Fig. 1. (a) Schematic configuration of the proposed III–V/Si₃N₄ hybrid integrated ECL with the WASR using the bent DC. Normalized transmission spectra of (b) the two MRRs and (c) the round-trip reflection for the MRRs-based Vernier filter. (d) The reflectivity of the MZI-based WASR changes with the phase shift in the MZI arm for three different PCCs of the couplers. (e) The simulated spectrum of the 3-dB power beam splitting ratio for the bar/cross port in the bent DC, and (f) the reflectivity of the WASR changes with wavelengths for three different phase shifts as an example in the MZI arms when using such bent DCs.

Fig. 2. Measured ASE spectra of the (a) RSOA1 and (b) RSOA2 chips under various injection currents. The ripple was induced by the residual reflection at the end of the received fiber. (c), (d) Normalized transmission spectra for both the AUX and OUT ports of the Si₃N₄ external cavity circuit. The Si₃N₄ chip was mounted on the stage with a thermoelectric cooler (TEC), which was set to a temperature of 25°C.

Fig. 3. (a) Microscope image of the testing setup for the hybrid integrated ECL. (b) The RSOA chip and the Si₃N₄ chip are butt-coupled and (c) a lensed fiber is placed behind the Si₃N₄ chip to collect the output light. A probe card is used to provide electric power for the Si₃N₄ chip.

Fig. 4. (a) Measured spectra for the AUX port under the injection currents of 20 and 30 mA. A dip is marked in a purple dashed box. (b) Measured spectra of single-mode lasing at 1578.56 nm. (c) Superimposed lasing spectra across a range of 81 nm measured by tuning MMR₂ and (d) measured SMSR as a function of wavelength. The RSOA1 was used as the gain chip, and the injection current was set to 50 mA.

Fig. 5. Measured (a) superimposed output lasing spectra and (b) the SMSRs across the entire wavelength tuning range of the ECL when the RSOA2 was used as the gain chip. The injection current of RSOA2 was 60 mA.

Fig. 6. (a) Experimental setup to measure the frequency noise of the laser using a self-heterodyne MZI with short delay. LUT, laser under test; BPD, balanced photodiode; AOM, acousto-optic modulator. (b) Typical frequency noise spectra of the fabricated ECL at 1568.52 nm for RSOA1 and 1615.8 nm for RSOA2. (c) The calculated intrinsic linewidths across most of the channels.