I have just entered the magical world of silicon photonics, and my first project is the design of an OPLL. As I read through the many papers, offset phase locking is the most common technique employed in this regard, unlike regular PLLs that often rely on only one external reference for phase and frequency detection.

I understand that regular electrical components are incapable of manipulating signals in the THZ regime and hence the idea of down conversion by the photo mixing process. Still, phase determination has no alternative other than the offset locking technique.