[Paper Review] A Survey of Spatio-Temporal Couplings throughout High-Power Ultrashort Lasers

Jeandet, Antoine, Jolly, Spencer W.+20 more|arXiv (Cornell University)|Sep 26, 2021

Laser-Matter Interactions and Applications44 citations

TL;DR

This paper presents the first systematic survey of spatio-temporal couplings (STCs) in high-power ultrashort-pulse lasers across multiple stages of chirped-pulse amplification (CPA) systems. Using advanced full-field diagnostics—TERMITES and INSIGHT—it reveals complex STC effects at various points in the laser chain, demonstrating that STC characterization throughout the entire system is essential for optimizing performance and understanding fundamental dynamics in ultrafast lasers.

ABSTRACT

The investigation of spatio-temporal couplings (STCs) of broadband light beams is becoming a key topic for the optimization as well as applications of ultrashort laser systems. This calls for accurate measurements of STCs. Yet, it is only recently that such complete spatio-temporal or spatio-spectral characterization has become possible, and it has so far mostly been implemented at the output of the laser systems, where experiments take place. In this survey, we present for the first time STC measurements at different stages of a collection of high-power ultrashort laser systems, all based on the chirped-pulse amplification (CPA) technique, but with very different output characteristics. This measurement campaign reveals spatio-temporal effects with various sources, and motivates the expanded use of STC characterization throughout CPA laser chains, as well as in a wider range of types of ultrafast laser systems. In this way knowledge will be gained not only about potential defects, but also about the fundamental dynamics and operating regimes of advanced ultrashort laser systems.

Motivation & Objective

• To investigate spatio-temporal couplings (STCs) in high-power ultrashort-pulse lasers beyond just the output stage.
• To identify and characterize STC sources at different stages of CPA laser chains, including after stretcher, amplifiers, and compressors.
• To demonstrate the feasibility and necessity of complete 3D spatio-temporal field characterization (amplitude and phase) throughout the laser chain.
• To motivate broader adoption of STC metrology in advanced ultrafast laser systems, including OPCPA, coherent combining, and future Petawatt-class systems.
• To provide a foundational dataset and methodology for diagnosing and optimizing complex STC effects in next-generation ultrafast lasers.

Proposed method

• Employed two advanced full-field diagnostic techniques: TERMITES (time-resolved spectral interferometry) and INSIGHT (spatially-resolved Fourier-transform spectroscopy).
• Measured the complex electric field E(x, y, ω) or Ẽ(x, y, ω) in the near-field across multiple planes along the beam propagation axis.
• Used iterative Gerchberg-Saxton-type phase retrieval algorithms to reconstruct the spatial phase from spectral intensity profiles measured at multiple axial positions.
• Performed numerical beam propagation using plane-wave decomposition to back-propagate the field to suspected STC sources (e.g., gratings, amplifiers).
• Applied both techniques to six distinct high-power CPA lasers with peak powers ranging from 0.1 TW to 1 PW, operating at various repetition rates.
• Combined experimental measurements with numerical simulations of a regenerative amplifier using a vectorial non-paraxial Fourier method and split-step algorithm with adaptive time slicing.

Experimental results

Research questions

• RQ1 What types of spatio-temporal couplings (STCs) are present at different stages of high-power ultrashort-pulse CPA lasers?
• RQ2 How do STCs evolve through the laser chain—from stretcher and amplifiers to compressor and final output?
• RQ3 To what extent do intrinsic components like gratings, prisms, and gain media contribute to STC formation?
• RQ4 Can full 3D spatio-spectral characterization (amplitude and phase) reveal previously undetected STC effects that impact beam quality and focusability?
• RQ5 How do measurement techniques like TERMITES and INSIGHT compare in resolving high-spatial-frequency STC features in real-world laser systems?

Key findings

• STC effects were observed at multiple stages across all six high-power CPA lasers, including after the stretcher, in amplifiers, and at the output, indicating that STCs are pervasive and not limited to final compression stages.
• The Salle Noire laser exhibited strong STC after post-compression, with spatially dependent spectral shifts and temporal distortions, affecting few-cycle pulse generation.
• In the UHI–100 system, STC was detected in the output beam due to residual phase distortions from the compressor, impacting particle acceleration efficiency.
• TERMITES demonstrated higher resolution in resolving high-spatial-frequency STC features compared to INSIGHT, particularly in the near-field.
• Numerical back-propagation of the measured field to the optical plane of suspected STC sources (e.g., gratings) confirmed that STC origins could be traced to specific components.
• The study revealed that STCs are not merely defects but are intrinsic to CPA systems due to broadband beam manipulation, necessitating full-field diagnostics for optimization.