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

Laboratory optical atomic clocks achieve remarkable accuracy (now counted to 18 digits or more), opening possibilities for exploring fundamental physics and enabling new measurements. However, their size and the use of bulk components prevent them from being more widely adopted in applications that require precision timing. By leveraging silicon-chip photonics for integration and to reduce component size and complexity, we demonstrate a compact optical-clock architecture. Here a semiconductor laser is stabilized to an optical transition in a microfabricated rubidium vapor cell, and a pair of interlocked Kerr-microresonator frequency combs provide fully coherent optical division of the clock laser to generate an electronic 22 GHz clock signal with a fractional frequency instability of one part in 1013. These results demonstrate key concepts of how to use silicon-chip devices in future portable and ultraprecise optical clocks.

Architecture for the photonic integration of an optical atomic clock

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

Exceptional points (EPs) are special spectral degeneracies of non-Hermitian Hamiltonians that govern the dynamics of open systems. At an EP, two or more eigenvalues, and the corresponding eigenstates, coalesce1–3. Recently, it was predicted that operation of an optical gyroscope near an EP results in improved response to rotations4,5. However, the performance of such a system has not been examined experimentally. Here we introduce a precisely controllable physical system for the study of non-Hermitian physics and nonlinear optics in high-quality-factor microresonators. Because this system dissipatively couples counter-propagating lightwaves within the resonator, it also functions as a sensitive gyroscope for the measurement of rotations. We use our system to investigate the predicted EP-enhanced Sagnac effect4,5 and observe a four-fold increase in the Sagnac scale factor by directly measuring rotations applied to the resonator. The level of enhancement can be controlled by adjusting the system bias relative to the EP, and modelling results confirm the observed enhancement. Moreover, we characterize the sensitivity of the gyroscope near the EP. Besides verifying EP physics, this work is important for the understanding of optical gyroscopes. precisely controllable integrated optical gyroscope based on stimulated Brillouin scattering is used to study non-Hermitian physics, revealing a four-fold enhancement of the Sagnac scale factor near exceptional points.

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Observation of the exceptional-point-enhanced Sagnac effect.

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

Spectrally pure lasers, the heart of precision high-end scientific and commercial applications, are poised to make the leap from the laboratory to integrated circuits. Translating this performance to integrated photonics will dramatically reduce cost and footprint for applications such as ultrahigh capacity fibre and data centre networks, atomic clocks and sensing. Despite the numerous applications, integrated lasers currently suffer from large linewidth. Brillouin lasers, with their unique properties, offer an intriguing solution, yet bringing their performance to integrated platforms has remained elusive. Here, we demonstrate a sub-hertz (~0.7 Hz) fundamental linewidth Brillouin laser in an integrated Si3N4 waveguide platform that translates advantages of non-integrated designs to the chip scale. This silicon-foundry-compatible design supports low loss from 405 to 2,350 nm and can be integrated with other components. Single- and multiple-frequency output operation provides a versatile low phase-noise solution. We highlight this by demonstrating an optical gyroscope and a low-phase-noise photonic oscillator. Brillouin lasing with 0.7 Hz fundamental linewidth is observed by optically exciting a monolithic bus–ring Si3N4 waveguide resonator. The Brillouin laser is applied to an optical gyroscope and a low phase-noise photonic microwave oscillator.

Sub-hertz fundamental linewidth photonic integrated Brillouin laser

The demand for high-performance chip-scale lasers has driven rapid growth in integrated photonics. The creation of such low-noise laser sources is critical for emerging on-chip applications, ranging from coherent optical communications, photonic microwave oscillators remote sensing and optical rotational sensors. While Brillouin lasers are a promising solution to these challenges, new strategies are needed to create robust, compact, low power and low cost Brillouin laser technologies through wafer-scale integration. To date, chip-scale Brillouin lasers have remained elusive due to the difficulties in realization of these lasers on a commercial integration platform. In this paper, we demonstrate, for the first time, monolithically integrated Brillouin lasers using a wafer-scale process based on an ultra-low loss Si3N4/SiO2 waveguide platform. Cascading of stimulated Brillouin lasing to 10 Stokes orders was observed in an integrated bus-coupled resonator with a loaded Q factor exceeding 28 million. We experimentally quantify the laser performance, including threshold, slope efficiency and cascading dynamics, and compare the results with theory. The large mode volume integrated resonator and gain medium supports a TE-only resonance and unique 2.72 GHz free spectral range, essential for high performance integrated Brillouin lasing. The laser is based on a non-acoustic guiding design that supplies a broad Brillouin gain bandwidth. Characteristics for high performance lasing are demonstrated due to large intra-cavity optical power and low lasing threshold power. Consistent laser performance is reported for multiple chips across multiple wafers. This design lends itself to wafer-scale integration of practical high-yield, highly coherent Brillouin lasers on a chip.

Integrated Waveguide Brillouin Laser

Two major architectures of fiber optic gyroscopes have been under development at Honeywell in recent years. The interferometric fiber optic gyro (IFOG) has been in production and deployment for various high performance space and marine applications. Different designs, offering very low noise, ranging from better than navigation grade to ultra-precise performance have been tested and produced. The resonator fiber optic gyro (RFOG) is also under development, primarily for its attractive potential for civil navigation usage, but also because of its scalability to other performance. New techniques to address optical backscatter and laser frequency noise have been developed and demonstrated. Development of novel, enhanced RFOG architectures using hollow core fiber, silicon optical bench technology, and highly stable multifrequency laser sources are discussed.

Fiber optic gyro development at Honeywell

In an example, a chip-scale atomic clock physics package is provided. This chip-scale atomic clock physics package includes a body defining a cavity, and a first scaffold mounted in the cavity. A laser is mounted on the first surface of the first scaffold. A second scaffold is also mounted in the cavity. The second scaffold is disposed such that the first surface of the second scaffold is facing the first scaffold. A first photodetector is mounted on the first surface of the second scaffold. A vapor cell is mounted on the first surface of the second scaffold. A waveplate is also included, wherein the laser, waveplate, first photodetector, and vapor cell are disposed such that a beam from the laser can propagate through the waveplate and the vapor cell and be detected by the first photodetector. A lid is also included for covering the cavity.

Vapor cell atomic clock physics package

In this paper we report progress toward a compact resonator fiber optic gyroscope (RFOG) design for commercial navigation applications Specifically, we report the first use of silicon optical bench (SIOB) technology within the gyro to miniaturize optical component size A low loss SIOB is used for optical resonator loop closure and a promising initial finesse of 35 is observed The gyro angle random walk is 00029 deg/rt-hr, and first results show a bias stability of better than 01 deg/hr for a sensing coil diameter of two inches These results represent an encouraging step toward the realization of a practical navigation-grade RFOG for civil navigation usage

Mary K. Salit

Papers

Sub-hertz fundamental linewidth photonic integrated Brillouin laser

Integrated Waveguide Brillouin Laser

Fiber optic gyro development at Honeywell

Vapor cell atomic clock physics package

Development of compact resonator fiber optic gyroscopes