Certain glasses in the As-S and As-Se binary and As-S-Se ternary systems satisfy the challenging combination of optical and physical properties that describe an athermal solid Fabry-Perot etalon. This means that, in the investigated temperature range (roughly 25-35<sup>o</sup>C), interference fringes undergo very little movement in resonant wavelength as the temperature changes (i.e. < 5 ppm/K). Further, by virtue of their unusual combination of properties, this suite of glasses will occupy unique coordinates in the “glass map” approach used by optical designers to fabricate thermally- and chromatically-corrected lens systems.
Retina-safe operation in open-air is of high interest to the next generation of lasers that are being utilized for many industrial, defense and medical applications. Those wavelengths that are considered to be the best for retina safe operations (also called eye-safe) fall in the range between 1400nm and1800nm. This wavelength region also coincides with the low loss window of fused silica fibers used for optical fiber communications , where the S and C bands near 1500nm are heavily utilized for long range communications due to the lowest attenuation losses possible in the fiber. The trivalent Er ion can produce direct emission into the 1540 nm wavelength, thus, it is the rare-earth emitter of choice for many eye-safe applications. In recent years, the need for high beam quality under passive operation in open air applications have renewed interest in Er-doped bulk glasses as the gain material of choice for solid-state eye-safe lasers.
The need for performance stability under a broad operating range from -400C to 1000C without active cooling is a challenge for amorphous gain materials. Moreover, there is very little known about how temperature may affect performance. In this study, we describe our first attempts to understand material behavior by systematically analyzing temperature driven variations exhibited in absorption and emission from the commercially available gain materials. As part of these investigations, we will also present our method for assessing quantum efficiency through measurements for critical evaluation from laser community at large.
SCHOTT has introduced IRG 27, a classic chalcogenide glass (As<sub>2</sub>S<sub>3</sub>), possessing transparency from approximately 0.65 μm to well into the long-wavelength IR (> 10 μm). Additionally, this glass exhibits an extremely small thermo-optic (dn/dT) value for most of its transparency range, a critical attribute in certain applications. Relatively low density and a good thermal expansion match to aluminum are further useful characteristics of IRG 27.
This paper focuses on the three main effects that can induce wave-front distortion due to thermal lensing in laser gain media: 1) thermo-optic (<i>dn/dT</i>); 2) stress-optic; and 3) surface deformation (e.g., “end-bulging” of a laser rod). Considering the simple case of a side-pumped cylindrical rod which is air- or water-cooled along its length, the internal temperature distribution has long been known to assume a simple parabolic profile. Resulting from this are two induced refractive index variations due to thermo-optic and stress-optic effects that also assume a parabolic profile, but generally not of the same magnitude, nor even of the same sign. Finally, a small deformation on the rod ends can induce a small additional lensing contribution. We had two goals in this study: a) use finite-element simulations to verify the existing analytical expressions due to Koechner1 and Foster and Osterink; and b) apply them to glasses from the SCHOTT laser glass portfolio. The first goal was a reaction to more recent work by Chenais et al. who claimed Koechner made an error in his analysis with regard to thermal stress, throwing into doubt conclusions within studies since 1970 which made use of his equations. However, our re-analysis of their derivations, coupled with our FE modeling, confirmed that the Koechner and Foster and Osterink treatments are correct, and that Chenais et al. made mistakes in their derivation of the thermally-induced strain. Finally, for a nominal laser rod geometry, we compared the thermally-induced optical distortions in LG-680, LG-750, LG-760, LG-770, APG-1, and APG-2. While LG-750, -760, and -770 undergo considerable thermo-optic lensing, their stress-optic lensing is nearly of the same magnitude but of opposite sign, leading to a small total thermal lensing signature.
The glass ceramic ZERODUR® from SCHOTT has an excellent reputation as mirror blank material for
earthbound and space telescope applications. It is known for its extremely low coefficient of thermal expansion
(CTE) at room temperature and its excellent CTE homogeneity. Recent improvements in CNC machining at
SCHOTT allow achieving extremely light weighted substrates up to 90% incorporating very thin ribs and face
sheets. In 2012 new ZERODUR® grades EXPANSION CLASS 0 SPECIAL and EXTREME have been released
that offer the tightest CTE grades ever. With ZERODUR® TAILORED it is even possible to offer ZERODUR®
optimized for customer application temperature profiles. In 2013 SCHOTT started the development of a new
dilatometer setup with the target to drive the industrial standard of high accuracy thermal expansion metrology
to its limit. In recent years SCHOTT published several paper on improved bending strength of ZERODUR® and
lifetime evaluation based on threshold values derived from 3 parameter Weibull distribution fitted to a multitude
of stress data. ZERODUR® has been and is still being successfully used as mirror substrates for a large number
of space missions. ZERODUR® was used for the secondary mirror in HST and for the Wolter mirrors in
CHANDRA without any reported degradation of the optical image quality during the lifetime of the missions.
Some years ago early studies on the compaction effects of electron radiation on ZERODUR® were re analyzed.
Using a more relevant physical model based on a simplified bimetallic equation the expected deformation of
samples exposed in laboratory and space could be predicted in a much more accurate way. The relevant
ingredients for light weighted mirror substrates are discussed in this paper: substrate material with excellent
homogeneity in its properties, sufficient bending strengths, space radiation hardness and CNC machining
Laser performance measurements (quasi-CW) were made of various Nd-doped SCHOTT catalog laser glasses: LG-680,
LG-750, LG-760, LG-770, APG-1, and APG-2; all but the first, a silicate, are phosphate glasses. Nominal Nd<sup>3+</sup> doping
was approximately 3 × 10<sup>20</sup> ions/cm<sup>3</sup>. An end-pumped, laser diode geometry was used and input powers, pump pulse
length, and pump rep-rates were kept low to avoid thermal lensing (4 W, 1 msec, and 0.1 Hz, respectively). As
expected, the phosphate glasses performed better than the silicate glass. Slope efficiencies ranged from 25% for LG-680
up to nearly 33% for LG-760. APG-1, designed for high rep-rate, high-power systems, performed nearly the same in
this particular configuration as glasses designed for high-energy applications (e.g., LG-770).
SCHOTT has developed eye-safe laser glasses for laser range finder and medical/ biophotonics applications. The
development described herein covers various combinations of key ions, Er, Yb, and Cr, with and without Ce, at
controlled ratios and their perspective reduction - oxidation (REDOX) states to improve glass lasing, thermal lensing,
and thermo-mechanical stability for field-based applications under high repetition rate operation. This report covers glass
property characterizations and selective modeling results using statistically designed compositions.
The zero-expansion glass ceramic ZERODUR® from SCHOTT is widely used for ground-based astronomical mirrors
and in industrial applications. This paper points out that it is also well suited for satellite applications, especially with
respect to the space radiation environment. Recent developments show that highly lightweighted components can be
manufactured and that such structures are strong enough to survive launch vibrations. A series of thirty reference
applications, where ZERODUR® has been or is currently used (including METEOSAT, SPOT, ROSAT, CHANDRA,
and HST), demonstrate the high and long lasting performance of ZERODUR® components in orbit. The ongoing
successful missions and upcoming new satellites continue to enlarge the space heritage of this unique material.
Coefficient of thermal expansion (CTE) measurements using small Fabry-Perot etalons were conducted on high and low
thermal expansion materials differing in CTE by a factor of nearly 400. The smallest detectable change in length was
~10<sup>-12</sup> m. The sample consisted of a mm-sized Fabry-Perot etalon equipped with spherical mirrors; the material-under-test
served as the 2.5 mm-thick spacer between the mirrors. A heterodyne optical setup was used with one laser locked
to an ~780 nm hyperfine line of Rb gas and the other locked to a resonance of the sample etalon; changes in the beat
frequency between the two lasers as a function of temperature directly provided a CTE value. The measurement system
was tested using the high-CTE SCHOTT optical glass N-KF9 (CTE = 9.5 ppm/K at 23 °C). Measurements conducted
under reproducibility conditions using five identically-prepared N-KF9 etalons demonstrate a precision of 0.1 ppm/K;
absolute values (accuracy) are within 2-sigma errors with those made using mechanical dilatometers with 100-mm long
sample rods. Etalon-based CTE measurements were also made on a high-CTE (~10.5 ppm/K), proprietary glass-ceramic
used for high peak-pressure electrical feedthroughs and revealed statistically significant differences among parts made
under what were assumed to be identical conditions. Finally, CTE measurements were made on etalons constructed
from SCHOTT's ultra-low CTE Zerodur<sup>(R)</sup> glass-ceramic (CTE about -20 ppb/K at 50 °C for the material tested herein).
EUV substrate materials have to meet enhanced requirements with respect to extreme low thermal expansion, high homogeneity and superior surface quality. A SCHOTT R&D program aims at the development of advanced materials covering these various aspects. The glass-ceramic Zerodur (registered trademark) of SCHOTT represents a substrate material currently used for EUV masks and optics of first generation tools due to its extremely low coefficient of thermal expansion (CTE) and its excellent homogeneity. Zerodur(registered trademark) even allows continuous shifting of the position of zero crossing of the CTE-slope to control the thermal expansion behavior according to varying customer requirements: As a result of specifically adjusted process parameters, samples of Zerodur (registered trademark) exhibit a coefficient of thermal expansion CTE < 5 ppb/K corresponding to the lowest expansion class of the SEMI standard P37 (19 to 25°C) for EUV mask blanks. By further variation of process parameters, the position of zero crossing, e.g. at 22.5°C or 30°C, can be varied, revealing an attractive attribute feature of Zerodur (registered trademark).
A new dilatometer type reveals an improved reproducibility of ~ 1ppb/K in the temperature range of 0 to 50°C. A series of CTE(0;50°C) measurements with a test-cube of Zerodur (registered trademark) provides information on CTE homogeneity on a cm-scale: no CTE variation was observed within the error of measurements (1ppb/K) for a block exhibiting ± 3.5*10<sup>-6</sup> variation in refractive index. CTE variation can cause surface deformations during changing temperature conditions. A Fizeau-Interferometer was used to record surface roughness at two different temperatures. This non- destructive metrology is regarded as a method to distinguish CTE variation < 1ppb/K. The surface deformation of Zerodur (registered trademark) due to elevated temperature was determined to be lower than the resolution. Both methods to analyze the CTE homogeneity of Zerodur (registered trademark) lead to the result of CTE variation below 1 ppb/K.
Surface treatment of glass-ceramic material is a major challenge as final finishing of EUV substrates may increase roughness of super-polished surfaces significantly. Improved new glass-ceramic materials demonstrate optimization of glass-ceramic compositions to nearly meeting the specification of surface roughness after a standard finishing process.
Recent achievements of material development reveal CTE-performance of this new glass-ceramic to also be adjustable to varying customer needs as already known for Zerodur (registered trademark).
These results are regarded as a promising milestone to develop an optimized glass-ceramic material, because the features of the modified New-Glass Ceramic now better match the key requirements of EUVL substrate materials.
All materials undergo some degree of compaction or expansion when exposed to radiation. Multi-component materials are more susceptible to this effect than single-component materials (e.g., fused silica). Nonetheless, the much lower expansion characteristics of multi-component materials -- such as the ultra-low expansion glass-ceramic Zerodur -- preserves the attractiveness of such materials for applications that require superior dimensional stability. In this study, we present a reanalysis of experimental data describing the compaction effects of electron radiation on Zerodur. These data include high-dose, high dose-rate bulk density measurements as well as lower-dose, interferometrically-measured surface figure changes. We show that previous attempts to deduce linear compaction from figure changes are in error and in fact have precluded earlier attempts to predict radiation effects for an arbitrary optical geometry. By interpreting surface figure measurements in light of a more relevant physical model -- a simplified bimetal equation -- we are able for the first time to accurately predict expected deformation as a function of prescribed dose for both laboratory and space-based experiments. Moreover, we show that a real discrepancy exists between compaction estimates from bulk density experiments and those from surface figure measurements.
The enhanced demands for substrate materials for next-generation optics and masks have initiated detailed investigations on Zerodur as a proposed EUVL substrate material with focus on thermal expansion behavior and surface roughness. As a result of specifically adjusted process parameters, the coefficient of thermal expansion (CTE) was tailored to be a minimum at 22.5°C. Laboratory samples of Zerodur exhibit a CTE < 5 ppb/K corresponding to the lowest expasnion class of the SEMI standard P37 (19 to 25°C) for EUV mask blanks. By further variation of process parameters, the position of zero crossing, e.g. at 30°C, can be varied, revealing an attractive attribute feature of Zerodur. A new dilatometer type was mounted in 2002 with first operatinoal results revealing an improved reproducibility of ~1ppb/K in the temperature range of 0 to 50°C. A series of CTE measurements with a small block of Zerodur provides information on CTE homogeneity on a cm-scale: No CTE variation was observed within the error of measurements for a block exhibiting ± 3.5*10<sup>-6</sup> vairtion in refractinve index. CTE variation can cause surface deformations during changing temperature conditions. A first setup of Fizeau-Interferometer with a current resolution of 0.3 nm rms was used to record surface deformation of Zerodur due to elevated temperature was determined to be lower than the current resolution. Both methods to analyze the CTE homogeneity of Zerodur lead to the result of CTE variation below 1 ppb/K, still identifying today's need to improve metrology further. Final finishing of EUV substrates may increase roughness of super-polished surfaces significantly. Using appropriate processes a to surface roughness < 0.25 nm rms under production conditions can be achieved after final finishing of Zerodur. As an improved Zerodur-type material, recent achievements of material development demonstrate the optimization of glass-ceramic composition to nearly meeting the specification of surface roughness after a standard finishing process. These results are regarded as a promising milestone to develop an optimized glass-ceramic material providnig adjusted thermal expansion behavior and surface processability according to the specific demands of EUV technology.
The enhanced demands for substrate materials for next- generation optics and masks have initiated detailed investigations on Zerodur as a proposed EUVL substrate material. The dependence of thermal expansion of Zerodur on process parameters is illustrated herein as well as its utility for EUV substrate material demands. As a result of specifically adjusted process parameters, the coefficient of thermal expansion (CTE) was tailored to be a minimum at 22.5 degrees C. Laboratory samples of Zerodur exhibit a CTE corresponding to the lowest expansion class of the SEMI standard P37. By further variation of process parameters, the position of zero crossing, e.g. at 30 degrees C, can be varied, revealing an attractive attribute feature of Zerodur. A series of CTE measurements with a small block of Zerodur provides information on CTE homogeneity on a cm- scale: No CTE variation was observed within the error of measurements for a block exhibiting +/- 2 * 10<SUP>-6</SUP> variation in refractive index. A new dilatometer type is in the course of development. First operational results are expected in Summer 2002 with an increased accuracy < ppb/K in the temperature range of 17 to 30 degrees C.
A proprietary, inorganic, sol-gel joining technique was used to fabricate Zerodur-Zerodur-SiO<SUB>2</SUB> joints at temperatures <EQ 120 degrees C. The low temperature joining of such dimensionally stable materials is of interest for light- weighted mirror blank and micro lithographic applications. Rigid, stress-free joints were achieved within 10 minutes after sandwiching 3 (mu) L of silicate-containing sol-gel solution between two cleaned, polished surfaces, which had a surface figure of 200-300 nm. The average four point flexural strength of the resulting Zerodur-Zerodur joints was > 5000 psi, regardless of the heat treatment temperature from 25 to 120 degrees C. The strength of Zerodur-SiO<SUB>2</SUB> joints was 4500 psi. The Zerodur-Zerodur joints exhibited excellent dimensional stability perpendicular to the joint interface as there was no statistically significant difference between the coefficient of thermal expansion measured for joined and monolithic samples. The inorganic, sol-gel joining technique is an attractive technology that could be employed when fabricating light-weighted, dimensionally stable mirror blanks and microlithography stages.
Detailed thermal expansion measurements and internal homogeneity measurements of the glass-ceramic material Zerodur were undertaken to examine its usefulness for EUVL. Repeat measurements on 100-mm long samples from three castings exhibit an expansion of approximately 12 +/- 2 ppb/K 2 (sigma) in the temperature range of interest for EUVL, corresponding to Class C of the draft SEMI 3148 standard. Internal homogeneity measurements reveal extremely small refractive index variations, suggesting comparably small compositional variations. This in turn is viewed as a necessary but not sufficient condition for high CTE uniformity, a factor required by EUVL applications.