Fig. 1. (Color online) (a) Evolution of photonic integration in terms of the number of devices in a single PIC. Silicon photonic integration (red circle) represents the “passive” integration without an on-chip laser solution; InP integration (blue squares) and heterogeneous silicon integration (green triangle) are solutions with on-chip lasers^[6]. (b) Schematic of the heterogeneous platform commercialized by Intel^[10].

Fig. 2. (Color online) High-Q heterogeneous laser device schematics (not to scale). (a) Two-dimensional cross-section of the heterogeneous platform, with superimposed optical transverse mode profile. (b) Perspective view of a high-Q heterogeneous laser. (c) Perspective view of the high-Q silicon resonator^[27]. (d) Frequency noise spectral density for three high-Q heterogeneous lasers (with different spacer thickness) and control laser^[30].

Fig. 3. (Color online) (a) High-Q widely tunable heterogeneous quad-ring tunable laser device schematics (not to scale). (b) Coarse tuning spectra showing the tuning range of 120 nm. (c) Frequency noise spectrum of the fabricated quad-ring mirror laser. A white noise level of 45 Hz²/Hz is drawn^[24].

Fig. 4. (Color online) (a) III–V/Si/Si₃N₄ laser schematic diagram. (b) Si–Si₃N₄ taper as well as the simulated mode profile. (c) Single-mode optical spectrum with gain current of 160 mA. The inset shows measured normalized reflection spectra of the Si₃N₄ spiral grating^[36].

Fig. 5. (Color online) (a) Schematic illustration of the typical epitaxial structure used for lasers and amplifiers including one period of the p-modulation doped active region and the III–V/Si buffer including defect filter layers and thermal cycle annealing (TCA) to reduce dislocation densities. (b) As-grown photoluminescence spectra for quantum dot lasers on GaAs and Si substrates^[47].

Fig. 6. (Color online) (a) Schematic diagram of the 20 GHz quantum dot mode-locked laser on silicon (not to scale). (b) Optical spectrum and corresponding optical linewidth of each mode within 10 dB. (c) Relative intensity noise of the whole O-band spectrum and certain filtered individual wavelength channels. (d) BER performance of the PAM-4 signal with different comb lines^[60].

Fig. 7. (Color online) Si-based QD-SOA (a) on-chip gain (TE polarization) mapping as a function of on-chip input power and wavelength at 20 °C. (b) On-chip small signal gain as a function of wavelength. (c) On-chip output power as a function of on-chip input power. (d) Wall-plug efficiency as a function of on-chip input power^[59]. (e) Bit error rate (BER) against the received optical power for the optical receiver (PD+TIA) with and without QD-SOA under 20 °C, eye diagrams of the receiver with and without QD-SOA are shown in the insets^[85].