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Multimode interference couplers (MMIs) are fundamental building blocks in photonic integrated circuits. Here it is experimentally demonstrated, for the first time, a two-fold length reduction in an MMI coupler without any penalty on device performance. The design is based on a slotted 2 × 2 MMI fabricated on a commercial silicon-on-insulator (SOI) substrate. The slot is implemented with a subwavelength grating (SWG) comprising holes fully etched down to the oxide cladding, thereby allowing single etch step fabrication. The device has been designed using an in-house tool based on the Fourier Eigenmode Expansion Method. It has a footprint of only 3.5 μm x 23 μm and it exhibits a measured extinction ratio better than 15 dB within the full C-band (1530 nm‒1570 nm). SWG engineered slots thus offer excellent perspectives for the practical realization of MMIs couplers with substantially reduced footprint yet with outstanding performance.

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. © 2011 IEEE.

In most integrated optics platforms, including silicon-on-insulator, only minor modifications in refractive index are possible. The geometry of the waveguiding structure is thus the only degree of freedom for the design of devices. The use of sub-wavelength gratings (SWGs), i.e. structures that are small enough to suppress diffraction effects, enables local engineering of both refractive index and dispersion, thereby opening new possibilities for device design. Here we present some of the recent advances in refractive index and dispersion engineering using silicon SWGs, focussing on ultra-broadband and compact multimode interference couplers and directional couplers. © 2013 IEEE., 2013 IEEE 10th International Conference on Group IV Photonics, GFP 2013, 28 August 2013 through 30 August 2013, Seoul

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. © 2013 SPIE., Integrated Photonics: Materials, Devices, and Applications II, April 24-26, 2013, Grenoble, Series: Proceedings of SPIE; no. 8767

Grating couplers have shown promising performance in terms of coupling efficiency and alignment tolerances as an interface between optical fibers and nanometric silicon-on-insulator (SOI) waveguides. In this paper we review our previous work, where the implementation of a fiber to chip grating coupler in micrometric rib SOI waveguide was demonstrated for the first time, showing measured coupling efficiency of -2.2dB for transverse-electric (TE) polarization. We also propose a new grating design that achieves a calculated coupling efficiency of -2dB for transverse-magnetic (TM) light. © 2012 SPIE., Silicon Photonics and Photonic Integrated Circuits III, April 16-19, 2012, Brussels, Series: Proceedings of SPIE; no. 8431

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 −2.2 dB was demonstrated experimentally. We also discuss the use of grating couplers to improve optical throughput (etendue) of a planar waveguide Fourier-Transform (FT) spectrometer implemented in thick rib waveguides.

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.

We review our advances in diffractive and subwavelength grating coupler structures for microphotonic waveguides. We present a subwavelength grating fiber-chip edge coupler with a loss as low as 0.9 dB and with minimal wavelength dependence over a broad wavelength range exceeding 200 nm. We also present fiber-to-chip surface grating couplers based on subwavelength effective medium. The effective medium refractive index is engineered to control the strength of the grating and thereby maximize coupling efficiency, while mitigating back reflections at the same time. We analyze the fabrication tolerances of the coupler, which are of particular relevance for large scale photonic fabrication. Furthermore, we present the first grating coupler for micrometric silicon rib waveguides, which is particularly challenging since the coupler waveguide region is multimode. We experimentally demonstrate grating couplers in 1.5 μm thick silicon ridge waveguides with a coupling efficiency of 2.2 dB and a 3 dB bandwidth of 40 nm. An inverse taper is used to match the fundamental mode of the interconnection waveguide with the optimum grating coupler excitation field, with negligible higher order mode excitation. The coupler was fabricated using i-line stepper lithography and single etch step. © 2012 IEEE., 14th International Conference on Transparent Optical Networks, ICTON 2012, 2 July 2012 through 5 July 2012, Coventry

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. © 2012 SPIE., Silicon Photonics VII, January 22-25, 2012, San Francisco, CA, USA, Series: Proceedings of SPIE; no. 8266

The use of grating couplers for fiber to chip coupling is a promising technique, allowing wafer scale testing with no need for facet preparation. Albeit grating couplers have been extensively studied for Si-wire waveguides, the implementation on SOI micrometric rib waveguides remains challenging, mainly due to excitation of higher order modes. In this work we show for the first time that monomode excitation of the grating can be achieved by properly designing the access waveguide. With the proposed approach the circuit can be fabricated using standard i-line stepper litography with a single etch step. The optimized design for a 1.5m thick silicon layer achieves a simulated coupling efficiency of 65.6% at 1.55m. Preliminary experimental results yield a coupler loss of 4dB. © 2011 IEEE., 2011 ICO International Conference on Information Photonics, IP 2011, 18 May 2011 through 20 May 2011, Ottawa, ON

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. © 2011 OSA., 37th European Conference on Optical Communication and Exhibition, ECOC 2011, 18 September 2011 through 22 September 2011, Geneva

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. © 2013 Optical Society of America.

Grating couplers are widely used as an efficient and versatile fiber-chip coupling structure in nanometric silicon wire waveguides. The implementation of efficient grating couplers in micrometric silicon-on-insulator (SOI) rib waveguides is, however, challenging, since the coupler waveguide region is multimode. Here we experimentally demonstrate grating couplers in 1:5 ?m-thick SOI rib waveguides with a coupling efficiency of ?2:2 dB and a 3 dB bandwidth of 40nm. An inverse taper is used to adiabatically transform the interconnection waveguide mode to the optimum grating coupler excitation field with negligible higher order Bloch mode excitation. Couplers are fabricated in the same etch step as the waveguides using i-line stepper lithography. The benefits of wafer-scale testing and device characterization without facet preparation are thus attained at no additional cost. © 2011 Optical Society of America.

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., 2013 Conference on Lasers & Electro-Optics Europe & International Quantum Electronics Conference 2013, CLEO EUROPE/IQEC, May 12-16 2013, Munich, Germany