The growing maturity of integrated photonic technology makes it possible to build increasingly large and complex photonic circuits on the surface of a chip. Today, most of these circuits are designed for a specific application, but the increase in complexity has introduced a generation of photonic circuits that can be programmed using software for a wide variety of functions through a mesh of on-chip waveguides, tunable beam couplers and optical phase shifters. Here we discuss the state of this emerging technology, including recent developments in photonic building blocks and circuit architectures, as well as electronic control and programming strategies. We cover possible applications in linear matrix operations, quantum information processing and microwave photonics, and examine how these generic chips can accelerate the development of future photonic circuits by providing a higher-level platform for prototyping novel optical functionalities without the need for custom chip fabrication. The current state of programmable photonic integrated circuits is discussed, including recent developments in their building blocks, circuit architectures, electronic control and programming strategies, as well as different application spaces.

https://biblio.ugent.be/publication/8677857/file/8677861.pdf

Programmable photonic circuits.

We look at the opportunities presented by the new concepts of generic programmable photonic integrated circuits (PIC) to deploy photonics on a larger scale. Programmable PICs consist of waveguide meshes of tunable couplers and phase shifters that can be reconfigured in software to define diverse functions and arbitrary connectivity between the input and output ports. Off-the-shelf programmable PICs can dramatically shorten the development time and deployment costs of new photonic products, as they bypass the design-fabrication cycle of a custom PIC. These chips, which actually consist of an entire technology stack of photonics, electronics packaging and software, can potentially be manufactured cheaper and in larger volumes than application-specific PICs. We look into the technology requirements of these generic programmable PICs and discuss the economy of scale. Finally, we make a qualitative analysis of the possible application spaces where generic programmable PICs can play an enabling role, especially to companies who do not have an in-depth background in PIC technology.

https://biblio.ugent.be/publication/8660108/file/8660112.pdf

Programmable Photonics: An Opportunity for an Accessible Large-Volume PIC Ecosystem

We investigate and demonstrate the thermal crosstalk problem in integrated photonic circuits with metal and silicon doped heaters. Further, we illustrate that due to the localized heating effect, integrated doped heaters are out-performed in terms of thermal crosstalk as compared to integrated metal heaters. To mitigate thermal crosstalk and enhance phase tuning efficiency further, a CMOS compatible air-filled trench region is realized between the doped heater and the adjacent element. The performances of three fundamental building blocks of integrated photonic circuits, namely, a PN phase shifter, an optical attenuator, and a ring resonator, are tested by full-wave thermal, charge, and optical simulations. Additionally, the impact of thermal crosstalk on the performance of integrated PN phase shifters and optical attenuators is examined thoroughly. The proposed low crosstalk thermal phase shifters might be very beneficial for densely routed complex integrated photonic circuits like photonic transceivers for data centers, optical phased array antennas, and photonic reservoirs.

Design and Simulation of Thermo-Optic Phase Shifters With Low Thermal Crosstalk for Dense Photonic Integration

Silicon photonic platforms offer relevance to large markets in many applications, such as optical phased arrays, photonic neural networks, programmable photonic integrated circuits, and quantum computation devices. As one of the basic tuning devices, the thermo-optic phase shifter (TOPS) plays an important role in all these applications. A TOPS with the merits of easy fabrication, low power consumption, small thermal time constant, low insertion loss, small footprint, and low crosstalk, is needed to improve the performance and lower the cost of the above applications. To meet these demands, various TOPS have been proposed and experimentally demonstrated on different foundry platforms In this paper, we review the state-of-the-art of TOPS, including metal heater, doped silicon, silicide, with silicon substrate undercut for heat insulation, folded waveguide structure, and multi-pass waveguide structure. We further compare these TOPSs and propose the directions of the future developments on TOPS. 

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Thermo-optic phase shifters based on silicon-on-insulator platform: state-of-the-art and a review

We report a thorough analysis of the thermal crosstalk issue for silicon photonic circuits using metal and doped silicon heaters for the two waveguide configurations, rib and strip waveguides. Full-wave thermal and optical simulations are performed and it is observed that integrated doped heaters with strip waveguides have the best thermal crosstalk performance. Also, the effect of metal contacts on the heater power consumption is investigated. Furthermore, to alleviate the phase crosstalk and improve the phase tuning efficiency, a deep trench is implemented to thermally shield an adjacent waveguide. Two well-known photonic components, namely, integrated Mach—Zehnder modulators and PN phase shifters are designed and the thermal crosstalk performance is analyzed rigorously. The proposed photonic phase shifter is outperformed in terms of thermal crosstalk over the existing designs. Such phase shifters are viable for compact silicon photonic circuits operating at a very high temperature.

CMOS-Compatible Photonic Phase Shifters With Extremely Low Thermal Crosstalk Performance

We demonstrate a time-multiplexed row-column addressing scheme to drive thermo-optic phase shifters in a silicon photonic circuit. By integrating a diode in series with the heater, we can connect $N \times M$ heaters in an matrix topology to $N$ row and $M$ column lines. The heaters are digitally driven with pulse-width modulation, and time-multiplexed over different channels. This makes it possible to drive the circuit without digital-to-analog converters, and using only $M+N$ wires. We demonstrate this concept with a $1 \times 16$ power splitter tree with 15 thermo-optic phase shifters that are controlled in a $3 \times 5$ matrix, connected through 8 bond pads. This technique is especially useful in silicon photonic circuits with many tuners but limited space for electrical connections.

https://biblio.ugent.be/publication/8660114/file/8660117.pdf

Column-Row Addressing of Thermo-Optic Phase Shifters for Controlling Large Silicon Photonic Circuits

Low-power and compact phase shifters are crucial for large photonic circuits, both to cope with variability and to create programmable waveguide circuits scaling to thousands of tuning elements. This work demonstrates a liquid crystal phase shifter where there is a lateral silicon electrode "rail" on one side of the waveguide core. Using this architecture, a strong quasi-static electric field <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:msub> <mml:mi mathvariant="bold-italic">E</mml:mi> <mml:mtext>actuation</mml:mtext> </mml:msub> </mml:math> can be applied over the gap, which is filled with liquid crystal cladding material, with modest voltages. Because the mode is largely confined in the waveguide, optical losses are limited, compared to earlier experiments with slot waveguides. The liquid crystal is deposited locally on three different device variations using inkjet printing. The local deposition avoids unwanted interference of the liquid crystal with other optical components such as grating couplers. Measurements show similar trends as simulations of the liquid crystal orientations. For one device with a length of 50 µm , a phase shift of almost 0.9 π is shown at 10 V RMS . We also discuss the challenges with this first demonstration of this phase shifter geometry using a silicon side-rail as an electrode. 

Low power optical phase shifter using liquidcrystal actuation on silicon photonics platform

We demonstrate a compact phase shifter using liquid crystal actuation, integrated in an established siliconphotonics platform. The devices are fabricated using IMEC’s iSiPP50G platform with a simpl ...

Liquid crystal phase shifter integrated in a silicon photonics platform

Programmable photonic circuits offer a new way to explore photonic chips: instead of hardwiring waveguide connections, one programs a mesh of tunable couplers. This requires electronics and software routines to govern the behavior of the chip, making it possible to rapidly develop new photonic applications.

/pdf/programmable-silicon-photonic-integrated-circuits-1um3avstg4.pdf

Programmable Silicon Photonic Integrated Circuits

Programmable photonic circuits are optical chips whose functionality can be (re)configured through electronics and software. This programmability enables new functionality, and opens the path to general-purpose photonic chips, similar to electronic FPGAs. This tutorial discusses the operational principles and technology stack for these chips.

https://biblio.ugent.be/publication/8689972/file/8689976.pdf

Lukas Van Iseghem

Papers

Column-Row Addressing of Thermo-Optic Phase Shifters for Controlling Large Silicon Photonic Circuits

Low power optical phase shifter using liquidcrystal actuation on silicon photonics platform

Liquid crystal phase shifter integrated in a silicon photonics platform

Programmable Silicon Photonic Integrated Circuits

Tutorial: Programmable Integrated Photonics