A. Maese-Novo

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.

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.

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.

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.

Re-inventing multimode interference couplers using subwavelength gratings

Multimode-Interference (MMI) devices are fundamental building blocks in photonic integrated circuits, where they are used for power splitting and combining, optical switches and modulators, Mach-Zehnder interferometers and 90o hybrids for coherent optical receivers. MMIs are based on the self-image principle, by which the guided modes of the multimode region interfere to form replicas of the input field with specific amplitude and phase relations. These relations are known to depend on i) the core/cladding refractive indexes (n1/n2), ii) the core width (W) and length (L) of the multimode region and iii) the number, width and position of the access ports. In this work, we show that by using sub-wavelength structures within an MMI, the self-imaging properties can be significantly altered, leading to ultra-short or ultra-broadband devices.

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.

Compact broadband directional coupler

Directional couplers are key building blocks for network-on-chip devices such as wavelength demultiplexers and switches. The operational bandwidth of directional couplers is, however, limited, and existing techniques to extend their bandwidth result in significantly increased device footprints and degraded phase response. Here we present a novel approach to enhance the bandwidth of directional couplers without affecting their size or phase response. Using dispersion engineering of sub-wavelength gratings we show by 3D FDTD simulations a fivefold increase in coupler bandwidth from 20nm to 100 nm.