Towards freeform manufacturing of ultra-low expansion glass optics

. Ultra-Low-Expansion glass (ULE®) has become an important technological enabler of advanced imaging for astronomy and for extreme-UV lithography. A major limitation though, is that ULE® cannot be poured from the fluid state unlike ZERODUR® which renders costly to produce large and/or complex shapes from it. Beside mirrors, optical components are rarely made of ULE® despite it sharing many properties of pure fused silica glass. Here we explore how femtosecond laser processing combined with laser induced reflow can be used to structure ULE® glass with the goal of producing miniature optical components. To fulfil optical roughness requirements, we adopt a strategy based on first producing elementary shapes, such as cubes or cylinders, that we further topologically transform into sphere, ellipsoids or curved surfaces, using a laser-reflow process. The structural modification of the glass matrix induced by the reflow were investigated using Raman spectroscopy. Our result points to a densification of the glass but no apparent sign of crystallization or devitrification. Furthermore, to understand whether the thermo-mechanical properties were affected or not, the thermal expansion coefficient was estimated using a dilatometry technic based on a pseudo-bimorph micro-cantilevers in a temperature-controlled chamber.


Introduction
ULE® glass from Corning has the record lowest coefficient of thermal expansion (CTE) as well as other interesting properties compared to other low-thermal expansion glass-ceramics, such as: a large optical transparency window covering the visible/mid-IR spectral range, a higher maximum operating temperature and a lower Poisson ratio.The unique properties of ULE® are conferred by the substitution of Ti 4+ cations in the SiO4 tetrahedral atomic structure of SiO2 in the glassy state.Such a solid solution can be obtained through flame hydrolysis (FHM) and sol-gel methods [1], but not from a melt, and hence prevent it from being poured out of a molten phase.Recent works have studied the mechanism and the types of bulk modification induced by femtosecond laser processing in ULE®.Through non-linear absorption processes, a local modification of the bulk glass matrix is achieved at the focal spot.A rich taxonomy of modification is observed, starting with photodarkening phenomena [2] at low energy dose, followed by the formation self-organized nanograting [3,4].The latter can be erased by melting through cumulative pulse laserexposure thermal loading [5,6]; all of which, being controlled by the exposure dose and the pulse parameters.We recently investigated the etching behavior of ULE® in function of exposure parameters for various pulse durations [7].A laser parameters window was found to significantly enhance the etching rate (~300 µm/hr), enabling the fabrication of high aspect ratio 3D structures.This manufacturing process is of interest for miniaturized space optical instruments, and similar to the LISA interferometer.Here, we specifically explored how femtosecond laser processing can be used to structure ULE® glass and in particular, for fabricating freeform optical components.To fulfil stringent optical roughness requirements (typically corresponding to an Ra in the nm range), we adopt a strategy based on the combination of two laser manufacturing processes.In this process flow, elementary shapes, such as cubes or cylinders, are first produced using femtosecond laser processing and chemical etching.In a second step, these elementary shapes, acting as preforms, are melted locally using a CW mid-IR laser and topologically transformed into spheres, ellipsoids or arbitrary curved surfaces.
It is known that in pure silicon dioxide glass, the rate of temperature change affects the glass structure.A short thermal cycling at high temperature with a rapid cooling, as induced by laser processing, results in a local densification characterized by a higher fictive temperature [8,9].In the case of titanium-silicate binary glass system, a high temperature annealing can, in certain conditions, trigger the formation of anatase and rutile phases [10].
The purpose of this work is to investigate the feasibility of this two-step process for producing freeform optics in ULE® glass.Various cylindrical and cuboidal preforms were produced in ULE® glass by means of femtosecond laser writing and wet etching.Using a CW mid-IR laser, a short laserreflow cycle (t < 1 min) was applied to the preforms to morph them into convex and quasi-spherical objects, with surface roughness (Ra and Rz) reduced to a few nanometers or less.

Results and discussion
Using Raman, we investigated whether the high temperature reached during the process may have caused the formation of anatase and/or rutile phase in the glass matrix.After reflow, we observed typical features of a Fig. 2. A cuboid preform produced by fs laser writing and wet etching in ULE®.On the left: the cube before exposure.On the right: the same cube after the laser induced morphing.The cube width was approx.102 µm, the pillar section is approx.13 µm thick.
densification phenomenon taking place, but no apparent signs of crystallization or devitrification.Densification was observed through the D1 and D2 peaks, which increased by approximatively 25% in intensity and by the main band, which became narrower.On the basis of preliminary Raman spectra characterization, we assume that the unique set of properties of ULE© are mostly preserved, at least their optical properties.
To provide a quantitative answer, we investigated the thermo-mechanical properties of the laser heat-affected zones, using a dilatometry technic based on pseudobimorph miniature cantilever.This approach, developed and applied in previous work related to the thermal expansion coefficients (CTEs) of fused silica [11], enables to study subtle CTE changes, induced locally in the bulk.