Fig. 1. Structure diagram of passive external cavity semiconductor laser

Fig. 2. Composite cavity can be equivalent to a mirror with equivalent reflectivity^[19]

Fig. 3. Structure diagram of a basically wide-tunable III-V/Si based on multi-ring mirrors^[37]

Fig. 4. Tunable laser with SOA and external resonator. (a) Structure diagram^[38]; (b) spectra of tunable lasers with different wavelengths^[38]; (c) schematic of laser structure with asymmetric MZI^[41]; (d) wavelength tuning of laser with asymmetry MZI at 90 mA injection current^[41]; (e) calculated and measured linewidth versus output power plot^[41]; (f) schematic of tunable laser integrated with SOA and silicon photonic filter chip^[43]; (g) laser spectrum in the 65 nm range^[43]

Fig. 5. Concept diagram of a wide tunable laser array covering the entire low-loss fiber optic telecommunications window^[46]. (a) Schematic of Si ring resonator lasers bonded with III-V gain materials in O-band, E-band, and S-C-L bands on common SOI wafers; (b) microscope images of ring resonators, phase shifters, and ring mirrors

Fig. 6. Structure diagram of tunable laser based on an air bridge structure waveguide ring cavity^[53]

Fig. 7. External cavity narrow linewidth laser^[12]. (a) Structure diagram; (b) spectrum of lasers (tuning range is from 1545.5 nm to 1550.4 nm); (c) frequency noise of a free-running DFB laser and the proposed external cavity laser corresponding to an intrinsic linewidth of 49.70 kHz and 27.17 Hz, respectively

Fig. 8. InP-Si₃N₄ hybrid laser.(a) Schematic diagram of a hybrid laser^[57]; (b) superimposed spectra when coarsely tuning over a 43 nm wide range (left), superimposed spectra when fine-tuning over a range of 0.8 nm via tuning both MRRs simultaneously (right), achieving a stepwise sweeping of the wavelength at the FSR of the whole laser cavity^[59]; (c) recorded beat signal at a driving current of 196 mA and a laser output wavelength of 1578.12 nm, Lorentz fitting linewidth is 87 kHz^[60]; (d) schematic diagram of a laser integrating InP and GaAs gain chips^[61]; (e) superimposed spectra when thermally tuning a microresonator, and the tuning range is 46 nm^[61]; (f) recorded RF beat spectrum (red dots), the blue line shows a Lorentzian fit corresponding to a laser linewidth of 18 kHz^[61]; (g) schematic diagram of the III-V/Si₃N₄ hybrid laser^[63]; (h) superimposed spectra, tuning range is 172 nm^[64]

Fig. 9. Vision of the integrated platform^[65](inset: microscope image of the ring resonator before microheater deposition, and the waveguide is fabricated using 175 nm thick Si₃N₄ surrounded by SiO₂)

Fig. 10. Narrow linewidth self-injection locked laser. (a) Schematic of laser structure^[75]; (b) comparison of linewidth between free-running DFB laser and laser locked into LN micro ring cavity by self-injection^[75]; (c) tuning of laser wavelength by applying electrical power^[75]; (d) schematic of narrow linewidth laser hybrid integration with external cavity chip and commercial DFB laser^[78]; (e) frequency noise of the laser using LN external cavity chip is reduced by 1/169 compared to free-running DFB, and linewidth is reduced to 2.5 kHz^[78]; (f) emission spectra^[78]