To further boost the peak power of the output laser pulse without adding additional amplifiers, a larger aperture Ti:sapphire crystal can be used in the amplifiers. However, using a larger aperture crystal will significantly increase the chance of inducing transverse amplified spontaneous emission (TASE) and parasitic lasing (PL), and the suppression of these two are essential to avoid deteriorating the efficiency of amplification. In this Optics Letters article, Yuxi Chu et al. use two traditional techniques, a matched index cladding and transverse gain control techniques, in the multipass (4 passes) amplifiers pumped with two high-energy pulses (527 nm), and search for optimal conditions to suppress TASE and PL effectively. First, the authors apply absorber-doped index-matching oil as the cladding of the Ti:sapphire crystal. This index-matching oil highly increases the transmittance at the crystal-oil interface by reducing the difference in index of refraction at the interface, and the absorber in the oil absorbs the transmitted emission from the crystal. This eventually results in reducing TASE and PL by enhancing the loss of emission. Next, the control of transverse gain in the crystal is used for further optimization by adjusting the pump-seed time delay during 4 passes of amplification. At each seed-pulse energy, the authors also show that the delay needs to be re-optimized, since the injected seed pulse affects the transverse gain. With these two optimization processes, considering a loss of 28% at the pulse compressor, the authors for the first time demonstrate an output pulse energy of 138.5 J at the maximum peak power of 5.13 PW with the Ti:sapphire laser system.
In summary, the authors successfully describe how to implement a large-aperture Ti:sapphire crystal in a petawatt laser system, and how to generate hundred-joule level pulses by effectively suppressing the TASE and PL through the optimization of the matched index cladding and transverse gain control techniques. As the authors note, the peak power of ultrafast laser pulses can potentially be increased further with these optimization techniques, because a larger aperture of Ti:sapphire crystal can be easily employed.
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