Compact Heterogeneously Integrated Optical Phase-Locked Loop for 10 GHz to 40 GHz Optical Frequency Difference Locking

An optical phase-locked loop (OPLL) is presented in this paper to lock the beat note generated by heterodyning two lasers to an electrical reference signal. The proposed OPLL is a heterogeneous integration of a photonic integrated circuit (PIC) and a CMOS electronic integrated circuit (EIC) to achieve a compact size and reduce loop delay. The photo-mixing is performed in the photodiode (PD) fabricated on the silicon photonic (SiP) chip. The generated beat note is first mixed down to a lower frequency by a millimeter-wave (mm-wave) mixer and then compared with a reference signal in the CMOS chip to generate feedback signal for the follower laser. With the help of the mm-wave mixer, the locking range is greatly extended. Moreover, impedance matching between the PIC and EIC is discussed in detail, which is critical to achieve contiguously wide frequency locking range. A prototype of the OPLL is fabricated and tested. During experiments, the frequency difference between two lasers is locked continuously from 10 GHz to 40 GHz with a significant improvement in the phase noise from -30 dBc/Hz to -70 dBc/Hz at 100 Hz offset frequency within the loop bandwidth.