Optical Coatings

Optical coatings typically consist of thin films made up of single or multiple layers of either metallic or dielectric materials. When properly designed and fabricated, these coatings can dramatically modify the reflection and transmission properties of an optical component. The properties can be controlled from the deep UV to the IR with narrowband, broadband, or multi-band response, and can be polarization sensitive. Optical coatings can be applied directly to the surface of an optical component to tailor its reflectivity, as in the case of an optical mirror or beamsplitter. For other components, such as lenses, the applied coatings may simply improve their overall transmission properties by reducing surface reflectivity. When optical coatings are integrated into a monolithic component for the express purpose of controlling the spectral transmission of light, the component is referred to as an optical filter.

Optical Coating Process

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.

Types of  Coatings

Metallic 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 (sometimes referred to as optical interference 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 1). 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. A variation of the formula given by the Reflectivity Equation (see Optical Mirror Physics) can be used to estimate the maximum reflectivity of the dielectric coating which increases with a greater number of layers but is accompanied by a concomitant reduction in the spectral bandwidth. Dielectric coatings are ubiquitous and their applications are discussed below. Other optical coatings include metal-dielectric hybrid films such as those in cube beamsplitters and absorptive coatings made of organic materials such as those found in certain optical filters.

Scanning electron microscope image and schematic of an optical interference coating
Figure 1. 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, these coatings are incorporated in broadband beamsplitters and dichroic filters (color-selective mirrors that transmit and reflect particular wavelengths, see filters section below). 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 beamsplitters. 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, beamsplitter, window, to markedly improve its transmission efficiency. Based on the reflectivity equation, 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 2). 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.
L-I curves for a laser diode
Figure 2. Typical broadband anti-reflection coating in the UV and VIS spectral regions.

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