To handle ever-increasing traffic volumes, data centers require optical transceivers with multiwavelength light sources. Increasing the number of communication channels needed to receive and transmit data efficiently can entail large laser arrays, and scaling laser array size can increase thermal crosstalk, which can affect laser efficiency and reliability. To minimize self-heating and thermal crosstalk, thermal-aware laser array designs are essential. A team from imec led by researcher David Coenen investigated the performance tradeoffs in large laser array design and developed a thermo-optic laser model that can be used with different approaches to laser array configuration. Imec’s 300-mm silicon photonic wafer with indium phosphide (InP) laser dies bonded on top. Courtesy of imec. The researchers conducted a thermal scaling analysis on hybrid, flip-chip, integrated indium phosphide-on-silicon (InP-on-Si) lasers. They experimentally validated a finite element thermal model of a single gain section laser by measuring the laser’s thermal resistance, and extrapolated the results to accommodate multi-section operation. The team looked at several parameters to identify the most energy-efficient, reliable configuration for large laser arrays, with the smallest footprint. It considered the number of lasers that could fit in one die, laser die size, output power per laser gain, ambient temperature, thermal management strategy, and integrated versus external light sources. The researchers also explored how adding a top-side heat sink and increasing laser length and width could impact laser array efficiency. The researchers used the results of their 3D simulation to build a compact, coupled thermo-optic model of a large, multiple-laser array that was sensitive to thermal crosstalk. They applied the model to a test case with eight wavelength division multiplexing channels and eight ports, requiring a transceiver with 64 output channels. They found that achieving an optimal footprint and energy efficiency with a laser array design depended on the ambient temperature of the array and whether the light source was configured to be integrated or external. Two variants of a laser array designed by imec researchers to be sensitive to thermal crosstalk. Courtesy of imec. The team observed that a smaller array area significantly increased the thermal crosstalk and temperature, indicating that there is a tradeoff between the size of the laser array and overall thermal resistance. By increasing the length of the laser, the researchers were able to generate more light per gain section and decrease laser thermal resistance. However, an increase in the gain section induced additional optical losses. Increasing laser width, and putting more lasers in one die, was found to increase thermal crosstalk significantly. External lasers, which must overcome fiber coupling losses, did not tolerate high ambient temperatures well, and had more difficulty achieving the required output power. However, an external laser can be thermally decoupled from any high-powered electronic chip — for example, from a network switch with co-packaged optics. The study offers insight into the advantages and drawbacks of large laser array design. It also provides tools to evaluate the impact of design choices and key performance metrics. The modeling framework demonstrated by the researchers can be applied to diverse types of lasers and systems. The research was published in the IEEE Journal of Selected Topics in Quantum Electronics (www.doi.org/10.1109/JSTQE.2024.3444923).
