Importance | Fabrication | Testing
In today's optics industry, many precision optical elements receive coatings in order to improve their transmission or reflection for certain wavelengths or polarization states. The most common types of coatings are anti-reflection (AR), high-reflective (mirror), beamsplitter, and filter. For more information on these coatings, view An Introduction to Optical Coatings.
As technology and industry evolve, many optical systems have come to rely on high-power laser sources. While standard coating technology can provide cost-efficient, easily reproducible precision results, there are limitations to the durability of standard coatings, particularly when subjected to high intensity irradiation. As a result, specialized high-power optical coatings are often required. High-power optical coatings can be applied to a range of optical elements, such as optical lenses, mirrors, windows, optical diffusers, optical filters, polarizers, beamsplitters, and diffraction gratings.
When considering what constitutes a high-power optical system, it is important to note that no universally-applicable energy threshold exists. Many manufacturers use rule-of-thumb estimates for the minimum intensity level, but the laser-induced damage threshold (LIDT) is largely dependent on the application. A reasonable definition is that a high-power optical system is one which has the potential to damage any of the coatings in use.
To understand the complexities of high-power optical coatings, consider their importance, fabrication methods, and testing procedures. Understanding each allows one to pick the best optics for the application at hand.
Importance of High-Power Optical Coatings
The optical coating is generally the limiting factor in the performance ability of a high-power laser system. For example, the most common failure mode of high-power optical coatings results from the presence of absorption sites within the coating or at the coating's interface with the substrate or air. These absorption sites are usually in the form of gross defects that absorb laser energy, resulting in heat generation that causes localized melting or thermal stress factors. Failure by this mechanism is usually catastrophic. Figures 1a – 1d show real-world images of coating failures at relatively low LIDTs due to poor process control and coating defects.
On the other hand, an example of non-catastrophic coating failure is plasma burn, which results from 1 - 5μm unoxidized metallic nodules within the coating. Interestingly, some manufacturers intentionally initiate plasma burns in order to remove these defect nodules.
Regardless of the type of damage, coating failure irrevocably and adversely affects the transmitted wavefront. This can have a drastic effect on system performance, but can also be costly when the time comes to replace the damaged optics. For these obvious reasons, it is crucial for optical designers to be aware of the source being used in conjunction with the coated elements in the system.
Figure 1a: Coating Failure from 11.77 J/cm3,
20ns Pulses @ 1064nm Wavelength Source due to Poor Process Control
Figure 1b: Coating Failure from 12.92 J/cm3,
20ns Pulses @ 1064nm Wavelength Source due to Poor Process Control
Figure 1c: Coating Failure from 14.3 J/cm3,
20ns Pulses @ 1064nm Wavelength Source due to Poor Process Control
Figure 1d: Coating Failure from 73.3 J/cm3
Source due to Coating Defect
