Integrated optical isolators have been a longstanding challenge for photonic integrated circuits (PIC). An ideal integrated optical isolator for PIC should be made by a monolithic process, have a small footprint, exhibit broadband and polarization-diverse operation, and be compatible with multiple materials platforms. Despite significant progress, the optical isolators reported so far do not meet all these requirements. In this article we present monolithically integrated broadband magneto-optical isolators on silicon and silicon nitride (SiN) platforms operating for both TE and TM modes with record high performances, fulfilling all the essential characteristics for PIC applications. In particular, we demonstrate fully-TE broadband isolators by depositing high quality magneto-optical garnet thin films on the sidewalls of Si and SiN waveguides, a critical result for applications in TE-polarized on-chip lasers and amplifiers. This work demonstrates monolithic integration of high performance optical isolators on chip for polarization-diverse silicon photonic systems, enabling new pathways to impart nonreciprocal photonic functionality to a variety of integrated photonic devices.

Narrow linewidth lasers have many applications, such as higher order coherent communications, optical sensing, and metrology. While semiconductor lasers are typically unsuitable for such applications due to relatively low coherence, recent advances in heterogeneous integration of III-V with silicon have shown that this is no longer true. In this tutorial, we discuss in-depth techniques that are used to drastically reduce the linewidth of a laser. The heterogeneous silicon-III/V platform can fully utilize these techniques, and fully integrated lasers with Lorentzian linewidth on the order of 100 Hz and tuning range of 120 nm are shown.

We demonstrate a fully integrated extended distributed Bragg reflector (DBR) laser with ∼1 kHz linewidth and over 37 mW output power, as well as a ring-assisted DBR laser with less than 500 Hz linewidth. The extended DBR lasers are fabricated by heterogeneously integrating III-V material on Si as a gain section plus a 15 mm long, low-kappa Bragg grating reflector in an ultralow-loss silicon waveguide. The low waveguide loss (0.16 dB/cm) and long Bragg grating with narrow bandwidth (2.9 GHz) are essential to reducing the laser linewidth while maintaining high output power and single-mode operation. The combination of narrow linewidth and high power enable its use in coherent communications, RF photonics, and optical sensing.

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