J. G. Wangüemert-Pérez

Waveguide grating coupler with subwavelength microstructures

We propose a silicon waveguide-fiber grating coupler that uses a subwavelength microstructure to achieve a continuously variable grating strength yet can be fabricated using only a single etch step. By adjusting the subwavelength microstructure at every point along the grating, the grating coupler can be optimized to give high field overlap with the optical fiber mode and also minimize backreflections along the incident waveguide path. Our design example is optimized for quasi-TM mode in a silicon photonic-wire waveguide, as required for waveguide evanescent-field-sensing applications. A field overlap of up to 94% with a standard single-mode optical fiber (SMF-28) is achieved by coupler apodization. Backreflection from the grating is reduced to ~0.1%, and the total predicted photonic wire to fiber coupling efficiency is 50%.

High-performance 90° hybrid based on a silicon-on-insulator multimode interference coupler.

We propose a multimode interference coupler (MMI) design for high-index-contrast technologies based on a shallowly etched multimode region, which is, for the first time to our knowledge, directly coupled to deeply etched input and output waveguides. This reduces the phase errors associated with the high-index contrast, while still allowing for a very compact layout. Using this structure, we fabricate a 2 × 4 MMI operating as a 90° hybrid, with a footprint of only 0.65 mm × 0.53 mm, including all the structures necessary to couple light to a fiber array. We experimentally demonstrate a common mode rejection ratio better than -20 dBe and phase errors better than ±5° in a ~50 nm bandwidth.

Colorless directional coupler with dispersion engineered sub-wavelength structure

Directional couplers are extensively used devices in integrated optics, but suffer from limited operational wavelength range. Here we use, for the first time, the dispersive properties of sub-wavelength gratings to achieve a fivefold enhancement in the operation bandwidth of a silicon-on-insulator directional coupler. This approach does not compromise the size or the phase response of the device. The sub-wavelength grating based directional coupler we propose covers a 100 nm bandwidth with an imbalance of ≤ 0.6 dB between its outputs, as supported by full 3D FDTD simulations.

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 × 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°, 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.

A Design Procedure for High-Performance, Rib-Waveguide-Based Multimode Interference Couplers in Silicon-on-Insulator

Single mode silicon-on-insulator rib waveguides offer a weak lateral confinement, which makes it difficult to design high-performance multimode interference couplers (MMIs) based on these waveguides. Here, a complete design procedure for single etch step MMIs with tapered rib access waveguides is presented, which yields low excess loss (0.1 dB) and good imbalance (0.02 dB). Furthermore, polarization dependence is low, and high fabrication tolerances are achieved, especially with respect to etch depth.

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 × 4 MMI that operates as a 90° 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°, 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.

Slot-Coupled Multisection Quadrature Hybrid for UWB Applications

We present a three-section quadrature hybrid based on slot-coupled directional couplers, which operates over a bandwidth from 3.1 to 10.6 GHz. A prototype has been fabricated which exhibits a return loss better than 20 dB, isolation around 20 dB, an amplitude imbalance between output ports of less than plusmn0.75 dB and a phase imbalance between +1deg and -3deg across the 3.1-10.6 GHz band. This design outperforms previously reported results for ultra wide band operation.

High-directionality fiber-chip grating coupler with interleaved trenches and subwavelength index-matching structure.

We present the first experimental demonstration of a new fiber-chip grating coupler concept that exploits the blazing effect by interleaving the standard full (220 nm) and shallow etch (70 nm) trenches in a 220 nm thick silicon layer. The high directionality is obtained by controlling the separation between the deep and shallow trenches to achieve constructive interference in the upward direction and destructive interference toward the silicon substrate. Utilizing this concept, the grating directionality can be maximized independent of the bottom oxide thickness. The coupler also includes a subwavelength-engineered index-matching region, designed to reduce the reflectivity at the interface between the injection waveguide and the grating. We report a measured fiber-chip coupling efficiency of -1.3  dB, the highest coupling efficiency achieved to date for a surface grating coupler in a 220 nm silicon-on-insulator platform fabricated in a conventional dual-etch process without high-index overlays or bottom mirrors.

Removal of the Gibbs phenomenon and its application to fast-Fourier-transform-based mode solvers

A simple strategy for accurately recovering discontinuous functions from their Fourier series coefficients is presented. The aim of the proposed approach, named spectrum splitting (SS), is to remove the Gibbs phenomenon by making use of signal-filtering-based concepts and some properties of the Fourier series. While the technique can be used in a vast range of situations, it is particularly suitable for being incorporated into fast-Fourier-transform-based electromagnetic mode solvers (FFT-MSs), which are known to suffer from very poor convergence rates when applied to situations where the field distributions are highly discontinuous (e.g., silicon-on-insulator photonic wires). The resultant method, SS-FFT-MS, is exhaustively tested under the assumption of a simplified one-dimensional model, clearly showing a dramatic improvement of the convergence rates with respect to the original FFT-based methods.

Fast-Fourier-based three-dimensional full-vectorial beam propagation method

An accurate and highly efficient fast-Fourier-based full-vectorial beam propagation method (BPM) with perfectly matched layer absorbing boundary conditions is presented. Full vectorial results obtained for highly guiding z-variant structures, such as a spot size converter and a Y-branch in rib technology, show that accurate results can be obtained with reduced computational effort, thus making this technique an interesting alternative to other existing BPM strategies.