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.
Interferometric measurements are often used to assess the homogeneity of optical glass. In order to determine the uniformity of refractive index of optical glass due to chemical fluctuations within the glass, residual stress from thermal processing must be low. We provide data showing the effects of residual stress, resulting from the annealing process, on the transmitted wavefront used to grade optical glass quality.
The performance and utility of optical systems can be significantly improved by using lightweight stable mirror components. Such components have been incorporated into a mirror assembly through a process that utilizes low temperature fusion to bond low CTE faceplates onto a lightweight core structure. The core structure is fabricated using an abrasive waterjet cutting technique that enables the designer to optimize core geometry to enhance the structural performance of the blank. The faceplates are bonded to the lightweight core resulting in a stiff structural mirror. A 15.125 inch Zerodur mirror was fabricated using this unique process and subjected to a test program to measure the optical stability of the mirror in support of either space-based applications. Strength testing was performed to verify the integrity of the fused joints and determine appropriate design allowable stress. A thermal test program was designed to assess performance at various temperature extremes and a mechanical load test was run to verify the capability of the blank to withstand operational loads without degrading the optical surface. The surface figure of the part was measured before, during, and after a 200 degree F temperature cycle with no change in figure quality. In addition, the mirror was subjected to a 10 g load for ten minutes with no change in the figure quality.
In large laser systems such as NOVA at Lawrence Livermore National Laboratory the active laser glass is surrounded by a cladding glass. The purpose of this glass is to absorb 1.06 micrometers laser light and prevent parasitic laser action from occurring due to amplified spontaneous emission (ASE). Currently, the cladding glass utilizes the base composition equivalent to the active laser glass with copper doping. The copper produces the required absorption coefficient, approximately 2.8/cm. The cladding glass has a high coefficient of thermal expansion which results in the degradation of the optical properties of the laser disc due to thermally induced strain. To eliminate this problem the concept of a near zero expansion glass-ceramic cladding material was developed.
The ability to vary glass properties by adjusting composition continues to make glass a leading material for application as both active and passive elements, in bulk as well as in guided-wave laser systems. We consider here how glass is engineered for specific intracavity and extracavity laser applications. Mention also is made of process and manufacturing techniques which result in glasses with improved or special properties critical for applications involving laser systems.
Historically, optical glasses were developed to provide desired optical characteristics; specifically, refractive index and dispersion. The batch composition required to achieve the optical performance demanded by industry sometimes resulted in glasses with weak chemical resistance. Due to the many different chemical constituents found in optical glass, it is impossible to define a single test method which can suffice as a means of describing the chemical behavior of all glasses. Since even a slight attack of the surface layer can render the optical element totally useless, the processor must gather and evaluate all the information available concerning the chemical behavior of the glass in order to minimize the risk of disastrous surface changes during processing.
Optical properties, especially refractive index, dispersion, and transmission, are discussed by glass family. The uniqueness of these properties is determined by the composition; an understanding of which makes evident why there often must be a compromise between the desired optical properties and undesirable secondary properties such as size limitations, glass quality, workability, and chemical resistance.
Hostile environments created by short wavelength electromagnetic radiation (UV, X-ray and gamma-radiation) or from particle fluxes (alpha-particles, beta-particles, protons, and neutrons), can produce discoloration within optical glasses. The associated loss in transmission is detrimental to the performance of any optical system and must be eliminated or reduced to a manageable level. For applications within these hostile environments, radiation-stabilized optical glasses have been developed. To optimize system performance, optical glasses which have been stabilized for applications within the particular radiation environment must be selected. If the environment is a mixture of radiation fields, compromises are called for.
18642 With the development of new bright narrow line sources in the form of lasers (see Table I) and better detectors the classical concept of optical glass has been expanded to include new glasses for applications in the ultraviolet and the infrared spectral regions. I would like to review briefly some recent developments in the spectral range from far UV to the end of the atmospheric window at 12 2 .
Interest in nonlinear optics has been growing rapidly. Today many materials are known to possess nonlinear effects and the intense search for even more efficient and cost-effective materials is continuing. Various crystals are known for large nonlinear coefficients however their limitations in size and high cost make other solid state materials such as glass more attractive despite their relatively smaller non-linear coefficients. We review the research on glasses with high nonlinearity by classifying them into homogenous type and heterogeneous type. 1.
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