameter, etches circular pinholes that sever the energy pool surrounding the filament, preventing energy exchange between the filament and the energy pool, thus making the rear end of the plasma filament unstable. Since the front end of the plasma filament lacks amplification and the rear end cannot form, the vortex seed light is amplified in the middle of the uniform plasma filament. The spatial structure and phase distribution of the vortex light are transmitted, inherited, and preserved, thereby generating a high-quality vortex air laser.

[0037] The steps of the embodiments of the present invention are as follows:

[0038] 1) Start the near-infrared femtosecond laser and output femtosecond laser;

[0039] 2) Adjust the phase matching angle of the BBO crystal to double the 800nm ​​output light to a 400nm seed light; rotate the first adjustable attenuator to adjust the seed pulse intensity; insert the spiral phase plate to obtain a vortex seed pulse;

[0040] 3) Move the retroreflector to synchronize the pumping and seed pulse timing;

[0041] 4) The pump and seed pulses pass through the beam combiner and the focusing lens and enter the gas cavity; the pump pulse is focused to form a plasma filament, and the vortex seed pulse is focused in the middle of the filament;

[0042] 5) Several minutes later, the plasma etched a micron-sized hole in the center of the steel sheet; the gas valve of the gas chamber was adjusted so that gas chamber I was filled with a fixed concentration of pure N2, while gas chamber II was in a vacuum state.

[0043] 6) Move the focusing lens and collimating lens, and observe through the side window to ensure that the steel sheet has cut off the rear end of the plasma filament. The vortex air laser is amplified in the middle of the plasma filament and output immediately, with complete phase information and uniform distribution of light spots on the ring.

[0044] 7) Rotate the second adjustable attenuator and insert the second bandpass filter to obtain a vortex air laser with suitable intensity and pure spectrum;

[0045] 8) Insert the cylindrical lens and measure the topological charge of the vortex air laser;

[0046] 9) The CCD camera records the spot and topological charge information of the vortex air laser.

Claims

1. A device for generating a vortex air laser, characterized in that, It includes a focusing lens, air cavity I, a steel sheet, air cavity II, and a collimating lens arranged along the beam transmission direction; air cavity I is filled with a fixed concentration of pure N2, air cavity II is in a vacuum state, and a side viewing window is provided above it; the steel sheet is used to separate the interior of air cavity I and air cavity II, so that a gradient air pressure is formed in air cavity I and air cavity II; When the pump pulse and the vortex seed pulse enter the air cavity I through the focusing lens, the pump pulse is focused to form a plasma filament and a circular pinhole is etched in the center of the steel sheet. The vortex seed pulse is focused in the middle of the filament. By observing through the side window, it is ensured that the steel sheet cuts off the rear end of the plasma filament. The vortex air laser formed is amplified in the middle of the plasma filament and output with complete phase information and uniform distribution of the light spot on the ring. Both the focusing lens and the collimating lens are placed on an electric displacement stage to adjust the position of the plasma filament relative to the steel sheet, thereby changing the filament cutting position; the side window is used to observe and determine the filament cutting position. It also includes a near-infrared femtosecond laser for generating pump pulses and vortex seed pulses, a beam splitter, a first reflector, a BBO crystal, a first dichroic mirror, a first bandpass filter, a first adjustable attenuator, a second reflector, a spiral phase plate, a retroreflector, and a beam combiner. The near-infrared femtosecond laser outputs a femtosecond laser with a center wavelength of 800 nm, which is split into two beams by a beam splitter. One beam is reflected by a first mirror and frequency-doubled by a BBO crystal to generate a seed pulse with a center wavelength of 400 nm. After being filtered by a first dichroic mirror, a first bandpass filter, and a first adjustable attenuator, a seed light with a center wavelength of 391 nm is obtained. Then, after passing through a second mirror and a spiral phase plate, it becomes a vortex seed pulse. The other beam serves as a pump pulse and is combined with the vortex seed pulse at a beam combiner by a retroreflector.

2. The vortex air laser generating device according to claim 1, characterized in that, It also includes a second dichroic mirror, a second adjustable attenuator, a second bandpass filter, a cylindrical lens, and a CCD camera; the vortex air laser is collimated by a collimating lens and filtered by the second dichroic mirror, the second adjustable attenuator, and the second bandpass filter, and then focused by the cylindrical lens onto the CCD camera.

3. The vortex air laser generating device according to claim 2, characterized in that, The first and second dichroic mirrors have high reflectivity in the 800 nm band and high transmittance in the 400 nm band, used to filter out residual pumping; the first and second bandpass filters have high transmittance in the 391 nm band, used to obtain pure vortex seed pulses or vortex air lasers; the first and second adjustable attenuators are used to adjust the pulse energy.

4. The vortex air laser generating device according to claim 2, characterized in that, The spiral phase plate transforms the Gaussian seed pulse into a vortex seed pulse with a spatial spiral phase structure and intrinsic photon orbital angular momentum.

5. The vortex air laser generating apparatus according to claim 2, characterized in that, The retroreflector is placed on an electric displacement stage and is used to adjust the time delay between the pump pulse and the vortex seed pulse.

6. The vortex air laser generating apparatus according to claim 2, characterized in that, The cylindrical lens is used to measure the topological charge of a vortex beam.

7. A method for generating a vortex air laser using the vortex air laser generating apparatus according to any one of claims 1-6, characterized in that, Including the following steps: S1. A steel sheet is placed close to the air outlet of air chamber I, separating the interior of air chamber I and air chamber II, thus constructing air chamber I and air chamber II with a pressure difference; S2. When the pump pulse and the vortex seed pulse enter the air cavity I through the focusing lens, the pump pulse is focused to form a plasma filament, and a circular pinhole is etched in the center of the steel sheet. The vortex seed pulse is focused in the middle of the filament. It is observed through the side window, and the position of the focusing lens is moved so that the steel sheet cuts off the rear end of the plasma filament. The resulting vortex air laser is amplified in the middle of the plasma filament and output. The phase information is complete and the light spot on the ring is evenly distributed.

8. The method for generating a vortex air laser according to claim 7, characterized in that, S1. The construction of air chambers I and II with pressure difference is specifically as follows: When pure N2 is continuously filled into the air chamber I, the pressure gauge reading remains unchanged and the pressure is constant; when air is continuously evacuated from the air chamber II, the pressure gauge reading is very small, close to a vacuum state.

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