G. Wanguemert-Perez

Fiber-chip edge coupler with large mode size for silicon photonic wire waveguides

Fiber-chip edge couplers are extensively used in integrated optics for coupling of light between planar waveguide circuits and optical fibers. In this work, we report on a new fiber-chip edge coupler concept with large mode size for silicon photonic wire waveguides. The coupler allows direct coupling with conventional cleaved optical fibers with large mode size while circumventing the need for lensed fibers. The coupler is designed for 220 nm silicon-on-insulator (SOI) platform. It exhibits an overall coupling efficiency exceeding 90%, as independently confirmed by 3D Finite-Difference Time-Domain (FDTD) and fully vectorial 3D Eigenmode Expansion (EME) calculations. We present two specific coupler designs, namely for a high numerical aperture single mode optical fiber with 6 µm mode field diameter (MFD) and a standard SMF-28 fiber with 10.4 µm MFD. An important advantage of our coupler concept is the ability to expand the mode at the chip edge without leading to high substrate leakage losses through buried oxide (BOX), which in our design is set to 3 µm. This remarkable feature is achieved by implementing in the SiO2 upper cladding thin high-index Si3N4 layers. The Si3N4 layers increase the effective refractive index of the upper cladding near the facet. The index is controlled along the taper by subwavelength refractive index engineering to facilitate adiabatic mode transformation to the silicon wire waveguide while the Si-wire waveguide is inversely tapered along the coupler. The mode overlap optimization at the chip facet is carried out with a full vectorial mode solver. The mode transformation along the coupler is studied using 3D-FDTD simulations and with fully-vectorial 3D-EME calculations. The couplers are optimized for operating with transverse electric (TE) polarization and the operating wavelength is centered at 1.55 µm.

Group IV mid-­infrared photonics

In this paper we present SOI, suspended Si, and Ge-on-Si photonic platforms and devices for the mid-infrared. We demonstrate low loss strip and slot waveguides in SOI and show efficient strip-slot couplers. A Vernier configuration based on racetrack resonators in SOI has been also investigated. Mid-infrared detection using defect engineered silicon waveguides is reported at the wavelength of 2-2.5 μm. In order to extend transparency of Si waveguides, the bottom oxide cladding needs to be removed. We report a novel suspended Si design based on subwavelength structures that is more robust than previously reported suspended designs. We have fabricated record low loss Ge-on-Si waveguides, as well as several other passive devices in this platform. All optical modulation in Ge is also analyzed.

New concepts in silicon component design using subwavelength structures

Subwavelength gratings (SWG) are periodically segmented waveguides with a pitch small enough to suppress diffraction. These waveguides can be engineered to implement almost any refractive between the refractive indices of the material that compose the waveguide, thereby opening novel design possibilities. In this communication we explore the use of SWGs in the design and optimization of a variety of integrated optical devices in the silicon-on-insulator platform: fiber-to-chip grating couplers, polarization splitters and high performance multimode interference couplers. We furthermore show that the dispersion properties of SWGs enable the design of novel filters, and discuss the design of low transitions between SWG waveguides of different characteristics.

Group IV Photonics for the Mid-Infrared

In this paper we present SOI, suspended Si, and Ge-on-Si photonic platforms and mid-IR passive and active devices based on these platforms. We demonstrate low loss waveguides, splitters, filters, interferometers and spectrometers in different material platforms. Mid-infrared detection using defect engineered silicon waveguides is reported at the wavelength of 2-2.5 µm. All optical modulation in Ge is also analyzed. Finally, theoretical investigation of electroabsorption and electrorefraction in Ge is reported.

Subwavelength nanophotonic structures for integration, sensing and spectroscopy

We report our advances in development of subwavelength engineered metamaterial structures in silicon waveguides, specifically high-efficiency fiber-chip couplers, ultra-broadband surface grating couplers and nanophotonic beam splitters, evanescent field waveguide sensors and on-chip Fourier-transform spectrometers.

Nanostructured silicon delivers unprecedented optical devices

On-chip optical and photonic devices are key to major advances in fields as diverse as optical communications, sensing, and quantum physics. These integrated devices enable complex optical functionalities on a single chip (i.e., within a few square millimeters) that might otherwise occupy an entire optical table when implemented with bulk optical components. Currently, many commercial photonic chips are made from group III–V materials (i.e., containing elements in groups 13 and 15 of the periodic table), such as indium phosphide. Over the past decade, integrated photonic systems based on group IV materials—elements in group 14, particularly silicon and germanium—have drawn a lot of attention and are being developed by research groups around the world as well as industrial players, such as IBM and Intel. The main advantage of silicon photonics is that the CMOS infrastructure of the micro-electronics industry can be leveraged, potentially leading to high-volume and low-cost fabrication. However, in terms of performance and optical bandwidth—the range of optical wavelengths (colors) that a device can process accurately—many integrated photonic devices cannot yet compete with their bulkoptics counterparts. Here, we present a new silicon optical waveguide device that offers high performance and ultra-broad bandwidth operation with a very compact footprint. In photonic devices, the flow of light is governed by variations in refractive index, which engineers exploit in a range of materials to enable optical functionalities (e.g., for optical waveguides). In silicon photonics, the choice of materials is limited to silicon (with a refractive index n 3.5), silicon dioxide (n 1.4), and several polymers (n 1.6), which hinders the fabrication of high-performance, high-bandwidth devices. This limitation can be overcome using layers of materials with different thicknesses, which produce different Figure 1. A schematic representation of a new on-chip optical beamsplitter based on a nanostructured silicon multimode interference coupler showing the input (left) and output (right) light waves.

Subwavelength metamaterial engineering for silicon photonics

Waveguides structured at the subwavelength scale frustrate diffraction and behave as optical metamaterials with controllable refractive index. These structures have found widespread applications in silicon photonics, ranging from sub-decibel efficiency fibre-chip couplers to spectrometers and polarization rotators. Here, we briey describe the design foundations for sub-wavelength waveguide devices, both in terms of analytic effective medium approximations, as well as through rigorous Floch-Bloquet mode simulation. We then focus on two novel structures that exemplify the use of subwavelength waveguides: mid-infrared waveguides and ultra-broadband beamsplitters.

Data for Suspended silicon waveguides for long-wave infrared wavelengths

This is the dataset used to create figures 2 and 4 on the paper: Soler Penades, J. et al (2017). Suspended silicon waveguides for long-wave infrared wavelengths. Optics Letters.

Silicon subwavelength structures for communications and sensing (Conference Presentation)

Silicon sub-wavelength structures have found widespread applications in devices ranging from fiber-to-chip couplers to spectrometers. So far, these structures have been mainly used to engineer the local refractive index. Here we focus on two further applications. We describe how to engineer the waveguide electromagnetic field distribution for enhanced evanescent field sensing, predicting a 6-fold enhancement of the sensitivity compared to conventional waveguides. We furthermore report experimental results on broadband multimode interference couplers, which, by leveraging the inherent anisotropy of the sub-wavelength structures, achieve virtually perfect operation over a bandwidth of more than 300nm at telecom wavelengths.

Subwavelength engineered structures for integrated photonics

We report our advances in development of subwavelength engineered structures for integrated photonics. This unique technology allows synthesis of an effective photonic medium with an unprecedented control of material properties, constituting a powerful tool for a designer of photonic integrated circuits. By locally engineering the refractive index of silicon by forming a pattern of holes at the subwavelength scale it is possible to manipulate the flow of light in silicon photonic waveguides. We have demonstrated a number of subwavelength engineered devices operating at telecom wavelengths, including fiber-chip couplers, waveguide crossings, WDM multiplexers, ultra-fast optical switches, athermal waveguides, evanescent field sensors, polarization rotators, transceiver hybrids and colorless interference couplers. The subwavelength metamaterial concept has been adopted by industry (IBM) for fiber-chip coupling and subwavelength engineered structures are likely to become key building blocks for the next generation of integrated photonic circuits. We present an overview of different implementations of these structures in silicon photonic integrated circuits, such as high-efficiency fiber-chip couplers, wavelength multiplexers, microspectrometers, waveguide crossovers, ultra-broadband splitters and mid-infrared waveguide components, to name a few.