Thermal challenges in Photonic Integrated Circuits

Photonics Integrated Circuits (PICs), a feature of contemporary optical communications technologies, can represent a stringent packaging challenge, particularly in terms of their requirements for thermal control. Devices such as laser arrays can demonstrate tight temperature limits, sub-ambient operating temperatures, moderate heat loads but high device-level heat fluxes. A key feature of many hybrid PICs is a multi-layer substrate which offers mechanical support, electrical interconnection and heat spreading for the devices that it carries; such substrates are typically mounted on a thermoelectric module (TEM) to achieve thermal control. This paper considers two thermal challenges associated with such PICs, in order to ensure their efficient operation: the requirement for aggressive heat sinking; and the imperative of adequate heat spreading within the substrate on the temperature controlled side of the TEM. To this end, a closed-form electrothermal model is developed for a representative PIC which captures the conductive heat transfer within the substrate, coupled with a constitutive representation of the TEM and its heat sink. An example of a laser array PIC is considered in order to illustrate the importance of heat sinking and spreading. This paper represents some initial results of an extensive programme of work on packaging-related aspects of next-generation PICs.