TDSU 3: Glass Studio

The Glass Studio has extensive glass melting and extrusion equipment for working with compound glass compositions such as tellurite, lead-oxide and germanate. This puts us in a strong position to produce rods, tubes and more complex structures directly from exotic bulk glasses made in the facility. These can then be used either to stack a PCF preform, or to draw the more complex structures directly to fibre. 

Facilities

The Studio has several laboratories, focusing on the development and characterization of special glasses for many different applications, including prisms, waveguides and microstructured optical fibres. Glasses such as heavy metal oxides, fluorides or chalcogenides, which are unavailable from the market, normally possess advantageous properties such as high linear/nonlinear indices, extended ultraviolet (UV) to mid-infrared (mid-IR) transmission, increased rare-earth ions doping concentration, etc. Such advantages bring novel properties to bulk materials and fibres, which have applications beyond the current limits of fused silica. The Studio is equipped with two induction furnaces, one resistance furnace with gas protection, and three annealing furnaces for glass melting. There is also a custom-designed extrusion machine, and now after two years of technical development we have a complete "die design/manufacturing and glass melting/preform extrusion" procedure for exotic waveguide structures. Most importantly, during the last year, we have built up extensive experience in using advanced 3D-printing techniques for extrusion dies, which gives huge flexibility for realising novel optical structures, well beyond the state-of-the-art. The glass studio also has glass cutting, grinding, polishing and etching facilities, as well as FTIR and  UV/Vis/NIR spectrometers and a prism coupler for the characterization of bulk materials.

Besides its own research activities (three on-going projects), the glass studio also provides a comprehensive service such as glass and fibre fabrication, melting and post-processing of bulk materials for optical components, for many groups within MPL and at external universities/institutes.

Glass melting, post-processing, and characterization

Glass melting is one of the most common activities in the glass studio. During the last few years, we have developed the skills and facilities necessary to prepare high-quality optical glasses with various compositions, such as lead-silicate, tellurite, phosphate, and germanate. Using the melting-casting-quenching-annealing procedure, bulk glasses with different forms can be fabricated. We have facilities for cutting, grinding, polishing, etching and extrusion, and so can manufacture for example cylinders, prisms and tubes, and even complicated preform structures. We can also produce active glasses by mixing rare-earth dopants into the raw powders before glass melting. Also available are UV/Visible/NIR and Fourier transform infrared (FTIR) spectrometers for glass characterisation, and a prism coupler for measuring the refractive indices of bulk or thin films. Further equipment at the Friedrich-Alexander-University in Erlangen (in the group of our collaborator Prof. Dominique de Ligny) includes a differential thermal analyser (DTA), a dynamic mechanical analyser (DMA), and both photoluminescence and Raman spectrometers.  

(a) Casting melts into a pre-heated mode. (b) A glass cylinder with a curved-wall profile, prepared by polishing. (c & d) Various bulk samples of lead-silicate, tellurite, and germanate glasses, doped or undoped with rare-earth ions.

Extrusion for micron-scale optical structures

The fabrication of photonic crystal fibres using the "stack-and-draw" method has become routine, most technical difficulties having been resolved, both for fused silica and soft glasses. The technique does, however, have its limits, being unsuitable for producing structures beyond simple symmetrical lattices (hexagonal, square, graphene-like, etc.) or membranes. Extrusion, widely used in industry, is much more suitable for producing structures with non-standard or asymmetrical geometries. In, the die material clearly must be able to withstand the temperatures needed to achieve the correct  glass viscosity, without contaminating the glass. The Glass Studio has an extrusion system with loading force up to 100 kN, temperature up to ~1100oC, and position control to ~1 μm stepping accuracy, with comprehensive data logging facilities (temperature, force, extrusion rate etc.). We have successfully developed techniques for extruding structures with many different geometries from tellurite, germanate and lead-silicate glasses. 

(a) Schematic of the extrusion system. (b) The extrusion unit is placed inside the furnace (the region within the green cylinder in (a)). Inset: photograph of an extrusion die manufactured by 3D printing. (c-e) Extrusion dies for X-shaped triple nanoweb fibre, multicore triple beam fibre, and single-ring fibre. (f-h) Scanning electron micrographs of fibres fabricated using extrusion dies depicted in (c-e).

Soft-glass PCFs

Soft-glasses such as heavy metal oxides, fluorides and chalcogenides have the potential to extend the transparency window of optical fibres into the deep ultraviolet and mid-infrared regions. In addition, PCFs fabricated from soft-glasses also possess favourable pro­per­ties such as high linear and nonlinear refractive indices, and in some cases offer freedom from UV-induced optical damage and high rare-earth solubility. The stack-and-draw tech­ni­que is well established for fabricating many types of PCF from fused silica. For PCFs made from low-melting-temperature soft glasses, however, techniques such as extrusion and etching provide  excellent alternatives. By careful optimization of the multistep stack-and-draw or extrusion procedures, we have successfully produced fibres for supercontinuum generation, photochemistry, optomechanics, triplet-photon generation, holographic optical tweezing, and other novel applications. A gallery of soft glass PCF structures is shown below.

Examples of solid-core (a-j) and hollow-core (k-n) soft glass PCFs made at MPL.