Tiled Titanium Sapphire Laser Amplification to Go Beyond the 10 Petawatt Limit to 40 PW or More

Coherently tiled titanium:sapphire laser amplification provides a relatively easy and inexpensive way to surpass the current 10-petawatt limit.

“By adding a 2×2 coherently tiled titanium:sapphire high-energy laser amplifier in China’s SULF or EU’s ELI-NP, the current 10-petawatt can be further increased to 40-petawatt and the focused peak intensity can be increased by nearly 10 times or more.

The method promises to enhance the experimental capability of ultra-intense ultrashort lasers for strong-field laser physics.

After reaching a world record of 10 PW, the peak power development of the titanium-sapphire (Ti:sapphire) PW ultraintense lasers has hit a bottleneck, and it seems to be difficult to continue increasing due to the difficulty of manufacturing larger Ti:sapphire crystals and the limitation of parasitic lasing that can consume stored pump energy. Unlike coherent beam combining, coherent Ti:sapphire tiling is a viable solution for expanding Ti:sapphire crystal sizes, truncating transverse amplified spontaneous emission, suppressing parasitic lasing, and, importantly, not requiring complex space-time tiling control. A theoretical analysis of the above features and an experimental demonstration of high-quality laser amplification are reported. The results show that the addition of a 2 × 2 tiled Ti:sapphire amplifier to today’s 10 PW ultraintense laser is a viable technique to break the 10 PW limit and directly increase the highest peak power recorded by a factor of 4, further approaching the exawatt class.

Femtosecond petawatt (fs-PW) ultraintense lasers, which are produced by chirped-pulse amplification (CPA), have opened and accelerated the research and development of plasma physics, particle physics, astrophysics, nuclear physics, etc…

Japan developed the first fs-PW Ti:sapphire ultraintense laser in 2003. Korea completed a 4.2 PW Ti:sapphire ultraintense laser in 2017. China and Europe accomplished two 10 PW (around 22 fs and 220 J) Ti:sapphire ultraintense lasers [i.e., Shanghai Super-intense Ultrafast Laser Facility (SULF) and Extreme Light Infrastructure – Nuclear Physics (ELI-NP)] in 2018 and 2020.

These fs-PW ultraintense lasers have produced 10^22 to 10^23 Watts per square centimeter high intensities providing the conditions for studying the relativistic high-field laser physics. However, strong-field quantum electrodynamics (SF-QED), such as vacuum birefringence and strong-field vacuum breakdown, require even higher intensities, over 10^23 Watts per square centimeter or close to the critical intensity (called the Schwinger limit) ∼
10^29 Watts per square centimeter beyond the capability of the current fs-PW ultraintense lasers.

Technically, two problems of the fs-PW Ti:sapphire ultraintense lasers, namely, small crystal size (Ti:sapphire) and parasitic lasing limit the significant increase in the maximum pulse energy, as well as the highest intensity. First, it is difficult to fabricate Ti:sapphire crystals over 300 mm now, and second, even with large Ti:sapphire crystals, the strong transverse amplified spontaneous emission (TASE) and its induced parasitic lasing can dramatically consume the stored pump energy in the case of strong pumping, thus significantly reducing the pump-to-signal conversion efficiency. Although there are many methods to suppress TASE and parasitic lasing, such as two-surface pumping, lightly doped Ti:sapphire crystal, refractive-index matching, TASE absorbing, pump-signal delay, and multiple-pulse pump, none of them can solve these problems completely. Because of this, the recently started project of the Station of Extreme Light 100 PW (SEL-100PW) at Shanghai, China, has chosen to use optical parametric chirped-pulse amplification (OPCPA) technology to reach 100 PW by increasing the energy to 1500 J and reducing the pulse to 15 fs. The OPCPA technology certainly has its advantages over the Ti:sapphire CPA technology, such as having a broader bandwidth, a larger crystal size (e.g., deuterated potassium dihydrogen phosphate), no parasitic lasing, and no saturation gain narrowing, but it also has its disadvantages, such as low-pump-to-signal conversion efficiency, poor beam quality, and low spatiotemporal stability.

The Ti:sapphire CPA, a proven and successful technology that has produced two 10 PW ultraintense lasers will still play an important role in enabling the future of 50 to 100 PW ultraintense lasers. For example, coherently combining 10 beams of 10 PW Ti:sapphire CPA ultraintense lasers is one way to reach 100 PW; tenfold reducing the pulses of a 10 PW Ti:sapphire CPA ultraintense laser with postcompression from ∼ 25 to 30 fs to 2.5 to 3 fs is another way to 100 PW. In engineering, each of these two approaches has its own difficulties.

Coherent Ti:sapphire tiling (CTT) proposed in 201432 and first demonstrated in a 1 Joule system in 2021, as a variant of the coherent beam combining (CBC), is also an important way to achieve 100 PW, which has significant advantages. The effective size of the tiled Ti:sapphire crystal is expanded and, importantly, parasitic lasing is suppressed as the TASE is truncated and limited within each subcrystal. In this paper, we analyzed the capability of the CTT in terms of enlarging crystal size, truncating TASE, and suppressing parasitic lasing; introduced the stability comparison with the CBC; demonstrated a four-pass 2× 2 tiled Ti:sapphire amplifier in a over 100 TW facility with a pump-to-signal conversion efficiency of 41.6%; and finally, discussed the solution to scale the current 10 PW Ti:sapphire ultraintense laser to 40 PW by adding a 2 X 2 tiled Ti:sapphire amplifier. This work provides an economical and convenient path to directly enhance the capability of the current 10 PW ultraintense lasers to meet the needs of experiments, such as SF-QED.

The CTT (Coherent Ti:sapphire tiling) technique is expected to break the bottleneck of current Ti:sapphire CPA ultraintense lasers and achieve higher output. Its advantageous effects in terms of TASE truncation, parasitic lasing suppression, and tiling error avoidance are theoretically analyzed and high conversion-efficiency energy amplification with high-quality spatial, temporal, and spectral homogeneity was successfully demonstrated in a less than 100 T laser facility. This work provides a technique to further improve current Ti:sapphire CPA ultraintense lasers from 10 to 40 PW or higher.