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.
Commercial introduction of emerging integrated photonics technologies requires a long and complex multi-layer product development, industrialization, and qualification cycles at all levels of value chain from initial product design, material sourcing, component-system-module manufacturing, and testing, through marketing and delivery of new products to the market. Scalable assembly and packaging of electronic-photonic integrated modules is important and may take more than a half of the entire product’s costs. In this paper, we will report on some of our industrial processes for scalable photonics packaging, as well as challenges and results obtained from our research and innovation projects.
In this paper, we present the development of a miniaturized Laser Doppler Vibrometer (LDV) system, based on the 3D hybrid integration of the Si3N4 platform of LioniX (TriPleX) and the polymer platform of FhG-HHI (PolyBoard). The photonic integrated circuit (PIC) supports all the functionalities of an LDV system including the splitting of the input light to the measurement and the reference beam, the introduction of an optical frequency shift up to 100 kHz, polarization handling and detection of the reflected measurement beam, using a heterodyne detection technique. The optical frequency shift is accommodated in the TriPleX section of the PIC based on a simple serrodyne scheme, where a phase modulator is driven with a sawtooth signal with the desired frequency. The modulation of the optical field is based on the stress-optic effect utilizing thin-films of PZT deposited on top of the waveguide structures of the TriPleX platform, capable of supporting modulation frequencies up to several MHz. The PolyBoard part enables polarization handling and heterodyne detection of the reflected beam using micro-optic elements on chip, including a polarization beam splitter (PBS), a half wave plate (HWP), and a pair of balanced detectors with four photodiodes that are flip chip bonded on the top. The TriPleX and the PolyBoard platform were brought together based on the 3D hybrid integration, using mode size converters and vertical directional couplers with coupling losses lower than 15 dB. On-chip beating, using the integrated photodiodes is experimentally demonstrated.
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.
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.
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.
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