Tuneability of monolithically integrated optical phase lock loop for THz generation

This work presents a monolithically integrated optical phase lock loop (OPLL) synchronised to an Optical Frequency Comb Generator (OFCG) without any prior filtering of the comb lines for the first time. The heterodyne phase locking is achieved at a tuneable offset frequency in the range of 2 to 6 GHz, while the optical comb lines are spaced by 12 GHz. This is of particular interest as it allows investigation of the influence of the nearest comb lines on OPLL performance. Moreover, the work discusses continuous tuneability limitations of a photonic THz source based on the single OPLL and proposes a solution to improve the frequency agility. Finally, the spectra of signals ranging from 2.5 up to 21.5 GHz, generated by heterodyning the locked slave laser and the subsequent comb lines, are presented and analysed. Phase noise performance of -80 dBc/Hz at 10 kHz offset across the frequency range is reported.