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Article

Multistage Smoothing Compressor for Multistep Pulse Compressors

by
Shuman Du
1,2,
Xiong Shen
1,3,*,
Wenhai Liang
1,2,
Peng Wang
1,3 and
Jun Liu
1,2,3,*
1
State Key Laboratory of High Field Laser Physics and CAS Center for Excellence in Ultra-Intense Laser Science, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China
2
University Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
3
Zhangjiang Laboratory, 100 Haike Road, Pudong, Shanghai 201210, China
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2023, 13(9), 5518; https://doi.org/10.3390/app13095518
Submission received: 13 March 2023 / Revised: 23 April 2023 / Accepted: 27 April 2023 / Published: 28 April 2023
(This article belongs to the Section Optics and Lasers)
Browse Figures
Versions Notes

Abstract

:
Ultrahigh peak-power lasers are important scientific tools for frontier laser physics research, in which both the peak power improvement and operating safety are very important. Based on spatial-chirp-induced beam smoothing in both the near field and far field, a multistage-smoothing-based multistep pulse compressor (MS-MPC) is proposed here to further improve safety and operating convenience. In the MS-MPC, beam smoothing is not simply executed in the pre-compressor or main compressor but is separated into multiple stages. As a result, important and expensive optics are directly protected in every stage. The prism-pair-based pre-compressor induces a small spatial chirp, making it both easier to achieve than the previous multistep pulse compressor and sufficient to protect the first grating directly. Furthermore, the asymmetric four-grating compressor, which serves as the main compressor, induces a spatial chirp that further smooths the laser beam, protecting the last grating. In this way, a 10 s to 100 s petawatt laser pulse can be compressed with a single laser beam using the currently available optics. Additionally, an extra beam-smoothing stage can be added before the main amplifier to safeguard the largest amplification crystal from damage. The MS-MPC can be easily integrated into all existing PW laser facilities to improve their potential compressed pulse energy and operational safety.

1. Introduction

Owing to the great progress in chirped-pulse amplification (CPA) [1] and optical parametric chirped-pulse amplification (OPCPA) techniques [2], ultrahigh peak-power lasers have achieved significant improvements in recent years; more than fifty lasers worldwide have achieved peak powers of several hundred terawatts (TWs) [3,4,5,6,7] and even petawatts (PWs) [8]. Very recently, 2 10-PW lasers were in operation in the Shanghai Super-intense Ultrafast Laser (SULF) facility in China and the Extreme Light Infrastructure (ELI) in Europe [9,10,11,12]. Furthermore, several 10 s to 100 s PW laser facilities have been proposed based on OPCPA [13,14,15,16]; for instance, the SEL-100PW facility [17,18] is currently being constructed and is expected to reach 100 PW in the next few years.
The CPA or OPCPA has broken through the bottleneck whereby amplification crystals are damaged due to their limited damage thresholds and confining size. However, as the laser’s peak power reaches 10 s to 100 PW, a new problem arises in the pulse compressor: damage to the compression grating due to the limited damage threshold and size. Previously, optical designs in XCELS-200PW, ELI-200PW, and SEL-100PW were all based on the multi-beam tiled-aperture combing method [19]. This beam-combing optical design is very complex and difficult to execute; it is highly sensitive to differences in the optical delay, pointing stability, wavefront, and dispersion among all of the beams [20,21]. Recently, a novel method called the multistep pulse compressor (MPC) was proposed to solve the compression problem, in which the limited input/output pulse energy problem is transferred to the spatiotemporal properties of the input/output laser beam [22]. Furthermore, an asymmetric four-grating compressor was proposed to simplify the MPC [23]. However, these methods only directly protect the first or last grating, and there is a risk of damage to other critical optics.
In this study, we propose a multistage-smoothing-based MPC (MS-MPC) not only to directly protect critical and costly optics such as the amplification crystal and compression gratings, but also to reduce the requirements for prism parameters. Unlike the typical symmetric four-grating compressor, the MS-MPC employs an asymmetric four-grating compressor as the main compressor, offering several notable advantages. First, the pre-compressor with a prism pair induces a relatively small and appropriate spatial chirp width, which is sufficient to smooth the laser beam for the safety of the first grating. It is easy and convenient to achieve this using a prism pair with a small apex angle and short prism-pair distance. Second, an asymmetric four-grating compressor can induce a suitable spatial chirp width with a small grating-pair distance difference to directly protect the last grating. Third, every grating in the asymmetric four-grating compressor is well protected, as the spatial chirp is induced directly in every stage. Therefore, even if there are defects/damages on one grating, they will not be transferred to the next grating due to the spatial chirp induced in every stage. However, in a traditional symmetric four-grating compressor, defects or wavefront-aberration at high/middling spatial frequencies before or on the first grating may cause hotspots to cause damage to the last grating. Furthermore, an additional prism-pair-based beam-smoothing stage can be added before the main amplifier to protect the important and expensive amplification crystal.

2. Beam Smoothing Owing to the Spatial Chirp

2.1. Beam Smoothing in the Near Field

Beam smoothing by inducing spatial chirp is the key method to reduce the spatial intensity modulation of a laser beam, and then improve the input/output laser pulse energy in the compressor of a PW laser system. To date, there are two typical optical designs, prism pairs and grating pairs, which can induce spatial chirp to smooth the laser beam [22,23]. In both setups, the first prism or grating induces angular dispersion to the laser beam, and the second parallel prism or grating is used to collimate the output laser beam. The induced spatial chirp width D is related to the laser spectral bandwidth and the distance between the prism pair or the grating pair. D can be expressed as D = L(tan(θs) − tan(θl))cosα, where L is the perpendicular distance of the prism pair or the grating pair; θs and θl are the diffraction angles of the shortest wavelength λs and the longest wavelength λl, respectively, of the input laser pulse; and α is the incident angle of the grating or the apex angle of the prism. For a laser pulse with a narrow-band spectrum, the difference in tan(θs) − tan(θl) is small; therefore, a much longer distance L is required to achieve the same width D compared with that required for a broadband spectrum. In comparison to a prism pair, a grating pair can induce a relatively large spatial chirp width with a relatively short distance L. The basic optical schemes of a prism pair and a grating pair for beam smoothing are shown in Figure 1a,b, respectively.
The beam-smoothing effect in one direction is directly related to the induced spatial chirp width D. For both the prism pair and the grating pair, the same beam-smoothing effect will be achieved if they induce the same D. Owing to the relatively weak angular-dispersion ability of prism, a distance L of nearly 70 m is required to smooth the beam effectively [22]. Although a prism pair with large apex angle can shorten the distance, the prism thickness will be increased, which is more expensive. Two perpendicular prism pairs can also shorten the distance [24]. Transmitting gratings may efficiently reduce the distance in the future. In this study, an asymmetric four-grating compressor was used, which has two grating pairs with reflective gratings. This design is able to induce a suitable spatial chirp width with a relatively short distance difference between the two grating pairs. Since an asymmetric four-grating compressor is able to smooth the output laser beam directly, the pre-compressor can induce a relatively small spatial chirp width to the input laser beam using a prism pair with a small apex angle, making it more practical and feasible.
To clearly explain the beam smoothing capability in the near field based on the induced spatial chirp, laser spatial intensity modulation (LSIM, the ratio of the maximum and average laser spatial intensities) with a random high spatial frequency of 1.0 mm−1 is intentionally induced into the input laser beam. Obvious beam smoothing is obtained for the output beam after either a prism pair or a grating pair, as shown in Figure 2. A pulse centered at 925 nm with a full spectral bandwidth of 200 nm and a 370 × 370 mm2 10th-order super-Gaussian beam is used as the simulation laser source.
As shown in Figure 2a, the LSIM experiences a rapid decline from approximately 2.0 to 1.3, when the spatial chirp width increases from zero to approximately 10 mm, and a further slight decline to approximately 1.1 when the spatial chirp width is continually increased to approximately 60 mm. Figure 2b shows the relationship between the induced spatial chirp width and the perpendicular distance L of the prism pair with a 15° apex angle and that of the grating pair with a groove density of 1400 lines/mm. It can be seen that the separated distance L for the prism pair is approximately ten times longer than that of the grating pair to achieve the same D. A spatial chirp width of approximately 8 mm can be obtained using the prism pair with the distance L of about 10 m.
Figure 2c shows the intensity curves of the center lines along the X−axis of the original 370 × 370 mm2 beam and those when 10 mm and 60 mm spatial chirp widths are introduced. Strong LSIM can be observed in the laser beam with no spatial chirp. If a 10 mm or 60 mm spatial chirp is introduced, the beam is well smoothed, and its LSIM is reduced to approximately 1.3 or 1.1, respectively. Note that the induced LSIM with high spatial frequency is random, then there is LSIM with low spatial frequency in the simulation.
Figure 2d,e show the experimental results of the beam-smoothing effect using the asymmetric four-grating compressor configuration in a Ti:sapphire PW laser [25,26]. A 2 mm-wide paper strip is placed directly before the asymmetric four-grating compressor, which is used to induce strong LSIM to the input laser beam due to the diffraction effect. Figure 2d shows the output laser beam with obvious LSIM when the compressor is a typical symmetrical four-grating compressor. Compared to Figure 2d, Figure 2e shows the output laser beam with a clear and smoothed spatial intensity when a distance difference of approximately 90 mm is induced between the two grating pairs. When considering a full spectral bandwidth of approximately 100 nm, the induced spatial chirp width on the last grating is about 9 mm.

2.2. Beam Smoothing in the Far Field

In large-sized ultrahigh peak-power laser systems, the primary risk for optical damage is a relatively high LSIM at the middle/high frequencies of the laser beam in the near field. Additionally, some of the wavefront aberrations at middling/high spatial frequencies can cause small-scale beam focusing in the far field during free propagation. These wavefront aberrations frequently occur due to imperfect optics, especially for large-sized optics. As for the wavefront distortion at low spatial frequencies, which affects the final focal diameter and then the focal intensity, deformable mirrors are commonly utilized to correct these wavefront distortions to achieve a small focal diameter. For wavefront aberrations with high spatial frequencies, a spatial filter based on a 4f optical system, with a small pinhole at the focal point, is usually used to filter out the wavefront aberration laser beam. This spatial filter can also be used to magnify the laser beam. To ensure that most of the laser beam can pass through the pinhole easily, the size of the pinhole is usually set to be approximately ten times the diffraction-limited size. As a result, some wavefront aberrations with high spatial frequencies and most of the wavefront aberrations with middling spatial frequencies remain in the output laser beam, which may self-focus or induce hotspots in the far field and damage important optics.
The wavefront aberrations at middling/high spatial frequencies cannot be completely eliminated by the induced spatial chirp. However, the induced spatial chirp can significantly reduce the damage risk of wavefront aberrations in the far field. Similarly to the principle of spatial intensity smoothing [22], the principle of wavefront aberration smoothing with spatial chirp can be explained using Figure 3. Without spatial chirp, the wavefront aberration with a high/middling spatial frequency induces a hotspot in the far field during free propagation, as shown in Figure 3a. Even if a suitable one-dimensional spatial chirp width is induced in the laser beam, the primary hotspot is smoothed into a line with D, as shown in Figure 3b. To experimentally show this property, a kHz Ti:sapphire femtosecond laser is used. A lens with 2 m focal lens is located in front of a four-grating compressor to simulate the wavefront aberration in the laser beam; a small round spot is achieved at the focal point or far field, as shown at the bottom of Figure 3a. However, if the laser beam is reflected out between G2 and G3, it becomes a line at the focal position, as shown at the bottom of Figure 3b. Since the diameter of the hotspot is usually about 1 mm, even for a spatial chirp width of 10 mm, the hotspot will be reduced ten-fold. From a different perspective, the spatial chirp actually smooths the wavefront aberrations with middling/high spatial frequencies because the wavefront at every position is an average of all wavelengths.

3. Multistage-Smoothing-Based MPC

To address the limitations of a single typical MPC or a single asymmetric four-grating compressor, a new approach, called a multistage-smoothing-based MPC (MS-MPC), has been developed. This approach combines two beam-smoothing optical designs into a single compressor, where beam smoothing is separated into multiple stages. The operation of an MS-MPC can be separated into two optical setups. First, the input is a laser beam with no spatial chirp, and the spatial chirp width is increased step by step at every stage. The largest spatial chirp width and smoothest beam are obtained in the compressed output laser beam. This is called the forward MS-MPC, as shown in Figure 4a. In contrast, the second setup starts with the input laser beam having the largest spatial chirp, and the spatial chirp width decreases step by step at every stage until the output laser beam has the smallest or even zero spatial chirp width. This setup is called backward MS-MPC, as shown in Figure 4b. Here, only forward MS-MPC is discussed in detail.
For the forward MS-MPC, a suitable spatial chirp is induced in the pre-compressor by using one or two prism pairs with a small apex angle and relatively short distances. According to the simulation results, the LSIM reduced rapidly from 2 to approximately 1.3 with a relative short distance between the 2 prisms [22]. Since the first grating has a relatively long nanosecond-pulse duration and has a high damage threshold, a relatively small spatial chirp is sufficient for protecting the first grating directly. In comparison to a spatial chirp line induced in one direction, the beam-smoothing effect is more effective when the spatial chirp induced in two directions [24]. To shield the compressor from all the random hotspots, one prism pair would be better located directly before the grating-based compressor. However, for laser beams larger than 500 mm, the prisms used here would be too thick and expensive. The prism pair can also be located before the relay imaging system for smaller beam sizes, imaging the well-smoothed laser beam directly onto the first grating.
The asymmetric four-grating compressor is employed as the main compressor, and it induced a suitable spatial chirp width on the last grating. This directly induced spatial chirp effectively protects the last grating from the risk of damage. After beam smoothing in the pre-compressor stage, a suitable spatial chirp width is sufficient, corresponding to a small difference in distance between the two grating pairs. Note that the asymmetric four-grating compressor will induce the same amount of spectral dispersion to the compressed laser pulse as the symmetric four-grating compressor if the total distance of the two grating pairs is equal. Finally, a post-compressor is utilized, based on the spatiotemporal self-focusing effect, to compensate for the induced spatial chirp from both the prism pairs and the asymmetric four-grating compressor. Owing to the smoothed beam profile, the self-compression process in bulk-medium plates using negatively chirped pulses can also be added to the post-compressor to shorten the output pulse duration in the near future [27].
In addition to the compressor, the main amplifier may also experience crystal damage risk. Therefore, an additional beam-smoothing stage based on prism pairs can be added before the main amplifier to mitigate this risk. The laser beam in the main amplifier of 10 s to100 s PW laser systems should be more than 200 mm in diameter or side length. An induced spatial chirp width of approximately 5 to 10 mm will not affect the main amplification. In fact, inducing a spatial chirp width of about 5 mm in one direction (X or Y) is enough to improve the LSIM of the amplified beam and protect the largest and most expensive crystal. Note that the beam smoothing at this stage is not essential because usually a 4f-system-based spatial filter is used before the main amplifier, and it is optional according to the requirements and conditions of every laser system.
The whole optical scheme of an MS-MPC is shown in Figure 5; it includes a pre-compressor, a main compressor, and a post-compressor. Since the induced spatial chirp can smooth the laser beam in both the near field and far fields, the MS-MPC will significantly improve the safety of the important and expensive optics, and then increase the long-term operational efficiency of the laser system. Here, the PS_I, together with BE_1, will protect the largest crystal in the main amplifier; the spatial chirp induced by PS_II will protect DM_2, G1 and other important optics after PS_II. The spatial chirp induced by the asymmetric four-grating compressor will protect G4 and DM_3 directly. Furthermore, the DM_1 ensures that the wavefront before PS_II is perfect; the DM_2 guarantees the reflective wavefront by G1 is flat; the CP will compensate the wavefront distortions from G2 and G3, which is optional; and, finally, the DM_3 will be well controlled to obtain the highest focal intensity on the focal point FP.

4. Sample Design for a 100 PW Laser

To date, the central wavelengths of the proposed 100 s PW femtosecond laser facilities have mainly been located at approximately 925 nm. The optical schematic of a sample design for a 100 PW laser is also shown in Figure 5. Since the PS_I is not indispensable in this MS-MPC, it will not be discussed in detail here. The laser beam after the main amplifier is assumed to be a 370 × 370 mm2 beam with a 10th-order super-Gaussian profiles in both the X and Y directions. The laser pulse is positively chirped to approximately 4 ns, of which the transform limited pulse duration is approximately 14.3 fs. The central wavelength is 925 nm, and the spectral bandwidth is about 200 nm.
A spatial chirp width of approximately 5 mm is applied to the laser beam using two perpendicular prism pairs (PS_II) with a 15° apex angle and a separation distance of approximately 6 m. Then, a smoothed laser beam (~375 × 375 mm2) is obtained, where the center of the 365 × 365 mm2 region has full spectral bandwidth. A beam-expander and relay-imaging system BE_2 is used to expand the laser beam by 1.73 times (approximately) on both sides to ~650 × 650 mm2, where the center 633 × 633 mm2 region possesses full spectral bandwidth. Since the first grating G1 has a relatively high damage threshold owing to the nanosecond chirped pulse, which is approximately 2.7-folds higher than that of the last grating G4 with a femtosecond pulse, the bearable LSIM of G1 should be 1.6-folds higher than that on the last grating G4, considering 60% total compression efficiency. Then, the reserved intensity space for G1 is ~2.0 times (1.6 × 1.3) that of its damages threshold, which is a typical design value for a safe operation of most PW laser systems, if 1.3 times of the damage threshold fluence is reserved for the last grating. This means that G1 should usually be safe if we only consider the near-field affect. The PS_II can also remove the far-field affect from the laser beam before PS_II. Except for beam smoothing, the two prism pairs with a small apex angle may also be used to precisely correct the pulse front tilt and angular dispersion of the whole PW laser system. This is because all the transmitted plane optics in PW laser system, such as crystals in the amplifier, intentionally induced a wedge with a small angle to avoid the temporal contrast of the output laser pulse decreasing [28]. These wedges induce angular dispersion and a pulse front tilt to the output laser beam, which affect the final focal intensity. The weak reflected beam from the surface of the prisms can also be used to monitor the spectrum, pointing stability, beam profile, time jitter, and wavefront of the laser beam.
After the pre-compressor, the spatiotemporal modified laser beam is guided into an asymmetric four-grating compressor with four identical 1400 lines/mm gold-coated gratings. The incident angle on the grating is set to approximately 61°, and this allows the beam size on the grating in the diffraction direction to be about 650/cos(61°) = 1340 mm. The perpendicular distances of the first and second grating pairs are set to 1215 mm and 1285 mm, respectively. As a result, there is a 70 mm length difference between the two grating pairs, and this induces a spatial chirp width of approximately 25 mm on the last grating. The approximate size (1365 × 650 mm2) of the beam on the last grating is still smaller than that of the largest grating with an approximate size of 1450 × 700 mm2. With this 25 mm spatial chirp width, the possible LSIM or hotspots, coming from both near-field and far-field effects of the optics after PS_II, can be effectively smoothed according to the previous simulation. When considering that the pre-compressor has already induced approximately 5 mm × 5 mm in the X and Y directions, which equals a 25 mm spatial chirp width in one direction, the LSIM with a period of less than 3 mm in the middling spatial frequency can also be smoothed well. As a result, a highly smoothing laser beam is achieved at the output of the asymmetric four-grating compressor. Note that the length difference between the two grating pairs can be conveniently varied according to different requirements; thus, different spatial chirp widths can be induced using the asymmetric four-grating compressor.
Since the total induced spatial chirp width on G2 is approximately 440 mm, it should be noted that the second grating G2 can cut light by a width of about 160 mm on both edges. The light cutting by the grating sharp edges induce diffraction and then LSIM to the laser beam after G2. According to the diffraction theory at a straight edge [29], the diffracted light intensity varies with the distance between the diffracted light source and the receiving screen. The maximum diffraction intensity can reach approximately 1.5 times that of the initial intensity. Based on the above input parameters, the red dotted line in Figure 6 shows the one-dimensional beam profile after G2 without light cutting. In the asymmetric four-grating compressor, the distance between G2 and G3 is variable. In our case, the shortest distance of the upper edge is set to 3 m, whereas the longest distance of the bottom edge is set to 5.5 m. The one-dimensional beam intensity profile on the G3 grating can be obtained by summing all the diffraction profiles at different wavelengths, as shown by the blue solid line in Figure 6. This shows that the intensity of the light cutting on both edges is about one-fifth of the maximum intensity of the laser beam. According to the calculation, the LSIM induced by the edge diffraction does not exceed or affect the top-hat region; therefore, it does not cause damage risks to G3. The energy loss induced by light cutting is calculated to be approximately 1.9%. Note that the diffraction-induced LSIM by G2 is absolutely smoothed after G4 owing to the large spatial chirp width induced by the grating pair of G3 and G4. The spectral cutting of G2 has been used in many grating-based compressors. A kHz Ti:sapphire laser system with a beam diameter of 15 mm is used to prove the light cutting induced diffraction effect, as shown in Figure 6c,e, where the light cutting occurs at the peak intensities of approximately 0.2 and 0.5. The simulation results of the edge diffraction are shown in Figure 6b,d. The red lines and blue lines are one-dimensional beam intensity profiles without and with light cutting, respectively. The insets of Figure 6c,e are the corresponding 2D beam profiles. The good agreement between the experimental and simulated results confirms the accuracy of the simulation program. It means the light-cutting-induced influence by G2 in the 100PW sample laser can be neglected.
Since the full-spectral-bandwidth regions on each grating will experience the strongest laser fluence, only the laser fluence of this region is considered. In the first grating, the effective beam size on the grating is approximately 1320 mm in the X-direction and 640 mm in the Y-direction. For an amplified laser pulse with 2500 J input, the laser fluence is 2500/(132 × 64) ≈ 296 mJ/cm2, which is twofold lower than the grating damage threshold of approximately 600 mJ/cm2 with a nanosecond pulse duration [22,30]. In the asymmetric four-grating compressor, the strongest laser fluence on the last grating G4 is decreased to approximately 296 × 0.60 = 178 mJ/cm2 if the total compression efficiency is 60%. It is 1.28-fold lower than the damage threshold of a compressed femtosecond laser pulse, which is approximately 229 mJ/cm2 [22,30]. Then, the 2500 × 0.60 = 1500 J compressed pulse can support 100 PW peak-power with a 15 fs pulse duration. Since the LSIM of the output laser beam can be smoothed to less than 1.1 after 3 stages of beam smoothing processes, it is theoretically possible to achieve 100 PW using this MS-MPC. However, in practice, achieving approximately 75 PW would be safer and more feasible for a real 10 s to 100 s PW laser system. This is because the laser fluence on the last grating would be almost 1.70 times lower than its damage threshold when the beam LSIM is smoothed to less than 1.2. Note that we only demonstrate the capabilities of this MS-MPC method for 10 s to 100 s of the PW system; the proposed parameters are not the optimized values, where enough margins have already been saved on many parameters, such as the beam size and light or spectral cutting.
After the main compressor with the asymmetric four-grating compressor setup, the output laser beam with smoothed spatial intensity and diffused wavefront aberration is focused by PM. At the focal point, the spatial chirp is automatically compensated owing to spatio- temporal focusing. Note that the damage threshold of the optics after the asymmetric four-grating compressor can be several times higher than that of the gratings. This is because the grating with a broad diffraction spectral bandwidth can only be coated with gold, which is the weakest film, whereas the reflective optics after asymmetric four-grating compressor can be coated with other kinds of coating film, of which the damage fluence threshold is several times higher. In the future, using a negative chirped pulse, this smoothed laser beam can be used to induce further pulse self-compression in a glass plate after the PM [22,27].

5. Discussion and Conclusions

Owing to the fixed and random hotspots induced by diffractions from dust, defects in the optics, and the spatially inhomogeneous pump beam, the LSIM of an amplified laser in a PW laser system is usually very high. To avoid laser-induced damage to important optics, especially gratings, the laser fluence on the grating must usually be kept below half of its damage threshold, which undoubtedly affects the maximum output pulse energy of a compressor. Aside from the peak power, the operating safety is also very important to maintain an economical PW laser system and increase its experimental efficiency.
As an improvement of MPC, the MS-MPC was proven to be able to achieve laser pulses with 10 s to 100 PW using a single laser beam. Based on the spatial chirp induced by grating pairs or prism pairs, both the LSIM in the near field and wavefront-aberration-induced hotspots in the far field can be effectively smoothed. Thus, the MS-MPC design can significantly improve the operating safety and increase the output peak power of PW laser systems. The proposed MS-MPC can directly protect important optics, such as the large crystal, all four gold-coated gratings, and deformable mirrors, from laser induced damage. Since the distances of the prism pairs or grating pairs can be easily varied, compressed laser beams with different spatial chirp widths can be conveniently achieved; these may induce different laser fields around the focal point and benefit laser physics experiments. Furthermore, this MS-MPC optical design can be easily applied to all existing PW laser facilities to improve their output pulse energy and operational safety.

Author Contributions

Conceptualization, S.D. and X.S.; Methodology, S.D. and J.L.; Software, S.D. and X.S.; Validation, S.D. and W.L.; Formal analysis, S.D.; Investigation, P.W.; Writing—original draft, S.D.; Writing—review & editing, J.L.; Project administration, J.L.; Funding acquisition, J.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported by the National Natural Science Foundation of China (NSFC) (61527821, 61905257, and U1930115) and the Shanghai Municipal Science and Technology Major Project (2017SHZDZX02).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Optical scheme of beam smoothing based on (a) a prism pair, and (b) a grating pair. θs and θl are the output angles of the shortest wavelength λs and the longest wavelength λl, respectively, from the first prism or grating of the input laser pulse; L is the distance of the prism pair or the grating pair; D is the introduced spatial chirp width on the output beam; and w is the full width of the output beam.
Figure 1. Optical scheme of beam smoothing based on (a) a prism pair, and (b) a grating pair. θs and θl are the output angles of the shortest wavelength λs and the longest wavelength λl, respectively, from the first prism or grating of the input laser pulse; L is the distance of the prism pair or the grating pair; D is the introduced spatial chirp width on the output beam; and w is the full width of the output beam.
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Figure 2. (a) LSIM ratio variations related to the spatial chirp width. (b) Spatial chirp width curves with respect to the distance of the prism pair or the grating pair. (c) Intensity curves of the center lines along the X−axis of the beams without spatial chirp (black) and those with 10 mm (red) or 60 mm (blue) spatial chirp width. The output laser beam using (d) a typical symmetrical four-grating compressor, and (e) an asymmetric four-grating compressor.
Figure 2. (a) LSIM ratio variations related to the spatial chirp width. (b) Spatial chirp width curves with respect to the distance of the prism pair or the grating pair. (c) Intensity curves of the center lines along the X−axis of the beams without spatial chirp (black) and those with 10 mm (red) or 60 mm (blue) spatial chirp width. The output laser beam using (d) a typical symmetrical four-grating compressor, and (e) an asymmetric four-grating compressor.
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Figure 3. Schematic diagram of wavefront aberration smoothing. (a) A near-field laser beam with a wavefront aberration without spatial chirp induces a hotspot in the far field; (b) the hotspot is smoothed into a line using a one-dimensional spatial chirp.
Figure 3. Schematic diagram of wavefront aberration smoothing. (a) A near-field laser beam with a wavefront aberration without spatial chirp induces a hotspot in the far field; (b) the hotspot is smoothed into a line using a one-dimensional spatial chirp.
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Figure 4. Scheme showing the principles of (a) forward MS-MPC with Lb2 > Lb1, and (b) backward MS-MPC with La1 > La2. The post-compressor based on spatiotemporal focusing is not shown.
Figure 4. Scheme showing the principles of (a) forward MS-MPC with Lb2 > Lb1, and (b) backward MS-MPC with La1 > La2. The post-compressor based on spatiotemporal focusing is not shown.
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Figure 5. Optical schematic of a typical MS-MPC. PS_I and PS_II: prism-pair system for beam smoothing; DM_1, DM_2, and DM_3: reflective deformable mirror for wavefront compensation at different stages; BE_1 and BE_2: 4f-system-based beam expander; asymmetric four-grating compressor: asymmetric four-grating compressor. M1–M5: plane-reflective mirrors; CP: compensation plate; PM: parabolic reflective mirror; FP: focal point.
Figure 5. Optical schematic of a typical MS-MPC. PS_I and PS_II: prism-pair system for beam smoothing; DM_1, DM_2, and DM_3: reflective deformable mirror for wavefront compensation at different stages; BE_1 and BE_2: 4f-system-based beam expander; asymmetric four-grating compressor: asymmetric four-grating compressor. M1–M5: plane-reflective mirrors; CP: compensation plate; PM: parabolic reflective mirror; FP: focal point.
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Figure 6. (a) One−dimensional beam profile (before G3) without light cutting (red dotted line) and with about 160 mm light cutting on both sides (blue solid line). The insets on the right are the enlarged diffraction-induced modulations. (b,d) The simulation results and (c,e) the experimental results with light cutting on G2.
Figure 6. (a) One−dimensional beam profile (before G3) without light cutting (red dotted line) and with about 160 mm light cutting on both sides (blue solid line). The insets on the right are the enlarged diffraction-induced modulations. (b,d) The simulation results and (c,e) the experimental results with light cutting on G2.
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Du, S.; Shen, X.; Liang, W.; Wang, P.; Liu, J. Multistage Smoothing Compressor for Multistep Pulse Compressors. Appl. Sci. 2023, 13, 5518. https://doi.org/10.3390/app13095518

AMA Style

Du S, Shen X, Liang W, Wang P, Liu J. Multistage Smoothing Compressor for Multistep Pulse Compressors. Applied Sciences. 2023; 13(9):5518. https://doi.org/10.3390/app13095518

Chicago/Turabian Style

Du, Shuman, Xiong Shen, Wenhai Liang, Peng Wang, and Jun Liu. 2023. "Multistage Smoothing Compressor for Multistep Pulse Compressors" Applied Sciences 13, no. 9: 5518. https://doi.org/10.3390/app13095518

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