Our anti–reflective coatings are optimized for a certain wavelength or wavelength ranges. They allow a high transmittance of the laser light and less absorption of energy in the lens for specific wavelengths. Low–absorption coatings are recommended for lasers with a high peak power, as they minimize thermal effects. These coatings are only available for fused silica lenses. Beside our standard coatings we also offer customized coatings. In the following table the LIDT (laser induced damage threshold) can be found for our coatings and are measured at 355nm, 532nm or 1064nm respectively. The used Laser had a pulse duration of 1ns at a pulse frequency of 50Hz. For detailed information about damage thresholds, plaese refer to our extra lexicon chapter about LIDT.

The following coating curves show measured reflections of our typical coatings per surface. If the transmission through a complete lens is of interest, the reflection value at the specific wavelength has to be multiplied with twice the number of lens elements (each element has two surfaces) and then subtracted from 100%. The number of lens elements can be found on the datasheets.

**Basic problem**

With the increasing powers of available lasers, commonly used optical glasses have been pushed to their limits in respect to acceptable thermal effects: Via the exposure of optical glasses to laser radiation, parts of the beam energy are absorbed into the material. This leads to a heating effect with two mayor influences onto optical properties. First, the heating changes the index of refraction of the glass and second, thermal expansion leads to changes in the surface curvatures and therefore changing the refraction of the laser beam. In the application, starting from average laser powers of about 50 W at 1064 nm, a resulting shift of the focal position can lead to decreasing process quality and make online adjustments necessary.

**Possible solution**

Another option lies in the use of fused silica as lens element material. It is a very resistive glass type which has also a very low absorption coefficient compared to optical glasses. Therefore it is commonly used to minimize thermal effects. Sill also uses special low-absorption coatings to minimize thermal effects further and increase typical damage thresholds.

**Other influences on absorption**

Cleanness of the optical components plays also an important role for absorption and therefore thermal effects. Of course there are any larger dust particles (e.g. finger prints) are an extreme absorber of laser radiation. But also time brings micro particles or other contaminations onto lens surfaces. These are not visible to the human eye, but they can be measured. Measurements show that the absorption increases with the duration to the last cleaning. A new lens cleaning resets the absorption values to the original state. Therefore regular cleaning of surfaces exposed to the environment is recommended. This effect could not be shown for internal lens elements, thus cleaning them is not necessary!

Here you can find further details on cleanness of lenses and lens elements:

When estimating the spot size of a diffraction limited lens, an additional apodization factor (APO) is needed. Its dependency on the truncation ratio T is shown in the following graph. The APO factor includes a dependency on the intensity distribution at the confining edges, where diffraction effects take place. Assuming a Gaussian beam, which is much smaller that the clear aperture, very few parts of the beam experience diffractive effects. By comparison, a Gaussian beam vignetted at the 1/e² beam diameter has larger portion of the beam influenced by diffraction.

The minimal adjustable focal spot size is calculated by the wavelength of the laser multiplied with the focal length of the scan lens, the APO factor and the diffraction parameter M² of the laser divided by the 1/e² beam diameter dL.

*d*_{F} = focal spot diameter*d*_{EP} = entrance pupil diameter*d*_{L} = beam diameter (1/e²)*f'* = focal length

In this example, the focal spot size will be calculated for a Gaussian beam with dL=6.0mm and dL=10.0mm. We assume the use of a f-theta lens S4LFT4010/292 with a frequency doubled Nd:YAG laser at 532nm and a diffraction value M²=1.2. The lens has an effective focal length of f’=100.0mm. Another very important value to determine in addition to the truncation ratio T, is the clear aperture or entrance pupil. This is not the clear aperture of the f-theta lens (Ø35mm), but typically the limiting factor is the beam entrance diameter or aperture of the scan system. Assume a very common value of dEP =10mm in this case.

Example 1

f’=100mm, λ=532nm, *d*_{EP} =10mm, M²=1.2, *d*_{L}=6.0mm

Example 2

f’=100mm, λ=532nm, *d*_{EP} =10mm, M²=1.2, *d*_{L}=10.0mm

In optics and especially laser science, the Rayleigh length or Rayleigh range is the distance along the propagation direction of a beam from the waist to the place where the area of the cross section is doubled.

The Rayleigh length is calculated by the focus area multiplied by a factor (depending on the APO-factor) divided by the wavelength and the diffraction value M² of the laser.

The depth of focus of the scan lens can be estimated by a doubled Rayleigh length. Be aware, that this is just a rough estimation and in many modern applications this value can be too large to still fulfill needed spot diameter requirements.

Sill Optics GmbH & Co. KG

Johann-Höllfritsch-Str. 13

DE-90530 Wendelstein

Tel. +49 (0) 9129 / 90 23 - 0

info@silloptics.de

Johann-Höllfritsch-Str. 13

DE-90530 Wendelstein

Tel. +49 (0) 9129 / 90 23 - 0

info@silloptics.de