Laser-Induced Damage by Thermal Effects

For all but the most highly transparent materials (e.g., fused silica, diamond, quartz, and sapphire), the damage threshold is more likely to be linked to thermal absorption in the laser pulse length region from 10−8 s to continuous wave (CW). In this scenario, the onset of damage, whether thought of as melting, catastrophic stress, drilling, or cutting, is related to the melting and/or vaporization of the material. ese mechanisms vary between two pulse length-related regimes-the rst regime is where the peak temperature is related to a steady-state process and is valid from CW to about 10−6 s. e second regime is where the peak temperature is governed by the relative size of the focused spot, the component diameter, and the thermal diusivity of the sample under test. is second regime is valid for all pulse lengths below 10−6 s. However, for highly transmitting materials (e.g., fused silica, diamond, and sapphire), the thermal damage threshold is so relatively high that other damage mechanisms come into play before strict thermal eects take place. ese are dielectric breakdown for pulse lengths of ~10−8-10−10 s, avalanche ionization for pulse lengths of 10−10-10−13 s, and multiphoton absorption for pulse lengths lower than ~10−13 s. ese mechanisms will be discussed in subsequent chapters.