Being able to survey the sky at ultraviolet and near-infrared wavelengths, astronomical spectrographs can study objects almost impossible to see in visible light alone. Very low light intensities of small objects require single-photon sensitivity. The unique optical properties of fused silica, e.g., highest transmission, best homogeneity, and lowest absorption, enable next-generation observations, space-bound as well as earth-bound. However light absorption in the range >800 nm limits the performance of IR spectrographs when using standard fused silica. Metallic impurities and OH groups embedded in the fused silica matrix need to be reduced to achieve best operation. The MOONS camera system at ESO’s Very Large Telescope observes the universe in the NIR range using low OH synthetic fused silica lenses up to a diameter of 900 mm. In the UV spectrum of 160-350 nm, standard fused silica performs in earth-bound systems extremely well. However, in space-bound applications, radiation damage reduces the transmissivity over time. Radiation hard fused silica windows for the PLATO M3 mission of the European Space Agency (ESA) protect the sensitive telescope from damage due to solar protons. Key optical instruments require calibration of the light source to normalize spectroscopic measurements. A radiation resistant fused silica diffusor with Lambertian scattering behavior supports ESA’s Copernicus program in the Sentinel 5 satellite.
Fused silica an optical material of choice for high energy laser applications offers a wide range of advantages in the UV and NIR wavelength range. Increasing laser performance (especially with critical fluences) require detailed information on laser induced damage threshold of the optical material itself, and not just the coating. Therefore, S-on-1 measurements according to ISO 21254-2 were commissioned from two different, experienced, and well-respected laboratories. The resulting LIDT (laser induced damage threshold) values are discussed in the context of current publications and differences are worked out. The measurements focus on the ns-pulse regime at 1064 and 355nm. The results differ up to a factor of 10 between different laboratories. In comparison to the literature study, we noticed a spread of measured values around a factor of 20. Non-binding recommendations of the standard as well as the optical setup have significant influence on the obtained LIDT values. However, depending on the optical design of the application, one or the other optical setup with the corresponding LIDT value can be relevant. In this respect there is no such thing as a single, generally applicable value. Nevertheless, a standard should enable material comparisons under sufficiently standardized conditions.
In over 100 years of quartz glass fabrication, the applications and the optical requirements for this type of optical
material have significantly changed. Applications like spectroscopy, UV flash lamps, the Apollo missions as well as the
growth in UV and IR applications have directed quartz glass development towards new products, technologies or
methods of measurement. The boundaries of the original measurement methods have been achieved and more sensitive
measurements with precise resolution for transmission, purity, radiation resistance, absorption, thermal and mechanical
stability as well as optical properties like homogeneity, stress birefringence, striae and bubbles/inclusions had to be
found. This article will provide an overview of the development of measuring methods of quartz glass, discuss their
limits and accuracy and point out the parameters which are of high relevance for today's laser applications.
Space bound as well as earthbound spectroscopy of extra-terrestrial objects finds its challenge in light sources with low intensities. High transmission for every optical element along the light path requires optical materials with outstanding performance to enable the measurement of even a one-photon event. Using the Lunar Laser Ranging Project and the LIGO and VIRGO Gravitational Wave Detectors as examples, the influence of the optical properties of fused silica will be described. The Visible and Infrared Surveillance Telescope for Astronomy (VISTA) points out the material behavior in the NIR regime, where the chemical composition of optical materials changes the performance. Special fibers are often used in combination with optical elements as light guides to the spectroscopic application. In an extended spectral range between 350 and 2,200 nm Heraeus developed STU fiber preforms dedicated for broad band spectroscopy in astronomy. STU fibers in the broad spectral range as well as SSU fibers for UV transmission (180 – 400 nm) show also high gamma radiation resistance which allows space applications.
Laser fusion projects are heading for IR optics with high broadband transmission, high shock and temperature resistance, long laser durability, and best purity. For this application, fused silica is an excellent choice. The energy density threshold on IR laser optics is mainly influenced by the purity and homogeneity of the fused silica. The absorption behavior regarding the hydroxyl content was studied for various synthetic fused silica grades. The main absorption influenced by OH vibrational excitation leads to different IR attenuations for OH-rich and low-OH fused silica.
Industrial laser systems aim for the maximum energy extraction possible. Heraeus Quarzglas developed an Yb-doped fused silica fiber to support this growing market. But the performance of laser welding and cutting systems is fundamentally limited by beam quality and stability of focus. Since absorption in the optical components of optical systems has a detrimental effect on the laser focus shift, the beam energy loss and the resulting heating has to be minimized both in the bulk materials and at the coated surfaces. In collaboration with a laser research institute, an optical finisher and end users, photo thermal absorption measurements on coated samples of different fused silica grades were performed to investigate the influence of basic material properties on the absorption level.
High purity, synthetic fused silica is as well the material of choice for optical components designed for DUV applications (wavelength range 160 nm - 260 nm). For higher light intensities, e.g. provided by Excimer lasers, UV photons may generate defect centers that effect the optical properties during usage, resulting in an aging of the optical components (UV radiation damage). Powerful Excimer lasers require optical materials that can withstand photon energy close to the band gap and the high intensity of the short pulse length. The UV transmission loss is restricted to the DUV wavelength range below 300 nm and consists of three different absorption bands centered at 165 nm (peroxy radicals), 215 nm (E’-center), and 265 nm (non-bridging oxygen hole center (NBOH)), which change the transmission behavior of material.
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