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While photonic integrated circuits (PICs) are being widely used inapplications such as telecommunications and datacenter interconnects, PICscapable of replacing bulk optics and fibers in high-precision, highly-coherentapplications will require ultra-low-noise laser sources to be integrated withother photonic components in a compact and robustly aligned format -- that is,on a single chip. Such PICs could offer superior scalability for complexfunctionalities and volume production, as well as improved stability andreliability over time. However, there are two major issues preventing therealization of such envisioned PICs: the high phase noise of semiconductorlasers, and the difficulty of integrating optical isolators directly on chip.PICs are still considered as inferior solutions in optical systems such asmicrowave synthesizers, optical gyroscopes and atomic clocks, despite theiradvantages in size, weight, power consumption and cost (SWaPC). Here, wechallenge this convention by introducing three-dimensional (3D) integration insilicon photonics that results in ultra-low-noise, isolator-free PICs. Throughmultiple monolithic and heterogeneous processing sequences, direct on-chipintegration of III-V gain and ultra-low-loss (ULL) silicon nitride (SiN)waveguides with optical loss around 0.5 dB/m are demonstrated. Consequently,the demonstrated PIC enters a new regime, such that an integrated ultra-high-Qcavity reduces the laser noise close to that of fiber lasers. Moreover, thecavity acts as an effective block for any downstream on-chip or off-chipreflection-induced destabilization, thus eliminating the need for opticalisolators. We further showcase isolator-free, widely-tunable, low-noise,heterodyne microwave generation using two ultra-low-noise lasers on the samesilicon chip.

Three-dimensional integration enables ultra-low-noise, isolator-free Si photonics