High-efficiency single etch step apodized surface grating coupler using subwavelength structure

Grating couplers are key elements enabling the coupling of light between planar waveguide circuits and optical fibers. In this work, it is demonstrated using simulations and experiments that a high coupling efficiency can be achieved for an arbitrary buried oxide thickness by judicious adjustment of the grating radiation angle. The coupler strength is engineered by subwavelength structure, allowing straightforward apodization and single etch step fabrication. The design has been implemented using Fourier-eigenmode expansion and finite difference time domain methods. The measured coupling loss of a continuously apodized grating is -2.16 dB with a 3 dB bandwidth of 64 nm, therefore opening promising prospects for low-cost and high-volume fabrication using 193 nm deep-ultraviolet lithography. It is also shown by simulations that a coupling loss as low as -0.42 dB is predicted for a modified coupler structure with bottom mirror.Peer reviewed: YesNRC publication: Ye

High-efficiency fully etched fiber-chip grating couplers with subwavelength structures for datacom and telecom applications

Surface grating couplers are key components to couple light between planar waveguide circuits in silicon-on-insulator (SOI) platform and optical fibers. Here, we demonstrate by using simulations and experiments that a high coupling efficiency can be achieved for an arbitrary buried oxide thickness by judicious adjustment of the grating radiation angle. The coupler strength is engineered by subwavelength structures, which have pitch and feature sizes smaller than the wavelength of light propagating through it, thereby frustrating diffraction effects and behaving as a homogeneous media with an adjustable equivalent refractive index. This makes it possible to apodize the grating coupler with a preferred single etch fabrication process. The coupling efficiency of the grating coupler is optimized for operating with the transverse electric (TE) polarization state at the wavelengths near 1.3 \u3bcm and 1.55 \u3bcm, which are the bands relevant for datacom and telecom interconnects applications, respectively. The design and analysis of the grating coupler is carried out using two-dimensional (2-D) Fourier-eigenmode expansion method (F-EEM) and finite difference time domain (FDTD) method. The simulations show a peak fiber-chip coupling efficiency of -1:61 dB and - 1:97 dB at 1.3 \u3bcm and 1.55 \u3bcm wavelengths, respectively, with a minimum feature size of 100 nm, compatible with 193 nm deep-ultraviolet (DUV) lithography. The measurements of our fabricated continuously apodized grating coupler demonstrate fiber-chip coupling efficiency of - 2:16 dB at a wavelength near 1.55 \u3bcm with a 3 dB bandwidth of 64 nm. These results open promising prospects for low-cost and high-volume fabrication of surface grating couplers in SOI using 193 nm DUV lithography, which is now used in several silicon photonics foundries. It is also predicted that a coupling efficiency as high as - 0:42 dB can be achieved for the coupler structure with a bottom dielectric mirror.Peer reviewed: YesNRC publication: Ye

New concepts in silicon component design using sub wavelength 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. \ua9 2012 SPIE.Peer reviewed: YesNRC publication: Ye

Subwavelength index engineered surface grating coupler with sub-decibel efficiency for 220-nm silicon-on-insulator waveguides

Surface grating couplers are fundamental components in chip-based photonic devices to couple light between photonic integrated circuits and optical fibers. In this work, we report on a grating coupler with sub-decibel experimental coupling efficiency using a single etch process in a standard 220-nm silicon-on-insulator (SOI) platform. We specifically demonstrate a subwavelength metamaterial refractive index engineered nanostructure with backside metal reflector, with the measured peak fiber-chip coupling efficiency of -0.69 dB (85.3%) and 3 dB bandwidth of 60 nm. This is the highest coupling efficiency hitherto experimentally achieved for a surface grating coupler implemented in 220-nm SOI platform.Peer reviewed: YesNRC publication: Ye

SWG dispersion engineering for ultra-broadband photonic devices

In most integrated optics platforms device design is restricted to variations in the lateral dimensions, and a small set of etch depths. Sub-wavelength gratings (SWGs) in silicon-on-insulator enable engineering of refractive index in a wide range. SWGs exhibit a pitch smaller than the wavelength of light propagating through them, thereby suppressing diffraction and acting as a homogenous medium with an equivalent refractive index controlled by the duty-cycle. Here, we propose to not only engineer refractive index, but to control SWG dispersion. We use this concept to design ultra-broadband directional couplers (DCs) and multimode interference couplers (MMIs) with a fivefold bandwidth enhancement compared to conventional devices. \ua9 2013 SPIE.Peer reviewed: YesNRC publication: Ye

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

Fiber-chip edge couplers are extensively used in integrated optics as one of the key structures for coupling of light between planar waveguide circuits and optical fibers. In this work, a new fiber-chip edge coupler concept with large mode size for coupling to submicrometer silicon photonic wire waveguides is presented. The coupler allows direct coupling to conventional SMF-28 optical fiber and circumvents the need for lensed fibers. We demonstrate by simulations a 95% mode overlap between the mode at the chip facet and a high numerical aperture single mode optical fiber with 6 \u3bcm mode field diameter (MFD). We also demonstrate a modified design with 89% overlap between the mode at the chip facet and a standard SMF-28 fiber with 10.4 \u3bcm MFD. The coupler is designed for 220 nm silicon-oninsulator (SOI) platform. An important advantage of our coupler is that large mode size is obtained without the need to increase buried oxide (BOX) thickness, which in our design is set to 3 \u3bcm. This remarkable feature is achieved by implementing in the SiO2 upper cladding two thin high-index Si3N4 layers. The high-index layers increase the effective refractive index of the upper cladding layer near the facet and are gradually tapered out along the coupler to provide adiabatic mode transformation to the silicon wire waveguide. Simultaneously, the Si-wire waveguide is inversely tapered along the coupler. The mode overlap at the chip facet is studied using a vectorial 2D mode solver and the mode transformation along the coupler is studied by 3D Finite-Difference Time-Domain simulations. The couplers are optimized for operating with transverse electric (TE) polarization and the operating wavelength is centered at 1.55 \u3bcm.Peer reviewed: YesNRC publication: Ye

A subwavelength structured multimode interference coupler for the 3-4 micrometers mid-infrared band

The mid-infrared is attracting increasing attention since many molecules, including potentially hazardous gases such as methane and carbon dioxide, exhibit very specific absorption spectra in this wavelength region. Integrated silicon photonics circuits are envisioned to enable compact and low-cost measurement solutions for these molecules. Multimode interference couplers (MMIs) are basic building blocks for photonic circuits and a broad operational bandwidth is key if flexible operation is to be achieved, e.g. to detect different gases. Here we overcome the bandwidth limitations found in classical MMIs by segmenting the multimode region at a sub-wavelength pitch to engineer its refractive index and dispersion. We achieve less than 0:5 dB imbalance and excess loss in the complete 3 - 4 \u3bcm wavelength range. The sub-wavelength MMI not only exhibits nearly threefold improvement in bandwidth, but is also about three times shorter than the conventional device.Peer reviewed: YesNRC publication: Ye

Subwavelength metastructures for dispersion engineering in planar waveguide devices

High contrast structures with a sub-wavelength pitch, small enough to suppress diffraction, exhibit extraordinary optical properties: depending on the design they may behave as perfect mirrors, anti-reflective interfaces, homogenous materials with controllable refractive index, or strongly dispersive materials. Here we discuss on the design possibilities such structures offer in planar waveguide devices in silicon-on-insulator. We briefly review the application of sub-wavelength structures in a variety of waveguide devices. We then focus on some of the latest advances in the design ultra-compact and ultra-wideband multimode interference couplers based on dispersion engineered sub-wavelength structures.Peer reviewed: YesNRC publication: Ye

Ultra-compact polarization mode converter implemented in a dual-trench silicon-on-insulator waveguide

We demonstrate an ultracompact polarization mode converter based on a silicon-on-insulator waveguide with two longitudinal subwavelength trenches. An extinction ratio of 16 dB at 1.5 \u3bcm is achieved for a device length of 10 \u3bcm. \ua9 2012 OSA.Peer reviewed: YesNRC publication: Ye

Evanescent field waveguide sensing with subwavelength grating structures in silicon-on-insulator

We explore, to the best of our knowledge, the potential of diffractionless subwavelength grating waveguides for sensing applications. We show that by subwavelength patterning of silicon-wire waveguides the field delocalization can be engineered to increase the sensitivity. Fully vectorial 3D-FDTD simulations confirm the sensitivity enhancement, achieving sensitivities of 0.83 RIU/RIU and 1.5 \ub7 10-3 RIU/nm for bulk and surface sensing, respectively, which compare favorably to state-of-the-art sensing waveguides.Peer reviewed: YesNRC publication: Ye