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 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 fi ...

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 (power per area), and

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, t ...

. 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 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 Thermal shock is a type of rapidly transient mechanical load. By definition, it is a mechanical load caused by a rapid change of temperature of a certain point. It can be also extended to the case of a thermal gradient, which makes different pa ...

Dielectric breakdown

Dielectric breakdown Electrical breakdown or dielectric breakdown is a process that occurs when an electrical insulating material, subjected to a high enough voltage, suddenly becomes an electrical conductor and electric current flows through it. All insulating mate ...

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 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 Avalanche breakdown (or avalanche effect) is a phenomenon that can occur in both insulating and semiconducting materials. It is a form of electric current multiplication that can allow very large currents within materials which are otherwise good ...

can occur. At these exceptionally high intensities, multiphoton absorption can cause the rapid ionization of atoms of the optic. This plasma 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 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 Chirped pulse amplification (CPA) is a technique for amplifying an ultrashort pulse, ultrashort laser pulse up to the petawatt level, with the laser pulse being stretched out temporally and spectrally, then amplified, and then compressed again. The ...

, a technique which permits the generation of petawatt class beams which was awarded the 2018

Nobel Prize in Physics ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...

Applications

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

Pulsed laser deposition Pulsed laser deposition (PLD) is a physical vapor deposition (PVD) technique where a high-power pulsed laser beam is focused inside a vacuum chamber to strike a target of the material that is to be deposited. This material is vaporized from the ...

* Certain

laser medicine Laser medicine consists in the use of lasers in medical diagnosis, treatments, or therapies, such as laser photodynamic therapy, photorejuvenation, and laser surgery. Lasers Lasers used in medicine include in principle any type of laser, bu ...

techniques * Plasma generation, especially for wakefield acceleration * Laser weapons

External links

Thorlabs - Laser Induced Damage Threshold Tutorial

Edmund Optics - Laser Damage Threshold Testing

Newport - Technical Note: Laser Damage Threshold

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