High-Energy Laser Optics Require Coatings in Their Own League

Lynn Savage, Features Editor, lynn.savage@photonics.com

Sure, your laser pointer doesn’t have the power to punch through the wall, but high-fluence CO₂ and other heavy-duty beasts of burden can carve through sheet metal, slice 1/4-in. steel into intricate shapes and drill smooth holes into parts that will become automobiles, electronic gadgets, weapons and medical devices. To deliver these most potent of laser beams to their targets requires optics that can withstand no small amount of punishment – even as they do their job of manipulating and focusing high-energy light.

All optics – and the thin films that coat them – contribute tremendously to overall system performance, but they degrade over time. Humidity, abrasion, extreme air temperatures and more can all take a toll. Blast a coated optic with a high-energy CO₂ laser, however, and you’re asking for trouble.

Continuous-wave lasers that deliver several watts per square centimeter to a surface are placing more than enough energy through the optical components to wither the parts, given enough time. (Pulsed lasers also can deliver dramatically high energy levels to their targets, although joules are the standard measurement tool for these applications.) The substrates themselves are prone to defects caused by mishandling or lack of protection, but the thin films used to coat the optics – generally to improve system performance and longevity – also are subject to premature performance degradation and destruction when high-energy lasers are involved.

Many companies, including Deposition Sciences Inc., Omega Optical Inc., JDSU Corp., Precision Photonics Corp., MLD Technologies LLC and CVI Melles Griot, specialize in producing high-end optics and coating materials to accompany them. For low-fluence applications such as infrared imaging, antireflection coatings are ubiquitous, but coatings designed to take the heat must be able to reflect as much of the energy delivered by a beam as possible, allowing transmission of only the most relevant portion of the laser output (see Figure 1).

Figure 1. Mirror coatings must exhibit extremely low absorbance when used with high-fluence lasers. AOI = angle of incidence. Courtesy of Precision Photonics Corp.

Coating technologies also must keep up with the fact that high-energy lasers no longer are limited to heavy-duty cutting and drilling operations. Additionally, they are being used in scientific, remote-sensing and display systems, and more every year.

For high-power laser optics, both the substrate and the coating must exhibit low losses resulting from absorption or scattering. In contrast, they also must offer high efficiency and a high damage threshold.

The damage threshold of optical coatings depends upon the substrate selected as well as upon the wavelength and pulse duration of the laser. Also important is handling of the substrate and of the coated optic during and after manufacture. Scratching or digging into a lens while cleaning it, for example, increases the chance of incurring damage during laser operation. Environmental issues, such as mold and mildew encouraged by high humidity and poor storage facilities, can negatively affect optical performance.

Deposition techniques

Electron-beam deposition, ion-assisted electron-beam deposition and ion-beam sputtering are the most used tools for getting thin-film materials onto optical substrates in quick, uniform batches. Coatings suitable for high-fluence applications require densely packed molecules. If the thin film’s density is too low, the resulting coating can be unstable enough to falter under the beam. One possible result is a coating that has a lower refractive index than desired. Unstable coatings also are more prone to damage caused by humidity, which collects in microscopic pockets, or voids, altering the refractive index and ultimately causing the coating to flake off prematurely.

Table 1. Ion-beam sputtering is the preferred technique for depositing optical thin films for high-energy laser applications.

Deposition techniques are worthy of their own examination, but, simply stated, electron-beam and ion-assisted electron-beam deposition techniques provide coatings that are not dense enough for high-energy applications. Ion-beam sputtering, however, provides densities adequate for the task. The technique is only one-third as fast, however, so the coating process takes longer.

In sputtering, energetic particles are directed at a target comprising the material to be deposited onto the optical substrate. The particles – typically electrons or ions as the techniques’ names suggest – chip away at the target, causing atoms of the deposition material to fly toward the substrate, where they build up layer by layer. Because the energy of ions is higher than that of electrons in these competing systems, the deposited atoms form tightly bonded molecules that are very dense, practically nonporous and quite uniform.

“While there is good technical data available on damage threshold causes and how to optimize for [them], most people don’t realize how important execution and process control are to achieving high damage thresholds consistently,” said Nick Traggis, vice president of Precision Photonics in Boulder, Colo. “Steps as simple as how you clean your substrates before coating, or the way you fixture them in your coating chamber, can have a significant impact on performance.”

Choosing coatings to protect a lens, mirror or other optical component from the upper range of powerful lasers is not a simple matter because you still must match all of your other performance requirements. Nonetheless, matching the right thin film to the substrate best suited for the job will ensure many hours of successful work.

Interview with Nick Traggis, vice president of photonics at Precision Photonics Corp.

Who is your typical client?

Our typical client is a solid-state laser manufacturer in an industry such as defense, medical or industrial. We find that our customers have become quite educated on various coating technologies and know what they want and how they want it done. For instance, they will often ask for a specific process, such as ion-beam sputtering [IBS] by name.

What are your criteria for choosing a deposition method?

Deposition method is often determined by the materials being deposited. For visible and near-IR coatings, we have found ion-beam sputtering to offer the highest performance in terms of packing density, adhesion and low absorption. However, IBS technology is typically limited to oxide-based coating materials. Therefore, other deposition methods are often utilized for long-wave-infrared and ultraviolet applications where other coating materials are necessary.

Do you have special criteria for choosing a material for high-damage-threshold coatings?

One needs to understand the laser application before determining which material is most appropriate. For instance, the damage mechanism in a continuous-wave laser system is very different than the damage mechanism in an ultrashort pulse system. The materials that offer low absorption and good CTE [coefficient of thermal expansion] matching in a CW system may be more susceptible to electron-field effects in a short pulse system and therefore no longer suitable. It is our job as the coating supplier to understand the customer’s application as well as possible and then make an educated determination as to the best material set.

Are there any new coating materials on the horizon? Will quantum dots or other particles be incorporated into them?

Precision Photonics Corp. currently has over $2 million in government funding for researching new coating materials and methods, and we certainly feel there is more to be done. Areas of interest include characterizing new dielectric materials as well as learning how to deposit materials with new deposition methods.

Do you have any anecdotes about coating technology?

While optical coating technology saw a big push during the telecom boom of the late ’90s for things like WDM [wavelength-division multiplexing] filters and no polarizing optics, the technology development has been a little slow since then. It is up to us as an industry to continue to innovate and rise to the needs of our customers, but right now too many suppliers are resting on the laurels of 15-year-old technology.

Aside from laser applications, are you seeing a need for high-damage-threshold optics/coatings in other areas?

Damage threshold considerations should not be limited to laser fluence. There are many needs for mechanically durable or abrasion-resistant optical coatings in deployed military and industrial applications. Temperature extremes are also an issue for some aerospace optics and even lighting or solar applications where lifetime can be affected. Precision Photonics was recently contracted to design a broadband antireflection coating that could withstand operating temperatures up to 1000 °C. This required a very detailed understanding of the material properties involved and the coating-to-substrate interaction.