Infrared Optics FAQ

A: The individual layers that make up optical coatings are typically a few tens of nanometers to a few hundred nanometers in thickness, while a single optical coating can be comprised of several hundred layers. Consequently, the techniques used to deposit these layers require a high degree of precision. Generally, the process begins with surface fabrication to minimize surface roughness and sub-surface damage. It continues with surface cleaning and preparation and is followed by deposition of high-performance thin film designs. The deposition technologies include thermal evaporation, electron-beam, ion-assisted deposition, and advanced plasma deposition. The most appropriate coating technology for the intended product design depends on the operating environment, spectral requirements, physical characteristics, application requirements, and economic targets. The optical coating process is completed with comprehensive performance testing using sophisticated metrology tools.

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Metal coatings used on optical mirrors typically consist of a single layer approximately 100 nm thick. This ensures that the broadband high reflectivity properties of the metal due to the complex index of refraction are present. In order to provide greater tuning of the reflectivity and over specific wavelengths of interest, dielectric coatings are used . These coatings consist of alternating high refractive index (nH = 1.8 &#; 4.0) and low refractive index (nL = 1.3 &#; 1.7)

dielectric layers (see Figure 3). The thickness of each layer is chosen such that the product of the thickness and the index of refraction of the layer is λ/4.

Figure 4. Scanning electron microscope image (top) and schematic (bottom) of an optical interference coating shown on left. Reflection and transmission of light by a filter consisting of an interference coating (right).

Dielectric optical coatings are used in a myriad of ways. In addition to highly reflective dielectric mirrors (see Figure 3), these coatings are incorporated in broadband beam splitters and IR wavelength lenses. When light is incident at an angle to a surface, i.e., not normal incidence, the reflectivity becomes polarization sensitive. This allows dielectric coatings to be polarization selective and such coatings are used in polarizing beam splitters (see Section III.A.5). In addition to enhancing the reflectivity, dielectric optical coatings can also be used to reduce surface reflections in the form of broadband anti-reflection coatings. These coatings can be applied to any optical component, e.g., lens, prism, beam splitter, window, to markedly improve its transmission efficiency. The reflection from an air-glass (n2 &#; 1.5) interface gives a reflectivity of 4%, which can be reduced considerably with a broadband anti-reflection coating (see Figure 5).

Figure 5. Typical broadband anti-reflection coating in the UV and VIS spectral regions.

These reflectivities can be reduced even more to improve transmission in laser systems with multiple optical elements, saving valuable laser energy from being lost to surface reflections. This superior performance, however, is achieved at the cost of reduced wavelength range.

Ophir® Optics Optical Coating Capabilities

*Content credit: MKS Instruments Handbook: Principals and Applications in Photonics Technologies, by the office of the CTO. 1/. https://www.mks.com/mks-handbook

The growth in popularity and acceptance of Fourier transform infrared (FT-IR) spectrometers for use in quality assurance (QA) laboratories and on manufacturing floors is one of the major developments affecting industrial environments in recent years. FT-IR spectroscopy offers almost unlimited analytical opportunities in many areas of production and quality control. It covers a wide range of chemical applications, especially in the analysis of organic compounds. In addition to its more classical role in qualitative analysis, its use in quantitative determinations has grown due to the improvements in signal-to-noise performance coupled with the development of advanced statistical analysis algorithms. Thanks to its compact design and ruggedness, the instrumentation can be located in the analytical laboratory or near the production line. Low cost, speed, and ease of analysis make FT-IR a method of choice for many industrial applications, including the analysis of polymeric materials.

FT-IR spectrometers offer many advantages over other analysis techniques. The most important include a drastic reduction of the time needed for data acquisition, component specificity, and sensitivity. Other benefits include the internal wavelength calibration, which ensures the precision of the analysis. With constant improvement in computing power, modern spectroscopy software, and advanced chemometric methodology, FT-IR is becoming ever more prevalent in addressing a wide variety of commercial applications. From simple identifications using library comparisons to sophisticated quantitative analysis, method development, spectrometer operation, and data manipulation are both simple and powerful.

Developments in FT-IR instrumentation and dramatic changes in FT-IR sample handling techniques resulted in an extensive range of new accessories that simplify and, in many cases, eliminate tedious sample preparation. Many of these sampling techniques feature constant optical path-length, regardless of the sample volume or thickness, making reproducible quantitative analysis simple and elegant. Depending on the sampling interface, the spectrometers can be used for gas, liquid, or solid sample evaluations.

Here are some examples of how FT-IR spectrometers address the needs of QC/QA laboratories in the polymers industry:

Toluene Diisocyanate in Pre-polymer Mixtures

Toluene diisocyanate (TDI) is used in various resin blends in the manufacture of polymeric foams. The TDI concentration in the pre-polymer mixture affects the quality of the final product. Attenuated total reflectance (ATR) FT-IR spectroscopy can be used to quantitatively determine the TDI concentration in resin blends prior to polymerization to ensure product quality. Once the calibrated method is developed, analysis can be performed with a single keystroke.

Monitoring of Fluorination Level of Polyethylene

Chemically reinforced polyethylene is used in many industrial applications. Fluorination of the polyethylene surface is one of the processes for improving its performance. The fluorination level can be conveniently monitored using an FT-IR spectrometer, offering price and performance advantages over neutron activation analysis (NAA) and electron scatter analysis (ESCA).

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Hydroxyl Number in Glycols

Knowledge of the hydroxyl group content of glycols is important for predicting the functional characteristics of the products. The hydroxyl value relates to molecular weight, viscosity, extent of reaction, and other parameters important to and dependent on the final application. Assessment of this value can be quickly and easily done using FT-IR.

Check out the FT-IR Applications page of the FT-IR Spectroscopy Learning Center to access application notes and webinars describing how FT-IR spectroscopy can be applied across all phases of the product lifecycle including design, manufacture, and failure analysis.

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