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99441 Mellingen

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Coatings for Laser Applications

LAYERTEC specializes in the production of optics for laser applications in the wavelength range from the VUV (157nm and below) to the NIR (~4μm).

The most common types of optical coatings for lasers are high reflecting mirrors (for normal incidence as resonator mirrors or for AOI=45° as turning mirrors), partial reflectors for output couplers and beam splitters and anti reflection coatings for windows and lenses. Coatings for more complex laser types combine up to three wavelength ranges of high reflectivity (e.g. for the laser wavelength and the harmonics) and up to three wavelength ranges with high transmission (e.g. for the pump wavelength, the harmonics or for the suppression of other laser lines). Broadband mirrors and mirrors which are optimized for smooth group delay and group delay dispersion spectra are required for lasers emitting in a large wavelength range, e.g. dye lasers, Ti:Sapphire lasers, optical parametric oscillators (OPOs) and femtosecond lasers, respectively.

Besides reflectivity and transmission, coatings for laser applications must be optimized for low optical losses and high laser induced damage thresholds.

Sputtered optical coatings for the VIS and NIR exhibit extremely low straylight and absorption losses (both in the order of some 10–5). The reflectivity of HR mirrors or the sum of reflectivity and transmission of partial reflectors produced by magnetron sputtering is well above 99.9%. Recently measured absorption losses in the NIR are in the order of 3–30ppm for both sputtered and evaporated coatings. Evaporated coatings show straylight losses in the order of some 10–3 in the VIS–NIR region and up to some 10–2 in the UV and VUV. Nevertheless, evaporated coatings show low absorption losses in the UV.

Damage in cw and ns laser optics is mainly related to thermal effects such as increased absorption – either intrinsic absorption in the coating materials or absorption by defects – or poor thermal conductivity and low melting temperatures of the coatings. High power coatings require both the controlling of the intrinsic properties of the coating materials and the reduction of defects in the layers. Laser damage of picosecond and femtosecond laser optics is mainly caused by field strength effects. High power coatings for these lasers require very special coating designs.

The determination of the laser induced damage threshold (LIDT) according to the standards ISO 11254-1 (cw- LIDT and 1 on 1–LIDT, i.e. single pulse LIDT), ISO 11254-2 (S on 1, i.e. multiple pulse LIDT) and ISO 11254-3 (LIDT for a certain number of pulses) requires laser systems operating in single modes, precise beam diagnostics as well as online and offline damage detection systems. This is the reason, why only a limited number of measurement systems with only a few types of lasers is available (e.g. for 1064nm at Laserzentrum Hannover). For some of the most prominent laser wavelengths such as e.g. Argon ion lasers (488nm or 514nm), there is no measurement system available and no certified LIDT data can be provided.

The 1-on-1-LIDT (i.e. 1 pulse on 1 site of the sample) is not representative for the normal operation conditions. However, these values can be used for comparison of different coatings and for optimization procedures. Moreover, the 1 on 1 values are directly related to the more practical S-on-1-LIDT (LIDT for a given number “S” of pulses on the same site of the sample) and can be interpreted as upper limit of the LIDT. Laser systems with high repetition rates (some kHz) require lifetime tests expressed by LIDT values for high numbers of pulses.

The limited number of measurement facilities and the need for lifetime tests for practical applications make it necessary to include also measurements, lifetime tests or cumulative irradiation tests of several customers into our catalog as well as into this website. Please take into account that these values cannot be compared with an LIDT measurement, because the laser parameters given there are those without damage. Moreover, there is always an uncertainty of these values, especially with respect to the determination of the spot size. Errors in the order of about 30% must be taken into account. Nevertheless, we think that information on parameters of successful operation of our optics will certainly help to decide to use LAYERTEC optics. Sometimes, however, tests at the customers laser system will be necessary. LAYERTEC supports such tests at the customers facility by a considerable discount for the test pieces.

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