The laser damage threshold (LDT) or laser induced damage threshold (LIDT) is the limit at which an optic or material will be damaged by a laser given the

fluence In radiometry, radiant exposure or fluence is the radiant energy ''received'' by a ''surface'' per unit area, or equivalently the irradiance of a ''surface,'' integrated over time of irradiation, and spectral exposure is the radiant exposure per un ...

(energy per area),

intensity Intensity may refer to: In colloquial use *Strength (disambiguation) *Amplitude * Level (disambiguation) * Magnitude (disambiguation) In physical sciences Physics *Intensity (physics), power per unit area (W/m2) *Field strength of electric, ma ...

(power per area), and wavelength. LDT values are relevant to both transmissive and reflective optical elements and in applications where the laser induced modification or destruction of a material is the intended outcome.

Mechanisms

Thermal

For long pulses or

continuous wave A continuous wave or continuous waveform (CW) is an electromagnetic wave of constant amplitude and frequency, typically a sine wave, that for mathematical analysis is considered to be of infinite duration. It may refer to e.g. a laser or particle ...

lasers the primary damage mechanism tends to be thermal. Since both transmitting and reflecting optics both have non-zero absorption, the laser can deposit thermal energy into the optic. At a certain point, there can be sufficient localized heating to either affect the material properties or induce thermal shock.

Dielectric breakdown

Dielectric breakdown occurs in insulating materials whenever the electric field is sufficient to induce electrical conductivity. Although this concept is more common in the context of DC and relatively low frequency AC

electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics, and electromagnetism. It emerged as an identifiable occupation in the l ...

the electromagnetic fields from a pulsed laser can be sufficient to induce this effect, causing damaging structural and chemical changes to the optic.

Avalanche breakdown

For very short, high power pulses, avalanche breakdown can occur. At these exceptionally high intensities, multiphoton absorption can cause the rapid ionization of atoms of the optic. This

plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral), a green translucent silica mineral * Quark–gluon plasma, a state of matter in quantum chromodynamics Biology * Blood pla ...

readily absorbs the laser energy, leading to the liberation of more electrons and a run-away "avalanche" effect, capable of causing significant damage to the optic.

Mitigation

Optical systems can mitigate the effects of laser damage both by increasing the LDT of the optics used and by changing the laser beam characteristics. The use of high reflectivity (HR)

dielectric mirrors A dielectric mirror, also known as a Bragg mirror, is a type of mirror composed of multiple thin layers of dielectric material, typically deposited on a substrate of glass or some other optical material. By careful choice of the type and thickne ...

instead of metal mirrors is a common strategy. Further, the beam can be expanded, reducing the fluence present on optics. Finally, the beam can be temporally stretched, i.e. " chirped", to reduce the power incident on the optic. The use of chirped beams was the key innovation in chirped pulse amplification, a technique which permits the generation of petawatt class beams which was awarded the 2018 Nobel Prize in Physics.

Applications

Some applications make use of laser breakdown directly, necessitating knowledge of the materials' LDT properties. Some examples include: *

Laser cutting Laser cutting is a technology that uses a laser to vaporize materials, resulting in a cut edge. While typically used for industrial manufacturing applications, it is now used by schools, small businesses, architecture, and hobbyists. Laser cutt ...

* Pulsed laser deposition * Certain laser medicine techniques * Plasma generation, especially for wakefield acceleration *

Laser weapons A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The firs ...

External links

Thorlabs - Laser Induced Damage Threshold Tutorial

Edmund Optics - Laser Damage Threshold Testing

Newport - Technical Note: Laser Damage Threshold

{{Lasers Lasers