Comparison of Optical Self-Phase Locked Loop Techniques for Frequency Stabilization of Oscillators

Self-phase locked loop (SPLL) through long optical delay lines is investigated for improving the phase noise of the optoelectronic oscillator (OEO) and the voltage-controlled oscillator (VCO). This paper features analytical modeling of close-in to carrier phase noise of oscillators employing SPLL technique using single and multiple delay loops. Experimental results are also provided to verify the analytical modeling and explore critical circuit design parameters to achieve substantial phase noise reduction. Performance comparison is presented for SPLL using electrical phase shifter, Mach-Zehnder modulator (MZM)-based phase shifting, and electrical VCO. Phase noise reduction of 65 dB at 1-kHz offset is achieved for an electrical VCO employing a dual-loop SPLL with 3- and 5-km optical fiber delay lines, whereas the phase locking based on MZM and electrical phase shifter provided 20- and 38-dB reductions at 1-kHz offset for an OEO, respectively.