Thin Film Optical Coatings
Within the scientific conception of the modern world, thin film optical coatings can be interpreted as one-dimensional photonic crystals. In general, they are composed of a sequence of single layers which consist of different transparent dielectrics with
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Thin Film Opt 6. Thin Film Optical Coatings
6.1
Theory of Optical Coatings ..................... 374
6.2
Production of Optical Coatings ............... 6.2.1 Thermal Evaporation .................... 6.2.2 Ion Plating and Ion-Assisted Deposition.......... 6.2.3 Sputtering ................................... 6.2.4 Ion-Beam Sputtering.................... 6.2.5 Chemical Vapor Deposition (CVD) .... 6.2.6 Other Methods ............................. 6.2.7 Process Control and Layer Thickness Determination
378 379 381 382 384 384 386 386
6.3 Quality Parameters of Optical Coatings................................ 388 6.4 Summary and Outlook........................... 391 References .................................................. 393 In order to keep pace with the rapid development of optical technology, innovations in the design, deposition processes and handling of optical coatings are some of the crucial factors. Also, high demands in respect to precision and reproducibility are imposed on the control of layer thickness during the production of the coating systems. For certain applications in fs lasers or optical measurement systems the individual layer thickness has to be controlled within the sub-nanometer scale, which can be only achieved on the basis of advanced in situ monitoring techniques of the growing layers. These skills have to be complemented by extended knowledge of characterization, because optimization and marketing of optical coatings can only be performed on the basis of reliable and standardized characterization techniques. The present chapter addresses these major aspects of optical coatings and concentrates on the essential topics of optical coatings in their theoretical modeling, production processes, and quality control.
Part A 6
Within the scientific conception of the modern world, thin film optical coatings can be interpreted as one-dimensional photonic crystals. In general, they are composed of a sequence of single layers which consist of different transparent dielectrics with a thickness in the nanometer scale according to the operation wavelength range. The major function of these photonic structures is to adapt the properties of an optical surface to the needs of specific applications. By application of optical thin film coatings with optimized designs, the spectral characteristics of a surface can be modified to practically any required transfer function for a certain wavelength range. For example, the Fresnel reflection of a lens or a laser window can be suppressed for a broad wavelength range by depositing an antireflective coating containing only a few single layers. On the basis of a layer stack with alternating highand low-refracting materials, high reflectance values up to 99.999% can be achieved for a certain laser wavelength. In addition to these basic functions, optical coatings can realize a broad variety of spectral filter characteristics according to even extremely sophisticated demands in modern precision optics and laser technology. Moreover, recent developments in optic
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