Open Access
Add to BibSonomy
Abstract
We present the performances of a broadband optical parametric chirped pulse amplification (OPCPA) using partially deuterated potassium dihydrogen phosphate (DKDP) crystals with deuteration levels of 70% and 98%. When pumped by a Nd:glass double frequency laser, the OPCPA system using the 98% deuterated DKDP crystal achieves a broad bandwidth of 189 nm (full width at 1/e2 maximum) from 836 nm to 1025 nm. For the DKDP crystal with length of 43 mm, the pump-to-signal conversion efficiency reaches 28.4% and the compressed pulse duration is 13.7 fs. For a 70% deuterated DKDP crystal with a length of 30 mm, the amplified spectrum ranges from 846-1021 nm, the compressed pulse duration is 15.7 fs, and the conversion efficiency is 25.5%. These results demonstrate the potential of DKDP crystals with higher deuteration as promising nonlinear crystals for use as final amplifiers in 100 Petawatt (PW) laser systems, supporting compression pulse duration shorter than 15 fs.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The intensity of 1024 W/cm2 can be generated by 100 PW (PW: petawatt) laser system, which can be applied to research in vacuum QED, high energy density physics, and other fields [1,2]. Most of PW or even 10 PW laser systems adopt CPA technology roadmap based on Ti: sapphire amplifier, such as an over 1 PW laser in the J-KAREN-P [3], 4.2 PW laser in CoReLS [4], a 10.2 PW laser in ELI-NP [5] and 12.9 PW in SULF [6]. Due to the limitation of amplified spectrum of Ti: sapphire-amplifiers from 750 nm to 850 nm, the compression pulse duration of these laser systems is usually 20-30 fs with the amplified energy from tens of joules to hundreds of joules. The unique characteristics of optical parametric chirped pulse amplification (OPCPA), such as ultra-broadband amplification spectrum, higher unidirectional gain and parasitic-free amplification, make it a highly promising technique for amplifying higher power lasers.
The size of deuterated potassium dihydrogen phosphate (DKDP) crystals can reach several hundred millimeters [7] and several research groups are developing ultra-intense laser systems to fully utilize the potential of DKDP in high-energy OPCPA. These works make DKDP an ideal nonlinear crystal for achieving thousands of joules and ultra-wideband amplification in 100 PW laser systems. In 2012, non-collinear optical parametric amplifiers (NOPAs) based on DKDP crystal produced a broadband gain for supporting pulses as short as 10 fs [8]. OPCPA systems based on DKDP crystals have achieved energy as high as 35 J near 1053 nm with a potential power of 300 TW by O.V. Chekhlov [9], and 38 J near 910 nm with a peak power of 0.56 PW by V. V. Lozhkarev [10]. In the MTW-OPAL laser system, a 0.35 PW laser, corresponding to 7.3 J and 20-fs pulses, was achieved based on a DKDP crystal with 70% deuteration level. Their long-term goal is to build two 25 PW laser pulses (500 J, 20 fs) by parametric amplification in DKDP crystals with high deuteration level [11]. A numerical investigation on a high-energy OPCPA for the amplification of 100 PW laser was presented by Hu, and a new method named optimization of zero-phase-mismatch wavelength (ZPMW) was theoretically proposed to achieve amplification with a bandwidth exceeding 200 nm in a DKDP crystal even with a moderate deuteration level of 70% [12]. However, as far as we know, there is no literature reporting on experimental research regarding the broadband OPCPA amplification based on high-deuterium-doped (≥80%) DKDP crystals with a central wavelength around 900 nm and output energy at the joule level.
In the design of the SEL (Station of Extreme Light)-100 PW laser system, OPCPA based on DKDP crystal will be utilized as the main amplifiers to generate a pulse with an energy of 2500 J and a spectral range of 825 nm to 1025 nm. Following compression, a 100 PW laser pulse with an energy of 1500 J and a duration of 15 fs will be achieved.
In this paper, we present the demonstration of a broadband OPCPA stage utilizing DKDP crystals with 70% and 98% deuteration levels. By employing a 98% deuteration level DKDP crystal, the OPCPA amplification achieved a spectral bandwidth exceeding 200 nm centered at 925 nm, surpassing the amplification bandwidth of DKDP crystals with a deuteration level of 70%. Furthermore, we achieved a pump-to-signal conversion efficiency [13] of 28. 4% and a compressed pulse duration of 13.7 fs using the 98% deuteration level DKDP crystal. To the best of our knowledge, this represents the first demonstration of an OPCPA stage center at 925 nm utilizing a high deuteration level DKDP crystal with a deuteration level of 98%.
2. OPCPA stage
2.1 Description
An OPCPA Front End for the 100 PW-Class Laser Facility was developed in 2021, and it has the potential to generate laser pulses with an output of 263 TW and a duration of 13.4 fs at a repetition rate of 0.1 Hz [14,15]. As shown in Fig. 1, the DKDP-OPCPA experiment was implemented on this laser system, utilizing the second-stage LBO-OPCPA output as the injection seed laser for the DKDP crystal, and utilizing the third-stage pump source as the pump laser pulse. The seed chirped pulse from the second-stage LBO-OPCPA output has an energy of 481.5 mJ, a broad spectrum of 925 ± 100 nm. And then, it was expanded to a diameter of 14 mm to inject to the DKDP crystal. The third-stage pump source is a homemade Nd:glass laser pumping system capable of generating 526.5 nm laser pulses and an energy of up to 25 J. In this experiment, to match the aperture of the DKDP crystals, the beam size of pump was down-collimated from 28 mm down to 11 mm. The pump laser energy was adjusted from 0-12 J by combining a half waveplate and a polarizer.
Fig. 1. Schematic of the experimental setup of DKDP-OPCPA based on the OPCPA front end for the 100 PW-class laser facility.
The detailed structure of the DKDP-OPCPA experiment is shown in Fig. 2. The left and right periscope tower were used to import and export the pump beam, respectively. The upper mirrors of both periscope towers both are 526.5 nm laser high reflection (HR) mirrors. The bottom mirrors of both periscope towers are dichroic mirrors (DM) which are designed to reflect the p-polarized pump beam at 526.5 nm and pass s-polarized broadband signal beam at 822-1032 nm. Most of the idler beam, along with 99% of the signal beam, can pass through the second dichroic mirror while maintaining a small angle between the idler beam and the signal beam. The signal beam and the idler beam were separated in space through long-distance transmission. By adjusting the incident angle of the signal beam using two HR mirrors in front of the first DM, the non-collinear angle ($\alpha $) between the signal beam and the pump beam can be adjusted.
The DKDP crystal (Type I) and a rotating platform were located between two periscope towers. The rotating platform was used to adjust the phase-matching angle ($\theta $) of crystal. Both $\theta $ and $\alpha $ are all at horizontal direction. The separated idler beam was terminated by a beam dump. The residual pump beam was used to be diagnosed. The amplified signal beam was injected to a two-grating compressor [13].
2.2 Simulation
To simulate the DKDP-OPA (Type-I), we solved the coupled wave equations using a combined split Fourier method with a fourth-order Runge-Kutta numerical solving method. A 16th order super-Gaussian beam spatial shape for the pump and a 6th order super-Gaussian beam spatial shape for the signal beams are set in the simulation model. Both of signal and pump beam have a spot diameter of 11 mm. The temporal waveform of original pump pulse is set as a 16th order super-Gaussian pulse and that of original signal pulse is set as a 6th order super-Gaussian pulse.The pulse duration of the pump beam is set to 4.5 ns with wavelength of 526.5 nm, while the signal beam is 4 ns with wavelength range from 820 nm to 1030 nm (full width at 1/e2). The setting of the pulse durations both for pump and signal in this simulation is consistent with that in the 100 PW design, but it differs from the pulse durations used in this experiment. The pulse shape and duration used in this experiment will be introduced in the next section. In simulation, maximum pump intensity is 2.3 GW/cm2. The simulation included six different DKDP crystals with varying deuterium doping rates and length. In this simulation, three DKDP crystals are 98% deuterated, with lengths of 43 mm (98%-43 mm DKDP), 35 mm (98%-35 mm DKDP) and 30 mm (98%-35 mm DKDP) respectively. The others three DKDP crystals are 70% deuterated and have length of 43 mm,35 mm and 30 mm (70%-43 mm, 70%-35 mm and 70%-30 mm DKDP). The lengths of these crystals are consistent with the crystal lengths that may potentially be applied in 100 PW high-energy main amplification systems in the future. However, in the experiment, we only have three types of crystals available: 98%-43 mm, 98%-35 mm and 70%-30 mm.
The phase matching angles and noncollinear angles of DKDP crystals are determined using wave-vector matching equations $\left( {\left|{\varDelta \mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}} \over k} } \right|= \left|{{{\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}} \over k} }_s} + {{\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}} \over k} }_i} - {{\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}} \over k} }_p}} \right|= 0} \right)$, ${\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}} \over k} _j}$ is the wave vector, $j = p,s,i$, p, $s$ and $i$ represent pump, signal and idler beam, respectively) and the group velocity matching (${v_{gs}} = {v_{gi}}\cos \beta $, ${v_{gs}}$ and ${v_{gi}}$ is the group velocity of signal and idler beam respectively, $\beta $ is the angle between ${\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}} \over k} _s}$ and ${\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}} \over k} _i}$). Sellmeier coefficients which we used is from Ref. [16]. For achieving the largest effective nonlinear coefficient (deff), the angle between the optical axis and the X-axis of the DKDP crystal is set to 45 degrees ($\varphi = 45^\circ $). The 98%-deuterated DKDP crystals have a phase matching angle $\theta $= 36.96° and a noncollinear angle $\alpha $=0.93°. The $\theta $ and $\alpha $ of the 70% DKDP crystals are 38.07° and 0.41°, respectively. Meanwhile, the deff of 70% DKDP and 98% DKDP crystal are 0.228 pm/V and 0.222 pm/V, respectively. The wave-vector mismatch values are shown in Fig. 3.
Fig. 3. Wave-vector mismatch value ($\varDelta \mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}} \over k}$) of 70% (blue line) and 98% (red line) DKDP crystal.
The calculations were performed using varying pump intensities, ranging from 0.23 to 2.34 GW/cm2. The results are shown in Fig. 4(a). We can see that different crystal lengths correspond to different pumping intensities for achieving the highest conversion efficiency. For crystals with a deuterium doping rate of 98%, the pumping intensities for achieving the highest conversion efficiency are 1.52 GW/cm2 and 0.82 GW/cm2 for crystal lengths of 35 mm and 43 mm, respectively. While for crystals with a deuterium doping rate of 70%, those are 1.40 GW/cm2 and 0.70 GW/cm2.The pump intensities for 30 mm lengths crystals both for 98% and 70% are higher than 2.34 GW/cm2
Fig. 4. (a)The conversion efficiencies of the 70%-30 mm,70%-35 mm, 70%-43 mm and 98%-30 mm, 98%-35 mm, 98%-43 mm DKDP crystals at various pump intensities are represented by the blue (70%) and red (98%) lines with different mark respectively. (b) The simulated amplified spectra of the 98% DKDP crystal (red line) and the 70% DKDP crystal (blue line).
Figure 4(b) shows the amplified spectra of the DKDP crystals with 98% and 70% deuteration. The bandwidth is defined as the full width at 1/e2 maximum of the spectrum. The amplified bandwidth of the 98%-43 mm DKDP crystal is 196 nm, spanning from 825 to 1021 nm. The amplified bandwidth of both 98%-35 mm and 98%-30 mm DKDP crystals is 195 nm, spanning from 826 to 1021 nm. Similarly, the amplified bandwidth of 70%-43 mm DKDP crystal is 190 nm, spanning from 834 to 1024 nm; the amplified bandwidth of 70%-35 mm and 70%-30 mm DKDP crystal is 189 nm, spanning from 833 to 1022 nm. It is noticeable that the amplified intensity near 825 nm is higher for the 98% DKDP crystal compared to the 70% DKDP crystal. This difference in amplified intensity near 825 nm can be attributed to the larger wave vector mismatch in the 70% DKDP crystal as compared to the 98% DKDP crystal.
3. Experimental result
In the experiment, we demonstrate broadband OPCPA centered at 925 nm using 70% and 98% DKDP crystals (provided by Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences) pumped by 526.5 nm pulses. The DKDP crystals were coated with anti-reflection films in the wavelength of 526.5 nm and wavelength range from 800 nm to 1050 nm. The damage threshold of DKDP is higher than 2.5 GW/cm2 at laser with wavelength of 526.5 nm and duration of 4 ns. The cut angles of both crystals are equal to their phase-matching angles as mentioned in section 2. The practical noncollinear angles was tuned by two broadband reflective mirrors for signal beam as shown in Fig. 2. The temporal waveforms of the input pump pulse and signal pulse in the experiment are shown in Fig. 5. The pump duration (full width at 1/e2) of 6.33 ns and the signal duration (full width at 1/e2) is 4.81 ns. The delay between signal and pump was modified by a 20-channel digital delay generator (adjustment accuracy of 100 picoseconds), which ensure that the pump pulse could cover signal pulse completely at temporal domain.
The relationship between the amplified signal energy (including the injected signal energy) and conversion efficiency at different pump intensities are shown in Fig. 6 for the following DKDP crystals:98%-35 mm (blue line), 98%-43 mm (red line), and 70%-30 mm (yellow line). Considering that the pump duration is wider than that of the signal laser, as shown in Fig. 5, the part of the pump that is not involved in the OPA process in the time domain is deducted when calculating the conversion efficiency. It is evident that the 98%-43 mm crystal exhibits higher gain compared to the other crystals for pump intensities ≤ 0.77 GW/cm2. From Fig. 6(a), it is shown that the amplified output energy continues to increase with the increase of the pump intensity. At the maximum pump intensity, the three crystals achieve maximum output energies of 1.46 J for 98%-43 mm DKDP, 1.60 J for 98%-35 mm DKDP, and 1.52 J for 70%-30 mm DKDP, respectively. From Fig. 6(b), it is shown that the 43 mm DKDP crystal reaches its highest conversion efficiency (28.4%) when the pump intensity reaches 0.62 GW/cm2. However, as the pump intensity further increases, there is a decreasing trend in the conversion efficiency, indicating the occurrence of backflow effects. For the 98%-35 mm and 70%-30 mm DKDP crystals, the conversion efficiencies are approximately 27.1% and 25.5%, respectively, at maximum pump intensity. Furthermore, the conversion efficiencies will increase with further increase in pump intensity. However, the increase in pump intensity was limited by the threshold of the dichroic mirror used in this experiment. Compared with the theoretically calculated conversion efficiency in Fig. 4, the experimentally measured conversion efficiency is generally lower. This is mainly because the temporal and spatial qualities of the pump laser and signal laser in the experiment are not as good as the ideal conditions set in the theoretical calculation.
The evolution of the signal spectrum for the 98%-43 mm crystals at different pump intensities is shown in Fig. 7 (a). It can be observed that the hollows in the seed spectrum at 894-917 nm and 933-944 nm are filled as the pump intensity increases. As a result, the final amplified spectrum is smoother compared to the seed spectrum. Figure 7(b) presents a comparison between the seed spectrum (red line) and the amplified signal spectrum for the 98%-35 mm (orange line), 98%-43 mm (yellow line), and 70%-30 mm (blue line) DKDP crystals. The spectra near 825 nm for both 98% DKDP crystals show significant amplification compared to the 70% DKDP crystal. The bandwidth (full width at 1/e2 maximum) of the amplified spectrum for the 98%-43 mm, 98%-35 mm, and 70%-30 mm DKDP crystals is 189 nm (836-1025 nm), 185 nm (836-1021 nm), and 175 nm (846-1021 nm), respectively. The amplified spectra of the three crystals were measured under the condition of a pump intensity of approximately 0.9 GW/cm2. The amplification spectral bandwidth of the 98% DKDP crystal ranges from 822 nm to 1030 nm, achieving a bandwidth of 208 nm (full width). Hence, the amplification bandwidth of the 98% DKDP crystal is larger than that of the 70% DKDP crystal, which is agreement with the simulated result.
Fig. 6. The measured output energies (a) and conversion efficiencies (b) with increasing pump intensity are shown for the following DKDP crystals:98%-35 mm (blue line), 98%-43 mm (red line), and 70%-30 mm (yellow line).
Figure 8 displays the normalized seed beam profile (a), pump profile (b), and the amplified signal beam profile (c) at a pump intensity of 0.9 GW/cm2. It can be observed that the amplified signal beam appears flatter compared to the seed profile.
Fig. 7. (a) The evolution of the amplified spectra of 98%-43 mm DKDP crystal with increasing pump intensity; (b) The spectra of seed and amplification with three DKDP crystals.
Fig. 8. The seed beam profile (a), input pump beam profile (b) and amplified signal beam profile (c).
At the pump intensity 0.6 GW/cm2, the compressed pulses were measured using WIZZLER, and the results are depicted in Fig. 9. The compressed pulses amplified by the 98%-43 mm DKDP crystal have a duration of approximately 13.7 fs at the FWHM, corresponding to the Fourier transform limit (FTL) duration of the amplified pulse of 13.4 fs. On the other hand, the compressed pulses amplified by the 70%-30 mm DKDP crystal have a duration of around 15.7 fs at the FWHM and the FTL duration of amplified pulse is 15.2 fs. It can be observed that the duration of the compressed pulses for the 98% DKDP crystal is narrower compared to the 70% DKDP crystal, which is attributed to a broader amplification bandwidth of 98% DKDP than that of the 70% DKDP crystal.
Fig. 9. Spectral phase (blue line) and compressed pulse (red line) for 98% DKDP (a) and 70% DKDP (b).
4. Summary
In conclusion, we have successfully demonstrated broadband OPCPA using both 98% and 70% DKDP crystals. This is the first reported achievement of ultra-broadband amplification (∼189 nm spectral full width at 1/e2 maximum), with a maximum conversion efficiency of 28.4% and a compressed pulse duration of 13.7 fs using the 98% DKDP crystal. Our experimental results indicate that the 98% DKDP crystal possesses a larger bandwidth compared to the 70% DKDP crystal. These findings emphasize the significance of the deuterium level in DKDP crystals for achieving broadband OPCPA. The results obtained in our experiments provide crucial insights and a valuable reference for setting up the main amplifier of a SEL-100 PW facility.
Funding
National Key Research and Development Program of China (2023YFA1608502); National Natural Science Foundation of China (11974367); Program of Shanghai Academic Research Leader (20SR014501); Youth Innovation Promotion Association of the Chinese Academy of Sciences (2019247).
Acknowledgments
We are greatly thankful to Prof. Guozong Zheng from Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences for providing us with the different deuterated DKDP crystals and the experimental assist from Shanghai High Repetition Rate XFEL and Extreme Light Facility.
Disclosures
The authors declare no conflicts of interest
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. Y. T. Wu, L. L. Ji, and R. X. Li, “On the upper limit of laser intensity attainable in nonideal vacuum,” Photonics Res. 9(4), 541–547 (2021). [CrossRef]
2. National Academies of Sciences, Engineering, and Medicine, Opportunities in Intense Ultrafast Lasers: Reaching for the Brightest Light (National Academies, 2018).
3. H. Kiriyama, A. S. Pirozhkov, M. Nishiuchi, et al., “High-contrast high-intensity repetitive petawatt laser,” Opt. Lett. 43(11), 2595–2598 (2018). [CrossRef]
4. J. H. Sung, H. W. Lee, J. Y. Yoo, et al., “4.2 PW, 20 fs Ti:sapphire laser at 0.1 Hz,” Opt. Lett. 42(11), 2058–2061 (2017). [CrossRef]
5. C. Radier, O. Chalus, M. Charbonneau, et al., “10 PW peak power femtosecond laser pulses at ELI-NP,” High Power Laser Sci. Eng. 10, e21 (2022). [CrossRef]
6. Z. B. Gan, L. H. Yu, C. Wang, et al., “The Shanghai Superintense Ultrafast Laser Facility (SULF) Project,” Progress in Ultrafast Intense Laser Science XVI. Topics in Applied Physics, vol 141. Springer, Cham. (2021).
7. M. Xu, B. Liu, L. Zhang, et al., “Progress on deuterated potassium dihydrogen phosphate (DKDP) crystals for high power laser system application,” Light: Sci. Appl. 11(1), 241 (2022). [CrossRef]
8. C. Skrobol, I. Ahmad, S. Klingebiel, et al., “Broadband amplification by picosecond OPCPA in DKDP pumped at 515 nm,” Opt. Express 20(4), 4619–4629 (2012). [CrossRef]
9. O. V. Chekhlov, J. L. Collier, I. N. Ross, et al., “35 J broadband femtosecond optical parametric chirped pulse amplification system,” Opt. Lett. 31(24), 3665–3667 (2006). [CrossRef]
10. V. V. Lozhkarev, G. I. Freidman, V. N. Ginzburg, et al., “Compact 0.56 Petawatt laser system based on optical parametric chirped pulse amplification in KD*P crystals,” Laser Phys. Lett. 4(6), 421–427 (2007). [CrossRef]
11. I. A. Begishev, C. Dorrer, S. W. Bahk, et al., “Final amplifier of an ultra-intense all-OPCPA system with 13-J output signal energy and 41% pump-to-signal conversion efficiency,” Opt. Express 31(15), 24785–24795 (2023). [CrossRef]
12. J. B. Hu, X. L. Wang, Y. Xu, et al., “Numerical analysis of the DKDP-based high-energy optical parametric chirped pulse amplifier for a 100 PW class laser,” Appl. Opt. 60(13), 3842–3848 (2021). [CrossRef]
13. .Vincent Bagnoud, Ildar A. Begishev, Mark J. Guardalben, et al., “5 Hz, >250 mJ optical parametric chirped-pulse amplifier at 1053 nm,” Opt. Lett. 30(14), 1843–1845 (2005). [CrossRef]
14. X. Wang, X. Liu, X. Lu, et al., “13.4 fs, 0.1 Hz OPCPA Front End for the 100 PW-Class Laser Facility,” Ultrafast Science 2022, 13 (2022). [CrossRef]
15. B. Shao, Y. Li, Y. Peng, et al., “Broad-bandwidth high-temporal-contrast carrier-envelope-phase-stabilized laser seed for 100 PW lasers,” Opt. Lett. 45(8), 2215–2218 (2020). [CrossRef]
16. V. V. Lozhkarev, G. I. Freidman, and V. N. Ginzburg, “Study of broadband optical parametric chirped pulse amplification in a DKDP crystal pumped by the second harmonic of a Nd: YLF laser,” Laser Phys. 15, 1319–1333 (2005).
