Smallest optical frequency comb to-date
Research teams led by Tobias J. Kippenberg at EPFL and Michael L. Gorodetsky at the Russian Quantum Center have now built an integrated soliton microcomb operating at a repetition rate of 88 GHz using a chip-scale indium phosphide laser diode and the silicon nitride (Si3N4) microresonator. At only 1 cm3 in size, the device is the smallest of its kind to-date.
The microresonator is fabricated using a patented photonic Damascene reflow process that delivers very low losses in integrated photonics. These ultra-low loss waveguides bridge the gap between the chip-based laser diode and the power levels required to excite the dissipative Kerr soliton states, which underly the generation of optical frequency combs.
The method uses commercially available chip-based indium phosphide lasers as opposed to conventional bulk laser modules. The rported work cites that a small portion of the laser light is reflected back to the laser due to intrinsic scattering from the microresonator. This direct feedback helps to both stabilize the laser and generate the soliton comb – showing that both resonator and laser can be integrated on a single chip and offering a marked improvement over past technology.
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“There is a significant interest in optical frequency comb sources that are electrically driven and can be fully photonically integrated to meet the demands of next-generation applications, especially LIDAR and information processing in data-centers,” says Kippenberg. “This not only represents a technological advancement in the field of dissipative Kerr solitons, but also provides an insight into their nonlinear dynamics, along with fast feedback from the cavity.”
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The whole system can fit in a volume of less than 1 cm3 and can be controlled electrically. “The compactness, easy tuning method, low cost and low repetition rate operation make this microcomb system interesting for mass-manufacturable applications,” says PhD student Arslan Sajid, the lead author of the study. “Its main advantage is fast optical feedback, which eliminates the need for active electronic or any other on-chip tuning mechanism.”
Looking forward, the scientists aim to demonstrate an integrated spectrometer and multi-wavelength source and to improve the fabrication process and the integration method further to push the microcomb source at a microwave repetition rate.
The samples were fabricated at EPFL’s Center of MicroNanoTechnology.
Reference
A. S. Raja, A. S. Voloshin, H. Guo, S. E. Agafonova, J. Liu, A. S. Gorodnitskiy, M. Karpov, N. G Pavlov, E. Lucas, R. R Galiev, A. E Shitikov, J. D. Jost, M. L. Gorodetsky, T. J. Kippenberg. Electrically pumped photonic integrated soliton microcomb. Nature Communications 10, 680 (2019).
https://dx.doi.org/10.1038/s41467-019-08498-2
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