Hybrid Integrated Low Noise Optical Phase-Locked Loop Based on Self-Injection Locked Semiconductor Laser

The combined spectral linewidth of master and slave laser and loop propagation delay are the trade-offs for suppressing residual loop noise in optical phase-locked loop systems. To reduce the slave laser’s linewidth and shorten the loop delay, a heterodyne hybrid optical phase-locked loop system that packages self-injection locked semiconductor laser, photodiode, the electronic circuit is proposed. By employing a DFB laser diode coupled to a microring resonator based on Si ₃ N ₄ integrated waveguide platform, the slave laser realizes a narrow spectral linewidth of 2.8 kHz. Due to the full use of the external microring cavity, the optical path delay is shortened to 30 ps. In this way, the residual loop noise has been reduced to −70 dBc/Hz at as low as 10 Hz offset from the carrier frequency, and phase noise variance is 0.0018 $rad^{2}$ at 600 MHz carrier frequency for offset between 10 Hz and 10 MHz.