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We have proposed an ultracompact mode-division (de)multiplexer ((de)MDMUX) based on an asymmetrical directional coupler (ADC) with SiO 2 top cladding, where a taper-etched silicon wire and a hybrid plasmonic waveguide (HPW) are used. By carefully selecting the structural dimensions, TE 0 mode from the input port 1 (I 1) is gradually coupled into the wide multimode waveguide (MMW) and then transformed to TE 1 mode while that from the input port 2 (I 2) just passes through the bar branch and outputs directly, forming an efficient two-mode (de)multiplexer. With the aid of the HPW as well as the taper-etched wire, coupling length is reduced and fabrication tolerance is enlarged to some extent. From the results, such device has a total length of only 17. 1 μ m with a short coupling length of 4. 1 μ m , and it provides a low crosstalk (CT) of -25.6 (-27.7) dB and a low insertion loss (IL) of 0.51 (0.04) dB at the wavelength of 1. 55 μ m when TE 0 mode is input in the I 1 (I 2). [ABSTRACT FROM AUTHOR]

An ultracompact polarization beam splitter based on silicon nitride-on-silicon platform is proposed and analyzed, where the two polarized modes are guided in two separated layers independently, almost no affecting each other. By using subwavelength grating structure in bottom silicon layer, the injected TE mode undergoes a strong coupling from input port to adjacent waveguide and then outputs from a bending waveguide. As to the injected TM mode, it is directly transmitted along the above silicon nitride layer. Moreover, the adjacent waveguide in bottom layer is cut by a right angle at one corner to further enhance the device performance. Results show that a coupling length of only 2.7~\mu \textm is achieved with an insertion loss of 0.22 dB (0.2 dB), an extinction ratio of 27.05 dB (25.73 dB), and a reflection loss of −26.94 dB (−37.79 dB) for TE (TM) mode at 1.55~\mu \textm . In addition, fabrication tolerances and field evolution are also presented. [ABSTRACT FROM PUBLISHER]

An ultracompact and broadband polarization beam splitter (PBS) is proposed by using an asymmetrical directional coupler composed of a hybrid plasmonic waveguide (HPW) and a regular silicon wire (RSW). By properly choosing the structural parameters, transverse-magnetic (TM) mode is cutoff for the RSW and phase-matching condition is only satisfied for transverse-electric (TE) mode. As a result, the input TM-mode in HPW directly passes through, while the input TE-mode in the same HPW is coupled into the RSW. Consequently, the two polarized modes can be effectively separated. Moreover, the present PBS shows fabrication-friendly design and its length can be significantly reduced, since the gap between the HPW and RSW can be shortened to zero. From the results, a ~2.6- \mu \textm -long PBS with a coupling length of 0.1~\mu \textm is achieved and its bandwidth is ~120 nm with a polarization extinction ratio over 14 dB, and an insertion loss below 1 dB for both polarizations. In addition, fabrication tolerances to the key structural parameters are analyzed in detail and field evolution through the designed PBS is also presented. [ABSTRACT FROM AUTHOR]

An ultracompact and polarization-insensitive silicon-based strip-to-slot power splitter based on multimode interference is proposed, where an inverse-tapered subwavelength grating (SWG) structure is embedded in the center of the multimode waveguide and two strip-to-slot mode converters are integrated at the end. With the optimized SWG structure, the length of the multimode waveguide is reduced by nearly half against without such a structure and polarization-insensitive transmission can be achieved. Tapered and cut-right-angled waveguides are also introduced to enhance the performance. Simulation results show that a length of 1.92 \mu \textm for the multimode region with an insertion loss of 0.39 dB (polarization-dependent loss (PDL) = 0), the reflection losses of −30.51 dB (transverse-electric mode) and −33.62 dB (transverse-magnetic mode) are obtained at 1.55 \mu \textm , and the bandwidth can be enlarged to $\sim 105$ nm for a \textrm {PDL}<0.1 dB. In addition, fabrication tolerances and mode-field evolution are also presented. [ABSTRACT FROM PUBLISHER]

A compact silicon-based polarization splitter (PS) is proposed using a three-guide directional coupler composed of a silicon nanowire in the center and two horizontal/vertical hybrid-plasmonic waveguides (HPWs) at outer sides. Utilizing the evanescent coupling, the injected TE/TM modes in the central silicon nanowire can be only coupled to the horizontal/vertical HPWs, leading to the separation of input polarizations. Moreover, to improve the extinction ratio (ER), an S-bend connecting with the vertical HPW is employed. Results show that a coupling length of 6.5 (4.5) \mu \textm is achieved at the wavelength of 1.55~\mu \textm with ER and insertion loss of $\sim 18.9$ (15.2) dB and $\sim 0.44$ (0.89) dB for TE (TM) mode, and the bandwidths can cover the entire C-band for both polarizations. In addition, fabrication tolerances to the structural parameters are presented and field evolution through the designed PS is also demonstrated. [ABSTRACT FROM PUBLISHER]

A compact crossing scheme for a silicon-based slot-to-strip (or vice versa) waveguide is proposed and analyzed by using a finite-difference time-domain method, where a strip–multimode waveguide (SMW) crossing is used at the center and two logarithmically tapered slot-to-strip mode converters are incorporated into the ports with slot waveguides. For the input ports with slot waveguides and output ports with strip waveguides, the guided modes are efficiently transformed through the mode converter, and then enter into the SMW, where the fields converge at the center of the intersection due to the self-imaging effect. Hence, the size of the input beam is much smaller than the width of the SMW at the crossing center, leading to significant reductions of both crosstalk and radiation loss. The numerical results show that a slot-to-strip waveguide crossing operating at a wavelength of 1.55??m can be achieved with insertion loss, crosstalk, and reflection of 0.134/0.182, ?36.18/?38.6, and ?35.8/?42.02?dB for input ports with slot/strip waveguides, respectively. [ABSTRACT FROM PUBLISHER]

Abstract: A silicon-based horizontal multiple-slotted waveguide with slanted sidewalls is analyzed by using a full-vectorial mode solver based on the finite element method. The results show that the mode classification of the present waveguide structures is different from that of the conventional ones (e.g., typical rib waveguides, optical fibers), both symmetric and anti-symmetric fundamental (and higher-order) quasi-TE and quasi-TM modes exist due to the strong coupling of high-index silicon layers. The field distributions, effective indexes, and optical field confinement factors, both in quasi-TE and quasi-TM modes, as functions of the top width and sidewalls angle of the waveguides, and the refractive index and thickness of the slots are investigated in detail, where the single-mode regime, non-birefringence condition, and maximum optical confinement factor are obtained. From the numerical results, a single-mode waveguide structure for fundamental symmetric quasi-TM modes with the power confinement factor of 80% and the normalized power density of 120μm−2 is achieved. [Copyright &y& Elsevier]

An ultracompact mode-order converting power splitter using a multimode interference (MMI) coupler with subwavelength gratings (SWGs) is proposed for mid-infrared (MIR) wavelength. Two symmetric oblique SWG-wires are embedded in the central input-region that is connected with the input waveguide. As a result, the input TE 0 mode is evenly coupled from the input waveguide into the MMI coupler due to the adiabatical mode evolution, and then two desired TE 1 modes are formed via self-imaging effect and further guided into the two output channels. Additionally, tapered structures and straight SWG wires are introduced into the two output channels to better obtain the required phase difference for the improvement of converting efficiency. Consequently, both TE 0 -to-TE 1 mode-order converting and power splitting, for the first time, are attained simultaneously in the present single device. Moreover, TE 0 -to-TE 2 power splitting can be readily achieved by flexibly changing the dimensions of the proposed device. Results show that a compact device length of 7 μ m with an insertion loss (IL) of 0.41 dB, a conversion efficiency (CE) of 99% and a crosstalk (CT) of -19.77 dB is obtained at the wavelength of 2 μ m; its bandwidth can be enlarged to 220 nm for IL < 0.79 dB and CE> 91%. Besides, fabrication tolerances are addressed and mode propagation profiles are illustrated. • Power splitting and mode-order conversion simultaneously in an individual component. • The total length of the proposed device power is only 7 microns. • The operating bandwidth can be enlarged to 220 nm. • The fabrication tolerances are large enough for fabrication. • The proposed device has the advantages of flexibility and expansibility. [ABSTRACT FROM AUTHOR]

A compact and polarization-independent three-guide power splitter using subwavelength gratings (SWGs) on the SOI platform is proposed and analyzed in detail, where the SWG structures are located at both sides of the central input waveguide and the inner sides of the two outside waveguides. The introduced SWG structures can effectively enhance the coupling strength for TE mode, while that for TM mode is hardly affected. Consequently, by carefully choosing the structural parameters, the input TE mode can be evenly coupled from the central input waveguide to the outside waveguides, with the same coupling length for TM mode, thus a polarization-independent power splitter is realized. Results show that a compact power splitter of 8.5 μ m in coupling length is achieved with an insertion loss (IL) of 0.1 dB (0.085 dB) and reflection loss (RL) of −39 dB (−46 dB) for TE (TM) mode at the wavelength of 1. 55 μ m , and the operation bandwidth is up to 170 nm (from 1.48 to 1. 65 μ m) when IL <0.8 dB, RL <−20 dB and splitting ratio (SR) ∼ 1.001 for both polarizations. In addition, fabrication tolerances to the key structural parameters are presented and field evolution through the device is illustrated [ABSTRACT FROM AUTHOR]

• The rectangular grooves are shallowly etched on the MMI waveguide to introduce the refractive-index perturbations on the MMI waveguide eigenmodes, leading to an effective refractive-index manipulation of the eigenmodes. • The refractive-index perturbation scheme provides a simple and scalable method to manipulate the four beat lengths of the two wavelengths at TE- or TM-polarization in a single waveguide via self-imaging effect, leading to a compact footprint. • The present refractive-index perturbation scheme realizes, for the first time, NIR/MIR wavelength demultiplexing and polarization independence simultaneously in an individual device. • A compact and polarization-independent wavelength diplexer for 1.31/2 μm is also realized by flexibly changing the dimensions, showing the flexibility and extensibility of our refractive-index perturbation scheme. Faced with the rapid growth of internet traffic demand, which has posed a huge challenge to the current fiber-based communications systems, expanding the telecommunications bands from traditional near-infrared (NIR) region to mid-infrared (MIR) region is emerging as an attractive solution. And on-chip wavelength division multiplexing (WDM) application for NIR/MIR wavelengths is an essential element in future optical telecommunication systems. Here, we propose a compact silicon-based polarization-independent wavelength diplexer for the NIR/MIR wavelengths of 1.55/2 μm, where multiple rectangular grooves are shallowly etched on the silicon MMI waveguide for introducing the refractive-index perturbations on the MMI waveguide eigenmodes. And this refractive-index perturbation scheme provides a simple and scalable method to manipulate the four beat lengths of the two wavelengths at TE- or TM-polarization in a single waveguide via self-imaging effect, leading to a compact footprint. By optimizing the structural parameters, both NIR/MIR wavelength demultiplexing and polarization independence, for the first time, are obtained simultaneously in the present single device. From the results, the proposed diplexer is only 21.6 μm in length, and shows a wide bandwidth of ∼ 100/120 nm around the wavelength of 1.55/2 μm for insertion loss (IL) < 1 dB and extinction ratio (ER) > 15 dB. Moreover, we also realize a compact and polarization-independent wavelength diplexer for 1.31/2 μm by flexibly changing the dimensions, showing the flexibility and extensibility of our refractive-index perturbation scheme. Besides, fabrication tolerances are analyzed and mode propagation profiles are also demonstrated. [ABSTRACT FROM AUTHOR]