Hybrid cavity-based slab regenerative amplifier and amplification method

Abstract

The application discloses a kind of based on wedge structure's ultrafast pulse hybrid cavity slab regenerative amplifier, including femtosecond seed laser, stretching unit, shaping unit, laser isolation unit, pulse selection unit, wedge slab regenerative amplification unit, compression unit;The shaping unit is used to shape seed in x, y direction, so that seed realizes round trip oscillation in hybrid cavity;The wedge slab regenerative amplification unit is constituted by trapezoidal slab crystal and two plane mirrors, which can make seed laser realize high-density folding arrangement inside slab crystal, while realizing self-folding return;The pulse selection unit is used to select frequency to seed, while the selected single pulse is limited in wedge slab regenerative amplification unit, to realize the regenerative amplification of seed.Compared with the traditional stable cavity regenerative amplifier, the hybrid cavity slab regenerative amplifier of the application can provide larger fundamental mode volume and good heat dissipation performance, and is more conducive to realizing high-power, high-beam-quality ultrafast pulse laser output.

Description

Technical Field

[0001] This invention belongs to the field of all-solid-state laser technology, and specifically relates to a slab regenerative amplifier based on a hybrid cavity and an amplification method thereon. Background Technology

[0002] In 2022, the Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, was granted a patent related to a thin-film regenerative amplifier, patent number CN115347449B. This invention mainly includes a thin-film crystal, a first concave mirror, and a second concave mirror. Its key feature is a special optical path design within the amplifier, which allows the seed laser to pass through the thin-film crystal a significantly greater number of times in a single round trip than a conventional seed laser, thereby significantly improving the gain for small and medium-sized signals.

[0003] However, the reason this patent pursues more amplification cycles is that it is difficult for a thin-film regenerative amplifier operating in a stable cavity to achieve a large eigenmode of several millimeters on a thin crystal. This results in very low single-pass gain, requiring an increase in the number of passes through the crystal to improve the gain. To directly achieve an eigenmode of several millimeters on a thin crystal, the entire optical path of the regenerative cavity would need to be extended to tens of meters, making the entire system extremely complex.

[0004] As is well known, in a compact design, an unstable cavity can provide a larger fundamental mode volume than a stable cavity, thereby achieving higher laser power. However, a crucial design principle for regenerative amplifiers is that the seed laser mode must match the eigenmode of the stable regenerative cavity. This ensures that the seed spot size remains constant during multiple round-trip oscillations within the cavity until the pulse energy reaches its maximum value. For regenerative amplifiers, the effective output comes from the continuously propagating and amplified laser within the stable cavity. Therefore, any beam leakage from the cavity reduces the output efficiency. Consequently, the use of unstable cavities for regenerative amplification is rarely considered due to unavoidable walk-out losses. In conclusion, achieving a larger fundamental mode volume within a regenerative amplifier remains a problem that needs to be solved by current technology. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention proposes a hybrid cavity slab regenerative amplifier based on a wedge structure. Compared to the stable cavity regenerative amplifier in the aforementioned patents, the hybrid cavity used in this invention can provide a larger intrinsic mode, thereby providing a larger gain volume and achieving the goal of increasing the output power of the regenerative amplifier. Furthermore, compared to the wedge amplifier structure used in the aforementioned patents, this solution can achieve a high-density optical path arrangement in a more compact and simpler structure, significantly increasing the number of passes through the slab crystal in a single pass, obtaining sufficient gain volume coverage, simplifying the structure while significantly improving the single-pass gain. In summary, this invention can solve the technical problems of small fundamental mode volume, low single-pass gain, and complex structure in thin-film regenerative amplifiers.

[0006] To solve the above problems, the technical solution of the present invention is as follows:

[0007] A slab regenerative amplifier based on a hybrid cavity is characterized in that it includes a femtosecond laser seed source, wherein the femtosecond laser seed source outputs a femtosecond seed laser that passes sequentially through a broadening unit, a shaping unit, a laser isolation unit and a pulse selection unit, and is incident on a wedge-shaped slab regenerative amplifier unit.

[0008] The stretching unit includes two parallel transmission gratings, one of which is fixed on a displacement platform to achieve positional adjustability, and is used to introduce quantitative dispersion to stretch the femtosecond seed laser to picosecond in the time domain.

[0009] The shaping unit is used to shape the femtosecond seed laser in the horizontal and vertical directions, so that the seed laser can oscillate back and forth in both the horizontal and vertical directions.

[0010] The laser isolation unit is used to rotate the polarization direction of the femtosecond seed laser by 90° to realize the injection and output of the seed laser;

[0011] The pulse selection unit is used to down-frequency the femtosecond seed laser and confine the selected single pulse within the wedge-shaped strip regeneration amplification unit to achieve regeneration amplification of reciprocating oscillations.

[0012] The wedge-shaped slab regeneration amplifier unit includes a slab crystal, a dichroic mirror module, and a pump module. The slab crystal is trapezoidal in shape and is placed inside the dichroic mirror module. The dichroic mirror module includes a first plane mirror and a second plane mirror. The first plane mirror and the second plane mirror have an included angle of 2β and are symmetrically placed on the left and right sides of the slab crystal at angles of -β and +β, respectively. The two plane mirrors and the left and right incident light end faces of the slab crystal form two wedge-shaped included angles, which together constitute a wedge-shaped regeneration amplifier structure.

[0013] The compression unit is used to compress the energy-amplified chirped pulse back to the same pulse width as the seed pulse, and adopts a negative dispersion system with a dispersion amount equivalent to but opposite in sign to that of the broadening unit.

[0014] Furthermore, the wedge-shaped slab regeneration amplification unit includes a slab crystal, a dual-color mirror module, and a pump module; the wedge-shaped slab regeneration amplification unit is characterized in that it operates in a hybrid cavity mode. Compared with the stable cavity of the prior art, the hybrid cavity of the present invention can significantly increase the fundamental mode volume and provide a larger gain volume, which is beneficial to achieving ultrafast pulsed lasers with higher output power.

[0015] Furthermore, the lath crystal, which is trapezoidal in shape, is placed inside the dichroic reflector module. Both the top and bottom faces of the lath crystal are isosceles trapezoids with the same γ-base angle, ensuring that the left and right incident light faces are not parallel, effectively suppressing ASE (associated photoreflection) and self-oscillation. In addition to the anti-reflection coatings for seed and pump light on the left and right incident light faces (width × height), the top and bottom faces (length × width) and the side faces (length × height) are roughened to eliminate specular reflection between the parallel surfaces of the lath crystal, further suppressing ASE and self-oscillation.

[0016] Furthermore, the dual-color reflector module includes a first plane reflector and a second plane reflector. The first plane reflector and the second plane reflector have an included angle of 2β and are symmetrically placed on the left and right sides of the slab crystal at angles of -β and +β, respectively. The two plane reflectors form two wedge-shaped included angles with the left and right light-incident end faces of the slab crystal, which together constitute a wedge-shaped slab regeneration and amplification unit.

[0017] Furthermore, the laser shaping unit includes a horizontal cylindrical lens and a vertical cylindrical lens, used to shape the seed laser into a collimated beam with a small divergence angle in the horizontal unstable cavity direction (x-direction), so that the size of the light spot does not change much after long-distance transmission in the x-direction, ensuring the round-trip oscillation of the seed laser in the unstable cavity direction. In the vertical stable cavity direction (y-direction), the thermal lensing effect of the slab crystal is used to achieve mode self-reproduction of the light spot. The beam waist of the shaped light spot in the y-direction is located at the plane mirror in the pulse selection unit, and its size is consistent with the fundamental mode size in the y-direction of the slab amplifier, achieving good mode matching while realizing mode self-reproduction in the y-direction.

[0018] Furthermore, when the seed laser is injected into the slab crystal, it should be incident from the narrower upper base edge of the trapezoidal slab crystal, and the incident angle θ should be controlled between 12-14°. With the same trapezoidal slab crystal base angle γ, the seed laser will fold multiple times between the first and second plane mirrors. Each time it is folded and reflected, its incident angle relative to the slab crystal will change by θ. i+1 =θ iThe velocity of -2[n-(90-γ+β)] gradually decreases, gradually forming a high-density folded arrangement within the slab crystal. This greatly improves the overlap efficiency between the pump light and the signal, significantly increasing the single-pass extraction efficiency of the seed for pump energy, thus solving the problem of low single-pass gain in the aforementioned patent. Furthermore, when the seed incident angle first decreases to a negative value, the seed laser will be folded back inside the amplifier and continue to propagate in the reverse direction along the original path. Combined with the pulse selection unit, this enables hybrid cavity slab regeneration amplification of the seed laser. Compared to the complex multi-mirror folded cavity of the aforementioned patent, this solution greatly simplifies the amplifier structure, making its operation more stable.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0020] Compared to the stable cavity thin-plate regenerative amplifier in the aforementioned patents, the slab regenerative amplifier in this invention operates in a hybrid cavity mode, which can significantly increase the fundamental mode volume and provide a larger gain volume, thereby increasing the amplifier's output power. Furthermore, this invention employs a special wedge-shaped amplifier structure, which enables high-density folding and arrangement of the seed laser within the amplifier, significantly improving the overlap efficiency between the laser and pump light, while also increasing the number of times the laser passes through the gain medium in a single pass, thus increasing the amplifier's single-pass gain. In this embodiment, the total number of round-trip folds reaches 16, as shown in Figure 3(a). Simultaneously, the wedge-shaped amplifier structure allows the seed to achieve optical path reversal within the amplifier; in this embodiment, the seed achieves reversal at the 8th reflection, as shown in Figure 3(a). Compared to the aforementioned patents, this invention reduces the number of reflectors used, greatly simplifying the amplifier structure and allowing the laser to operate in a more stable state.

[0021] In summary, this invention combines seed laser shaping, a wedge-shaped hybrid cavity slab amplifier, and pulse selection to achieve slab crystal regenerative amplification operating in hybrid cavity mode. It provides a solution to the technical problems of small fundamental mode size, low single-pass gain, and complex structure in regenerative amplifiers. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of an embodiment of the slab regenerative amplifier based on a hybrid cavity according to the present invention;

[0023] In the diagram: 1 - Femtosecond seed laser; 2 - Stretching unit; 3 - Shaping unit; 4 - Laser isolation unit; 5 - Pulse selection unit; 6 - Wedge-shaped slab regeneration and amplification unit; 7 - Compression unit; 8 - Amplified femtosecond laser.

[0024] Figure 2 This is a schematic diagram showing the location of each unit in an embodiment of the present invention.

[0025] Figure 3(a) is a detailed structural diagram of each unit in an embodiment of the present invention;

[0026] 1-Seed laser; 21-1, 21-2-Parallel transmission gratings; 101-45° mirror; 31-1, 31-2-Horizontal cylindrical lenses; 32-Vertical cylindrical lens; 41-1, 41-2-First and second thin-film polarizers; 42-Faraday rotator; 102-Half-wave plate; 51-Thin-film polarizer; 52-Pockell cell; 53-Quarter-wave plate; 54-Plane mirror; 61-Trapezoidal slab crystal; 62-1, 62-2-Two-color mirror modules; 63-Pump module; 71-1, 71-2-Parallel transmission gratings; 8-Magnified ultrafast pulsed laser.

[0027] Figure 3(b) is a schematic diagram of the angle within the enlarged wedge-shaped slat regeneration unit in this embodiment. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0029] Please see Figure 1 , Figure 1 This is a schematic diagram of an embodiment of the hybrid cavity-based slab regenerative amplifier of the present invention. As shown in the figure, it comprises a femtosecond seed laser 1, a stretching unit 2, a shaping unit 3, a laser isolation unit 4, a pulse selection unit 5, a wedge-shaped slab regenerative amplification unit 6, and a compression unit 7. For the location of each unit and a detailed structural diagram, please refer to [reference needed]. Figure 2 As shown in Figure 3.

[0030] The stretching unit 2 includes two parallel transmission gratings 21-1 and 21-2, one of which is fixed on a displacement platform to achieve positional adjustability. This is used to introduce a quantitative positive dispersion to stretch the femtosecond seed laser 1 to picoseconds in the time domain, thereby reducing the peak power of the femtosecond pulse and minimizing the risk of damage to optical devices.

[0031] The shaping unit 3 includes horizontal cylindrical lenses 31-1 and 31-2 and a vertical cylindrical lens 32, arranged sequentially along the light transmission direction, for shaping the seed laser in the horizontal (x-direction) and vertical (y-direction) directions, respectively. The horizontal cylindrical lenses 31-1 and 31-2 shape the seed laser in the horizontal unstable cavity direction (x-direction) into a collimated beam with a small divergence angle, ensuring that the beam spot size does not change significantly after long-distance transmission in the x-direction, thus guaranteeing that the seed laser can achieve reciprocating oscillation in the unstable cavity direction. In this embodiment, the shaped x-spot is ω. in-x=4mm. In the vertical cavity direction (y-direction), to utilize the thermal lensing effect of the lath crystal to achieve pattern self-reproduction of the light spot, the beam waist, shaped by the vertical cylindrical lens 32, is located at the plane mirror 54 in the pulse selection unit 5, with a beam waist size of ω. in-y =0.3mm is consistent with the fundamental mode size in the y direction of the slab amplifier, so as to achieve good mode matching while ensuring that the size of the light spot on the surface of the plane mirrors 62-1 and 62-2 remains basically unchanged each time it is folded in the y direction.

[0032] The laser isolation unit 4 consists of a first thin-film polarizer 41-1, a Faraday rotator 42, and a second thin-film polarizer 41-2. As shown in Figure 3(a), the horizontally polarized laser emitted from the seed light source 1 passes through polarizer 41-2, is rotated 90° by half-wave plate 102, and then enters the pulse selection unit 5. The pulse isolation unit 5 can isolate reverse laser pulses to prevent them from damaging the seed laser 1, and is also used to rotate the polarization direction of the seed laser by 90° to realize the injection and output of the seed laser.

[0033] The pulse selection unit 5 consists of a second thin-film polarizer 51, a Pockel cell 52, a quarter-wave plate 53, and a plane mirror 54. By controlling the pressurization frequency of the Pockel cell 52, the seed laser can be frequency-selected. The selected single pulse will be confined within the laser regeneration amplification unit 6 to achieve regeneration amplification through reciprocating oscillation. Controlling the single pressurization time of the Pockel cell 52 can control the number of reciprocating oscillations of the seed within the regeneration amplification unit 6. When the seed laser power reaches its maximum value, the seed can be extracted from the cavity.

[0034] The wedge-shaped slab regeneration amplification unit 6 includes a slab crystal 61, two-color mirror modules 62-1 and 62-2, and a pump module 63. The slab crystal is trapezoidal and placed inside the two-color mirror module. The key feature is that the upper and lower large faces of the slab crystal are isosceles trapezoids with the same base angle γ, making the left and right incident light faces non-parallel, effectively suppressing ASE (associated lasing) and self-oscillation. In this embodiment, γ = 89°. In addition to the left and right incident light faces (width × height) being coated with anti-reflection films for seed and pump light, the upper and lower large faces (length × width) and side faces (length × height) are roughened to eliminate specular reflection between the parallel surfaces of the slab crystal, further suppressing ASE and self-oscillation. The two-color mirror module includes a first plane mirror 62-1 and a second plane mirror 62-2, both coated with a high-reflection film for seed laser and an anti-reflection film for pump light. The two plane mirrors have an included angle of 2β and are symmetrically placed on the slab crystal at angles of -β and +β, respectively. At 5mm on both sides of the slab crystal 61, in this embodiment, β=±2°, the two plane mirrors and the left and right light-incident end faces of the slab crystal form two wedge-shaped angles (the wedge angles are 90-γ+β=3°). The four planes are not parallel to each other and together form a wedge-shaped regenerative amplifier structure. The pump module 63 is placed parallel to the outside of the second plane mirror 61-02. The pump light is shaped and focused by the pump module 63 and then passes through the second plane mirror 61-2 to form a uniform rectangular pump area on the right light-incident end face of the crystal 61.

[0035] As shown in Figure 3(b), the seed laser 1 is injected into the wedge-shaped regeneration amplification unit 6. It is to be incident from the narrower upper bottom edge of the trapezoidal slab crystal 61. In this embodiment, the initial incident angle θ1 = 12.9°, combined with the trapezoidal slab crystal with a bottom angle γ = 89°, the seed laser will fold multiple times between the first plane mirror 62-1 and the second plane mirror 62-2. Each time it is folded and reflected, its incident angle relative to the slab crystal will change by θ. i+1 =θ i The velocity of -2[n-(90-γ+β)] gradually decreases, gradually forming a high-density folded arrangement, which greatly improves the overlap efficiency between the pump light and the signal. When the incident angle first decreases to a negative value, the seed laser will be folded back inside the amplifier and continue to propagate in the opposite direction along the original path, realizing a complete round-trip oscillation. In this embodiment, the slab crystal uses Nd:YVO4 with a refractive index of n=2.16, and the total number of folds achieved in the round trip is as high as 16, with internal folding achieved on the 8th fold.

[0036] The compression unit 7 includes two parallel transmission gratings 71-1 and 71-2, one of which is fixed on a displacement platform to achieve positional adjustment. It is used to introduce a quantitative negative dispersion to compress the amplified chirped pulse back to the same pulse width as the seed in the time domain, and finally obtain a high-power femtosecond pulse laser 8.

[0037] In summary, this invention proposes an ultrafast pulse hybrid cavity slab regenerative amplifier based on a wedge-shaped structure. Compared to traditional stable cavity regenerative amplifiers, the hybrid cavity of this scheme can provide a larger gain volume, which is more conducive to realizing high-power ultrafast pulse laser output. Furthermore, the special wedge-shaped amplifier structure can achieve high-density folding arrangement within the slab crystal, greatly improving the overlap efficiency of the signal light and pump light, significantly improving pump energy utilization while effectively suppressing ASE (associated oscillation) in the pump "dead zone." Simultaneously, this structure allows the optical path to be folded back within the amplifier and continue propagating along the original path. Combined with a pulse selection unit and a special laser shaping unit, reciprocating oscillation slab regenerative amplification can be achieved in a more compact and simple structure. Moreover, this invention can match the amplifier's fundamental mode with the seed laser mode by adjusting the distance between the reflector and the slab crystal. Adjusting the angle of the reflector allows for flexible control of the beam arrangement and folding number within the amplifier.

[0038] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A hybrid-cavity-based stripline regenerative amplifier, characterized by, The femtosecond laser seed source outputs femtosecond seed laser which sequentially passes through a stretching unit, a shaping unit, a laser isolation unit and a pulse selection unit, is incident to a wedge-shaped slab regenerative amplifier unit for amplification, and is input to a compression unit; The stretching unit includes two mutually parallel transmission gratings, one of which is fixed on a displacement platform to achieve adjustable position, for introducing a quantitative dispersion to stretch the femtosecond seed laser to picoseconds in time domain; The shaping unit is used for shaping the femtosecond seed laser in horizontal and vertical directions, so that the seed laser can realize back-and-forth oscillation in horizontal and vertical directions. The laser isolation unit is used for rotating the polarization direction of the femtosecond seed laser by 90° to realize injection and output of the seed laser. The pulse selection unit is used for reducing the frequency of the femtosecond seed laser, and limiting the selected single pulse in the wedge-shaped slab regenerative amplifier unit to realize regenerative amplification of back-and-forth oscillation. The wedge-shaped slab regenerative amplifier unit includes a slab crystal, a double-color mirror module and a pumping module, the slab crystal is in the shape of a trapezoidal slab and is placed inside the double-color mirror module; the double-color mirror module includes a first plane mirror and a second plane mirror, the first plane mirror and the second plane mirror have an included angle of 2β and are symmetrically placed at the left and right sides of the slab crystal at angles of -β and +β respectively, and form two wedge-shaped angles with the left and right input end faces of the slab crystal to jointly constitute a wedge-shaped regenerative amplifier structure. The compression unit is used for compressing the chirp pulse after energy amplification to the same pulse width as the seed pulse, and adopts a negative dispersion system with a dispersion amount corresponding to the stretching unit and opposite in sign.

2. The hybrid-cavity-based slab regenerative amplifier of claim 1, wherein, The laser shaping unit includes a horizontal cylindrical lens and a vertical cylindrical lens, which are used for shaping the seed laser into a collimated light beam with a small divergence angle in the horizontal unstable cavity direction, so that the spot size changes little after long-distance transmission in the horizontal unstable cavity direction, and ensures the back-and-forth oscillation of the seed laser in the unstable cavity direction; the thermal lens effect of the slab crystal is used to realize mode self-reproduction in the vertical stable cavity direction, and the waist of the shaped spot in the vertical stable cavity direction is located at the plane mirror in the pulse selection unit, and the size is consistent with the size of the fundamental mode in the vertical stable cavity direction of the slab amplifier, so as to realize good mode matching and mode self-reproduction in the vertical stable cavity direction.

3. The hybrid-cavity-based slab regenerative amplifier of claim 1, wherein, The upper and lower large faces of the slab crystal are isosceles trapezoids with the same γ base angle, so that the left and right input end faces are not parallel to each other, effectively suppressing ASE and self-oscillation; the left and right input end faces are coated with an antireflection film for seed and pump light, and the upper and lower large faces and the side faces are roughened to eliminate mirror reflection between mutually parallel surfaces of the slab crystal, further suppressing ASE and self-oscillation.

4. The hybrid-cavity-based slab regenerative amplifier of claim 3, wherein, The seed laser is injected into the slab crystal from the narrow upper base of the trapezoidal slab crystal, and the incident angle θ is controlled between 12-14°, matched with the trapezoidal slab crystal with the same γ base angle, the seed laser will be folded multiple times between the first and second plane mirrors, and the incident angle of the seed laser relative to the slab crystal will gradually decrease by θ i+1 = θ i -2[n-(90-γ+β)] each time the seed laser is reflected, gradually forming a high-density folded arrangement, until the incident angle is first reduced to a negative value, the seed laser will realize backward folding inside the amplifier, continue to propagate in the original path in the reverse direction, and realize a complete round-trip oscillation.

5. The hybrid-cavity-based slab regenerative amplifier of claim 1, wherein, The first and second plane mirrors are both coated with high reflection film for seed laser and anti-reflection film for pump light, and are symmetrically arranged at a distance of 5-15 mm from the left and right sides of the slab crystal at angles of -β1 and +β1, and the β angle of the single-side mirror ranges from 0.5 to 2.5°, the distance of the mirror is adjusted according to the size of the crystal thermal lens, the matching between the fundamental mode of the amplifier and the seed laser mode is realized, and the arrangement mode and the folding number of the light beam in the amplifier can be flexibly and variably controlled by adjusting the angle.

6. The hybrid-cavity-based slab regenerative amplifier of claim 1, wherein, The pump module is arranged in parallel outside the second plane mirror, the pump light is shaped and focused by the pump module, and a uniform rectangular pump region is formed on the right light-in end face of the crystal after the pump light passes through the second plane mirror.

7. The hybrid-cavity-based slab regenerative amplifier of claim 1, wherein, The stretching unit and the compression unit each include two mutually parallel transmission or reflection gratings, and one of the gratings is fixed on a displacement platform to be position-adjustable.

8. The hybrid-cavity-based slab regenerative amplifier of claim 1, wherein, The slab crystal material is Yb:YAG, Nd:YVO4, Nd:YAG, Nd:GdVO4 or Tm:YLF.

Images