Repetition Rate Stabilization and Optical Axial Mode Linewidth Reduction of a Chip-Scale MLL Using Regenerative Multitone Injection Locking

A novel injection locking architecture is demonstrated to simultaneously improve the axial mode linewidth and stabilize the repetition rate of a monolithically integrated modelocked laser. First, linewidth reduction is demonstrated via multitone injection locking. Then, the passive mode-locked laser is used as a source to make a coupled optoelectronic oscillator, and its RF spectral characteristics are investigated for different optical delays. A novel combination of these two techniques is established by creating a coupled optoelectronic oscillator via multitone injection locking, using an intracavity element as photodetector to generate the feedback signal to a Mach-Zehnder modulator. Using this combination of methods, the axial mode linewidth is reduced by over 6000× to ~100 kHz and the RF linewidth is reduced by a factor of 70 at -30 dBc, resulting in a 3 dB RF linewidth of 400 Hz. The improvement in RF phase noise at 200 kHz offset is >40 dB. Finally, the system is referenced to a RF synthesizer using a variable voltage phase-shifter and a PID controller. Allan deviation measurements show a resolution limited stability in the repetition rate of 10 -10 at 1 second following a 1/τ trend.