Ultra-intense lasers have been developed rapidly over the world since they can create extreme conditions in laboratories. The pulse energy gets higher and the pulse width gets shorter to achieve higher peak power. Chirped-pulse amplification (CPA) is one of the most successful techniques to enhance the pulse peak power. Based on the CPA technique, a number of ultra-short pulse laser systems with petawatt (PW)-class output power have been established over the world [1]. Amplification media that can realize PW-class output power include neodymium glass (Nd:glass) (1.5PW) [2], Ti:sapphire [3], and nonlinear crystals used in optical parametric chirped-pulse amplification (OPCPA) [4–6]. However, limited by the spectrum gain narrowing in neodymium glass and the transverse amplified spontaneous emission (ASE) in Ti:sapphire [7], Nd:glass and Ti:sapphire laser systems may be unsuitable for producing tens-of-PW laser pulses. The OPCPA technique combines the advantages of optical parametric amplification (OPA) and CPA techniques: high gain, wide gain bandwidth, high temporal contrast, small thermal effect, and tunable wavelength [7–10]. Theoretically, large-energy and short-pulse-width laser pulses can be obtained by OPCPA; thus, it has great potential in achieving dozens-of-PW output peak power. By this technique, PW or sub-PW pulses have been generated at 1053 nm, 910 nm, and 800 nm [1,4,11]. As to PW-class laser output, the potential of OPCPA in providing higher power, higher energy, and shorter pulse is not fully exploited.

In this Letter, we present a multi-PW laser facility named CAEP-PW based on the all-OPCPA technique, which is used for strong-field physics researches in the China Academy of Engineering Physics (CAEP). A 3-cascaded OPA system was developed. The seed pulse was amplified from 0.5 nJ to 168.7 J. The optimization of pulse compression was completed by wide-range, high-precision dispersion diagnosis and control based on space-spectrum interference [12]. A compressed pulse shorter than 20 fs is achieved in a PW-class laser facility for the first time. The highest output peak power of OPCPA is promoted to 4.9 PW. The temporal contrast is better than ${10}^{10}$ at $-20\mathrm{ps}$ before the main pulse. The capability of the OPCPA system in providing efficient amplification, near-Fourier-transform-limited pulse compression, and high temporal contrast is presented in detail.

The CAEP-PW laser facility is illustrated in Fig. 1. It consists mainly of a femtosecond oscillator, a short-pulse optical parametric amplifier (which is used to control the temporal contrast), two efficient nanosecond optical parametric amplifiers, a pulse stretcher, a pulse compressor, and pump lasers for the amplifiers.

 

Fig. 1. Schematic of the CAEP-PW laser facility.

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The duration, central wavelength, power, and repetition rate of the mode-locked Ti:sapphire oscillator are 10 fs [the corresponding bandwidth is about 100 nm in full width at half maximum (FWHM)], 800 nm, 330 mW, and 77.76 MHz, respectively. The output laser pulse is split into three for different purposes. The first one (30 mW) is used as a trigger pulse for the synchronization control system, so that the mode-locked oscillator is phase-synchronized with the control system. The second one (100 mW) is used as the seed of the whole laser facility, which is stretched by SF10 glass and DAZZLER to 0.1 ps/nm (chirp rate) and then injected to OPA-1 for amplification. The third one (200 mW) is coupled into the photonic crystal fiber to generate a super-continuum optical soliton pulse, which is then stretched by a fiber Bragg grating (the central wavelength is 1053 nm), amplified by a fiber amplifier and a Nd:glass amplifier, compressed by a pair of gratings and frequency doubled to act as the pump for OPA-1. By this means, the seed and the pump of OPA-1 are from the same source and the pump can be compressed to about 1 ps. OPA-1 is a short-pulse OPA using a type-I phase-matching beta-barium borate (BBO) crystal. The repetition rate is 1 shot per 5 s. The crystal is $10\mathrm{mm}\times 10\mathrm{mm}\times 10\mathrm{mm}$ in size with a 2.4-deg nonlinear angle between the pump and the seed pulse inside it. The duration of the pump is set to 10 ps to balance the requirements of energy stability, temporal contrast and the conversion efficiency. When the pump energy is 13 mJ, the gain of the seed is above ${10}^6$, which means OPA-1 amplifies the seed from 0.5 nJ to 2 mJ. The spectrum after OPA-1 is shown in Fig. 2.

After amplification in OPA-1, the seed is stretched to 3 ns (FWHM) with a chirp rate of 40 ps/nm. The energy loss is about 10 times in the stretcher. Another type-I phase-matching BBO crystal is employed in OPA-2 for amplification, which provides ${10}^3$ saturated gain for the seed and amplifies the seed from 0.2 mJ to 200 mJ at a repetition rate of 1 shot per 40 s. The pump laser for OPA-2 is first generated by a single longitudinal mode fiber oscillator, then injected to an arbitrary waveform generator (AWG) for single-mode selection and temporal pulse shaping and amplified to 3 ns/500 nJ by a fiber amplification chain. Further amplification is done by a Nd:glass amplification chain, which includes near-field pulse shaping by a liquid crystal spatial light modulator. A $15\mathrm{mm}\times 15\mathrm{mm}\times 15\mathrm{mm}$ type-I phase-matching BBO crystal is used as the optical parametric crystal. Both sides of the crystal are coated with 527 nm and 800 nm broadband anti-reflection films. The wedge angle of each end is 0.5 deg. The non-collinear angle between the pump and the seed is about 2.4 deg (inside the crystal) and the phase matching angle is about 24 deg. The output energy and optical parametric transfer efficiency are shown in Fig. 3. When the energy, aperture, and duration of the pump are 640 mJ, $\mathrm{\varphi }8\mathrm{mm}$, and 3 ns, respectively, the output energy of the signal is about 221 mJ with a spectral width (FWHM) of 70 nm (see Fig. 2). The highest optical parametric conversion efficiency is up to 34.5%.

 

Fig. 2. Typical output spectrum of three amplifiers.

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Fig. 3. Amplified energies and conversion efficiency of OPA-2.

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OPA-3 is a high-energy optical parametric amplifier (ns-OPCPA). The shot-to-shot duration is 2 h. The seed is expanded and shaped by a spatial aperture and two spatial filters to match the size of the ${\text{LiB}}_2{\text{O}}_3$ (LBO) crystals. The pump of OPA-3 uses the same single-longitudinal-mode fiber oscillator and arbitrary waveform generator as the ones in OPA-2. Further amplification of the pump is also done by fiber amplifiers and Nd:glass amplifiers. The output energy is around 1200 J/3 ns/527 nm. The pump is shaped to be super Gaussian shape in both space and time domains, which facilitates promotion of the energy transfer efficiency. $165\mathrm{mm}\times 120\mathrm{mm}\times 10\mathrm{mm}$ and $130\mathrm{mm}\times 130\mathrm{mm}\times 10\mathrm{mm}$ type-I phase-matching LBO crystals are employed in OPA-3. Both sides of the crystals are coated with 527 nm and 800 nm broadband anti-reflection films. The wedge angle of each end is 0.5 deg. The non-collinear angle between the pump and the seed is about 1.2 deg (inside the crystal) and the phase matching angle is about (90, 13.8) deg. The output energy and optical parametric transfer efficiency are shown in Fig. 4. In the beginning, the seed power increases as the pump power increases. Then it enters the saturated amplification phase and obvious energy backflow is found. But the highest seed peak power and highest energy transfer efficiency are not obtained at the same time, which is mainly due to the near-field nonuniformity of the pump laser. When the beam size and the pump energy are $105\mathrm{mm}\times 105\mathrm{mm}$ and 694 J ($2.1\mathrm{GW}/{\mathrm{cm}}^2$), respectively, the seed energy reaches the maximum: 168.7 J. When the pump energy is 600 J, the energy transfer efficiency reaches the maximum: 25.0%. The near-field profile of the signal pulse after OPA-3 is shown in Fig. 5.

 

Fig. 4. Amplified energies and conversion efficiency of OPA-3.

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Fig. 5. Near-field profile after OPA-3.

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As shown in Fig. 2, the spectral width (FWHM) of OPA-3 is about 65 nm. The spectral narrowing after OPA-3 results from the shorter pulse width of the pump laser. The output pulse of OPA-3 is first expanded by 3 times in a spatial filter then injected into a pulse compressor, which consists of 4 large-scale gold-plated 1740 grooves/mm gratings ($370\mathrm{mm}\times 970\mathrm{mm}$), whose incident angle is 53 deg and the distance between grating pair is 1.6 meter. The optimization and spectral phase measurement were completed by wide-range, high-precision dispersion diagnosis and control based on space-spectrum interference [12]. We measured the pulse width and temporal contrast of the pulse. The pulse shape of the output laser measured by an autocorrelator is shown in Fig. 6. The pulse width is 18.6 fs after Gaussian profile fitting. The transmission efficiency of the compressor (including the dichroic mirror and spatial filter after OPA-3) is around 54%. The maximum pulse energy is 91.1 J after the compressor, and the laser peak power is up to 4.9 PW. This is the first time that the pulse duration of a PW-class laser facility is compressed to be shorter than 20 fs. The temporal contrast was measured by a single-shot third-order autocorrelator [13]. The temporal profile after the compressor is shown in Fig. 7. The temporal contrast at $-20\mathrm{ps}$ before the main pulse reaches ${10}^{10}$, which is the measurement limit of the autocorrelator. The obvious change of temporal contrast in 20 ps before the main pulse is mainly attributed to the optical parametric fluorescence and spectral-phase distortion. After the compressor, the beam is focused by an on-axis parabolic mirror with a focal length of 800 mm ($f/1.7$) in the target chamber. The wavefront aberrations are corrected by a deformable mirror. After correction, the far-field quality is improved greatly (see Fig. 8 for the focal spot images before and after correction). The focal spot size (FWHM) is $\mathrm{\varphi }6.5\mathrm{\mu m}$ after correction [see Fig. 8(b)] and the enclosed energy in $\mathrm{\varphi }10\mathrm{\mu m}$ is around 30%.

 

Fig. 6. Compressed duration of 18.6 fs (single-shot autocorrelation trace, by applying a Gaussian deconvolution factor).

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Fig. 7. Temporal profile after the compressor measured by a single-shot third-order autocorrelator.

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Fig. 8. Focal spot images before (a) and after correction (b).

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In conclusion, we demonstrate a multi-PW laser facility based on the all-OPCPA technique. The pump laser and seed laser of the short-pulse OPA were synchronized based on the super-continuum-generated pulse spectrum broadening technique. High-gain and stable amplification of the short-pulse OPA have been achieved. High temporal contrast at $-20\mathrm{ps}$ before the main pulse is assured due to the short-pulse OPA. The measured temporal contrast at $-20\mathrm{ps}$ before the main pulse reaches ${10}^{10}$, which demonstrates the high temporal contrast output ability of the all-OPCPA system. We shaped the pump pulse in both time and space domains. The maximum transfer efficiency achieved is 34.5% and 25.0% for OPA-2 and OPA-3 using BBO and LBO crystals, respectively. Measures to further improve the transfer efficiency include shaping the seed in both time and space domains and improving the near-field quality of the pump. We developed a wide-range, high-precision dispersion diagnosis and control method based on space-spectrum interference and completed the optimization and spectral phase measurement of the compressor. A compressed pulse shorter than 20 fs is achieved in a PW-class laser facility for the first time. The maximum output peak power of the all-OPCPA system is promoted to 4.9 PW. With the increase of the effective diameter of LBO crystals, the output power can be promoted further. In the next step, output energy of several-hundreds-joule and peak power of 15 PW will be achieved using $200\mathrm{mm}\times 200$ LBO crystals.