Thin film basics

Laser optics are optical components specialized for laser applications. They are optimized for the specific wavelengths used and for high power densities.

They are needed to build a laser (e.g. cavity mirror or coated laser crystal) or to get the laser light with the right shape to the right place with the right intensity.

A dielectric coating is made of electrically insulating materials.

A dielectric coating usually consists of an alternating layer stack of two electrically insulating materials with different refractive index, such as metal oxides, SiO2 or fluorides.

The desired optical properties are achieved via interference at the interfaces of the individual layers.

The thickness of each layer and the number of layers in the entire system depend on the application. For example, anti-reflective coatings have only a few layers, while complex filters can have more than a hundred layers.

https://www.laseroptik.com/en/coating-guide/thin-film-basics/bases-of-calculation

We have more than 40 years of experience in the field of coatings and only use deposition materials from long-standing and reliable suppliers who can guarantee consistent quality.

In the VUV range only fluorides are used, in the VIS/NIR mainly metal oxides in combination with SiO2 and in the MIR, special IR materials, e.g. zinc sulfide, can be needed.

Due to the principle of conservation of energy, the following applies to each optic:

Reflection (R) + Transmission (T) + Losses = 1

The losses are due to Scattering (S) and Absorption (A).

For reflection, this results in: R = 1 - T - S - A

Therefore, for the highest reflection, the lowest possible scattering and absorption must be achieved. In addition to the right choice of coating materials and process, substrates with the lowest possible surface roughness must also be used.

When laser optics absorb light energy, it is converted into heat. The greater the absorption, the greater the heat. Depending on the beam profile, laser power and coating type, the layer structure can be damaged or even completely destroyed. There are essentially three defect patterns: color change, molten layer material, layer delamination due to thermally induced stress.

On rough surfaces and layer interfaces, but also on defects within the substrate and coating layers, light is scattered in all directions, which means that parts of the incident light leave the geometrically predetermined path.

Because of the relation R + T = 1 - A - S, absorption (A) and scattering (S) reduce the desired performance of an optic. For the highest quality of laser optics, substrates and coatings have to be as absorption-free and low-scattering as possible.

With the right choice of the number and thickness of the layers, any desired reflection can be achieved for a wavelength. The greater the reflection, the more layers are needed.

https://www.laseroptik.com/en/coating-guide/thin-film-basics/number-of-layers

If further wavelengths or larger angular ranges are used, a larger number of layers is usually required.

A dielectric mirror has a coating with high reflection, using electrically insulating materials, such as metal oxides, SiO2 or fluorides. The coating materials must be chosen carefully, depending on the wavelengths and laser parameters used.

It is often used in a resonator, for beam guidance or as a dichroic mirror.

Unfortunately, no. Even with the best coating and the best polished substrates, there will still be some losses due to transmission, scattering and absorption.

In the NIR the losses can be reduced to < 10 ppm in total, so R > 99.999% can be achieved here.

https://www.laseroptik.com/en/coatings/coatings-for-special-applications/extremely-low-loss-laser-optics

There is no value above which one speaks of high power, especially because the maximum laser power depends strongly on the wavelength and the laser data (pulse width, repetition rate, beam size). Perhaps it makes the most sense to speak of high power when the used materials and deposition technology must be adapted to avoid damage due to laser irradiation. With over 40 coating machines and 6 different processes, LASEROPTIK can offer the right coating for every requirement.

According to the Fresnel equations, reflection and transmission depend on polarization. As a result, s-polarized light always has a larger reflection bandwidth and also higher reflection.

https://www.laseroptik.com/en/coating-guide/thin-film-basics/hr-standards

With increasing angle of incidence (AOI) the central wavelength of a dielectric coating shifts to shorter wavelengths. For example, a standard mirror HR 1064 nm /0° shifts about 10% when using AOI = 45° but only less than 1% when using AOI < 13°.

When tilting the optic, metal coatings change their reflection behavior only very slightly.

Porous layers absorb moisture from the air, which is released again when heated. This changes the refractive index of the layers, but also their optical thickness, and thus the coating drifts spectrally to shorter wavelengths, which is referred to as wet-dry shift. Depending on the type of production and the coating materials used, this shift is up to 0.02% per °C (referring to the central wavelength). Ion assisted and sputtered coatings have dense layers and therefore show no wet-dry shift.

There will be a shift of the central wavelength for every coating if the surface is curved, the larger the curvature the bigger the shift. For compensation there are mainly two different options, using a planetary rotating system for the holding of the lenses while coating or just working with a broader coating design, optimized for the radius of curvature and used aperture.

We use a planetary rotating system only for larger optics (Ø > 220 mm), therefore for lenses with more standard sizes we can offer only the second option. Roughly speaking, an optimized coating is usually needed when the ratio of the radius of curvature to the used aperture is less than 2.

To give you an estimation: A normal human hair has a thickness of about 50-100 µm, a dielectric mirror coating HR 1064 nm / 0° has about 5 µm and a single layer AR 248 nm / 0° about 0.05 µm.

No, our coatings cannot be removed without damaging the substrates, not even with acids. The only way to recoat is to polish the substrates again, which might be problematic for the thickness tolerance of the substrate.