Tunable amplitude and frequency modulated optical frequency combs in external-cavity-locked quantum dot lasers

Optical frequency combs (OFCs) are highly promising candidates as multichannel light sources for photonic integrated circuits (PICs). We present a tunable on-chip OFC source based on quantum dot colliding-pulse mode-locked lasers (QD-CPMLs), capable of generating both amplitude-modulated (AM) and frequency-modulated (FM) combs through external-cavity locking. A free-running fourth-order QD-CPML with a 100 GHz repetition rate is demonstrated to produce FM and AM combs under different bias conditions, achieving an ultra-wide comb with a 3-dB bandwidth of 1.8 THz and a 10-dB bandwidth of 2.5 THz. By leveraging external-cavity locking, the modulation dynamics of the comb are finely tuned, significantly expanding the AM comb range while reducing pulse width and chirp. The shortest pulse width achieved is 0.6 ps, with a minimum time-bandwidth product of 0.33, approaching the transform limit for hyperbolic secant pulses. The near-zero linewidth enhancement factor of the QD-CPML effectively suppresses coherence collapse under optical feedback, whereas its low group velocity dispersion facilitates the generation of narrow pulses and broad bandwidths. The ability to dynamically control AM and FM comb regions through external-cavity locking represents an innovative strategy for tunable OFC generation, offering potential for applications in sensing, spectroscopy, and optical communications within PICs.