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What is optical thin film design?
"Optical thin film design" is a blue print of the structure and materials of optical coating based on the principles of interference. This blue print determines total number of thin film layer, coating materials, layer pattern, layer thickness, coating process, etc. in order to realize the required optical characteristics requested by customers.
Main procedure of optical thin film design
| Consideration of the intended application of the filter, the mechanical strength, environmental durability, optical specification, difficulty, etc. |
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Determination of
coating process |
| Consideration of its refractive index, film stress, mechanical strength, optical characteristics, uniformity, etc. with respect to various materials options. |
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Determination of
coating materials |
| Considering specification, repeatability ease of production, chemical durability, etc. |
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Determination of
coating pattern |
| Calculating transmittance rate, reflection rate considering filter type such as BPF, LPF, SPF, AR, etc., and calculating the optimal thin film thickness setting required, with the specification as target, using OCJ's simulation software. |
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Determination of each
layer thickness |
Main theory used for optical thin film design
There is a theoretical basis in order to design optical filter by optical calculation and optimised simulation. We are using very famous theories such as Snell's law, Young's experiment on interference, Maxwell's equation, etc., which are well known even to those who are not optical coating engineers. We are even astonished to learn those were already established at the end of 19th century. Based on those theories a matrix method will be used for analysing the performance of optical multi layer coating. Reflection rate, transmission rate, absorption rate and phase change of optical multi layer coating can be given by vector elements using a matrix method. Let us explain this part a little further here. A complex multiplexed reflection will occur since each layer has reflection at each boundary of an optical multi layer coating, which is made of several materials with different refractive indices and thicknesses. Total overall transmission rate T of whole multi layer coating will be given by the next formula which multiplies 2 x 2 transfer matrix in order to satisfy the boundary conditions of electric and magnetic fields at each layer.
Most of the current analytical calculation program for the optical performance of multi layer coating is based on this method. OCJ also adopts this in our own simulation software.
Other than this method, there is an automatic design method which gives the optical multi layer structure to satisfy the required optical characteristics. There are many optimal theories for this automatic method such as Simplex method, hereditary algorithm, needle method, etc., each of which has different advantages and features. Many engineers and researchers worldwide are now studying the optimized theory. OCJ designs filters using both those optimizing methods and the calculation method.
OCJ design technology is :
When you seek the design value from design specification, it may have only one answer, or no answer, or several answers. It is also true that it still has a dependency on the initial value though the optimizing theory is improving. Optical design is more important for making a filter stably with good reproducibility not only satisfying the required optical specifications. OCJ has accumulated a huge database of design pattern, experience, know-how, and coating materials over 45 years business. OCJ designs filters with optical simulation software developed in -house besides that available in the market.
From design to coating
Recently, many advanced coating machines have come out in the market. However, the optical spec achieved does not always satisfy the target design value even using such high spec coating machines. This occurs even more the more stringent the spec. Filters are usually made by trial and error iterative methods, feeding back the results calculating tooling factor and optical constants for all the coating materials, which may be required for each coating machine.
The following is a comparison chart of design value and actual value.
Product : Beam Splitter
Coating materials : Ta2O5/SiO2
Coating process : Ion Assisted Deposition
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