Photograph of the silicon nitride photonic chips used for frequency comb and photonic microwave generation. [Image: Junqiu Liu and Jijun He, EPFL]
Microwave photonics—miniaturized wideband circuitry using carrier waves in between the radio and optical bands—will play a role in emerging 5G communications networks and the Internet of Things. Frequency combs serve as the bridge between the radio and optical domains, so miniaturizing them to fit into photonic integrated circuits is crucial to future development of the signal-bottleneck-busting technology.
So-called “soliton” frequency combs, which form dissipative Kerr solitons inside tiny resonators, exist, but combs with practical repetition rates have not been compatible with the CMOS manufacturing processes needed to make widely available products. Now a team at a Swiss laboratory has fabricated compatible soliton combs that operate in the two most common bands for radar and 5G communications (Nat. Photon., doi: 10.1038/s41566-020-0617-x).
Tiny but crucial
Silicon nitride, Si3N4, is an excellent material for many CMOS applications, but it raises the threshold power level to create solitons. This effect keeps the repetition rate of soliton frequency combs too high to form signals in the X-band (around 10 GHz) and the K-band (roughly 20 GHz). The only way to get the repetition rate down to microwave levels was to use air cladding.
Physicist Tobias Kippenberg and his colleagues at the Swiss Federal Institute of Technology Lausanne (EPFL) employed their photonic Damascene process for manufacturing patterned, ultra-low-loss silicon nitride waveguides with a Q-factor as high as 23 × 106. The deep-ultraviolet lithographic patterning of the waveguide substrate makes the waveguides less likely to crack under stress. The waveguides look like circles, with diameters of 2.3 mm for K-band signals and 4.6 mm for X-band signals. The overall chips are squares 5 mm on a side.
A low-noise continuous-wave fiber laser operating at 193 THz generated the solitons entering the photonic chip, creating combs with more than 300 frequency lines within a bandwidth of 3 dB.
According to the EPFL researchers, the devices still have room for improvement. Their phase noise is still well above the fundamental limit set by temperature fluctuations. Nevertheless, the soliton combs are poised for integration into dense wavelength-division multiplexing (DWDM) systems and atomic clocks.