Two-dimensional multi-beam steering for parallel free-space optical communication based on a silicon nitride optical phase array

Optical phased array (OPA) structures are designed and fabricated for the optical addressing of trapped <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">88</sup> Sr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ions. Laser light of 1092 nm wavelength corresponding to the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{2}\text{D}_{3/2}$ </tex-math></inline-formula> to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{2}\text{P}_{1/2}$ </tex-math></inline-formula> of <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">88</sup> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sub> ion is used, where silicon nitride (SiN) platform with integrated titanium nitride (TiN) thermos-optic heaters are selected. Beam steering of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 33~\mu \text{m}$ </tex-math></inline-formula> , which corresponds to ~11.6° steering angle, is obtained by applying 10V on alternative TiN heaters in the 8-Channel OPA structure.

Optimization and comprehensive comparison of thermo-optic phase shifter with folded waveguide on SiN and SOI platforms

In this paper, a 128-channel non-uniform optical phased array is proposed. The antenna is based on a silicon nitride waveguide with a large cross-sectional area (3 μm × 1.2 μm) and a silicon nitride grating with a small diffraction window (grating width of 200 nm), enabling high optical power transmission and a wide 1 dB field of view. As a result, the designed sparse optical phased array achieves less than 1 dB of main lobe attenuation over a 94° field of view. Within this field of view, the main lobe will not fall below 80% of the maximum main lobe. This allows the minimum detection distance to still be about 89% of the maximum detection distance without increasing the input power. In this field of view, the maximum side-lobe suppression of the designed sparse optical phased array is 13.4 dB, and the minimum side-lobe suppression is higher than 11.9 dB. This is useful for simultaneously achieving a large field of view, low main lobe attenuation, stable side-lobe suppression, and long detection distance.

Optical phased array (OPA) beam scanners for light detection and ranging (LiDAR) are proposed by integrating polymer waveguides with superior thermo-optic effect and silicon nitride (SiN) waveguides exhibiting strong modal confinement along with high optical power capacity. A low connection loss of only 0.15 dB between the polymer and SiN waveguides was achieved in this work, enabling a low-loss OPA device. The polymer-SiN monolithic OPA demonstrates not only high optical throughput but also efficient beamforming and stable beam scanning. This novel integrative approach highlights the potential of leveraging heterogeneous photonic materials to develop advanced photonic integrated circuits with superior performance.

Enhancing wireless optical communication through multi-beam scanning and bandwidth optimization using an optical phased array

The silicon photonic optical phased array (OPA) featured by high integration and agile beam steering exhibits greatly potential application in optical wireless communication (OWC). In this article, we demonstrate high-data-rate and long-range OWC using an integrated Silicon-Silicon Nitride OPA chip with 12 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> large aperture. The OPA chip constituted by 128 waveguide grating antennas at nonuniform spacing is employed for wide-steering and small-divergence solid beam steering. In the experiment, we demonstrated 100° wide range communications in horizontal direction via antenna phase tuning. To our best knowledge, 100° is the widest cover range in the reported OPA-based OWC systems. Moreover, the OPA chip is also utilized to achieve up to 32 Gb/s low BER transmission of non-return-to-zero (NRZ) signal over 54 m free space distance. In addition, we also successfully demonstrated more than 50 Gb/s PAM4 signal transmission by measuring clear eye diagrams at the horizontal direction of 0° and 50° over 10 m. Thanks to agile beam steering of the OPA, switching time among arbitrary positions in multi-targets communication can be less than 27 μs. By virtue of tiny beam divergence 0.021° × 0.029° emitted from the OPA, data transmission switching between two targets spaced at 3.5 cm over 20 m can be achieved accurately. The experimental measurement shows the sidelobe crosstalk from the adjacent target channel is less than −25.2 dB. Our demonstrations validate the feasible implementation of the silicon-based OPA in the high-data-rate and wide-steering-range OWC system.

In this work, we demonstrate a 1024-channel grating-based antennas phased array fabricated on silicon nitride platform. The beam steering angle range can reach to 56from 635 nm to 1064 nm in the longitudinal direction while the divergence angle is only 0.023°.

We demonstrate an ultra-compact waveguide taper on a silicon nitride platform. The proposed taper provides a coupling efficiency of 95% at a length of 19.5μm in comparison to the standard linear taper of length 50μm, which connects a 10μm wide waveguide to a 1μm wide photonic wire. The taper has a spectral response >75% spanning over 800nm and resilience to fabrication variations; ±200 nm change in taper and end waveguide width varies transmission by <5%. We experimentally demonstrate taper insertion loss of <0.1 dB/transition for a taper as short as 19.5μm, and reduce the footprint of the photonic device by 50.8% compared to the standard adiabatic taper. To the best of our knowledge, the proposed taper is the shortest waveguide taper ever reported in silicon nitride.

Silicon nitride (SiN x ) is an appealing waveguide material choice for large-scale, high-performance photonic integrated circuits (PICs) due to its low optical loss. However, SiN x PICs require high electric power to realize optical reconfiguration via the weak thermo-optic effect, which limits their scalability in terms of device density and chip power dissipation. We report a 6-mode programmable interferometer PIC operating at the wavelength of 1550 nm on a CMOS-compatible low-temperature inductance coupled plasma chemical vapor deposition (ICP-CVD) silicon nitride platform. By employing suspended thermo-optic phase shifters, the PIC achieves 2× improvement in compactness and 10× enhancement in power efficiency compared to conventional devices. Reconfigurable 6-dimensional linear transformations are demonstrated including cyclic transformations and arbitrary unitary matrices. This work demonstrates the feasibility of fabricating power-efficient large-scale reconfigurable PICs on the low-temperature ICP-CVD silicon nitride platform.

We demonstrate passive large-scale nanophotonic phased arrays in a CMOS-compatible silicon photonic platform. Silicon nitride waveguides are used to allow for higher input power and lower phase variation compared to a silicon-based distribution network. A phased array at an infrared wavelength of 1550 nm is demonstrated with an ultra-large aperture size of 4 mm×4 mm, achieving a record small and near diffraction-limited spot size of 0.021°×0.021° with a side lobe suppression of 10 dB. A main beam power of 400 mW is observed. Using the same silicon nitride platform and phased array architecture, we also demonstrate, to the best of our knowledge, the first large-aperture visible nanophotonic phased array at 635 nm with an aperture size of 0.5 mm×0.5 mm and a spot size of 0.064°×0.074°.

Silicon photonics enables the development of optical components on a chip with the potential for large-scale optical integrated circuits that can be fabricated at the wafer-scale using foundries similar to those used in the electronics industry. Although silicon is a passive optical material with an indirect bandgap, reconfigurable devices have been demonstrated using thermo-optic effects (large phase shifts, but relatively slow with large power consumption) and carrier plasma dispersion effects (high-speed, but small phase shifts). We recently demonstrated a low-power approach for inducing large phase shifts (>2π) using a technique that we call micro-opto-electro-mechanical index perturbation (MOEM-IP). In this initial work we characterized silicon nitride waveguides in which the propagating optical mode’s evanescent field is vertically coupled to silicon nitride microbridges. This interaction leads to an effective index tuning that is a strong function of the waveguide-microbridge separation. We now extend our MOEM-IP approach to different configurations (i.e. in-plane coupling) and material systems (i.e. silicon-oninsulator). Mode perturbation simulations indicate that the MOEM-IP approach is widely applicable to many configurations and material systems enabling large effective index tuning (Δneffective>0.1) requiring microbridge displacements of only a few hundred nanometers. We also examine several device applications that take advantage of MOEM-IP. These include tunable optical filters using high-Q microring cavities and optical phased arrays that enable chip-scale beam steering in two-dimensions using low-power phase shifting enabled by MOEM-IP.

As the optical analogue to integrated electronics, integrated photonics has already found widespread use in data centers in the form of optical interconnects. As global network traffic continues its rapid expansion, the power consumption of such circuits becomes a critical consideration. Electrically tunable devices in photonic integrated circuits contribute significantly to the total power budget, as they traditionally rely on inherently power-consuming phenomena such as the plasma dispersion effect or the thermo-optic effect for operation. Here, we demonstrate ultra-low-power refractive index tuning in a hybrid barium titanate (BTO)-silicon nitride (SiN) platform integrated on silicon. We achieve tuning by exploiting the large electric field-driven Pockels effect in ferroelectric BTO thin films of sub-100 nm thickness. The extrapolated power consumption for tuning a free spectral range (FSR) in racetrack resonator devices is only 106 nW/FSR, several orders of magnitude less than many previous reports. We demonstrate the technological potential of our hybrid BTO-SiN technology by compensating thermally induced refractive index variations over a temperature range of 20 {\deg}C and by using our platform to fabricate tunable multiresonator optical filters. Our hybrid BTO-SiN technology significantly advances the field of ultra-low-power integrated photonic devices and allows for the realization of next-generation efficient photonic circuits for use in a variety of fields, including communications, sensing, and computing.

We present novel design of thermally driven acousto-optic modulation with surface acoustic wave (SAW) in silicon nitride (Si3N4) platform. Buried waveguide with metal grating on the surface is optimized for intramodal and intermodal modulation optomechanics.

Photonic crystal (PhC) waveguides on the silicon nitride (SixNy) platform currently are capable only of transmission of a single light polarization, severely limiting polarization diversity applications. In this work, we propose a scheme that combines a single-polarization photonic band gap (SPBG) formed by the first two bands with the same polarization, along with large birefringence between the first two bands with different polarizations. In simple terms, one polarization in the PhC slab waveguide is guided by the SPBG, while the other polarization is index guided, and overlap between the frequency bands for the two polarizations in which propagation is forbidden, dubbed the no-polarization-mode region (NMR), tuned by the aspect ratio of PhC slabs. This approach theoretically achieves PhC waveguides on asymmetric SixNy slabs that guide both polarizations within the NMR. Three-dimensional (3D) planewave-method calculations show that through simple adjustment of the aspect ratio of the PhC slab, the first two bands with different polarizations can be adjusted to be shifted in frequency to form the NMR, where neither TE- nor TM-like modes can propagate along the in-plane directions. The NMR is then systematically investigated and optimized for various refractive indices and slab thicknesses, and using the optimized-NMR design, line-defect waveguides supporting propagation of both polarizations are theoretically demonstrated. A 3D finite-difference time-domain simulation shows that an all-polarization bandwidth as large as 180 nm can be obtained in an optimized PhC waveguide with SixNy refractive index of 2.4, which can be tuned by x and y. In addition, our investigation shows that an NMR can also be obtained in an asymmetric rod-type PhC slab even when the refractive index is as low as 1.8, offering significant flexibility in engineering of the structure and refractive index of all-polarization devices on SixNy and, by extension, in other moderate-index platforms.

Optical arbitrary waveform generation (OAWG) plays a critical role in many applications, such as generating optical ultra-wide band (UWB) signal, optical pulse radar, all-optical temporal differentiator, and test of optical communication system. Although lots of OAWG schemes were reported using mature fiber grating techniques, one of the most promising solutions is prone to be the miniaturization and integration with photonic integrated circuits, such as using indium phosphide (InP) platform, silica on silicon, silicon nitride or silicon platform. In this talk, we review our recent progress on some OAWG schemes using silicon platform and their applications in photonic computing. First, we report an OAWG and high-order photonic differentiator based on a four-tap FIR silicon integrated circuit. By thermally controlling the amplitude and phase of each tap, we obtain several typical waveforms such as triangular waveform, sawtooth waveform, square waveform and Gaussian waveform, etc. Furthermore, we demonstrate first-, second- and third-order differentiation based on the optical pulse shaper, whose spectra were tailored to the transfer functions of temporal differentiators. Especially, our scheme can switch the differentiator patterns from first- to third-order freely on a fixed photonic chip, and this is unable in our previous works such as cascaded microrings or cascaded Mach-Zehnder interferometers (MZIs). Second, we demonstrate radio-frequency arbitrary waveform generation using temporal synthesis. We compare two schemes of cascaded microring resonators and finite impulse response. There functional photnic chip has only millmeter size, low power consumption, and high frequency response. Third, we will demonstrate an OAWG scheme based on superposition of differentiated waveforms. Compared to the first two schemes, this one shows a more compact size and a more flexible operation.