【Preface】
After Professor Gérard Mourou, the inventor of chirped pulse amplification technology, returned to Europe from the University of Michigan in the United States, he suggested that the EU build a 100PW ultra-powerful laser device. After discussions among EU countries, it was finally decided to establish ultra-intense lasers and user equipment with different scientific application focuses in the Czech Republic, Hungary and Romania, three countries with the second gradient of European economic development. Starting from this issue, this public account will invite laser scientists who have worked in the field of ultra-intense lasers for many years to give detailed introductions to the three sub-devices of ELI. We will introduce the parameters of ELI's laser device in three phases. Later, we will successively introduce the introduction of ELI laser physics experiments and the recruitment requirements of each sub-device, so stay tuned.
The ELI-BEAMLINE laser device introduced in this issue is located in the Czech Republic [1]. The laser system of this device is divided into four independent beams. The naming method is simple and crude. L1, L2, L3, and L4 respectively correspond to: high repetition frequency 15fs OPCPA system, 10Hz OPCPA petawatt laser system, 10Hz titanium sapphire petawatt laser system, and single-shot kilojoule petawatt laser system. These four systems will be introduced below.
L1 laser beam: optical parametric amplification system (100mJ/15fs)
The laser is based on short-pulse pumped OPCPA technology and adopts a homologous seed optical synchronization scheme. The design index is 100mJ/15fs/1kHz. The seed source of the system is an ultra-wide-spectrum titanium sapphire oscillator. The special feature of this oscillator is that it can not only provide the broadband signal light of OPA, but also the spectrum can be extended to 1030nm, providing a seed source for the pump light of OPA. The natural optical synchronization scheme ensures the strict time synchronization required by the OPA process. With active synchronization feedback control, the synchronization accuracy can reach hundreds of attoseconds. OPA's pump laser uses Thin-disk technology, which is currently the first choice for high-repetition-frequency, high-energy lasers and has great advantages in thermal management of lasers. Germany's TRUMPF is the industry leader in this core technology. L1 Laser and TRUMPF's Scientific Laser Division have cooperated to develop a pump source with this performance advantage. In addition, one thing that needs to be highlighted is that the Czech Republic has invested heavily in the independent research and development of all-semiconductor-pumped Yb lasers and has reached the international leading level. For details, please refer to the HiLASE project website: http://www.hilase.cz/en/about-project/.
After the broadband signal light is amplified by 7-level OPA, it is compressed in vacuum, and finally reaches the design target: 100mJ/15fs/1kHz. In addition to some technologies of the pump source, the laser system was independently developed by the Czech side. The project is currently progressing smoothly and can already reach an output of 10mJ/11fs/1kHz [2].
L2 laser beam: petawatt OPCA laser system (1PW/20J/10Hz)
The UK's Rutherford Laboratory is famous in the field of ultra-powerful lasers, especially its unique high-energy short-pulse OPCPA technology. ELI-BEAMLINE's L2 laser is a collaboration with Rutherford Laboratory. As we all know, one of the biggest difficulties in OPA technology is how to obtain a high-quality pump source. Therefore, L2 laser beam has made great efforts in the pump source [3]. The technical solution adopts the Yb:YAG amplification method of low-temperature cooling all-semiconductor pumping. The final fundamental frequency energy can reach 60J, and the repetition frequency can reach 50Hz. It fully demonstrates the triple power of low-temperature cooling, all-semiconductor pumping, and low Stokes loss Yb medium! After 4 levels of OPA amplification, the signal pulse energy can reach 26J. After compression, the energy is 18J, the pulse width is 15fs, and the pulse contrast reaches 1:1011. This is the most ideal tool for generating high-quality laser accelerated proton sources.
L3 laser beam: petawatt titanium sapphire laser system (1PW/10Hz/30J)
There are rumors that because Lawrence Berkeley Laboratory in the United States purchased the PW laser system from Thales in France, the United States was dissatisfied and lobbied the European Union to purchase a high-repetition-frequency PW laser. This is the ELI-BEAMLINE L3 laser beam built by Livermore Laboratory. This may be just gossip, because if Livermore Laboratory develops industrialized lasers, it will definitely kill any commercial company in the world. With its strength, it is definitely a small case to build just one PW laser [4]. The design index of this laser is 30J/1PW/10Hz. Its biggest highlight is the pump source: it adopts helium cooling and all-semiconductor pump technology, and the gain medium selects neodymium glass. The reason is probably based on the mature glass laser amplification technology of NIF. The front stage of this pump source uses the DPSS laser of the American CEO Company. The front stage of the PW laser system uses a former Femtolasers titanium sapphire kHz amplifier to assist XPW technology in time purification and spectral broadening. The subsequent broadening, amplification and compression adopt mature commercial technologies and are highly reliable.
L4 laser beam: Nd:glass/OPCPA hybrid amplification (10PW/130fs/single shot)
Todd Ditmire of the University of Texas at Austin is not only a heavyweight in the laser field but also a pioneer in the industrialization of ultra-powerful lasers. The company he founded, National Energetics, successfully won a large order from ELI to build a 10PW, kilojoule-level femtosecond laser device. This is Beamline's L4 laser. National Energetics cooperated with Lithuania's EKSPLA company to build this laser system. SCHOTT Corporation and Livermore Laboratory of the United States also participated, integrating the superior resources of ultra-powerful laser design, flash lamp pumping, neodymium glass materials, and OPCPA to ultimately provide users with a high-quality laser system.
The laser system has two working modes:
(1) 10PW/single shot mode: All the energy of the entire system is used to generate 10PW peak power, and the nanosecond chirp pulse before compression can be exported separately to serve specific experiments. The seed laser in this mode is a broadband femtosecond oscillator. It first performs picosecond OPCPA prevention, widens to the nanosecond level and then continues parametric amplification. Finally, the plate-like neodymium glass pumped by an ultra-large aperture flash lamp is used to amplify to the kJ level. The compressor uses a dielectric film grating.
(2) 2kJ/narrowband nanosecond output mode: This beamline is similar to NIF or China’s Shenguang device. The front end of this mode is an independent laser system, which can realize flexible transformation of time and space modes, ensure the safe operation of the laser system, and serve different types of basic physics experiments.
[Afterword]
Judging from the ELI-BEAMLINE sub-device alone, it integrates the most cutting-edge laser technologies in the world: optical parametric amplification, all-semiconductor pumping, cryogenic cooling, thin-film amplifiers, etc. The use of these technologies will surely promote the rapid development of the entire ultrafast laser field. According to the author's many years of experience in the ultra-intense laser industry, the daily maintenance and stable operation of these laser systems will pose great challenges, and the demand for laser scientists and engineers will continue to increase. This is a good opportunity for researchers who are interested in engaging in ultra-intense lasers.
One of ELI's purposes of placing the three sub-devices in less developed countries in Europe may be to "technological poverty alleviation." However, in summary, the core technologies of the main laser systems come from Germany, the United States, France and the United Kingdom. This change actually stimulates the scientific and technological development of these developed countries and has a limited effect on promoting the scientific and technological development of the host country. This also has reference significance for China's development of ultra-powerful laser technology. Self-sufficiency can lead to long-term sustainability. Fortunately, the Shanghai Institute of Optics and Mechanics, the Institute of Physics of the Chinese Academy of Sciences, Tsinghua University and a group of outstanding returnees are working tirelessly to develop China's own ultra-powerful laser technology. I wish them success!
References
[1] ELI-BEAMLINE website: https://www.eli-beams.eu
[2]František Batysta, Roman Antipenkov, Jakub Novák, Jonathan T. Green, Jack A. Naylon, Jakub Horáček, Martin Horáček, Zbyněk Hubka, Robert Boge, Tomáš Mazanec, Bedřich Himmel, Pavel Bakule, and Bedřich Rus, "Broadband OPCPA system with 11 mJ output at 1 kHz, compressible to 12 fs," Opt. Express 24, 17843-17848 (2016)
[3]Saumyabrata Banerjee, Paul D. Mason, Klaus Ertel, P. Jonathan Phillips, Mariastefania De Vido, Oleg Chekhlov, Martin Divoky, Jan Pilar, Jodie Smith, Thomas Butcher, Andrew Lintern, Steph Tomlinson, Waseem Shaikh, Chris Hooker, Antonio Lucianetti, Cristina Hernandez-Gomez, Tomas Mocek, Chris Edwards, and John L. Collier, "100 J-level nanosecond pulsed diode pumped solid state laser," Opt. Lett. 41, 2089-2092 (2016)