Large-scale production of optical frequency combs, integrated into a single system

Breakthrough in Chip-Scale Soliton Microcomb Technology

A significant advancement in the field of integrated photonics has been made, with researchers successfully integrating semiconductor lasers onto a silicon nitride (Si3N4) substrate, paving the way for large-scale production of chip-based frequency combs.

The new technology, which has been widely deployed for optical interconnects to replace the traditional copper-wire ones linking servers at data centers, was first demonstrated by labs at the University of California, Santa Barbara (UCSB) and the École Polytechnique Fédérale de Lausanne (EPFL) in an article published in Science.

Each device produced is entirely electrically controlled, with distributed feedback (DFB) lasers being fabricated on the silicon and indium phosphide layers. The Si3N4 microresonator, known for its ultralow loss, wide transparency window, absence of two-photon absorption, and high power-handling capability, creates tens of new frequency lines during soliton microcomb formation.

The single-frequency output from one DFB laser seeds soliton microcomb formation in a Si3N4 microresonator. This integration of microcombs and lasers on the same chip can enable high-volume production of soliton microcombs using well-established CMOS techniques developed for silicon photonics.

However, achieving ultralow-loss Si3N4 microresonators is still insufficient for high-volume production of chip-scale soliton microcombs, as co-integration of chip-scale driving lasers are required. The heterogeneous fabrication process combines silicon, indium phosphide, and Si3N4 for integrated photonics, with the process being based on multiple wafer bonding of these materials onto a Si3N4 substrate.

The wafer-scale heterogeneous process can produce over a thousand chip-scale soliton microcomb devices from a single 100-mm-diameter wafer. This scalability opens up possibilities for large-volume, low-cost manufacturing of chip-based frequency combs for high-capacity transceivers, data centers, sensing, and metrology.

One outstanding challenge in the generation of soliton microcombs is the integration of laser sources, which are typically off-chip and bulky. This breakthrough addresses this issue, bringing us one step closer to realizing the potential of soliton microcombs in various applications such as terabit-per-second coherent communication in data centers, astronomical spectrometer calibration for exoplanet searches, neuromorphic computing, optical atomic clocks, absolute frequency synthesis, and parallel coherent LiDAR.

Dr. Junqiu Liu, who leads the Si3N4 fabrication at EPFL's Center of MicroNanoTechnology (CMi), made this statement regarding the technology: "This heterogeneous integration laser technology has the potential to revolutionize the field of integrated photonics."

Fifteen years ago, Professor John Bowers's lab at UCSB pioneered a method for integrating semiconductor lasers onto a silicon wafer. The researcher group that first demonstrated heterogeneous integration of ultralow-loss Si3N4 photonic integrated circuits and semiconductor lasers using wafer-scale CMOS techniques in a publication in Science has not been explicitly identified in the provided search results. The closest relevant work involves heterogeneous integration using lithium tantalate-on-silicon nitride photonics and Si3N4 photonic Damascene process showing ultralow loss, but without explicit mention of semiconductor laser integration or a named research group in Science.

This breakthrough in chip-scale soliton microcomb technology promises to significantly reduce the cost and increase the efficiency of optical interconnects, making data centers and high-speed communication networks more accessible and sustainable.