arXiv subjects

Matteo Cherchi

Publications and source records attributed to Matteo Cherchi.

At least 19 recordsLinked to original sources

arXiv2025

Multimode interface between optical free-space- and waveguide modes

Free-space and on-chip photonic systems are key components in optical communication networks. While free-space beams allow for the flexible generation and manipulation of spatial modes, integrated waveguides provide compact and stable platforms for on-chip signal processing. Bridging these two domains is essential for scalable multi-mode communication networks. Here, we present an efficient, broadband interface capable of converting multiple higher-order free-space Laguerre-Gauss (LG) modes into corresponding waveguide modes using the multi-plane light conversion (MPLC) scheme. We experimentally demonstrate low-crosstalk mode conversion between various set of three LG modes, and the first three TE modes of a multimode silicon waveguide across the telecom C-band. The system operates passively without active switching and can be adapted to different spatial mode sets. This platform provides a pathway to increased data capacities and may enable more compact and efficient multi-mode optical communication and on-chip processing schemes.

physics.optics↗

arXiv2024

Stimulated Brillouin scattering in a non-suspended ultra-low-loss thick-SOI platform

Silicon photonics, with its CMOS compatibility and high integration density, has enabled a wide range of novel applications. Harnessing stimulated Brillouin scattering (SBS), an optomechanic interaction between optical and GHz acoustic waves, in silicon-on-insulator (SOI) platforms attracts great interests for its potential in narrow-linewidth lasers and microwave photonics. However, the poor optoacoustic overlap in silicon nanowires on conventional SOI platforms has previously restricted the observation of SBS signals to suspended silicon waveguide structures. In this work, we report, for the first time, the SBS response in a non-suspended ultra-low-loss thick-SOI waveguide platform. The SBS process in this 3~$\mu$m thick SOI platform is enabled by a leaky acoustic mode that coexists with the optical mode in the waveguide core, resulting in enhanced optoacoustic overlap. We measured a Brillouin gain coefficient of 2.5 m$^{-1}$W$^{-1}$ and 1.9 m$^{-1}$W$^{-1}$ at 37.6 GHz for the rib and strip waveguide, respectively. This work paves the way for Brillouin-based applications in non-suspended ultra-low-loss silicon photonics systems.

physics.optics↗

arXiv2024

InP optical amplifiers with Euler U-bend waveguide geometry for low-loss flip-chip hybrid integration

We report on the development of InP-based semiconductor amplifiers with a U-bend waveguide geometry having the input and output ports on one facet only. This waveguide geometry simplifies the chip alignment during the hybrid integration on silicon photonics platforms ultimately reducing the coupling losses, improving the integration yield, and minimizing the length of the optoelectronic chip. To achieve low loss U-bends with small footprint, we utilize the Euler bend geometry previously demonstrated on silicon and GaAs platforms. We analyze the gain properties of the devices by operating them as laser diodes at room temperature. Low loss U-bend performance with a 0.56 dB for a 50 $\mu$m effective bending radius bend in a single-mode strip InP waveguide is demonstrated. The interface between bend and straight waveguides was studied by comparing deep etched waveguides to a combination of shallow straight waveguides and deep etched bends. The effects of this interface on the device losses, electric properties and spectrum are reported. The implications related to having a bend section on the carrier injection and gain are discussed. Finally, results on the integration trials on silicon-on-insulator platform are presented.

physics.app-ph↗

arXiv2024

A Silicon Photonic 32-Input Coherent Combiner for Turbulence Mitigation in Free Space Optics Links

A photonic integrated circuit (PIC) for the coherent combination of 32 input optical signals into a single output fiber is reported. The PIC was fabricated using a low-loss thick silicon-on-insulator (SOI) process and packaged with 32 input and 1 output fibers. The basic building block is a 2x2 Mach-Zehnder interferometer (MZI) with an external (to the MZI branches) and an internal thermal phase shifter, and a bandwidth in excess of 80 kHz. The PIC monolithically integrates 31 MZIs and 31 germanium photodetectors, and is suitable in principle for turbulence mitigation in LEO-ground and horizontal free space optics links. Improvements to the device for the coherent combination of 64 inputs and for the reduction of insertion losses are also discussed

physics.optics↗

arXiv2023

Second order add/drop filter with a single ring resonator

We show theoretically and experimentally how a flat-top second-order response can be achieved with a self-coupled single add-drop ring resonator based on two couplers with different splitting ratios. The resulting device is a 1x1 filter, reflecting light back in the input waveguide at resonating wavelengths in the passbands, and transmitting light in the output waveguide at all other non-resonating wavelengths. Different implementations of the filter have been designed and fabricated on a micron-scale silicon photonics platform. They are based on compact Euler bends - either U-bends or L-bends - and Multi-Mode Interferometers as splitters for the ring resonators. Different finesse values have been achieved by using either 50:50 MMIs in conjunction with 85:15 MMIs or 85:15 MMIs in conjunction with 95:05 double MMIs. Unlike ordinary lowest order directional couplers, the MMIs couple most of the power in the cross-port which make them particularly suitable for the topology of the self-coupled ring, which would otherwise require a waveguide crossing. Experimental results are presented, showing good agreement with simulations. The proposed devices can find applications as wavelength-selective reflectors for relatively broad-band lasers or used as 2x2 add-drop filters when two exact replicas of the device are placed on the arms of a Mach-Zehnder interferometer.

physics.optics↗

arXiv2023

Tapered MMI splitters with unconstrained splitting ratio on a thick SOI platform

We have systematically studied multimode interferometer (MMI) splitters made from multiple tapered sections. The goal is to create a library of robust and low-loss splitters covering all splitting ratios (SR) for our silicon photonics platform based on 3 $\mu$m thick waveguides. The starting point is always a non-tapered canonical MMI either with general symmetry (canonical SRs 50:50, 100:0, and reciprocal ratios), with mirror symmetric restricted symmetry (canonical SRs 85:15, 50:50, 100:0, and reciprocal ratios), and with point-symmetric restricted symmetry (canonical SRs 72:28 and 28:72). Splitters of these three types are then divided into one to four subsections of equal length, leading to 12 possible different configurations. In each of these subsections, the width is first linearly tapered either up or down and then tapered back to its starting value ensuring mirror symmetry. For all twelve configurations, we carried out an extensive campaign of numerical simulations. For each given width change, we scanned the splitter length and calculated the power in the fundamental mode at the output as well as its relative phase. We then selected the designs with sufficiently low loss and mapped their SR as a function of either the change in width change or length, therefore creating systematic maps for the design of MMI splitters with any SR. Eventually, we selected and fabricated a subset of designs with SRs ranging from 5:95 to 95:5 in steps of 5% and validated their operation through optical measurements.

physics.optics↗

arXiv2022

Supporting quantum technologies with an ultra-low loss silicon photonics platform

Photonic integrated circuits (PICs) are expected to play a significant role in the ongoing second quantum revolution, thanks to their stability and scalability. Still, major upgrades are needed for available PIC platforms to meet the demanding requirements of quantum devices. In this paper, we present a review of our recent progress in upgrading an unconventional silicon photonics platform towards such goal, including ultra-low propagation losses, low fibre coupling losses, integration of superconducting elements, Faraday rotators, fast and efficient detectors, as well as phase modulators with low loss and/or low energy consumption. We show the relevance of our developments and of our vision in two main applications: quantum key distribution - to achieve significantly higher key rates and large-scale deployment - and cryogenic quantum computers - to replace electrical connections to the cryostat with optical fibres.

physics.optics↗

arXiv2022

Autocorrective interferometers for photonic integrated circuits

Extensive literature has shown that finite impulse response (FIR) interferometers can be engineered to be insensitive under variations of different physical parameters, e.g., to ensure flat-top response and/or tolerance to fabrication errors. In this context, I will show how the Bloch sphere representation can be a very powerful design tool providing superior physical insight into the working principle of autocorrective devices like broadband 50:50 splitters or flat-top interleavers, that can be therefore designed through simple analytical formulas. I will eventually review the recent progress in practical implementation of the autocorrective designs in the micron-scale silicon photonics platform of VTT.

physics.optics↗

arXiv2020

Flat-top interleavers based on single MMIs

We demonstrate for the first time flat-top interleavers based on cascaded Mach-Zehnder interferometers (MZIs) which use only single multimode interferometers (MMIs) as power splitters. Our previous designs were based on 4-stage cascades of MZIs, where we used single MMIs and double MMIs to achieve 85:15 splitting ratio and 31:69 splitting ratio respectively. This time, we propose instead a greatly simplified 2-stage configuration using only single MMIs, including a standard 50:50 MMI, and two tapered MMIs to achieve 71:29 and 92:08 splitting ratios. We have designed the interleaver based on its geometrical representation on the Bloch sphere, then confirmed by efficient 2D simulations of the building blocks and of the whole structure, based on the eigenmode expansion method. We show how important is to take into account the phase relations between the outputs of all MMIs in order to make a working design. We have successfully fabricated devices with different channel spacing on our micron-scale silicon photonics platform, and measurement results confirmed their expected flat-top operation on a broad band. Using only single MMI splitters we can not only greatly outperform the bandwidth achieved by standard directional couplers, but we can also ensure much higher robustness to fabrication errors, also compared to previous demonstrations based on double MMIs. Indeed, when compared to those previous attempts, the new results prove tapered MMIs to be the most robust approach to achieve arbitrary splitting ratios.

physics.optics↗

arXiv2018

3$\times$3 transfer matrix modelling of integrated optical devices

Unlike common devices based on ring resonators, the structure in Fig. 1.a involves not only 2$\times$2 couplers but also a 3$\times$3 coupler, which means that a 3$\times$3 transfer matrix approach is required to model the system. To the best of our knowledge, no such a model has been developed before. The only model available in the literature is based on a clever recursive 2$\times$2 transfer matrix model [1], which requires lengthy calculations that depend on the chosen boundary conditions and on the particular geometry chosen. The scope of this document is instead to show how to generalize the standard 2$\times$2 transfer matrix approach to cover any system with 3$\times$3 couplers, and calculate the transfer matrix of any complicated system just as a product of simple 3$\times$3 matrices.

physics.optics↗

arXiv2013

New silicon photonics integration platform enabled by novel micron-scale bends

Even though submicron silicon waveguides have been proposed for dense integration of photonic devices, to date the lightwave circuits on the market mainly rely on waveguides with micron-scale core dimensions. These larger waveguides feature easier fabrication, higher reliability and better interfacing to optical fibres. Single-mode operation with large core dimensions is obtained with low lateral refractive index contrast. Hence, the main limitation in increasing the level of integration and in reducing the cost of micron-scale waveguide circuits is their mm- to cm-scale minimum bending radius. Fortunately, single-mode rib waveguides with a micron-scale silicon core can be locally transformed into multi-mode strip waveguides that have very high lateral index contrast. Here we show how Euler spiral bends realized with these waveguides can have bending radii below 10 μm and losses below 0.02 dB/90° for the fundamental mode, paving way for a novel densely integrated platform based on micron-scale waveguides.

physics.optics↗

arXiv2012

Guided-wave frequency doubling in surface periodically poled lithium niobate: competing effects

We carried out second-harmonic generation in quasi-phase-matched α-phase lithium niobate channel waveguides realized by proton exchange and surface periodic poling. Owing to a limited ferroelectric domain depth, we could observe the interplay between second-harmonic generation and self-phase modulation due to cascading and cubic effects, resulting in a nonlinear resonance shift. Data reduction allowed us to evaluate both the quadratic nonlinearity in the near infrared as well as the depth of the uninverted domains.

physics.optics↗

arXiv2009

The role of nonlinear optical absorption in narrow-band difference frequency terahertz-wave generation

We present a general analysis of the influence of nonlinear optical absorption on terahertz generation via optical difference frequency generation, when reaching for the quantum conversion efficiency limit. By casting the equations governing the process in a suitably normalized form, including either two-photon- or three-photon-absorption terms, we have been able to plot universal charts for phase matched optical-to-terahertz conversion for different values of the nonlinear absorption coefficients. We apply our analysis to some experiments reported to date, in order to understand to what extent multiphoton absorption could have played a role and also to predict the maximum achievable conversion efficiency at higher peak pump intensities.

physics.optics↗

arXiv2009

Universal charts for optical difference frequency generation in the terahertz domain

We present a universal and rigorous approach to study difference frequency generation in the terahertz domain, keeping the number of degrees of freedom to a minimum, through the definition of a suitable figure of merit. The proposed method relies on suitably normalized charts, that enable to predict the optical-to-terahertz conversion efficiency of any system based on wave propagation in quadratic nonlinear materials. The predictions of our approach are found to be in good agreement with the best experimental results reported to date, enabling also to estimate the d22 nonlinear coefficient of high quality GaSe.

physics.optics↗

arXiv2009

Bloch Analysis of Finite Periodic Microring Chains

We apply Bloch analysis to the study of finite periodic cascading of microring resonators. Diagonalization of the standard transfer matrix approach not only allows to find an exact analytic expression for transmission and reflection, but also to derive a closed form solution for the field in every point of the structure. To give more physical insight we analyze the main features of the transmission resonances in a finite chain and we give some hints for their experimental verification

physics.optics↗

arXiv2009

Exact analytic expressions for electromagnetic propagation and optical nonlinear generation in finite one-dimensional periodic multilayers

Translation Matrix Formalism has been used to find an exact analytic solution for linear light propagation in a finite one-dimensional (1D) periodic stratified structure. This modal approach allows to derive a closed formula for the electric field in every point of the structure, by simply imposing a convenient form for the boundary conditions. We show how to apply this result to Second Harmonic Generation (SHG) in the undepleted pump regime.

physics.optics↗

arXiv2009

Wavelength flattened directional couplers for mirrorsymmetric interferometers

In the context of guided optics, we derive, analytically and geometrically, a rigorous general criterion to design wavelength insensitive interferometers with mirror symmetry, which are needed for wavelength multiplexing/demultiplexing. The criterion is applied to a practical case, resulting in an interferometer that works on a band wider than 70 nm.

physics.optics↗

arXiv2009

Wavelength flattened directional couplers: a geometrical approach

A new approach to design wavelength insensitive optical power splitter is presented. First coupled-mode theory is cast in operatorial form. This allows to solve the equivalent of coupled differential equations as simple limits. The operators are then represented on a generalized Poincare sphere, and the resulting graphical tool is applied to different structures, giving a clear interpretation of previous results in literature as well as hints on how to find improved solutions.

physics.optics↗