As technology advances, extremely powerful lasers with precise characteristics are needed for a range of fields, from the medical industry, scientific research industry and the military. With these developments comes the need for optical coatings with precisely tailored optical properties able to withstand the constant barrage of power required. In any facility with a laser, you will likely find a drawer filled with mirrors and lenses destroyed by burn marks, but scientists are working to make coatings that last perform better and last longer.
Most optical elements like mirrors, beamsplitters or filters employ thin layers of alternating dielectric materials, sometimes up to 200 layers, depending on the nature of the element. Each layer has a different index of refraction, so at each interface, some of the light is transmitted and some is reflected. In antireflective coatings, for example, the thickness of the coatings is designed so that reflected light from one layer will destructively interfere with reflected light from the next layer so that no light is reflected back.
More complex processes can require hundreds of coating layers to accomplish their task.
The success of an optical element is generally determined by three things: the substrate, the choice of coating and the deposition of the coating. Any imperfections in the substrate tend to be magnified by the application of a coating, so uncoated lenses have higher damage thresholds. Simple mirrors may just have one coating layer while more complex processes can require hundreds of coating layers to accomplish their task. They are manufactured by applying dielectric coatings to a carefully polished substrate. It is imperative that both the substrate and the coating be free of flaws and inconsistencies.
Damage can occur almost anytime the laser beam encounters an inconsistency. Any location that absorbs more laser energy than it was designed for can heat up, creating more widespread damage. Damage can also occur during routine transport, storage or cleaning, which can create microscopic flaws that can eventually lead to failure.
The precise nature of the substrate and coating are determined by the function to be performed and the wavelength range and power of the laser. Simple, reflective metal coatings like aluminum, gold, and silver are still used in many laser systems.
Advances in coating technology came during the 1990s when the use of fiber optic cables to transport information became more widespread. An influx of research took place to create coatings able to withstand changes in temperature and humidity and to allow more precise control over the beam. Many modern coatings use metal oxides that are more impervious to environmental effects than sulfide or fluoride high index coatings, but require more expensive deposition techniques.
Coatings by laser type
The type of laser also plays an important role in choosing a laser coating. A continuous wave (CW) laser has very different energetic properties from a pulsed laser, which imparts a very short, high-energy pulse onto the surface. CW lasers tend to have much lower peak field strength, but the power over time is higher. With a pulsed laser, there is a much higher peak electric field, but the material will have the chance to microscopically recover between pulses. A CW laser beam can heat up an optical element over time, so the thermal expansion of the substrate and coating is extremely important. For a pulsed laser system, microscopic flaws in the dielectric can amplify and change local electric fields.
In order to perform properly, optical coatings must have precise and consistent densities throughout their surface. If the density is too low, water molecules can invade voids in the surface lowering the damage threshold. Low packing densities also lead to a lower index of refraction as compared to the bulk material. Areas where the packing density is too high may absorb too much of the beam energy leading to damage, or just not react to the beam per its design.
The density of the film is proportional to the energy used in the coating process.
The density of the film is proportional to the energy used in the coating process. The higher the energy of the optical material when deposited, the more even and dense the coating will be; thus reducing the chance for temperature or humidity-related damage.
While it is desirable to use the highest energy possible, these methods are usually more expensive, time consuming, and there is the possibility of altering or stressing the coating. In all processes, the coating material is excited into the gas phase, and then condenses on the substrate. In these techniques, the coating material is vaporized into a gas in a vacuum chamber. The evaporated material then condenses on the substrate held or spun in the same vacuum chamber.
The earliest deposition technique involves heating the coating material either thermally using resistive heating or using an electron beam. Evaporative deposition is the fastest and cheapest coating method, but often results in uneven or thin coatings which may be sufficient for simple anti-reflective or mirror coatings but fail for high precision and multilayer coatings.
Advances in sputtering
Sputtering techniques came about during the 1970s and use a high-energy ion gun fired at the coating material to sputter off the coating into the vacuum chamber and onto the substrate. Ion bean sputtering is considered to generate the most even and high quality coating, but the capital equipment costs associated with it are very high. It is also labour and time intensive and small batches make the produced elements more expensive. Most often, sputtering techniques are followed by oxidation of the surface of the substrate.
With the advent of new plasma sputtering techniques, scientists are able to model complex optical element designs.
Likely one of the most exciting advancements in the field of optical coatings in recent years is advanced plasma reactive sputtering techniques. With the advent of new plasma sputtering techniques, scientists are able to model complex optical element designs, and then quickly fabricate and apply coatings able to precisely match their designs. Plasma sputtering also has the speed and lower cost of traditional deposition. It generates repeatable, predictable thin films that allow the optical elements’ attributes to more closely match those predicted by designed theory. Both sputtering techniques are usually limited to oxide coatings.
With developments in deposition techniques, scientists are also looking toward new advanced materials like quantum dots and nanoparticles that may allow even greater laser control. Coatings will no doubt continue to play a pivotal role in our advances in laser applications.
Written by Sydney Kaufman, Contributing Editor, Novus Light Technologies Today