Fig. 1. Structural comparison of OPLL and PLL. (a) Diagram of PLL structure; (b) Diagram of OPLL structure

Fig. 2. Complex frequency domain model of OPLL

Fig. 3. OPLL structure diagram using analog phase discrimination technology

Fig. 4. OPLL structure diagram using digital phase discrimination technology

Fig. 5. OPLL offset locking system based on FPGA

Fig. 6. Schematic diagram of all digital OPLL based on FPGA^[28]

Fig. 7. Integrated optical path for OPLL^[38]

Fig. 8. Integrated OPLL offset locking system^[39]

Fig. 9. OPLL offset locking system for EIT experiment^[25]

Fig. 10. OPLL offset locking system for atomic coherence experiments^[24]

Fig. 11. OPLL offset locking system for He-Ne laser source^[23]

Fig. 12. OPLL offset locking system for ECDL^[26]

Fig. 13. OPLL locking system for laser heterodyne interferometry source^[45]

Fig. 14. OPLL system for coherent beam combining^[46]

Fig. 15. Improved OPLL system for coherent power combining^[47]