High-contrast, 30 fs, 1.5 PW laser pulses are generated from a chirped-pulse amplification (CPA) Ti:sapphire laser system at 0.1 Hz repetition rate. The maximum output energy of 60.2 J is obtained, at a pump energy of 120 J, from a booster amplifier that is pumped by four frenquency-doubled Nd:glass laser systems. During amplification, parasitic lasing is suppressed by index matching fluid with absorption dye and the careful manipulation of the time delay between the seed and pump pulses. An amplified pulse passes through a pulse compressor consisting of four gold-coated gratings. After compression, the measured pulse duration is 30 fs, and the output energy is 44.5 J, yielding a peak power of about 1.5 PW. The output energy of 44.5 J and output power of 1.5-PW are the highest values ever achieved from the femtosecond CPA Ti:sapphire laser system. To maintain a sufficiently high temporal contrast, a saturable absorber is installed in the front-end system with two ultrafast Pockels cells in order to minimize the amplified spontaneous emission (ASE) and pre-pulse intensity. An adaptive optics system is implemented for PW laser pulses and a focused intensity of about 1 × 1022 W/cm2 can be obtained when an f/3 optic is used.
© 2012 OSA
Over the past decades, there has been significant progress in developing femtosecond high-power laser systems by using chirped pulse amplification (CPA) technique . Recently, femtosecond, petawatt (PW)-class Ti:sapphire laser systems using the CPA technique have been demonstrated worldwide [2–5]. A PW laser system can produce a focused intensity of 1021 to 1022 W/cm2 [6,7], and it is thought that this level of focused intensity is suitable for high-field physics, for example, proton beam acceleration in a radiation pressure regime [8,9]. Currently, the development of femtosecond high power laser systems having a 10 PW or even 100 PW level are being pursued through the European Extreme Light Infrastructure (ELI) project . However, technical bottlenecks related to the high-energy amplification, high temporal contrast, and high-repetition rate are quickly encountered in developing a PW-class high-power laser system .
When amplifying a chirped laser pulse to high energy (over few tens of J) levels, the output energy and the beam quality are affected by the transverse amplified spontaneous emission (TASE) or parasitic lasing , which is accompanied by issues pertaining to the commercially available size of the Ti:sapphire gain medium and the pump laser fluence. For this reason, the maximum output energy from a final booster amplifier in a femtosecond high-power Ti:sapphire laser system still remains at about 47 J [4,5]. The use of an index matching fluid with an absorption dye can significantly suppress parasitic lasing , but at a higher pump fluence and energy, the index matching fluid and absorption dye are not sufficient enough to totally suppress parasitic lasing. Thus, additional technique to suppress parasitic lasing should be devised when the index matching fluid with absorption dye is not sufficient to block parasitic lasing.
The intensity level of the pre-pulse and amplified spontaneous emission (ASE) in the nanosecond and picosecond ranges, which is defined as the contrast ratio, should be well suppressed below a certain level in laser-plasma experiments using PW laser pulses. As such, several contrast-enhancement techniques, including optical parametric chirped-pulse amplification (OPCPA) , cross-polarized wave (XPW) generation , plasma mirror (PM) , and saturable absorber (SA) , have been proposed and demonstrated in attempts to improve the temporal contrast of ultrashort high-power laser pulses. Of these techniques, OPCPA has been widely used for high-contrast, ultrashort high-power laser systems, though it is more complex and sensitive to optical alignment than conventional stimulated amplification techniques. XPW generation provides a better contrast ratio; however, it requires a double CPA scheme and suffers from a low extraction efficiency of less than 25% . In contrast, though a PM can be easily implemented after a pulse compressor in an ultra-short high-power laser system without a serious modification of a laser system, it significantly reduces the output energy and produces a poor beam quality . Similarly, although inserting an SA into a front-end system is simple and does not seriously modify the system, the use of an SA is not sufficient for achieving the contrast ratio required in a PW regime, and thus additional contrast enhancement techniques are required for laser-plasma interaction experiments.
High-repetition rate can be obtained using laser-diode (LD)-pumped laser systems as pump lasers. However, because of the huge cost of laser diodes, flashlamp-pumped high-energy Nd:glass laser systems were widely used for optically pumping PW laser systems. But, for the low thermal conductivity of Nd:glass rod, the maximum repetition rate of the flashlamp-pumped Nd:glass laser system still remained 0.1 Hz. Recently, the HiLASE project  is launched for the development of a 100 J / 10 Hz laser system scalable to 1 kJ level as pump lasers for PW-class laser systems.
In this paper, we report on a high-contrast, 1.5–PW, CPA Ti:sapphire laser system that is operating at a repetition rate of 0.1 Hz. In the laser system, a reliable PW booster amplifier can amplify the output energy of a laser pulse up to 60.2 J. This output energy of 60.2 J is the highest value ever achieved from a femtosecond high-power laser system. To achieve this output energy, the time delay between the input pulse and pump pulses is precisely manipulated in the booster amplifier, in conjunction with the use of an index-matching fluid and absorption dye. The amplified laser pulse is subsequently recompressed by a gold-coated grating compressor with 74% efficiency, and the measured pulse duration is 30 fs, yielding a peak power of about 1.5 PW. A saturable absorber and two ultrafast Pockels cells (UPCs) are installed in the front-end system in order to maintain the high contrast ratio. An adaptive optics (AO) system is also employed, which can compensate for the wavefront aberration in PW laser pulses. With an f/16 focusing optic, the measured focal spots are 20 µm and 20 µm in the horizontal and vertical directions, respectively. This implies that a focused intensity of about 1 × 1022 W/cm2 can be obtained with an f/3 focusing optic. The 1.5 PW CPA laser system will be beneficial for laser-plasma interaction experiments in the 1022 W/cm2 intensity regime.
2. 0.1 Hz, 1.5 PW CPA Ti:sapphire laser system
Figure 1(a) presents a schematic of the Petawatt Ultrashort Laser Source for Extreme science Research (PULSER) installed in the Advanced Photonics Research Institute (APRI). The PULSER consists of two PW beamlines (PULSER I and II). The first PW beamline (PULSER I) was demonstrated in 2010, and its detailed output performance is described elsewhere . In this section, we describe the output performance of the second PW beamline
(PULSER II), which generates 1.5 PW laser pulses (30-fs and 44.5-J output energy) at 0.1 Hz.
Figure 1(b) shows the three-pass booster amplifier installed in the second PW beamline. In brief, an output laser pulse from the second power amplifier is injected into the PW booster amplifier; the output energy from the power amplifier is about 4.5 J and the repetition rate is 10 Hz. The beam size of the laser pulse is expanded, before injection, from 23 mm to 65 mm using an achromatic beam expander, which minimizes the chromatic aberration. Four frequency-doubled Nd:glass laser systems (Atlas + , THALES) operating at 0.1 Hz are then used to optically pump the PW booster amplifier. Each Nd:glass pump laser system provides a maximum output energy of 30 J with two green (527 nm) beamlines. Thus, the total energy of 120 J from eight beamlines could be irradiated on a 100-mm-diameter Ti:sapphire crystal (Crystal Systems). The pump laser system is designed to have a flat-top beam profile and the beam size is about 20 mm at the laser output position. An image relay system was also designed and installed in order to magnify the pump beam size from 20 mm to 65 mm on the Ti:sapphire crystal. The average pump fluence was 1.7 J/cm2 per face. The pulse duration of the pump laser pulse was about 13 ns, and the time delay of each beamline could be independently controlled. As a result, 92% of the pumping energy was absorbed in a Ti:sapphire crystal having a thickness of 25 mm.
Figure 2(a) shows the measured (red circles) and calculated (grey rectangles, black diamonds) output energies from the final booster amplifier with respect to the pump energy. Here, the output energy is calculated from the parameters used in the three-pass booster amplifier. The maximum output energies and extraction efficiencies are 73.3 J and 63% for the 1-D Frantz-Nodvick simulation (grey rectangles), 66.2 J and 56% for the 2-D Frantz-Nodvick simulation (black diamond), and 60.2 J and 51% for the experimental measurement (red circles). The main reason for the difference between the 1-D and 2-D simulations is the spatial beam distribution of the pump laser considered; the difference between the 2-D simulation and experimental measurement was due to the effect of parasitic lasing, which was not considered in the simulation. The maximum output energy of 60.2 J was obtained at a pump energy of 120 J. The output energy was measured by an energy-meter (PE100BF-DIF, Ophir) with a beam splitter having a transmission of 23%. During amplification, parasitic lasing was efficiently suppressed by using an index-matching fluid with an absorption dye and by precisely controlling the time delay between the input pulse and eight-pump laser pulse. Without precise control of the time delay, parasitic lasing is obviously seen by degrading output beam profiles at an accumulated pump energy of over 60 J, due to the slight mismatch in refractive indexes between the index matching fluid and Ti:sapphire crystal.
The transverse parasitic lasing is generated when the gain in the transverse direction reaches transverse parasitic lasing threshold . In our experiment, the transverse gain was carefully controlled in the saturation regime, by manipulating the time delay between the pump pulse and input seed pulse. A more detailed analysis and explanation on the suppression of parasitic lasing are described elsewhere . Precisely controlling the time delay between the pump and input seed pulses makes it possible to use the total pump energy of 120 J with significantly reduced parasitic lasing. Here, the overall pump beam profile was optimized by adjusting the time delay of each pump beam to minimize the pulse duration of the superposed eight-pump beam [See the inset of Fig. 2(a)]. As such, the time delay (τd) was optimized to maximize the amplified output energy. In the experiment, by increasing the time delay (τd) from 0 ns to 5 ns, the amplified output energy increased from 43 J to 53 J. When the time delay was further increased to 10 ns, the amplified output energy decreased to 51 J due to the effect of parasitic lasing. Parasitic lasing was significantly reduced in the range from 12 ns to 17 ns, and the maximum amplified energy of 60.2 J was obtained at a time delay of 15 ns, as shown in the inset of Fig. 2(a); increasing the time delay to over 18 ns again brought about strong parasitic lasing.
Figure 2(b) shows the shot-to-shot energy fluctuation of the amplified laser pulses. The shot-to-shot fluctuation had a 0.6% rms value for single-shot mode, and 1.2% rms value for 0.1-Hz mode, measured from 50 successive laser pulses. In the figure, the output energyslightly decreases as the shot number increases, though may be sufficient for the laser-plasma interaction experiments. It is posited here that the degradation of stability and output energy for 0.1-Hz mode operation might be from a decrease of the pump laser energy.
Figure 3 shows beam profiles for the measured and calculated output beam profiles, as well as the residual pump beam after three-pass amplification at different pump energies. In the figure, the measured beam profile of the amplified laser pulse is close to that of a flat-top beam profile and agrees well with the calculated profile for the 2-D Frantz-Nodvick simulation . With the output beam profile, we performed a super-Gaussian fit with a nth-order super-Gaussian profile, exp(−2r2N/w02N). From the super-Gaussian fit, it was found that the super-Gaussian orders, N, were 13 and 9 in the vertical and horizontal direction respectively. To minimize the doughnut-shaped residual pump profile or maximize the output energy from a Ti:sapphire gain medium, the size of the seed beam should be well optimized to the size of the pump beam profile. In our case, we set the size of the input seed beam at 65 mm for the first pass and 70 mm for the third pass, with a divergence of 0.3 mrad, By doing this, the output energy from the Ti:sapphire gain medium could be maximized at 60.2 J. The measured output energies at input beam divergences of 0 mrad, 0.15 mrad, 0.3 mrad are 57.8 J, 59.3 J, 60.2 J, respectively. The higher input beam divergence over 0.3 mrad was not practical because of a higher chance for optical damage in the vacuum beam expander in front of the PW booster amplifier.
3. Temporal and spatial characteristics
After passing though another achromatic beam expander, the amplified laser pulse is incident on a pulse compressor that consists of four gold-coated holographic gratings (1480 groove/mm). In the achromatic beam expander, the laser beam size was expanded from 65 mm to 200 mm to avoid optical damage on the gold-coated compression gratings. The average input fluence on the first grating was 0.1 J/cm2. Figure 4 presents the schematic of the four-grating pulse compressor. The sizes of the first and fourth gratings are 275 mm (V) × 390 mm (H), and the second and third gratings are 360 mm (V) × 565 mm (H). In the grating compressor, the incident angle and grating separation are 50.1° and 1002 mm, respectively, and the measured efficiency of the pulse compressor was 74%, yielding a final output energy of 44.5 J. The compressor efficiency is the average of values measured at five different positions. After compression, the pulse duration was measured using single-shot spectral interferometry for direct electric field reconstruction (SPIDER).
Figure 5(a) shows the output spectrum (red line) of the laser pulse amplified in the booster amplifier and the measured spectral phase (black dotted line) in the range of 750 nm to 870 nm. Compared to the input spectrum (blue line), the spectral distribution of the output spectrum is red-shifted and broadened due to the effect of saturation during the amplification. The measured spectral bandwidth of the amplified pulse is 46 nm at FWHM. The measured spectrum (red line) agreed well with the calculated one (green line) based on the 2-D Frantz-Nodvick model as shown in Fig. 5 (a).
The gain narrowing effect is compensated for with an acousto-optic programmable dispersive filter (AOPDF). The reconstructed pulse profile is shown in Fig. 5(b), where the stretched pulse of 900 ps is recompressed to 30.2 ± 1.8 fs (mean ± standard deviation for 30 successive pulses) by the pulse compressor. The transform-limited pulse duration is 26 fs for the measured spectrum. When considering an output energy of 44.5 J after pulse compression with a transmission efficiency of 74%, the peak power is about to 1.5 PW, which is the highest output power ever achieved with a femtosecond Ti:sapphire CPA laser system.
The temporal contrast is an important parameter to be considered in laser-plasma interaction experiments involving such high power laser systems. By using a beam splitter with a transmission of 2% and taking a small part of laser beam to the third-order autocorrelator, we could directly measure the contrast ratio of PW laser pulses because of its relatively high repetition rate of 0.1 Hz. Here, the contrast ratio was measured with a third-order autocorrelator (Amplitude, Sequoia). To enhance the contrast ratio, we implemented a saturable absorber (RG850, CVI laser) and two ultrafast Pockels cells (UPCs) which had rise times of 150 ps and 800 ps, respectively. The saturable absorber and two UPCs were installed between the front-end system and the stretcher. The 5-ps laser pulses form the front-end system were selected at 10-Hz repetition rate and temporally cleaned in the range of a-few-hundred ps by two UPCs. Furthermore, the contrast ratio of the cleaned pulses was enhanced by an order of 2 with the saturable absorber [22,23].
Figure 6 presents the measured contrast ratios for 1.5 PW laser pulses. The contrast ratios at 500 ps and 50 ps before the main pulse are about 2.3 × 10−12 and 4.8 × 10−10. There are several peaks observed before the main pulse at 220 ps, 170 ps, 63 ps, 35 ps, and 25 ps. The prepulse, in the 100 TW laser pulse, with a contrast ration of 2.7 × 10−10 at 220 ps is a ghost due to the effect of the third-order correlation between the main pulse and the post-pulse at 220 ps. A post pulse was generated from an AR-coated window with a thickness of 2 cm, which corresponded at 220 ps after the main pulse, in a vacuum beam expander. The contrast ratio induced by the reflection of the post-pulse was calculated to be 1.6 × 10−5 due to the double reflection from the AR-coated window. Thus, the contrast ratio measured at 220 ps was originated from the post-pulse. However, the prepulse at 170 ps is real, and we think that it is from the nonlinear interference [24,25] between the main pulse and the post-pulse originating from the high-damage-threshold broadband polarizer (PBSK, CVI). The origins of the prepulses, in the 100 TW laser pulse, at 63 ps, 35 ps, and 25 ps are currently under investigation. We think the contrast ratio for 1.5 PW pulses would be worse than that for 100 TW pulses in the range over 300 ps when considering the energy increase in the booster amplifier. However, because of the instrument detection limit, it is hard to compare contrast ratios between 1.5 PW pulses and 100 TW pulses. We could not observe any serious degradation in the contrast ratio for 1.5 PW pulses less than less than 300 ps. The reason is that the gain of ASE inside the stretched pulse was similar to the gain experienced by the main pulse.
An adaptive optics (AO) system is employed for wavefront correction in the PW laser pulses, and a wavefront sensor (SID-4, Phasics) is installed after the PW pulse compressor. A bimorph deformable mirror, which has a clear aperture of 70 mm, is installed between the PW booster amplifier and the achromatic beam expander; thus, a pre-corrected wavefront is incident on the PW pulse compressor. Figure 7 shows the residual wavefront aberration after correction and the focal spot measured for the PW laser pulse.
The residual wavefront aberration has a 0.097 µm rms value for Zernike coefficients of 10-th radial order Zernike polynomials. The Strehl ratio calculated from the wavefront measurement is about 0.67, which means the focal spot is close to the diffraction-limited one. The measured sizes of the focal spots are 20 µm and 20 µm in the horizontal and vertical directions, respectively. After wavefront correction, a focused intensity of 3.3 × 1020 W/cm2 can be reached for an f/16 focusing optic. This means that a focused intensity of about 1 × 1022 W/cm2 is possible when an f/3 focusing optic is used.
In conclusion, a PW booster amplifier that capable of producing 60.2 J at a repetition rate of 0.1 Hz was demonstrated. To avoid parasitic lasing during amplification, two techniques using an index-matching fluid with an absorption dye and the precise control of the time delay between the input pulse and pump laser pulses were simultaneously implemented. The amplified laser pulse was recompressed to 30 fs in a four-grating pulse compressor. Thus, the maximum peak power could reach about 1.5 PW, which is the highest output power ever achieved. Contrast measurement showed that contrast ratios for the 1.5 PW laser pulse were 2.3 × 10−12 and 4.8 × 10−10 at 500 ps and 50 ps before the main pulse, respectively. For our 1.5 PW laser system, the contrast ratio measured might be acceptable until 50 ps before the main pulse. However, the contrast ratio for the 170-ps pre-pulse and ASE background in the range below 50 ps should be improved in order to accelerate proton beams from nanometer-scale ultrathin targets. In our case, a double plasma mirror system  is a candidate for further improving the contrast ratio. Focusability was also investigated for the PW laser pulse. Here, the calculated Strehl ratio based on the wavefront measurement reached 0.67, and a focused intensity of about 1 × 1022 W/cm2 will be possible when an f/3 focusing optic is used.
We thank C. W. Lee, S. W. Kang, J. M. Lee, J. M. Yang, and J. Jeong for many helps on this experiment. This work was supported by the Ministry of Knowledge and Economy of Korea through the Ultrashort Quantum Beam Facility Program
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