Detlev Ristau

Highly efficient transmission gratings in fused silica for chirped-pulse amplification systems

We report on highly efficient transmission gratings in fused silica with a grating period of 800 nm generated by electron-beam lithography. At a wavelength of 1060 nm, 95% diffraction efficiency is achieved under Littrow conditions. The damage threshold, extremely enhanced compared with conventional gold-coated diffraction gratings, makes these gratings the key elements in high average power (>100 W) femtosecond fiber chirped-pulse amplification systems.

Measuring the absolute absorptance of optical laser components.

The precise determination of the absolute absorptance of a laser component is of high scientific and commercial importance. Our intention is to demonstrate that laser calorimetry can be a reliable and sensitive characterization tool for this purpose. Furthermore, the limitations of laser calorimetry are discussed and suggestions for possible revisions of the ISO 11551 (International Organization for Standardization, Geneva, Switzerland) standard are made. Finally, laser calorimetry is compared with photothermal deflection methods with respect to their practicability in different fields of laser optic characterization.

Calorimetric measurement of optical absorption at 532 nm and 1064 nm according to ISO/FDIS 11551

The intention of this paper is to contribute to the discussion on the characterization methods for laser components within the framework of CHOCLAB. We report on absorption measurements performed with a highly sensitive laser calorimeter at the wavelengths 532 nm and 1064 nm. Several types of sample holders have been tested for their performance and the results were studied with special regard to the influence of scatter light and other signal distortions. For the evaluation of the temperature data, different methods are employed and compared. Based on our practical and theoretical investigations, the practicability of the ISO/FDIS 11551 for calorimetric measurements in the visible and near infrared spectral range is discussed and possible refinements are suggested.

Calorimetric measurement of optical absorption and transmissivity with sub-ppm sensitivity

We report on the development of a laser calorimeter with sub ppm sensitivity. Besides the adaptation of the sample holder for low loss measurements, different ways of evaluating calorimetric data and methods for the detection and compensation of scatter irradiation effects as well as other signal distortions are discussed. As laser sources, an AR- laser and a Nd-YAG laser with frequency doubling option are employed for the measurements. The system is used for the characterization of different types of KTP crystals for frequency conversion. The results show a distinct polarization dependency of the absorption, throughout all crystals investigated.

Laser-induced damage measurements according to ISO/DIS 11 254-1: results of a national round robin experiment on Nd:YAG laser optics

The standardization of laser damage testing is an important prerequisite for development and quality control of optical components exposed to high-power laser beams. The ISO working group ISO/TC 172/SC9/WG6 is currently engaged with the preparation of a standard for laser damage testing. The first part of this standard which is concerned with 1-on-1 damage testing has reached the status of a draft international standard: ISO/DIS 11254-1. To test for the practicability of this characterization method, a round robin experiment was performed within the framework of the EUROLASER-CHOCLAB project. Identical sets of typical Nd:YAG laser components were distributed to the above-mentioned laboratories for LIDT measurements in conformance with the standard. The results are summarized in this paper. Additionally, damage morphologies were examined by microscopic techniques like Nomarksi differential interference contrast and scanning force microscopy.

Absorptance measurements of optical coatings: a round robin

An international round robin study was conducted on the absorption measurement of laser-quality coatings. Sets of optically coated samples were made by a reactive DC magnetron sputtering and an ion beam sputtering deposition process. The sample set included a high reflector at 514 nm and a high reflector for the near infrared (1030 to 1318 nm), single layers of silicon dioxide, tantalum pentoxide, and hafnium dioxide. For calibration purposes, a sample metalized with hafnium and an uncoated, superpolished fused silica substrate were also included. The set was sent to laboratory groups for absorptance measurement of these coatings. Whenever possible, each group was to measure a common, central area and another area specifically assigned to the respective group. Specific test protocols were also suggested in regards to the laser exposure time, power density, and surface preparation.

Measurement of thermal conductivity in dielectric films by the thermal pulse method

Thermal conductivity of dielectric films can be measured by determining the travelling time of a thermal pulse propagating through the film. In the approach to thermal conductivity measurements described here, the energy of a laser-pulse is deposited into the test sample consisting of a totally absorbing substrate and a thin transparent test layer. As a consequence, a temperature profile builds up at the substrate-film interface. The time delay of the temperature rise at the surface of the test layer is determined by the propagation time of the thermal pulse through the layer, and is directly related to the thermal diffusivity and the thickness of the layer. Measurements were carried out with a preliminary setup and evaluated by calculations on the basis of the finite differences method. Thermal conductivity was determined for single layers of Al2O3, SiO2, and Ta2O5 films. The measurements indicate that the thermal diffusivity is dependent on the film thickness and are essentially smaller than the corresponding bulk values.

National round-robin test on laser induced damage at 1.064 μm: revised data reduction and correlation analysis

This paper presents a second look at analyzing the results from the recently held German National damage test round robin experiment. This experiment was performed under the auspices of CHOCLAB to validate the method in ISO/DIS 11254- 1.2. The result of the experiment are re-examined in terms of an improved threshold determination procedure, and the result are correlated for each sample type. Differences in detailed experimental set-up are examined and their effect on the outcome evaluated. The measured threshold are shown to agree well with one another intramurally. Results from two of the three laboratories involved show good extramural correlation.

Continuous-wave CO2-laser-induced damage thresholds in optical components

The measurement of intrinsic laser induced damage thresholds (LIDT) in optical components for continuous wave (CW) CO2 radiation has been investigated. A combination of analytical and numerical models showed that the temperature rise is mainly determined by the surface absorption in transmissive as well as reflective components, and is proportional to the ratio of power to linear dimension (P/d) of the irradiated spot rather than to the conventional power/area (P/d2) parameter. The former ratio therefore represents the correct power scaling law for LIDT measurement in CW laser systems. The precise time domain within which this law holds is a function of spot diameter. This prediction has been confirmed by experimental LIDT tests on well characterized uncoated ZnSe substrates and copper mirrors, and on coated ZnSe windows and copper mirrors. Measured P/d values, though lower than predicted by modelling are considerably higher than those inferred from the technical literature, and show that transmissive components may be used at much higher powers than are at present believed. The results indicate that surface absorption occurs primarily in the sub-surface processing layer. This has been shown by transmission electron microscopy and spectroscopic ellipsometry to be a few hundred nm in depth.

Thermal conductivity of e-beam and IBS coatings

The development of coatings which show a good thermal conductivity plays an important role in the realization of new laser schemes like the Yb:YAG disk laser. We present results on investigations on the thermal conductivity of different thin film materials produced by conventional thermal evaporation and ion beam sputtering. With the optimized IBS-process high reflecting coatings are produced on the end faces of Yb:YAG disk lasers. A significant reduction of the thermal gradient in the laser disk was observed, which is an essential requirement for high power lasers besides the excellent optical quality of the coating.