Fiber ring interferometer

The silicon nitride (Si3N4) planar waveguide platform has enabled a broad class of low-loss planar-integrated devices and chip-scale solutions that benefit from transparency over a wide wavelength range (400–2350 nm) and fabrication using wafer-scale processes. As a complimentary platform to silicon-on-insulator (SOI) and III–V photonics, Si3N4 waveguide technology opens up a new generation of system-on-chip applications not achievable with the other platforms alone. The availability of low-loss waveguides (<1 dB/m) that can handle high optical power can be engineered for linear and nonlinear optical functions, and that support a variety of passive and active building blocks opens new avenues for system-on-chip implementations. As signal bandwidth and data rates continue to increase, the optical circuit functions and complexity made possible with Si3N4 has expanded the practical application of optical signal processing functions that can reduce energy consumption, size and cost over today’s digital electronic solutions. Researchers have been able to push the performance photonic-integrated components beyond other integrated platforms, including ultrahigh Q resonators, optical filters, highly coherent lasers, optical signal processing circuits, nonlinear optical devices, frequency comb generators, and biophotonic system-on-chip. This review paper covers the history of low-loss Si3N4 waveguide technology and a survey of worldwide research in a variety of device and applications as well as the status of Si3N4 foundries.

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Silicon Nitride in Silicon Photonics

A recent renaissance in Brillouin scattering research has been driven by the increasing maturity of photonic integration platforms and nanophotonics. The result is a new breed of chip-based devices that exploit acousto-optic interactions to create lasers, amplifiers, filters, delay lines and isolators. Here, we provide a detailed overview of Brillouin scattering in integrated waveguides and resonators, covering key concepts such as the stimulation of the Brillouin process, in which the optical field itself induces acoustic vibrations, the importance of acoustic confinement, methods for calculating and measuring Brillouin gain, and the diversity of materials platforms and geometries. Our Review emphasizes emerging applications in microwave photonics, signal processing and sensing, and concludes with a perspective for future directions. Acousto-optical interactions within integrated optics platforms are reviewed with a discussion of the useful chip-based devices such as lasers, amplifiers, filters, isolators and more besides that can result.

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Brillouin integrated photonics

Optical frequency combs have a wide range of applications in science and technology1. An important development for miniature and integrated comb systems is the formation of dissipative Kerr solitons in coherently pumped high-quality-factor optical microresonators2–9. Such soliton microcombs10 have been applied to spectroscopy11–13, the search for exoplanets14,15, optical frequency synthesis16, time keeping17 and other areas10. In addition, the recent integration of microresonators with lasers has revealed the viability of fully chip-based soliton microcombs18,19. However, the operation of microcombs requires complex startup and feedback protocols that necessitate difficult-to-integrate optical and electrical components, and microcombs operating at rates that are compatible with electronic circuits—as is required in nearly all comb systems—have not yet been integrated with pump lasers because of their high power requirements. Here we experimentally demonstrate and theoretically describe a turnkey operation regime for soliton microcombs co-integrated with a pump laser. We show the appearance of an operating point at which solitons are immediately generated by turning the pump laser on, thereby eliminating the need for photonic and electronic control circuitry. These features are combined with high-quality-factor Si3N4 resonators to provide microcombs with repetition frequencies as low as 15 gigahertz that are fully integrated into an industry standard (butterfly) package, thereby offering compelling advantages for high-volume production. A turnkey regime for soliton microcombs is demonstrated, in which solitons are generated by switching on a co-integrated pump laser, eliminating the need for photonic and electronic control circuitry.

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Integrated turnkey soliton microcombs

Driven by narrow-linewidth bench-top lasers, coherent optical systems spanning optical communications, metrology and sensing provide unrivalled performance To transfer these capabilities from the laboratory to the real world, a key missing ingredient is a mass-produced integrated laser with superior coherence Here, we bridge conventional semiconductor lasers and coherent optical systems using CMOS-foundry-fabricated microresonators with a high Q factor of over 260 million and finesse over 42,000 A five-orders-of-magnitude noise reduction in the pump laser is demonstrated, enabling a frequency noise of 02 Hz2 Hz−1 to be achieved in an electrically pumped integrated laser, with a corresponding short-term linewidth of 12 Hz Moreover, the same configuration is shown to relieve the dispersion requirements for microcomb generation that have handicapped certain nonlinear platforms The simultaneous realization of this high Q factor, highly coherent lasers and frequency combs using foundry-based technologies paves the way for volume manufacturing of a wide range of coherent optical systems Using CMOS-ready ultra-high-Q microresonators, a highly coherent electrically pumped integrated laser with frequency noise of 02 Hz2 Hz−1, corresponding to a short-term linewidth of 12 Hz, is demonstrated The device configuration is also found to relieve the dispersion requirements for microcomb generation that have limited certain nonlinear platforms

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Hertz-linewidth semiconductor lasers using CMOS-ready ultra-high-Q microresonators

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.

High-power sub-kHz linewidth lasers fully integrated on silicon

Heterogeneous silicon photonics using wafer bonding is reaching maturity with commercial products for the data center market being shipped in volume Here we give an overview of recent research in the area showing record device performance by using the best of both worlds: III–V for light generation and Si for guiding the light Utilizing the flexibility of the heterogeneous silicon platform, narrow-linewidth widely tunable lasers as well as fully integrated mode locked lasers with record pulse powers and pulse duration were demonstrated The ability to perform multiple die bonding with optimized epitaxially grown layer stacks was used to realize high-performance photonic integrated circuits both for communications and sensing In addition to III–V materials, nonreciprocal materials such as, for example, Ce:YIG can be bonded, providing additional functionality such as on-chip isolators and reconfigurable on-chip circulators On-chip isolation will become necessary with the increase in complexity of photonic integrated chips as photonic components such as lasers are sensitive to feedback effects

Photonic Integrated Circuits Using Heterogeneous Integration on Silicon

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.

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Tutorial on narrow linewidth tunable semiconductor lasers using Si/III-V heterogeneous integration

Heterogeneous silicon photonics is uniquely positioned to address the photonic sensing needs of upcoming autonomous cars and provide the necessary cost reduction for widespread deployment. This is because it allows for wafer-scale active/passive integration, including optical sources. We present our recent research and the development of interferometric optical gyroscopes and LiDAR sensors. More specifically, we show a fully integrated gyroscope front-end occupying an area of only 4.5 mm2. We also show the first dense pitch optical phased array using heterogeneous phase shifters. The 4 µm pitch heterogeneous phase shifters provide very low V2π of only 0.35-1.4 V across 200 nm, low residual amplitude modulation of only 0.1-0.15 dB for 2π phase shift, extremely low static power consumption ( 1 GHz). All of these factors make them ideal for next-generation LiDAR systems that employ optical phased arrays.

Heterogeneous silicon photonics sensing for autonomous cars.

Determination of laser frequency with high resolution under continuous and abrupt tuning conditions is important for sensing, spectroscopy, and communications. We show that a single microresonator provides rapid and broadband measurement of optical frequencies with a relative frequency precision comparable to that of conventional dual-frequency comb systems. Dual-locked counterpropagating solitons having slightly different repetition rates were used to implement a vernier spectrometer, which enabled characterization of laser tuning rates as high as 10 terahertz per second, broadly step-tuned lasers, multiline laser spectra, and molecular absorption lines. Besides providing a considerable technical simplification through the dual-locked solitons and enhanced capability for measurement of arbitrarily tuned sources, our results reveal possibilities for chip-scale spectrometers that exceed the performance of tabletop grating and interferometer-based devices.

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Minh A. Tran

Papers

High-power sub-kHz linewidth lasers fully integrated on silicon

Photonic Integrated Circuits Using Heterogeneous Integration on Silicon

Tutorial on narrow linewidth tunable semiconductor lasers using Si/III-V heterogeneous integration

Heterogeneous silicon photonics sensing for autonomous cars.

Vernier spectrometer using counterpropagating soliton microcombs