A tunable central point pulsed vortex ring continuous laser system

Abstract

An adjustable central point pulsed vortex ring continuous laser system. It belongs to the field of laser technology, and specifically relates to the technical field of vortex light point ring pulsed continuous laser systems. It solves the problems that the volume of the device for simultaneously obtaining a mode-tunable vortex light point ring pulsed continuous laser output is too large and requires a large number of components. The system includes a 792nm pump source, a coupling lens group, a plano-concave total reflector, a Tm:YLF crystal, a beam expander, and a transmissive pulsed continuous laser modulator; the 792nm pump source is used to emit pump light, the coupling lens group is used to shape the pump light, the receiving surface of the plano-concave total reflector for the pump light is a plane mirror, and the emitting surface for the pump light is a concave mirror. The plano-concave total reflector, the Tm:YLF crystal, the beam expander, and the transmissive pulsed continuous laser modulator form a gain cavity, and the pump light is amplified in the gain cavity and generates ultrashort pulsed laser and vortex light output.

Description

Technical Field

[0001] The present invention belongs to the field of laser technology, and specifically relates to the technical field of a central point pulsed vortex ring continuous laser system. Background Art

[0002] The progress of laser technology has promoted the development of modern society and made it an important part of modern science and technology and industry. Compared with ordinary light sources, lasers have the advantages of high coherence, high energy density, good monochromaticity, and good directivity. These advantages have enabled lasers to thrive in the past few decades, and their application fields have become increasingly extensive, including laser cutting, laser welding, laser communication, national defense and military, laser medicine, etc. This has promoted the development of related industries and is also increasingly worthy of research.

[0003] In the field of laser welding, one of the challenges is welding defects such as spatter, hump, and pores generated during welding. In order to improve such defects, various laser hybrid welding and beam shaping welding technologies have been proposed one after another. With the in-depth research, it is found that the dot-ring light spot has a more uniform temperature distribution, smaller central temperature and temperature gradient changes compared with the heat source with Gaussian distribution. This indicates that the dot-ring light spot has great application prospects in reducing welding defects.

[0004] Vortex light has a unique annular light intensity distribution and helical wavefront structure, and this characteristic makes it perform excellently in laser welding. Specifically, through the computer-generated holography method of optical field regulation, a vortex array with arbitrary positions, ring radii, and energy distributions can be designed for the vortex beam, thereby significantly improving the welding efficiency and welding quality; in addition, the application of the vortex beam in laser welding also involves the real-time observation and quality control of the welding process. By observing the turbulence of the molten pool and vapor, the real-time monitoring of the weld formation process can be realized, which is of great significance for the improvement and enhancement of welding quality.

[0005] In summary, in order to improve welding defects such as spatter, hump, and pores generated during laser welding, if a tunable vortex light dot-ring pulsed continuous laser output can be obtained simultaneously, the welding effect can be better improved.

[0006] At present, there are still many technical difficulties in obtaining a tunable vortex light dot-ring pulsed continuous laser output simultaneously. The more prominent ones are that when realizing the laser output of two modes simultaneously, the device volume is too large and the required components are more. Summary of the Invention

[0007] In order to solve the above technical problems, the present invention provides an adjustable central point pulsed vortex ring continuous laser system.

[0008] The system includes a 792nm pump source, a coupling lens group, a plane-concave total reflector, a Tm:YLF crystal, a beam expander, and a transmissive pulsed continuous laser modulator;

[0009] The 792 nm pump source is used to emit pump light. The coupling lens group is used to shape the pump light. One side of the plano-concave total reflector that receives the pump light is a plane mirror, and the side that emits the pump light is a concave mirror. The plano-concave total reflector, Tm:YLF crystal, beam expander, and transmissive pulsed continuous laser modulator form a gain cavity. The pump light is amplified in the gain cavity, and ultrashort pulsed laser and vortex light are output.

[0010] The coupling lens group consists of a set of Keplerian collimating and focusing lens groups.

[0011] In the set of Keplerian collimating and focusing lens groups, the focal length of one lens is 25 mm, and the focal length of the other lens is 75 mm.

[0012] The transmissive pulsed continuous laser modulator includes a liquid crystal panel, a graphene saturable absorber, and a ring circuit board. There is an opening in the middle of the liquid crystal panel. The graphene saturable absorber is placed at the opening in the middle of the liquid crystal panel. The ring circuit board is connected end to end around the liquid crystal panel.

[0013] Both the upper and lower surfaces of the liquid crystal panel are flat and are attached with PI alignment layers. Through rubbing alignment and drying and curing operations, the internal liquid crystals all have pretilt angles.

[0014] A film layer with a transmittance of 5% for 1908 nm laser is deposited on the PI alignment layer on the optical path output side of the liquid crystal panel.

[0015] The ring circuit board is controlled by a computer. The computer controls the voltage of the ring circuit board to change the alignment direction vector of the local liquid crystals in the liquid crystal panel, thereby achieving the effect of optical field regulation.

[0016] The beneficial effects of the system of the present invention are as follows:

[0017] The area of the entire target surface of the traditional spatial light modulator is 18×20 mm 2 , and in addition to the liquid crystal target surface, it also has an internal electronic control system and other devices, and the overall size is 60×60×35 mm 3As mentioned above, from the perspective of optical path construction, it appears extremely cumbersome. The working principle of a traditional spatial light modulator is to fix the orientation of internal liquid crystal molecules by applying a voltage to a light field in a specific wavelength band. Macroscopically, it turns the liquid crystal panel into an isotropic medium, and changes the original light field into light with different phases according to the orientation degree of each unit of liquid crystal and then outputs it. Since a specific voltage needs to be applied, different phase diagrams must be transmitted from a computer to the spatial light modulator. Each phase diagram represents a different form of electric field. Combining the above working principle, the indispensable components are only the liquid crystal on silicon layer and the electrically controlled panel. The present invention retains the key liquid crystal on silicon layer and electrically controlled panel of the traditional spatial light modulator, and changes the shape of the liquid crystal panel. Both the upper and lower surfaces of the liquid crystal panel are flat. A graphene saturable absorber is arranged inside the liquid crystal panel, and the structure of the electrically controlled panel that was previously attached to the bottom surface of the liquid crystal on silicon layer is improved to a circular circuit board that surrounds the liquid crystal panel and is connected end to end. The above improvements have two advantages: First, after the improvement, the overall device structure becomes smaller, saving both the space occupancy and material occupancy while retaining the necessary laser modulator function required by the present invention; Second, the structural improvement of the liquid crystal panel enables multifunctional multiplexing. The liquid crystal panel is both a device for generating ultrashort pulse lasers and vortex light, and can also be used as a flat output mirror, enabling the device to be used for the oscillation gain of oscillating light in the resonator cavity while also using the graphene saturable absorber at the center for pulse modulation and the regulation of the surrounding light field, outputting a pulsed laser at the central point and a continuous vortex ring laser.

[0018] The addition of the beam expander and graphene realizes the tunable vortex spot ring pulsed continuous laser output required by the present invention. The beam expander expands the laser beam and forms a resonator cavity with other components to increase the light intensity. When the transmissive pulsed continuous laser modulator starts to work, a circular specified electric field is formed by applying a voltage to the liquid crystal panel through an external computer to control the circular circuit board. In this way, both the absorption and release of light by graphene and the control of the orientation of liquid crystal molecules can be utilized to generate ultrashort pulse lasers and vortex light respectively, realizing the tunable mode of vortex spot ring ultrashort pulse continuous laser output. Description of the Drawings

[0019] Figure 1 It is a structural diagram of the tunable central point pulsed vortex ring continuous laser system described in the embodiment of the present invention;

[0020] Figure 2 It is a structural diagram of the transmissive pulsed continuous laser modulator described in the embodiment of the present invention;

[0021] Figure 3 It is an overall view of the transmissive pulsed continuous laser modulator described in the embodiment of the present invention. Detailed Embodiments

[0022] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0023] Embodiment 1

[0024] This embodiment provides a tunable central point pulsed vortex ring continuous laser system. As Figure 1 shown, the system includes a 792 nm pump source 1, a coupling lens group 2, a plano-concave total reflector 3, a Tm:YLF crystal 4, a beam expander 5, and a transmissive pulsed continuous laser modulator 10.

[0025] The 792 nm pump source 1 is used to emit pump light. The coupling lens group 2 is used to shape the pump light. One side of the plano-concave total reflector 3 that receives the pump light is a plane mirror, and the side that emits the pump light is a concave mirror. The plano-concave total reflector 3, the Tm:YLF crystal 4, the beam expander 5, and the transmissive pulsed continuous laser modulator 10 form a gain cavity. The pump light is gain-amplified in the gain cavity and generates ultrashort pulsed laser and vortex light output.

[0026] The plano-concave total reflector 3 and the transmissive pulsed continuous laser modulator 10 form a laser resonator. Without the addition of the beam expander 5, the oscillation spot in the cavity is very small, resulting in a small spot area on the surface of the transmissive pulsed continuous laser modulator 10, and only a small number of liquid crystal molecules are fully utilized, making it impossible to modulate efficiently. However, with the addition of the beam expander 5 in the cavity, the area of the oscillation spot can be enlarged, thereby expanding the utilization range of the liquid crystal panel 6 and achieving the best light field modulation effect.

[0027] The coupling lens group 2 consists of a set of Keplerian collimating and focusing lens groups. Since the numerical aperture of the optical fiber used in the fiber-coupled pump source is generally about 0.22, according to the divergence angle formula, the spot diameter of the 792 nm laser will expand by 4 mm at a position 10 mm away from the output. In order to shape the laser beam, a set of Keplerian collimating and focusing lens groups is used to form a coupling system to shape the pump light, thereby improving the pump coupling efficiency and the output beam quality, and expanding the LD pump light with a core diameter of 200 μm into a circular spot with a diameter of 600 μm and incident on the Tm:YLF crystal 4.

[0028] In the set of Keplerian collimating and focusing lens groups, the focal length of one lens is 25 mm, and the focal length of the other lens is 75 mm.

[0029] In the experiment, the Tm:YLF crystal is wrapped with a metal heat sink and connected to a water chiller to dissipate heat and keep its side temperature at about 20 °C.

[0030] The transmissive pulsed continuous laser modulator 10 includes a liquid crystal panel 6, a graphene saturable absorber 8, and an annular circuit board 11. The middle of the liquid crystal panel 6 has an opening, the graphene saturable absorber 8 is placed at the opening in the middle of the liquid crystal panel 6, and the annular circuit board 11 is wound around the liquid crystal panel 6 and connected end to end. Its structure is as shown in Figure 2 shown, and the overall combined structure is as shown in Figure 3 shown.

[0031] Both the upper and lower surfaces of the liquid crystal panel 6 are flat and are both attached with PI alignment layers 7. Through rubbing alignment and drying and curing operations, the internal liquid crystals all have a pretilt angle.

[0032] On the PI alignment layer 7 on the light path output side of the liquid crystal panel 6, a film layer 9 with a transmittance of 5% for 1908 nm laser is plated. The Tm:YLF crystal 4 has a strong absorption peak at 792 nm different from the light in other bands. The lower-level particles absorb photon energy to the upper level, and this stage is called stimulated absorption. The particles in the upper level are restricted by the energy level lifetime and the external radiation field. After a certain time, they will transition to the lower level and generate coherent photons. During the transition process, a certain amount of energy will be released, which is called stimulated emission. After the action of the resonant cavity, this part of the light is amplified back and forth, and this is the laser generated after stimulated emission amplification, corresponding to the strong emission peak of the crystal itself. This laser is at 1.9 μm. Three elements are required for laser generation: a specific band excitation source, a working medium, and a resonant cavity. As long as the resonant mirror film system is selected correctly and the distance between them is within the stable region, laser can be generated.

[0033] The annular circuit board 11 is controlled by a computer. The computer controls the voltage of the annular circuit board 11 to change the alignment direction vector of the local liquid crystals in the liquid crystal panel 6, so as to achieve the effect of light field regulation.

[0034] Both the upper and lower surfaces of the liquid crystal panel 6 are flat, so it can be used as a flat total reflector. This device can be used to oscillate the gain of the light in the cavity, and at the same time, it can use the saturable absorber at the center for pulse modulation and the regulation of the surrounding light field, and emit pulsed laser at the central point and continuous vortex ring laser.

[0035] Specifically, the pulsed laser at the central point is the result of the work of the graphene saturable absorber 8 at the center. The normal laser emission is continuous, and it will become pulsed light after passing through it. Without the participation of the graphene saturable absorber 8 around, it still remains in the continuous mode state, but it will be controlled by the liquid crystal panel 6 after the application of the electric field for light field regulation, generating an ideal vortex ring, which is modulated simultaneously.

[0036] Further introduce the application principle of graphene. When the incident light is strong enough, the optical waveguide of the graphene material exhibits nonlinear characteristics; when the incident light intensity is weak, valence band electrons can absorb photons and transition to the conduction band. However, as the light intensity further increases, the conduction band is easily filled. Due to the Pauli exclusion principle, valence band electrons can no longer absorb photons. Therefore, photons can pass through graphene without loss, that is, at high light intensities, graphene has a high optical conductivity. It can be simply understood that when absorbing light intensity, it brings great losses, and no light can be output during this period. But after reaching the saturated absorption state, there is no loss of light, and the laser can pass through and be output. After the electrons of the material itself recover, it can absorb light again. This period is the time for recovering the absorption ability, called the recovery time. Therefore, by using the above nonlinear optical properties of graphene, it can be designed as a saturable absorber and used to generate ultrashort pulses to obtain an ultrashort pulse laser output. This part is a phenomenon that occurs at the initial stage of the substance and when the input light intensity is weak, where it can absorb photon energy and cause spontaneous energy level transitions inside it, with electrons transitioning from low energy levels to high energy levels. At this time, the loss of light is very large and it cannot pass through the substance. As the input light intensity increases, the upper energy level of the substance reaches the saturated state, making the substance macroscopically approach the transparent state and not absorb light. The result is that light can pass through the substance without loss. Then, due to the fact that the substance itself has a certain recovery time, it can return from the transparent state to the light-absorbing state. This recovery stage is the recovery time. Then, the interval between each pulse depends on the length of this recovery time. On a long time scale, an ultrashort pulse sequence is formed, which is what we call ultrafast light, generally in the picosecond order of magnitude or even up to the femtosecond order. This pulse form is different from the millisecond, microsecond, and nanosecond output forms of ordinary pulsed lasers.

Claims

1. A tunable center point pulse vortex halo continuous laser system, characterized in that: The system comprises a 792 nm pump source (1), a coupling lens group (2), a plano-concave total reflection mirror (3), a Tm:YLF crystal (4), a beam expander (5) and a transmission-type pulse continuous laser modulator (10); The 792nm pump source (1) is used to emit pump light, the coupling lens group (2) is used to shape the pump light, the side of the plano-concave total reflection mirror (3) that receives the pump light is a plane mirror, and the side that emits the pump light is a concave mirror, the plano-concave total reflection mirror (3), the Tm:YLF crystal (4), the beam expander (5) and the transmission-type pulse continuous laser modulator (10) form a gain cavity, the pump light is gain-amplified in the gain cavity, and ultrashort pulse laser and vortex light output are generated; The transmission-type pulse continuous laser modulator (10) comprises a liquid crystal panel (6), a graphene saturable absorber (8) and an annular circuit board (11); the liquid crystal panel (6) has an opening in the middle, the graphene saturable absorber (8) is placed at the opening in the middle of the liquid crystal panel (6), and the annular circuit board (11) is connected end to end around the liquid crystal panel (6).

2. The tunable center point pulse vortex light ring continuous laser system according to claim 1, characterized in that: The coupling lens group (2) consists of a group of Kepler collimating and focusing lenses.

3. The tunable center point pulse vortex light ring continuous laser system according to claim 2, characterized in that: In the group of Kepler collimating and focusing lenses, the focal length of one lens is 25 mm, and the focal length of the other lens is 75 mm.

4. The tunable center point pulse vortex light ring continuous laser system according to claim 3, characterized in that: The upper and lower surfaces of the liquid crystal panel (6) are both flat and are both provided with a PI orientation layer (7). Through friction orientation and drying and curing operations, the liquid crystals inside are all given a pre-tilt angle.

5. The tunable center point pulse vortex light ring continuous laser system according to claim 4, characterized in that: The liquid crystal panel (6) is plated with a film layer (9) having a transmittance of 5% for 1908 nm laser light on the PI orientation layer (7) on the light path output side.

6. The tunable center point pulse vortex light ring continuous laser system according to claim 5, characterized in that: The annular circuit board (11) is controlled by a computer, and the computer controls the voltage of the annular circuit board (11) to change the arrangement director of the localized liquid crystal in the liquid crystal panel (6), thereby achieving the effect of light field regulation.

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