Measuring methods

LASEROPTIK has more than 30 years experience with high quality coatings. In these years we were able to collect valuable knowledge about the properties of our optics from the best laboratories we could find: the experiences and feed­backs of our customers.

Additionally we developed new coating techni­ques, measurement devices and improved coa­ting materials in co-operation with research partners like Laser Zentrum Hannover e.V. (LZH), Fraunhofer Institute for Applied Optics and Precision Engineering (IOF) in Jena or Laser-Labo­ra­torium Göttingen e.V. (LLG).

To guarantee stable high quality of our optics LASEROPTIK owns a wide range of measuring equipment.

Optical surface inspection
Uncoated and coated optics are inspected with standard OLYMPUS microscopes or ZEISS differential interference contrast microscopes and for larger surfaces with a Keyence flexible microscope.

Surface roughness
For the production of low loss components it is necessary to quantify the surface roughness. This rms-value is measured by a whitelight-inter­ferometer which is combined with a microscope. The measuring sensitivity goes below 0.1 nm rms.

Focal length and wedge
Optical standard parameters like effective focal length (EFL), back focal length (BFL), radius of curvature, modulation transfer function (MTF) or centring errors are inspected using an OptiSpheric®. Radii of 5 mm up to 500 mm and centering errors in the ran­ge of 10” to 5’ can be detected. Additionally it is possible to measure wedges from 10” to 1.5°.

To inspect the accuracy of the surface we use a Twyman-Green interferometer with a measuring wavelength of 632.8 nm. It can work with a maximum aperture of 4" and a flatness down to λ/20 at room temperature. The flatness can be measured in transmission and reflection referring to ISO 10110.

Thin film stress
For many applications it is important to know exactly the thin film stress, respectively to compensate for the bending of the optics due to this stress. LASEROPTIK uses an in-situ stress measurement system and interferometric mea­surements for the quantification of the induced stress.

Reflectance and transmittance
The most important quality tool in optical thin film production is the spectrophotometer. LASEROPTIK uses different types to measure transmittance respectively reflectance from va­cuum-UV (120 nm) to infrared up to 50 µm.

Besides a VUV-spectrometer built by LZH we mainly use PERKIN-ELMER spectrometers (Lambda 19/883/900/950/1050/ Frontier Optica). To give the coating engineers always a prompt access to a spectrometer for getting a fast feedback on the actual production pro­ces­ses we provide one spectrometer for every two coating machines.

Cavity ring-down (CRD)
The accuracy of reflectance measurements with spectrophotometers is limited. Therefore a cavity ring-down setup is employed to measu­re high reflectance at least for the wavelengths 635 nm and 1064 nm.

By analyzing the power decay in the cavity reflectance can be measured in a range from 99.5% to 99.999%. Typical application examples for cavity ring-down measurement are low loss mirrors.

Group delay (GD) and
group delay dispersion (GDD)

For fs-optics the control of the phase respecti­ve­ly the group delay and group delay dispersion of the coatings is of great importance. Besides a custom built one LASEROPTIK uses a white light interferometer from KMLabs (Chromatis).

The measurements can be done in the wavelength range 600-1650 nm, an angle of inciden­ce 0-60° and with both polarizations at the same time. It is possible to measure single mirrors as well as mirror pairs.

Environmental testing
A climate chamber and other equipment for en­­vironmental testing enable us to deliver optics fully certified for customer applications according to MIL and ISO standards.

Forward and backward scattering can be detec­ted in the wavelength range 190-800 nm by a self-constructed measuring device with a Coblentz sphere. Additional external qualifi­cations can be done by IOF or LZH.

Absorption and laser induced damage threshold (LIDT)
LASEROPTIK has a long-standing cooperation with the LZH for the characterisation of optical coatings. The most important measurements are laser damage threshold (LIDT according to ISO 21254), optical losses, i.e. absorptance (ISO 11551) and scattering (ISO 13696). Especially the LIDT has gained increasing importance for optical components.

To produce high power optics you have to consider the different mechanisms of laser indu­ced damages and the influences of the substra­tes and coating materials. Therefore we would like to provide some background information on topic "Laser Induced Damage Threshold (LIDT)"