J. Eur. Opt. Society-Rapid Publ. 2026, 22, 24 Ó The Author( s), published by EDP Sciences, 2026 https:// doi. org / 10.1051 / jeos / 2026012 Available online at: https:// jeos. edpsciences. org
Journal of the European Optical Society-Rapid Publications
RESEARCH ARTICLE
Optimum asymmetric spatial and temporal distribution of femtosecond laser pulses in refractive surgery
Samuel Arba Mosquera * and Shwetabh Verma SCHWIND eye-tech-solutions GmbH, Research and Development, Kleinostheim D-63801, Germany
Received 12 December 2025 / Accepted 8 February 2026
Abstract. Femtosecond laser-assisted refractive correction relies on temporally and spatially separated pulses that generate coalescent cavitation bubbles, forming a cleavage plane for tissue separation. Achieving optimal outcomes requires balancing laser-induced stress, mechanical dissection stress, and surface roughness. This work introduces a nonlinear absorption model and a theoretical framework to identify the optimum spatial and temporal distribution of single pulses. The analysis, based on inequalities involving scaling factors for spot size and track distance, defines a bounded solution space. Within this domain, the most favorable setting corresponds to minimum dose with maximum asymmetry, ensuring energy efficiency while enhancing surface smoothness, whereas the least advantageous of the optimum conditions occurs for higher dose and minimum asymmetry( compatible with optimum conditions), both enabling a theoretical bridge-free dissection. Bubble overlap emerges as a key determinant of cutting efficiency and smoothness, and an optimal window for overlap factors is delineated, minimizing treatment dose while preserving corneal quality through smoother stromal cuts.
Keywords: Asymmetric spacing, Spatial and Temporal Pulse Distribution, Dose optimization, Surface smoothness, Femtosecond laser, Refractive Surgery.
1 Introduction
Refractive surgery has advanced with the introduction of ultrashort-pulse laser systems [ 1, 2 ]. Under high concentrated peak irradiances and shorter exposure times, in the picosecond and femtosecond( fs) range, one can not only break molecules as during photoablation, but even strip electrons from their atoms and accelerate them resulting in the generation of dense free electron plasma. If a critical value is exceeded, laser-induced optical breakdown( LIOB) occurs with a very fast increase of temperature and pressure, which leads to a rapid plasma expansion. This results in a shock wave propagating into the surrounding medium, causing the formation of a cavitation bubble, which may undergo a series of oscillations before ending in a small persistent gas bubble after some microseconds. Plasmamediated ablation [ 3, 4 ] and photodisruption [ 5, 6 ] are the key laser – tissue interaction mechanisms underlying femtosecond laser-assisted techniques, enabling precise intrastromal dissection. In Photodisruption, microjoulerange pulses at high repetition rates and micrometer-scale spacing create cavitation bubbles that mechanically
* Corresponding author: Samuel. Arba. Mosquera @ eye-tech. net separate stromal lamellae, whereas in plasma-mediated ablation, lower pulse energies delivered at multi-MHz rates with strong pulse overlap result in tissue separation through cumulative free-electron-mediated effects without cavitation. The( asymptotic) size of the cavitation bubble leading to photodisruption grows with the cubic root of the applied( suprathreshold) pulse energy [ 7, 8 ]. Theoretical models and empirical data have substantiated the importance of identifying the fine differences between the two interaction mechanisms to explain the cavitation bubble dynamics and reliably estimating its size at any given single pulse energy [ 9 ].
Femtosecond lasers generate plasma and cavitation bubbles with substantially lower energy thresholds than longer-pulse systems due to nonlinear multiphoton absorption [ 10 ]. In corneal tissue, femtosecond optical breakdown typically occurs at pulse energies on the order of tens to hundreds of nanojoules, depending on numerical aperture, pulse duration, and tissue hydration, with reported breakdown thresholds in the range of approximately 25 – 200 nJ for commonly used clinical focusing conditions [ 10 – 12 ]. The resulting cavitation bubbles in intrastromal corneal surgery exhibit characteristic radii typically in the range of 0.7 – 3 lm, with upper clinical values approaching 4 – 5 lm at higher pulse energies [ 13 – 15 ]. Clinical systems
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