To model a photonic component, you need to consider material and optical properties. At the device level, these properties are usually:
- The refractive index of the material stack versus the wavelength, represented by the real and imaginary part of it (also known as optical performance)
- Values of pre-characterized components or validated components
- Analytical models to predict the performance (when real data are not available)
The material and optical properties for the photonic component can be represented as a building block by the S-matrix; the S-matrix describes the signal transfer between the ports of the component from the device level to the circuit/layout level to simulate the PIC.
These building blocks are represented as white and black boxes. The black box is represented by a geometric shape, usually a rectangle with input and output ports, and it contains linear and non-linear models (or values from pre-characterized components) that represent the performance of the component; they are called black boxes because the designer can see only the input and output ports, while the foundry owns the relevant information of the component for fabrication. This information is loaded into the white box component when the foundry is assembling the mask (putting together all the photonic integrated circuit layout designs in the wafer) for fabrication. White and black boxes protect the foundry’s intellectual property because its process flow (masks, layers, and materials) cannot be seen by the designer.
Foundries use different material platforms, such as silicon photonics, InP, LiNbO3, polymer, and glass. Each material platform and foundry has (up to now) its own process flow or fabrication process which, based on the material and optical properties, determines the performance of the PICs at the physical level.
Designing a photonic integrated circuit (PIC) is not straight forward; its performance is linked to material and optical properties, which in turn are linked to geometrical shapes (light travels more or less efficiently in different geometries). The ability to design an efficient PIC comes with experience. There is a vast library of examples and papers that describe performance and design properties to help improve PIC designs. The following figure shows how a designer can create a very complex integrated circuit with hundreds of components from the basic and composite building blocks of a PDK. The layout shown on the right is a MDM (Mode Division Multiplexer).