Optical frequency combs consist of light frequencies made of equidistant laser lines. They have already revo­lutionized the fields of frequency metrology, timing and spectro­scopy. The discovery of 'soliton microcombs by Tobias Kippenberg's lab at Swiss Federal Institute of Technology (EPFL) in the past decade has enabled frequency combs to be generated on chip. In this scheme, a single-frequency laser is converted into ultra-short pulses, the dissipative Kerr solitons. Soliton microcombs are chip-scale frequency combs that are compact, consume low power, and exhibit broad bandwidth. Combined with large spacing of comb teeth, microcombs are uniquely suited for a wide variety of applications, such as terabit-per-second coherent communi­cation in data centers, astronomical spectro­meter calibration for exoplanet searches and neuro­morphic computing, optical atomic clocks, absolute frequency synthesis, and parallel coherent Lidar.

Yet, one outstanding challenge is the integration of laser sources. While microcombs are generated on-chip via parametric frequency conversion, the pump lasers are typically off-chip and bulky. Integrating 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 this has been an outstanding challenge for the past decade. For the nonlinear optical micro­resonators, where soliton micro­combs are formed, silicon nitride (Si₃N₄) has emerged as the leading 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. 

Fifteen years ago, John Bowers's lab at University of California, Santa Barbara, pioneered a method for integrating semi­conductor lasers onto a silicon wafer. Since silicon has an indirect bandgap and cannot emit light, scientists bond 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 centers. This trans­formative laser technology has been already commer­cialized, and Intel ships millions of transceiver products per year. 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. 

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 100-mm-diameter 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 300-mm-diameter substrates. “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 Junqiu Liu who leads the Si₃N₄ fabrication at EPFL's Center of MicroNano­Technology (CMi). (Source: EPFL / UCSB)

Reference: C. Xiang et al.: Laser soliton microcombs heterogeneously integrated on silicon, Science 373, 99 (2021); 
DOI: 10.1126/science.abh2076

Links: Laboratory of Photonics and Quantum Measurements, Institute of Physics, Swiss Federal Institute of Technology Lausanne EPFL, Lausanne, Switzerland • Optoelectronics, Dept. of Electrical and Computer Engineering, University of California, Santa Barbara, Santa Barbara, USA