Spatiotemporal coupling investigations for Ti:sapphire-based multi-PW lasers

During the past decade, parallel with the considerable advances of laser materials and laser technologies, several PW-class lasers have been built in the world. These systems allow the envisioning of extreme light applications, such as high field physics, particle acceleration to relativistic velocity, and medical applications. Producing extreme laser intensity on the target is the main objective of PW laser systems. Both in the temporal and the spatial domain, these systems need to meet strict requirements to assure high-quality, high contrast pulses, focused to nearly diffraction-limited spots. However, the temporal and spatial domains are not totally separated in ultra-short PW lasers. The induced space–time/spectral coupling effects are very specific features that could dramatically impact the targeted laser intensity even for the most carefully designed systems.

 

In the work published on High Power Laser Science and Engineering, Vol. 10, Issue 1(Ji Ping Zou, Hervé Coïc, Dimitris Papadopoulos. Spatiotemporal coupling investigations for Ti:sapphire-based multi-PW lasers[J]. High Power Laser Science and Engineering, 2022, 10(1): 010000e5), the authors present a synthesis of simulation studies, showing the need for careful consideration of a large number of parameters and the necessity of global optimization of PW-class systems, throughout the different sub-systems of the laser down to the end-chain target area. For our analysis a typical Ti:sapphire-based, multi-PW-class laser configuration is considered. New complex spatiotemporal (S-T) coupling effects that have not been investigated up until now are discussed in detail.

 

As a first example the authors consider sub-aperture local dependence effects of the gain and the saturation in the laser chain. In general, the amplification process will lead to the progressive transformation of the beam profile from a Gaussian input to a super-Gaussian output. However, the radial dependence of the gain/saturation balance will impact the spatial homogeneity of the output beam, with the external parts of the beam experiencing more gain (less saturation) in contrast to the central part of the beam reaching the full saturation level at the early stages of the amplification. As a result, the spectral gain narrowing and red-shifting are expected to evolve also radially leading to a non-constant FTL (Fourier transform limited) pulse duration over the beam, reaching variation up to 20-30% for typical PW systems. In the following figure they present the spatio-spectral evolution of the beam in the case of the Apollon laser assuming amplification from 5 mJ to 300 J with a corresponding beam diameter from 2.5 to 140 mm, respectively. The impact of such inhomogeneity should not be underestimated, especially regarding the focused beam intensity. In the specific case they find a non-negligible 12% difference in favor of a theoretical fully homogeneous beam.

 

Normalized near-field intensity evolution in the Ti:Sapphire-based amplification chain with a Gaussian beam input. It illustrates clearly the spatial dependence of the spectrum evolution.

 

Pulse stretching and compressing are two important processes for Ti:sapphire-based multi-PW lasers. Grating misalignment and surface quality effects have been at some extent previously discussed. Here the authors focus on a completely disregarded effect related to the gratings limited size and beam diffraction effect at their edges. Since the beam is spectrally dispersed on the 2nd and the 3rd grating it is nearly impossible to completely prevent a spatio-spectral variation as the impact of the diffraction effect. Even though no significant energy transmission losses (<1%) and no modification of the compressed pulse duration are observed, the spectral diffraction has a considerable impact on the output pulse contrast. They estimate a contrast decreases to 10-7-10-6 in the region of ±350 fs around the main peak. Furthermore, in the laser far field the spatiotemporal structure of the intensity evolution is quite unusual. An important secondary, highly dispersed structure sweeping across the focus both in space and in time is predicted. Such an effect could be highly disturbing for all high intensity experiments sensitive to the contrast in space and time.

 

In the next example the authors investigate, to the best of our knowledge for the first time, the impact of the optical Kerr effect on the spectral homogeneity of the beam wavefront and its contribution to the chromatic focal shift. In a CPA system the intensity of the stretched pulses varies in time in an almost proportional manner to the spectral intensity. This means that under the assumption of an instantaneous response of an NL medium the Kerr NL phase shift is a time/wavelength varying function. Analytical estimation of the impact of this effect is far from trivial in the case of real-world laser systems where local gain saturation dynamics result in a highly complex spatial repartition of the Kerr NL phase shift. Thanks to their 3D model they have been able to estimate the impact of the effect in the case of the Apollon laser corresponding to a rather important total chromatic focal shift of about 20% of the Rayleigh range and therefore comparable to other chromatic effects in the chain such as the longitudinal chromatism due to lens based telescopes.

 

As a last example the authors investigate the mirror induced dephasing issues especially in the case of ultrashort pulses requiring complex multilayer coating structures. They more specifically focus on 'out-of-plane' beam propagation configurations usually required when the incidence plane on the target is neither parallel nor perpendicular to the polarization of the beam. In this case, two orthogonal polarization components of the reflected beam may accumulate different spectral phase resulting in complex chromatic depolarization defects in the focus. In particular, they study the impact of two types of mirrors (a multi-layer dielectric coating and a hybrid metallic-dielectric one), both designed to provide large bandwidth reflectivity and low dispersion characteristics. They show the non-negligible impact both on the effective pulse duration as well as the temporal contrast of pulses in the interaction area with the target.

 

In conclusion the authors show that ultra-intense laser performances can be considerably degraded by S-T coupling effects that are most of the times disregarded. To optimize the performance of multi-PW-class laser systems and to achieve targeted laser intensity, the examined effects should be carefully taken into consideration for the better understanding, the accurate experimental characterization and the mitigation of these detrimental effects. Further research and development efforts are required to evaluate experimentally the impact of cumulated S-T coupling effects on the focused laser intensity, particularly in the direction of adequate metrology technology for their precise characterization and control.