1. Introduction

Benefiting from the development of nonlinear crystals, optical parametric chirped-pulse amplification (OPCPA) has become a promising technique to build ultra-short ultra-intense laser system toward the peak power of multi-petawatt or even higher at various spectral ranges [1]. In the visible region, especially the most common wavelength of 800 nm which seeded by Ti: sapphire mode-locked oscillator, two multi-petawatt laser systems fully based on OPCPA have been reported [2,3]. In these two laser systems, beta-barium borate (β-BaB₂O₄, BBO) and lithium triborate (LiB₃O₅, LBO) are the mainly used nonlinear crystals for OPCPA amplifiers. Due to the high nonlinear coefficient and broad gain bandwidth near 800 nm wavelength, BBO is more favorable in the high gain OPCPA amplifier either in the pulse duration of picosecond or nanosecond domain [4–6]. However, what limits its application to high energy OPCPA amplifier is that the available crystal aperture of BBO does not exceed few centimeters. Then, LBO which has crystal aperture more than 100 millimeters and higher damage threshold becomes the mainstream option for the boost and final OPCPA amplifiers of multi-petawatt laser system [2,3,7,8]. Another nonlinear crystal which has been widely studied and utilized in OPCPA is potassium dideuterium phosphate (KD₂PO₄, DKDP). It is found that its gain bandwidth at 800 nm strongly depends on the deuteration level, a 95% deuterated DKDP has been successfully verified to generate sub-30 fs pulses with high energy [9]. But the biggest advantage of DKDP is the large crystal aperture which can be grown to 300∼400 mm [10], and the extensive gain bandwidth of over 190 nm for parametric amplification around 910 nm [11,12]. Despite these advantages, the DKDP has a low nonlinear coefficient and poor thermo-optical properties because of the optical absorption above 800 nm for low deuteration, so some countries and laboratories have proposed to build fully OPCPA-based, tens to one hundred petawatt laser systems at low repetition rate [13,14].

Recently, the rare-earth doped crystal, yttrium calcium oxyborate (YCOB), has gained growing attention for being used as the nonlinear medium of OPCPA. The reasons are as follows. For the capability of parametric amplification, the theoretical analysis has revealed that YCOB and BBO exhibit comparable OPCPA gain bandwidth at the 808 nm wavelength [15,16]. Secondly, compared to the crystals mentioned above, YCOB features remarkable thermal properties, such as large temperature acceptance, high thermal conductivity and low coefficient of thermal expansion. In addition, YCOB can also be grown to large size (very recently 5 in. crystals were grown, leading to a usable aperture of 100 mm × 100 mm [17]) to support high energy amplification. Consequently, the comprehensive performance makes this nonlinear crystal become a promising choice to build high average power petawatt laser with pulses of tens of joule and few-cycle duration. The first joule-level OPCPA in YCOB at 800 nm was demonstrated by Yu et al. They produced 3.36 J amplified pulses that were then compressed to 44.3 fs [18]. However, their injected signal bandwidth to the OPCPA amplifier is 37 nm (FWHM) which greatly limits YCOB from reaching its gain bandwidth capacity at 800 nm. The largest gain bandwidth of near-IR OPCPA laser system using YCOB was reported by H. Pires et al., recently [19]. They used two short YCOB crystals (5 mm and 7.4 mm) and 690 fs pump pulse to obtain a gain bandwidth of 150 nm with moderate signal net gain (14 and 45 for two stages respectively).

For the generation of extreme intensity laser in few cycle pulse duration at 808 nm, the optical parametric amplifier is demanded to simultaneously achieve gain of billion scale and spectrum bandwidth over hundred nanometer, which has been theoretically predicted while not been experimentally realized in YCOB crystals by now. In this work, the OPCPA capability of YCOB crystal is exploited greatly by realizing much high signal gain with ultra-broadband gain spectrum. For both high gain amplification (HGA) region and saturated amplification (SA) region, the properties are carefully studied through an OPCPA system consisting of four YCOB crystals. We finely tuned the non-collinear angle to study the variation of gain spectrum at a certain phase-matching angle of YCOB crystal. After the amplification in four YCOB crystals, a total gain of 0.9×10⁹ and spectral bandwidth beyond 100 nm were simultaneously obtained. The amplified signal energy of 182 mJ was delivered and the OPA conversion efficiency of ∼40% for SA stage was realized. Finally, a pulse duration of 20 fs was measured after the master compressor in SG-II 5PW facility [3].

2. Experimental setup

In this section, we describe the overall structure of the YCOB-based OPCPA system whose detail schematic is shown in Fig. 1. The pump pulse for this OPCPA stage is roughly the same as the one used in the OPCPA frontend of SG-II 5PW facility except for a longer pulse duration. The seed of pump laser is a continuous wave single longitudinal mode (CW-SLM) laser operating at 1064 nm. An acousto-optic chopper trims the CW laser to 1 Hz repetition rate and an electro-optic modulator with an adjustable rising edge is used to make the pulse time pre-compensation for the subsequent amplification gain saturation. The laser pulse output from electro-optic modulator has 3 ns pulse duration of full width and 1 nJ energy. This temporally shaped pump pulse is amplified to 1 mJ in a diode-pumped Nd: YAG regenerative amplifier, then it is injected into the following Nd: YAG rod amplifiers chain, after which the output pump pulses go through a PPKTP crystal for a second harmonic generation (SHG). Finally, a 520 mJ pump laser at 532 nm with super-Gaussian spatial-temporal profiles is available at repetition rate of 1 Hz, and the stability is measured at 1.1% RMS. The FWHM of laser beam diameter and temporal pulse width are 10 mm and 3 ns respectively.

 

Fig. 1. Schematic of the YCOB based OPCPA system, EOM: electro-optical modulator.

Download Full Size | PDF

The signal is seeded by a commercial Ti: sapphire mode-locked oscillator, which delivers 2 nJ, 10 fs pulse at 85 MHz repetition rate. The time synchronization of pump and signal is realized through electronic synchronization devices triggered by the pulse sequence from mode-locked oscillator. The seed pulses are temporally expanded to 2.5 ns FWHM in an Öffner stretcher with -25.5 ps/nm chirped ratio and the chirped signal energy per pulse is about 0.2 nJ. The pump duration of 3 ns and stretching factor of -25 ps/nm indicate an overlap of a signal spectrum region of 749 nm ∼ 866 nm, which almost takes the whole seed spectrum to be included. A longer pump pulse duration for researching a broader gain bandwidth can’t be achieved because of the large phase-mismatching in the short wave side (<755 nm) and negligible incident seed pulse in the long wave side (>865 nm). In order to verify the OPCPA capability of YCOB, both HGA and SA are experimented with total four YCOB crystals. All these crystals are phase-matched at XOZ principle plane with an angle of θ = 27.1°, φ = 180° which can theoretically achieve the largest gain bandwidth in the type-I phase matching configuration with an internal non-collinear angle of 2.74° [15]. The HGA is tested in YCOB-1 and YCOB-2 and the SA in YCOB-3 and YCOB-4, the lengths of four crystals are 22 mm, 27 mm, 25 mm and 25 mm respectively. As adopted in [6], the crystal lengths were optimized by the numerical calculations based on coupling wave equations. To guarantee the spatial beam quality, the pump pulses for HGA and SA are all image relayed in the center of two YCOB crystals, using the telescope systems that decrease the beam diameter to 2 mm and 5.6 mm respectively. The energy for two OPCPA amplifiers is 80 mJ and 440 mJ, corresponding to the power density of ∼0.85 GW/cm² and ∼0.6 GW/cm² respectively. The signal after HGA will also be image relayed to the SA stage with beam expansion ratio of three times for which the pump and signal can be well spatially overlapped.

3. OPCPA output performance

Both OPCPA amplifiers at HGA and SA adopt “L” type amplifier configuration to compensate the walk-off effect induced by pump pulse (extraordinary light in type-I phase matching) and to prevent the signal to pump energy back conversion at SA [6]. All the spectrums in our experiment were measured by focusing the full-aperture laser beam into a spectrometer (OceanOptics-HR2000+).

Figure 2(a) presents the normalized spectrums before and after various crystals. The full bandwidth of incident signal seed before YCOB-1 is ∼120 nm and FWHM is 100 nm centered at 808 nm wavelength as the red line shown in Fig. 2(a). After amplified in HGA, the energy of signal is boosted to 5 mJ corresponding to a gain of 2.5×10⁷. The blue line is the spectrum of single signal pulse from HGA amplifier. Comparing to the incident spectrum, it shows bandwidth narrowing under such high gain. However, FWHM bandwidth of 60 nm is still maintained and the spectrum near 760 nm and 850 nm are also obviously amplified while gain is smaller, which is due to phase mismatching and indicated by Fig. 5(b) in [15]. Besides, the temporal waveform was also measured by a photodiodes (EOT pin detector) and oscilloscope (Tektronix TDS694C), as illustrated in Fig. 2(b). The amplified signal pulse after YCOB-2 shows a narrowed pulse width ∼1.8 ns compared to the 2.55 ns of incident seed.

 

Fig. 2. The normalized signal and optical parametric fluorescence spectrums for various cascaded YCOB crystals, as well as the spectrum (cyan line) from BBO-based OPCPA (a), and the waveforms of incident pump pulse and signal pulses after different YCOB crystals (b).

Download Full Size | PDF

In the SA stage, after the OPCPA amplification in YCOB-3 and YCOB-4, the spectrums are further broadened and smoothed. As illustrated in Fig. 2(a), both YCOB-3 (green line) and YCOB-4 (black line) express spectrums with bandwidth of >100 nm. In particular, the spectrum after YCOB-4 can span from 750 nm to 863 nm, which can support a compression of ultra-short pulse duration of 20 fs. Figure 3 shows that with pump laser increasing to 440 mJ, a conversion efficiency of ∼40% with 182 mJ signal energy can be measured after YCOB-4. For comparison, the spectrum delivered by BBO-based front end of SG-II 5PW when pump with the identical 3 ns pump pulses, is presented by the cyan line (BBO-4) in Fig. 2(a) [6]. The maximized signal pulse energy from BBO-4 is ∼175 mJ, which is of the same scale with that of YCOB-4. Thus, it is demonstrated that the YCOB-based OPCPA is comparable with that of BBO in terms of the ultra-broadband spectrum and the high-gain amplification. The optical parametric fluorescence (OPF) is measured to be 45 mJ after YCOB-4, under the condition that the seed pulse is blocked before YCOB-1, and the OPF spectrum is presented by the yellow line in Fig. 2(a), which indicates an optimal phase matching at ∼808 nm for the whole amplification chain. The corresponding pulse width of amplified signal approaches to ∼2.3 ns and ∼2.5 ns after YCOB-3 and YCOB-4, as shown by the green line and black line in Fig. 2(b), respectively. The spatial distribution of signal beam from YCOB-4 was also measured. As given in Fig. 3(c), the signal beam quality was not deteriorated by the walk-off effect after SA, resulting in a relatively uniform spatial profile.

 

Fig. 3. Conversion efficiency (a), energy (b) and spatial profile (c) of signal after YCOB-4.

Download Full Size | PDF

Considering the sensitivity of OPCPA on the configuration [15], the amplification under various non-collinear angles changing around 2.74° was also researched after YCOB-3 with YCOB-4 uninstalled. By adjusting crystal to realize the identical gain at central wavelength of 808 nm, the output signal characteristic including the amplified spectrum and energy was measured against non-collinear angle. As presented in Fig. 4, the maximal energy of 95 mJ and largest bandwidth with the smoothest spectrum spanning from 755 nm to 860 nm (FWHM ∼100 nm) were obtained at 2.74°. And the fluctuation of amplified energy as well as bandwidth is small at the range of 2.74°±0.02°. However, with the angle deviation from 2.74° further increasing, the amplified energy starts to decrease obviously and the inhomogeneous amplification on different wavelength gets severe, which results in a decline of FWHM even the full bandwidth is nearly unchanged. Hence it proves the high dependence of OPCPA in SA stage on the non-collinear angle, which can be deduced to have much stronger impact on the amplification in HGA stage.

 

Fig. 4. The amplified spectrums (a) and signal energy (b) after YCOB-3 under different non-collinear angles.

Download Full Size | PDF

Finally, the amplified pulse after four YCOB crystals was delivered into the master compressor of SG-II 5PW facility. The compressed pulse traces, measured by our home-made second-order autocorrelator [20], are shown by the black line in Fig. 5. The Gaussian profile fitting shows that the FWHM pulse duration is ∼20 fs, which is close to the Fourier-limited (FL) pulse width of 18.5 fs, shown by the red line. With an additional dynamic chromatic aberration pre-compensation module [21], the pulse energy after compression is about 36 mJ and the power can be estimated ∼1.8 TW.

 

Fig. 5. The autocorrelation (AC) traces of signal pulse (black line) after master compressor and FL pulse (red line) by calculation.

Download Full Size | PDF

4. Summary

In summary, the YCOB-based OPCPA system in this paper realized a total signal gain of 0.9×10⁹ with pulse energy up to 182 mJ, and the conversion efficiency of ∼40% was achieved in the SA stage. Specially, the smooth spectrum with wavelength range of 750-863 nm was also obtained after YCOB-4. The compressed pulse width of ∼20 fs is measured and nearly coincident with the FL duration calculated from the spectrum after the cascaded amplifiers. Above all, the YCOB-based OPCPA has realized comparably excellent characteristic with that of BBO at the wavelength of 808 nm. And comparing with the widely used LBO in high-energy amplification, the gain broadband of YCOB is more remarkable. Considering the availability of large aperture (>100 mm) of YCOB, it indicates great application prospect for the petawatt laser facilities in the future.