(Invited) Temperature Stable III-V/Si/Si3N4 Heterogeneous Integrated Laser
Authors: Xiang Chao , Jin Warren , Guo Joel , Morton Paul , Bowers John E 
Published: December 2020

Silicon nitride (Si3N4) is a widely used complementary metal–oxide–semiconductor (CMOS) compatible material in modern IC industry and recent years have witnessed tremendous development of Si3N4 in photonics. The benefits of Si3N4 in photonics include low loss, low thermo optic coefficient and negligible nonlinear absorption at telecom wavelengths. Devices based on Si3N4 waveguides innovates in the field of nonlinear photonics, sensing, microwave photonics and so on. However, they are all dependent on the excellent passive properties of Si3N4. There are no successful demonstrations of fully integrated laser with Si3N4 yet.
For an external cavity laser design, it is beneficial to have a low loss waveguide. A low loss waveguide platform can enable a long optical cavity which can greatly reduce the laser phase noise. Butt-coupled hybrid integrated lasers are extensively studied using Si3N4 cavity, but they are based on separate chips and are not scalable for large volume integration.
Heterogeneous integration of III-V material and Si has achieved great success in the past years, offering the silicon photonics industry with low-cost, high-performance silicon photonic integrated circuits fully integrated with lasers. For the heterogeneous integration of III-V material with Si3N4, the biggest challenge is the large index mismatch. Typical III-V epi used for heterogeneous lasers is about 2 μm thick and extreme tapering cannot facilitate a high coupling efficiency between the III-V layer and Si3N4 layer.
In this work, we used an intermediate Si layer to bridge the index mismatch between the III-V and Si3N4 layer. By using multilayer heterogeneous integration shown in Fig.1 (a), we enable a III-V/Si/Si3N4 structure to efficiently couple the mode between III-V/Si and Si/Si3N4. Based on this structure, we designed the III-V/Si/Si3N4 laser using an external Si3N4 spiral grating as the feedback cavity. This spiral grating provides the narrow-band optical feedback and determines the laser wavelength. The laser configuration is shown in Fig. 1(b).
The laser performance is summarized in Fig. 1 (c)-(e). The lasing wavelength is at 1546 nm and the laser has over 58 dB side mode suppression ratio (SMSR). This shows the excellent mode selectivity from the 20 mm long Si3N4 spiral grating. We measured the frequency noise of the laser. With optimized loop mirror reflectivity, the laser fundamental linewidth is about 4 KHz. This linewidth can be further improved by lowering the Si3N4 waveguide loss. We anticipate the laser fundamental linewidth can be reduced to below 100 Hz using this design.
Another advantage of integrating Si3N4 into the laser cavity is to have much higher temperature stability. By comparison, III-V/Si/Si3N4 laser wavelength shifts by only 0.47 nm for 45 oC change, while for a III-V/Si DBR laser this wavelength shift is 3.3 nm. This is over 7x difference and is a result of the low thermo-optic coefficient of Si3N4. When combined with Si3N4 based arrayed waveguide gratings, our laser can be potentially used in dense wavelength-division-multiplexing (DWDM) systems.
The successful demonstration of this multilayer heterogeneous integration will enable a whole new class of devices that require low loss waveguides and are presently not integrated. This is a key step in fully exploiting the capabilities of wafer-bonding technologies. We anticipate this platform can take narrow-linewidth semiconductor laser performance to a new level.

Figure 1