UC Santa Barbara

Silicon photonics (SiPh) has emerged as a photonic integrated circuit (PIC) platform, especially for high volume applications. Integrated laser sources, however, remain a challenge. SiPh foundries have existed for more than 10 years but still don’t offer a qualified process with integrated lasers. Direct heteroepitaxy of group III-V materials on silicon is still immature and suffers from reliability issues. Heterogeneous and hybrid integration techniques, however, have been pursued in research and by industry and present a practical near-term solution for laser integration. Heterogenous approaches based on wafer bonding involve the bonding of bare III-V epitaxial material to silicon on insulator (SOI), co-fabrication, and evanescent light coupling. The laser active medium is thermally isolated from the silicon substrate by the buried oxide layer limiting the laser efficiency at high temperature. Hybrid integration approaches, such as the butt coupling of fabricated III-V lasers to SOI waveguides, may address the thermal issue. However, the main limitation for butt coupling is the significant mode mismatch of the waveguides that imposes a strict alignment requirement.

In this thesis the novel 3D hybrid integration technique for SiPh, addressing the aspects of thermal performance and alignment tolerance, was proposed and demonstrated for the first time. This approach is based on the flip-chip integration of indium phosphide (InP) reflective semiconductor optical amplifiers (RSOAs) containing total internal reflection turning mirrors for surface emission. Light is coupled to the SOI waveguides through surface grating couplers. This technique yields increased alignment tolerance compared to butt coupling. Flip-chip integration also allows the RSOA chip to be bonded P-side down directly to the silicon substrate. In this way, the heat generated in the active region can dissipate more efficiently in the silicon. 3D hybrid integration can be carried out at wafer level in a backend step for high throughput manufacturing, and also allow for the integration of InP PICs on silicon interposers for large-scale electronic-photonic integration.

A tunable laser was demonstrated with 3D hybrid integration demonstrating a side-mode suppression ratio up to 43 dB. Greater than 4 mW of optical power was coupled into SiPh waveguide and more than 20 nm wavelength tuning range was achieved. A linewidth of 1.5 MHz and relative intensity noise of -132 dB/Hz were demonstrated. A low thermal impedance of 6.2 ℃/W was extracted experimentally from a 3D hybrid laser that was bonded to the silicon substrate, demonstrating a factor of three improvement over a laser that was bonded above the SOI layer. To improve coupling efficiency, various advanced silicon surface grating couplers as well as dilute waveguide RSOAs were investigated. Coupling efficiency up to 85\% can be achieved while also maintaining an alignment tolerant implementation.