In photonic integrated circuits, there are various photonic devices. For example, several waveguides, cavities, channel drop filters and connecting parts for the fibers can be on one photonic integrated circuit. If there are two different devices, we must have an interface between them. Therefore we need to understand the interface phenomena of photonic devices. One of such interfaces is that of the hetero-structure photonic crystal waveguide(HS-PC-WG).
We have studied the transmission properties of HS-PC-WG which consists of two different triangular lattice air hole structure photonic crystal waveguides. They have the same lattice constant but different radii. First, we get the band diagram of the guide modes for each waveguide by the plane wave expansion method. Then, we calculated the transmittance and the reflectance of HS-PC-WG's by the finite-difference time-domain method. And we also suggest a model to explain the transmittance and the reflectance caused by the hetero-structure. The model is based on the concept of the effective phase refractive index and the effective group refractive index.
We show that the model agrees with the simulation results. We found that the model of effective phase refractive index shows the better agreement in the region where the wave transmits in both waveguides, while the model of effective group refractive index does in the region where the wave transmits in the first waveguide only. So the model may be helpful for the design of photonic integrated circuits.
And we have also found that the reflection in HS-PC-WG's doesn't wholly occur exactly at the interface. The reflection occurs while the wave experiences several periodicities after the interface. Some possible explanations with the band diagrams are given.
One possible application of HS-PC-WG is the band pass filter. For the guiding frequency region for both the first and the second waveguides, HS-PC-WG's have the transmittance nearly 1. For the frequency region guided only in the first waveguide, HS-PC-WG's have high reflectance. Therefore, HS-PC-WG's can be used as band pass filters with high performance.
