Gravitational wave detectors (GWDs) rely heavily on low mechanical and optical loss mirror coatings to detect cosmic events happening in the universe. This work discusses optical losses through light absorption and scattering mechanisms in silicon nitride (SiN x ) and silicon oxynitride (SiO x N y ) thin films deposited by the plasma-enhanced chemical vapor deposition technique. We report an efficient and repeatable procedure to tune the refractive index of both SiN x and SiO x N y thin films, while preserving a low optical absorption and scattering in the range of ppm. Finally, we demonstrate the design, fabrication, and characterization of an SiN x /SiO x N y multi-layer stack composed of 20 layers with a total thickness of 3.9 µm, achieving a reflectance above 99% (and a low absorbance of 0.259%) in the near-infrared region, which is a promising step toward meeting the optical requirements of the third generation of GWDs.

Gravitational wave detectors rely on low mechanical and optical loss mirror coatings. This work discusses optical losses through absorption and light scattering mechanisms in SiN x and SiO x N y coatings deposited by plasma-enhanced chemical vapour deposition.

Absolute measurement of residual absorption in optical coatings is steadily becoming more important in thin film characterization, in particular with respect to high power laser applications. A summary is given on the current ability of the laser induced deflection (LID) technique to serve sensitive photo-thermal absorption measurements combined with reliable absolute calibration based on an electrical heater approach. To account for different measurement requirements, several concepts have been derived to accordingly adapt the original LID concept. Experimental results are presented for prominent UV and deep UV laser wavelengths, covering a variety of factors that critically can influence the absorption properties in optical coatings e.g., deposition process, defects and impurities, intense laser irradiation and surface/interface engineering. The experimental findings demonstrate that by combining high sensitivity with absolute calibration, photo-thermal absorption measurements are able to be a valuable supplement for the characterization of optical thin films and coatings.

The Sandwich concept of the laser induced deflection (LID) technique is applied to characterize nonlinear optical crystals from the near infrared to the deep UV wavelengths. In particular, nonlinear bulk absorption at 355nm and 266nm is investigated in BBO, CLBO and LBO, respectively.

The laser induced deflection (LID) technique for absolute absorption measurements is introduced. From the general principle, several concepts are derived for measurements in optical materials, coatings and fibers. Experimental results will cover nonlinear optical crystals, optical coatings and fiber Bragg gratings.

A simplified absolute calibration for the photothermal laser-induced deflection technique is introduced and applied to characterize different nonlinear optical crystals at common diode pumped solid state laser wavelengths. In particular, intensity-dependent absorption measurements at 355 nm are performed to investigate the nonlinear absorption in lithium triborate crystals. From the results, it is verified that at least to a certain extent the nonlinear absorption is related to impurities or defects, i.e., to sequential three-photon absorption in addition to a potential intrinsic three-photon absorption process. (C) 2018 Society of Photo-Optical Instrumentation Engineers (SPIE)

The surface absorption of CaF2 at 193 nm after surface treatment by ion beam figuring (IBF) and the subsequent antireflective (AR) coating process was investigated by using the laser-induced deflection technique. In comparison to a standard pitch polishing, enhanced surface absorption (subsurface damage) is observed after IBF treatment. However, after AR coating deposition, the total surface absorption of IBF treated samples does not exceed values obtained for pitch polished samples. The experimental results provide indication that IBF treatment yields both fluorine displacement and deficiency in the CaF2 subsurface region, which is annealed during the coating process by the high temperature and the fluorine supply from the fluoride coating materials, respectively.

We present the results of comprehensive bulk absorption studies on numerous LBO crystals at 1070nm, 532nm and 355nm using the sandwich-LID measurement concept. One aim of the studies is answering the question whether there is a correlation between bulk absorption coefficients at the different applied wavelengths. Further, intensity dependent absorption measurements at 355nm are performed to investigate the nonlinear absorption in the LBO crystals. From the results it is verified, that at least to a certain extent the nonlinear absorption is related to impurities or defects, i.e. to sequential three photon absorption in addition to a potential intrinsic 3-photon absorption process.

An overview is given on the different concepts to absolutely measure very low absorption in optical materials and coating by using the laser induced deflection (LID) technique. Besides the individual pros and cons of the concepts we focus on the absolute calibration and the achievable sensitivity. In particular for the Sandwich-LID-concept experimental results are given that cover low loss materials and coatings in a wide wavelength range.

An overview is presented of the characteristic features for the sandwich concept used for NLO crystal bulk absorption measurements. The sandwich concept is a photo-thermal absorption measurement concept based on the laser induced deflection (LID) technique. Besides a strong sensitivity enhancement for photo-thermally insensitive materials, the focus of the paper is on the absolute calibration, one of the key criteria for photo-thermal techniques. Based on experimental results it is proven that absolute bulk absorption calibration is simplified by using the sandwich concept since it is insensitive to sample orientation or dopants. Furthermore, experimental results on a variety of materials reveal that in general the bulk absorption calibration sample can be made of just one material, e.g. Aluminum which is favorable because of its easy mechanical handling However, for surface/coating calibration a different result is found. Finally, the sandwich concept is applied to characterize the bulk absorption of different nonlinear crystals at the wavelengths 1064, 532, 355 and 266nm.

The CaF2 surface absorption at 193nm is investigated as function of surface treatment by different ion beam figuring (IBF) parameters and susequent AR coating deposition using laser induced deflection (LID) technique.

We report on the first realization of direct absorption measurements in thin rods and optical fibers using the laser induced deflection (LID) technique. Typically, along the fiber processing chain more or less technology steps are able to introduce additional losses to the starting material. After the final processing, the fibers are commonly characterized regarding losses using the so-called cut-back technique in combination with spectrometers. This, however, only serves for a total loss determination. For optimization of the fiber processing, it would be of great interest to not only distinguish between different loss mechanisms but also have a better understanding of possible causes.For measuring the absorption losses along the fiber processing, a particular concept for the LID technique is introduced and requirements, calibration procedure as well as first results are presented. It allows to measure thin rods, e.g. during preform manufacturing, as well as optical fibers. In addition, the results show the prospects to also apply the new concept to topics like characterizing unwanted absorption after fiber splicing or Bragg grating inscription.

A new concept enhances the capability of absolute photo-thermal absorption measurements with transversal probe beam guiding. The so-called sandwich concept is particularly designed to investigate nonlinear and laser crystals as well as high reflective coatings.

We report on the continuation of a comparative study of different fused silica materials for ArF laser applications. After selecting potentially suited fused silica materials from their laser-induced absorption and compaction obtained by a short-time testing procedure, accelerated lifetime tests have been undertaken by sample irradiating at liquid nitrogen temperature and subsequent direct absorption measurements were made using the laser-induced deflection technique. The obtained degradation acceleration strongly differs between fused silica materials, showing high and low oxygen hole (OH) contents, respectively. As a result, a difference in the absorption degradation mechanism between high and low OH-containing fused silica is proposed. Consequently, two different scenarios for an acceleration of the absorption degradation are derived. (C) 2014 Society of Photo-Optical Instrumentation Engineers (SPIE)

A new concept enhances the capability of photo-thermal absorption measurements with transversal probe beam guiding by overcoming drawbacks such as a lack of sensitivity for materials with low photo-thermal response and/or round substrate geometry. The sandwich concept using the laser-induced deflection technique is introduced and tested for the investigation of highly reflecting (HR) coatings. The idea behind the sandwich concept is based on the decoupling of the optical materials for the pump and probe beams. This is realized by either placing a HR coated rectangular substrate in between two optical (sandwich) plates or attaching a HR coated thin round substrate onto one optical plate. For both configurations, the sandwich concept results in a strong increase in sensitivity for the measurement of HR coatings deposited onto photo-thermally insensitive substrates. Experiments reveal that for a CaF2 substrate, up to two orders of magnitude enhancement in sensitivity can be achieved.

A new concept strongly enhances the capability of photo-thermal absorption measurements with transversal probe beam arrangement by overcoming several drawbacks like sensitivity lack for materials with low photo-thermal response, non-transparent substrates and round sample geometries.

In the initial stage of low fluence ArF laser long term irradiation the absorption behavior in medium-to-high OH containing fused silica materials ([OH] = 250 ... 1400 wt-ppm) depends on the OH content as well as on the initial content of oxygen deficiency centers (ODC). The absorption at a particular fluence (1 mJ/cm(2)) and its change after 193 nm irradiation (20 Mio. Pulses, 5 mJ/cm(2)) correlate to the OH content whereas the fluence dependent absorption dk/dH(1 ... 3 mJ/cm(2)) and its change for the same irradiation conditions follow the initial ODC content.For a more narrow range of the OH content ([OH] >= 1000 wt-ppm) both, the absorption at a particular fluence and the fluence dependent absorption are correlated to the initial amount of molecular hydrogen in the fused silica. (C) 2013 Elsevier B.V. All rights reserved.