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Laser Damage FAQ

Frequently Asked Questions

Laser Damage: what is it?

Any temporary or permanent change of an optical element caused by exposure to intense laser radiation.  The most common type of laser damage is physical deterioration at coated optical surfaces (e.g. pitting, erosion, melting, or delamination).  Fracture and discoloration of the bulk optical material are also often encountered.  Subtle modifications of refractive index or other bulk optical properties may be considered damage in sensitive applications.

Why is Laser Damage important?

Laser damage can degrade or completely destroy the performance of high power laser systems.  This can be avoided by using optics with sufficiently high damage thresholds.

What is the Laser Damage Threshold?
“Threshold” refers to the energy density or power density of laser radiation above which the probability of causing damage is non-zero.  Generally speaking, the weakest parts of an optic will just begin to fail when exposed at this level.
Which lasers are most susceptible to Laser Damage?
Q-switched solid state lasers (e.g. Nd:YAG, ruby) are the most likely candidates.  Their short pulse durations (~10 ns), moderate output energies (~1 J/pulse), and small beams (~6 mm diameter) combine to generate potentially destructively peak power densities on every pulse.
What is Peak Power Density?
It’s the maximum local irradiance (power per unit area) that a laser pulse can apply to a surface, so it’s a measure of how destructive a pulse can be.  The more energy a pulse carries, and the more tightly it is concentrated in space and time, the greater its Peak Power Density.  Laser Damage becomes a real possibility when Peak Power Densities reach a few hundred MW/cm2.
What causes Laser Damage?
Usually small defects or impurities in the optic absorb laser radiation preferentially.  As they heat up and expand, they may cause melting or thermal stress fracturing of the surrounding material.  Or, they may just vaporize violently, physically pitting the surface.  Ionized plasma may form, further eroding the surface.  Reducing impurity levels and controlling surface fabrication defects are key strategies for making high damage threshold optics.
What else affects Laser Damage?
Almost everything you can think of.  Important factors are the intrinsic strengths and impurities of the optical materials; design of the coatings; care and cleanliness in fabrication and coating; surface contamination; laser radiation parameters (e.g. wavelength, pulse duration, repetition rate, angle of incidence, polarization state, number of pulses); and the operating environment of the laser itself.
How can I tell if my laser has Laser Damage?
If your laser works about as well as it did when new, then your optics are fine.  If performance has degraded, Laser Damage may be the reason.  The only way to be sure is to inspect the optical components.
How do I inspect for Laser Damage?
The International Standard method is microscopic inspection at 100X-150X magnification, using Nomarski/Darkfield illumination.  This very stringent method will reveal the most subtle laser-induced effects.  For engineering purposes, a careful visual inspection under bright white-light illumination with a low power magnifier will usually reveal laser damage severe enough to degrade laser performance.
How can I buy high Damage Threshold optics?
If you’re ordering from a catalog, ask your suppliers for Laser Damage Certification test results for your particular coating runs.  If you’re writing a procurement specification, add a Bulletproof Laser Damage Requirement.
What is a Laser Damage Certification test?
In this “Pass/Fail” test, an optic is exposed to calibrated laser radiation at a specified peak power density and then inspected for damage.  This test is often used to verify that an optic meets or exceeds a particular Laser Damage requirement.  Download a standardized Certification Test Specification.
What is a Laser Damage Threshold test?
In this destructive test, the optic is exposed to calibrated laser radiation at several different peak power densities to determine the minimum level necessary to cause damage.  It’s appropriate when you’re trying to understand why optics fail, develop stronger optics, or estimate the safety margin ina laser system.  The test procedure is governed by an International Standard (ISO 11254).  Download a standardized Threshold Test Specification.
What are some “typical” Laser Damage Thresholds?
There’s no such thing as a “typical” number, because commercially available laser optics vary all over the map when tested.  A common rule of thumb is that “satisfactory” dielectric mirrors and antireflection coatings will pass a 500 MW/cm2 test at 1064 nm, or 250 MW/cm2 at 532 nm.  A computer-searchable Database of more than 7000 Laser Damage Threshold test results is available from Quantel.
Where can I get more information about Laser Damage?
Download “Damage Test Descriptions” from Quantel.  Check out the tutorial article “Using High-Energy Exposure to Gauge Optical Components” in the 1998 Photonics Design & Applications Handbook (Laurin Publishing Company, Inc., Pittsfield MA, page 291) or request a reprint from  A helpful but hard-to-find textbook is Laser Damage in Optical Materials, by Roger M. Wood (Adam Hilger, Bristol and Boston, 1986).  Current research is published annually by SPIE in the Proceedings of the Boulder Damage Symposium.  Ask an expert.
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