374 research outputs found
High performance and small footprint spot size converters based on SWG metamaterial lenses
Spot size converters with high expansion ratio are required in a variety of situations. This is the case of non-focusing Silicon on Insulator (SOI) fiber-to-chip grating couplers, which typically require long adiabatic tapers (Ltaper >100μm) from the narrow single-mode waveguides (WSi-wire ~ 500nm) to the wide grating region (Wgrating ~ 15μm). Here, we explore the potential of subwavelength grating (SWG) dielectric metamaterials to implement integrated GRaded INdex (GRIN) lenses to expand the mode field. Our designs achieve the desired Beam Expansion (BE) with insertion losses below 1dB over a distance of only LBE ~ 17μm.Ministerio de EconomÃa y Competitividad, Programa Estatal de Investigación, Desarrollo e Innovación Orientada a los Retos de la Sociedad (cofinanciado FEDER), (TEC2016-80718-R);
Universidad de Málaga.
Ministerio de Educación, Cultura y Deporte (MECD) (FPU16/06121);
Universidad de Málaga. Campus de Excelencia Internacional AndalucÃa Tech
Tilted subwavelength gratings: controlling anisotropy in metamaterial nanophotonic waveguides
Subwavelength grating (SWG) structures are an essential tool
in silicon photonics, enabling the synthesis of metamaterials
with a controllable refractive index. Here we propose, for the
first time to the best of our knowledge, tilting the grating elements
to gain control over the anisotropy of the metamaterial.
Rigorous finite difference time domain simulations
demonstrate that a 45° tilt results in an effective index variation
on the fundamental TE mode of 0.23 refractive index
units, whereas the change in the TM mode is 20 times smaller.
Our simulation predictions are corroborated by experimental
results. We furthermore propose an accurate theoretical
model for designing tilted SWG structures based on rotated
uniaxial crystals that is functional over a wide wavelength
range and for both the fundamental and higher order modes.
The proposed control over anisotropy opens promising venues
in polarization management devices and transformation
optics in silicon photonics.Universidad de Málaga (UMA); Ministerio de
EconomÃa y Competitividad (MINECO) (IJCI-2016-30484,
TEC2015-71127-C2-R, TEC2016-80718-R); Ministerio de
Educación, Cultura y Deporte (MECD) (FPU16/06762);
European Regional Development Fund (ERDF); Comunidad
de Madrid (SINFOTON-CM S2013/MIT-2790); European
Association of National Metrology Institutes (EURAMET)
(H2020-MSCA-RISE-2015:SENSIBLE, JRP-i22 14IND13
Photind)
Designing polarization management devices by tilting subwavelength grating structures
Subwavelength gratings (SWG) are periodic structures which behave as controllable homogeneous metamaterials.
SWGs are extremely interesting when they are used in platforms with a limited choice of material refractive
indices, enabling the design of a myriad of high-performance devices. Here we present a novel technique to
gain control over the intrinsic anisotropy of the synthesized metamaterial. We show that tilting the silicon
segments in a SWG structure mainly affects the in-plane (TE) modes, with little impact on the out-of-plane
(TM) modes. Moreover, we present a methodology to quickly but accurately calculate the modes of a tilted
periodic structure modeling the structure as a rotated uniaxial crystal which can be solved with an anisotropic
mode solver. Measurements on a set of fabricated tilted SWG waveguides validate our simulation results. By
using the presented technique, we design a polarization beam splitter based on a 2x2 multimode interferometer.
The design is based on the optimization of the tilting angle to tone the beat length of the TE modes to be a half
of the beat length of the TM modes.Universidad de Málaga. Campus de Excelencia Internacional AndalucÃa Tech;
Ministerio de EconomÃa y Competitividad (MINECO) (IJCI-2016-30484, TEC2015-71127-C2-R, TEC2016-80718-R); Ministerio de Educación, Cultura y Deporte (MECD) (FPU16/06762); European Regional Development Fund (ERDF); Comunidad
de Madrid (SINFOTON-CM S2013/MIT-2790); European Association of National Metrology Institutes (EURAMET) (H2020-MSCA-RISE-2015:SENSIBLE, JRP-i22 14IND13 Photind)
Design of arbitrary optical filters in silicon-on-insulator using evanescently-coupled Bragg gratings
Spectral filters are experiencing an increasing demand in several applications of the silicon-
on-insulator (SOI) platform. Many works have demonstrated that arbitrary frequency
responses can be synthesized by apodizing the coupling coefficient profile of an integrated
Bragg grating. However, the high index contrast of the SOI platform hinders their
practical implementation, due to the difficulty of achieving the precise control required in
the Bragg strength. In this paper, we propose the implementation of spectral filters using
an architecture based on placing loading segments within the evanescent field region of a
photonic wire waveguide. The Bragg coupling coefficient can be accurately controlled by
simply moving the segments away from, or closer to, the waveguide core. The layerpeeling
algorithm, in conjunction with a Floquet-Bloch modal analysis, allows to determine
the spatial distribution of the segments that synthesizes the desired spectrum. The
proposed topology is verified by designing a filter with five arbitrary passbands.Universidad de Málaga. Campus de Excelencia Internacional AndalucÃa Tech
Fabrication tolerant and broadband polarization splitter and rotator based on a taper-etched directional coupler.
We propose a fabrication tolerant polarization splitter and rotator (PSR) on the silicon-on-insulator platform based on the mode-coupling mechanism. The PSR consists of a silicon wire waveguide coupled to a taper-etched waveguide. Compared to previously reported PSRs based on directional couplers which are sensitive to fabrication variations, the partially etched taper structure can compensate for fabrication inaccuracies. In addition, the taper-etched geometry breaks both the horizontal and vertical symmetries of the waveguide, introducing an additional degree of design freedom to accommodate different upper cladding layers. The proposed PSR can be readily integrated in a planar waveguide circuit using e.g. SiO(2) cladding, making it compatible with typical metal back-end-of-line processes. Our simulation results show that the PSR has a low TM-to-TE polarization conversion loss of -0.09 dB in the C-band (or a conversion efficiency of 98%). A low TE-to-TE through insertion loss (-0.07 dB) and a very low polarization crosstalk (-30 dB) over a wide wavelength range exceeding 160 nm with a large fabrication tolerance (50 nm) are numerically demonstrated
Silicon waveguide modulator based on carrier depletion in periodically interleaved PN junctions
We present the design and numerical simulation results for a silicon waveguide modulator based on carrier depletion in a linear array of periodically interleaved PN junctions that are oriented perpendicular to the light propagation direction. In this geometry the overlap of the optical waveguide mode with the depletion region is much larger than in designs using a single PN junction aligned parallel to the waveguide propagation direction. Simulations predict that an optimized modulator will have a high modulation efficiency of 0.56 V.cm for a 3V bias, with a 3 dB frequency bandwidth of over 40 GHz. This device has a length of 1.86 mm with a maximum intrinsic loss of 4.3 dB at 0V bias, due to free carrier absorption. (C) 2009 Optical Society of Americ
Optical wavefront phase-tilt measurement using Si-photonic waveguide grating couplers
Silicon photonic wavefront phase-tilt sensors for wavefront monitoring using
surface coupling grating arrays are demonstrated. The first design employs the
intrinsic angle dependence of the grating coupling efficiency to determine
local wavefront tilt, with a measured sensitivity of 7 dB/degree. A second
design connects four gratings in an interferometric waveguide circuit to
determine incident wavefront phase variation across the sensor area. In this
device, one fringe spacing corresponds to approximately 2 degree wavefront tilt
change. These sensor elements can sample a wavefront incident on the chip
surface without the use of bulk optic elements, fiber arrays, or imaging
arrays. Both sensor elements are less than 60 um across, and can be combined
into larger arrays to monitor wavefront tilt and distortion across an image or
pupil plane in adaptive optics systems for free space optical communications,
astronomy and beam pointing applications
Sub-wavelength grating mode transformers in silicon slab waveguides.
We report on several new types of sub-wavelength grating (SWG) gradient index structures for efficient mode coupling in high index contrast slab waveguides. Using a SWG, an adiabatic transition is achieved at the interface between silicon-on-insulator waveguides of different geometries. The SWG transition region minimizes both fundamental mode mismatch loss and coupling to higher order modes. By creating the gradient effective index region in the direction of propagation, we demonstrate that efficient vertical mode transformation can be achieved between slab waveguides of different core thickness. The structures which we propose can be fabricated by a single etch step. Using 3D finite-difference time-domain simulations we study the loss, polarization dependence and the higher order mode excitation for two types (triangular and triangular-transverse) of SWG transition regions between silicon-on-insulator slab waveguides of different core thicknesses. We demonstrate two solutions to reduce the polarization dependent loss of these structures. Finally, we propose an implementation of SWG structures to reduce loss and higher order mode excitation between a slab waveguide and a phase array of an array waveguide grating (AWG). Compared to a conventional AWG, the loss is reduced from -1.4 dB to < -0.2 dB at the slab-array interface
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