Both the carrier offset frequency, fceo, and the repetition rate, frep, of a mode-locked laser (MLL) are referenced to a 10 MHz reference signal through an fceo locking block and an frep locking block. The output of the MLL and a TL are combined using a 50:50 fiber coupler and beaten in a balanced photodetector (BPD). The output of the BPD is sent to the TL-to-comb locking block, which also references the 10 MHz signal. The TL-to-comb locking output is sent to the TL to lock the frequency of TL to one comb line of the MLL. Bottom: schematic of the TL (not to scale). The laser cavity is located in the Al2O3:Er3+ layer and the Si3N4 layer. A >4-cm-long bent gain waveguide in the Al2O3:Er3+ layer is used to provide sufficient gain. In the Si3N4 layer, there are two microring filters in a Vernier configuration and two longitudinal-mode phase shifters, which are controlled by the metal heaters on their top layer, to tune the wavelength of the TL.