2.1 Experimental setup and incidence laser

The schematic of the experiment, based on the SG-II 5 PW laser facility, is presented in Figure 1. The incidence signal pulses are originally generated by a commercial Ti:sapphire mode-locked oscillator. A small portion of the pulse chain is used as a trigger for the electronic synchronization system, which ensures synchronization accuracy of 14 ps root mean square (RMS) for OPCPA-I and 100 ps RMS for OPCPA-II. The majority of the seed pulses are stretched by an Öffner stretcher, delivered into a pre-compressor and amplified by the preamplifier. A chromatic aberration pre-compensator (CAPC) was installed for pre-compensation of the chromatic aberration deriving from the large aperture transmission spherical elements in the spatial filters (SFs), SF3, SF4 and SF5. It was aligned before SF3 and the capability of high enhancement on the focusing characteristics at the target was verified^[ Reference Cui, Kang, Gu, Zhu, Yang, Zhu, Sun, Gao, Liu, Ouyang, Zhang, Wei, Liang, Zhang, Yang, Zhang, Xie and Zhu ^{31 ]}. The seventh beam of the SG-II facility was used as the pump for the PW-scale amplifier, where the YCOB crystal was located. The high-energy chirped pulse after amplification was delivered into the adaptive optics (AO), by which the wavefront was measured by a Hartmann sensor and corrected by a deformable mirror. Finally, the chirped pulse was injected into the master compressor, after which the compressed pulse was measured with the entire diameter by a home-made autocorrelator.

Figure 1 Schematic of the experiment. SHG, second harmonic generation; AO, adaptive optics; OAP, off-axis parabolic mirror.

Compared with the entire system illustrated in Ref. [Reference Zhu, Xie, Sun, Kang, Yang, Guo, Zhu, Zhu, Gao, Liang, Cui, Yang, Zhang and Lin9], several changes were made for a better performance. The pre-compressor, which was aligned in a Treacy configuration for high-order dispersion control, was moved after the Öffner stretcher, because energy loss deriving from the degradation of the transmission efficiency of the gratings was intolerable when located after OPCPA-I. The grating pairs in the pre-compressor also provided negative linear component dispersion (group velocity dispersion, GVD), and the chirped ratio of seed pulses injected into OPCPA-I at present was deduced to be 21.3 ps/nm, while the energy per pulse was reduced to approximately 0.1 nJ. The pump for OPCPA-I was provided by the Nd:YAG laser, which delivered pulses of 450 mJ in 2.2 ns duration at 532 nm with 1 Hz repetition rate, and four BBO crystals were aligned as before^[ Reference Liang, Xie, Kang, Yang, Wei, Sun and Zhu ^{32 ]}. In our previous work, the pump energy allocation for the prior and the lateral two BBO amplifiers was set (52 and 398 mJ) for optimizing the temporal contrast in pure nanosecond amplifiers^[ Reference Sun, Xie, Zhu, Zhang, Zhang, Zhu, Guo, Kang, Zhu, Yang and Liang ^{33 ]}. However, in the current condition the injected chirped signal pulse energy was half that of before, so the pump energy allocation was reset (90 and 360 mJ) to ensure a delivery capacity of 150 mJ. The lower energy seed and high-energy pump would degrade the contrast. The waveform and spectrum of the signal pulse after OPCPA-I are represented by the blue lines in Figures 2 and 3, respectively. With a smaller chirped ratio, a broader spectrum bandwidth was temporally overlapped by the pump pulse, and the amplified signal pulse was in the spectrum ranging from 770 to 860 nm and with temporal duration of approximately 1.8 ns. The SFs for image relay were used as before^[ Reference Zhu, Xie, Sun, Kang, Yang, Guo, Zhu, Zhu, Gao, Liang, Cui, Yang, Zhang and Lin ^{9 ]}. However, owing to the reflection on 25 element surfaces and bulk losses in the polarized beam splitter (PBS) in the CAPC^[ Reference Cui, Kang, Gu, Zhu, Yang, Zhu, Sun, Gao, Liu, Ouyang, Zhang, Wei, Liang, Zhang, Yang, Zhang, Xie and Zhu ^{31 ]}, the total transmission efficiency was limited to approximately 17% and a chirped pulse of approximately 25 mJ was injected into OPCPA-II. The signal after the CAPC was image relayed and beam expanded to $\Phi$ 90 mm by SF3 to ensure that the pump and signal can be well spatially overlapped.

Figure 2 Normalized waveforms of the incidence signal (blue), amplified signal (red) and incidence pump (green) for OPCPA-II.

Figure 3 Normalized spectra of the incidence (blue) and amplified (red) signal, compared with the theoretical gain of YCOB-based OPCPA-II (green).

A thicker crystal as well as higher pump peak power was required to realize a delivery capacity of tens of joules. The duration of the pulse from the seventh beam of the SG-II facility was 2.1 ns and the energy was around 125 J at 1053 nm. The beam aperture was measured as approximately 60 mm square. A large aperture potassium dihydrogen phosphate (KDP) crystal was installed for second harmonic generation and the optimal conversion efficiency was about 80%. Those specifications above ensured a pump pulse of energy of approximately 100 J and intensity of approximately 1.3 GW/cm² for OPCPA-II. The waveform of the pump, measured by a phototube and oscilloscope (Tektronix TDS694C), is presented by the green line in Figure 2.

A non-collinear geometry based on YCOB crystal was adopted in OPCPA-II. The crystal was phase-matched at (θ = 26.37°, φ = 180°) in the type-I phase matching configuration with an internal non-collinear angle of 2.74°. The non-collinear configuration was set in the XOZ principal plane with an effective nonlinear coefficient d _eff of approximately 0.95 pm/V because a much larger gain bandwidth can be theoretically achieved than that achieved out of principal planes with d _eff of approximately 1.4 pm/V^[ Reference Pires, Galimberti and Figueira ^{22 ,} Reference Ma, Xiong, Yuan, Tu, Wang, Xie, Zheng and Qian ^{25 ]}. Numerical simulation based on coupling wave equations was carried out with the parameters of both incident laser pulses and phase-matching conditions^[ Reference Zhu, Xie, Sun, Kang, Yang, Guo, Zhu, Zhu, Gao, Liang, Cui, Yang, Zhang and Lin ^{9 ]}. It indicated an optimal crystal length of 27 mm to realize a high gain as well as stability. The normalized gain curve, presented by the green line in Figure 3, indicated a spectrum ranging from 750 to 880 nm, which was much larger than that of the incident signal.

A YCOB crystal boule, with a size of approximately $\Phi$ 108 mm × 100 mm, was grown by the Bridgeman method, as reported elsewhere^[ Reference Tu, Wang, Xiong, Zheng and Shi ^{27 ]}. Based on the phase-matching parameters above, the crystal was oriented and cut into an element at a size of $\Phi$ 100 mm × 27 mm. A wedge angle of 1° was processed to prevent parasitic oscillation. The two clear surfaces were polished without coating. The reflection and transmission properties were measured by a ZYGO interferometer. As shown in Figure 4, the peak to volley (PV) of the reflection wavefront was 0.15λ in a circular region of $\Phi$ 85 mm, and the PV of the transmission wavefront was smaller than 0.3λ, which indicated a good material uniformity.

Figure 4 The reflection (a) and transmission (b) properties of YCOB crystal measured by a ZYGO interferometer.

2.2 OPCPA performance of YCOB crystal

Numerical simulation, in which the spatial profiles of the signal and pump were assumed to be ideal super-Gaussians, for the signal pulse energy versus pump pulse energy, is represented by the red solid line in Figure 5(a). The maximal amplified signal energy would be up to 46 J in theory, corresponding to pump pulse energy of 100 J. In the experiment, the signal pulse energy was smaller than that in the simulation, shown by the red circles in Figure 5(a). The energy of the signal pulse increased rapidly with pump pulse energy, and the highest energy was 40.3 J when the pump pulse energy was 96 J, which indicated a conversion efficiency of 41.98%. The net gain of YCOB-based OPCPA-II was up to 1600. The pump-to-signal conversion efficiency is also provided by the blue solid line in theory and blue circles in the experiment in Figure 5(b). Maximum conversion efficiency of 42.3% was achieved corresponding to pump energy of 92.3%, and the conversion efficiency remained higher than 40% when the pump pulse energy exceeded 88 J.

Figure 5 (a) Amplified signal energy achieved by numerical simulation (red solid line) and measured in the experiment (red circles); (b) conversion efficiency obtained by numerical simulation (blue solid line) and experimental measurement (blue circles).

It was concluded in Ref. [Reference Yang, Liang, Xie, Yang, Tu, Zheng, Zhang, Zhang, Guo, Zhu, Kang, Sun and Zhu23] that both the amplified spectrum and output energy were very sensitive to the non-collinear angle. In this experiment, we measured the amplified signal energy and obtained the conversion efficiency by precisely tuning the phase-matching angles with a fixed non-collinear angle. As illustrated in Figure 6, the acceptance angle, referring to the full width at half maximum (FWHM) of the conversion efficiency in practice, could be calculated to be approximately 0.08°, which was larger than the 0.02° achieved in the numerical analysis. For comparison, an acceptance angle of a 19 mm LBO crystal was measured as 0.2° for the same sort of gain levels^[ Reference Zhu, Xie, Sun, Kang, Yang, Guo, Zhu, Zhu, Gao, Liang, Cui, Yang, Zhang and Lin ^{9 ]}.

Figure 6 OPCPA conversion efficiency versus the deviation from the optimal phase-matching angle, when the non-collinear angle was set at 2.74°.

The waveform of the amplified signal is represented by red line in Figure 2 and the pulse width was broadened to 1.94 ns, compared with the 1.8 ns of incidence. The pulses for spectrum measurement in our experiment were sampled and attenuated by reflection with optical polished substrate glasses. They were measured by focusing the full-aperture laser beam into a spectrometer (Ocean Optics HR4000+), shown by the red line in Figure 3. A spectrum ranging from 770 to 860 nm was fully amplified in the YCOB-based OPCPA-II with an FWHM spectral bandwidth of 87 nm, which is the largest among PW-scale amplifiers ever reported, to the best of our knowledge^[ Reference Li, Gan, Yu, Wang, Liu, Guo, Xu, Xu, Hang, Xu, Wang, Huang, Cao, Yao, Zhang, Chen, Tang, Li, Liu, Li, He, Yin, Liang, Leng, Li and Xu ^{3 ,} Reference Lureau, Matras, Chalus, Derycke, Morbieu, Radier, Casagrande, Laux, Ricaud, Rey, Pellegrina, Richard, Boudjemaa, Simon-Boisson, Baleanu, Banici, Gradinariu, Caldararu, De Boisdeffre, Ghenuche, Naziru, Kolliopoulos, Neagu, Dabu, Dancus and Ursescu ^{4 ,} Reference Kiriyama, Pirozhkov, Nishiuchi, Fukuda, Ogura, Sagisaka, Miyasaka, Mori, Saksaki, Dover, Kondo, Koga, Esirkepov, Kando and Kondo ^{7 ,} Reference Zeng, Zhou, Zuo, Zhu, Su, Wang, Wang, Huang, Jiang, Jiang, Guo, Xie, Zhou, Wu, Mu, Peng and Jing ^{10 ,} Reference Sung, Lee, Yoo, Yoon, Lee, Yang, Son, Jang, Lee and Nam ^{12 ,} Reference Papadopoulos, Ramirez, Genevrier, Ranc, Lebas, Pellegrina, Le Blanc, Monot, Martin, Zou, Mathieu, Audebert, Georges and Druon ^{13 ,} Reference Lozhkarev, Freidman, Ginzburg, Katin, Khazanov, Kirsanov, Luchinin, Mal'shakov, Martyanov, Palashov, Poteomkin, Sergeev, Shaykin and Yakovlev ^{16 ]}.

The spatial profile of the amplified signal pulse was measured with an online diagnosis system, sharing the same light path with AO assemblies. It is presented in Figure 7(a); the full aperture was equal to that of the pump beam. For comparison, an experiment using 19-mm-thick LBO crystal was accomplished. To ensure the output energy and conversion efficiency with LBO being the same scale as YCOB, the pump pulse duration was reset to 1.6 ns so that the peak intensity was approximately 2 GW/cm², which was still well below the damage threshold of LBO. However, the output bandwidth with LBO was only 75 nm. Figure 7(b) provides the spatial profile of the amplified signal pulse after the LBO-based OPCPA-II for comparison. We used the fill factor (FF) and fluence beam contrast (FBC) to quantitatively describe the near-field distribution characteristics^[ Reference Zhu, Xie, Sun, Kang, Yang, Guo, Zhu, Zhu, Gao, Liang, Cui, Yang, Zhang and Lin ^{9 ]}. A numerical analysis showed that the [FF, FBC] were [0.56, 0.12] for YCOB and [0.45, 0.2] for LBO, respectively. The YCOB-based OPCPA was capable of attaining a good near-field distribution, which was comparable to that achieved based on LBO.

Figure 7 Near fields of the amplified signal in OPCPA-II based on YCOB (a) and LBO (b) crystals.

To investigate the influence of material uniformity of YCOB crystal presented by the transmission property in Figure 4(b) on the parametric process, the wavefront was measured by the AO system^[ Reference Zhu, Xie, Sun, Kang, Yang, Guo, Zhu, Zhu, Gao, Liang, Cui, Yang, Zhang and Lin ^{9 ]}. Wavefront aberration as the static quantity was firstly measured when only OPCPA-I was operated at a 1 Hz repetition rate. After that, it was corrected by the deformable mirror. The shape of the deformable mirror was kept and a new wavefront was measured by the Hartmann sensor when both OPCPA-I and OPCPA-II were operated. The dynamic wavefront aberration derived from the YCOB-based OPCPA–II is shown in Figure 8(a), and the PV and RMS were 2.452λ and 0.388λ, respectively. For comparison, the dynamic wavefront aberration from the LBO-based OPCPA–II is also presented under the same measurement condition, and the PV, RMS values of 2.098λ, 0.351λ were of the same scale as that of YCOB. This indicates a comparable focusing property between YCOB and LBO.

Figure 8 The dynamic wavefront aberrations measured by a Hartmann sensor in the adaptive optics assembly for OPCPA–II with YCOB crystal (a) and LBO crystal (b).

With our home-made second-order autocorrelator^[ Reference Yang, Cui, Kang, Guo, Zhu, Sun, Zhu, Gao, Xie and Zhu ^{34 ]}, the compressed pulse traces after the master compressor were measured and are shown in Figure 9. The sampled pulse for measurement was of the whole beam size of 290 mm × 290 mm. The Gaussian profile fitting showed that the FWHM pulse duration was 22.3 fs. The total transmission efficiency of the master compressor and off-axis parabolic mirror (OAP) was measured to be 65.6%, on average. The pulse energy after compression exceeds 26 J and the peak power can be estimated as 1.17 PW.

Figure 9 The autocorrelation (AC) trace of a signal pulse after the master compressor.