How Does Incubation Affect Laser Material Processing?

In the last decades, the subject of laser-induced damage (LID) moved from a topic of scientific interest toward laser material processing for technical applications. When using pulsed lasers, the cumulative effect known as incubation is one of the most fundamental features of the processing event. Incubation manifests by the fact that the critical fluence for LID (known as laser-induced damage threshold, LIDT) depends on the number of pulses N exciting one spot on the sample. In most cases, the threshold fluence decreases with N starting from the single-shot ablation threshold and remains constant for large N. No ablation or damage occurs for any N, if the fluence is kept below the multiple-pulse threshold. In contrast, examples where the LIDT increases with N have been reported. The latter effect is known as laser conditioning and is advantageously used when ramping up the power of high-power laser systems. Incubation has been described for many types of solids; the motivation for these comprehensive efforts is twofold. Firstly, one tries to prevent the damage of optical materials in the beam path of high-energy or high-peak-power lasers. Secondly, one deliberately uses this damage for the sculpting of components. In this chapter, we introduce the reader to the mechanisms of incubation. We are going to look at the parameters controlling LID and highlight the peculiarity of the parameter “number of pulses”. We will give an overview of the experimental work done in a variety of materials. There are several physical and chemical mechanisms proposed that govern incubation, and there are several mathematical models to describe the behavior of threshold fluence and ablation rate in dependence on the number of pulses. In a few cases, the two classes of mechanisms are even related to each other. Eventually, we will show the implications that incubation has on real-world laser machining.

💡 Research Summary

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The chapter provides a comprehensive overview of the phenomenon of incubation in laser‑induced damage (LID) and its implications for laser material processing. Incubation is defined as the cumulative effect whereby a series of identical laser pulses, each below the single‑shot damage threshold, can together cause material damage when applied to the same spot. The key observable is that the laser‑induced damage threshold (LIDT), expressed as a fluence ϕ_th, is not a constant material property but a function of the number of pulses N. In most materials the threshold fluence decreases with increasing N, starting from the single‑shot threshold ϕ₁ and asymptotically approaching a constant “safe” fluence ϕ_∞ for N≫N_C. Below ϕ_∞, an unlimited number of pulses can be applied without damage. In contrast, a minority of cases exhibit an increase of the threshold with N, a process termed laser conditioning, which can be exploited when ramping up high‑power laser systems.

The historical development of the concept is traced from early observations in the 1960s (fatigue, accumulation, pre‑threshold phenomena) through the introduction of the term “incubation” in the early 1980s. Early experiments demonstrated a reciprocal relationship between the number of exposures and the critical energy, and later studies identified sub‑threshold modifications of optical properties (e.g., IR‑spectroscopic evidence of Si‑O bond breaking in quartz). Zhurkov’s thermally activated failure model was adapted to laser damage, leading to an exponential dependence of the failure probability on fluence, temperature, and exposure time.

Two broad classes of mechanisms are distinguished:

1. Thermal accumulation – Relevant for long pulses (nanoseconds to milliseconds). Each pulse deposits energy that is partially converted to heat; if the inter‑pulse interval is shorter than the thermal diffusion time, temperature builds up. When the local temperature reaches melting or vaporization criteria, material removal or damage occurs. Key parameters include thermal conductivity, specific heat, pulse repetition rate, and spot size.

2. Electronic (defect) accumulation – Dominant for ultrashort pulses (femtoseconds to picoseconds). Multiphoton absorption creates dense free‑electron plasmas; subsequent relaxation can generate lattice defects, color centers, or mid‑gap states. The defect density D(N) grows with pulse number, often following a saturation law D(N)=D₀