Volker Dietrich

Cladding YAG crystal fibers with high-index glasses for reducing the number of guided modes

Yttrium aluminium garnet (YAG) crystal fibers with a core diameter of 40 μm were cladded by high index glasses using the co-drawing laser-heated pedestal growth method. Due to the extremely large cooling rates in the fabrication processes, unexpected and phenomenally large index drops of 0.018 and at least 0.02 were found from the as-grown capillary and the YAG crystal fiber cladding compared with bulk N-SF57’s, respectively. The high-index glass cladding is effective in reducing the number of guided modes, and the intensity profiles of the crystal fiber show there are only four guided modes at 532 nm.

Large optical glass blanks for the ELT generation

The upcoming extremely large telescope projects like the E-ELT, TMT or GMT telescopes require not only large amount of mirror blank substrates but have also sophisticated instrument setups. Common instrument components are atmospheric dispersion correctors that compensate for the varying atmospheric path length depending on the telescope inclination angle. These elements consist usually of optical glass blanks that have to be large due to the increased size of the focal beam of the extremely large telescopes. SCHOTT has a long experience in producing and delivering large optical glass blanks for astronomical applications up to 1 m and in homogeneity grades up to H3 quality in the past. The most common optical glass available in large formats is SCHOTT N-BK7. But other glass types like F2 or LLF1 can also be produced in formats up to 1 m. The extremely large telescope projects partly demand atmospheric dispersion components even in sizes beyond 1m up to a range of 1.5 m diameter. The production of such large homogeneous optical glass banks requires tight control of all process steps. To cover this demand in the future SCHOTT initiated a research project to improve the large optical blank production process steps from melting to annealing and measurement. Large optical glass blanks are measured in several sub-apertures that cover the total clear aperture of the application. With SCHOTTs new stitching software it is now possible to combine individual sub-aperture measurements to a total homogeneity map of the blank. In this presentation first results will be demonstrated.

Bending strength measurements at different materials used for IR-cut filters in mobile camera devices

Digital cameras are present everywhere in our daily life. Science, business or private life cannot be imagined without digital images. The quality of an image is often rated by its color rendering. In order to obtain a correct color recognition, a near infrared cut (IRC-) filter must be used to alter the sensitivity of imaging sensor. Increasing requirements related to color balance and larger angle of incidence (AOI) enforced the use of new materials as the e.g. BG6X series which substitutes interference coated filters on D263 thin glass. Although the optical properties are the major design criteria, devices have to withstand numerous environmental conditions during use and manufacturing - as e.g. temperature change, humidity, and mechanical shock, as wells as mechanical stress. The new materials show different behavior with respect to all these aspects. They are usually more sensitive against these requirements to a larger or smaller extent. Mechanical strength is especially different. Reliable strength data are of major interest for mobile phone camera applications. As bending strength of a glass component depends not only upon the material itself, but mainly on the surface treatment and test conditions, a single number for the strength might be misleading if the conditions of the test and the samples are not described precisely,. Therefore, Schott started investigations upon the bending strength data of various IRC-filter materials. Different test methods were used to obtain statistical relevant data.

Results of a polishing study for SCHOTT XLD glasses

Extremely low dispersion glasses (e.g. SCHOTT XLD glasses) play an essential role in the color correction of optical systems. Together with short flint glasses (KZFS Types) they can be used for apochromatic color correction in the visible spectrum or even for broadband color correction in combination with lanthanum crown glasses (LAK Types). Unfortunately the chemical composition of those glasses leads to a high coefficient of thermal expansion, low hardness and low resistance against chemical attacks. As a consequence these glasses tend to be difficult in processing. Therefore the glass engineer’s task is to improve processing characteristics while keeping their special optical properties. N-FK58 XLD is an example of a new generation of XLD glasses from SCHOTT with improved workability. In 2014 a processing study has been conducted to optimize the polishing of XLD glasses. This presentation will show the results of this study for N-FK58 XLD and the application to other fluorophosphates glasses.

Single crystalline YAG-core fiber with a lanthanum dense flint glass cladding

The crystalline YAG-core fiber cladded by high-index N-LaSF9 glass has been fabricated to significantly reduce the number of guided modes. The selective propagation of the LP01 and LP11 modes was successfully demonstrated at 633 nm.