Measuring methods for laser optics

LASEROPTIK has nearly 40 years of experience with high quality coatings. During these years we collected 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 Institut für Nanophotonik Göttingen e.V. (IFNANO, formerly 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, ZEISS differential interference contrast microscopes and for larger surfaces with a Keyence flexible microscope. For automated surface characterization we use an Argos 200.

Zeiss differential interference contrast microscope

Argos 200

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 Zygo NewView 9000 white light interferometer. The measuring sensitivity goes below 0.1 nm rms.

Zygo NewView 9000

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.

Focal length and wedge
Optical standard parameters like effective focal length (EFL), back focal length (BFL), radius of curvature or centering 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°.

OptiSpheric 1000 AF

Flatness
To check the accuracy of the surface, we use an interferometer with a measuring wavelength of 632.8 nm and a Zygo Verifire MST with a measuring wavelength of 1053 nm. 12" is possible as the maximum aperture. These devices can measure flatness up to λ/20 at room temperature. The flatness can be measured in transmission and reflection according to ISO 10110.

Zygo Verifire MST

Interferometer for smaller optics

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 current production processes we provide one spectrometer for every two coating machines.

PERKIN-ELMER spectrometers

Group delay (GD) and group delay dispersion (GDD)
For ultrafast optics used with femtosecond pulsed lasers, the control of the phase respectively the group delay and group delay dispersion of the coatings is of great importance. We employ a custom built white-light interferometer as well as commercial systems to benefit from a wide range of dispersion measurements covering the UV up to the mid infrared.

Measurements can be performed in a wavelength range from 350­­–2400 nm, with an angle of incidence 0–60° and for both polarizations simultaneously. It is possible to measure single mirrors as well as mirror pairs.

Custom-built white-light interferometer

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 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.


Cavity ring-down setup

Environmental testing
A climate chamber and other equipment (e.g. abrasion-test-set) for environmental testing enable us to deliver optics fully certified for customer applications according to MIL and ISO standards.


Climate chamber and abrasion-test-set

Scattering
Forward and backward scattering can be de­tected in the wavelength range from 190–800  nm by a custom-made measuring device based on a Coblentz sphere. We also offer external qualifi­cations at the IOF or LZH.

Absorption and laser induced damage threshold (LIDT)
Absorption measurements allow us to constantly verify and improve the quality of our coatings. Our in-house measurement capabilities cover the irradiation wavelengths 308 nm, 355 nm, 532 nm and 1064 nm.

Moreover, LASEROPTIK benefits from a long-standing cooperation with domestic and inter­national partners for characterizing optical com­ponents. The most important measurements are laser damage threshold (LIDT according to ISO 21254), optical losses, i.e. absorptance (ISO 11551) and scattering (ISO 13696).

Absorption measurement

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).

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