The Smallest Optical Frequency Comb

Part of a photonic integrated, compact, and portable soliton microcomb source. The device is less than 1 cubic centimeter in size, and is driven by an on-chip indium phosphide laser consuming less than 1 Watt of electrical power. It can be used in LIDAR, data center interconnects, and even satellites. (Source: EPFL)

Optical frequency combs are laser sources whose spectrum consists of a series of discrete, equally spaced frequency lines that can be used for precise measure­ments. In the last two decades, they have become a major tool for appli­cations such as precise distance measure­ment, spectro­scopy, and telecommu­nications. Most of the commercially available optical frequency comb sources based on mode-lock lasers are large and expensive, limiting their potential for use in large volumes and portable appli­cations. Although chip-scale versions of optical frequency combs using micro­resonators were first demon­strated in 2007, a fully integrated form has been hindered by high material losses and complex excitation mechanisms.

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 micro­resonator. At only 1 cubic centi­meter in size, the device is the smallest of its kind to-date. The silicon nitride micro­resonator is fabricated using a patented photonic Damascene reflow process that yields unprece­dentedly 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. In the reported work, 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. This shows that both resonator and laser can be integrated on a single chip offering a unique improve­ment over past tech­nology.

“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.”

The whole system can fit in a volume of less than 1 cubic centimeter 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 Raja. “Its main advantage is fast optical feedback, which eliminates the need for active electronic or any other on-chip tuning mechanism.” The scientists now 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. (Source: EPFL)

Reference: A. S. Raja et al.: Electrically pumped photonic integrated soliton microcomb, Nat. Commun. 10, 680 (2019). DOI: 10.1038/s41467-019-08498-2

Link: Center of MicroNanoTechnology, École Polytechnique Fédérale de Lausanne EPFL, Lausanne, Switzerland • Laboratory of Photonics and Quantum Measurements, EPFL, Lausanne, Switzerland 

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