Polarization-independent grating coupler for micrometric silicon rib waveguides

Grating couplers are a promising approach to implement efficient fiber-chip coupling. However, their strong polarization dependence makes dual-polarization operation challenging. In this Letter we propose, for the first time, a polarization-independent grating coupler for thick rib silicon-on-insulator (SOI) waveguides. Coupling efficiency is optimized by designing the grating pitch and duty cycle, without varying the bottom oxide thickness, which significantly simplifies practical implementation. Directivity of the grating coupler is enhanced by a high reflectivity layer under the bottom oxide after the selective removal of the Si substrate. Dual-polarization coupling efficiency of -2.8 dB is shown.Peer reviewed: YesNRC publication: Ye

Fiber-chip grating coupler based on interleaved trenches with directionality exceeding 95%

We propose a fiber-chip grating coupler that interleaves the standard full and shallow etch trenches in a 220 nm thick silicon layer to provide a directionality upward exceeding 95%. By adjusting the separation between the two sets of trenches, constructive interference is achieved in the upward direction independent of the bottom oxide thickness and without any bottom reflectors, overlays, or customized etch depths. We implement a transverse subwavelength structure in the first two grating periods to minimize back-reflections. The grating coupler has a calculated coupling efficiency of CE ~ -1.05 dB with a 1 dB bandwidth of 30 nm and minimum feature size of 100 nm, compatible with deep-UV lithography.Peer reviewed: YesNRC publication: Ye

High-performance multimode interference coupler in silicon waveguides with subwavelength structures

The performance of multimode interference (MMI) couplers in silicon waveguides is limited by the high lateral refractive index contrast. Here we propose the use of subwavelength gratings (SWGs) in the lateral cladding regions of the MMI to reduce the index contrast. Our approach significantly reduces the mode phase error while at the same time allowing a single etch step process. Using a $z$-periodic lateral SWG, we design a 2 7 4 MMI that operates as a 90\ub0 hybrid for a coherent optical receiver. This complex device exhibits a common mode rejection ratio (CMRR) and a phase error of less than-24 dBe and 2\ub0, respectively, over the full C-band. Compared to MMI with a homogenous lateral cladding, using subwavelength refractive index engineering effectively extends the receiver bandwidth from 36 to 60 nm. \ua9 2011 IEEE.Peer reviewed: YesNRC publication: Ye

Grating couplers for thick SOI rib waveguides

The use of grating couplers in high index contrast waveguides like silicon on insulator (SOI) offers several advantages over other coupling approaches, including better alignment tolerances and allowing for wafer-scale testing. The grating couplers were developed for nanometric SOI waveguides (Si-wires), and recently also for micrometric rib waveguides. In this paper we review our work in fiber-to-chip grating couplers for thick SOI rib waveguides, where a coupling efficiency of 122.2 dB was demonstrated experimentally. We also discuss the use of grating couplers to improve optical throughput (\ue9tendue) of a planar waveguide Fourier-Transform (FT) spectrometer implemented in thick rib waveguides.Peer reviewed: YesNRC publication: Ye

Highly tolerant tunable waveguide polarization rotator scheme

Integrated polarization rotators are known to exhibit stringent fabrication tolerances, which severely handicap their practical application. Here we present a general polarization rotator scheme that enables both the compensation of fabrication errors and wavelength tunability. The scheme is described analytically, and a condition for perfect polarization conversion is established. Simulations of a silicon-on-insulator polarization rotator show polarization extinction ratios in excess of 40 dB even in the presence of large fabrication errors that in a conventional rotator configuration degrade the extinction ratio to below 5 dB. Additionally, wavelength tuning over \ub130 nm is shown.Peer reviewed: YesNRC publication: Ye

Efficient fibre-chip grating coupler for thick SOI rib waveguides

In this work a fiber to chip grating coupler in micrometric SOI rib waveguides is experimentally demonstrated, for the first time, with -4dB coupling efficiency when an inverse taper is used as excitation stage. \ua9 2011 OSA.Peer reviewed: YesNRC publication: Ye

Wavelength independent multimode interference coupler

We propose an ultra-broadband multimode interference (MMI) coupler with a wavelength range exceeding the O, E, S, C, L and U optical communication bands. For the first time, the dispersion property of the MMI section is engineered using a subwavelength grating structure to mitigate wavelength dependence of the device. We present a 2 7 2 MMI design with a bandwidth of 450nm, an almost fivefold enhancement compared to conventional designs, maintaining insertion loss, power imbalance and MMI phase deviation below 1dB, 0.6dB and 3\ub0, respectively. The design is performed using an in-house tool based on the 2D Fourier Eigenmode Expansion Method (F-EEM) and verified with a 3D Finite Difference Time Domain (FDTD) simulator. \ua9 2013 Optical Society of America.Peer reviewed: YesNRC publication: Ye

Single etch grating couplers for mass fabrication with DUV lithography

Surface grating couplers enable efficient coupling of light between optical fibers and planar waveguide circuits. While traditional grating designs require two etch steps for efficient coupling to silicon-on-insulator waveguides, recently proposed subwavelength structured gratings can achieve the same coupling efficiencies with a single etch step, thereby significantly reducing fabrication complexity. Here we demonstrate that such couplers can be fabricated on a large scale with ultra-violet lithography, achieving a 5 dB coupling efficiency at 1,550 nm. Through both simulations and experiments we give physical insight on how pattern fidelity impacts the performance of these couplers, and propose strategies to deal with inevitable process variations.Peer reviewed: YesNRC publication: Ye

High efficiency blazed fiber-chip grating coupler with interleaved trenches

We demonstrate a fiber-chip surface grating coupler that interleaves standard full and shallow-etched trenches to maximize directionality in the upward direction. The coupler is implemented in a regular SOI substrate with 220 nm silicon thickness and etch depths of 220 nm (full etch) and 70 nm (shallow etch), as offered by silicon photonic foundries. The blazing effect is controlled by adjusting the separation between the two sets of trenches. This way, grating directionality exceeding 95% is achieved independently of the bottom oxide (BOX) thickness. Couplers have been fabricated at LETI using 193 nm DUV lithography on 200 mm SOI wafers with 2 \u3bcm BOX. The measured coupling efficiency is-2.1 dB with a 3 dB bandwidth of 52 nm.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