Laser-induced Damage

Laser light can have very high optical intensities because it is usually delivered in the form of a laser beam with small transverse dimensions, and in addition it is often generated in the form of short or even ultrashort laser pulses. Therefore, laser light often causes laser-induced damage of optical components like laser mirrors, fibers, nonlinear crystal materials, prisms, optical filters, optical modulators and saturable absorbers, photodetectors and SESAMs. For various applications, one needs to select optical components with a sufficiently high optical damage threshold (LIDT). On the other hand, light-induced changes can be exploited in the context of various techniques for laser material processing, for example.

Optical damage is in principle a more general term than laser-induced damage. However, optical damage rarely occurs with other light sources.

Continuous-wave laser radiation can usually cause damage only at rather high power levels, except when materials are hit which exhibit a substantial absorption of laser light. Here, the typical damage mechanism is simply that some material is overheated, which causes a chemical degradation, in some cases even burning. Also, it is possible that parts fracture due to thermally induced stress. Such types of damage are called thermal damage. A high tendency for thermal damage occurs for parts with significant absorption — for example, for metal-coated mirrors and for absorbing neutral density filters. Catastrophic optical damage at the facets of laser diodes also involves thermal effects.

Laser-induced damage is more likely for intense pulses, as can be generated e.g. with Q-switched lasers and with ultrafast amplifiers. For example, even if the pulse energy is only 10 mJ, but the pulse duration is as short as 10 ns, the peak power is on the order of 10 mJ / 10 ns = 1 MW. If such a beam is focused to a beam waist with a beam radius of 10 μm, an enormous optical peak intensity on the order of 300 GW/cm² is reached. For amplified ultrashort pulses, the peak intensities can become even higher; however, the damage threshold in terms of optical intensity may then also be higher (see below).

For very high intensities, nonlinear self-focusing may occur within a sample, which may lead to even further increased intensities. For example, catastrophic self-focusing in silica fibers can occur for power levels of a couple of megawatts.

Optical components differ greatly in terms of damage threshold. Laser mirrors can exhibit very high values, but there are huge differences between different kinds of dielectric mirror coatings, which can be caused by different materials but also by different layer designs. For example, dielectric mirror designs for dispersive mirrors, as required for dispersion compensation, tend to lead to substantial optical intensities within the layer structure and therefore often lead to lower damage thresholds. Cemented components (e.g., some polarizers and waveplates) are also normally more sensitive to laser radiation than non-cemented parts. Sometimes, there is a trade-off between highest damage threshold and best performance in other respects.

The potential of intense laser pulses to cause damage is often based on nonlinear processes which increase the fraction of absorbed energy far above the value which would be experienced at lower intensity levels. In particular, many media are transparent at low or moderate optical intensities, but become strongly absorbing once the threshold for laser-induced breakdown is exceeded. The involved process may start with multiphoton absorption, which generates some free carriers, and these can subsequently provide strong linear absorption for further radiation. As the generated carriers have a limited lifetime, a sufficiently high optical energy must be provided within a short time. If the initial carriers are generated by multiphoton absorption in the used material, the related intrinsic damage process can be rather deterministic: damage sets in above a well-defined damage threshold. In some cases, however, most initial carriers are generated at randomly distributed faults or impurities; in such cases, the tendency for damage involving such extrinsic effects substantially depends on the exact spot which is illuminated on a sample.

The physical mechanisms of laser damage may depend on the pulse duration. For example, one may have quite deterministic laser-induced damage for femtosecond pulses, which reliably sets in when the damage threshold is reached. For nanosecond and longer pulses, however, non-deterministic damage is often observed, where the probability of damage increases smoothly around the threshold. In the latter case, a larger safety margin is required for safely avoiding damage.

Normally, doubling the laser beam area should allow one to apply two times more power until damage occurs. Note, however, that a larger beam makes it more likely to hit a “bad spot” on a sample. Therefore, this kind of scaling may not work perfectly for some optical components. For large beam spots, one may more frequently see damage at a given intensity or fluence level.

For pulse trains with high pulse repetition rate, thermal effects may come into play, similar to those for continuous-wave beams.

Optical damage can arise within a bulk piece of some material, but also on its optical surfaces. Particularly for surfaces, the exact treatment of those (preparation procedures, later contaminations, etc.) can have a strong influence on the laser damage threshold. Applied coatings, such as anti-reflection coatings as often needed on optical parts, can substantially decrease the damage threshold. That can depend both on the used materials and the exact optical fabrication techniques.

As surfaces often have a substantially higher density of microscopic defects than bulk material, they more often exhibit damage related to impurities or defects. The laser damage threshold for optical surfaces can be substantially lower than for bulk material, and it may substantially depend on the exact surface treatment. For example, even careful grinding and polishing of a surface can generate sub-surface defects within a layer of e.g. 0.1 μm thickness, and it may also contaminate the surface with polishing materials, e.g. containing cerium. Both issues can lead to an increased damage tendency. Cleaning procedures can usually not eliminate such problems, but various refined techniques have been developed which may substantially improve the results: magnetorheological finishing, wet chemical etching, ion beam or plasma etching and UV laser conditioning. Their effectiveness can greatly depend on the materials to which they are applied, and to tiny details of their application.

Some optical components are embedded into mounts consisting of materials which should never be hit by laser beams. For example, there are plastic materials which easily melt and produce fumes. If this leads to the deposition of materials on optical surfaces, these may subsequently be easily damaged by a laser beam. Similar problems can arise if traces of machine oils remain on mechanical parts. Even if these are not hit by a laser beam, they may exhibit some outgassing. The gaseous substances may then be chemically altered when getting through a laser beam, and cause depositions on optical surfaces, which thus get darker and darker. The result can be a gradual degradation or laser-induced real damage.

For such reasons, it is advisable to use very clean parts and to avoid hitting mechanical mounts with a laser beam during alignment procedures, for example. Such issues are particularly important in the context of ultraviolet lasers.

It can happen that although the material does not exhibit any easily detectable changes after having been hit with a single intense laser pulse, it does react to multiple pulses with the same parameters. This implies that the first pulses create only small hidden defects, which increase the tendency of the material for damage caused by further pulses. In some cases, such cumulative effects (also called fatigue effects) become apparent even after only a few pulses, whereas in other cases a large number of pulses is required. (One may obtain some kind of accelerated aging.) The involved physical mechanisms can be difficult to analyze in detail.

It is not obvious how damage should be defined, and how exactly to determine in practice whether damage has occurred under certain conditions. In principle, any significant permanent change in the characteristics of an irradiated sample should be considered as damage. In some cases, one examines a sample after irradiation with a microscope, searching for any visible changes. In other cases, increased levels of stray light are monitored; for example, the total integrated scattering (TIS) is checked. Alternatively, one may monitor some property which is essential for the function of a component — for example, the reflectivity of a mirror or the saturation characteristics of a SESAM. Depending on how damage is evaluated, one may obtain different values for the damage threshold.

A typical type of damage which can be observed on dielectric coatings (e.g. of laser mirrors) and in bulk media is the occurrence of small isolated defects (see Figure 3), which cause increased scattering losses. They can be linked to even tinier defects (e.g. nanometer-sized absorbing inclusions) introduced already in the manufacturing process. Such pinpoint defects may be too small to be seen even under a microscope, and nevertheless act as seeds for laser-induced damage (preferential damage sites).

Coatings may also partially lose the contact with the mirror substrate, or the contact between coating layers is lost; this is called delamination. It can happen e.g. when too high optical intensities occur within the coating as a result of interference effects.

Particularly with high-energy nanosecond pulses, one may also obtain larger fractures and cracks. In extreme cases, a whole crystal or glass piece falls apart or even explodes. This results from a runaway process, where increasing damage leads to increased energy deposition. Even small, isolated cracks can affect laser beams at some distance because of frozen-in mechanical stress. Depending on the processes and materials, the local refractive index may be increased or decreased.

With ultrashort pulses, one can also obtain other damage morphologies. Generally, thermal effects like melting and re-solidification are less frequently observed than with longer pulses, essentially because the heat has little time within a pulse to diffuse, and less energy needs to be deposited to start dielectric break-down. Once the break-down starts, one quickly gets into a regime of plasma formation and laser ablation. This is called "cold ablation because there is hardly any heat-affected zone.

Basically, the laser damage threshold is some optical intensity or fluence level below which no optical damage occurs for certain material. (For continuous-wave beams, it is always an intensity; for pulses, both peak intensity or fluence may be specified, but the fluence is more common.) Damage thresholds of materials or components can essentially be measured by evaluating the damage caused by laser light at different intensity or fluence levels. This is unavoidably a destructive testing method.

Although the definition and measurement task are simple in principle, a variety of aspects can come into play which cause complications for the measurement and specification of laser damage thresholds. These are discussed in the following sections.

Standardized test procedures — particularly those defined in ISO 21254 — are essential for obtaining results that are comparable between laboratories. They specify aspects such as beam characterization, inspection methods, acceptable damage criteria, and pulse-count requirements.