Characterization of spatiotemporal couplings in ultrashort pulses

Abstract

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.

Popular Abstract

Ultrashort laser pulses are like a camera flash that blinks in a mere fraction of a trillionth of a second. Achieving such a short duration requires the laser to simultaneously contain a wide blend of different colors, also known as a broad spectrum. These ultrashort and energetic pulses can be useful scientific tools in lots of areas. They can capture the fastest processes in nature, such as the movement of electrons within atoms, and generate enormous bursts of peak power necessary for driving advanced technologies like compact particle accelerators and high-harmonic generation.

However, this multi-colored nature introduces a significant physical challenge. When this broad spectrum of light travels through standard optical components, such as glass lenses or gratings, the different colors bend and propagate at slightly different speeds and angles. As a result, the colors lose their perfect alignment, smearing out in both space and time. This complex distortion is known as a spatiotemporal coupling (STC). Simply put, STCs prevent the laser from focusing perfectly into a single, intensely tight spot, which dramatically reduces the maximum power the laser can deliver. Before these distortions can be corrected, they must be accurately mapped and measured. This thesis focuses on diagnosing STCs in a massive, multi-terawatt high-power laser system. To achieve this, a novel diagnostic technique called IMPALA (Iterative Multispectral Phase Analysis for LAsers) was used. The method works by placing a special mask with a pattern of tiny pinholes directly into the laser beam before focusing optics. This mask splits the large beam into several smaller beamlets. A camera then captures the complex interference patterns created when these smaller beamlets overlap at the focal point. Using customized computer algorithms, these recorded patterns are decoded to reconstruct the distortion maps for individual wavelengths inside the laser pulse. By applying this technique, the research characterized aberrations within the high-power laser. The results showed that while the high-power laser was focused by a deformable mirror, the colors at the outer edges of the spectrum were clearly tilted and distorted. Characterizing these hidden STCs is an important first step toward correcting them, paving the way for building sharper and more powerful lasers for next-generation scientific breakthroughs.

• author: Zou, Yuhao
• supervisor: Olle Lundh
• organization: Atomic Physics; Department of Physics
• course: PHYM03 20261
• year: 2026
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: Ultrashort pulses, Lasers, Spatiotemporal couplings, Phase retrieval
• publication/series: Lund reports on atomic physics (LRAP)
• report number: LRAP628
• language: English
• id: 9241077
• date added to LUP: 2026-06-18 09:00:40
• date last changed: 2026-06-18 09:00:40