Silicon-on-insulator Waveguide Research Articles

Distributed temperature sensing on silicon-on-insulator chip by optical frequency domain reflectometry

This study introduces a novel distributed temperature sensing (DTS) technique on silicon-on-insulator (SOI) chips by the optical frequency domain reflectometry (OFDR) technology. In contrast to traditional on-chip silicon photonics temperature sensors which rely on transmission spectrum detection, this method is based on Rayleigh backscatter induced by the sidewall roughness of as-fabricated waveguides, eliminating the need for a specially designed structure. On-chip DTS results with a remarkable sensing spatial resolution of 200 μm and a high temperature sensitivity of 88 pm/K are demonstrated within the temperature range from 22.8 ∘C to 200 ∘C. Furthermore, the technology is employed to measure non-uniform temperature distributions along an SOI waveguide generated by integrated heaters. Importantly, this approach offers a straightforward sensing structure, opening new possibilities for investigating temperature profiles and thermal crosstalk across the chip.

Active manipulation for Goos-Hänchen shift of guided-wave via a metasurface of silicon-nanoscale semi-spheres on SOI waveguide

Goos-Hänchen shift of total internal reflection (TIR) is the light beam movement without external driving, so envisioned to have potential manipulation of optical beams. In this article, with a silicon-on-insulator (SOI) waveguide corner structure, a variable equivalent permittivity of guided wave is modelled, and then the equivalent electric polarizabilities and the Goos-Hänchen shift of guided wave are modelled. Consequently, with a 2.0-µm SOI waveguide corner structure and an abrupt phase change of ∼0.5π caused by a vertically inserted metasurface of nanoscale semi-spheres having a 450-nm radius can reach the GH shifts of 2.1 µm for TE- and TM-mode, respectively, which are verified by both the FDTD simulation results of 1.93 µm with a reflectivity of about 62% and the experimental results of 2.0 µm with ∼60%. Therefore, this work has efficiently modelled the optical feature response of semi-sphere metasurface to guided wave and the active manipulation for the GH shifts of guided-wave, opening more opportunities to develop the new functionalities and devices for Si-based photonic integrated circuit (PIC) applications.

Optimization of Grating Coupler over Single-Mode Silicon-on-Insulator Waveguide to Reach < 1 dB Loss through Deep-Learning-Based Inverse Design

Grating couplers are essential components in silicon photonics that facilitate the coupling of light between waveguides and fibers. Optimization of the grating couplers to reach <1 dB loss when coupling to single-mode fibers (SMFs) has been reported in the literature, but this was based on silicon-on-insulator (SOI) waveguides supporting multi-modes. In this paper, using a deep-learning model combined with an inverse-design process, we achieve <1 dB losses for grating couplers implemented over single-mode SOI waveguides, i.e., a maximum efficiency of 80.5% (−0.94 dB) for gratings constrained with e-beam (EB) lithography critical dimension (CD), and a maximum efficiency of 77.9% (−1.09 dB) for gratings constrained with deep ultraviolet (DUV) lithography CD. To verify these results, we apply covariance matrix adaptation evolution strategy (CMA-ES) and find that while CMA-ES yields slightly better results, i.e., 82.7% (−0.83 dB) and 78.9% (−1.03 dB) considering e-beam and DUV, respectively, the spatial structures generated by CMA-ES are nearly identical to the spatial structures generated by the deep-learning model combined with the inverse-design process. This suggests that our approach can achieve a representative low-loss structure, and may be used to improve the performance of other types of nanophotonic devices in the future.

Broadband and Low-Loss Silicon Photonic Directional Coupler for Signal Power Tapping on the 3 μm SOI Waveguide Platform

Silicon photonics (SiPh) has emerged as a promising technology for photonic integrated circuits (PICs). One of the basic components in SiPh is the directional coupler (DC), which plays an important role in signal monitoring with the requirement of low wavelength dependence and low loss. This paper proposes a broadband and low-loss DC designed for signal power tapping on the 3 μm silicon-on-insulator (SOI) waveguide platform. By utilizing the advantages of multi-micron waveguides and replacing one of the straight waveguides with an optimized arc-shaped waveguide in the coupling region, the proposed DC enhances spectral stability and improves transmission with negligible loss. Experimental evidence indicates that the proposed DC showcases a minimal variation in the tapping ratio. From 1470 nm to 1630 nm, the largest deviation away from the tapping ratio at 1550 nm is 1.433%. Additionally, the device exhibits a low excess loss of −0.27 dB. These results suggest that the proposed device is well-suited to reliable signal power tapping and monitoring, particularly within PICs.

High frequency characterization of PZT thin-films deposited by chemical solution deposition on SOI for integrated high speed electro-optic modulators

The increasing demand for high data rates and low power consumption puts silicon photonics at the edge of its capabilities. The heterogeneous integration of optical ferro-electric materials on silicon enhances the functionality of the silicon on insulator (SOI) platform to meet these demands. Lead zirconate titanate (PZT) thin films with a large Pockels coefficient and good optical quality can be directly integrated on SOI waveguides for fast electro-optic modulators. In this work, the relative permittivity and dielectric loss of PZT thin films deposited by chemical solution deposition on SOI substrates are analyzed at high frequencies. We extract ε r =1650−2129 and tan (δ) = 0.170 − 0.209 for the PZT thin films in the frequency range 1-67GHz. We show the possibility of achieving bandwidths beyond 60GHz via a Mach-Zehnder modulator with V π = 7V, suitable for next generation data communication systems.

Optimization and Performance Evaluation of Single-Mode SOI Waveguides for Ultra-Broadband C-to-S Wavelength Conversion

In order to keep up with expected growth in data traffic, using multiple bands beyond the C band is an appealing solution as it can partly reuse deployed optical fibers. One way of handling multi-band data generation and reception is by using all-optical wavelength converters (AOWCs) to translate to or from the C band, where commercial equipment is easily available. In this work, we therefore designed and realized an AOWC targeting C-to-S band conversion. To respect the requirements of commercial applications, we aimed for a low-complexity high-bandwidth implementation with reasonably low power consumption, tolerance for typical process variations and potential for further integration. Thus we implemented the AOWC as a strip waveguide in a Silicon-on-insulator (SOI) technology. An optimization of the conversion bandwidth was conducted during design and initial characterizations confirmed a conversion efficiency of -32.8 dB and a high bandwidth of about 35 nm with grating couplers and likely beyond 50 nm for edge couplers. Using the fabricated converters, we were able to demonstrate a marginal 0.3 dB optical signal-to-noise ratio (OSNR) penalty for a back-to-back (b2b) transmission of a single-channel single-polarization 32 GBd quadrature-phase-shift keying (QPSK) signal converted from C to S band. Additionally, this converted signal was successfully transmitted over 100 km of standard single-mode fiber (SSMF). Finally, crosstalk issues arising in multi-channel conversion were investigated.

Pulsed heterodyne interferometry for nonlinear SOI waveguide characterization

Silicon waveguides are a promising candidate for integrated nonlinear optics applications. Nonlinear coefficients of Silicon on Insulator (SOI) waveguides have been previously measured using techniques such as Z-scan, D-scan, Four Wave Mixing (FWM) and Self-phase modulation. However, they have several drawbacks such as they operate at high power or are cumbersome to setup and require multiple measurements to determine all the coefficients. In this work, we develop a direct and single measurement technique to characterize the nonlinear processes in SOI waveguides. This is achieved by employing a heterodyne interferometric technique to accurately measure minute nonlinear response. The measured nonlinear amplitude and phase shifts are fit to extract third-order nonlinear coefficients of Two-photon absorption, Kerr nonlinear index, Free carrier absorption and Free carrier dispersion. The obtained coefficients for SOI waveguides are close to that found in literature measured using the above-mentioned techniques. The advantages of this method include easy interpretation of the output signal and relatively low power of operation. It is especially advantageous for studying materials such as Phase Change Materials (PCM) in which phase changes occur dynamically. This aspect is quite promising for characterizing emerging materials for integrated photonics applications.

Spectrally-Pure Integrated Telecom-Band Photon Sources in Silicon

We demonstrate theoretically the direct (filter-free) generation of spectrally uncorrelated photon pairs, and consequently high-purity heralded single photons at telecommunication wavelengths, in SOI (silicon-on-insulator) nanowaveguides via spontaneous four-wave mixing (SFWM). The inherent strong waveguide dispersion in such high-index contrast structures is employed for discrete-band phase-matching in order to suppress the pair spectral correlations, and to produce photons in the fundamental spatial mode for easy in-out coupling to single-mode optical fibers. Additionally, by optimizing the source parameters like waveguide length, dispersion and pump bandwidth, the group-velocity of generated photons is also engineered to further eliminate spectral correlations. Extensive numerical mode simulations for various waveguide widths are used to design intrinsically pure photon sources via joint spectral amplitude (JSA) engineering. Purity <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$&gt;$</tex-math></inline-formula> 95% is shown to be achievable in a silica-clad SOI waveguide with a cross-section 220 × 600 nm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{2}$</tex-math></inline-formula> for typical values of length and pump bandwidth. The wavelength of these heralded single photons can be tuned in the range of 1266–1363 nm ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> O-band) by varying the pump wavelength. Considering the high accuracy provided by silicon nanofabrication techniques, the feasibility of generating single photons across the wavelength range 1333–1425 nm (E-band) and 1437–1510 nm (S-band) for slightly different waveguide widths (625 nm & 650 nm) is also demonstrated. These filter-free integrated photon sources could be important for large-scale linear optical quantum computation (LOQC) and long-distance fiber-optic quantum key distribution (QKD) networks.

On-chip mid-infrared silicon-on-insulator waveguide methane sensor using two measurement schemes at 3.291μm.

Portable or even on-chip detection of methane (CH4) is significant for environmental protection and production safety. However, optical sensing systems are usually based on discrete optical elements, which makes them unsuitable for the occasions with high portability requirement. In this work, we report on-chip silicon-on-insulator (SOI) waveguide CH4 sensors at 3.291 μm based on two measurement schemes including direct absorption spectroscopy (DAS) and wavelength modulation spectroscopy (WMS). In order to suppress noise, Kalman filter was adopted in signal processing. By optimizing the waveguide cross-section structure, an etch depth of 220 nm was selected with an experimentally high power confinement factor (PCF) of 23% and a low loss of only 0.71 dB/cm. A limit of detection (LoD) of 155 parts-per-million (ppm) by DAS and 78 ppm by WMS at an averaging time of 0.2 s were obtained for a 2 cm-long waveguide sensor. Compared to the chalcogenide (ChG) waveguide CH4 sensors at the same wavelength, the reported sensor reveals the minimum waveguide loss and the lowest LoD. Therefore the SOI waveguide sensor has the potential of on-chip gas sensing in the mid-infrared (MIR) waveband.

Hybrid silicon photonics DBR laser based on flip-chip integration of GaSb amplifiers and µm-scale SOI waveguides.

The development of integrated photonics experiences an unprecedented growth dynamic, owing to accelerated penetration to new applications. This leads to new requirements in terms of functionality, with the most obvious feature being the increased need for wavelength versatility. To this end, we demonstrate for the first time the flip-chip integration of a GaSb semiconductor optical amplifier with a silicon photonic circuit, addressing the transition of photonic integration technology towards mid-IR wavelengths. In particular, an on-chip hybrid DBR laser emitting in the 2 µm region with an output power of 6 mW at room temperature is demonstrated. Wavelength locking was achieved employing a grating realized using 3 µm thick silicon-on-insulator (SOI) technology. The SOI waveguides exhibit strong mode confinement and low losses, as well as excellent mode matching with GaSb optoelectronic chips ensuring low loss coupling. These narrow line-width laser diodes with an on-chip extended cavity can generate a continuous-wave output power of more than 1 mW even when operated at an elevated temperature of 45°C. The demonstration opens an attractive perspective for the on-chip silicon photonics integration of GaSb gain chips, enabling the development of PICs in a broad spectral range extending from 1.8 µm to beyond 3 µm.

Characterizing THz Scattering Loss in Nano-Scale SOI Waveguides Exhibiting Stochastic Surface Roughness with Exponential Autocorrelation

Electromagnetic (EM) scattering may be a significant source of degradation in signal and power integrity of high-contrast silicon-on-insulator (SOI) nano-scale interconnects, such as opto-electronic or optical interconnects operating at 100 s of THz where two-dimensional (2D) analytical models of dielectric slab waveguides are often used to approximate scattering loss. In this work, a formulation is presented to relate the scattering (propagation) loss to the scattering parameters (S-parameters) for the smooth waveguide; the results are correlated with results from the finite-difference time-domain (FDTD) method in 2D space. We propose a normalization factor to the previous 2D analytical formulation for the stochastic scattering loss based on physical parameters of waveguides exhibiting random surface roughness under the exponential autocorrelation function (ACF), and validate the results by comparing against numerical experiments via the 2D FDTD method, through simulation of hundreds of rough waveguides; additionally, results are compared to other 2D analytical and previous 3D experimental results. The FDTD environment is described and validated by comparing results of the smooth waveguide against analytical solutions for wave impedance, propagation constant, and S-parameters. Results show that the FDTD model is in agreement with the analytical solution for the smooth waveguide and is a reasonable approximation of the stochastic scattering loss for the rough waveguide.

Design of a half-ring plasmonic tweezers for environmental monitoring

An integrated near-field plasmonic tweezers, based on two gold half-ring elements, is proposed. The device consists of a silicon-on-insulator (SOI) waveguide on which two symmetric half-ring-shaped elements are deposited. The SOI waveguide, having dimensions 500 × 220 nm, enables single mode light propagation at 1.55 μm. The structure of the half-ring plasmonic tweezers exhibits up to five design parameters: inner radius ri, outer radius re, gap g, height h and angle ϑ. Gold permittivity is modeled by means of a Drude equation. The trapping performance is investigated by evaluating the optical force exerted on a nanoparticle having a radius of 25 nm, together with the stiffness and the trapping potential. Simulation results show that stable trapping is achieved for an input power as low as 27 mW at 1.55 μm, with a stiffness of −4.06 fN mW−1 nm−1.

SOI Waveguide Bragg Grating Photonic Sensor for Human Body Temperature Measurement Based on Photonic Integrated Interrogator.

A waveguide Bragg grating (WBG) provides a flexible way for measurement, and it could even be used to measure body temperature like e-skin. We designed and compared three structures of WBG with the grating period, etching depth, and duty cycle. The two-sided WBG was fabricated. An experimental platform based on photonic integrated interrogator was set up and the experiment on the two-sided WBG was performed. Results show that the two-sided WBG can be used to measure temperature changes over the range of 35–42 °C, with a temperature measurement error of 0.1 °C. This approach has the potential to facilitate application of such a silicon-on-insulator (SOI) WBG photonic sensor to wearable technology and realize the measurement of human temperature.

Design of an erbium-doped Al[formula omitted] optical waveguide amplifier with on-chip integrated laser pumping source

Erbium-doped optical waveguide amplifier (EDWA) is an important building block for conventional and integrated optical communication and light signal processing systems. In recent years remarkable advancement has been achieved for EDWA to meet the demand for silicon photonic chips and integrated photonic circuits. However, in the currently demonstrated EDWAs all the laser pumping sources are external or off-chip, which not only limits the application field of EDWA, complicates the amplification scheme, but also does not fulfill the expectation of monolithic integration. In this work we propose a design of EDWA using high-gain Er-doped Al2O3 waveguide, low-loss silicon-on-insulator (SOI) waveguide, and on-chip integrated semiconductor laser pumping source. The 1550 nm signal light and 1480 nm pump light are transmitted and combined in SOI waveguide and coupled into the EDWA waveguide embedded in the adjacent layer. The 1480 nm pump light is coupled into the SOI waveguide from a bonded hybrid III–V semiconductor laser. The pump and signal waveguide building scheme, the coupling method and structure, and the integration process of laser pumping source are shown and simulated. The system performance was evaluated by using the device’s structure parameters and Er3+ rate equation model. More than 1 dB/cm net gain and up to 35 dB maximum net gain can be achieved at about 100 mW pump power. It indicates that such an on-chip integration design is feasible and fully on-chip integrated optical waveguide amplifier could be achieved.

Nonlinear pulse reshaping in a typically designed silicon-on-insulator waveguide and its application to generate a high repetition rate pulse train

A silicon-on-insulator (SOI) planar waveguide is designed with a small group velocity dispersion (β 2) ∼2.212 (ps2 m−1) and a quite high nonlinear coefficient (γ) of ∼360.57 (W−1 m−1). The designed waveguide is capable of reshaping a super-Gaussian input optical pulse into a parabolic pulse (PP), without any use of external gain. The same waveguide with a relatively longer length can also generate a triangular pulse (TP) by using a positive chirp at the input. In both cases, PP and TP are created at a much shorter optimum length (L opt) of a few millimeters when compared to previously reported works on normal dispersion optical fibers. The interaction of a pulse pair inside such an SOI waveguide is investigated, for the first time as per our knowledge to generate a high-frequency (∼4.8 THz) pulse train, while lower repetition rate (∼180 GHz) pulses are used at the input. This study as a whole enables potential device applications in the domains of tunable high frequency (THz) pulse generators, optical signal processing, and many more.

Intraguide mode conversions of silica-on-silicon transfer waveguides and SOI tapered structures for highly efficient chip-fiber butt coupling.

An advanced fiber-waveguide coupling scheme is crucial to the effective developments and applications of silicon-on-insulator (SOI) waveguide photonic integrated chips. In this paper, a regime of the mode conversions within guided channels is investigated to realize a highly efficient fiber-chip coupling with a silica-on-silicon (SOS) transfer waveguide that can be realized with the local oxidation of silicon (LOCOS) technique. With this regime, the optical-field overlap process of two optical guided modes between the fiber and the SOI waveguide through a gap is simplified to the two loss contributions: (i) the ultralow coupling loss of the fiber-SOS transfer waveguide and (ii) the Fresnel loss of the SOS transfer waveguide and an SOI tapered structure. As a result, all the mode conversions in this system are operated within the guided channels, so it is referred to as an intraguide mode conversion. Further, the overall mode conversion efficiency of such a fiber-SOS-SOI system is modeled, in which the three key processes, the fiber-SOS gap mode conversion, the SOS-SOI interface mode conversion, and the SOI taper profile mode conversion, are individually investigated and optimized. The simulation values and the experimental results are agreeable with each other for both fiber-SOS waveguide coupling loss and fiber-SOI waveguide coupling loss. Consequently, a fiber-chip butt-coupling loss of 0.8-1.0 dB/facet is demonstrated. Finally, the conditions for realizing the effective LOCOS structure and process are discussed.

Near-infrared photoresponse in Ge/Si quantum dots enhanced by localized surface plasmons supported by aluminum nanodisks

An array of plasmonic nanoparticles can sustain surface plasmon modes from visible to infrared spectral range and thus offers effective surface light trapping, enhancement of local fields, and interaction with the thin active regions of optical devices. We report the fabrication and optical characterization of a planar Ge/Si quantum dot (QD) detector grown on silicon-on-insulator (SOI) substrate for photodetection in the near-infrared telecommunication wavelength range. The multilayer Ge/Si QD heterostructures are near-field coupled to the adjacent layers of aluminum nanodisks on the detector top. The periodic Al disk arrays have the square lattice symmetry with a lattice constant of 400 nm and the disk diameter varying from 150 to 225 nm. A significant enhancement in the room-temperature detector sensitivity is achieved due to the excitation of localized surface plasmons supported by the metallic disks and radiative coupling to the SOI waveguide modes. Through extinction spectroscopy and numerical modeling, we confirm the emergence of nanoparticle-induced plasmon resonances near the Si–Al interface. We demonstrate that an appropriate choice of the array periodicity and the size of the metal disks is able to increase the photodetector's efficiency by ∼ 40× at λ=1.2μm and by 15× at λ≈1.55μm relative to a bare detector with no plasmonic structure. These outcomes pave the way toward the use of Al as a low-cost plasmonic material with potential applications in infrared photodetection similar to those of the noble metals.

Angle-Sensitive Detector Based on Silicon-On-Insulator Photodiode Stacked with Surface Plasmon Antenna.

We present a pixel-level angle sensitive detector composed of silicon-on-insulator (SOI) photodiode (PD) stacked with a gold surface plasmon (SP) antenna to affect the direction of the incoming light. The surface plasmons are excited in the grating-type SP antenna and enhance the diffraction efficiency of the grating. The diffracted light is coupled strongly with the propagation light in the SOI waveguide when the phase matching condition is satisfied. The phase matching takes place at a specific angle of light incidence, and the discrimination of the light based on the incident angle is achieved. As spatial patterns in the polar coordinate of the elevation-azimuth angles (θ, ϕ) of the incident light, we present the phase matching condition theoretically, the absorption efficiency in the SOI by simulation, and also the quantum efficiency of the SOI PD experimentally for different SP antennas of one-dimensional (1D) line-and-space (L/S) and two-dimensional (2D) hole array gratings under various polarization angles. 1D grating offers a polarization sensitive angle detection and 2D grating exhibits angle detection in two orthogonal directions, enabling a polarization independent angle sensitivity. A good agreement among the theory, simulation, and experiment are attained. The proposed device features relatively high quantum efficiency as an angle-sensitive pixel (ASP) and gives wider opportunities in applications such as three-dimensional (3D) imaging, depth-of-field extension, and lensless imaging.

(Invited) Monolithic Ge Integration on the 3 μm SOI Platform for 40 GHz Photodiodes

Silicon photonics offers the possibility to integrate a large number of photonic components on a small Si chip in a similar fashion as microelectronic circuits have been realized for many decades. This enables low power consumption, small footprint and low-cost mass production. Application areas for Si photonics reach from optical communication and computing to optical sensing and imaging. Monolithic germanium PIN detectors are one of the key components in photonic integrated circuits (PICs) and convert light into electrical signals. Germanium is the preferred material for photodetectors in silicon photonics as it absorbs light in the datacom and telecom wavelength range (1.2-1.7 μm) due to its small bandgap. Germanium is also tolerated in the IC-clean manufacturing lines, unlike many other potential semiconductor materials. Electron and hole mobilities in germanium are high, enabling high-bandwidth detectors. Challenges in Ge integration on silicon-on-insulator (SOI) wafers include unintentional oxidization and etching in many of the standard silicon technology processing steps, fast diffusion of n-type dopants in Ge and relatively large lattice mismatch between Ge and Si.High refractive index contrast between silicon (3.5) and silicon dioxide (1.5) is the key to achieve ultra-dense photonics integration. Most of the silicon photonics research is carried out using submicron-thick SOI waveguides and latest microelectronics fabrication technologies that involve (and require) very small wafer topography. In this paper, we present Ge photodiode integration on a 3 µm SOI platform with 3 µm thick Si waveguides. Those offer ultra-low propagation losses (~0.1 dB/cm), ultra-dense integration (μm-scale bends), small polarization dependency (down-to-zero birefringence) and ability to tolerate relatively high optical powers (>1W). One major limitation on the 3 µm SOI platform has been the lack of as fast modulators and photodetectors as those integrated on sub-µm SOI platforms. In this work, we monolithically integrated a fast horizontal PIN photodiode on the 3 µm SOI platform. The 3 dB cutoff frequency was 40 GHz and the responsivity was 1.0 A/W at -1 V bias and 1.55 µm wavelength. This high bandwidth indicates that the detector is limited by the transit time of the carriers over the i-region, rather than the junction capacitance. The electric field in the i-region at -1 V is sufficient to keep the carrier drift speed close to the maximum carriers velocity in Ge.The design of the 40 GHz PD included analytical calculations and numerical simulations. Physical dimensions of the detector were chosen so that they are compatible with the 3 µm SOI process flow and meet the bandwidth and responsivity targets. Germanium was selectively grown into cavities in the 3 µm SOI layer, leading to very low amount of stress-induced crystal defects. The Ge waveguide detector and the Si waveguides were patterned with a common hard mask to achieve self-alignment between them. The n- and p-contacts were directly made into Ge using Ti/Al metallization. The vertical sidewalls of the Ge detector were implanted in order to create a horizontal PIN structure. Due to high confinement of light into the 3 µm thick SOI and Ge waveguides, the 3 µm thick and 1 µm wide Ge detector could be made as short as 9 µm. This led to small junction capacitances. The electrical output pulse shape was not distorted by the slow diffusion current of electrons and holes because the incoming light does not enter the doped regions. The development of 40 GHz PDs was an important step in improving the feasibility of the 3 µm SOI platform in high-bandwidth applications. Figure 1

(Invited) Directly Silicon Nitride Waveguide Coupled Ge Photodiode for Non-SOI PIC and Epic Platforms

We demonstrate a Ge photodiode that is directly coupled to a silicon nitride waveguide and shows more than 67 GHz bandwidth. This device paves the way for utterly new SiN waveguide platform based applications. By light feeding through SiN waveguides, the new photodiode can also be a key enabler for a bulk-Si based, monolithically integrated electronic-photonic integrated circuit platform.Silicon nitride (SiN) waveguides (WGs) outperform silicon-on-insulator (SOI) based waveguides in various aspects: they offer an enhanced optical bandwidth, allowing for low loss waveguides in the classic communication bands (C- and O- bands), widely addressed by SOI based photonics, as well as in the visible wavelength range, were Si WGs have already become opaque. Moreover, due to the absence of two-photon absorption in SiN, the optical power handling is superior compared to Si WGs [1–3]. Various platforms with outstanding optical loss performance have been demonstrated over the last years and are meanwhile offered for commercial purposes, as summarized in [1]. However, despite all these advantages, so far, commercially offered SiN based photonic integrated circuit (PIC) platforms neither provide monolithically integrated high speed modulators, nor fast SiN WG coupled detectors. In this work we focus on the detector side of a high-speed SiN PIC platform.Communication applications for upcoming 400 Gbps standards require 56 Gbd symbol rates, which makes photodiodes (PDs) with more than 40 GHz opto-electrical (OE) bandwidth indispensable. Previous works with focus on Ge PDs in SiN platforms have either shown devices directly SiN WG fed or indirectly coupled via multi-layers (SiN on SOI WGs) where the Ge PDs are actually still located on SOI. However, the former ones showed only inferior bandwidth of less than 10 GHz [4] while the latter ones provide at least a bandwidth of about 30 GHz [5]. Here, we demonstrate for the first time a bulk-Si based, directly SiN WG coupled Ge PD with very high OE -3 dB bandwidth of more than 67 GHz which opens new perspectives for opto-electronic platform technologies:(1) The combination of truly high-speed PDs with SiN waveguides could be the start for a novel active SiN platform e.g. aiming at integrated coherent receivers at wavelengths far below the O-band.(2) The demonstrated devices are in principle compatible for co-integration with IHP’s photonic BiCMOS process [6], which opens various opportunities for novel sensing or spectroscopic applications as well.(3) This work could pave the way towards a non-SOI based technology as an alternative to established SOI based PIC or electronic-photonic integrated circuit (ePIC) platforms, e.g. IHP’s photonic BiCMOS technology. Circuit fabrication on pure bulk-Si wafers would significantly relieve process complexity, time and thus costs.We show that the new devices, fabricated on bulk-Si, provide the same bandwidths as Si waveguide coupled SOI based reference Ge photodiodes. However, their O-band responsivity is 0.3 A/W, which is about three times lower compared to the SOI waveguide coupled devices. We attribute this effect to substrate losses and few specific layout features but see some potential for improvement by design and technological optimizations. We further demonstrate that the diodes can be fabricated with high yield and low metrics tolerances. Figure 1 shows SEM cross-sections (perpendicular to the light incidence direction) of Ge photodiodes fabricated on SOI waveguide (left) as well as on bulk-Si (right).[1] P. Muñoz et al., “Silicon Nitride Photonic Integration Platforms for Visible, Near-Infrared and Mid-Infrared Applications,” Sensors, vol. 17, no. 9, 2017.[2] A. Rahim et al., “Expanding the Silicon Photonics Portfolio With Silicon Nitride Photonic Integrated Circuits,” J. Lightwave Technol., vol. 35, no. 4, pp. 639–649, 2017.[3] K. Ikeda et al, “Thermal and Kerr nonlinear properties of plasma-deposited silicon nitride/ silicon dioxide waveguides,” Optics express, vol. 16, no. 17, pp. 12987–12994, 2008.[4] D. Ahn et al., “High performance, waveguide integrated Ge photodetectors,” Optics express, vol. 15, no. 7, pp. 3916–3921, 2007.[5] W. D. Sacher et al., “Monolithically Integrated Multilayer Silicon Nitride-on-Silicon Waveguide Platforms for 3-D Photonic Circuits and Devices,” Proc. IEEE, vol. 106, no. 12, pp. 2232–2245, 2018.[6] D. Knoll et al. “(Invited) SiGe BiCMOS for Optoelectronics,” ECS Transactions, vol. 75, no. 8, pp. 121–139, 2016. Figure 1