Pulse stretcher

Abstract

The invention provides a stretcher which can be used for a femtosecond laser amplifier. The stretcher can realize large spreading amount of optical pulse. In one embodiment, the pulse stretcher can comprise a block-shaped element, the block-shaped element comprises an incident surface and an emergent surface, laser enters the block-shaped element through the incident surface and is transmitted out through the emergent surface after being totally internally reflected for multiple times in the block-shaped element, and therefore the stretching effect is achieved. The stretcher provided by the invention is stable in structure, and is beneficial to realizing a large spreading amount in a limited space.

Description

Technical Field

[0001] The present application generally relates to the technical field of laser chirped pulse amplification, and in particular to a laser pulse stretcher. Background Art

[0002] Femtosecond laser chirped pulse amplification (CPA) is a technology that combines laser pulse width control and laser energy amplification to achieve ultrafast and ultra-strong laser output. Based on this technology, peak powers of 10PW (1PW = 10 15 W) or more, has greatly promoted research and discovery in fields such as laser accelerators, free electron lasers, and laboratory astrophysics. The principle of ultrafast laser chirped pulse amplification is: the femtosecond laser oscillator is mode-locked to produce a femtosecond seed pulse of relatively weak intensity. Before the seed light of the femtosecond laser enters the optical amplifier, the laser pulse is first time-stretched, and then the stretched chirped pulse is introduced into the optical amplifier for energy amplification. This process will increase the pulse energy by several orders of magnitude. Finally, a pulse compressor is used to compress the laser pulse stretched on the time scale to obtain a high-energy femtosecond laser pulse. This method can avoid damage to optical devices caused by directly amplifying the energy of femtosecond pulses, and thus can obtain higher pulse energy and relatively good spatial and temporal beam quality. One of the key steps of chirped pulse amplification technology is the broadening of the laser pulse, which is achieved by controlling the temporal dispersion of the femtosecond pulse. The use of a dispersion device to make the optical path of light of different wavelengths in the pulse different can achieve changes in the width of the optical pulse.

[0003] The stretchers currently used include Martinez stretcher and Offner stretcher, which mainly use gratings and curved spatial light transmission structures to stretch the pulse in time. The problem with this technology is that the femtosecond amplifier containing multiple modules is large in size, especially in the pulse stretcher part. In addition, the problem with this stretching method is that the optical path transmission distance is very long, making it susceptible to vibration or thermal drift, and large-scale optical components are required to support the separation of the pulse spectrum in space. At the same time, the broadening of the wide spectrum seed light places high demands on the processing requirements and coating process of the optical components. These factors limit the application of femtosecond amplifiers. Summary of the invention

[0004] In view of the above problems, the present application is proposed. The embodiments of the present application provide a pulse stretcher that can be used in a femtosecond laser amplifier, which can achieve a large stretching amount of an optical pulse.

[0005] According to one aspect of the present application, a stretcher is provided, characterized in that it includes a block element, the block element includes an incident surface and an exit surface, laser enters the block element through the incident surface, and is transmitted through the exit surface after multiple total internal reflections in the block element, thereby achieving a stretching effect.

[0006] In some embodiments, the block element is in the shape of a rectangular block with diagonals cut off vertically, and the surfaces formed by cutting off the diagonals serve as the incident surface and the exit surface respectively.

[0007] In some embodiments, the incident surface and the exit surface of the block element are coated with a high-transmittance film, and / or the surface where the light beam is totally reflected inside the block element is coated with a high-reflection film.

[0008] In some embodiments, the pulse stretcher further comprises a reflecting mirror arranged along the propagation direction of the laser.

[0009] In some embodiments, along the laser propagation direction, the reflectors include a first group of reflectors arranged in front of the block element and a second group of reflectors arranged behind the block element, wherein the first group of reflectors is used to adjust the laser incident optical path, and the second group of reflectors is used to adjust the laser output optical path.

[0010] In some embodiments, the pulse stretcher further includes a first wedge angle, wherein one of the cut sides of the rectangular parallelepiped block coincides with a side of the first wedge angle.

[0011] In some embodiments, the pulse stretcher further includes a second wedge angle, which is disposed at the other of the cut side surfaces of the rectangular block and partially overlaps with the cut side surface, and a length of the overlapping portion in the vertical direction is half of the height of the rectangular block.

[0012] In some embodiments, the first wedge angle and the second wedge angle are triangular prisms, the end faces of the triangular prisms are shaped like isosceles right triangles, and the height of the rectangular block is equal to the length of the base of the isosceles right triangle.

[0013] In some embodiments, the pulse stretcher includes a plurality of block-shaped elements overlapped in a vertical direction and a corresponding number of first wedge angles and second wedge angles.

[0014] According to another aspect of the present application, a laser amplification device is also provided, comprising the stretcher described above.

[0015] Based on some embodiments, the stretcher of the present application is intended to achieve wide spectrum femtosecond pulse stretching, by making the laser vertically incident on the incident surface of the bulk material element, achieving multiple total internal reflections of the laser pulse in the bulk material, and then achieving a large stretching amount of the light pulse. Compared with the existing grating stretcher (for example, Martinez stretcher) or multi-pass cavity (MPC) bulk material stretcher, the pulse stretcher of the embodiment of the present application can avoid the problems of complex optical path adjustment, truncation of the spectrum bandwidth after stretching, etc. The stretcher based on total internal reflection has a compact structure, a simple optical path, high stability, and no complex adjustment is required. It can stretch the pulse to the order of tens of ps, which is very suitable for the stretching of wide spectrum short pulse width seeds. In addition, in some embodiments of the present application, the light beam can be propagated in multiple layers in a stretcher composed of multiple block materials, effectively reducing the volume of the stretcher. Based on the overlap design structure, the number of overlap layers can be selected as needed to adjust the optical path and the stretching effect.

[0016] The above and other features and advantages of the present invention will become apparent from the following description of exemplary embodiments in conjunction with the accompanying drawings. It should be understood that the exemplary embodiments do not necessarily achieve all of these advantages. Therefore, the present invention can be embodied or implemented in a manner that achieves or optimizes one advantage or a group of advantages as taught herein, without having to achieve other advantages as taught or illustrated herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The following describes the embodiments of the present invention in detail in conjunction with the accompanying drawings. It is worth mentioning that the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative work. Among them:

[0018] Figure 1 A schematic structural diagram of a pulse stretcher provided in an embodiment of the present application is shown.

[0019] Figure 2 A schematic structural diagram of a pulse stretcher provided in another embodiment of the present application is shown.

[0020] Figure 3 A schematic structural diagram of a pulse stretcher in overlapping form provided in one embodiment of the present application is shown.

[0021] Figure 4 A schematic diagram showing the calculation of the dispersion introduced by a wide spectrum pulse provided in an embodiment of the present application is shown.

[0022] Figure 5 A schematic diagram of pulse width calculation before and after stretching provided in an embodiment of the present application is shown.

[0023] Figure 6 A schematic diagram of the spectrum before and after broadening measured by a spectrometer provided in an embodiment of the present application is shown. DETAILED DESCRIPTION

[0024] Exemplary embodiments of the present invention are described below with reference to the accompanying drawings. In the accompanying drawings, the same reference numerals usually represent the same parts. It should be understood that the size and dimensions of the parts shown in the accompanying drawings are not necessarily drawn to scale, and they may be different from the embodiments shown here for implementation. In addition, some embodiments may combine any suitable combination of features from two or more accompanying drawings.

[0025] Figure 1 A schematic diagram of the structure of a total internal reflection block material stretcher according to an embodiment of the present application is shown. As shown in the figure, the main part of the pulse stretcher is a block element 10, which includes an incident surface 12 and an exit surface 14. The laser can enter the block element 10 through the incident surface 12, and transmit through the exit surface 14 after multiple total internal reflections in the block element 10, thereby achieving a stretching effect. Although the side 12 of the block element 10 is used as the incident surface and the side 14 is used as the exit surface in the figure, it is only an example and not a limitation. In some embodiments, the side 14 can also be used as the incident surface and the side 12 can be used as the exit surface. In addition, as described in detail below, in some embodiments, the incident surface 12 can also serve as the exit surface, that is, a part of the side serves as the incident surface of the light beam, and another part serves as the exit surface of the light beam on the subsequent propagation path. At the same time, the exit surface 14 can also serve as the incident surface, that is, a part of the side serves as the exit surface of the light beam, and another part serves as the incident surface of the light beam on the subsequent propagation path.

[0026] In one embodiment, the material of the block element 10 may be optical glass, such as SF57. For example, the block element may be formed by cutting a single block material. In this example, a portion of the diagonal vertical cut of the rectangular block material is cut off, and the surfaces formed by cutting off the diagonal are used as the incident surface 12 and the exit surface 14 respectively. The cut portion is a prism with a cross section of an isosceles right triangle, and can be cut to obtain a specific size. Figure 1 The overall size of the block element 10 is shown to be 72.8 mm×52 mm×20 mm. In other embodiments, the appropriate size may be determined as required, and the present application does not impose any specific limitation on this.

[0027] In order to reduce the loss of the laser beam, in one embodiment, an anti-reflection or high-transmittance film for a light beam in the wavelength band of 650-1100 nm, such as a silicon nitride layer (the transmittance T is preferably greater than 99.5%), may be coated on the incident surface 12 and the exit surface 14. The side surface of the block material 10 may be polished so that the light beam incident on the block material 10 at a specific angle may be totally internally reflected at the boundary between the block material and the air. In some embodiments, a high-reflection film (for example, a high-reflection film layer such as a gold-plated layer) may be coated on the surface where the light beam is totally reflected inside the block material, thereby further reducing the loss of the light beam.

[0028] In one example, in order to facilitate the adjustment of the optical path, one or more reflectors may be arranged along the propagation direction of the laser. For example, along the propagation direction of the laser beam, the reflectors may include a first group of reflectors arranged in front of the block element, and / or a second group of reflectors arranged behind the block element, wherein the first group of reflectors is used to adjust the incident optical path of the laser, and the second group of reflectors is used to adjust the exit optical path of the laser.

[0029] In one specific embodiment, see Figure 1 A first silver mirror 21 and a second silver mirror 22 are arranged on the optical path before the bulk glass material 10. The two silver mirrors are arranged in parallel for adjusting the input laser optical path, that is, the input light beam can be guided to enter the interior of the bulk material 10. The light beam undergoes multiple total reflections in the bulk glass material to achieve a large width expansion of the input laser pulse. A third silver mirror 23 and a fourth silver mirror 24 are arranged in sequence on the optical path after the bulk glass material 10. The two silver mirrors are also arranged in parallel for adjusting the output laser optical path after expansion, ensuring that the incident light and the outgoing light are on the same axis, thereby not affecting the overall optical path.

[0030] Preferably, the first silver mirror 21 and the second silver mirror 22 are both plane mirrors with a diameter of 1 inch, the laser beam is incident on the surface at an angle of 45°, the reflection bandwidth is 650-1100nm, and the reflectivity is preferably greater than 98%, and the second-order dispersion (also called group delay dispersion GDD) provided in the wavelength range of 750-1000nm is 0±10fs 2 The optical path of the incident laser pulse can be collimated by two silver mirrors so that the laser is adjusted to be perpendicularly incident on the incident surface 12 of the single block material.

[0031] Preferably, the third silver mirror 23 and the fourth silver mirror 24 can also be plane mirrors with a diameter of 1 inch. After the laser beam is emitted from the exit surface 14 of the bulk material 10, it is incident on the third silver mirror and the fourth silver mirror at an angle of 45°. The reflection bandwidth is 650-1100nm, and the reflectivity is preferably greater than 98%. The second-order dispersion provided in the wavelength range of 750-1000nm is 0±10fs. 2 The outgoing laser can be adjusted to the same axis as the incident laser through two silver mirrors.

[0032] for Figure 1 In the example shown, the incident laser passes through the first silver mirror 21 and is reflected to the second silver mirror 22, and then reflected to the incident surface 12 of the monolithic block material 10. The laser is vertically transmitted into the monolithic block material, and 10 total internal reflections occur at the boundary between the block material and the air, and the total optical path is 500 mm. Then, the laser is transmitted from the exit surface 14 to the third silver mirror 23, and then reflected to the fourth silver mirror 24, thereby achieving the broadening of the femtosecond pulse. It can be seen that the pulse stretcher according to this embodiment can achieve total internal reflection of the laser, and thus achieve a large amount of broadening in a limited space. Compared with the existing grating stretcher or MPC block material stretcher, the advantages of this embodiment are: the number of optical elements of the entire stretcher is small, which is conducive to improving the overall stability, and due to the total internal reflection in the block material, the laser spectrum bandwidth loss is small, and less space is occupied, which is conducive to modular design, and therefore it is very suitable for broadening wide spectral bandwidth pulses.

[0033] Figure 2 FIG. 1 shows a schematic diagram of the structure of a pulse stretcher provided by another embodiment of the present invention, which mainly includes a block element 100 and a pulse stretcher 100 of FIG. 1 . Figure 1 Although not shown, in some embodiments, the block element 10 may also be as shown in FIG. Figure 1 The illustrated method arranges a plurality of reflectors on the optical path so that the incident light and the outgoing light are on the same axis.

[0034] like Figure 2 As shown, the pulse stretcher further includes a wedge angle 200 (also referred to herein as a first wedge angle), the first wedge angle 200 is located at one of the side surfaces 102 and 104 (shown as 104) formed after the bulk element 100 is cut, and one side surface of the wedge angle 200 coincides with the side surface 104 of the bulk element 100. In this figure, as described in detail below, parts of the side surfaces 102 and 104 serve as the incident surface or the exit surface of the bulk element 100, respectively, and parts of the side surfaces serve as the exit surface and the incident surface, respectively. It can be understood that in some embodiments, the wedge angle 200 can also be located at the side surface 102 and coincide with the side surface 102.

[0035] In one example, the wedge angle 200 can be made of the same material as the block element 100 (e.g., SF57), so the two can be processed into a whole optical element. Figure 2 As shown, the wedge angle 200 and the block element 100 can also be processed and formed separately and combined into one by bonding or the like. In this case, a high-transmittance film layer can be plated on the overlapping sides of the two.

[0036] In one example, if Figure 2As shown, the wedge angle 200 has a triangular prism shape, the end face of the triangular prism is an isosceles right triangle, and the height of the rectangular parallelepiped block 100 is equal to the length of the base (hypotenuse) of the isosceles right triangle. Among the three side faces of the triangular prism, the side face corresponding to the base of the isosceles right triangle coincides with the side face 104 of the block 100, and the two side faces corresponding to the two right-angled sides of the isosceles triangle serve as the internal reflection surface of the light beam. In one embodiment, the two side faces may be coated with a high reflection film to reduce the loss of the light beam.

[0037] Continue to refer to Figure 2 In one example, the pulse stretcher may further include a wedge angle 300 (also referred to herein as a second wedge angle), which is disposed at the other of the cut sides 102 and 104 of the rectangular parallelepiped block 100 (shown as 102). Unlike the first wedge angle 200, the second wedge angle 300 does not overlap with the side 102 of the rectangular parallelepiped block 100, but partially overlaps. In a specific embodiment, the length of the overlapping portion of the two in the vertical direction (i.e., the height direction of the rectangular parallelepiped block 100) is half the height of the rectangular parallelepiped block 100, which is beneficial to the output of the light beam and the multi-layer overlap of the rectangular parallelepiped block 100, which will be described later. In the example shown in FIG. Figure 2 When only one layer is shown, the second wedge angle 300 can also be replaced by an optical element such as a reflector to adjust the output laser light path after widening. In some embodiments, the side of the second wedge angle 300 that partially overlaps with the rectangular block 100 can be coated with a high-transmittance film, and additionally, the other two side surfaces of the second wedge angle 300 can be coated with a high-reflection film to reduce beam loss.

[0038] In one example, the second wedge angle 300 and the first wedge angle 200 may have the same shape and size, that is, both are in the shape of a triangular prism, and the end face shape of the triangular prism is an isosceles right triangle, but the heights of the two are different.

[0039] The following Figure 2 The working principle of the stretcher shown in FIG. 1 is described, the lower half of the side surface 102 of the rectangular block 100 is used as the incident surface, the laser beam is vertically transmitted into the lower half layer of the single block material (for example, the incident point height is less than half the height of the block material), and 10 total internal reflections occur at the boundary between the block material and the air (for details, see Figure 1 ), transmitted from the side 104 into the first wedge angle 200 and re-transmitted into the upper layer of the bulk material 100 after two total internal reflections at the boundary between the first wedge angle 200 and the air. Ten total internal reflections also occurred in the upper layer of the bulk material, and transmitted from the side 102 into the second wedge angle 300, where two total internal reflections occurred before emitting out of the crystal. It can be seen that after a total optical path of 1000 mm, the laser beam is transmitted from the exit surface, thereby achieving the broadening of the femtosecond pulse.

[0040] Figure 3FIG. 1 is a schematic diagram showing a structure of a pulse stretcher in a lapped state provided by an embodiment of the present application. Figure 3 The stretcher includes a plurality of block elements overlapped in the vertical direction and a corresponding number of first wedge angles and second wedge angles. For example, the block elements 110, 120, and 130 are overlapped in sequence in the vertical direction, and the structure and shape of each block element are similar to the reference Figure 1 The block element 10 described is the same and will not be described again. The block element 110 is configured with wedge angles 210 and 310, the block element 120 is configured with wedge angles 220 and 320, and the block element 130 is configured with wedge angles 230 and 330. For the stretcher, a relatively small stretcher volume and a relatively large magnitude stretching effect are achieved through multi-layer propagation, and the number of overlapping layers is the same as the increase in the optical path of the stretcher. Although Figure 3 A three-layer overlapping structure is shown in the figure, but it can be understood that this is only an example and not a limitation. The user can set fewer (for example, 2 layers) or more layers (for example, 4 layers, 5 layers) of block materials according to actual conditions.

[0041] The following Figure 3 The working principle of the stretcher shown is described as follows. The lower half of the right side surface of the rectangular block 110 is used as the incident surface. The laser is vertically transmitted into the lower half layer of the single block material (for example, the incident point height is located at 1 / 4 of the block material height), and 10 total internal reflections occur at the boundary between the block material and the air. It is transmitted from the left side into the first wedge angle 210 and undergoes 2 total internal reflections at the boundary between it and the air, and then re-transmitted to the upper half layer of the block material 110. After 10 total internal reflections also occur in the upper half layer of the block material 110, it is transmitted from the right side and then enters the second layer of the block material 120 through the wedge angle 310. Similarly, the laser also undergoes 20 total internal reflections in the second layer of the block material, and then enters the third layer of the block material 130 through the wedge angle 320 of the block material, undergoes 20 total internal reflections again, and finally exits the crystal through the wedge angle 330. After a total optical path of 3000 mm, it is transmitted from the exit surface, thus achieving the broadening of the femtosecond pulse.

[0042] In order to verify the technical advantages and application prospects of the present invention, the prepared Figure 1 The bulk material stretcher of the structure was optically tested and calculated. Figure 4-6 The test performance provided by the embodiment of the present application is shown, wherein: Figure 4 This is a schematic diagram for calculating the dispersion introduced by a wide spectrum pulse. Figure 5 This is a schematic diagram of pulse width calculation before and after broadening. Figure 6 This is a schematic diagram of the spectrum before and after stretching measured by a spectrometer for an example stretcher.

[0043] from Figure 4It can be seen that the stretcher made of bulk materials is used for femtosecond pulses with a total optical path of 500 mm. The second-order dispersion introduced for a wavelength of 800 nm is +111790 fs. 2 , so the laser pulse can be broadened. Figure 5 It can be seen that the stretcher can stretch the laser pulse width of 10fs to 52ps. In addition, based on the above description, the laser is totally reflected in the stretcher of the embodiment of the present application, so there is basically no loss of spectral bandwidth. Figure 6 As shown, the wavelength range of the incident laser beam before broadening and the laser beam after broadening are both 600-1000 nm.

[0044] According to the technical solution of the embodiment of the present invention, the stretcher based on total internal reflection has a compact structure and high stability, and can achieve pulse stretching to the order of tens of ps without complex adjustment, so it is suitable for stretching wide-spectrum short-pulse seeds. In addition, a stretcher with a larger magnitude of stretching effect can be formed by combining multiple block materials in the vertical direction, and the number of overlapped layers can be selected as needed to adjust the optical path and the stretching effect.

[0045] An exemplary embodiment of the present invention further provides a laser amplifier device including the above-mentioned stretcher. In addition to the stretcher, the laser amplifier device may also include components such as a pulse seed source, a compressor and an amplifier, wherein the amplifier is composed of a gain medium and a pump, etc. The laser amplifier device can be applied to fields such as circuit manufacturing, radar detection and optical fiber communication, such as laser accelerators, laboratory astrophysics research, etc.

[0046] It should be understood that although terms such as "first", "second", "third", and "fourth" may be used herein to describe different parts or features, these parts or features are not limited to these terms. Using the above terms, only one part will be distinguished from another part, and it is not used to emphasize the order, positional relationship, etc. For example, the first part may be referred to as the second part without departing from the scope of the present disclosure; and the second part may also be referred to as the first part. That is, modifiers such as "first" and "second" without quantifiers are interchangeable.

[0047] As used herein, words such as "include," "comprises," "have," and the like are open ended words that mean "including but not limited to," and are used interchangeably therewith. The words "or" and "and" as used herein mean the words "and / or," and are used interchangeably therewith, unless the context clearly indicates otherwise. The word "such as," as used herein, means the phrase "such as but not limited to," and is used interchangeably therewith.

[0048] The embodiments of the present application have been described above, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Without departing from the scope and spirit of the described embodiments, many combinations, modifications and changes are obvious to those of ordinary skill in the art. Therefore, the scope of protection of the present application should be based on the scope of protection of the claims.

Claims

1. A pulse stretcher, characterized in that The invention comprises a block-shaped element, wherein the block-shaped element comprises an incident surface and an exit surface. Laser enters the block-shaped element through the incident surface and is transmitted through the exit surface after being totally internally reflected multiple times in the block-shaped element, thereby achieving a widening effect.

2. The pulse stretcher according to claim 1, wherein: The block element is in the shape of a rectangular block with its diagonal edges cut off vertically, and the faces formed by cutting off the diagonal edges serve as the incident face and the exit face respectively.

3. The pulse stretcher according to claim 1, wherein: The incident surface and the exit surface of the block-shaped element are coated with a high-transmittance film, and / or the surface where the light beam is totally reflected inside the block-shaped element is coated with a high-reflection film.

4. The pulse stretcher according to claim 1, wherein: The pulse stretcher also includes a reflecting mirror arranged along the propagation direction of the laser.

5. The pulse stretcher according to claim 4, wherein: Along the laser propagation direction, the reflectors include a first group of reflectors arranged in front of the block element and a second group of reflectors arranged behind the block element, wherein the first group of reflectors is used to adjust the laser incident light path, and the second group of reflectors is used to adjust the laser output light path.

6. The pulse stretcher according to claim 2 or 4, wherein: The pulse stretcher further includes a first wedge angle, wherein one of the cut side surfaces of the rectangular parallelepiped block coincides with a side surface of the first wedge angle.

7. The pulse stretcher according to claim 6, wherein: The pulse stretcher further includes a second wedge angle, which is disposed at the other of the cut side surfaces of the rectangular parallelepiped block and partially overlaps with the cut side surface, and a length of the overlapping portion in the vertical direction is half of the height of the rectangular parallelepiped block.

8. The pulse stretcher according to claim 7, wherein: The first wedge angle and the second wedge angle are triangular prisms, the end face shape of the triangular prism is an isosceles right triangle, and the height of the rectangular block is equal to the length of the base of the isosceles right triangle.

9. The pulse stretcher according to claim 7 or 8, wherein: The pulse stretcher includes a plurality of block-shaped elements overlapped in a vertical direction and a corresponding number of first wedge angles and second wedge angles.

10. A laser amplification device, comprising the pulse stretcher according to any one of claims 1 to 9.

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