J. Eur. Opt. Society-Rapid Publ. 22, 24( 2026) 245
generally avoid pulse energies that generate bubbles exceeding ~ 5 lm radius due to the increased risk of stromal disruption, endothelial stress, and irregular interface formation [ 16 ].
The affected diameter of a single spot is typically on the order of 1 – 10 lm, whereas the treatment zone diameter ranges from 6 – 9 mm, yielding a scale separation exceeding three orders of magnitude [ 15, 17 ]. Because of this extreme scale separation, the corneal surface may be regarded as locally planar both radially and azimuthally, independent of whether the global scan pattern is spiral, raster, or meander. Thus, the local geometry governing bubble overlap is effectively Cartesian.
Due to the scale separation, multiple laser pulses are sequentially delivered onto the cornea. Each laser pulse locally creates a small cavitation bubble separating the corneal tissue. The global process is an integral effect of the local process of each laser pulse. A larger bubble gives a faster treatment, but a lower resolution. On the other hand, a smaller bubble increases the resolution at the cost of increasing the treatment time due to an increased number of laser pulses invested in cutting the same area. Additionally, using laser pulses too close to the threshold of the material( for generating cavitation bubbles) would mean only imparting thermal effects instead of generating stable bubbles. All these factors make the energy selection a sensitive criterion.
The laser pulses in femtosecond laser-assisted refractive correction techniques are separated both spatially( spot and track separation conforming a pattern, Fig. 1) and temporally( pulse repetition rate and track frequency). The spatially and temporally scanned process eventually leads to a confluent( or coalescent) bubble( conforming a cleavage plane), enabling the separation of the tissue. The residual tissue bridges between cavitation cites have a significant clinical relevance. Histological and surgical experience suggests that residual tissue bridges remaining after femtosecond laser lamellar ablation are associated with increased interface roughness and less smooth cleavage surfaces; when the uncut gaps between cavitation sites approach a significant fraction of the cavitation bubble diameter, surface irregularities become more pronounced [ 18, 19 ]. While precise clinical thresholds are not universally defined, surgical practice suggests residual gaps larger than a few microns can influence interface smoothness and biomechanical stability postoperatively.
Individual pulses and cumulative effects may be governed by different underlying mechanisms. For instance, very different regimes have been observed for the interaction mechanisms of cavitation bubbles induced by spatially and temporally separated fs-laser pulses( at least for pulse energies well above the breakdown threshold) [ 20, 21 ]. Tinne et al. [ 8, 17 ] first investigated the interaction mechanisms of cavitation bubbles induced by two laser pulses that were both spatially and temporally separated. In a more recent study, Freidank et al. [ 15 ] presented high-speed videographic investigations performed at up to 50 million frames per second, of multiple pulse effects during intrastromal dissection in corneal tissue. They found that the bubble dynamics in corneal tissue are much more complex than in
Fig. 1. Illustrative representation of the spatial distribution of cavitation bubbles determined by spot and track distances in femtosecond laser – assisted refractive correction.
water. The laser-induced bubbles exist longer in tissue and are not spherical, instead multiple bubble lobes are formed along the elongated laser plasma within the cornea. These lobes expand preferentially along the corneal lamellae, which are approximately 2 – 3 lm thick, creating overlapping layers of bubbles aligned with the stromal architecture suggesting that spot spacing and pulse overlap, both in depth and across lamellae, play a critical role in determining whether the resulting cut is smooth or fragmented.
Several studies have investigated the impact of optimizing pulse energy, and temporal and spatial relationships between single pulses. Adapting the laser settings in femtosecond laser-assisted cataract surgery with increased vertical spacing( i. e., fewer pulses in the vertical direction) has shown to significantly reduce the number and size of“ tags”( incomplete / misplaced cuts) at the capsule edge, thereby improving the cut quality [ 22 ]. Lombardo et al. [ 23 ] investigated the morphology of posterior stromal lenticules created with a femtosecond laser. Their results showed that lower pulse energy( 0.50 lJ) produced much smoother posterior stromal lenticule surfaces, while roughness increased with increasing energy. Very fine spot spacing( 2 lm) with low energy resulted in a surface comparable to mechanical microkeratome. In the study by Amann and Arba Mosquera [ 24 ], the authors conducted detailed simulations of femtosecond LIOB to assess how variations in spot spacing and track spacing influence corneal surface roughness. Their optimization framework consistently favored asymmetric spacing patterns, delivered at laser energies just above the LIOB threshold, to reduce surface irregularities.
In this regard and at least in the context of this work, we use the terminology symmetric vs. asymmetric spacings with respect to spot and track distances, as well as asymmetry ratio, according to previous publications [ 24 ]. Figure 2 provides a visual representation of symmetric and