What this application demands
Laser optics are limited by absorption, not by reflection. A coating that performs perfectly at low power can be destroyed at high power because a fraction of a percent of the energy is absorbed in the layers and turned into heat, which shifts the coating's own spectrum and accelerates the damage. Everything in a laser path is therefore specified around damage threshold, absorption and wavefront quality rather than around nominal reflectance alone.
Substrate quality matters as much as the coating. Internal defects and inclusions become damage sites under high fluence, and surface scratches concentrate the field. This is the application where we most often recommend fused silica over a cheaper material, and where we ask for the actual pulse duration and repetition rate rather than only the wavelength.
Specifications we recommend
| Component | Typical specification | Why |
|---|---|---|
| Laser windows | UV-grade fused silica, both faces AR-coated, high LIDT design | Low absorption and low inclusion density keep the damage threshold high |
| High-reflectance mirrors | Dielectric HR above 99.5 %, optimised over the specified angle | Dielectric stacks avoid the absorption of metallic coatings at high power |
| Focusing and delivery optics | Tight wavefront control, flatness to λ/10 after coating | Prevents distortion of the beam profile at the work surface |
| Protective windows | AR-coated fused silica, replaceable, sealed to the housing | Sacrificial protection for the more expensive optics behind it |
Failure modes we design against
Thermal lensing appears when a coating absorbs enough energy to heat the substrate, changing its figure and therefore the beam it delivers. The symptom is a focus that moves as the laser warms, and it is resolved by reducing absorption rather than by increasing the coating thickness. Coating damage usually begins at a defect — a nodule, a pinhole or a cleaning mark — which is why the cleaning and handling protocol matters as much as the deposition itself.
Angle and polarisation sensitivity is the third factor. A stack optimised at 45° for unpolarised light will separate into s and p curves that can differ substantially. Where the application uses a polarised beam, the coating has to be optimised for that state specifically.
Frequently asked questions
What damage threshold can you achieve?
It depends on the coating type, the wavelength and the pulse regime. Supply the operating parameters and we will quote against a design qualified for them rather than quote a generic figure.
Why is fused silica preferred over borosilicate for lasers?
Lower absorption, higher damage threshold, better UV transmission and far lower thermal expansion — all of which matter once the optic carries real power.