Understanding Laser-Induced Damage Threshold (LIDT)

Aug. 11, 2025
2 min read

I. Laser-Induced Damage Threshold: Concept, Mechanism, and Technical Importance

  1. Definition of Laser-Induced Damage Threshold (LIDT)

Laser-Induced Damage Threshold (LIDT) is formally defined as the minimum laser energy density (J/cm² for pulsed lasers) or power density (W/cm² for continuous-wave lasers) at which permanent and irreversible damage—such as melting, ablation, cracking, or coating delamination—occurs on or within an optical element under specific laser irradiation conditions. These conditions include wavelength, pulse duration, repetition rate, beam spot size, and irradiation duration. LIDT is a critical quantitative measure of a material’s resistance to laser-induced damage.

1.1 Core Damage Mechanisms

Thermal Damage: Laser energy absorbed by the material is converted into heat, resulting in a sharp local temperature rise. This is especially relevant for CW or high-repetition-rate lasers, where heat accumulation can cause melting, thermal stress cracking, decomposition, thermal lensing, or even thermal explosion. Key material parameters include melting point, thermal conductivity, and thermal expansion coefficient.

Field-Induced (Electric Breakdown) Damage: Predominantly observed under ultrashort-pulse lasers (picosecond or femtosecond), where high peak power generates strong electric fields. These fields disrupt atomic/molecular bonds via multiphoton and avalanche ionization, creating plasma and causing ablation. This is a non-thermal process, with sharply defined damage edges.

Defect-Initiated Damage: Microscopic defects (impurities, microcracks, voids, dislocations, coating imperfections) act as local hot spots or field enhancement zones. These defects significantly lower the actual LIDT compared to theoretical values.

Shock Wave Damage: Intense pulsed lasers can induce rapid ablation or plasma expansion, generating shock waves that propagate through the material and potentially cause internal delamination or fracture, even at points distant from the laser spot.

1.2 Technical Importance of LIDT

System Safety and Reliability: Knowing the LIDT is essential for designing safe laser systems, selecting compatible optics, and setting safe operational parameters. This prevents catastrophic failure or safety hazards due to optical damage.

Performance and Cost Optimization: By understanding LIDT constraints, engineers can select components that strike a balance between safety margins and system performance. This supports optimized component selection at a competitive cost.

Materials and Process R&D: LIDT serves as a benchmark for evaluating new optical materials, coatings (e.g., laser-resistant films), fabrication methods, and post-processing treatments, such as ion-beam polishing.

To read the entire article, visit Avantier Inc.

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