Opt. Express 25, 681–695 (2017)

Reducing the size of optical frequency synthesizers (OFSs) to the chip scale is technically challenging. Optical frequency comb (OFC) generators are a key element of OFSs, and microresonator-based OFCs lend themselves to on-chip integration. However, even these generators require pump laser powers in the hundred milliwatt range, thus hampering the successful thermal management of integrated OFSs. Now, Shamsul Arafin and co-workers from the USA have demonstrated a chip-scale OFS based on an optical heterodyne phase-locked loop. Light emitted from a semiconductor distributed feedback laser with an output power of 20 mW was sent to a high-Q MgF2 whispering-gallery-mode resonator to generate an optical frequency comb spanning the 1,535–1,575 nm range. The comb lines were used as the reference for the heterodyne optical phase-locked loop, where the latter consisted of a photonic integrated circuit, an electronic integrated circuit and a loop filter. Arbitrary frequency synthesis between the comb lines was demonstrated by tuning the radio frequency offset source, and better than 100 Hz tuning resolution within an accuracy of 5 Hz was achieved.