Fraunhofer HHI’s hybrid photonic integration technology based on SiN and polymer waveguiding platforms enables photonic integrated circuits operating at wavelengths from the infrared down to the visible. Hybrid photonic integration processes allow integrating active photonic building blocks such as lasers and active sections, as well as non-reciprocal and non-linear functionalities. Those features prove the large potential of Fraunhofer HHI’s hybrid photonic integration technology in application domains such as sensing and quantum technologies.
A photonic engine for the integration of multi-lane optical transceivers is presented. The building blocks are InP-based electro-absorption modulated lasers and photodiodes capable of operating at 50 GBaud with PAM-4 modulation, and a low-cost polymer waveguiding chip providing routing of the multiple lanes and connectivity towards standard single-mode fibers. An automatic process for the hybrid assembly of the different building blocks has been developed, and photonic integrated circuits with up to 16 lanes have been demonstrated. Furthermore, high-frequency flexible interconnects with bandwidths beyond 100 GHz provide a connectivity solution between photonics and high-speed electronics.
Photonic integrated circuits (PICs) are one of the key enablers for beyond 5G networks. A novel generation of fully integrated photonic-enabled transceivers operating seamlessly in W- D- and THz-bands is developed within the EU funded project TERAWAY. Photonic integration technology enables key photonic functionalities on a single PIC including photonic up/down conversion. For efficient down-conversion at the photonic integrated receiver, we develop the first waveguide-fed photoconductive antenna for THz communications. Finally, we report on the experimental implementation of a fully photonic-enabled link operating across W- D- and THz-bands.
We describe the assembly of a 5G transceiver leveraging photonics for the generation, emission and detection of THz wireless signals. The transceiver and all associated control electronics and power supplies are designed for mounting in a mobile aerial unit. A photonics motherboard concept that brings together polymer, III-V and SiNbased photonic platforms and provides optical fiber connectivity is used for the assembly. In addition, scalable integration of 3D components, in this case an antenna rod or rod array, is demonstrated. Thermal considerations arising from the dense integration of photonic and electronic components and the resulting concentrated heat load are also discussed.
Fraunhofer HHI's hybrid integration platform PolyBoard combines polymer passive waveguides with InP and other materials. We present new functionalities integrated in PolyBoard:
Isolation: With a microoptical bench integrated into polymer isolators can be built.
Quantum and sensing: By integrating nonlinear materials into the microoptical bench, 2nd (775 nm), 3rd (515 nm), and 4th (387 nm) harmonic generation could be observed
3D: First results for a 2x4 phased array have been achieved
Flip-chip laser active alignment: We have developed an active alignment process, which also works for flip-chip lasers which are impossible to electrically contact during the alignment process.
First automation results show the potential for cost effective volume scaling.
We present the current challenges for high frequency interconnects, especially for calibrated measures of the frequency response of components operating above 100 GHz. This is the challenge addressed by the TERAmeasure Future and Emerging Technologies project, aiming to combine photonics and electronics to develop new paradigm in the millimetre and Terahertz frequency ranges, overcoming the current obstacles to better measurements, eliminating the frequency banded nature of rectangular waveguides and providing metrology-grade results across the full frequency range.
Nonreciprocal optical functionalities like optical isolators and circulators are key components for the suppression of unwanted optical feedback in lasers and are also widely used for light routing in fiber-based measurement systems such as optical coherence tomography. Therefore, they are important building blocks in integrated optics, which promises further miniaturization and cost reduction of optical elements for telecom, datacom, and sensing applications. In this work, we experimentally demonstrate a four-port polarization independent optical circulator on a polymer-based hybrid integration platform. The circulator consists of polymer waveguides and two thin-film polarization beam splitters (PBSs) inserted into waveguides via etched slots. Crystalline, pre-magnetized bulk Faraday rotators (FRs) and half-wave plates (HWPs) are inserted into free-space sections, formed by pairs of waveguide butt-coupled GRIN lenses. For a first demonstrator, on-chip losses down to 5 dB and optical isolations up to 24 dB were measured, depending on the different input and output constellations, as well as the polarization. By applying an external magnetic field opposite to the magnetization of the faraday rotators, it is possible to repole the magneto-optic material, leading to reversely circulating light inside the device. This enables optical switching between ports in form of a latching switch, which maintains its state after removing the external magnetic field.
Existing transceiver technology inside data centers will soon reach its limits due to the enormous traffic growth rates driven by new, bandwidth-hungry applications. Efforts to develop the next generation of 800Gbps and 1.6Tbps transceivers for intra-DC optical interconnects have already kicked-off to address the demands in traffic, the exhaustion of the ports at the digital switches and the power consumption limitations inherent to the use of many lower capacity modules. The new generation of optical modules must also provide Terabit capacities at low cost, necessitating the use of high-volume manufacturing processes. TERIPHIC is an EU funded R and D project that aims at developing transceiver modules with up to 1.6 Tbps capacity over 16 lanes in duplex fiber and cost less than 1 € per Gbps for distances up to 2 km, utilizing PAM-4 modulation for 100Gbps per lane and high-volume production compatible transceiver designs. At the component level, TERIPHIC will rely on arrays of high-speed electronics, InP Externally Modulated Lasers (EMLs) and InP photodetectors, and at the integration level it will rely on a polymer photonic platform as a host motherboard, leveraging its flexibility and powerful toolbox. A summary of the progress on the TERIPHIC transceiver modules concept, both at the component level and integration level is presented in this paper.
3D photonic integration introduces a new degree of freedom in the design of photonic integrated circuits (PICs) compared to standard 2D-like structures. Novel applications such as large-scale optical switching matrices, e.g. for top-of- rack cross connect switches in data centers, benefit from the additional design flexibility due to their waveguide crossing-free architecture and compact footprint. In this work, a novel 3D 4×4 multi-mode interference coupler (MMI) based on HHI’s polymer-based photonic integration platform PolyBoard is presented. The fabrication process of the PolyBoard platform allows for the realization of vertically stacked polymer waveguide layers. Cascading two of the presented 3D 4×4 MMIs will form the building block of future large-scale 3D switching matrices. The 3D 4×4 MMI structure comprises two waveguide layers separated by a distance of 7.2 μm, with two input and two output waveguides in each layer, and a multimode interference (MMI) section in between. The vertical MMI section serves as the interconnection between the different waveguide layers and distributes the incoming light from each input waveguide across the four output ports of the 4×4 MMI. Design rules and fabrication methodology of these novel structures are presented in detail. Preliminary measurements demonstrate the proof-of-concept indicating an insertion loss below 9.3 dB, including fiber-chip coupling loss and the 6 dB intrinsic loss.
Recent developments in versatile polymer-based technologies and hybrid integration processes offer a flexible and cost-efficient alternative for creating very complex photonic components and integrated circuits. The fast and efficient test, optimization and verification of new ideas requires an automated and reproducible simulation and design process supporting flexible layout-driven and layout-aware schematic-driven methodologies. Targeting very complex designs, even small fabrication tolerances of one building block could make a huge difference on the performance and manufacturability of the whole structure. To reduce risk of failure and to make performance predictions by virtual prototyping reliable, the simulation model of each single building block needs to be working correctly based not only on the appropriate mathematical and physical equations, but also on adequate information provided by the foundry where the final structure will be manufactured.
The PolyPhotonics Berlin consortium targets to address these design challenges and establish a new versatile integration platform combining polymer with Indium-Phosphide and thin-film filter based technologies for numerous photonics applications in the global communications and sensing market. In this paper we will present our methodologies for modelling and prototyping optical elements including hybrid coupling techniques, and compare them with exemplary characterization data obtained from measurements of fabricated devices and test structures. We will demonstrate how the seamless integration between photonic circuit and foundry knowledge enable the rapid virtual prototyping of complex photonic components and integrated circuits.
We demonstrate the hybrid integration of a multi-format tunable transmitter and a coherent optical receiver based on optical polymers and InP electronics and photonics for next generation metro and core optical networks. The transmitter comprises an array of two InP Mach-Zehnder modulators (MZMs) with 42 GHz bandwidth and two passive PolyBoards at the back- and front-end of the device. The back-end PolyBoard integrates an InP gain chip, a Bragg grating and a phase section on the polymer substrate capable of 22 nm wavelength tunability inside the C-band and optical waveguides that guide the light to the inputs of the two InP MZMs. The front-end PolyBoard provides the optical waveguides for combing the In-phase and Quadrature-phase modulated signals via an integrated thermo-optic phase shifter for applying the pi/2 phase-shift at the lower arm and a 3-dB optical coupler at the output. Two InP-double heterojunction bipolar transistor (InP-DHBT) 3-bit power digital-to-analog converters (DACs) are hybridly integrated at either side of the MZM array chip in order to drive the IQ transmitter with QPSK, 16-QAM and 64-QAM encoded signals. The coherent receiver is based on the other side on a PolyBoard, which integrates an InP gain chip and a monolithic Bragg grating for the formation of the local oscillator laser, and a monolithic 90° optical hybrid. This PolyBoard is further integrated with a 4-fold InP photodiode array chip with more than 80 GHz bandwidth and two high-speed InP-DHBT transimpedance amplifiers (TIAs) with automatic gain control. The transmitter and the receiver have been experimentally evaluated at 25Gbaud over 100 km for mQAM modulation showing bit-error-rate (BER) performance performance below FEC limit.
Photonic devices and new functions based on HHI’s hybrid integration platform PolyBoard are presented providing lowloss thin-film-element-based light routing, an on-chip micro-optical bench and flexible chips comprising optical and electrical waveguides. The newly developed transfer and integration of graphene layers enables the fabrication of active optoelectronic devices in the intrinsically passive polymer waveguide networks with bandwidths in the GHz range. These novel functionalities in combination with the mature thermo-optic components of the PolyBoard platform such as tunable lasers, switches and variable attenuators pave the way towards new applications of photonic integrated circuits in communications and sensors.
Graphene with its high carrier mobility as well as its tunable light absorption is an attractive active material for highspeed electro-absorption modulators (EAMs). Large-area CVD-grown graphene monolayers can be transferred onto arbitrary substrates to add active optoelectronic properties to intrinsically passive photonic integration platforms. In this work, we present graphene-based EAMs integrated in passive polymer waveguides. To facilitate modulation frequencies in the GHz range, a 50 Ω termination resistor as well as a DC blocking capacitor are integrated with graphene EAMs for the first time. Large signal data transmission experiments were carried out across the O, C and L optical communications bands. The fastest devices exhibit a 3-dB bandwidth of more than 4 GHz. Our analytical model of the modulation response for the graphene-based EAMs is in good agreement with the measurement results. It predicts that bandwidths greater than 50 GHz are possible with future device iterations. Owing to the absorption properties of the graphene layers, the devices are expected to be functional at smaller wavelengths of interest for optical interconnects and data-communications as well, offering a novel flexibility for the integration of high-speed functionalities in optoelectronic integrated circuits. Our work is the first step towards an Active Optical Printed Circuit Board, hiding the optics completely inside the board and thus removing entry barriers in manufacturing. We believe this will lead to the same success as observed in Active Optical Cables for short range optically wired connections.
A hybrid polymer/InP dual DBR laser at 1.5μm is presented as an optical source for heterodyne generation and detection of cw-THz signals. The device consists of an active InP chip as an active gain element, end-fire coupled to a polymer chip with thermo-optically tunable phase shifters and Bragg gratings. Mode-hop-free tuning of 1.1 THz has been achieved on the single DBR lasers. The usability of such sources for heterodyne cw-THz generation has been demonstrated in a coherent cw-THz setup. Scans in the THz range show a resolution of the H2O absorption lines comparable to the results achievable with commercially-available external-cavity diode lasers.
Hybrid photonic integration allows individual components to be developed at their best-suited material platforms without sacrificing the overall performance. In the past few years a polymer-enabled hybrid integration platform has been established, comprising 1) EO polymers for constructing low-complexity and low-cost Mach-Zehnder modulators (MZMs) with extremely high modulation bandwidth; 2) InP components for light sources, detectors, and high-speed electronics including MUX drivers and DEMUX circuits; 3) Ceramic (AIN) RF board that links the electronic signals within the package. On this platform, advanced optoelectronic modules have been demonstrated, including serial 100 Gb/s [1] and 2x100 Gb/s [2] optical transmitters, but also 400 Gb/s optoelectronic interfaces for intra-data center networks [3]. To expand the device functionalities to an unprecedented level and at the same time improve the integration compatibility with diversified active / passive photonic components, we have added a passive polymer-based photonic board (polyboard) as the 4th material system. This passive polyboard allows for low-cost fabrication of single-mode waveguide networks, enables fast and convenient integration of various thin-film elements (TFEs) to control the light polarization, and provides efficient thermo-optic elements (TOEs) for wavelength tuning, light amplitude regulation and light-path switching.
Recent progress on polymer-based photonic devices and hybrid photonic integration
technology using InP-based active components is presented. High performance thermo-optic
components, including compact polymer variable optical attenuators and switches are powerful
tools to regulate and control the light flow in the optical backbone. Polymer arrayed waveguide
gratings integrated with InP laser and detector arrays function as low-cost optical line terminals
(OLTs) in the WDM-PON network. External cavity tunable lasers combined with C/L band thinfilm
filter, on-chip U-groove and 45° mirrors construct a compact, bi-directional and color-less
optical network unit (ONU). A tunable laser integrated with VOAs, TFEs and two 90° hybrids
builds the optical front-end of a colorless, dual-polarization coherent receiver. Multicore polymer
waveguides and multi-step 45°mirrors are demonstrated as bridging devices between the spatialdivision-
multiplexing transmission technology using multi-core fibers and the conventional PLCbased
photonic platforms, appealing to the fast development of dense 3D photonic integration.
In this work, a direct DQPSK receiver was fabricated, which comprises a polymer waveguide based delay-line
interferometer (DLI); a polymer based optical hybrid, and two monolithic pairs of > 25 GHz bandwidth photodiodes that
are vertically coupled to the polymer planar lightwave circuit (PLC) via integrated 45° mirrors. The common mode
rejection ratio (CMRR) is used to characterize the performance of coherent receivers, by indicating the electrical power
balance between the balanced detectors. However, the standard CMRR can only be measured when the PDs can be
illuminated separately. Also, the standard CMRR does not take into account the errors in the relative phases of the
receiver outputs. We introduce an adapted CMRR to characterize the direct receiver, which takes into account the
unequal responsivities of the PDs, the uneven split of the input power by the DLI and hybrid, the phase error and the
extinction ratio of the DLI and hybrid.
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