Open Access
Add to BibSonomy
Abstract
A high-energy, high-beam-quality, high-contrast picosecond optical parametric chirped-pulse amplification (ps-OPCPA) laser system was demonstrated. The pulse from a femtosecond oscillator was stretched to 4 ps, after which it was amplified from 140 pJ to 600 µJ by an 8 ps/6 mJ pump laser in two non-collinear OPCPA stages. The total gain was >106, and the root mean square of the energy stability of the laser system was 1.6% in 10 h. The contrasts of the solid and fiber mode-locked femtosecond oscillator-seeded ps-OPCPA systems were compared, and a signal-to-noise ratio of >1011 was achieved. Using this system, the contrast of the front end in high-power picosecond petawatt laser facility was improved by ∼40 dB to >1011, beyond ∼200 ps ahead of the main pulse with an output level of 60 mJ.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The contrast of high-power laser facilities has become a bottleneck challenge, which restricts fast-ignition and other high-field physics experiments (X-ray and high-order harmonic generation and plasma physics). Many methods have been used to improve the contrast of ultra-short, ultra-intense laser pulses. The noise generated during ultra-intense and ultrashort pulse amplification is mainly generated in the high-gain pre-amplification stage of optical parametric chirped-pulse amplification (OPCPA) [1]. OPCPA has become a key technology in present high-energy, short-pulse laser systems and the future development of exawatt (1018 W) laser systems because of its advantages, such as high contrast, high gain, high beam quality, and wide gain bandwidth. [2–7] Related theories and experiments have proven that the contrast of OPCPA is mainly affected by spectral clipping led by the stretcher [8,9], spectral modulation caused by spectral phase noise [10], amplified spontaneous emission (ASE) noise of the pump [11,12], and parametric fluorescence from optical parametric amplification [13–15]. Concurrently, many methods, such as saturable absorption [16–18], nonlinear Sagnac interferometry [19], nonlinear elliptical polarization rotation [20–23], filtration of the generated cross-polarized wave [24,25], plasma mirroring [26–29], and dual-stage chirped-pulse amplification, have been proposed to improve the signal-to-noise ratio (SNR) [30]. Although these nonlinear pulse-cleaning techniques can improve the contrast to 1011, the conversion efficiency is still limited to ∼20%, and most of the energy is lost.
Parametric fluorescence is limited to the temporal window of the pump laser during parametric amplification. Therefore, the replacement of conventional high-gain ns-OPCPA pre-amplification using ps short-pulse-pumped OPCPA can restrict the parametric fluorescence in the ps temporal range and reduce the ASE noise energy in the nanosecond temporal range by reducing the ns-OPCPA gain. Dorrer of the University of Rochester [31] proposed the dual-stage OPCPA method, which combines high-gain ps-OPCPA and low-gain ns-OPCPA. This can improve the contrast of ultra-short, ultra-intense petawatt laser facilities. Compared with the above methods, this is the only method that effectively improves the SNR of the system without losing energy. Thus, high-energy, high-beam-quality, high-contrast ps-OPCPA laser system is widely used as seeds in high-power petawatt laser facilities, as shown in Table 1.
Table 1. Configuration of the front end in petawatt laser facilities
In the OMEGA EP petawatt laser system, a 1053 nm Nd:glass solid-state mode-locked oscillator afforded a short-duration seed for the front end, which is composed of one ps-OPCPA and two ns-OPCPA stages. The output energies of the ps-OPCPA and ns-OPCPAs were 10 µJ and ∼100 mJ, respectively. The contrast of the output from the front end is greater than 108 beyond ∼200ps ahead of the main laser pulse [31]. In the Orion petawatt facility, the seed is a Ti:sapphire solid-state Kerr-lens mode-locking oscillator with 80 MHz pulses of 3 nJ and 160 fs. A single pulse selected from the oscillator is amplified to 70 $\mu $J after injection into one ps-OPCPA stage, and 100 mJ of energy is achieved after two ns-OPCPA stages in the front end. Additionally, a contrast of more than 109 beyond ∼300ps ahead of the main laser pulse is measured after the front end [32]. In the Cerberus petawatt laser facility, the front end is composed of two ps-OPCPA and two ns-OPCPA stages, and a 5 nJ, 250 fs, and 70 MHz solid-state oscillator provided the pulse trains with a near-ideal contrast of up to the maximum dynamic range of 2.5 × 109. Furthermore, after compression, the output energies of the ps-OPCPAs and ns-OPCPAs were 300 µJ and ∼20 mJ, respectively. A contrast of more than 108 at ∼200 ps ahead of the main laser pulse is achieved after the front end [33]. In the Texas petawatt laser facility, the chain begins with a 100 fs pulse from a tunable Ti:sapphire solid-state oscillator, a pulse of 700 mJ was obtained after it was injected into two ps-OPCPA and ns-OPCPA stages. After that, the contrast is greater than 108, beyond 100 ps ahead of the main pulse [34]. The contrast enhanced by short-pulse OPCPA is also demonstrated in fs PW laser facilities such as Apollon and ELI [35,36].
In these laser systems, almost all seeds were solid-state mode-locked laser systems. The temporal intensity contrasts of these solid-state lasers that functioned as seeds of high-intensity laser systems were more than 108 ahead of the main laser pulse at the sub-picosecond level, while the contrast of the fiber mode-locked oscillators is ≤106 [37]. The two central topics of the study were introduced in this article: the result of offline contrast comparison with two oscillators and the online results of contrast improvement with the ps-OPCPA seed system. First, the contrasts of two-stage ps-OPCPA systems containing solid-state and fiber mode-locked oscillators were first compared to analyze the effect of the noise level of the oscillators on the pulse contrast. The measured results of the contrast revealed that two types of oscillators could achieve >1011, and the spectral width of the fiber oscillator is clipped to provide further proof. Although the fiber mode-locked oscillator is barely adopted directly as the seed of the front end in high-intensity laser systems, the 60 dB high-gain ps-OPCPA employing the fiber mode-locked oscillator as the seed of high-power laser facilities could also achieve high contrast. Second, the fiber mode-locked oscillator is adopted to inject the ps-OPCPAs because it offers significant advantages, such as compactness, lack of misalignment, and environmental insensitivity, compared with a solid-state laser. This high-gain, high-contrast two-stage ps-OPCPA laser seed is tested online, combined with two low-gain ns-OPCPA at the front end of the high-power ps PW laser facility. The results showed that the contrast increased from 107 to more than 1011 beyond 200 ps ahead of the main pulse at an output level of 60 mJ, which is the highest contrast front-end system for high-power laser facilities reported for this energy level employing only the ps-OPCPA technology. This paper is introduced according to the process of simulation, system design and construction, measurement of output parameters, and analysis of results. Two topics of offline contrast comparison with two oscillators and the online contrast improvement with the ps-OPCPA seed system were analyzed and explained in detail in the analysis section.
2. Simulation
The design and optimization of the two-stage ps-OPCPAs was based on the numerical modeling of the coupled-wave equations for nonlinear OPCPAs. In the non-collinear case, the relationships between the k-vectors of the signal, the idler, and the pump can be expressed as follows:
The method of non-collinear seed and pump pulses could be set up to counter the Poynting vector walk-off. Therefore, the nonlinear conversion efficiencies are optimized by increasing the optical intensity of the crystal and applying the minimum-required crystal lengths to reduce the general vector machine and Poynting vector walk-off. Since smaller beams are more sensitive to walk-off effects, the walk-off effect impacts them when they are transmitted inside the crystal. The walk-off angle of the pump can be expressed as follows:
Numerical simulations of the BBO crystals were conducted using a two-stage noncollinear optical parametric chirp-pulse amplifier. The energy, pulse widths, and beam size of the pump and signal beams in the two-stage ps-OPCPAs were determined, as listed in Table 2. A 527 nm monochromatic pump beam was used with type-I phase matching in non-collinear geometries. This designed pump with two stages afforded two intensities (0.38 GW/cm2 with pump energy of 3 mJ), as well as a full width at half maximum (FWHM) pulse width and a nominal beam diameter of 8 ps and 6 mm, respectively. The pump possessed a 10-order super-Gaussian spatial shape and a Gaussian temporal waveform, and the seed was Gaussian in time and space with a FWHM of 4 ps and 6 mm, respectively, possessing an input energy of 140 pJ. Figure 1 shows the output conversion efficiencies versus the lengths of the crystals in two-stage preamplifier designs containing two BBO crystals. The conversion efficiency of the first stage achieved a peak point at a crystal length of 14 mm. To maintain the best stability for the OPCPA stages, the length of the first crystal was selected as 15 mm for pre-amplification, and the gain was ∼5 × 106 with an energy of 60 $\mu $J, as shown in Fig. 1(a). The peak point of the conversion efficiency was located at a crystal length of 4 mm in the second stage. Thus, 5 mm was selected as the length of the second crystal, and the gain was ∼10, as shown in Fig. 1(b). If the previous seed in the front end is replaced by this high-gain ps-OPCPA, the gain of the subsequent ns-OPCPAs would decrease by 104. Therefore, the ASE noise energy in the nanosecond temporal range would decrease, and the output contrast of ns-OPCPAs was predicted to increase by 104.
Fig. 1. Calculated conversion efficiencies of the signals with the lengths of the crystal employing the (a) first and (b) second ps-OPCPA stages
Table 2. Parameters of two-stage ps-OPCPAs
A two-stage ps-OPCPA laser system without spatiotemporal noise in the input beams was considered, and it was demonstrated that the operation of the system in the supersaturated region could degrade the near-field beam quality and slightly reduce the conversion efficiency [7]. The BBO preamplifier consisted of two BBO crystals, which were configured for type-I phase matching. The extraordinary axes of the two crystals are oriented in opposite directions to compensate for the walk-off. The spatial pattern of the signal after the amplification was nearly super-Gaussian because the spatial pattern of the pump was preset by a 10th-order super-Gaussian. However, the spatial variations in the saturation and reconversion produced beam asymmetry in the walk-off direction, as shown in Fig. 2. Since the calculated walk-off length of the pump in the first crystal was 0.75 mm, whereas that in the second crystal was 0.25 mm, the walk-off effect was not completely compensated because of the different lengths of the two BBO crystals. The spatial flatness in the y-direction was ∼20% (peak-to-peak value).
3. Method
The ps-OPCPA system consists of a ps-pump laser system and a two-stage ps-OPCPA system. A mode-locked laser affords a 73 fs pulse with a central wavelength of 1053 nm, and a spectral bandwidth of 17 nm was generated at a repetition rate of 77.76 MHz, which corresponds to a pulse energy of 0.5 nJ. Further, the mode-locked seed was synchronized with a homemade external reference clock for frequency locking. The pulse trains, which were obtained from the oscillator, were split by combining a half-wave plate with a polarizer in the two parts employing energies of 80% and 20%. An acousto-optic modulator (AOM) reduced the repetition rate of the 20% part from 77.76 MHz to 1 Hz, after that a regenerative and multi-pass amplifier was used to amplify it, which was then frequency-doubled for a narrow spectral bandwidth pump with a pulse width of approximately 8 ps and an energy of 6 mJ. The pulse train of the 80% energy was stretched by a Dazzler (acousto-optic programmable dispersive filter, AOPDF, FASTLITE) to ∼4 ps and used it as a seed for two-stage OPCPA.
The pump system is composed of an AOM pulse picker, an optical delay line, an Nd:YLF regenerative amplifier, a spatial shaping module, a four-pass Nd:glass power amplifier, and a frequency doubler, as shown in Fig. 3. A pulse was selected for regenerative amplification, after which it was passed through the AOM and a fiber delay line. After many roundtrips in the regenerative amplifier, the selected pulse was amplified as a pump to 1.3 mJ with a delay of 1.6 µs, after which it was passed to the multi-pass amplifier and frequency doubler to amplify the pulse in the 80% part of the pulse train of the seed. For the ps-OPCPAs, the signal was chirped, and the time corresponded to the spectrum. Furthermore, the delay between the pump and signal was sensitive in such a manner that the thermal and mechanical drifts in optical fibers and regenerative cavity caused a drift in the central wavelength. Additionally, a fiber-delay line (MDL Series from AFR) was added after AOM to compensate for the slow delay drifts and maintain the stability of the energy and spectrum.
An experimental diagram of the regenerative amplifier is shown in Fig. 4. The laser crystal was a 1% doped a-cut Nd:YLF rod with a diameter and length of 4 mm and 70 mm, respectively, and was pumped by a laser head consisting of vertical-cavity surface-emitting laser arrays. The regenerative amplifier was a plane–plane cavity, which possessed a concave mirror at the center of the cavity to ensure stability, with a cavity length of 5 m, which corresponded to a roundtrip time of 16.5 ns. After 100 rounds in the regenerative amplifier, the spectral width of the pump was narrowed to nearly 0.2 nm, corresponding to the transform limit pulse width of approximately 8 ps. Therefore, the pump laser has almost no chirp, and the change in the input seed spectral width does not affect the pump pulse width because of the gain narrowing during regenerative amplification. To reduce the sensitivity of the cavity length, a thermally stable structure in which all the optical elements that affected the cavity length were fixed on a metal framework comprising three Invar rods was adopted. The thermal expansion coefficient of the Invar rod was 1.88 × 10−5/°C, which is approximately 1/30 that of aluminum. Thus, the effect of temperature on the length of the regenerative cavity was minimized. Furthermore, the roundtrip length of the regenerative cavity was ∼5 m, and the pulse was passed 100 times through the cavity. When the temperature changed by 1°C, the cavity length also changed by 0.5 mm, and the pulse delay between the pump and signal changed by 1.7 ps. Furthermore, the measured timing jitter in the input and output of the regenerative amplifier was 340 fs for 8 h with a standard deviation owing to our experimental environment. The stability of the regenerative amplifier was improved as follows: energy stability was 0.53% root mean square (rms), and the directional stability was 2 µrad during the measurement, which lasted 8 h.
To ensure high-quality, flat-top, pump-beam distribution during the power amplification, as well as alleviate the self-focusing effect and compensate for the thermal effect, three approaches, including the beam-shaping module to suppress the self-focusing phenomenon and control the size of the B-integral and compensation for thermal lensing and thermal-induced depolarization, were adopted [38]. First, the small-signal gains of the laser diode-pumped Nd:glass laser head (1.2% neodymium by atomic percent) at different currents are shown in Fig. 5(a), and the single-pass small-signal gain of 4 was selected in the system, while the pump current was 210 A. The spatial gain distribution was not relatively flat, as shown in Fig. 5(b), and the gain at the center of the amplifier exhibited a litter, which was higher than the edge. Thus, the spatial intensity distribution of the injected beam was shaped into an inverse super-Gaussian structure with a weak center and a strong edge by an optically addressed liquid-crystal light modulator to increase the energy-extraction efficiency and prevent the self-focusing effect during the four-pass power amplification. Second, the distance between the imaging lenses L1 and L2 was tuned to pre-compensate the thermal-lensing effect by collimating the divergence of the output beam with a shear interference plate under the power amplification condition. Finally, a 45° Faraday rotator, which was located between the laser rod and M2 of the four-pass amplifier, was employed to compensate for the distortion of the thermal polarization. The “beam shaping” in Fig. 3 is composed of a liquid crystal light modulator and a polarized beam splitter. The liquid crystal plane in the liquid crystal light modulator became the reference plane, after which it was relay-imaged onto M1, then to M2, and finally onto the frequency-doubled crystal and parametrically amplified crystal to ensure good beam quality. A 1053 nm pulse with a 10 mJ output energy and a 6 mm diameter flat-top beam distribution was obtained after four-pass power amplification. Finally, a stable 6 mJ energy, 8 ps duration, and 526.5 nm pump pulse were generated by a 3-mm-thick type-I BBO crystal (10 × 10 × 3 mm3, $\theta = 29^\circ $, $\phi = \textrm{0}^\circ $) with a conversion efficiency of >60%. Furthermore, the frequency-doubled laser pulse was filtered by three 0° dichroic mirrors (DM: anti-reflection at 527 nm and high reflection at 1053 nm) to prevent the amplification of the residual 1053 nm laser from the fundamental pump.
Fig. 3. Scheme of experimental setup VSF: vacuum spatial filter, TFP: thin-film polarizer, HWP: half-wave plate, FR: Faraday rotator, L1 and L2: a plano-convex lens with a focal length of 0.5 m; DM: 0° dichroic mirrors, AMP: 35 KW LD Laser head, as the amplifier, Dazzler: acousto-optic programmable dispersive filter, M1 and M2: 0° mirror, BS: 5:5 beam splitter, DM1–DM5: 45° dichroic mirror, 527 nm reflected, 1053 nm transmitted, OPCPA1: BBO crystal = 10 × 10 × 15 mm3, $\theta = \textrm{23}\textrm{.8}^\circ $, $\phi = \textrm{0}^\circ $, OPCPA2: BBO crystal, 10 × 10 × 5 mm3, $\theta = \textrm{23}\textrm{.8}^\circ $, $\phi = \textrm{0}^\circ $.
Fig. 4. Structure of the regenerative amplifier. M1:$45^\circ $ reflecting mirror; PBS: polarization beam splitter; ISO: isolator; HWP: half-wave plate; QWP: quarter-wave plate; FR: Faraday rotator; TFP: thin-film polarizer; PC: Pockels cell; M2, M4, M5, and M6:$0^\circ $ mirror; M3: concave mirror.
Fig. 5. (a) Small-signal gain of the laser head in the four-pass amplifier. The red mark on the curve is the operating point of the laser head. (b) Gain distribution of the laser head at 210 A.
The output of the 80% part of the mode-locked laser, which was employed as the seed, was stretched by the AOPDF to ∼4 ps by adding a dispersion of 1 × 105 fs2 to the pulse, with a single pulse energy of 120 pJ and a spectral width of 17 nm. The signal beam was expanded to a Gaussian-shaped diameter of 6 mm to match the pump pulse and achieve maximum energy extraction efficiency. The signal pulse was amplified by two ps-OPCPA stages employing the BBO crystals (BBO1: 10 × 10 × 15 mm3, $\theta = \textrm{23}\textrm{.8}^\circ $, $\phi = \textrm{0}^\circ $; BBO2: 10 × 10 × 5 mm3, $\theta = \textrm{23}\textrm{.8}^\circ $, $\phi = \textrm{0}^\circ $) with type-I phase matching, and the non-collinear angle was 2.9°, as shown in Fig. 6. The output energy of the first stage was ∼60 $\mu $J, whereas the energy of the second stage was ∼600 $\mu $J when the pump pulse was split into two 3 mJ portions to pump the two ps-OPCPA stages.
Fig. 6. Diagram of the two-stage ps-OPCPAs setup. BS: 5:5 beam splitter; DM1–DM4:$\textrm{45}^\circ $DM, 527 nm reflected, 1053 nm transmitted; OPCPA1: BBO crystal, 10 × 10 × 15 mm3, $\theta = \textrm{23}\textrm{.8}^\circ $, $\phi = \textrm{0}^\circ $; OPCPA2: BBO crystal, 10 × 10 × 5 mm3, $\theta = \textrm{23}\textrm{.8}^\circ $.
4. Results and discussion
The ps-OPCPAs were run in a stable region, which was slightly above the saturation point. The gain of the two-stage short-pulse-pumped OPCPAs was >106, and the long-term energy stability is shown in Fig. 7. The output of the regenerative amplifier was 1.3 mJ with a rms of 0.53% in 10 h, as shown in Fig. 7(a). The energy stability after the frequency-doubling of the pump laser was ∼1.18% rms in 10 h, as shown in Fig. 7(b). The mean value of the output signal energy was 540 µJ, and the energy-conversion efficiency was ∼10%, which is consistent with the results obtained via the simulation. Furthermore, the energy stability of the output signal was ∼1.6% rms for 10 h, as shown in Fig. 7(c).
Fig. 7. Long-term energy stability measurements of the (a) regeneration output, (b) pump laser, and (c) amplified signal.
The spatial distributions of the pump and signal are presented in Fig. 8. An optically addressed liquid-crystal light modulator was adopted in the pump beam to suppress the self-focusing phenomenon, and near-field modulation of the pump before the frequency-doubling was achieved 1.13, as shown in Fig. 8(a). The near-field profile after frequency-doubling with a near-field modulation of 1.17, is shown in Fig. 8(b). The pump–signal walk-off increased the spatial gain along the walk-off direction. The crystals in the two ps-OPCPA stages were set along different walk-off directions. However, the output beam revealed slight asymmetry because of the different lengths of the two BBO crystals (15 and 5 mm in the first and second ps-OPCPA stages, respectively), as shown in Fig. 8(c), which is consistent with the simulated results. A 500-mm focal lens was employed to measure the far-field beam profile, as shown in Fig. 8(d), thereby obtaining a size that was smaller than twice the diffraction limit.
Fig. 8. Near-field beam profiles of the (a) four-pass amplifier output, (b) pump laser after frequency-doubling, and (c) amplified signal laser. (d) Far-field beam profile of the signal laser.
The amplified signal-pulsed width was compressed from 4 to 98 fs by two parallel 1000 groove/mm transmission gratings with >80% transmittance. The spectral phase information was obtained using a high-dynamic single-shot spectral phase measurement system (Wizzler, FASTLITE) after compression, as shown in Fig. 9(a). After that, the phase information was sent back to the Dazzler to optimize and compensate for higher-order dispersion and achieve the Fourier-transform limited-pulse width. As shown in Fig. 9(b), the 98 fs compressed pulse width (black curve) is consistent with the Fourier-transform limited-pulse width (red curve), which was calculated from the spectrum measured by Wizzler. The pedestal of the compressed pulse was obtained from the residual high-order dispersion that was not compensated for.
Fig. 9. Measurement of the compressed pulse by Wizzler: (a) spectrum and phase and (b) temporal waveform (black: measured, red: Fourier-transform limited-pulse width calculated from the spectrum).
Owing to the chirp of the OPCPA seed, changes in the pump–signal relative timing caused changes in the output spectrum of the central wavelength via air disturbance and thermal-induced cavity-length change. The corresponding relation between the delay drifts of the pump and signal lasers and the change in the spectrum was estimated by adjusting the fiber delay line. As shown in Fig. 10(a), the central wavelength of the amplified laser (red curve) after the two ps-OPCPA stages was 1053 nm, which is the same as that of the input signal pulse (blue curve). Therefore, the temporal center of the pump laser is consistent with that of the signal pulse. By adjusting the fiber delay line to change the relative delay between the pump and signal pulses, the central wavelength increased by 3 nm (from 1053 to 1056 nm), as shown in Fig. 10(b), as the center of the pump laser was moved forward by 1 ps relative to the center of the signal laser. In contrast, the central wavelength decreased by 3 nm (from 1053 to 1050 nm), as shown in Fig. 10(c), when the center of the pump laser was moved backward by 1 ps relative to the center of the signal laser. This phenomenon proves that the amplified laser was a positive chirp and reveals that the change in the pump pulse delay by 1 ps corresponds to the 3 nm drift of the spectral center when the temperature changed by 0.6 °C (mentioned in Chapter 3). Thus, a homemade closed-loop feedback system with a measured spectrum and fiber delay line was adopted in the system to ensure the long-term stability of the outputs of the central wavelength and energy of the amplified laser for engineering applications. Owing to the limited resolution of the spectrometer (Ocean Optics 2000+), the central wavelength of the amplified signal was accurate, although the top modulation of the spectrum was high.
Fig. 10. Changes in the output spectra of the central wavelength in different delays. Blue curves: input of the signal pulse centered at 1053 nm. Red curves: amplified pulse centered at (a) 1053, (b) 1056, and (c) 1050 nm.
The temporal pulse contrast was measured using the SEQUOIA 1000 system, which is a high-dynamic-range third-order fs cross-correlator (Amplitude Technologies). As shown in Fig. 11(a), the contrast of the compressed amplified pulse after two ps-OPCPA stages could achieve more than 1011 beyond the 18 ps temporal range ahead of the main pulse when seeded by a fiber mode-locked oscillator (Onefive Origami-10), which is restricted by a maximum dynamic range of 1011 by a Sequoia instrument. By changing the ps-OPCPA seed to a higher-contrast solid mode-locked oscillator with an SNR of 108 (Montfort Laser M-FEMTO), the contrast of the amplified pulse achieved >1010 in the same temporal range. In this condition, to ensure the same parameters of two-stage ps-OPCPAs with two oscillators, the dispersion factor of the dazzler was changed to maintain the pulse width of the seed at 4 ps, and the pump pulse width nearly does not change because of the narrowing of the gain during regenerative amplification. The energy of the solid oscillator decreased to the same value as that of the fiber oscillator, and the parameters of the two-stage ps-OPCPAs did not change during the comparison of the SNR.
Fig. 11. Normalized contrast of the compressed amplified pulse seeded by the fiber mode-locked [(a) before and (b) after clipping the spectral width] and solid-state mode-locked oscillators.
The parameters of the two oscillators are listed in Table 3. The compressed-pulse width of the fiber seed after amplification was ∼98 fs, and that of the solid seed was ∼200 fs. Although the contrast of the solid mode-locked oscillator was 20 dB higher than that of the fiber mode-locked oscillator, the measured contrast of the fiber mode-locked laser-seeded system was a few times higher than that of the solid mode-locked laser at the same amplified energy, because the compressed amplified pulse width of the solid oscillator was twice that of the fiber oscillator. Considering the restricted dynamic range of Sequoia 1000, the measured peak intensity of the compressed amplified pulse from the fiber oscillator was several times that of the solid-state oscillator, but the restricted dynamic range of Sequoia limited the measured noise intensities of the two types of compressed amplified pulses. Therefore, the measurements of contrast after normalization showed several times difference. To demonstrate the restriction of the dynamic range in the contrast measurement of the compressed amplified pulse from the solid-state oscillator, the spectral width of fiber oscillator was clipped by Dazzler to obtain a longer compressed pulse width which was close to the solid-state oscillator for the comparison of the SNR (the dispersion factor of the dazzler was changed to maintain the pulse width of the seed at 4 ps). The contrast of the compressed amplified pulses from the fiber (after clipping the spectral width) was consistent with that of the solid-state mode-locked oscillators, as shown in Fig. 11(b). This proved that there was a restricted dynamic range for measuring Sequoia. According to the above inference, the contrast of the ps-OPCPA system with a solid-state mode-locked oscillator was certain >110 dB. However, the noise pedestal starting at −18 ps was caused by the uncompressed high-order dispersion with the stretcher, the compressor, or other optical components during ps-OPCPAs.
Table 3. Parameters of the fiber and solid-state mode-locked oscillators
Owing to the advantages of the fiber mode-locked oscillator (compactness, lack of misalignment, and environmental insensitivity) compared with the solid-state oscillator, it was employed to inject the two-stage ps-OPCPA system based on offline test results. The high-contrast high-energy ps-OPCPAs seed with a fiber oscillator was injected into our ps, kJ PW laser system to verify the enhancement of the contrast. The original PW front-end system comprised an fs solid mode-locked oscillator, a stretcher, two ns-OPCPA stages, and pre-compression. The output energy was ∼60 mJ with an nJ input, and the obtained gain of the two-stage ns-OPCPAs was >80 dB, which was mainly in the ns-OPCPA I with a high gain of 60 dB (Table 4). After changing the seed to high-contrast, high-energy ps-OPCPAs, the input energy increased by 40 dB, and the gain of ns-OPCPA I decreased by 40 dB. Therefore, the contrast was improved by four orders of magnitude outside the 200 ps temporal scale ahead of the main pulse, whereas the amplified spontaneous emission intensity level was reduced to 10−11, as shown in Fig. 12, which was consistent with the predicted result. It was demonstrated that replacing conventional high-gain ns-OPCPA pre-amplification with ps short-pulse-pumped OPCPA could inhibit parametric fluorescence in the ps temporal range and reduce the ASE noise energy in the nanosecond temporal range by reducing the gain of ns-OPCPA. The exponentially increasing pedestal within 200 ps of the main pulse is an uncompressed high-order dispersion, as shown in Fig. 12, where the extended leading edge within hundreds of ps of the compressed pulse is a coherent contrast pedestal, which accrued from the scattering of the diffraction gratings in the stretcher [39] and high-frequency spectral phase modulation taken by optical component surface roughness within pulse stretchers and compressors [10,40]. The noise pedestal from ps-OPCPAs was covered by this stronger and wider pedestal, and the noise pedestal caused by uncompressed high-order dispersion with different oscillators did not affect the contrast of the ns-OPCPA front end; therefore, the fiber mode-locked oscillator can be adopted in a ps-OPCPA system. In our facility, the grating surface was oxidized by exposure to air for a long time without protection. Thus, the diffraction efficiency was reduced to ∼60%, thereby ensuring that the transmissivity of the eight-pass Offner stretcher was only 3%, and the scattering of the oxidized grating was high. Replacing the gratings with new components with improved quality would cause an order-of-magnitude reduction in the intensity of the pedestal. Moreover, the pre-pulse at the 300 ps temporal scale ahead of the main pulse was generated by the post-pulse of the Faraday rotator with a length of ∼30 mm owing to its nonlinear nature, which was proposed by N. V. Didenko [41]. Fortunately, the noise energy of the pre-pulse was <10−8; it was eliminated by replacing the rotator crystal with a split angle.
Fig. 12. Comparison of the contrast of the PW front end system injected with the original oscillator (red curve) and the high contrast of ps-OPCPAs (blue curve).
Table 4. Energy distribution of the PW front end system before and after changing the injection seed
5. Conclusion
In this study, a high-energy, high-beam-quality, high-contrast two-stage ps-OPCPA laser system is reported. The fs oscillator was stretched to 4 ps, after which it was amplified from pJ to 600 µJ by a 6 ps/6 mJ pump laser in two non-collinear ps-OPCPA stages. Both fiber and solid mode-locked oscillators achieved more than 1011 contrast ratio outside 18 ps temporal scale ahead of the main pulse, which was restricted by the dynamic range of the Sequoia 1000 instrument. The high-contrast, high-energy ps-OPCPA seed containing a fiber oscillator was injected into the ps, kJ PW laser, thereby enhancing the contrast of the front end in the SG-II PW laser facility by 40 dB to more than 1011 beyond 200 ps temporal scale ahead of the main pulse with an output level of 60 mJ. Thus far, this contrast ratio is the highest that can be reported for a front end in ps PW facilities containing a ps-OPCPA seed at this energy level. Some improvement approaches would be adopted in the next experiment to achieve an order-of-magnitude promotion in the intensity of the pedestal, and a large range of single SNR-measuring instruments would be employed in the kJ, ps PW laser facility output to compare the contrast improvements across a temporal range of hundreds of ps.
Funding
Technology Research Leader (19XD1404000); National Natural Science Foundation of China (11604350).
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. M. D. Perry and G. Mourou, “Terawatt to petawatt sub-picosecond lasers,” Science 264(5161), 917–924 (1994). [CrossRef]
2. R. Butkus, R. Danielius, A. Dubietis, A. Piskarskas, and A. Stabinis, “Progress in chirped pulse optical parametric amplifiers,” Appl. Phys. B 79(6), 693–700 (2004). [CrossRef]
3. F. Tavella, A. Marcinkevičius, and F. Krausz, “Investigation of the superfluorescence and signal amplification in an ultrabroadband multiterawatt optical parametric chirped pulse amplifier system,” New J. Phys. 8(10), 219 (2006). [CrossRef]
4. G. Cerullo and S. De Silvestri, “Ultrafast optical parametric amplifiers,” Rev. Sci. Instrum. 74(1), 1–18 (2003). [CrossRef]
5. J. Moses, C. Manzoni, S.-W. Huang, G. Cerullo, and F. X. Kärtner, “Temporal optimization of ultrabroadband high-energy OPCPA,” Opt. Express 17(7), 5540–5555 (2009). [CrossRef]
6. J. Bromage, S. -W. Bahk, I. A. Begishev, C. Dorrer, M. J. Guardalben, B. N. Hoffman, J. B. Oliver, R. G. Roides, E. M. Schiesser, M. J. Shoup, M. Spilatro III, B. Webb, D. Weiner, and J. D. Zuegel, “Technology Development for ultraintense all-OPCPA systems,” High Power Laser Sci. Eng. 7, e4 (2019). [CrossRef]
7. M. Galletti, P. Oliveira, M. Galimberti, M. Ahmad, G. Archipovaite, N. Booth, E. Dilworth, A. Frackiewicz, T. Winstone, I. Musgrave, and C. Hernandez-Gomez, “Ultra-broadband all-OPCPA petawatt facility fully based on LBO,” High Power Laser Sci. Eng. 8, e31 (2020). [CrossRef]
8. B. Hou, K.-H. Hong, E. Power, G. Mourou, and J. Nees, “Generation and measurement of >10 8 intensity contrast ratio in a relativistic kHz chirped-pulse amplified laser,” Appl. Phys. B 81(4), 447–457 (2005). [CrossRef]
9. M. Kalashnikov, A. Andreev, and H. Schönnagel, “Limiting characteristics of temporal contrast for high aperture CPA lasers,” AIP Conf. Proc. (1228) (2010).
10. J. Bromage, C. Dorrer, and R. Jungquist, “Temporal contrast degradation at the focus of ultrashort pulses from high-frequency spectral phase modulation,” J. Opt. Soc. Am. B 29(5), 1125–1135 (2012). [CrossRef]
11. S. Keppler, M. Hornung, R. Bödefeld, A. Sävert, H. Liebetrau, J. Hein, and M. C. Kaluza, “Full characterization of the amplified spontaneous emission from a diode-pumped high-power laser system,” Opt. Express 22(9), 11228–11235 (2014). [CrossRef]
12. Y. H. Cha, Y.-W. Lee, S. M. Nam, J. M. Han, Y. J. Rhee, B. D. Yoo, B. C. Lee, and Y. U. Jeong, “Simple method for the temporal characterization of amplified spontaneous emission in femtosecond terawatt Ti:sapphire lasers,” Appl. Opt. 46(28), 6854–6858 (2007). [CrossRef]
13. A. Picozzi and M. Haelterman, “Influence of walk-off, dispersion and diffraction on the coherence of parametric fluorescence in the (2 + 1)D geometry,” in Nonlinear Guided Waves and Their Applications OSA Technical Digest Series (Optical Society of America, 2001), WA6.
14. J. Chwedeńczuk and W. Wasilewski, “Intensity of parametric fluorescence pumped by ultrashort pulses,” Phys. Rev. A 78(6), 063823 (2008). [CrossRef]
15. C. Manzoni, J. Moses, F. X. Kärtner, and G. Cerullo, “Excess quantum noise in optical parametric chirped-pulse amplification,” Opt. Express 19(9), 8357–8366 (2011). [CrossRef]
16. J. Itatani, J. Faure, M. Nantel, G. Mourou, and S. Watanabe, “Suppression of the amplified spontaneous emission in chirped-pulse-amplification lasers by clean high-energy seed-pulse injection,” Opt. Commun. 148(1-3), 70–74 (1998). [CrossRef]
17. M. Nantel, J. Itatani, A.-C. Tien, J. Faure, D. Kaplan, M. Bauvier, T. Buma, P. Rompay, J. Nee, P. Pronko, D. Umstadter, and G. A. Mourou, “Temporal contrast in Ti:sapphire lasers: Characterization and control,” IEEE J. Sel. Top. Quantum Electron. 4(2), 449–458 (1998). [CrossRef]
18. J. Sung, S. Lee, T. Jeong, and C. Nam, “Enhancement of temporal contrast of high-power femtosecond laser pulses using two saturable absorbers in the picosecond regime,” Appl. Phys. B 116(2), 287–292 (2014). [CrossRef]
19. A. Renault, F. Augé-Rochereau, T. Planchon, P. D’Oliveira, T. Auguste, G. Chériaux, and J. Chambaret, “ASE contrast improvement with a non-linear filtering Sagnac interferometer,” Opt. Commun. 248(4-6), 535–541 (2005). [CrossRef]
20. D. Homoelle, A. L. Gaeta, V. Yanovsky, and G. Mourou, “Pulse contrast enhancement of high-energy pulses by use of a gas-filled hollow waveguide,” Opt. Lett. 27(18), 1646–1648 (2002). [CrossRef]
21. J.-L. Tapié and G. Mourou, “Shaping of clean, femtosecond pulses at 1.053 µm for chirped-pulse amplification,” Opt. Lett. 17(2), 136–138 (1992). [CrossRef]
22. A. Jullien, F. Augé-Rochereau, G. Chériaux, J.-P. Chambaret, P. d’Oliveira, T. Auguste, and F. Falcoz, “High-efficiency, simple setup for pulse cleaning at the millijoule level by nonlinear induced birefringence,” Opt. Lett. 29(18), 2184–2186 (2004). [CrossRef]
23. H. J. Liu, Q. B. Sun, N. Huang, J. Wen, and Z. L. Wang, “Highly efficient pulse cleaner via nonlinear ellipse rotation in liquid CS2 for ultrashort pulses,” Opt. Lett. 38(11), 1838–1840 (2013). [CrossRef]
24. N. Minkovski, G. I. Petrov, S. M. Saltiel, O. Albert, and J. Etchepare, “Nonlinear polarization rotation and orthogonal polarization generation experienced in a single-beam configuration,” J. Opt. Soc. Am. B 21(9), 1659–1664 (2004). [CrossRef]
25. A. Jullien, O. Albert, G. Chériaux, J. Etchepare, S. Kourtev, N. Minkovski, and S. M. Saltiel, “Nonlinear polarization rotation of elliptical light in cubic crystals, with application to cross-polarized wave generation,” J. Opt. Soc. Am. B 22(12), 2635–2641 (2005). [CrossRef]
26. S. K. Mishra, A. Andreev, and M. P. Kalashinikov, “Reflection of few cycle laser pulses from an inhomogeneous overdense plasma,” Opt. Express 25(10), 11637–11651 (2017). [CrossRef]
27. H. C. Kapteyn, M. M. Murnane, A. Szoke, and R. W. Falcone, “Prepulse energy suppression for high-energy ultrashort pulses using self-induced plasma shuttering,” Opt. Lett. 16(7), 490–492 (1991). [CrossRef]
28. A. Lévy, T. Ceccotti, P. D’Oliveira, F. Réau, M. Perdrix, F. Quéré, P. Monot, M. Bougeard, H. Lagadec, P. Martin, J.-P. Geindre, and P. Audebert, “Double plasma mirror for ultrahigh temporal contrast ultraintense laser pulses,” Opt. Lett. 32(3), 310–312 (2007). [CrossRef]
29. S. Inoue, K. Maeda, S. Tokita, K. Mori, K. Teramoto, M. Hashida, and S. Sakabe, “Single plasma mirror providing 104 contrast enhancement and 70&per; reflectivity for intense femtosecond lasers,” Appl. Opt. 55(21), 5647–5651 (2016). [CrossRef]
30. Z. Wang, C. Liu, Z. Shen, Q. Zhang, H. Teng, and Z. Wei, “High-contrast 1.16PW Ti:sapphire laser system combined with a doubled chirped-pulse amplification scheme and a femtosecond optical-parametric amplifier,” Opt. Lett. 36(16), 3194–3196 (2011). [CrossRef]
31. C. Dorrer, A. Consentino, D. Irwin, J. Qiao, and J. D. Zuegel, “OPCPA front end and contrast optimization for the OMEGA EP kilojoule, picosecond laser,” J. Opt. 17(9), 094007 (2015). [CrossRef]
32. N. Hopps, C. Danson, S. Duffield, D. Egan, S. Elsmere, M. Girling, E. Harvey, D. Hillier, M. Norman, S. Parker, P. Treadwell, D. Winter, and T. Bett, “Overview of laser systems for the Orion Facility at the AWE,” Appl. Opt. 52(15), 3597–3607 (2013). [CrossRef]
33. N. H. Stuart, D. Bigourd, R. W. Hill, T. S. Robinson, K. Mecseki, S. Patankar, G. H. C. New, and R. A. Smith, “Direct fluorescence characterisation of a picosecond seeded optical parametric amplifier,” Opt. Commun. 336, 319–325 (2015). [CrossRef]
34. E. Gaul, T. Toncian, M. Martinez, J. Gordon, M. Spinks, G. Dyer, N. Truong, C. Wagner, G. Tiwari, M. E. Donovan, T. Ditmire, and B. M. Hegelich, “Improved pulse contrast on the Texas Petawatt Laser,” J. Phys.: Conf. Ser. 717, 012092 (2016). [CrossRef]
35. F. Batysta, R. Antipenkov, T. Borger, A. Kissinger, J. T. Green, R. Kananavičius, G. Chériaux, D. Hidinger, J. Kolenda, E. Gaul, B. Rus, and T. Ditmire, “Spectral pulse shaping of a 5 Hz, multi-joule, broadband optical parametric chirped pulse amplification frontend for a 10-PW laser system,” Opt. Lett. 43(16), 3866–3869 (2018). [CrossRef]
36. D. N. Papadopoulos, P. Ramirez, A. Pellegrina, N. Lebas, C. Le Blanc, G. Chériaux, J. P. Zou, G. Mennerat, P. Monot, F. Mathieu, P. Audebert, P. Georges, and F. Druon, “High-contrast 10-fs OPCPA-based Front-End for the Apollon-10PW laser,” in Advanced Solid State Lasers, OSA Technical Digest (online) (Optical Society of America, 2015), ATu4A.3.
37. N. Stuart, T. Robinson, D. Hillier, N. Hopps, B. Parry, I. Musgrave, G. Nersisyan, A. Sharba, M. Zepf, and R. A. Smith, “Comparative study on the temporal contrast of femtosecond mode-locked laser oscillators,” Opt. Lett. 41(14), 3221–3224 (2016). [CrossRef]
38. J. Guo, J. Wang, H. Wei, W. Huang, T. Huang, G. Xia, W. Fan, and Z. Lin, “High-power, Joule-class, temporally shaped multi-pass ring laser amplifier with two Nd: Glass laser heads,” High Power Laser Sci. Eng. 7, e8 (2019). [CrossRef]
39. C. Hooker, Y. Tang, O. Chekhlov, J. Collier, E. Divall, K. Ertel, S. Hawkes, B. Parry, and P. P. Rajeev, “Improving coherent contrast of petawatt laser pulses,” Opt. Express 19(3), 2193–2203 (2011). [CrossRef]
40. L. Ranc, C. Le Blanc, N. Lebas, L. Martin, J.-P. Zou, F. Mathieu, C. Radier, S. Ricaud, F. Druon, and D. Papadopoulos, “Improvement in the temporal contrast in the tens of ps range of the multi-PW Apollon laser front-end,” Opt. Lett. 45(16), 4599–4602 (2020). [CrossRef]
41. N. V. Didenko, A. V. Konyashchenko, A. P. Lutsenko, and S. Y. Tenyakov, “Contrast degradation in a chirped-pulse amplifier due to generation of prepulses by postpulses,” Opt. Express 16(5), 3178–3190 (2008). [CrossRef]
