Thermal Modelling of Silicon Photonic Ring Modulator with Substrate Undercut

Ring modulators for silicon photonic (SiPho) optical transceivers are extremely thermally sensitive and require thermal tuning for stable operation. This tuning is achieved with integrated heaters and in this work two aspects of the heaters are investigated: firstly, how to thermally model the heater-waveguide system in a SiPho ring modulator. Secondly, how to improve the energy efficiency by adapting the design in order to minimize the thermal tuning power consumed by the heater. Silicon substrate undercut (UCUT) is done in order to achieve out-of-plane thermal isolation and prevent heat flow directly into the Si substrate below the device. Introducing the UCUT increases the heater efficiency (
$\eta _{h}$
) with a factor X3.08. It is shown that the influence of the UCUT cross section on the heater efficiency can be neglected, only the total area and metals significantly influence the thermal behaviour. Comparison between electro-thermal and pure thermal simulation shows very similar behaviour. Finally, the UCUT causes an increase in thermal time constant with factor X3.68.