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The performance of silicon photonic components and integrated circuits has improved dramatically in recent years. As a key enabler, heterogeneous integration not only provides the optical gain which is absent from native Si substrates and enables complete photonic functionalities on chip, but also lays the foundation of versatile integrated photonic device performance engineering. This paper reviews recent progress of high-performance silicon photonics using heterogeneous integration, with emphasis on ultra-low-loss waveguides, single-wavelength lasers, comb lasers, and photonic integrated circuits including optical phased arrays for LiDAR and optical transceivers for datacenter interconnects.

Silicon nitride (SiN) waveguides with ultra-low optical loss enable integrated photonic applications including low noise, narrow linewidth lasers, chip-scale nonlinear photonics, and microwave photonics. Lasers are key components to SiN photonic integrated circuits (PICs), but are difficult to fully integrate with low-index SiN waveguides due to their large mismatch with the high-index III-V gain materials. The recent demonstration of multilayer heterogeneous integration provides a practical solution and enabled the first-generation of lasers fully integrated with SiN waveguides. However, a laser with high device yield and high output power at telecommunication wavelengths, where photonics applications are clustered, is still missing, hindered by large mode transition loss, non-optimized cavity design, and a complicated fabrication process. Here, we report high-performance lasers on SiN with tens of milliwatts output power through the SiN waveguide and sub-kHz fundamental linewidth, addressing all the aforementioned issues. We also show Hertz-level fundamental linewidth lasers are achievable with the developed integration techniques. These lasers, together with high-Q SiN resonators, mark a milestone towards a fully integrated low-noise silicon nitride photonics platform. This laser should find potential applications in LIDAR, microwave photonics and coherent optical communications., Achieving high output power and low noise integrated lasers is a major challenge. Here the authors experimentally demonstrate integrated lasers from a Si/SiN heterogeneous platform that shows Hertz-level linewidth, paving the way toward fully integrating low-noise silicon nitride photonics in volume using real devices for lasing.

Lasers with hertz linewidths at time scales of seconds are critical for metrology, timekeeping, and manipulation of quantum systems. Such frequency stability relies on bulk-optic lasers and reference cavities, where increased size is leveraged to reduce noise but with the trade-off of cost, hand assembly, and limited applications. Alternatively, planar waveguide–based lasers enjoy complementary metal-oxide semiconductor scalability yet are fundamentally limited from achieving hertz linewidths by stochastic noise and thermal sensitivity. In this work, we demonstrate a laser system with a 1-s linewidth of 1.1 Hz and fractional frequency instability below 10 −14 to 1 s. This low-noise performance leverages integrated lasers together with an 8-ml vacuum-gap cavity using microfabricated mirrors. All critical components are lithographically defined on planar substrates, holding potential for high-volume manufacturing. Consequently, this work provides an important advance toward compact lasers with hertz linewidths for portable optical clocks, radio frequency photonic oscillators, and related communication and navigation systems.

Narrow-linewidth lasers are important to many applications spanning precision metrology to sensing systems. Characterization of these lasers requires precise measurements of their frequency noise spectra. Here we demonstrate a correlated self-heterodyne (COSH) method capable of measuring frequency noise as low as 0.01 Hz$^2$/Hz at 1 MHz offset frequency. The measurement setup is characterized by both commercial and lab-built lasers, and features low optical power requirements, fast acquisition time and high intensity noise rejection., 10 pages, 6 figures

A semiconductor laser is self-injection-locked to a spiral, ultra-high-Q silicon-nitride resonator. The laser attains record-low frequency noise noise floor (0.006 Hz2/Hz or 40 mHz white-noise-linewidth) with stability at low-offset frequencies comparable to high-performance fiber lasers.

We demonstrated mode-hop-free tuning over 225 GHz (1.8 nm) in an integrated external-cavity diode laser operating around 1550 nm and with an intrinsic linewidth of 4.7 kHz.

The use of silicon nitride in integrated photonics has rapidly progressed in recent decades. Ultra-low-loss waveguides based on silicon nitride are a favorable platform for the research of nonlinear and microwave photonics and their application to a wide variety of fields, including precision metrology, communications, sensing, imaging, navigation, computation, and quantum physics. In recent years, the integration of Si and III-V materials has enabled new large-scale, advanced silicon nitride-based photonic integrated circuits with versatile functionality. In this perspective article, we review current trends and the state-of-the-art in silicon nitride-based photonic devices and circuits. We highlight the hybrid and heterogeneous integration of III-V with silicon nitride for electrically pumped soliton microcomb generation and ultra-low-noise lasers with fundamental linewidths in the tens of mHz range. We also discuss several ultimate limits and challenges of silicon nitride-based photonic device performance and provide routes and prospects for future development.

Within the context of globalization, the development of renewable energy is critical for attaining sustainable development, and the digital economy is also a critical driver for achieving it. This article incorporates globalization, renewable energy development, and the digital economy into its research framework, investigates the relationship between globalization and renewable energy development, and explores the moderating effect of the digital economy, using panel data from countries along the Belt and Road (B&R) from 2001 to 2018. It is found that globalization facilitates the development of renewable energy. The 1% increase in globalization results in a 1.06% increase in renewable energy development; the level of globalization has a significant effect on renewable energy development in high-income countries, upper-middle-income countries, and low-income countries, but not in lower-middle-income countries; the digital economy has a moderating effect on the impact of globalization on renewable energy development in countries along the B&R. Simultaneously, the effect of globalization on renewable energy development in B&R countries is influenced by the digital economy’s single threshold effect, and the effect of globalization on renewable energy development is more pronounced when the level of digital economy development is less than the threshold of 0.061. The conclusions of this article have significant implications for the B&R countries’ sustainable development in the contexts of globalization and the digital economy.

Mainstream silicon photonic integrated circuits are based on compact and low-loss silicon-on-insulator (SOI) waveguide platforms. However, monolithic SOI-based photonics provides only a limited number of functional device types. Here, to extend the on-chip capabilities, we propose a general heterogeneous integration approach to embed highly nonlinear III-V (AlGaAs) photonics into the SOI platform. We develop low-loss AlGaAs-on-SOI photonic circuits with integrated Si waveguides and showcase sub-milliwatt-threshold ( ∼ 0.25 mW ) Kerr frequency comb generation in ultrahigh- Q AlGaAs microrings ( Q over 10 6 ) at the telecom bands. Our demonstration complements existing mature Si photonics technology with efficient nonlinear functionalities provided by III-V and propels conventional Si photonics into emerging nonlinear photonic applications towards fully chip-based nonlinear engines.