KEYWORDS: Waveguides, Microelectromechanical systems, Phase shifts, Liquid crystals, Silicon photonics, Silicon, Photonic integrated circuits, Electrodes, Oxides, Back end of line
The demand for efficient actuators in photonics has peaked with increasing popularity for large-scale general-purpose programmable photonics circuits. We present our work to enhance an established silicon photonics platform with low-power micro-electromechanical (MEMS) and liquid crystal (LC) actuators to enable largescale programmable photonic integrated circuits (PICs).
We give an overview the progress of our work in silicon photonic programmable circuits, covering the technology stack from the photonic chip over the driver electronics, packaging technologies all the way to the software layers. On the photonic side, we show our recent results in large-scale silicon photonic circuits with different tuning technologies, including heaters, MEMS and liquid crystals, and their respective electronic driving schemes. We look into the scaling potential of these different technologies as the number of tunable elements in a circuit increases. Finally, we elaborate on the software routines for routing and filter synthesis to enable the photonic programmer.
We present our work in the European project MORPHIC to extend an established silicon photonics platform with low-power and non-volatile micro-electromechanical (MEMS) actuators to demonstrate large-scale programmable photonic integrated circuits (PICs).
The miniaturization of optical coherence tomography (OCT) systems could open up potential new markets, such as point-of-care application, home OCT to regularly monitor disease and treatment progress, and in low-resource settings. Photonic integrated circuits (PIC) are considered an attractive approach to miniaturize OCT. We present our recent achievements in in vivo retinal imaging with a PIC-based Mach-Zehnder interferometer integrated in a state-of-the-art ophthalmic OCT system. The system achieves 94 dB at 750 µW on the sample, running at 50 kHz. Preliminary results of a fully packaged 4-channel opto-electronic OCT engine further demonstrate the potential of PIC-based OCT.
We report on a swept-source OCT system based on a photonic-electronic integrated circuit. It enables a parallelization of data acquisition resulting in an effective A-scan rate of 4x100 kHz at a central wavelength of 840 nm.
The monolithic co-integration of photonic elements forming the multiplexed interferometers and the system electronics on one chip allows a very compact OCT engine in a photonic package. Integrated in an ophthalmic system, the maximum sensitivity was estimated to be 91 dB with an optical power of 4x520 µW at the model eye. An eye phantom was imaged at 400 kHz showing its layered structure.
Silicon nitride waveguide based photonic integrated circuits (PICs) are intensively investigated for a wide range of sensing applications in the visible to sub 1-µm near-infrared spectral region. The monolithic co-integration of silicon photodiodes and read-out electronics offers additional benefits in terms of performance and miniaturization. We discuss challenging aspects related to the efficient coupling and routing of light to, from, and within PICs and present interfacing photonic building blocks offering potential solutions. We demonstrate the suitability of these interfacing building blocks by using them for the realization of a PIC-based multi-channel optical coherence tomography concept at 840 nm.
We present our work to extend silicon photonics with MEMS actuators to enable low-power, large scale programmable photonic circuits. For this, we start from the existing iSiPP50G silicon photonics platform of IMEC, where we add free-standing movable waveguides using a few post-processing steps. This allows us to implement phase shifters and tunable couplers using electrostatically actuated MEMS, while at the same time maintaining all the original functionality of the silicon photonics platform. The MEMS devices are protected using a wafer-level sealing approach and interfaced with custom multi-channel driver and readout electronics.
Silicon (Si) photonic micro-electro-mechanical systems (MEMS), with its low-power phase shifters and tunable couplers, is emerging as a promising technology for large-scale reconfigurable photonics with potential applications for example in photonic accelerators for artificial intelligence (AI) workloads. For silicon photonic MEMS devices, hermetic/vacuum packaging is crucial to the performance and longevity, and to protect the photonic devices from contamination. Here, we demonstrate a wafer-level vacuum packaging approach to hermetically seal Si photonic MEMS wafers produced in the iSiPP50G Si photonics foundry platform of IMEC. The packaging approach consists of transfer bonding and sealing the silicon photonic MEMS devices with 30 μm-thick Si caps, which were prefabricated on a 100 mm-diameter silicon-on-insulator (SOI) wafer. The packaging process achieved successful wafer-scale vacuum sealing of various photonic devices. The functionality of photonic MEMS after the hermetic/vacuum packaging was confirmed. Thus, the demonstrated thin Si cap packaging shows the possibility of a novel vacuum sealing method for MEMS integrated in standard Si photonics platforms.
In the European project MORPHIC we develop a platform for programmable silicon photonic circuits enabled by waveguide-integrated micro-electro-mechanical systems (MEMS). MEMS can add compact, and low-power phase shifters and couplers to an established silicon photonics platform with high-speed modulators and detectors. This MEMS technology is used for a new class of programmable photonic circuits, that can be reconfigured using electronics and software, consisting of large interconnected meshes of phase shifters and couplers. MORPHIC is also developing the packaging and driver electronics interfacing schemes for such large circuits, creating a supply chain for rapid prototyping new photonic chip concepts. These will be demonstrated in different applications, such as switching, beamforming and microwave photonics.
High bandwidth density silicon photonic interconnects offer the potential to address the massive increase in bandwidth demands for data center traffic and high performance computing. One of the major challenges in realizing silicon photonics transceivers is the integration and packing of photonic ICs (PIC) with electronic ICs (EIC). This paper presents our version one, 2.5D integrated multi-chip module (MCM) transceiver for 4 channel wavelength division multiplexing (WDM) operation, targeting 10 Gbps per channel. We identify five key areas critical to successful integration of MCM transceivers, which we have used in developing our version two MCM transceiver: integration architecture, equivalent circuit model development, PIC to EIC interface modelling, MCM I/O design, and design for assembly.
A swept source optical coherence tomography (SS-OCT) system with the interferometer engine being a photonic integrated circuit (PIC) has been developed. Furthermore, an Arrayed Waveguide Grating (AWG), representing a grating on a PIC, for spectral domain OCT (SD-OCT) has been integrated in a fiber-based OCT system. With measured sensitivities of ~87 dB (SS-OCT) and ~80 dB (SD-OCT), scattering tissue imaging becomes feasible for OCT-on-chip systems. In this study, we present two OCT-on-chip systems and first results of biological tissue imaging in-vivo and exvivo.
The growth of the global photonics market and technology investment creates a need for skilled professionals with multidisciplinary knowledge. The development of a successful multidisciplinary training for these professionals requires special attention to the implemented educational approaches. Current methods of multidisciplinary teaching and learning include tutoring by a team of experts of different backgrounds and development of projects among multidisciplinary teams of students. In these cases, a detailed feedback is needed from students and teachers in order to improve the course and keep a consistent alignment among the learning outcomes, teaching strategy, and evaluation methods. In this study, we describe the implementation of a method based on continuous improvement of multidisciplinary outreach activities and undergraduate programs. We illustrate the results of this implementation by showing samples of the feedback we received from students and teachers. The overall quality of the teaching and the content of the subject matter was perceived as good by students. Teachers’ evaluation showed students’ knowledge and behavior was satisfactory and the learning outcomes were achieved. Based on this, we believe our educational approach could improve the development and implementation of multidisciplinary activities.
The packaging of high speed Photonic Integrated Circuits (PICs) should maintain the electrical signal integrity. The standard packaging of high speed PICs relies on wire bonds. This is not desirable because wire bonds degrade the quality of the electrical signal. The research presented in this paper proposes to replace wire bonds with an interposer with multilevel transmission lines. By attaching the PIC by flip chip onto the interposer, the use of wire bonds is avoided. The main concern for designing an interposer with multilevel transmission lines is the vertical transition, which must be designed to avoid return and radiation losses. In this paper, a novel design of a high speed vertical transition for Low Temperature Co-fired Ceramic (LTCC) is presented. The proposed vertical transition is simpler than others recently published in the literature, due to eliminating the need for additional ceramic layers or air cavities. A LTCC board was fabricated with several variations of the presented transition to find the optimal dimensions of the structure. The structures were fabricated then characterized and have a 3 dB bandwidth of 37 GHz and an open eye diagram at 44 Gbps. A full wave electromagnetic simulation is described and compared with good agreement to the measurements. The results suggest that an LTCC board with this design can be used for 40 Gbps per channel applications. Keywords: Photonics packaging, Low Temperature Co-Fired Ceramics.
Designing photonic integrated circuits (PICs) with packaging in mind is important since this impacts the performance of the final product. In coherent optical communication applications there are a large number of DC and RF lines that need routed to connect the PIC to the outer packaging. These RF lines should be impedance matched to the devices, isolated from each other, low loss and protected against electromagnetic interference (EMI) over the frequency range of interest to achieve the performance required for the application. Multilevel low temperature co-fired ceramic (LTCC) boards can be used as a carrier board connecting the PIC to the packaging due to its good RF performance, machinability, compatibility with hermetic sealing, and ability to integrate drivers into the board. Flexibility with layer numbers enables additional layers for shielding against electromagnetic interference or increased space for routing electrical connections. In this paper the design, simulations, and measured results for a set of 4 phase matched transmission lines in LTCC that would be used with an IQ MZM are presented. The measured 3dB bandwidth for a set of four phase matched transmission lines for an IQ MZM was measured to be 19.8 GHz.
To compensate for velocity mismatch in travelling wave opto-electronic devices, the microwave velocity of the propagating RF signal is reduced by introducing capacitively loaded elements. For high speed operation, these elements must be electrically isolated from one another, which is typically achieved by using ion-implantation to render the p-doped material non-conducting. We propose and demonstrate through optical and electrical simulations that ion-implantation can be avoided by using a quasi-shallow etch to electrically isolate the capacitive elements. High isolation can be achieved using such an etch without introducing additional losses to the propagating optical signal.
We present a study of the total internal reflection of a Helmholtz-Gauss beam at a plane interface between
two dielectric media. The derivation is based on the decomposition of the Helmholtz-Gauss beams in terms of
its constituent plane waves components. We determine the shift predicted by the classical theory of the Goos-
Hänchen shift and analyze the transverse intensity patterns of the reflected waves for a variety of Helmholtz-Gauss
beam including Bessel-Gauss and Cosine-Gauss beams.
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