Si3N4 Waveguides Research Articles

Unlocking the monolithic integration scenario: optical coupling between GaSb diode lasers epitaxially grown on patterned Si substrates and passive SiN waveguides

Silicon (Si) photonics has recently emerged as a key enabling technology in many application fields thanks to the mature Si process technology, the large silicon wafer size, and promising Si optical properties. The monolithic integration by direct epitaxy of III–V lasers and Si photonic devices on the same Si substrate has been considered for decades as the main obstacle to the realization of dense photonics chips. Despite considerable progress in the last decade, only discrete III–V lasers grown on bare Si wafers have been reported, whatever the wavelength and laser technology. Here we demonstrate the first semiconductor laser grown on a patterned Si photonics platform with light coupled into a waveguide. A mid-IR GaSb-based diode laser was directly grown on a pre-patterned Si photonics wafer equipped with SiN waveguides clad by SiO2. Growth and device fabrication challenges, arising from the template architecture, were overcome to demonstrate more than 10 mW outpower of emitted light in continuous wave operation at room temperature. In addition, around 10% of the light was coupled into the SiN waveguides, in good agreement with theoretical calculations for this butt-coupling configuration. This work lift an important building block and it paves the way for future low-cost, large-scale, fully integrated photonic chips.

A heterogeneously integrated lithium niobate-on-silicon nitride photonic platform

The availability of thin-film lithium niobate on insulator (LNOI) and advances in processing have led to the emergence of fully integrated LiNbO3 electro-optic devices. Yet to date, LiNbO3 photonic integrated circuits have mostly been fabricated using non-standard etching techniques and partially etched waveguides, that lack the reproducibility achieved in silicon photonics. Widespread application of thin-film LiNbO3 requires a reliable solution with precise lithographic control. Here we demonstrate a heterogeneously integrated LiNbO3 photonic platform employing wafer-scale bonding of thin-film LiNbO3 to silicon nitride (Si3N4) photonic integrated circuits. The platform maintains the low propagation loss (<0.1 dB/cm) and efficient fiber-to-chip coupling (<2.5 dB per facet) of the Si3N4 waveguides and provides a link between passive Si3N4 circuits and electro-optic components with adiabatic mode converters experiencing insertion losses below 0.1 dB. Using this approach we demonstrate several key applications, thus providing a scalable, foundry-ready solution to complex LiNbO3 integrated photonic circuits.

Optically modified second harmonic generation in silicon oxynitride thin films via local layer heating

Strong second harmonic generation (SHG) in silicon nitride has been extensively studied—among others, in terms of laser-induced SHG enhancement in Si3N4 waveguides. This enhancement has been ascribed to the all-optical poling induced by the coherent photogalvanic effect. Yet, an analogous process for Si3N4 thin films has not been reported. Our article reports on the observation of laser-induced threefold SHG enhancement in Si3N4 thin films. The observed enhancement has many features similar to all-optical poling, such as highly nonlinear power dependence, cumulative effect, or connection to the Si3N4–Si interface. However, identical experiments for low-oxygen silicon oxynitride thin films lead to complex behavior, including laser-induced SHG reduction. Following a thorough experimental study, including the effects of repetition rate or pulse length, the observed results were ascribed to heat-induced SHG variation. In addition to revealing a new mechanism of laser-induced SHG variation, our results also provide a means to identify this mechanism.

High-efficiency broadband fiber-to-chip coupler using a 3D nanoprinting microfiber.

We propose a method for coupling a tapered optical fiber to an inverted tapered SiN waveguide by fabricating a microfiber using 3D nanoprinting lithography. The microfiber consists of three parts: a tapered cladding cap, an S-bend, and a straight part, all composed of high-refractive-index material. Light is adiabatically coupled from the tapered fiber to the printed microfiber through the cladding cap. The light is then transmitted through the S-bend and the straight part with low loss and is finally coupled to the waveguide through the evanescent field. In the simulation, our design can achieve a high coupling efficiency (TE mode) of ∼97% at a wavelength of 1542nm with a wide bandwidth of ∼768n m at the 1-dB cutoff criterion.

Comprehensive Investigation of Thermo-Optic Phase Shifters on a Multi-Layered SiN-on-SOI Platform

Thermo-optic phase-shifter is an important component widely used in various photonic applications such as optical neural network and LiDAR. Different methods have been proposed to improve the performance of the device. However, there is a lack of detailed quantitative comparison of such methods due to differences in fabrication platform ad material properties. In this article, detailed investigation and comparison of 14 different thermal phase shifter designs on the same fabrication platform is provided. This includes the dimensions, material and heat dissipation approaches of the thermo-optic phase shifter. The comparison indicates that both suspended heater and doped silicon design could significantly improve thermo-optic phase modulation efficiency. On the choice of waveguide material, the electrical power consumption of SiN waveguide is about eight times higher than that of Si waveguide. Furthermore, the characterisation of thermal crosstalk and breakdown current of the thermo-optic phase shifters is also detailed. Such comprehensive investigation of thermo-optic phase shifters can serve as a useful design guideline for PIC designers.

SiN waveguides for ultra-broadband visible-telecom photon pairs

We propose simple rectangular cross-section, few-cm long SiN waveguide designs capable of generating ultra-broadband frequency-correlated photon pairs via SFWM, exceeding an octave in bandwidth spanning the visible-to-telecom wavelengths ( $$\sim$$ 700–1700 nm for $$\sim$$ 1- $$\upmu$$ m pump) with a spectral brightness of $$>2.5\times 10^4$$ photon pairs/s/nm/mW $$^2$$ . Such compact sources, spanning the entire telecom spectrum (across O, E, S, C, L, and U bands) on one side and covering most of the major atomic transitions used in ultracold gases and solid-state based quantum memories on the other side, could potentially be employed in large-scale multi-channel hybrid (free space + fiber) quantum networking schemes in addition to the heralding of telecom single photons by utilizing high efficiency visible detectors.

Quantum Dot Lasers Directly Grown on 300 mm Si Wafers: Planar and In-Pocket

We report for the first time the direct growth of quantum dot (QD) lasers with electrical pumping on 300 mm Si wafers on both a planar template and in-pocket template for in-plane photonic integration. O-band lasers with five QD layers were grown with molecular beam epitaxy (MBE) in a 300 mm reactor and then fabricated into standard Fabry–Perot ridge waveguide cavities. Edge-emitting lasers are demonstrated with high yield and reliable results ready for commercialization and scaled production, and efforts to make monolithically integrated lasing cavities grown on silicon-on-insulator (SOI) wafers vertically aligned and coupled to SiN waveguides on the same chip show the potential for 300 mm-scale Si photonic integration with in-pocket direct MBE growth.

Heterogeneous Integration of III-V Lasers in a 300 mm CMOS Manufacturing Line

AIM Photonics has developed a high precision laser attach process using an ultra-precise ASM Amicra NANO 300 mm die-to-wafer flip-chip bonder and integrated solder stamping system. Laser die are placed with high accuracy (&lt;1 µm) and soldered to form a AuSn eutectic bond. This photonic platform has low optical losses, with SiN waveguides measuring on the order of 0.4 dB/cm and a calculated laser-to-waveguide coupling loss of 1.86 dB.

Widely Tunable (2.47–2.64 µm) Hybrid Laser Based on GaSb/GaInAsSb Quantum‐Wells and a Low‐Loss Si3N4 Photonic Integrated Circuit

AbstractPhotonic integrated circuits fabricated using a Si3N4 waveguide platform exhibit low losses in a wide wavelength region extending from visible to beyond 2 µm. This feature is exploited to demonstrate a high‐performance integrated laser exhibiting broad wavelength tuneability near a 2.6 µm wavelength region. The laser is based on a Si3N4 photonic integrated circuit incorporating a tunable reflector and a AlGaInAsSb/GaSb quantum‐well gain element. A tuning range of 170 nm (2474–2644 nm) and single‐mode CW operation with a maximum power of 6.4 mW at room temperature are demonstrated. The performance is enabled by exploitation of several essential building blocks realized in Si3N4, namely low‐loss Y‐branches, inverse tapers, and a double‐ring resonator with a free spectral range of ≈160 nm. Moreover, the limits of wavelength coverage are explored using Si3N4 waveguides and show that the platform supports low propagation loss up to 3.5 µm. Finally, the possibility to achieve improved mode matching between Si3N4 and GaSb waveguides is analyzed, further enabling enhancing the performance of such a hybrid laser platform and supporting wavelength extension beyond a 3 µm range.

Low temperature sensitivity on-chip Fourier-transform spectrometer based on dual-layer Si3N4 spiral waveguides

On-chip Fourier-transform spectrometers (FTSs) based on Mach–Zehnder interferometer (MZI) arrays suffer from severe central wavelength and fringe contrast variation due to fabrication errors. Even though a calibration matrix can be employed to correctly retrieve the input spectra, environmental temperature variation greatly degrades the retrieving performance. In this paper, we devise a dual-layer Si3N4 waveguide interferometer to reduce the temperature sensitivity. The beating of the even and odd supermodes in the dual-layer waveguide generates periodic intensity fluctuations in the spectrum. Since these two modes have similar modal profiles, their thermal sensitivity and propagation loss are relatively balanced, leading to a low temperature sensitivity and a high interference extinction ratio. We designed and fabricated a passive FTS based on a 32-channel dual-layer Si3N4 waveguide array. Experimental results show that the temperature sensitivity is reduced to 10 pm/°C, which is almost half that of single-layer Si3N4 MZI-based FTSs. With this chip, we accurately reconstructed various types of optical spectra, including single and two sparse laser lines, and broadband optical spectra. Our method can fit a wide wavelength range, which is a promising technology to improve the practical applications of on-chip FTSs.

Three-dimensional inter-layer optical signal transmission realized by a monolithically integrated semiconductor-based carrier transport structure.

In this study, we proposed and demonstrated a brand new type of monolithic photonic devices which realizes the three-dimensional (3D) all-optical switching for inter-layer signal transmission. This device is composed of a vertical Si microrod which serves as optical absorption material within a SiN waveguide in one layer and as an index modulation structure within a SiN microdisk resonator lying in the other layer. The ambipolar photo-carrier transport property in the Si microrod was studied by measuring the resonant wavelength shifts under continuous-wave laser pumping. The ambipolar diffusion length can be extracted to be 0.88 µm. Based on the ambipolar photo-carrier transport in a Si microrod through different layers, we presented a fully-integrated all-optical switching operation using this Si microrod and a SiN microdisk with a pump-probe technique through the on-chip SiN waveguides. The switching time windows for the on-resonance operation mode and the off-resonance operation mode can be extracted to be 439 ps and 87 ps, respectively. This device shows potential applications for the future all-optical computing and communication with more practical and flexible configurations in monolithic 3D photonic integrated circuits (3D-PICs).

High-Speed Waveguide-Integrated Avalanche Photodiodes for Near-Infrared Wavelengths on SiN-on-SOI Photonic Platform

Recently, there have been efforts on developing monolithically integrated avalanche photodiodes (APDs) for visible and near-infrared (NIR) wavelengths. However, the fabrication of these detectors in commercial photonic foundries often requires custom integration flow due to many design rule violations in standard integration process. This increases the fabrication cost, hinders the use of highly robust integration platform, and prevents access to well-mature process design kits. Here, we report on waveguide-integrated APDs operating at 850 nm by utilizing a commercial silicon nitride-on-silicon-on-insulator (SiN-on-SOI) photonics platform with no process modifications. Our devices comprise a SiN waveguide for input light routing and an SOI layer underneath to form APDs. We examine the effect of different junction parameters and device length on DC and high-speed device performance. We demonstrate high gain-bandwidth product up to 120 GHz with low dark current of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{&lt; }$</tex-math></inline-formula> 25 nA and high responsivity that can reach 0.71 A/W, and observe open eye diagrams at up to 40 Gbps for all device variations. The performance parameters are on-par or even superior when compared to other integrated APDs operating in the NIR wavelengths. Being compatible with the standard integration flow of a multi-project wafer service of a commercial foundry, our devices are readily accessible, and they expand the functionalities of well-mature SiN-on-SOI photonics platform for applications ranging from optical interconnects, LIDAR, visible light communication to biosensing and quantum applications.

Widely tunable 2 µm hybrid laser using GaSb semiconductor optical amplifiers and a Si3N4 photonics integrated reflector

Tunable lasers emitting in the 2-3 µm wavelength range that are compatible with photonic integration platforms are of great interest for sensing applications. To this end, combining GaSb-based semiconductor gain chips with Si3N4 photonic integrated circuits offers an attractive platform. Herein, we utilize the low-loss features of Si3N4 waveguides and demonstrate a hybrid laser comprising a GaSb gain chip with an integrated tunable Si3N4 Vernier mirror. At room temperature, the laser exhibited a maximum output power of 15 mW and a tuning range of ∼90 nm (1937-2026 nm). The low-loss performance of several fundamental Si3N4 building blocks for photonic integrated circuits is also validated. More specifically, the single-mode waveguide exhibits a transmission loss as low as 0.15 dB/cm, the 90° bend has 0.008 dB loss, and the 50/50 Y-branch has an insertion loss of 0.075 dB.

High‐Performance Electro‐Optic Modulator on Silicon Nitride Platform with Heterogeneous Integration of Lithium Niobate

AbstractSilicon nitride (SiN) emerges as an important platform for ultralow loss photonic integrations with complementary metal‐oxide‐semiconductor compatibility. However, active devices, such as modulators, are difficult to realize on pure SiN due to the lack of any electro‐optic (EO) properties of the material. Here, an SiN and lithium niobate (LN) heterogenous integration platform supporting high‐performance EO modulators on SiN waveguide circuits is introduced. An efficient evanescent coupling structure is realized for low‐loss light transitions between the SiN waveguide and the LN ridge waveguide with a measured mode transition loss of only 0.4 dB. Based on this heterogeneous platform, an EO Mach–Zender interference modulator on SiN is built with unprecedented loss, efficiency, and bandwidth performances. A half‐wave voltage of 4.3 V with a modulation bandwidth of 37 GHz and an overall insertion loss of 1 dB is measured for a 7‐mm long device. Data transmission up to 128 Gb s−1 with a bit‐error‐rate of &lt;2.4 × 10‐4 is also demonstrated.

Spatially resolved multimode excitation for smooth supercontinuum generation in a SiN waveguide.

We propose a method of supercontinuum light generation enhanced by multimode excitation in a precisely dispersion-engineered deuterated SiN (SiN:D) waveguide. Although a regularly designed SiN-based nonlinear optical waveguide exhibits anomalous dispersion with the fundamental and first-order multimode operation, the center-symmetric light pumping at the input edge has so far inhibited the full potential of the nonlinearity of SiN-based materials. On the basis of numerical analysis and simulation for the SiN:D waveguide, we intentionally applied spatial position offsets to excite the fundamental and higher-order modes to realize bandwidth broadening with flatness. Using this method, we achieved an SNR improvement of up to 18 dB at a wavelength of 0.6 µm with an offset of about 1 µm in the Y-axis direction and found that the contribution was related to the presence of dispersive waves due to the excitation of TE10, and TE01 modes.

Broadband and CMOS-compatible polarization splitter and rotator built on a silicon nitride-on-silicon multilayer platform.

A broadband and CMOS-compatible polarization beam splitter and rotator (PSR) built on the silicon nitride-on-silicon multilayer platform is presented. The PSR is realized by cascading a polarization beam splitter and a polarization rotator, which are both subtly constructed with an asymmetrical directional coupler waveguide structure. The advantage of this device is that the function of PSR can be directly realized in the SiN layer, providing a promising solution to the polarization diversity schemes in SiN photonic circuits. The chip is expected to have high power handling capability as the light is input from the SiN waveguide. The use of silicon dioxide as the upper cladding of the device ensures its compatibility with the metal back-end-of-line process. By optimizing the structure parameters, a polarization conversion loss lower than 1dB and cross talk larger than 27.6dB can be obtained for TM-TE mode conversion over a wavelength range of 1450 to 1600nm. For TE mode, the insertion loss is lower than 0.26dB and cross talk is larger than 25.3dB over the same wavelength range. The proposed device has good potential in diversifying the functionalities of the multilayer photonic chip with high integration density.

Pulsed Four-Wave Mixing at Telecom Wavelengths in Si3N4 Waveguides Locally Covered by Graphene.

Recently, the nonlinear optical response of graphene has been widely investigated, as has the integration of this 2D material onto dielectric waveguides so as to enhance the various nonlinear phenomena that underpin all-optical signal processing applications at telecom wavelengths. However, a great disparity continues to exist from these experimental reports, depending on the used conditions or the hybrid devices under test. Most importantly, hybrid graphene-based waveguides were tested under relatively low powers, and/or combined with waveguide materials that already exhibited a nonnegligible nonlinear contribution, thereby limiting the practical use of graphene for nonlinear applications. Here, we experimentally investigate the nonlinear response of Si3N4 waveguides that are locally covered by submillimeter-long graphene patches by means of pulsed degenerate four-wave mixing at telecom wavelength under 7 W peak powers. Our measurements and comparison with simulations allow us to estimate a local change of the nonlinearity sign as well as a moderate increase of the nonlinear waveguide parameter (γ∼-10 m-1W-1) provided by graphene. Our analysis also clarifies the tradeoff associated with the loss penalty and nonlinear benefit afforded by graphene patches integrated onto passive photonic circuits, thereby providing some guidelines for the design of hybrid integrated nonlinear devices, coated with graphene, or, more generally, any other 2D material.

FDTD investigation of adiabatic optical coupling between Silicon nitride and Germanium-based optical structures for energy-efficient photonic integration

This paper aims to investigate the potential of a direct integration between Si3N4 waveguides and Germanium(Ge)-based structures, which are considered two of the most promising silicon(Si)-compatible materials for passive and active functions for Si-based energy-efficient photonics, respectively. This paper focuses on investigating the vertical coupling scheme between these materials employing a tapered structure of the Ge-based layer using 2D FDE and 3D FDTD simulations, to match the optical mode from a passive Si3N4 waveguide to the Ge-based layer and vice versa. The expected optical coupling performance will be reported, and its dependence on the unavoidable fabrication variation in the coupling region will be also investigated. Additionally, the coupling performance of the vertical coupling approach compared to the previous investigation between a Si waveguide and a Ge layer will be addressed.

Integration of Edge-Emitting Quantum Dot Lasers with Different Waveguide Platforms using Micro-Transfer Printing

O-band InAs/GaAs Quantum Dot edge-emitting lasers are integrated onto a number of waveguiding platforms using micro-transfer printing. These are deep recesses in 220 nm Si, 3 µm Si and 300 nm SiN waveguide circuits. The processing technology to achieve release and high-yield accurate transfer of laser coupons up to 2.4 mm long and < 5 µm thick onto the target substrates is described. At 85 mA, waveguide coupled powers of 1.0 mW and 0.95 mW are measured after out-coupling from the 220 nm SOI and 300 nm SiN waveguides, respectively while 1.7 mW total power was measured from the 3 µm SOI waveguide.

Numerical study on flat dispersion in a single-layer Si3N4 coupled-waveguides with four zero-dispersion wavelengths for supercontinuum generation

We achieve a flat broadband dispersion profile in a dual-core silicon nitride (Si3N4) waveguide utilizing the mode coupling between the quasi-TE00 mode in a narrow waveguide and the quasi-TE10 mode in a wide waveguide. Mode coupling extends the anomalous dispersion region and flattens the entire dispersion, creating four zero dispersion wavelengths. The generated dispersion profile varies between −10 and 13 ps/(nm∙km) over an octave bandwidth from 1150 nm to 2425 nm. A two-octave supercontinuum spectrum from 750 nm to 3001 nm can be generated through the 5 mm long Si3N4 waveguide by launching a 50-fs 5-kW hyperbolic secant input pump pulse at anomalous dispersion region. Significant spectral broadening is also observed, by pumping in the normal dispersion region. The proposed Si3N4 waveguide has only a single-layer core structure and is compatible with the photonic Damascus process. Moreover, the obtained dispersion profile is insensitive to the gap distance between the narrow and wide waveguides, making it easier to fabricate than slot-assisted waveguide structures.