Low Expansion Glass Research Articles

Comprehensive review of the effects of ionizing radiations on the ZERODUR® glass ceramic

The low thermal expansion glass ceramic ZERODUR® has a long and successful history in spaceborne optical applications. This material is especially used where precise shape invariance is required, i.e., for the mirror substrates dimensional stability when subject to temperature gradients and transients. In space, temperature may not be the exclusive driving force impacting the form stability; influence from ionizing radiations require considerations. The real impact of ionizing radiation on ZERODUR® has become a matter of reconciling, on the one hand, in situ experience, e.g., that the secondary mirror of low Earth orbit (LEO) Hubble Space Telescope crossing the South Atlantic Anomaly or the overall optics of geosynchronous equatorial orbit (GEO) Chandra X-ray Observatory are not reporting any specific problems related to dimensional stability at the optical form level. On the other hand, finite element simulation based on early lab experiments of ZERODUR® compaction are suggesting the opposite. This debate was brought to the forefront with the SILEX mission, where radiative ageing models were significantly overestimating the deformation experimentally observed on the lab replicas and were in even stronger disagreement with the observations collected over the mission. It has been speculated that an erroneous form factor in the physical model used to derive the phenomenological compaction law was responsible for these discrepancies. Following this hypothesis, we readdressed the effect of ionizing radiation induced by γ, electron, and proton fluences on ZERODUR® compaction. For each of these, we present and discuss the irradiation source, the experimental setup, the sample design, and the measurement procedure as well as the observations. Consistent with the feedback gathered over many different space missions, we confirm that the compaction observed is significantly smaller than the estimations available in the prior literature.

Glass ceramic ZERODUR®: Even closer to zero thermal expansion: a review, part 1

Observational astronomy has striven for better telescopes with higher resolutions from its start. This needs ever-larger mirrors with stable high precision surfaces. The extremely low expansion glass ceramic ZERODUR® enables such mirrors with more than 50 years of significant improvements in size and quality since its development. We provide a survey of the progress achieved in the last 15 years. The narrowest coefficient of thermal expansion (CTE) tolerance is now ±7 ppb / K. It is possible to adapt the material for lowest expansion to temperature-time courses given for special environments. At cryo temperatures, expansion is low and adaptable. Improved measurement capabilities allow for absolute CTE uncertainty of 3 ppb / K and reproducibility of 1 ppb / K (2σ). The influence of ionizing radiation on the surface figure integrity is subject to new investigations. Improved measurement capabilities increase the reliability of structure designs. Some outstanding examples are given for applications of ZERODUR in astronomy and in the very important high technology industry. The progress of thermal expansion homogeneity, the mechanical strength of ZERODUR, production capabilities in melting, precision machining, light-weighting, and dimensional metrology is presented in the second part of the paper.

Glass ceramic ZERODUR®: Even closer to zero thermal expansion: a review, part 2

Observational astronomy has sought better telescopes with higher resolution from its beginning. This needs ever-larger mirrors with stable, high-precision surfaces. The extremely low-expansion glass ceramic ZERODUR® has enabled such mirrors for more than 50 years with significant improvements in size and quality since then. We provide a survey of the progress achieved in the last 15 years. Equally important as the thermal expansion coefficient CTE is its homogeneity. The CTE variation in 4-m mirror blanks lies below 5 ppb / K in radial and axial directions on large and short scales. Improved measurement capabilities allow reduced bias, which in the past made variations look greater than they were. Isotropy and uniformity of ZERODUR are outstanding. A method for lifetime calculation increases reliability considerably with respect to mechanical loads. The production and metrology capability and capacity are greatly extended. Surface figure and texture of large blanks allow starting directly with polishing. Filigree structures with up to 90% weight reduction are well-suited for space mirrors. The progress with low thermal expansion and its measurement, the insensitivity of ZERODUR against ionizing radiation in space, and outstanding application examples are presented in the first part of our review.

Multi-layer Laser Mirrors for Optical Sensors Deposited by Electron Beam Deposition and Ion Assisted Electron Beam Deposition

High reflectivity and low loss mirrors are very much essential for state-of art sensors like Ring Laser Gyroscopes widely used for navigation of moving platforms. Surface scattering is the main source of loss for the Laser mirrors of high reflectivity. Surface scattering also lead to coupling of back reflected light leading to dead band of optical sensors at low input rotation. Super polished glass substrates have been prepared from low expansion glass ceramic material. Surface roughness is achieved about 3-5Å (RMS value) for the ceramic glass substrates prior to thin film coating. The substrates are deposited with 41 alternative layers of SiO2 and Ta2O5 of Quarter-Wave optical thickness with Electron Beam Deposition (EBD) technique. Ion beam Assisted Electron Beam Deposition (IAEBD) is used for another batch of substrates to coat identical stack of layers on glass substrates of the same surface quality. The key process parameters of both the coating processes are presented. The optical constants of the films are evaluated with spectroscopic Ellipsometer from 400nm to 1600nm. Total Integrated Scattering is estimated from the achieved surface finishing of mirrors deposited. The multi-layer coatings are characterized with UV-Vis-IR Spectrophotometer and Cavity Ring Down(CRD) loss meter. The quantitative optical loss of the mirrors is measured at 45o angle of incidence with CRD loss meter. The merits of ion assisted EB process over simple EB coating processes are evaluated against optical constants of thin films and performances of mirrors. Fitness of the processes for realizing Laser mirrors for optical sensors at 632.8nm is evaluated.

Correction to: 22 W average power multiterawatt femtosecond laser chain enabling 1019 W/cm2 at 100 Hz

In the original publication, we erroneously reported that the substrate of the diffraction gratings used in the experiments was made of low expansion glass (zerodur). In fact, the substrate of the diffraction gratings was made of Pyrex, which does not have the same thermomechanical properties as zerodur. This is an important correction to mention as the thermally induced wavefront distortions depend on the thermomechanical properties of the substrate.

Low Li2O content study in Li2O-Al2O3-SiO2 glass-ceramics

The price of lithium-containing minerals and other chemical materials continues to increase, resulting in an increase in the production cost of Li2O-Al2O3-SiO2 (LAS) system glass-ceramics. In the LAS glass-ceramics component, the reduction in the amount of Li2O used can reduce the cost of the product. It is worthwhile to study whether it is possible to prepare glass-ceramics with low expansion properties under low Li2O content. The effect of Li2O content on the glass-ceramics of LAS system was studied. In this paper, spodumene was used as the main raw material, and TiO2 and ZrO2 were added as crystal nucleating agents to prepare transparent glass-ceramics with low expansion coefficient. The effects of the change of Li2O content on the crystal phase and microstructure of glass-ceramics were investigated by XRD, DSC, FTIR and SEM. The results show that the main crystalline phase of the low expansion transparent glass-ceramics is β-quartz solid solution. When Li2O content is in the range of 2.99 wt% to 4.13 wt%, low expansion glass ceramics can be prepared by an appropriate method. With the increase of Li2O content, the average coefficient of thermal expansion (CTE) in the temperature range of 30 °C–300 °C shows a decreasing trend. When Li2O content is in the range of 3.51 wt% to 4.13 wt%, the thermal expansion coefficient of the glass ceramics is extremely small, and even a negative expansion coefficient occurs.

Low-expansion glass for telescopes

Low-expansion glass for telescopes

Medium-finesse optical cavity for the stabilization of Rydberg lasers.

We describe the design, construction, and characterization of a medium-finesse Fabry-Perot cavity for simultaneous frequency stabilization of two lasers operating at 960 and 780 nm wavelengths, respectively. The lasers are applied in experiments with ultracold rubidium Rydberg atoms, for which a combined laser linewidth similar to the natural Rydberg linewidth (≈10 kHz) is desired. The cavity, with a finesse of ≈1500, is used to reduce the linewidth of the lasers to below this level. By using a spacer made of ultra low expansion (ULE) glass with active temperature stabilization, the residual frequency drift is limited to ≲1 MHz/day. The design optimizes for ease of construction, robustness, and affordability.

Mechanism of compound fracture and removal in grinding process for low-expansion glass ceramics

The mechanism of fracture and its removal for low-expansion glass ceramics processing was studied through grinding experiments. A dynamic coefficient Kd was proposed to characterise the brittle material processing property. Using surface roughness as the evaluation index, the limitations of the empirical model and the brittle fracture model were analysed. On the basis of simple single-factor tests, two-step changes on the processing surface of brittle materials were observed for the first time, which could be described in terms of the critical depth of ductile and ductile–brittletransition (hc1) and the critical depth of ductile–brittle and brittle transition (hc2). The brittle material fracture processing and removal could be divided into three phases: ductile fracture, ductile–brittle fracture and brittle fracture. Furthermore, a compound grinding factor Q and a model of compound regime were proposed in this study, in order to comprehensively test the variation tendency of surface roughness and improve the model accuracy. A multi-factor cross-validation test also was conducted, and the results of the three models were compared with the test results. The model of compound regime (Ra3) showed the highest precision.

Chemical Mechanical Polishing on Extremely Low Expansion Glass Ceramic Wafers

Extremely low expansion glass ceramics are widely used in integrated circuit (IC), liquid crystal display (LCD) lithography, high-precision measurement and astronomy, due to their excellent mechanical properties and chemical stability at higher temperatures. Nevertheless, the extremely low expansion glass ceramics are hard-to-machine materials due to their hard-brittle nature, resulting in cracking, chipping and scratching induced in conventional machining. This leads to higher surface roughness, and is not qualified for high-performance devices. In this study, surface roughness of 0.447 and 4.904 nm are achieved for Ra and peak-to valley (PV), respectively with a measurement area of 70×53 μm2 after chemical mechanical polishing (CMP). Firstly, the glass ceramic wafers are lapped using silicon carbide (SiC) abrasives on a cast-iron plate. After lapping, the wafers are polished by CeO2 slurry in a sequence of 3 μm and 500 nm in diameter, and polyurethane and floss pads are used correspondingly. Finally, CMP is employed on the glass ceramic wafers. Floss pad and silica slurry are used in CMP in an alkaline solution with a pH value of 8.5. After CMP, the wafers are cleaned and dried by deionized wafer and compressed air, respectively.

Construction of an optical test-bed for eLISA

In the planned eLISA mission a key part of the system is the optical bench that holds the interferometers for reading out the inter-spacecraft distance and the test mass position. We report on ongoing technology development for the eLISA optical system like the back-link between the optical benches and the science interferometer where the local beam is interfered with the received beam from the distant spacecraft. The focus will be on a setup to investigate the tilt-to-pathlength coupling in the science interferometer. To test the science interferometer in the lab a second bench providing a laser beam and a reference interferometer is needed. We present a setup with two ultra-stable low expansion glass benches and bonded optics. To suppress the tilt-to-pathlength coupling to the required level (few μm/rad) imaging optics are placed in front of the interferometer photo diodes.

DTA를 통한 LAS계 투명 결정화 유리의 결정화 조건 최적화

The basic characteristics of glass are highly fragile and brittle consequences the ultimate purpose of glass manufacturing is to make a transparent glass with complex shape. In order to solve this problem, mechanical properties of glass can be increased by crystallization of its amorphous glass. However, glass-ceramics has become opaque through crystallization process due to the distracted interface of glass by precipitated particles. This study has been investigated thermal processing conditions of LAS transparent glass-ceramic by using DTA (differential thermal analysis), in order to control size of precipitated particle and then fabricate transparent glass-ceramic. DTA indicated that crystallization peak area was declined with increased nucleation temperature. Subsequently, we have been established optimum temperature for crystallization depending on the nucleation temperature. The transmission and thermal expansion were measured after crystallization, and the size of precipitated particle was identified in range of 20~100 nm by FE-SEM. In addition, by setting the optimized crystallization condition, with high transmission and low thermal expansion glass was synthesized through this experiment.

Universal gate-set for trapped-ion qubits using a narrow linewidth diode laser

We report on the implementation of a high fidelity universal gate-set on optical qubits based on trapped 88Sr+ ions for the purpose of quantum information processing. All coherent operations were performed using a narrow linewidth diode laser. We employed a master-slave configuration for the laser, where an ultra low expansion glass Fabry–Perot cavity is used as a stable reference as well as a spectral filter. We characterized the laser spectrum using the ions with a modified Ramsey sequence which eliminated the affect of the magnetic field noise. We demonstrated high fidelity single qubit gates with individual addressing, based on inhomogeneous micromotion, on a two-ion chain as well as the Mølmer–Sørensen two-qubit entangling gate.

Fracture toughness of hydroxide catalysis bonds between silicon carbide and Zerodur low thermal expansion glass-ceramic

In many optical and precision engineering applications, low thermal distortion materials need to be bonded together reliably. Since high temperature bonding process ultimately introduce stresses in the bond, rendering it dimensionally instable, room temperature or near room temperature processes are preferred. Low thermal distortion materials such as silicon carbide and low thermal expansion glass ceramics are bonded at room temperature using hydroxide catalysis bonding with a silicate bonding material. The bonding procedure is explained and fracture toughness results are presented for SiC–SiC, Zerodur–Zerodur and SiC–Zerodur bonds. A surface treatment technique for hydrating the SiC surface is presented, which eliminates the need for pre-oxidized SiC surfaces when using HCB bonding. The bonds between surface treated bare SiC surfaces and thermally oxidized SiC surfaces are found to have comparable fracture toughness.

Chemical bonding of ultra low expansion glass substrates with aqueous NaOH solution

We report on our research towards determining a reliable and reproducible method of bonding ULE ® components for space-based instruments. Such bonded components are required to withstand considerable accelerations during launch and can be exposed to significant temperature variations. The results of tensile and shear testing of a large number of bonded samples are reported, as well as the impact of thermal cycling on bond strength. A study on the feasibility of de-bonding and re-bonding ULE parts is also described.

Low Energy Xe+ Ion Beam Machining of the Ultralow Expansion Glass Substrates for Aspherical Extreme Ultraviolet Lithography Projection Optics

Aspherical substrates for extreme ultraviolet lithography (EUVL) optics require an ultrahigh shape accuracy of less than 0.15 nm rms and a high-spatial frequency roughness (HSFR; spatial wavelength: less than 1 µm) of 0.12 nm rms. Generally, the ultra low expansion glass (ULE®) substrate with HSFR of 0.06–0.08 nm rms can be produced by mechanical machining methods. However, it is difficult to obtain the shape accuracy of less than 0.12 nm rms using mechanical machining methods. Therefore, ion beam figuring (IBF) may be adapted to final shape correction of the substrates for the projection optics of EUVL tools. In this study, we investigated the HSFR and machining rate of the ULE® substrate machined by a 0.3–1.0 keV Xe+ ion beam at off normal ion incidence angles and obtained the following results: the HSFRs of the ULE® substrate machined by a 1.0 keV Xe+ ion beam at a ion incidence angle of lower than 30° and a 0.3–0.5 keV Xe+ ion beam at an ion incidence angle of 0–45° are below 0.12 nm rms, which is smaller than the required HSFR specification of EUVL projection optics. From our experimental result and discussion, we concluded that the scan fine beam and tilt target mode smoothing for processing of the ULE® substrate meets the required specification of the HSFR (0.12 nm rms) of hemispherical ULE® substrates of EUVL projection optics.

Tuning the thermal expansion properties of optical reference cavities with fused silica mirrors

We investigate the thermal expansion of low thermal noise Fabry–Pérot cavities made of low thermal expansion (LTE) glass spacers and fused silica (FS) mirrors. The different thermal expansion of mirror and spacer deforms the mirror. This deformation strongly contributes to the cavity’s effective coefficient of thermal expansion (CTE), decreasing the zero crossing temperature by about 20 K compared to an all-LTE glass cavity. Finite element simulations and CTE measurements show that LTE rings optically contacted to the back surface of the FS mirrors allow to tune the zero crossing temperature over a range of 30 K.

Diode-laser system for high-resolution spectroscopy of the2S1/2→2F7/2octupole transition in171Yb+

A diode-laser system at $467$ nm is built in order to drive the ${}^{2}{S}_{1/2}\ensuremath{\rightarrow}{}^{2}{F}_{7/2}$ electric octupole transition at $467$ nm in ${}^{171}{\mathrm{Yb}}^{+}$. The frequency of the laser is stabilized to a reference cavity made of ultra low expansion glass and is demonstrated to have a relative instability of better than $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ at $1$ s and a stable linear drift rate with variations below $10$ mHz/s over several hours. The system is applied for spectroscopy of a single trapped laser-cooled ${}^{171}{\mathrm{Yb}}^{+}$ ion. We obtain excitation spectra of the octupole transition with a resonant excitation probability of about $65%$ and an essentially Fourier transform-limited resolution of $13$ Hz.

The effect of aluminum sources on synthesis of low expansion glass–ceramics in lithia–alumina–silica system by sol–gel route

This paper investigated the effect of different aluminum sources on the crystallization behaviors, thermal expansion and morphology of lithium aluminosilicate glass–ceramics sintered at different temperatures. Specimens were prepared by sol–gel technology. The crystalline phase in the specimen with Al(NO 3) 3⋅9H 2O aluminum source sintered at 1300 °C has shown a major phase of Li 2Al 2Si 3O 10 accompanied by minor phase of LiAl 5O 8 and trace amount of SiO 2. The β-eucryptite appeared as the only phase in the specimens with aluminum isopropoxide or aluminum isopropoxide mixed with Al(NO 3) 3⋅9H 2O aluminum sources sintered at 1300 °C. The sintering temperature has significant effect on the thermal expansion behavior of all the specimens. SEM indicated that microstructures were spherulitic crystals and no distinct differences were observed in the specimens with different aluminum sources.

Temperature analysis of low-expansion Fabry-Perot cavities

Temperature dependent structural distortion at the contacted mirrors of low-expansion glass cavities can introduce changes to the cavity length independent of the length of the spacer. There are resulting temperature sensitivities of the path (m/K) at each end of a cavity that are proportional to the difference of the coefficient of thermal expansion (alpha) at the contact. The temperature sensitivity of the resonant frequency can be a minimum at a temperature T(C) if the product of length times alpha(T(C)) of the spacer approximately offsets the combined sensitivities of the ends, even if alpha(T(C)) of the spacer is significantly nonzero.