Optical Soliton in Quantum Dot Comb Laser

Optical frequency comb sources have attracted significant attention for their properties of coherent waveform generation. As an alternative to Kerr micro-resonators, semiconductor mode-locked lasers are also considered as an energy efficient optical comb source. In particular, semiconductor quantum dot (QD) mode-locked lasers are one of ideal multi-wavelength laser sources for data communication and optical interconnects, attributed to unique physical properties of ultra-fast carrier dynamics and extended photon lifetime. Here, we demonstrate InAs QD 4 ^th -order colliding pulse mode-locked (CPM) laser epitaxially grown on silicon, generating stable O-band optical frequency combs with 100 GHz spacing upto 90 ℃. By adjusting operating conditions (injection current and reversed bias voltage), different optical spectral modes (Gaussian, soliton and flat-top) can be realized corresponding to different pulse train evolution. Systematical analysis in frequency-domain and time-domain shows that at low reverse bias voltage of saturable absorbers (V _SA ), the laser is frequency modulation dominated, generating weak and stretched pulses, while transfers into amplitude modulation dominated at high V _SA . At specific operation point (injection current ~ 105 mA and V _SA ~ -5 V), the zero dispersion, small frequency chirp, squared hyperbolic secant shaped pulse profile and close to limit time-bandwidth product (0.3176) reveal soliton state discovered in active semiconductor laser cavity. A flat-top optical comb with 8+1 channels within 3 dB optical bandwidth is achieved. By integrating with high speed thin-film lithium niobate (TFLN) Mach-Zender modulators, single QD CPM laser directly grown on silicon substrate can provide high capacity data transmission through 100 Gbit/s NRZ or 50 Gbaud/s PAM-4 format modulation with bit error rate (BER) reaching hard-decision forward error correct (HD-FEC) level for all channels.