RP Coating – Design of Optical Multilayer Structures
RP Coating is a powerful software for designing all kinds of optical multilayer structures, e.g.
- dielectric mirrors (laser mirrors), including chirped mirrors (e.g. most advanced double-chirped mirror designs with extremely high bandwidth) or other kinds of dispersive mirrors (e.g. GTIs)
- anti-reflection coatings, including advanced multilayer structures, found e.g. with a Monte-Carlo technique
- fiber Bragg gratings
- semiconductor structures, such as SESAMs, including saturable absorption characteristics, chromatic dispersion, etc., and also VECSEL gains structures
It has been developed by Dr. Rüdiger Paschotta. So far, it is not for sale, but allows RP Photonics to do a wide variety of calculations within consulting contracts, requiring a rather limited amount of time. Note that Dr. Paschotta has a very deep experience with the physics of multilayer structures, interference effects, dispersion, etc.
Main Features
- RP Coating offers different methods to define a multilayer structure. Apart from simply listing all layer materials and thickness values one by one, it is also possible to specify a structure with equations (e.g. to calculate a chirped mirror structure according to a few given parameters) or read a structure from a file.
- The program can calculate a large number of relevant properties of a multilayer structure, including its reflection and transmission amplitudes and phases (with a variable angle of incidence), chromatic dispersion of any order, dissipation in absorbing layers, field distribution inside the structure, etc.
- The software allows for various kinds of optimizations. A figure of merit can be defined which may refer to arbitrary combinations of different properties, including fabrication error tolerances. The optimization can be a local one or use a Monte-Carlo algorithm. It can affect all layer thickness values separately, or some parameters which indirectly determine a layer structure.
- By fitting to experimental data, deviations from specified thickness values or material data can be calculated.
- All results can be delivered in many different forms, including various kinds of plots and tabulated data, e.g. for exporting data to other software.
You will hardly find a competing software which is similarly flexible and suitable for such a wide range of applications.
Examples of Graphical Output
The following graphs have all been made with RP Coating and illustrate some of its features.
Bragg Mirror
This example shows the characteristics of a simple Bragg mirror for reflection at wavelengths around 1000 nm. The group delay dispersion is also shown; it is small over much of the reflection range, but becomes large in the wings.
Dual-Wavelength Anti-Reflection Coating
A four-layer anti-reflection coating on BK7 glass has been designed with RP Coating for minimum reflectivity at 1064 nm and 532 nm. The thick black curve shows the reflectivity spectrum for the nominal design, while the thin gray curves indicate spectra for random deviations of 2% rms from the design due to fabrication errors.
Double-Chirped Dispersive Mirror
This example is fairly demanding, and would be hard to realize with most commercially available design software. A double-chirped mirror has been designed to compensate the dispersion of 5 mm of fused silica (dotted curve) in the wavelength range from 1000 nm to 1200 nm, and to achieve a high reflectivity over the same wavelength range. A sophisticated optimization algorithm is needed for finding such a high performance design within a reasonable time. With its high flexibility, RP Coating allowed to implement a very efficient strategy – in the script language, not on the source code level. The beginning can be a course optimization, operating not on the single layer thickness values but rather on a very limited number of parameters determining the whole structure via a formula. In a second step, the single thickness values are further refined, and excellent performance is achieved. On an ordinary PC, the whole process takes at most a few minutes, while other software often requires many hours.
The plot above shows the reflectivity spectrum and the group delay dispersion, while the one below indicates the field penetration into the structure as a function of wavelength. This shows that within the wavelength range of 1000-1200 nm, the field penetrates more deeply in the structure for longer wavelengths, and this causes the anomalous dispersion.
Semiconductor Saturable Absorber Mirror
RP Coating can also be convenient applied to the design of semiconductor saturable absorber mirrors (SESAMs). The following plots apply to a simple antiresonant design consisting essentially of a Bragg mirror and an absorber structure with a single quantum well absorber, the position of which is indicated with a vertical gray line in the first plot. The black curve shows the refractive index distribution, indicating the Bragg mirror at the left-hand side and the thin absorber (at the gray vertical line). There is a local field maximum at the absorber.
The second plot shows the reflection spectrum (where the small saturable absorption is not apparent) and the field intensity in the absorber layer. This shows that the design can work over the full reflection range.
