Optical-based rotation sensors have revolutionized precision, high-sensitivity inertial navigation systems. At the same time these sensors use bulky optical fiber spools or free-space resonators. A chip-based, micro-optical gyroscope is demonstrated that uses counterpropagating Brillouin lasers to measure rotation as a Sagnac-induced frequency shift. Preliminary work has demonstrated a rotation-rate measurement that surpasses prior micro-optical rotation-sensing systems by over 40-fold.

Microresonator Brillouin gyroscope

As the only method for all-weather, all-time and long-distance target detection and recognition, radar has been intensively studied since it was invented, and is considered as an essential sensor for future intelligent society. In the past few decades, great efforts were devoted to improving radar's functionality, precision, and response time, of which the key is to generate, control and process a wideband signal with high speed. Thanks to the broad bandwidth, flat response, low loss transmission, multidimensional multiplexing, ultrafast analog signal processing and electromagnetic interference immunity provided by modern photonics, implementation of the radar in the optical domain can achieve better performance in terms of resolution, coverage, and speed which would be difficult (if not impossible) to implement using traditional, even state-of-the-art electronics. In this tutorial, we overview the distinct features of microwave photonics and some key microwave photonic technologies that are currently known to be attractive for radars. System architectures and their performance that may interest the radar society are emphasized. Emerging technologies in this area and possible future research directions are discussed.

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Microwave Photonic Radars

On-chip optical waveguides with low propagation losses and precisely engineered group velocity dispersion (GVD) are important to nonlinear photonic devices such as soliton microcombs. Yet, despite intensive research efforts, nonlinear integrated photonic platforms still feature propagation losses orders of magnitude higher than in standard optical fiber. The tight confinement and high index contrast of integrated waveguides make them highly susceptible to fabrication induced surface roughness. Therefore, microresonators with ultra-high Q factors are, to date, only attainable in polished bulk crystalline, or chemically etched silica based devices, that pose however challenges for full photonic integration. Here, we demonstrate the fabrication of silicon nitride ($\mathrm{Si_3N_4}$) waveguides with unprecedentedly smooth sidewalls and tight confinement with record low propagation losses. This is achieved by combining the photonic Damascene process with a novel reflow process, which reduces etching roughness, while sufficiently preserving dimensional accuracy. This leads to previously unattainable \emph{mean} microresonator Q factors larger than $5\times10^6$ for tightly confining waveguides with anomalous dispersion. Via systematic process step variation and two independent characterization techniques we differentiate the scattering and absorption loss contributions, and reveal metal impurity related absorption to be an important loss origin. Although such impurities are known to limit optical fibers, this is the first time they are identified, and play a tangible role, in absorption of integrated microresonators. Taken together, our work provides new insights in the origins of propagation losses in $\mathrm{Si_3N_4}$ waveguides and provides the technological basis for integrated nonlinear photonics in the ultra-high Q regime.

Probing the loss origins of ultra-smooth $\mathrm{Si_3N_4}$ integrated photonic waveguides

We report on fabrication of high-confinement and low loss silicon nitride ( $\text{Si}_{3}\text{N}_{4}$ ) waveguides using the photonic Damascene process. This process scheme represents a novel fabrication approach enabling reliable, wafer-scale fabrication of high-confinement optical waveguides. A reflow step of the silica preform reduces sidewall scattering to values not attainable with conventional etching, and reduces losses and backscattering significantly, resulting in a waveguide attenuation of 5.5 dB/m. We discuss the critical aspects of the process in detail and demonstrate the fabrication of high stress $\text{Si}_{3}\text{N}_{4}$ waveguides with unprecedentedly large dimensions ( $\text{1.75}\,\mu \text{m} \times \text{1.425}\,\mu \text{m}$ ) providing high-confinement at midinfrared wavelengths. A device characterization strategy allowing for systematic extraction of statistically relevant loss values is discussed and reveals the effects of the sidewall smoothing.

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Photonic Damascene Process for Low-Loss, High-Confinement Silicon Nitride Waveguides

Optical gyroscopes are among the most accurate rotation measuring devices and are widely used for navigation and accurate pointing. Since the advent of photonic integrated components for communications, and with their increasing complexity, there has been interest in the possibility of chip-scale optical gyroscopes1. Besides the potential benefits of integration, such solid-state systems would be robust and resistant to shock. Here, we report a gyroscope using Brillouin ring lasers on a silicon chip. Its stability and sensitivity enable measurement of Earth’s rotation, representing a major milestone for this new class of gyroscope. A Sagnac gyroscope based on Brillouin ring lasers on a silicon chip is presented. The stability and sensitivity of this on-chip planar gyroscope allow measurement of the Earth’s rotation, with an amplitude sensitivity as small as 5 deg h−1 for a sinusoidal rotation, an angle random walk of 0.068 deg h−1/2 and bias instability of 3.6 deg h−1.

Earth rotation measured by a chip-scale ring laser gyroscope

Low-cost chip-scale optoelectronic gyroscopes having a resolution ≤ 10 °/h and a good reliability also in harsh environments could have a strong impact on the medium/high performance gyro market, which is currently dominated by well-established bulk optical angular velocity sensors. The R&D activity aiming at the demonstration of those miniaturized sensors is crucial for aerospace/defense industry, and thus it is attracting an increasing research effort and notably funds. In this paper the recent technological advances on the compact optoelectronic gyroscopes with low weight and high energy saving are reviewed. Attention is paid to both the so-called gyroscope-on-a-chip, which is a novel sensor, at the infantile stage, whose optical components are monolithically integrated on a single indium phosphide chip, and to a new ultra-high Q ring resonator for gyro applications with a configuration including a 1D photonic crystal in the resonant path. The emerging field of the gyros based on passive ring cavities, which have already shown performance comparable with that of optical fiber gyros, is also discussed.

https://www.jeos.org/index.php/jeos_rp/article/download/14013/1170

Recent advances in miniaturized optical gyroscopes

The design of an integrated graphene-based fine-tunable optical delay line on silicon nitride for optical beamforming in phased-array antennas is reported. A high value of the optical delay time (τg=920  ps) together with a compact footprint (4.15  mm2) and optical loss <27  dB make this device particularly suitable for highly efficient steering in active phased-array antennas. The delay line includes two graphene-based Mach–Zehnder interferometer switches and two vertically stacked microring resonators between which a graphene capacitor is placed. The tuning range is obtained by varying the value of the voltage applied to the graphene electrodes, which controls the optical path of the light propagation and therefore the delay time. The graphene provides a faster reconfigurable time and low values of energy dissipation. Such significant advantages, together with a negligible beam-squint effect, allow us to overcome the limitations of conventional RF beamformers. A highly efficient fine-tunable optical delay line for the beamsteering of 20 radiating elements up to ±20° in the azimuth direction of a tile in a phased-array antenna of an X-band synthetic aperture radar has been designed.

Graphene-based fine-tunable optical delay line for optical beamforming in phased-array antennas.

A rigorous mathematical method to simulate an ultra-high Q-factor 1D PhC ring resonator (1D PhCRR) is proposed. The integration of a 1D PhC in a ring cavity resonator allows an enhancement of the Q-factor of a single ring resonator up to 109 or more, without enlarging the device footprint, with an improvement of at least two orders of magnitude in comparison with the values obtained without the grating. Rigorous modelling and simulations of such ultra high-Q resonant cavities are very challenging, because of the structure complexity, due to the large device footprint and the requirements for an accurate spectral analysis. Conventional numerical methods, such as Finite Element Method (FEM) and Finite Difference Time Domain (FDTD) require very long simulation time and huge computing resource. Therefore, a general mathematical method, able to reduce the computing time, assuring accurate solutions, and investigating the effect of the grating in the coupling region, is a real need. Several configurations of 1D PhCRRs have been investigated. An ultra-high Q-factor (> 109) for a 1D PhCRR in Si 3 N 4 technology with a footprint of 64 mm2 has been calculated. Similar performance makes the 1D PhC ring resonator suitable for several applications, such as optical gyroscopes for space environment and ultra-sensitive biosensors.

Rigorous model for the design of ultra-high Q-factor resonant cavities

In the last few years, the interest towards chip-scale microphotonic resonant label-free biosensor is quickly growing. In this context, novel low-cost platforms able to simultaneously detect, with high resolution, several target molecules in an ultra-small volume of biologic fluid are strongly demanded. In this paper, the recent advances in the field of resonant microphotonic platforms for label-free biosensing are critically reviewed, with a special emphasis on the devices exhibiting an ultra-high sensitivity due to a strong light-matter interaction. A new multi-analyte biosensing platform in silicon nitride technology is reported. The platform is based on a properly designed configuration of planar ring resonator and exhibits a record limit-of-detection of 0.06 pg/mm2. The applications of the platform in the field of medical diagnostics and environmental monitoring are discussed.

New microphotonic resonant devices for label-free biosensing

An integrated graphene-based fine-tunable optical delay line in silicon nitride has been designed. A high optical delay time up to 920 ps has been calculated with a device footprint A ~ 4 mm2. The device includes two graphene-based Mach-Zehnder interferometer (MZI) switches and two vertically-stacked microring resonators. Graphene capacitors have been integrated in the optical delay line to exploit the electro-optic effect aiming at fast tuning (response time of few nanoseconds) with a low power dissipation. Such performance, together with optical loss α < 27 dB, makes this device suitable for beam-steering up to 20 radiating elements, with an angle of ±20° in the azimuth direction, of a tile in a phased-array antenna included in an X-band SAR.

T. Tatoli

Papers

Recent advances in miniaturized optical gyroscopes

Graphene-based fine-tunable optical delay line for optical beamforming in phased-array antennas.

Rigorous model for the design of ultra-high Q-factor resonant cavities

New microphotonic resonant devices for label-free biosensing

Reconfigurable optical beamformer with graphene-based fine-tunable optical delay line