Interferometers are one of the basic devices in many photonics applications. Interferometers can be used in the design of optical filters, wavelength de-multiplexing (WDM), electro-optical modulators and optical sensors. They can also form the building block of optical digital signal processor (DSP). In this work, we propose novel integrated Michelson interferometer based on the Silicon on Insulator (SOI) technology with 220nm silicon device layer and working in the near infrared region. The Interferometer consists of input splitter directional coupler, two waveguide arms and directional coupler combiner with loop reflector. The interferometer transfer function and its parameters including the free spectral range (FSR), the full width half maximum (FWHM) and sensitivity were derived analytically. Using our proposed interferometer instead of the conventional Mach Zehnder Interferometer (MZI) as optical filter, electro-optical modulator or sensor will reduce the size of the device needed by a factor of two while achieving the same performance. Here, we use our Michelson Interferometer with four different path length differences resulting in FSR from 0.8nm to 6.4nm. A strip waveguide with 500nm width platform is used. These devices are suitable for optical filtering as well as wavelength de-multiplexing WDM applications. The simulation results of the proposed designs are extracted using Lumerical MODE and INTERCONNECT software tools that use scattering matrices of optical components to determine the transfer function of photonic integrated circuits (PICs). The designs were verified with three-dimensional finite-difference-time-domain (3D-FDTD) solver and show good agreement. Finally, the designs were fabricated using Electron Beam Lithography (EBL) and characterized showing also good matching with the numerical simulations results.
Using transparent conducting oxides, such as indium-tin-oxide (ITO), for optical modulation has attracted research interest because of their epsilon-near-zero (ENZ) characteristics at the telecom wavelengths. Utilizing ITO in multilayer structure modulators, optical absorption of the active ITO layer can be electrically modulated over a large spectrum range. Although they show advances over common silicon electro-optical modulators (EOMs), they suffer from high insertion losses (ILs). To reduce ILs and device footprints without sacrificing bandwidth and modulation strength, slot waveguides are promising options because of their high optical confinement. We present the study and design of an electro-optical absorption modulator based on an electrically tuning ITO carrier density inside an MOS structure. The device structure is based on the dielectric slot waveguide with an ITO plasmonic waveguide modulation section. By changing the dimensions, the effective refractive indices for the slot mode and the off-state mode of the plasmonic section can be matched. When applying electric field to the plasmonic section (on-state), carriers are generated at the ITO-dielectric interface that results in changing the layer where the electric field is confined from a transparent layer into a lossy layer. A finite difference time-domain method with perfect matching layer absorbing boundary conditions is taken up to simulate and analyze this design. An extinction ratio of 15.5 dB is achieved for a 10-μm-long modulation section, at the telecommunications wavelength (1.55 μm). This EOM has advantages of simple design, easy fabrication, compact size, compatibility with existing silicon photonics platforms, as well as broadband performance.
An optical modulator is considered one of the most fundamental components in an optical data communication system as it acts as a linking device between the optical and electrical parts of the system. Electro-absorption (i.e. electro-optical) modulation is one popular scheme in designing optical modulators; however, minimizing the device footprint in siliconbased platforms acted as a challenge. Few years ago, “plasmonics” field emerged as a good candidate that could possibly further reduce silicon-based modulators’ footprint. Unfortunately, existence of metals introduced huge propagation losses. Recently, transparent conducting oxides (e.g. indium tin oxide “ITO”) have been intensively used as active media in electro-optical (EO) modulators. They have a metal-like plasmonic behavior with extremely lower losses.
Under no biasing voltage, ITO acts almost as a dielectric. However, by carefully tuning the biasing voltage, the free carrier concentration beneath the ITO surface is changed. This allows a dramatic alteration in the complex permittivity of the ITO reaching an epsilon-near-zero (ENZ) value at some point. At this region, the ITO acts as a metal and a plasmonic mode is present at an ITO-dielectric interface. A heavily doped silicon slab can be used as a contact for the gating voltage to be applied on in order to accumulate free carriers on the ITO surface.
In this work, an all-silicon indium tin oxide-integrated electro-optical modulator is designed. The modulator exhibits superior parameters (e.g. insertion loss and extinction ratio) that outperform the current modulators based on the same technology.
Using transparent conducting oxides (TCOs), like indium-tin-oxide (ITO), for optical modulation attracted research interest because of their epsilon-near-zero (ENZ) characteristics at telecom wavelengths. Utilizing indium-tin-oxide (ITO) in multilayer structure modulators, optical absorption of the active ITO layer can be electrically modulated over a large spectrum range. Although they show advances over common silicon electro-optical modulators (EOMs), they suffer from high insertion losses. To reduce insertion losses and device footprints without sacrificing bandwidth and modulation strength, slot waveguides are promising options because of their high optical confinement. In this paper, we present the study and the design of an electro-optical absorption modulator based on electrically tuning ITO carrier density inside a MOS structure. The device structure is based on dielectric slot waveguide with an ITO plasmonic waveguide modulation section. By changing the dimensions, the effective refractive indices for the slot mode and the off-sate mode of the plasmonic section can be matched. When applying electric field to the plasmonic section (on-state), carriers are generated at the ITO-dielectric interface that result in changing the layer where the electric field is confined from a transparent layer into a lossy layer. A finite difference time domain method with perfect matching layer (PML) absorbing boundary conditions is taken up to simulate and analyze this design. An extinction ratio of 2.3 dB is achieved for a 1-μm-short modulation section, at the telecommunications wavelength (1.55 μm). This EOM has advantages of simple design, easy fabrication, compact size, compatibility with existing silicon photonics platforms, as well as broadband performance.
Ion-exchange process is one of the most common techniques used in glass waveguide fabrication. This has many advantages, such as low cost, ease of implementation, and simple equipment requirements. The technology is based on the substitution of some of the host ions in the glass (typically Na+) with other ions that possess different characteristics in terms of size and polarizability. The newly diffused ions produce a region with a relatively higher refractive index in which the light could be guided. A critical issue arises when it comes to designing such waveguides, which is carefully and precisely determining the resultant index profile. This task has been proven to be hideous as the process is generally governed by a nonlinear diffusion model with no direct general analytical solution. Furthermore, numerical solutions become unreliable—in terms of stability and mean squared error—in some cases, especially the K+−Na+ ion-exchanged waveguide, which is the best candidate to produce waveguides with refractive index differences compatible with those of the commercially available optical fibers. Linearized finite-element method formulations were used to provide a reliable tool that could solve the nonlinear diffusion model of the ion-exchange in both one- and two-dimensional spaces. Additionally, the annealed channel waveguide case has been studied. In all cases, unprecedented stability and minimum mean squared error could be achieved.
Due to its low cost and simplicity, ion exchange is considered one of the most commonly used processes to produce glass waveguides nowadays. This fabrication technology is based on the substitution of some ions already present in the glass with other ions having different sizes and polarizabilities. A careful study of the resultant refractive index profile is crucial in the impact on the waveguide characteristics. In this paper, we introduce, for the first time, a novel solution of the nonlinear diffusion equation that model this process using finite element method (FEM) approach. The ion exchange can be modelled as a nonlinear diffusion equation, as the exchanged ions Ka+ diffuse into their new sites where the original ions were existing. Numerical instabilities are encountered when solving for the exchanged ions with similar diffusion coefficients as in the case of Ka+/Na+, which is used in fabricating integrated optical devices with refractive index differences compatible with those of the optical fibers. Different novel FEM techniques are proposed in solving the problem in 1D space. The stability and accuracy of the different methods outperforms the current numerical methods and provide a good tool for highly nonlinear diffusion problem.
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