Laser engineers are leveraging new materials, unusual gain mechanisms, and innovative cavity designs to push laser performance into new regimes. Pulse lengths are getting shorter, peak power is getting larger, and photon energy is getting higher as systems move to short ultraviolet (UV) wavelengths. System designers looking to work with any laser need to take care to use optics designed to accommodate the peak power and energy density of their laser source. UV wavelengths are more energetic than their visible or infrared counterparts, so the concerns become more magnified for UV laser optics.
Surface roughness and surface quality are especially important for both intra-cavity and external optical surfaces. Any energy absorbed by imperfections in the optic can result in rapid heating, which degrades the optic further and can lead to component failure. In addition, with high energy laser systems, there can be serious consequences when any element scatters light to where it shouldn't be. Reducing scattering also leads to low loss systems with high throughput, which is critical for many laser applications. When evaluating component suppliers, designers should look at more than just the advertised specifications; they should also make certain their vendor can measure the characteristics they claim to create.
Smooth, Real Smooth
Much of the work of any optical system is done at the surfaces, so correct curvature is critical. A lens or mirror that has no deviations from its design curvature will perform exactly as predicted, but it is also impossible to achieve. There will always be deviations from perfection during manufacturing. Low-spatial frequency departures from perfection are classified as surface figure errors. Surface figure errors reduce the performance of an optical system by shifting beam propagation, deviating the path of portions of a beam. This can blur images or send parts of a beam away from its target.
Deviations at higher spatial frequencies create a different class of performance degradations and are commonly referred to as surface finish errors. One type of surface finish error is called surface roughness, which is a measure of the variation of a surface profile from its ideal profile measured over a given high spatial frequency range. Allowable surface roughness is specified by the maximum divergence of the surface profile at a given spatial frequency or range of spatial frequencies. The primary effect of surface roughness is scattering light, sending parts of a light beam off at high angles with respect to the primary propagating beam. Scatter can be a problem with any optical system, as it reduces the effective transmission and can even end up creating spurious signals at the system detector. In laser systems, scattered light can be even more detrimental because it can carry significant amounts of energy, which can damage components within a system and even present a safety hazard.
Another type of surface finish error comes in the form of single-point defects. These types of errors are often classified as “scratches” and “digs,” and are covered under scratch-dig standards that define both the allowable size and the number of these cosmetic defects. Scratch-dig defects also scatter light, degrading a system's performance in similar fashion to surface roughness. Laser damage often initiates at scratches and digs.
Most of the time, surface quality ends up being synonymous with coating quality. Almost all laser optics are coated, so — although substrate quality is not unimportant — the surface of the optic is the surface of the coating. Laser optics are typically designed to be used at a single wavelength or a set of harmonic wavelengths, so coatings will often be designed to reflect or transmit a high percentage of those target wavelengths. Coating durability is also important, particularly under conditions of high humidity or wide temperature variation.
In general, scatter is greater at shorter wavelengths. Because of this, short-wavelength laser optics are affected the most by surface roughness and quality. It is incumbent upon UV laser system engineers to carefully determine the specifications for surface roughness and scratch/dig. However, it does not matter how good a specification is if the supplier's metrology can't verify it.
Trust, But Verify
Many well-intentioned suppliers quote very tight specifications on surface roughness and cosmetic defects and, in general, modern fabrication equipment can deliver these tolerances. But when pushing the capabilities of optical fabrication, even the slightest lapse can significantly degrade the surface quality. That is why optical system designers should verify that their vendor uses the proper metrology to actually measure the performance they claim to achieve.
Atomic force microscopy (AFM) is a valuable tool for measuring surface finish — characterizing the roughness and defects of an optic. AFM uses a scanning probe to measure surface profile at very high spatial frequency. AFM is a time-consuming measurement, so its greatest value is in the essential task of process validation and monitoring, where a fraction of the surface is sampled to provide a statistically significant representation of the process performance.
Cavity ring-down spectroscopy is another important tool for verifying high-quality optical coatings. In a cavity ring-down measurement, light is reflected or transmitted many times by an optical element. Each time light interacts with the optical element, it will suffer whatever losses occur at those surfaces. This measurement is very sensitive to the coating performance, and equally as sensitive to the quality of the surface finish. Because this measurement is based on a decay time, it can be performed with a very high accuracy.
AFM and cavity ring-down spectroscopy are two metrology methods that can instill confidence in the capabilities of a component supplier. Experienced optical fabricators will also understand the nuances of visual examination, both unaided and on a microscopic scale. Competent metrology is critical when dealing with high peak powers common in laser systems, especially for laser systems in the high-energy UV spectral region. This is not only a system performance issue, but a system lifetime issue as well.
LIDT - What You Don't Know Will Hurt You
Laser Induced Damage Threshold (LIDT) is a specification that quantifies the maximum peak power or energy an optical element can tolerate before failing. Typically, laser damage occurs when energy is absorbed or concentrated at an imperfection in an optical element. The extra energy absorbed leads to heating at the defect site, which in turn leads to higher absorption and consequent heating, resulting in permanent damage to the optical surface. A variety of imperfections can act as laser damage sites, including scratches and digs, which is why surface finish is of particular concern for laser optics. Damage-causing imperfections are not always visible upon visual inspection, as laser damage can be caused by nanometer scale defects, residues, or deposits.
In previous years, LIDT was thought to be primarily due to one-time exposure to energy that exceeds a single threshold value, which could be determined for a given design. Now it is recognized that LIDT is a complex phenomenon in which cumulative exposure to lower energy levels, or perhaps short peaks from statistical power fluctuations in a lower energy beam, can contribute to degradation that eventually leads to permanent damage. This means that more than a single number might be necessary to quantify the safe level of exposure to laser radiation, and work is in progress to provide a more complete definition of the proper metrics for LIDT. Again, this problem is exacerbated at short, high-energy wavelengths, because UV exposure directly affects the structure of chemical bonds.
What to Watch For
Designers of any precision optical system must carefully specify optical components to reach their desired system performance. Proper specification becomes even more important in laser systems, where designers must pay particular attention to surface quality. Designers working with UV systems have an even tougher task, as UV radiation scatters more and carries more energy than its longer wavelength visible and infrared counterparts. It is important not only to specify components properly, but also to ensure that the component supplier can measure to the quoted specifications.
Even with all these challenges, system designers can take comfort in the facts that manufacturing and metrology technology continue to improve and suppliers who merge new technology with their existing proven expertise will be well-positioned to meet the most demanding specifications. Designers should begin a conversation with their potential suppliers early to make sure all important factors are being considered. If a fabricator's customer service department cannot courteously and quickly answer questions about specifications and metrology prior to a purchase, they are unlikely to be able to help with questions further down the line. Designers should look for an experienced supplier who understands the issues associated with laser optics, knows how to make the relevant measurements to ensure parts meet demanding specifications, and is willing to talk about how their expertise can meet the designers’ needs.