Optical leaky fin waveguide for long-range optical antennas on high-index contrast photonic circuit platforms

Long-distance light detection and ranging(LiDAR)applications require an aperture size in the order of 30 mm to project 200–300 m.To generate such collimated Gaussian beams from the surface of a chip,this work presents a novel waveguide antenna concept,which we call an“optical leaky fin antenna,”consisting of a tapered waveguide with a narrow vertical“fin”on top.The proposed structure(operating aroundλ=1.55μm)overcomes fundamental fabrication challenges encountered in weak apodized gratings,the conventional method to create an offchip wide Gaussian beam from a waveguide chip.We explore the design space of the antenna by scanning the relevant cross section parameters in a mode solver,and their sensitivity is examined.We also investigate the dispersion of the emission pattern and angle with the wavelength.The simulated design space is then used to construct and simulate an optical antenna to emit a collimated target intensity profile.Results show inherent robustness to crucial design parameters and indicate good scalability of the design.Possibilities and challenges to fabricate this device concept are also discussed.This novel antenna concept illustrates the possibility to integrate long optical antennas required for long-range solid-state LiDAR systems on a high-index contrast platform with a scalable fabrication method.

Automatic synthesis of light-processing functions for programmable photonics:theory and realization

Linear light-processing functions(e.g.,routing,splitting,filtering)are key functions requiring configuration to implement on a programmable photonic integrated circuit(PPIC).In recirculating waveguide meshes(which include loop-backs),this is usually done manually.Some previous results describe explorations to perform this task automatically,but their efficiency or applicability is still limited.In this paper,we propose an efficient method that can automatically realize configurations for many light-processing functions on a square-mesh PPIC.At its heart is an automatic differentiation subroutine built upon analytical expressions of scattering matrices that enables gradient descent optimization for functional circuit synthesis.Similar to the state-of-the-art synthesis techniques,our method can realize configurations for a wide range of light-processing functions,and multiple functions on the same PPIC simultaneously.However,we do not need to separate the functions spatially into different subdomains of the mesh,and the resulting optimum can have multiple functions using the same part of the mesh.Furthermore,compared to nongradient-or numerical differentiation-based methods,our proposed approach achieves 3×time reduction in computational cost.

Femtojoule electro-optic modulation using a silicon– organic hybrid device

Energy-efficient electro-optic modulators are at the heart of short-reach optical interconnects,and silicon photonics is considered the leading technology for realizing such devices.However,the performance of all-silicon devices is limited by intrinsic material properties.In particular,the absence of linear electro-optic effects in silicon renders the integration of energy-efficient photonic–electronic interfaces challenging.Silicon–organic hybrid(SOH)integration can overcome these limitations by combining nanophotonic silicon waveguides with organic cladding materials,thereby offering the prospect of designing optical properties by molecular engineering.In this paper,we demonstrate an SOH Mach–Zehnder modulator with unprecedented efficiency:the 1-mm-long device consumes only 0.7 fJ bit^(-1) to generate a 12.5 Gbit s^(-1) data stream with a bit-error ratio below the threshold for hard-decision forward-error correction.This power consumption represents the lowest value demonstrated for a non-resonant Mach–Zehnder modulator in any material system.It is enabled by a novel class of organic electro-optic materials that are designed for high chromophore density and enhanced molecular orientation.The device features an electro-optic coefficient of r33<180 pm V^(-1) and can be operated at data rates of up to 40 Gbit s^(-1).

Silicon and silicon nitride photonic circuits for spectroscopic sensing on-a-chip [Invited]

There is a rapidly growing demand to use silicon and silicon nitride（Si3N4） integrated photonics for sensing applications, ranging from refractive index to spectroscopic sensing. By making use of advanced CMOS technology,complex miniaturized circuits can be easily realized on a large scale and at a low cost covering visible to mid-IR wavelengths. In this paper we present our recent work on the development of silicon and Si3N4-based photonic integrated circuits for various spectroscopic sensing applications. We report our findings on waveguide-based absorption, and Raman and surface enhanced Raman spectroscopy. Finally we report on-chip spectrometers and on-chip broadband light sources covering very near-IR to mid-IR wavelengths to realize fully integrated spectroscopic systems on a chip.

Design of a single all-silicon ring resonator with a 150 nm free spectral range and a 100 nm tuning range around 1550 nm

We present a novel and simple method to obtain an ultrawide free spectral range(FSR) silicon ring resonator together with a tuning range covering the entire spectrum from 1500 to 1600 nm. A ring resonator with a large FSR together with a high Q factor, high tuning efficiency, and low fabrication cost and complexity is desired for many applications. In this paper, we introduce a novel way to make such a ring resonator, which takes advantage of the well-known resonance-splitting phenomenon. It is a single ring resonator with an FSR of more than 150 nm around 1550 nm and which has an easy thermo-optic tunability that can produce a tuning range around 90 nm or even more. Moreover, the device is simple to implement and can be fabricated in standard complementary metal-oxide semiconductor technology without requiring any kind of complicated processing or extra materials.The potential applications include single mode laser cavities, wavelength division multiplexing filters,(de)multiplexers, optical sensors, and integrated reflectors.

Effects of coupling and phase imperfections in programmable photonic hexagonal waveguide meshes

We present a study of the effect of imperfections on the transmission and crosstalk in programmable photonic meshes with feedback loops consisting of tunable couplers and phase shifters.The many elements in such a mesh can generate a multitude of parasitic paths when the couplers and phase shifters deviate even slightly from their nominal value.Performing Monte Carlo simulations,we show that small stochastic imperfections in the phase and coupling(<1.0%)can introduce unwanted interferences and resonances and significantly deteriorate the frequency response of the circuit.We also demonstrate that,in the presence of imperfections,the programming strategy of the unused couplers can reduce effects of such parasitics.

Stochastic collocation for device-level variability analysis in integrated photonics

We demonstrate the use of stochastic collocation to assess the performance of photonic devices under the effect of uncertainty. This approach combines high accuracy and efficiency in analyzing device variability with the ease of implementation of sampling-based methods. Its flexibility makes it suitable to be applied to a large range of photonic devices. We compare the stochastic collocation method with a Monte Carlo technique on a numerical analysis of the variability in silicon directional couplers.

Accurate extraction of fabricated geometry using optical measurement

We experimentally demonstrate extraction of silicon waveguide geometry with subnanometer accuracy using optical measurements. Effective and group indices of silicon-on-insulator(SOI) waveguides are extracted from the optical measurements. An accurate model linking the geometry of an SOI waveguide to its effective and group indices is used to extract the linewidths and thicknesses within respective errors of 0.37 and0.26 nm on a die fabricated by IMEC multiproject wafer services. A detailed analysis of the setting of the bounds for the effective and group indices is presented to get the right extraction with improved accuracy.

Backcoupling manipulation in silicon ring resonators

In this paper, we theoretically propose and experimentally demonstrate the manipulation of a novel degree of freedom in ring resonators, which is the coupling from the clockwise input to the counterclockwise propagating mode(and vice versa). We name this mechanism backcoupling, in contrast with the normal forward-coupling of a directional coupler. It is well known that internal reflections will cause peak splitting in a ring resonator. Our previous research demonstrated that the peak asymmetry will be strongly influenced by the backcoupling. Thus, it is worth manipulating the backcoupling in order to gain full control of a split resonance for the benefit of various resonance-splitting-based applications. While it is difficult to directly manipulate the backcoupling of a conventional directional coupler, here we design a circuit explicitly for manipulating the backcoupling. It can be potentially developed for applications such as single sideband filter, resonance splitting elimination, Fano resonance, and ultrahigh-Q and finesse.

Numerical modeling of a linear photonic system for accurate and efficient time-domain simulations

In this paper, a novel modeling and simulation method for general linear, time-invariant, passive photonic devices and circuits is proposed. This technique, starting from the scattering parameters of the photonic system under study, builds a baseband equivalent state-space model that splits the optical carrier frequency and operates at baseband, thereby significantly reducing the modeling and simulation complexity without losing accuracy.Indeed, it is possible to analytically reconstruct the port signals of the photonic system under study starting from the time-domain simulation of the corresponding baseband equivalent model. However, such equivalent models are complex-valued systems and, in this scenario, the conventional passivity constraints are not applicable anymore. Hence, the passivity constraints for scattering parameters and state-space models of baseband equivalent systems are presented, which are essential for time-domain simulations. Three suitable examples demonstrate the feasibility, accuracy, and efficiency of the proposed method.

Time-domain compact macromodeling of linear photonic circuits via complex vector fitting

In this paper, a novel baseband macromodeling framework for linear passive photonic circuits is proposed, which is able to build accurate and compact models while taking into account the nonidealities,such as higher order dispersion and wavelength-dependent losses of the circuits. Compared to a previous modeling method based on the vector fitting algorithm, the proposed modeling approach introduces a novel complex vector fitting technique. It can generate a half-size state-space model for the same applications, thereby achieving a major improvement in efficiency of the time-domain simulations. The proposed modeling framework requires only measured or simulated scattering parameters as input,which are widely used to represent linear and passive systems. Three photonic circuits are studied to demonstrate the accuracy and efficiency of the proposed technique.

Wafer-level hermetically sealed silicon photonic MEMS

The emerging fields of silicon(Si) photonic micro–electromechanical systems(MEMS) and optomechanics enable a wide range of novel high-performance photonic devices with ultra-low power consumption, such as integrated optical MEMS phase shifters, tunable couplers, switches, and optomechanical resonators. In contrast to conventional SiO;-clad Si photonics, photonic MEMS and optomechanics have suspended and movable parts that need to be protected from environmental influence and contamination during operation. Wafer-level hermetic sealing can be a cost-efficient solution, but Si photonic MEMS that are hermetically sealed inside cavities with optical and electrical feedthroughs have not been demonstrated to date, to our knowledge. Here, we demonstrate wafer-level vacuum sealing of Si photonic MEMS inside cavities with ultra-thin caps featuring optical and electrical feedthroughs that connect the photonic MEMS on the inside to optical grating couplers and electrical bond pads on the outside. We used Si photonic MEMS devices built on foundry wafers from the iSiPP50G Si photonics platform of IMEC, Belgium. Vacuum confinement inside the sealed cavities was confirmed by an observed increase of the cutoff frequency of the electro-mechanical response of the encapsulated photonic MEMS phase shifters, due to reduction of air damping. The sealing caps are extremely thin, have a small footprint, and are compatible with subsequent flip-chip bonding onto interposers or printed circuit boards. Thus, our approach for sealing of integrated Si photonic MEMS clears a significant hurdle for their application in high-performance Si photonic circuits.