Scalable CMOS manufacturing of integrated optical frequency combs
Researchers in Switzerland and the US have developed a technique to produce soliton optical microcombs and diode lasers together in volume on CMOS wafers.
Optical frequency combs consist of light frequencies made of equidistant laser lines. They have already revolutionized the fields of frequency metrology, timing and spectroscopy. Professor Tobias Kippenberg at EPFL has been working on microcombs to where a single-frequency laser is converted into ultra-short pulses called dissipative Kerr solitons.
Soliton microcombs consume low power and support terabit-per-second coherent communication in data centres and neuromorphic computing as well as optical atomic clocks, absolute frequency synthesis, and parallel coherent LiDAR.
The challenge has been to integrate the pump lasers on the same chip in high volume as silicon nitride (Si3N4) is the leading silicon platform due to its ultralow loss, wide transparency window from visible to mid-infrared, absence of two-photon absorption, and high power-handling capability. But 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.
Meanwhile Professor John Bowers at the University of California Santa Barbara (UCSB) on a method for integrating semiconductor lasers onto a silicon wafer. This bonds indium phosphide semiconductors on silicon wafers to form laser gain sections. This heterogeneous integration laser technology has now been widely deployed for optical interconnects to replace the copper-wire ones that linked servers at data centres.
Belgian research lab imec has also been working with Sivers Photonics (formerly CST Global) in the UK and ASM Amicra Microtechnologies to integrate indium-phosphide (InP) distributed feedback (DFB) lasers on the silicon wafers.
The two labs at EPFL and UCSB now demonstrate the first heterogenous integration of ultralow-loss Si3N4 photonic integrated circuits (fabricated at EPFL) and semiconductor lasers (fabricated at UCSB) through wafer-scale CMOS techniques.
“Our heterogenous fabrication technology combines the three mainstream integrated photonics platforms, namely silicon, inidium phosphate and Si3N4, and can pave the way for large-volume, low-cost manufacturing of chip-based frequency combs for next-generation high-capacity transceivers, data centers, sensing and metrology,” says Dr Junqiu Liu who leads the Si3N4 fabrication at EPFL’s Center of MicroNanoTechnology (CMi).
The method is mainly based on multiple wafer bonding of silicon and indium phosphide onto the Si3N4 substrate. Distributed feedback (DFB) lasers are fabricated on the silicon and indium phosphide layers. The single-frequency output from one DFB laser is delivered to a Si3N4 microresonator underneath, where the DFB laser seeds soliton microcomb formation and creates tens of new frequency lines.
This wafer-scale heterogeneous process can produce more than a thousand chip-scale soliton microcomb devices from a single 100mm wafer, lending itself to commercial-level manufacturing. Each device is entirely electrically controlled. Importantly, the production level can be further scaled up to the industry standard 200 or 300mm wafers.
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