Characterization of spatiotemporal couplings in ultrashort pulses

Sammanfattning

When ultrashort pulses propagate through common optical elements, such as gratings and thick lenses, they inevitably acquire spatiotemporal couplings (STCs), which are nonseparable chromatic aberrations. This phenomenon fundamentally stems from the broad spectral bandwidth inherent to ultrashort pulses. STCs can significantly reduce peak intensity, enlarge the focal spot, and thus limit the optimal performance of ultrashort pulses in their applications such as high-harmonic generation (HHG) and laser-driven particle acceleration. This thesis aims to characterize the STCs in a multi-terawatt optical parametric chirped-pulse amplification (OPCPA) laser system. The primary method employed is the Iterative Multispectral Phase Analysis for LAsers (IMPALA) technique, which combines far-field beamlet cross-correlation with a Gerchberg-Saxton (GS) phase-retrieval algorithm. To achieve this, we developed and optimized stable data-processing programs, including spatial Fourier filtering and complex phase averaging. The technique was first validated using a continuous-wave fiber laser and subsequently applied to the OPCPA system using both a single-shot mask with 12 pinholes and a multi-rotation mask with 10 pinholes. The results reveal that while the system's wavefront is well optimized by a deformable mirror near its central wavelength (850 nm), it exhibits significant chromatic aberrations at the spectral edges (e.g., 830 nm), primarily exhibiting chromatic spatial tilt (pulse-front tilt) and oblique astigmatism. Furthermore, the quantitative accuracy of the IMPALA algorithm was verified by introducing a defined amount of aberrations, such as astigmatism, x-coma, and spherical aberration via the deformable mirror, demonstrating agreement with theoretical predictions.