Kavli Affiliate: Kerry J. Vahala
| First 5 Authors: Chao Xiang, Warren Jin, Osama Terra, Bozhang Dong, Heming Wang
| Summary:
While photonic integrated circuits (PICs) are being widely used in
applications such as telecommunications and datacenter interconnects, PICs
capable of replacing bulk optics and fibers in high-precision, highly-coherent
applications will require ultra-low-noise laser sources to be integrated with
other photonic components in a compact and robustly aligned format — that is,
on a single chip. Such PICs could offer superior scalability for complex
functionalities and volume production, as well as improved stability and
reliability over time. However, there are two major issues preventing the
realization of such envisioned PICs: the high phase noise of semiconductor
lasers, and the difficulty of integrating optical isolators directly on chip.
PICs are still considered as inferior solutions in optical systems such as
microwave synthesizers, optical gyroscopes and atomic clocks, despite their
advantages in size, weight, power consumption and cost (SWaPC). Here, we
challenge this convention by introducing three-dimensional (3D) integration in
silicon photonics that results in ultra-low-noise, isolator-free PICs. Through
multiple monolithic and heterogeneous processing sequences, direct on-chip
integration 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-Q
cavity reduces the laser noise close to that of fiber lasers. Moreover, the
cavity acts as an effective block for any downstream on-chip or off-chip
reflection-induced destabilization, thus eliminating the need for optical
isolators. We further showcase isolator-free, widely-tunable, low-noise,
heterodyne microwave generation using two ultra-low-noise lasers on the same
silicon chip.
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