Photonic circuit design, coming of age in a fabless ecosystem

April 04, 2016 // By Iñigo Artundo
Optical integration is a technology that enables the miniaturization of multiple optical components like lasers, photodetectors, splitters or filters into a single photonic integrated chip.

While it is still lagging about 30 years behind electronic integration in terms of maturity, it is a quickly evolving technology. It experienced its greatest development at the telecom bubble around 2000, where millions of passive optical components for fiber networks started to be integrated into planar lightwave circuits (PLC) made out of silica.

Nowadays, there are several mature material platforms available for fabless chip development, each of them excelling at different features: PLC because of its low loss and low cost passive circuits, silicon (Si) because of its compactness and CMOS compatibility, indium phosphide (InP) because of its capability of generating and amplifying light on a chip, and silicon nitride (Si3N4) because of its low loss and compactness.

Fig. 1: Photonic integrated circuit fabricated in Si3N4 for sensing applications with metallic micro-heaters.

While InP and Si platforms are usually optimized to operate at the optical fiber telecommunication wavelength ranges of C-band (around 1550 nm) and O-band (around 1310 nm), PLC and Si3N4 can also work on the visible wavelength range, down to 400nm where many sensing and biomedical applications operate.

Once the material platform has been selected to meet the requirements of the target application, designers must select a specific foundry, or let an experienced design house like VLC Photonics assist them in choosing the most appropriate one. The design of a photonic integrated circuit starts by solving the optical modes that will be guided along the circuit depending on the waveguide geometry.

Fig. 2: Compact interferometers for quantum optics integrated on a silicon photonics microchip.

A thorough frequency domain analysis is usually performed at this stage, to calculate optical parameters like dispersion, group velocity, group index, propagation loss, effective refractive index, etc., considering certain boundary conditions (periodic, symmetric, asymmetric, metallic, etc.). Common methods for this are FD (Finite differences), FMM (Film Mode Matching), FEM (Finite Element Method), Correlation Method or Gaussian mode fiber solver, and there are several commercial software tools that implement them, like PhoeniX Software or Photon Design.