Crystals Materials
Optical Thin Film Principles, Design Software, and Coating Techniques


The unique functionalities of optical thin films, such as anti-reflection, high reflection, or spectral splitting, are fundamentally based on the interference of light within the thin films. Typically, thin films are composed of alternating layers of high refractive index materials and low refractive index materials, such as oxides, metals, or fluorides. By setting the number of layers, thickness, and refractive index differences between layers, the interference of light between the layers can be controlled, thereby achieving the desired functionality.

For the example of an anti-reflection (AR) coating, the optical thin films thickness of the layers is typically set to one-quarter (1/4 QWOT) or one-half (1/2 HWOT) of the wavelength of light used. In the diagram below, where the refractive index of the incident medium is n0 and the refractive index of the substrate is ns, the refractive index of the coating material that can produce interference cancellation conditions can be calculated n f=sqrt(n0 * ns). The light reflected from the upper surface of the coating is denoted as R1, and the light reflected from the lower surface of the coating is denoted as R2. When the optical thickness of the coating is one-quarter of the wavelength, the optical path difference between R1 and R2 is half of the wavelength, satisfying the interference condition and resulting in interference cancellation.

AR Coating Theory

1. Optical Thin Film Design Software

To facilitate the design of thin film systems that meet various specific functionalities, thin film design software has been developed. These software packages integrate commonly used coating materials and their parameters, layer simulation and optimization algorithms, and analysis functions, facilitating the development and analysis of various thin film systems. Commonly used thin film design software includes:

A. TFCalc
TFCalc is a versatile tool for optical thin film design and analysis, capable of designing various types of coatings such as anti-reflection, high reflection, bandpass, spectral splitting, and phase coatings. TFCalc can design double-sided coatings with up to 5000 layers for single-sided coatings, supports input of coating formulas, and can simulate various types of light sources such as collimated beams and random radiation. Additionally, the software has optimization capabilities using methods such as extremum and variational calculus to optimize coating properties such as reflectance, transmittance, absorption, phase, and ellipsometric parameters. The software integrates various analysis functions including reflectance, transmittance, absorption, ellipsometric parameter analysis, electric field intensity distribution curves, coating reflection and transmission color analysis, crystal control curve calculations, coating tolerance and sensitivity analysis, yield analysis, etc. TFCalc is shown below:

TFCalc Coating Design Software Operation Interface

B. Essential Macleod
The transportation of vapor-phase deposition material must be carried out in a vacuum of less than 1.33x10-2Pa. This is done to: Essential Macleod is a comprehensive software package for optical thin film analysis and design, with a truly multi-document interface. It can meet various requirements in optical coating design, from simple single-layer coatings to rigorous spectral splitting coatings, and can also evaluate wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) filters. It can design from scratch or optimize existing designs, survey errors in the design, and is rich in features and powerful. The design interface of this software is shown in the figure below:

Essential Macleod Coating Design Software Operation Interface

C. OptiLayer
OptiLayer software supports the entire process of optical thin films: parameter-design-production-inverse analysis. It includes OptiLayer, OptiChar, and OptiRE, and there is also an OptiReOpt dynamic link library (DLL) that can enhance the functionality of the software. OptiLayer examines the evaluation function from design to target, optimizes to achieve design goals, and performs pre-production error analysis. OptiChar examines the difference function between the spectral characteristics of the layer material under various thin film theories and its measured spectral characteristics, obtains a layer material model that fits well with reality, and identifies the factors that affect the current design, indicating which factors need to be considered when designing with this layer material. OptiRE examines the spectral characteristics of the design model and the experimentally measured spectral characteristics after production, and obtains some errors generated during production through engineering inversion, which are fed back to the production process to guide production. Through the dynamic link library function, the above modules can be linked together to achieve functions such as design, modification, and real-time monitoring in a series of processes from thin film design to production.

2. Optical Coating Technology

Internal Chambers of Vacuum Coating Machines

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Key words: Coating Service; AR Coating; Optical Coating Technology