Design and fabrication of surface trimmed silicon-on-insulator waveguide with adiabatic spot-size converters
TL;DR: The proposed surface trimming technique can be potentially used to tune the waveguide cross-section/geometry for phase error correction and/or to avail stronger light-matter interactions at a desired location of an integrated optical circuit.
Abstract: Theoretical and experimental studies reveal that a predefined single-mode rib waveguide fabricated in silicon-on-insulator (SOI) substrate with a device layer thickness of 2 μm can be adiabatically trimmed down to submicron waveguide dimensions (<1 μm), resulting in regional modification of waveguide properties. The fabrication process involves physical trimming/removal of a waveguide surface by plasma etchants that is spatially filtered by a shadow mask with a rectangular aperture inside a reactive ion etching system. The exact position of a shadow mask above a sample surface has been optimized (∼500 μm) to obtain the desired adiabatic spot-size converters of length up to 1 mm at both ends of the trimmed waveguides. For experimental demonstration, three different sets of 15-mm-long single-mode waveguides fabricated in 2-μm SOI were adiabatically trimmed in the middle for three different lengths of 3, 5, and 7 mm, respectively. Excess propagation loss and group index of a trimmed submicron waveguide section were extracted by analyzing the wavelength-dependent Fabry–Perot transmission characteristics of the device with polished input/output end facets. The insertion loss of a typical spot-size converter designed for the guidance of TE-like polarization has been recorded to be ∼0.25 dB for a wide range of wavelengths (1500 nm≤λ≤1600 nm). As predicted by numerical simulation, no polarization rotation has been observed in all the trimmed submicron waveguides. The proposed surface trimming technique can be potentially used to tune the waveguide cross-section/geometry for phase error correction and/or to avail stronger light-matter interactions at a desired location of an integrated optical circuit.
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Dissertation•
01 Jan 2018
TL;DR: In this article, an ultra-subwavelength grating coupler has been developed with an engineered grating structure which exhibits high coupling efficiency and bandwidth without the need for bottom mirrors.
Abstract: In recent years silicon photonics has become a considerable mainstream technology, especially in telecommunications fields to overcome the limitations imposed by copper-based technology. Nanoscale photonic technologies have attracted a lot of attention to co-develop photonic and electronic devices on silicon (Si) to provide a highly integrated electronic–photonic platform. Silicon-on-insulator (SOI) technology that relies heavily on the contrasted indices of Si and SiO2, enables the design and integration of these photonic devices in submicronic scales, similar to the devices produced by a standard CMOS fabrication platform in the electronics industry. One of the key challenges with these submicronic waveguide devices is to enable efficient coupling with fibre, which is mainly due to the mode-field differences between fibre and the waveguide, and their relative misalignments. To overcome this challenge, various techniques including prism, butt and grating coupling have been proposed. Among them, although butt coupling is an elegant solution for low loss and wideband operation, it often requires post-processing for accurate polishing and dicing to taper the waveguide edges. Therefore, it is not suitable for wafer-scale testing. Grating couplers, which mostly perform out of the plane coupling between a fibre and a waveguide, are also an attractive solution as light can be coupled in and out everywhere on the chip, opening the way for wafer-scale testing. However, despite such advantages, grating couplers often exhibit low coupling efficiency (CE) due to downward radiation of light that propagates towards substrate through buried oxide (BOX) which comprises 35%-45% of total incident light. Grating couplers are also very sensitive to the wavelength of the light as different wavelengths exhibit specific diffraction properties at the grating, which cause a narrow coupling bandwidth. In this thesis we have studied various techniques to improve the coupling efficiency and coupling bandwidth of the grating couplers. We have used the finite difference time domain (FDTD) and Eigenmode Expansion (EME) methods to study the interaction of light with grating. The directionality of the coupler which determines the coupling efficiency has been improved by means of silicon mirrors in the BOX layer that essentially redirect the light propagates toward substrate. For improvement of directionality, an ultra-subwavelength grating coupler has also been developed with an engineered grating structure which exhibits high coupling efficiency and bandwidth without the need for bottom mirrors. The grating coupler only converts vertical dimension into nano scale, leaving the lateral width in micrometre range typically >15 μm. In order to connect the grating coupler with a nanophotonic waveguide, the grating structure needs to be matched in dimensions both vertically and laterally. Conventionally, to meet the requirement the width of grating structure is gradually tapered to nano scale. The coupling efficiency relies highly on the taper length, which is typically hundreds of micrometres. Such a long taper waveguide causes an unnecessarily large footprint of the photonic integrated circuits. In order to minimise the length of the taper while retaining high coupling efficiency, we have designed two different types of tapered waveguides. One of them is a partially overlaid tapered waveguide and the other is a hollow tapered waveguide.
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...The design and fabrication procedures are described in [111] for surface-trimmed silicon-on-insulator waveguide with adiabatic SSCs where the insertion loss of ∼0....
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30 May 2017
TL;DR: In this paper, diffusion doped p-i-n/p-n diodes in SOI substrate are proposed for the fabrication of active silicon photonics devices with scalable waveguide cross-sections.
Abstract: Diffusion doped p-i-n/p-n diodes in SOI substrate is proposed for the fabrication of active silicon photonics devices with scalable waveguide cross-sections. The p-type and n-type diffusion doping parameters are optimized for the fabrication of tunable single-mode waveguide phase-shifters with microns to submicron cross-sectional dimensions. The simulations results show that the shape of depletion layer can be effectively engineered by suitably positioning the rib waveguide with respect to the gap between doping windows. We could thus introduce an additional control parameter to optimize over-all figure of merits of the phase-shifter for various applications. For an optimized set of diffusion parameters, the VπLπ of single-mode waveguides designed with 1μm, 0.5μm, and 0.25μm device layers (under reverse bias operating in TE-polarization at λ ~ 1550 nm) are found as 2.7 V-cm, 2.1 V-cm, and 1.6 V-cm, respectively. The typical p-n junction capacitance of an optimized 0.25μm single-mode waveguide is estimated to be < 0.5 fF/μm, which is comparable to that of ion-implanted p-n waveguide junctions.
2 citations
TL;DR: In this article , the authors focus on the Optomechanical bichromatic wavelength switching system as an indirect two-color up-conversion process that relies on optical force and nanorod scattering effects.
Abstract: This study focuses on the Optomechanical bichromatic wavelength switching system as an indirect two-color up-conversion process that relies on optical force and nanorod scattering effects. This system is used to control light coupling between four parallel optical waveguides made of silicon nitride (Si3N4) which form two identical parts. The parallel waveguides with 0.5 µm × 0.5 µm cross-section and 220 µm lengths are suspended on a silica (SiO2) substrate embedded with the array of square silicon (Si) nanorods. By mid-IR plane wave illumination, as control light, with different intensities and different wavelengths on nanorods, scattering would increase and result in an improvement in attractive gradient optical force exerted on waveguides. Via bending waveguides toward each other, caused by optical gradient force, two different visible lights, as probe signals, propagating in the first waveguide of each section would couple to the adjacent waveguide. Simulation results reveal that when the distance between the parallel waveguides in the equilibrium position is 100 nm and the intensity of mid-IR light is 1.28 mW/µm2 total coupling would occur in two situations: 1- when the control light is 4.5 µm, the probe light with 713 nm wavelength is transmitted to the output, 2- when the control light is 3 µm, the probe light with 609 nm wavelength is transmitted to the output. In the first case 1.92 pN/µm optical force is needed to bend each waveguide by 9 nm and in the second one, 1.28 pN/µm optical force is needed to bend each waveguide by 6 nm for total coupling. The efficiency of the coupled waveguides system is %88.6 for 609 nm probe light injection and %96.5 for 713 nm probe light injection.
1 citations
TL;DR: In this paper , the authors focus on the Optomechanical bichromatic wavelength switching system as an indirect two-color up-conversion process that relies on optical force and nanorod scattering effects.
Abstract: This study focuses on the Optomechanical bichromatic wavelength switching system as an indirect two-color up-conversion process that relies on optical force and nanorod scattering effects. This system is used to control light coupling between four parallel optical waveguides made of silicon nitride (Si3N4) which form two identical parts. The parallel waveguides with 0.5 µm × 0.5 µm cross-section and 220 µm lengths are suspended on a silica (SiO2) substrate embedded with the array of square silicon (Si) nanorods. By mid-IR plane wave illumination, as control light, with different intensities and different wavelengths on nanorods, scattering would increase and result in an improvement in attractive gradient optical force exerted on waveguides. Via bending waveguides toward each other, caused by optical gradient force, two different visible lights, as probe signals, propagating in the first waveguide of each section would couple to the adjacent waveguide. Simulation results reveal that when the distance between the parallel waveguides in the equilibrium position is 100 nm and the intensity of mid-IR light is 1.28 mW/µm2 total coupling would occur in two situations: 1- when the control light is 4.5 µm, the probe light with 713 nm wavelength is transmitted to the output, 2- when the control light is 3 µm, the probe light with 609 nm wavelength is transmitted to the output. In the first case 1.92 pN/µm optical force is needed to bend each waveguide by 9 nm and in the second one, 1.28 pN/µm optical force is needed to bend each waveguide by 6 nm for total coupling. The efficiency of the coupled waveguides system is %88.6 for 609 nm probe light injection and %96.5 for 713 nm probe light injection.
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TL;DR: In this paper, a reactive ion etching chemistry has been optimized for trimming of rib waveguides and an empirical model has been developed to obtain the resulting waveguide geometries.
Abstract: Surface trimming of rib waveguides fabricated in 5-μm SOI substrate has been carried out successfully without any significant increase of propagation losses. A reactive ion etching chemistry has been optimized for trimming and an empirical model has been developed to obtain the resulting waveguide geometries. This technique has been used to demonstrate smaller footprint devices like multimode interference based power splitters and ring resonators after defining them photolithographically with relatively large cross-section rib waveguides. We have been also successful to fabricate 2D tapered spot-size converter useful for monolithic integration of waveguides with varying heights and widths. The taper length is again precisely controlled by photolithographic definition. Minimum insertion loss of such a spot-size converter integrated between waveguides with 3-μm height difference has been recorded to be ∼2 dB. It has been also shown that the overall fiber-to-chip coupling loss can be reduced by >3 dB by using such spot-size converters at the input/output side of the waveguides.
3 citations
TL;DR: In this article, the authors reported the realization of an eleventh order distributed bragg reector (DBR) on the surface of single-mode rib waveguides, and the characterization results of the fabricated DBRs showed a reectivity R = 8832% and FWHM = 243 nm.
Abstract: The distributed Bragg reector (DBR) plays a major role in integrated optics Because of the recent advances ofsilicon photonics and CMOS electronics in SOI platform, various types of DBR structures are being investigatedfor integrated optical couplers, lters, (de-)multiplexers, interleavers, Fabry-Perot micro-cavities, laser sources,etc The rst order di raction gratings in SOI waveguide requires surface relief gratings of period = 225 nmfor a DBR response at = 1550 nm Fabrication of such sub-micron gratings with a uniform period over alength of several mm is really a challenging issue Here we report the realization of an eleventh order Bragggrating ( = 26 m, B 1564 nm, L = 262 mm) on the surface of single-mode rib waveguides The DBRsand waveguide structures were de ned by conventional photolithography and subsequent reactive ion etchingprocesses The waveguide end-facets were polished suitably before they were taken for characterizations usinga tunable laser source (tunability 10 pm) in our free-space waveguide coupling set-up The characterizationresults of the fabricated DBRs showed a reectivity R = 8832% and FWHM = 243 nm Higher reectivityand narrower grating response can be achieved by further increasing the grating length The waveguide loss hasbeen increased ( 05 dB/mm) because of the enhanced surface roughness during the RIE process for gratingfabrication It can be reduced if conventional RIE is replaced by ICP RIEKeywords: silicon photonics, DBR, silicon waveguides, integrated optics, lters, higher order gratings, siliconon insulator, di raction
2 citations
TL;DR: A semi-analytical model using Gaussian beam approximation of guided mode profiles has been developed to analyze the output spectrum of arrayed waveguide grating and to estimate the phase errors due to waveguide inhomogeneities.
Abstract: The arrayed waveguide grating structure can be used as an important component in high-speed CMOS optical interconnects in silicon-on-insulator (SOI) platform. However, the performance of such device is found to be extremely sensitive to the fabrication-related errors in defining the critical features. In the absence of an appropriate analytical model, one needs to rely on numerical computation to analyze the device characteristics and fabrication tolerances. Because compact design of such a device structure has foot-print ∼mm2 and the smallest features can be as small as ∼500 nm×220 nm (waveguide cross section), it demands a huge computational budget to optimize the design parameters. A semi-analytical model using Gaussian beam approximation of guided mode profiles has been developed to analyze the output spectrum of arrayed waveguide grating and to estimate the phase errors due to waveguide inhomogeneities. This model has been validated with existing numerical methods and published experimental results. It has been observed that a probabilistic waveguide width variations of ΔW∼5 nm can cause a cross-talk degradation of about 40 dB (25 dB) for a device (operating at λ∼1550 nm) fabricated on SOI substrate with 220 nm (2 μm) device layer thickness.
1 citations