We examine the current performance and future demands of interconnects to and on silicon chips. We compare electrical and optical interconnects and project the requirements for optoelectronic and optical devices if optics is to solve the major problems of interconnects for future high-performance silicon chips. Optics has potential benefits in interconnect density, energy, and timing. The necessity of low interconnect energy imposes low limits especially on the energy of the optical output devices, with a ~ 10 fJ/bit device energy target emerging. Some optical modulators and radical laser approaches may meet this requirement. Low (e.g., a few femtofarads or less) photodetector capacitance is important. Very compact wavelength splitters are essential for connecting the information to fibers. Dense waveguides are necessary on-chip or on boards for guided wave optical approaches, especially if very high clock rates or dense wavelength-division multiplexing (WDM) is to be avoided. Free-space optics potentially can handle the necessary bandwidths even without fast clocks or WDM. With such technology, however, optics may enable the continued scaling of interconnect capacity required by future chips.

Device Requirements for Optical Interconnects to Silicon Chips

Wave interference is a fundamental manifestation of the superposition principle with numerous applications. Although in conventional optics, interference occurs between waves undergoing different phase advances during propagation, we show that the vectorial structure of the near field of an emitter is essential for controlling its radiation as it interferes with itself on interaction with a mediating object. We demonstrate that the near-field interference of a circularly polarized dipole results in the unidirectional excitation of guided electromagnetic modes in the near field, with no preferred far-field radiation direction. By mimicking the dipole with a single illuminated slit in a gold film, we measured unidirectional surface-plasmon excitation in a spatially symmetric structure. The surface wave direction is switchable with the polarization.

/pdf/near-field-interference-for-the-unidirectional-excitation-of-3so5gt6vn3.pdf

Near-Field Interference for the Unidirectional Excitation of Electromagnetic Guided Modes

Periodic structures with a sub-wavelength pitch have been known since Hertz conducted his first experiments on the polarization of electromagnetic waves. While the use of these structures in waveguide optics was proposed in the 1990s, it has been with the more recent developments of silicon photonics and high-precision lithography techniques that sub-wavelength structures have found widespread application in the field of pho- tonics. This review first provides an introduction to the physics of sub-wavelength structures. An overview of the applications of sub-wavelength structures is then given including: anti-reflective coatings, polarization rotators, high-efficiency fiber-chip cou- plers, spectrometers, high-reflectivity mirrors, athermal waveg- uides, multimode interference couplers, and dispersion engi- neered, ultra-broadband waveguide couplers among others. Particular attention is paid to providing insight into the design strategies for these devices. The concluding remarks provide an outlook on the future development of sub-wavelength structures and their impact in photonics.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/lpor.201400083

Waveguide sub-wavelength structures: a review of principles and applications

We have developed a silicon photonics platform that allows monolithic integration with electronic circuits in a CMOS-compatible process. In this platform, vertical couplers are found to be a superior solution compared to traditional edge-coupling techniques. Grating couplers are an essential element in developing the optical wafer-scale test infrastructure, which in turn, enables the development of the photonic device library. The photonic devices were assembled into a 4 × 10 Gb/s transceiver die that also contains modulator drive, control, and receive electronics.

A Grating-Coupler-Enabled CMOS Photonics Platform

A new generation of Silicon-on-Insulator fiber-to-chip grating couplers which use a silicon overlay to enhance the directionality and thereby the coupling efficiency is presented. Devices are realized on a 200mm wafer in a CMOS pilot line. The fabricated fiber couplers show a coupling efficiency of −1.6dB and a 3dB bandwidth of 80nm.

https://biblio.ugent.be/publication/1147439/file/1147459.pdf

High-efficiency fiber-to-chip grating couplers realized using an advanced CMOS-compatible Silicon-On-Insulator platform

We report experimental results on compact and broadband focusing grating couplers, both in silicon-on-insulator (SOI) and gold on SOI. An eight-fold length reduction of the coupling structure from fiber to photonic wire in SOI, as compared to a linear grating and adiabatic taper, is obtained, without performance penalty. A proof of principle is given for a focusing grating coupler in gold on SOI, with 20% fiber-to-focus efficiency.

/pdf/compact-focusing-grating-couplers-for-silicon-on-insulator-29seicm6br.pdf

Compact Focusing Grating Couplers for Silicon-on-Insulator Integrated Circuits

Compact metal grating couplers for efficient and broadband coupling between optical fibers and silicon-on-insulator waveguides are proposed and demonstrated. Simulation results for silver, gold, aluminum and copper grating couplers are presented. Using a uniform silver grating, coupling efficiencies are calculated as high as 60 % at a wavelength of 1.55 mum. Metal grating couplers require only one single etching or lift-off step and are therefore very easy to fabricate. As a proof of principle, a gold grating coupler for near-vertical fiber-to-waveguide coupling was fabricated using e-beam writing and lift-off, demonstrating 34 % coupling efficiency and a 1 dB-bandwidth of 40 nm. Measurement data and simulation results are in good agreement.

https://biblio.ugent.be/publication/388973/file/584755.pdf

Efficient, broadband and compact metal grating couplers for silicon-on-insulator waveguides.

We propose a novel configuration for a highly integrated and highly sensitive optical biosensor. The basic element of our sensor is a surface plasmon interferometer consisting of a thin layer of gold embedded in a silicon membrane. We investigate the performance of our sensor by simulation using eigenmode expansion. We calculate that refractive index changes in the order of 10-6 RIU (refractive index unit) are detectable for a component of length 10 mum. Moreover, we illustrate that the operation wavelength of our sensor can be tuned to a desired wavelength for a wide range of environmental refractive indices, making our device suitable for chemo- and biosensing

/pdf/surface-plasmon-interferometer-in-silicon-on-insulator-novel-1fya5502vs.pdf

Surface Plasmon Interferometer in Silicon-on-Insulator: Novel Concept for an Integrated Biosensor

An optical probe for photonic integrated circuits is proposed and demonstrated. The device is based on a single-mode fiber containing a subwavelength period metal grating on the facet. When approaching an integrated waveguide, light can be efficiently coupled between probe and waveguide without the need for integrated coupling structures, paving the way for wafer-scale circuit testing. A nanoimprint-and-transfer process were developed for fabricating this probe in a single step. We report 15% coupling efficiency between a gold grating fiber probe and a 220nm×3μm silicon-on-insulator waveguide and demonstrate testing of an integrated microring resonator using two probes.

Flexible metal grating based optical fiber probe for photonic integrated circuits

In this paper, the use of diffractive grating structures to efficiently interface between a single mode fiber and a high index contrast waveguide circuit is outlined. We show that high index contrast grating structures allow for broadband and high efficiency coupling. Since no polished facet is required on the photonic integrated circuit to interface with the optical fiber, fiber-to-chip grating couplers enable wafer-scale testing, reducing the cost for testing large scale integrated optical circuits. We show that two-dimensional grating structures can solve the problem of the huge polarization dependence of high index contrast photonic integrated circuits. Finally, an optical probe is presented, which allows testing individual components of a photonic integrated circuit, analogous to the electrical probes used in micro-electronics.

/pdf/bridging-the-gap-between-nanophotonic-waveguide-circuits-and-3xx0d8hhcv.pdf

Stijn Scheerlinck

Papers

Compact Focusing Grating Couplers for Silicon-on-Insulator Integrated Circuits

Efficient, broadband and compact metal grating couplers for silicon-on-insulator waveguides.

Surface Plasmon Interferometer in Silicon-on-Insulator: Novel Concept for an Integrated Biosensor

Flexible metal grating based optical fiber probe for photonic integrated circuits

Bridging the gap between nanophotonic waveguide circuits and single mode optical fibers using diffractive grating structures.