We report on a 2.0 PW femtosecond laser system at 800 nm based on the scheme of chirped pulse amplification using Ti:sapphire crystals, which is the highest peak power ever achieved from a femtosecond laser system. Combining the index-matching cladding technique and the precise control of the time delay between the input seed pulse and pump pulses, the parasitic lasing in the final booster amplifier is effectively suppressed at the pump energy of 140 J at 527 nm. The maximum output energy from the final amplifier is 72.6 J, corresponding to a conversion efficiency of 47.2% from the pump energy to the output laser energy. The measured spectral width of the amplified output pulse from the final amplifier is 60.8 nm for the full width at half-maximum (FWHM) by controlling the spectral evolution in the amplifier chain, and the recompressed pulse duration is 26.0 fs. The technology of cross-polarized wave (XPW) is applied in a broadband front-end, and the pulse contrast is improved to ~1.5 × 10−11 (−100 ps before the main pulse) which is measured at 83 TW power level with a repetition rate of 5 HZ.
© 2013 Optical Society of America
The invention of chirped pulse amplification (CPA) in the mid-1980s  leads to a rapid increase in the peak power attainable with high energy ultrashort pulse lasers. In particular, the Ti:sapphire (Ti:S) based CPA laser systems have become standard table-top sources of powerful femtosecond laser pulses because of their capability to support broadband amplification, high energy storage, and large thermal conductivity. Peak power up to petawatt (PW)-class Ti:S laser systems using CPA technique have been developed by several groups worldwide [2–9]. Now, a 10 PW Ti:S laser system is being constructed in the French APOLLON/ILE laser as a single beam line prototype for the ELI project . Accordingly, larger-aperture Ti:S crystals pumped at higher pump fluence and energy are inevitable. In this case, transverse parasitic lasing (PL) within the booster amplifier volume severely clamps the signal gain, gain lifetime , and the pulse energy that can be extracted from the amplifier. Therefore, the suppression of PL is a very important task that has to be solved for the next generation of the ultrahigh power laser systems. However, the investigation for PL suppression at pump energy of over 120 J for 100 mm diameter Ti:S crystal is rarely reported at present.
Nowadays, the techniques for suppressing transverse PL are in two major ways. First, materials with matched index as external coating for the crystal are used to limit reflections at the edge of the crystal and increase the threshold of PL [12,13]. We describe this method as a passive technique. Second, the lightly-doped Ti:S crystal and the optimization of the pump beam homogenization are used to minimize transverse gain . Additionally, optimization of the time delay between the input seed pulse and pump pulses has been proved to be an effective method to suppress PL [7,9,14]. We describe these methods as active techniques. Thanks to the development of index-matching liquid, we make use of Cargille serie M refractive index liquid (n = 1.76 at laser wavelength)  as a cladding for our 100 mm Ti:S crystal. Thus, the use of the index matching cladding can effectively suppress PL . However, for a larger-aperture Ti:sapphire crystal pumped at a higher pump fluence and energy, this method is not sufficient to totally suppress PL. To overcome this restriction, combining the active method for PL suppression will be an important research direction in the future.
In this paper, we report an ultrafast and intense Ti:S CPA laser system with an output energy of 72.6 J at pump energy of 140 J. This is the first time to investigate the output properties in the final booster amplifier pumped at such high pump energy. The PL in our final booster amplifier is effectively suppressed at pump energy of 140 J by combining index-matching cladding and the accurate control of the time delay between the input seed pulse and pump pulses. However, the PL becomes more complex and cannot be suppressed regardless of the optimization of the time delay when the pump energy reaches 150 J. After the final booster amplifier, a homogeneous flattop spatial beam profile is attained. To improve the contrast and keep a very short pulse duration after the grating compressor, high-contrast, broadband seed pulses are generated by the front-end, which can support 15 fs pulse duration, and the seed pulse contrast is enhanced from original ~10−8 to 1.5 × 10−11 (−100 ps before the main pulse) at 83 TW power level. The measured spectral width of the amplified output pulse from the final booster amplifier is 60.8 nm (FWHM) by controlling the spectral evolution in the amplifiers, and the recompressed pulse duration is 26.0 fs. The compressor throughput efficiency is measured to be 72%, yielding a peak power of 2.0 PW, which is the highest peak power pulse yet produced in Ti:S laser systems.
2. High-energy pumping and PL suppression in final booster amplifier
2.1 Experimental setup
Figure 1 shows a schematic of the PW laser facility. A robust, high-contrast front-end based on cross-polarized wave (XPW) generation is used as the seeding source, and high-contrast seed pulses with energy of 105 uJ can be generated at a repetition of 1 kHz. The spectral width of the clean seed pulses is over 65 nm (FWHM), which can support sub-15 fs pulse duration. Injecting the high-contrast seed pulses into the following amplifiers, the pulse contrast can be improved to be 1.5 × 10−11 at 83 TW power level. A more detailed describe on the high-contrast front end is described elsewhere . The clean high-energy signal pulse is expanded by an all-reflective Offner type stretcher to a width of about 1.4 ns. Following the stretcher, the chirped pulses are boosted to energy of about 1.8 mJ at a 10 Hz repetition rate after a regenerative amplifier (RA). A spectral shaping filter (Alpine Research Optics) is inserted into the RA to suppress spectrum gain narrowing and control spectral shape. The beam is then enlarged to 4 mm for further amplification in the first six-pass amplifier I, which is pumped by a frequency-doubled Nd:YAG laser at a 10 Hz repetition rate, with single pulse energy of 204 mJ at 532 nm. The output laser energy is 40 mJ. This beam is then expanded to 20 mm and is injected into the next four-pass amplifier II. After four-pass amplification, the output laser pulse energy is about 3 J at pump energy of 9 J at a repetition rate of 5 Hz (using two frequency double Q-switched Nd:YAG lasers, TITAN 5, Amplitude Technologies). The laser beam is then expanded to 28 mm for the next-stage amplifier.
To increase the injected seed energy for the final booster amplifier, we further amplify the seed energy by a two-pass power amplifier pumped by a frequency-doubled Nd:glass laser, operating at a repetition of 20 min per shot. The amplified output energy is about 6.5 J at pump energy of 10 J. The side surfaces of Ti:S with size of 42 × 50 × 23 (mm3) used in the power amplifier are coated with black pen ink (ink from ZEBRA, Japan, catalog number MO-120-MC-BK) for PL suppression. The laser beam from the power amplifier is expanded to 82 mm with a uniform profile and is injected to the final four-pass booster amplifier. The Ti:S crystal used in the final booster amplifier is a commercially available disk (Crystal System, Inc) with 100 mm in diameter and 30 mm in thickness. The pump absorption coefficient of our Ti:S crystal is 1.0/cm. The crystal is antireflection coated at pump and signal wavelengths on two main surfaces, but fine-ground for the cylindrical edges. The PL suppression is mainly implemented in the final booster amplifier.
2.2 Index-matched cladding in the final booster amplifier
To minimize reflectivity on the interface of the Ti:S crystal, several index-matched cladding materials are used. In our case, we replaced the previous 1-BN with the Cargille Series M refractive index liquid (n = 1.76 at laser wavelength) as cladding material. The variation of refractive index along with wavelength is very small. However, the liquid is corrosive, and incompatible with commonly-used materials, such as steel and copper alloys. Based on the compatibility report of Cargille Laboratories , we chose aluminum after oxidation treatment as the material of the Ti:S mounting cell, whose broadsides were processed into arrises to prevent the mounting cell from scattering the transmitted spontaneous emission, as shown in Fig. 2. Thus, the problem of compatibility was well solved. We then used the laser dye of IR140 which can be readily dissolved into the index-matching liquid as a broadband absorber to absorb the transmitted spontaneous emission. The thickness of the liquid cladding layer is 1 cm. Based on the index-matching liquid, the threshold of PL can be increased effectively to a high value.
2.3 Optimization of seed-pump time delay in the final booster amplifier
According to the theoretical model in , we first calculated the surface transverse gains as a function of pumping time to predict the influence of the time delay on transverse PL in the final four-pass booster amplifier. The time interval from the seed pulse of the first pass to the front edge of the pump pulse is defined as the time delay. The energy of the pump beam is 140 J with 82 mm in diameter. The pulse duration of the pump beam is 21 ns for FWHM and approximately 28 ns for 1/e2 full width. We set the time interval between the two passes at 9 ns (ΔT12), 10 ns (ΔT23), and 11 ns (ΔT34) according to the practical optical system, as shown in Fig. 2. Figures 3(a) and 3(b) show the surface transverse gains at an injected seed energy of 6.5 J, when the time delay is 20 ns and 25 ns, respectively. In Fig. 3, the blue solid curve represents the surface transverse gains after the injection of the seed pulse. The green dashed curve is the practical profile of the pump beam. The green asterisks are the amplified output energies through each pass. Simulation results show that the surface transverse gains drastically increase when the Ti:S crystal is pumped some time later, especially in ΔT23, because of the exponential growth relationship between the gains and the pump energy. Simultaneously, the transverse gains can be suppressed effectively by optimizing the injected time of the seed pulse. The optimization of the time delay depends on two principles: that the mean values of the transverse gains during the whole pumping time are minimal, and that inverted population can be extracted completely after the amplification of the final pass. In this case, the optimal time delay for the seed injection of 6.5 J is 20 ns (Fig. 3(a)). Furthermore, the energy of the injected seed pulse has a significant influence on transverse gains. Figures 3(c) and 3(d) show the surface transverse gains at an injected seed energy of 10 J, when the time delay is 18 ns and 20 ns, respectively. The optimal time delay for the seed injection of 10 J is 18 ns. The results indicate the surface transverse gains are further reduced compared with the injection of 6.5 J. This result was also confirmed in our experiment. The PL threshold was about 125 J when the energy of the injected seed pulse was 3 J, although the time delay was optimized. For this reason, we added a power amplifier to enhance the injection for the final booster amplifier.
Based on the two principles for the optimization of the time delay mentioned above, we also simulated the optimal time delay at different pump energies when the energy of the seed pulse was 6.5 J, as presented in Fig. 4. The choice of optimal time delay in region I depends only on the second principle. The transverse PL can be suppressed only by the cladding technique. The influence of time delay on the transverse gain becomes obvious in region II. The higher the pump energy, the shorter the optimal time delay. However, the range of variation in the time delay is reduced in region III. The surface transverse gains are more sensitive to pumping time.
2.4 Results and discussions
In our experiment, the pump laser used for the final amplifier was split into two beams, which were spatially filtered, with the image relayed to the Ti:S crystal. The time delay between the seed and pump pulses was precisely synchronized by a Master Clock (THALES, Inc) circuit. The time delay was manipulated to maximize the amplified output energy at a fixed pump energy of 140 J. The maximum amplified output energy of 72.6 J was obtained at a time delay of 22 ns, which corresponds to a conversion efficiency of 47.2%. Figure 5 shows the measured (black rectangles) output energies and the conversion efficiencies from the final booster amplifier with respect to the pump energy at the time delay of 22 ns. The black solid curve indicates a polynomial fit. The red dashed curve is the calculated result based on a Frantz-Nodvick simulation .
The experimental results indicate that output energies and conversion efficiencies increase with pump energy, and appear clamped when pump energy is close to 140 J. Meanwhile, the difference between experimental and simulated results becomes more obvious due to the effect of PL. During amplification, PL can be effectively suppressed by combining index-matching cladding and the precise control of the time delay before the pump energy reaches 140 J. From this point, output energies begin to drop slightly. When the pump energy increases further up to about 150 J, the PL occurs strongly. The output energy was not improved significantly, although we continued to change the time delay at pump energy of 150 J. The optimization of the time delay to suppress the PL was also limited at this higher pump energy.
To investigate more details, we measured the output energies of each pass at pump energy of 140 J, and with the time delay of 22 ns. The output energies of each pass were 12.5, 35.4, 62.8, and 70.5 J, respectively. The results indicate that the gain of the first pass was low because of the time delay. Consequently, we can increase the energy of the injected seed pulse to extract inverted population stored before the seed pulse is injected more adequately. Moreover, the amplified seed energy through the first pass is stronger, and transverse gains can be suppressed effectively during the whole amplification process. Therefore, for larger-aperture Ti:S crystals pumped at higher pump fluence and energy, using multistage amplifiers to enhance the injection can be an effective approach to suppress PL based on index-matching cladding and the optimization of the time delay.
3. Output beam profile and energy stability of the final booster amplifier
After the final booster amplifier, the amplified beam profile was measured by a CCD (Spiricon, USB L230). A homogeneous flattop spatial beam profile was attained due to gain saturation, as shown in Fig. 6(a). Relative RMS intensity modulations are 10% and 12% for the horizontal and vertical directions, respectively. The amplified signal energy from the final booster amplifier was continuously recorded with the pump energy and time delay fixed to 140 J and 22 ns, respectively. Figure 6(b) shows the shot-to-shot energy fluctuation of the amplified laser pulses. The shot-to-shot fluctuation had a 1.8% rms value, measured from eight successive laser pulses. The maximum energy was 72.6 J and the minimum energy was 68.8 J.
4. Pulse width and contrast ratio
The front-end in our experiment can generate clean seed pulses with a broad spectrum. Spectrum gain-narrowing suppression and spectral shaping can be achieved by inserting a spectral shaping filter into the RA. The strong red-shift of the central wavelength, induced by the strong gain saturation in the final booster amplifier, was compensated for by changing the injected spectral shaping, as shown in Fig. 7(a). The blue line in Fig. 7(a) shows the injected spectrum with energy of 3 J at 5 Hz for the final booster amplifier. The measured spectral width of the amplified output pulse from the final booster amplifier was 60.8 nm for FWHM (dark line in Fig. 7(a)). The compressed pulse width was measured to be 26.0 fs using a single-shot autocorrelator as shown in Fig. 7(b). The measured transmission efficiency of the compressor was about 72%, indicating output energy of 52.3 J for a compressed pulse, corresponding to a peak power of 2.0 PW.
The contrast ratio was further measured by a third-order scanning cross correlation (Sequoia, Amplitude Technologies) at 83 TW power level, without pumping the power amplifier and final amplifier, and the seed pulse was through the entire system chains at 5 HZ repetition rate. Figure 8 shows that the contrast ratio is around 10−11 within the time scale of −100 ps. Although it is very difficult to measure the pulse contrast in single-shot mode, the contrast should stay the same level while all the amplifiers are pumped and the peak power is 2.0 PW, according to the measurement and numerical simulation in the similar laser systems [7, 19]. The contrast enhancement method in the PW Ti:S laser system is described in detail in another paper . The high-contrast PW Ti:S system can provide a powerful tool for investigating laser-matter interactions in the relativistic regime.
In conclusion, we successfully produced a 2.0 PW femtosecond laser system at 800 nm based on the Ti:S CPA technique, and investigated the output properties of the final booster amplifier pumped at a high energy for the first time. By combining excellent index-matching cladding and the precise optimization of the time delay, transverse PL in the final booster amplifier was suppressed effectively at pump energy of 140 J when the energy of the injected seed pulse was 6.5 J. Maximum output energy of 72.6 J at pump energy of 140 J was achieved by the Ti:S CPA laser system. However, the PL becomes more serious and cannot be suppressed when the energy of the pump beam reaches 150 J, although the time delay has been optimized. With the combination of the techniques mentioned above, multistage amplifiers yielding higher energy seed pulse as injection will be an effective approach to suppress PL in the future. Meanwhile, based on a high-contrast broadband front end, pulse contrast was improved to 1.5 × 10−11 (−100 ps before the main pulse) at 83 TW power level. The recompressed pulse duration was 26.0 fs by controlling the spectral evolution in the amplifiers. These results are valuable for developing ultra-power, large-aperture, and high-performance Ti:S amplifiers with minimal PL effects.
The authors acknowledge support from the National Science Foundation of China under grant (No.61221064, 61078037, 61378030), the National Basic Research Program of China (Grant No.2011CB808101), the National Natural Science Foundation of China (Project No.11127901), and the International S&T Cooperation Program of China (Grant No. 2011DFA11300).
References and links
1. D. Strickland and G. Mourou, “Compression of amplified chirped optical pulses,” Opt. Commun. 56(3), 219–221 (1985). [CrossRef]
3. X. Liang, Y. Leng, C. Wang, C. Li, L. Lin, B. Zhao, Y. Jiang, X. Lu, M. Hu, C. Zhang, H. Lu, D. Yin, Y. Jiang, X. Lu, H. Wei, J. Zhu, R. Li, and Z. Xu, “Parasitic lasing suppression in high gain femtosecond petawatt Ti:sapphire amplifier,” Opt. Express 15(23), 15335–15341 (2007). [CrossRef] [PubMed]
4. V. Yanovsky, V. Chvykov, G. Kalinchenko, P. Rousseau, T. Planchon, T. Matsuoka, A. Maksimchuk, J. Nees, G. Cheriaux, G. Mourou, and K. Krushelnick, “Ultra-high intensity- 300-TW laser at 0.1 Hz repetition rate,” Opt. Express 16(3), 2109–2114 (2008). [CrossRef] [PubMed]
6. Z. Wang, C. Liu, Z. Shen, Q. Zhang, H. Teng, and Z. Wei, “A high contrast 1.16 PW Ti:sapphire laser system combined with DCPA scheme and femtosecond optical-parametric amplifier,” Opt. Lett. 36(16), 3194–3196 (2011). [CrossRef] [PubMed]
7. T. J. Yu, S. K. Lee, J. H. Sung, J. W. Yoon, T. M. Jeong, and J. Lee, “Generation of high-contrast, 30 fs, 1.5 PW laser pulses from chirped-pulse amplification Ti:sapphire laser,” Opt. Express 20(10), 10807–10815 (2012). [CrossRef] [PubMed]
8. S. Laux, F. Lureau, C. Radier, O. Chalus, F. Caradec, O. Casagrande, E. Pourtal, C. Simon-Boisson, F. Soyer, and P. Lebarny, “Suppression of parasitic lasing in high energy, high repetition rate Ti:sapphire laser amplifiers,” Opt. Lett. 37(11), 1913–1915 (2012). [CrossRef] [PubMed]
9. Y. X. Chu, X. Y. Liang, L. H. Yu, L. Xu, X. M. Lu, Y. Q. Liu, Y. X. Leng, R. X. Li, and Z. Z. Xu, “Parasitic lasing suppression in large-aperture Ti:sapphire amplifiers by optimizing the seed-pump time delay,” Laser Phys. Lett. 10(5), 055302 (2013), http://iopscience.iop.org/1612-202X/10/5/055302/. [CrossRef]
10. G. Chériaux, F. Giambruno, and A. Fréneaux, “Apollon-10P: Status and implementation,” Proc. AIP. 1462, 78–83 (2012). [CrossRef]
13. F. Ple, M. Pittman, G. Jamelot, and J.-P. Chambaret, “Design and demonstration of a high-energy booster amplifier for a high-repetition rate petawatt class laser system,” Opt. Lett. 32(3), 238–240 (2007). [CrossRef] [PubMed]
14. V. Chvykov and K. Krushelnick, “Large aperture multi-pass amplifiers for high peak power lasers,” Opt. Commun. 285(8), 2134–2136 (2012). [CrossRef]
16. D. C. Brown, S. D. Jacobs, and N. Nee, “Parasitic oscillations, absorption, stored energy density and heat density in active-mirror and disk amplifiers,” Appl. Opt. 17(2), 211–224 (1978). [CrossRef] [PubMed]
17. Y. Xu, X. Guo, X. Zou, Y. Li, X. Lu, C. Wang, Y. Liu, X. Liang, Y. Leng, R. Li, and Z. Xu, “Pulse temporal quality improvement in a petawatt Ti:sapphire laser based on cross-polarized wave generation,” Opt. Commun. 313, 175–179 (2013).
18. T. A. Planchon, F. Burgy, J.-P. Rousseau, and J.-P. Chambaret, “3D modeling of amplification processes in CPA laser amplifiers,” Appl. Phys. B 80(6), 661–667 (2005). [CrossRef]
19. H. Kiriyama, T. Shimomura, H. Sasao, Y. Nakai, M. Tanoue, S. Kondo, S. Kanazawa, A. S. Pirozhkov, M. Mori, Y. Fukuda, M. Nishiuchi, M. Kando, S. V. Bulanov, K. Nagashima, M. Yamagiwa, K. Kondo, A. Sugiyama, P. R. Bolton, T. Tajima, and N. Miyanaga, “Temporal contrast enhancement of petawatt-class laser pulses,” Opt. Lett. 37(16), 3363–3365 (2012). [CrossRef] [PubMed]