Optical phase lock loop as high-Q filter for optical frequency comb line selection

This paper describes the first optical phase lock loop (OPLL) based on a photonic integrated circuit (PIC) fabricated using a generic foundry process and off-the-shelf electronic components. The PIC contains a DBR laser that is for the first time frequency and phase stabilized in reference to an Optical Frequency Comb Generator (OFCG) line. The OFCG used in this demonstration offers 19 highly coherent lines spaced by 15 GHz. The OPLL can attenuate all adjacent comb lines and noise by more than 50 dB below the power of the selected comb line. Hence, the OPLL can be considered as an ultra-selective optical filter. The OPLL output is a single mode DBR laser line, with frequency offset from the reference comb line exactly selectable over the frequency range 4 GHz to 12 GHz. The laser can be current tuned over a 1 THz (8 nm) range enabling the different comb lines to be selected. The coherence between DBR laser and comb lines is demonstrated by measurements of the heterodyne signal residual phase noise level, which is below -100 dBc/Hz at 5 kHz offsets from the carrier. This is a record low value for an OPLL based on a monolithically photonic integrated circuit. Such an OPLL could be used for high purity, tuneable millimetre and THz wave generation.