Laser Induced Damage
LASER INDUCED DAMAGE
What is it?
In the most general terms, laser induced damage is any undesirable degradation of the coating, surface, or bulk material which causes a loss of function, generally to an optical element, and results from exposure to a laser beam.
Laser damage, on the other hand, is what happens when you drop your laser or leave it out in the rain. But "laser damage" is often verbal shorthand for laser induced damage. We will stick with the more precise term here.
A key property of lasers is that they concentrate a substantial amount of energy on a very small area or volume. This concentrated energy may exceed the capability of material in the beam path to survive the exposure. That laser energy may be absorbed over a localized small spot, or even throughout the volume, of material exposed to the laser beam. The result is laser induced damage. It may be immediate and catastrophic, or gradual and cumulative.
What Causes Laser Induced Damage?
Most often, laser induced damage occurs at a limited number of sites. But in some cases, there may be bulk damage, or sites scattered throughout the volume or a surface. In most cases it occurs due to absorption. It may also result from a direct breakdown of the material in the high energy field associated with high power laser pulses. The usual culprits are intense short laser pulses, although high power continuous wave (CW) lasers sometimes cause laser induced damage, to both the bulk and coatings of a material.
Also, if the bulk material expands due to heating, the coating may be stressed far enough to damage in the form of crazing or even peeling. This is predicated upon there being a difference in the coefficient of thermal expansion (CTE) between that of the substrate and one or more coating layer materials.
What is Damage Threshold, and what Reduces the Threshold?
There is an intrinsic value for energy absorption that may be withstood by a material, but certain conditions will reduce it. That value is known as the damage threshold, the limit of laser energy that can be withstood. Every material has a damage threshold. For certain materials it is very high. For example, a typical LBO element can easily stand up to 45 GW/cm2 tested in the nanosecond regime.
That threshold may depend on the purity of the material, the wavelength of the laser irradiating it versus its absorption spectra, and the condition of the material resulting from processing.
Surface defects such as scratches or digs are often the cause of reduced damage threshold. Inadequate cleaning is also a common culprit. They may have occurred prior to thin film coating, or later during handling. Impurities in the bulk material absorb energy and reduce the damage threshold. One example is platinum inclusions in laser glass that come from the vessels used to melt the glass components. Some laser crystals are grown at elevated temperature in precious metal crucibles. Iridium inclusions in YAG crystals are an example of that. In some cases, an impurity or component of an otherwise transparent material selectively absorbs the energy associated with a particular laser wavelength. The laser irradiance must be limited to allow for this.
Finally, processing aids such as cerium based polishing compounds may diffuse into the polished surface (a Beilby layer); that causes a multitude of damage sites in a high-power UV laser system. This was also a problem for lasers that operated around 1-micron wavelengths when iron oxide (rouge) was used to polish.
It is imperative that the manufacturer of these critical laser component understands the composition of all materials used in the fabrication process and how they relate to the conditions of the laser light (such as wavelength, pulse width, etc.). Furthermore, they must control these materials through a rigorous quality system.
What Form Does Laser Induced Damage Take?
Laser induced damage on a surface appears as burning, pitting, and fracturing. Internal damage takes the form of spikey fractures; to the unaided eye, they look much like bubbles. At a surface there may be pits surrounded by ejecta from the material that has been superheated by the laser beam.
What are Damage Initiators?
Localized defects, most often on the surface of an optic, result in laser induced damage. Scratches and digs, some too small to resolve even with a microscope, are frequently the sites of laser damage. Internal faults, such as voids or inclusions, and other impurities within an optic, lead to damage. Platinum inclusions are most often responsible for damage to laser glasses. Platinum vessels are used to melt, then condition, laser glass. The initiators may be microscopic, but if they damage initially, the damage will typically grow in size with continued exposure to the laser beam.
What is the Effect of Surface Finish?
There are two aspects of surface finish, surface roughness and scratch-dig quality. Any surface defect may become filled (and thus visually undetectable) with an energy absorbing contaminant that causes premature laser damage. The defects themselves reduce the structural integrity, and may consequently damage. Interior damage can result as self-focusing caused by diffraction effects around a pit or an approximately circular defect. Also, the initial defects on the surface can grow with further exposure to the laser beam.
How Do Scratches and Micro-cracks Reduce Damage Threshold?
Scratches and micro-cracks are typically filled with contaminants that could act as damage initiators. Light is also slightly distorted by the surface aberrations and the random intensity changes as a result could initiate damages.
What Are Their Sources?
Scratches occur during grinding or polishing, as well as from mishandling.
Under sufficient magnification scratches are revealed as continuous rows of small fractures. A contaminant or even an oversize particle of polishing or grinding compound bears more pressure than the surrounding area and penetrates below the average depth of grinding pits, and deeper than the polished surface. These scratches may be latent defects, that are ordinarily impossible to detect. However, they may be revealed by etching with an acid or alkaline solution, depending on the material’s susceptibility to corrosive solutions.
There are three common modes of laser operation that cause laser induced damage.
These are CW (continuous wave) and pulsed lasers. The latter can be divided into nanosecond and ultra-short pulsed lasers which includes picosecond and femtosecond lasers. CW and high average power nanosecond light tend to keep material exposed for a long period of time; absorption from impurities incrementally builds up until material succumbs to thermal damage. Ultra-short pulse light with intense peak power produces an electro-magnetic force that can readily breakdown chemical bonds in coatings and bulk material. Strict process control and material selection are necessary to develop the right coatings/material tailormade for each of these modes of laser operations.
What Causes Laser Induced Damage in the CW Domain
Any material has an inherent laser damage threshold. If it is exceeded, laser induced damage will occur. Optical materials must be selected to withstand the maximum output of a continuous wave laser, or the laser output must be controlled so that it is not greater than the limits of the optic. Still, if the material absorbs even a little of the laser illumination, its temperature will rise, the material will expand, and bulk damage occurs. This is often catastrophic damage, which seems to fracture or even explode the unsuspecting optic.
Proper material selection is critical. Visibly transparent optics can absorb energy that is outside the transmissive range. Fused silica has a lower absorption wavelength limit than other optical glass, such as BK7. The best grades of fused silica are better yet. If fused silica won’t suffice, there are single crystal materials that have better transmission in the UV region. For longer wavelengths in the IR (infrared), special glasses or crystals transmit the light safely, yet may be colored or opaque to the eye.
What Causes Laser Damage in the Nanosecond Domain
In a pulsed laser, energy is often stored in the laser material, and then the user triggers the laser to emit light in a short intense burst. Applications of these bursts include laser machining, welding, or ablation (including medical applications). There may be very little heat absorbed compared to what would occur with a CW laser operating at the same intensity.
Many pulsed lasers operate in the nanosecond region. As for CW lasers, the principal damage initiators are surface defects, and internal defects. Bulk absorption is not normally an issue.
What Causes Laser Damage in the Ultra-short Pulse Domain
Ultra-short pulses are those whose duration is on the order of a picosecond (10-12 second) or less. Often, the term is used to refer to pulses that last only tens of femtoseconds or less. Research has led to pulses of even just an attosecond duration.
These can induce a novel form of laser damage. Clearly, surface defects, contamination, and internal inconsistencies will give rise to laser induced damage, as they do for CW and nanosecond pulse lasers. Also, intense peak power breaks down chemical bonds, altering physical characteristics of the materials in its path.
Nonlinear processes resulting from ultrafast laser pulses increase the amount of energy absorbed far beyond the value that would occur at lower intensity. The result is laser induced damage that substantially exceeds what would happen with the same energy in a longer pulse.
DENNIS J. GARRITY, AUTHOR
Dennis is an engineer with over 45 years of experience in fabrication, testing, and material evaluation for high precision optics, with extensive hands-on experience. More on the author can be found here.