High-performance and ultra-wide bandwidth silicon TM polarizer with suppressed reflection utilizing a tailored contra-mode conversion Bragg grating.

We present a compact TM polarizer with high polarization extinction ratio (PER), low loss, and suppressed reflection over an ultra-wide bandwidth on a 220 nm silicon-on-insulator (SOI) platform. The device utilizes a contra-mode conversion Bragg grating (CMC-BG) with strong polarization dependence embedded in a multimode waveguide. Through a sophisticated grating design incorporating tailored chirping and apodization profiles to match modal properties, we have achieved, by simulation, a compact device footprint of 34.

Ultra-compact and polarization-insensitive silicon waveguide 3 × 3 star-crossing based on composite subwavelength grating metamaterials.

We present what we believe is the first report on a polarization-insensitive 3 × 3 silicon star-crossing utilizing a composite subwavelength metamaterial waveguide structure. Two different types of subwavelength grating metamaterials (nanohole grating and fan-shaped bent subwavelength grating) are respectively used to address diffraction issues in the crossing region and mode interference issues caused by a compact non-adiabatic design. This approach results in a device with an ultra-compact footprint of 12.

Local-field engineering in slot waveguide for fabricating on-chip Bragg grating filters with high reflectivity across a flat broadband.

On-chip Bragg gratings with high reflectivities have been found to have widespread applications in filters, resonators, and semiconductor lasers. However, achieving strong Bragg reflections with flat response across a broad bandwidth on the popular 220 nm silicon-on-insulator (SOI) platform still remains a challenge. In this paper, such a high performance device is proposed and fabricated, which is based on a slot waveguide with gratings etched on the inner sidewalls of the slot.

Compact and efficient polarization rotator using laterally asymmetric rib waveguides on a lithium-niobate-on-insulator platform.

Photonic devices based on a lithium-niobate-on-insulator (LNOI) are current research hotspots; however, owing to the high refractive index contrast of the LNOI platform and inherent birefringence of lithium niobate itself, such photonic devices are generally polarization sensitive, affecting their further wide application. This paper proposes a simple, compact, and efficient polarization rotator (PR) based on a laterally asymmetric rib waveguide by depositing a layer of semi-infinite silicon nitride dielectric material on one side of the rib waveguide. The results show that a PR with a polarization rotation region length of 15.

All-silicon TM polarizer covering the 1260-1675 nm bandwidth using a band engineered subwavelength grating waveguide.

A TM polarizer working for whole optical communication bands with high performance is proposed on a 220-nm-thick silicon-on-insulator (SOI) platform. The device is based on polarization-dependent band engineering in a subwavelength grating waveguide (SWGW). By utilizing an SWGW with a relatively larger lateral width, an ultra-broad bandgap of ∼476 nm (1238 nm-1714nm) is obtained for the TE mode, while the TM mode is well supported in this range.

Experimental demonstration of a flexible and high-performance mode-order converter using subwavelength grating metamaterials.

Mode-order converters, transforming a given mode into the desired mode, have an important implication for the multimode division multiplexing technology. Considerable mode-order conversion schemes have been reported on the silicon-on-insulator platform. However, most of them can only convert the fundamental mode to one or two specific higher-order modes with low scalability and flexibility, and the mode conversion between higher-order modes cannot be achieved unless a total redesign or a cascade is carried out.

Highly scalable and flexible on-chip all-silicon mode filter using backward mode conversion gratings.

Mode filters are fundamental elements in a mode-division multiplexing (MDM) system for reducing modal cross-talk or realizing modal routing. However, the previously reported silicon mode filters can only filter one specific mode at a time and multiple modes filtering usually needs a cascade of several filters, which is adverse to highly integrated MDM systems. Here, we propose a unique concept to realize compact, scalable and flexible mode filters based on backward mode conversion gratings elaborately embedded in a multimode waveguide.

Ultra-compact and broadband all-silicon TM-pass power splitter using subwavelength holey-structured metamaterial waveguides.

Power splitters with polarization management features are highly desired to construct high-density silicon photonic integrated circuits. However, few attempts have been made to design a single device that can act as both a power splitter and a TE- or TM-pass polarizer. In this paper, for the first time, we experimentally demonstrate an ultra-compact and broadband all-silicon TM-pass power splitter, where a triple-guide directional coupler (TGDC) composed of three parallel subwavelength holey-structured metamaterial waveguides (SHMWs) is located at central coupling region and three regular strip waveguides are connected at the input/output ports.

Compact and ultra-broadband all-silicon TM-pass and TE-reflected polarizer using grating based weakly coupled nanowires.

On-chip silicon polarizers with broad operating bandwidth and compact footprint have recently attracted increasing attention for their applications in large capacity and high density integrated optical systems. However, strong waveguide dispersion usually limits the bandwidth of the silicon polarizers, especially for the TM-pass polarizers. In this paper, we overcome the bandwidth limit of the TM polarizer by utilizing a novel waveguide structure composed of two weakly coupled nanowires with gratings sandwiched in between.

Compact and broadband silicon-based polarization beam splitter using asymmetric directional couplers embedded with subwavelength gratings and slots.

A compact and broadband silicon-based polarization beam splitter (PBS) is proposed and investigated in detail, where the two arms of the directional coupler (DC) are, respectively, embedded with subwavelength gratings (SWGs) and vertical slots so that field distributions for the TE mode are significantly changed, effectively weakening coupling strength, whereas those for the TM mode are almost unaffected, nearly analogous to the DC with strip waveguides. By carefully optimizing structural parameters, efficient coupling will emerge between the two waveguides for the TM mode, while TE mode will be confined in the SWG-assisted strip waveguide. Consequently, the two modes can be effectively separated, and thus the realization of a PBS is accomplished.

Full-vectorial meshless finite cloud method for an anisotropic optical waveguide analysis.

By using the meshless finite cloud method, an efficient full-vectorial mode solver based on the transverse-magnetic-field components is developed to analyze the optical waveguides made of anisotropic materials, in which the waveguide cross-section enclosed by the perfectly matched layers is divided into an appropriate number of homogeneous clouds. The point collocation technique is utilized to create a scattered set of nodes over the cloud, and then the continuity conditions of the longitudinal field components are imposed to appropriately deal with the discrete nodes at the interfaces shared by the adjacent clouds. In comparison with conventional mesh-based numerical techniques, the distributions of solution nodes of the present method can be applied to the area of complexity in a completely free manner.

Compact silicon-based on-chip wavelength triplexer using directional couplers with double bridged subwavelength gratings.

A compact silicon on-chip wavelength triplexer engineered by double bridged subwavelength gratings (BSWGs) is proposed based on cascaded symmetric directional couplers (SDCs). The SDC consists of a pair of symmetric and identical BSWGs with another SWG waveguide implemented in the middle of the SDC section. It functions well provided that the coupling lengths can match even or odd times of beat lengths of the three wavelengths.

Ultracompact polarization-insensitive power splitter using subwavelength-grating-based MMI couplers on an SOI platform.

An ultracompact and polarization-insensitive power splitter using a subwavelength-grating-based multimode interference (MMI) coupler on an SOI platform is proposed and analyzed in detail. By properly tailoring the structural parameters of the subwavelength gratings embedded in the center of the MMI coupler, the effective reflective indices for TE and TM modes supported by this MMI coupler can be engineered, leading to equal coupling lengths for the two polarizations and an efficient reduction in length for the used MMI coupler. As a result, an ultracompact polarization-insensitive power splitter can be realized.

Compact silicon-based polarization-independent directional coupler using subwavelength gratings.

A compact and high extinction ratio polarization-independent directional coupler (PIDC) using silicon-based subwavelength gratings (SWGs) is proposed and analyzed in detail, where the SWG structures are located between the two input/output strip waveguides together with input/output SWG-based transitions. The introduced SWG-based structures can effectively enhance the coupling strength for the TE mode, while that for the TM mode is hardly affected. According to the coupled-mode theory, by carefully choosing the structural parameters, the coupling length for both polarizations can be identical; thus a PIDC is realized.

Ultracompact and broadband silicon-based TE-pass 1 × 2 power splitter using subwavelength grating couplers and hybrid plasmonic gratings.

An ultracompact TE-pass 1 × 2 power splitter, using subwavelength grating (SWG) couplers and hybrid plasmonic gratings (HPGs), is proposed and analyzed in detail. As the input strip waveguide is tapered to a narrow wire, where TE mode is cutoff, the launched TE-mode can be evenly divided with high efficiency with the help of the SWG couplers, which are placed between the central silicon wire and two nearby output branches. The injected TM-mode will be perfectly reflected by the carefully designed HPG, whose periodically varied metal layer is located above the bottom strip and SWG waveguides.

Broadband TE-pass slot waveguide polarizer using an asymmetrical directional coupler.

A compact TE-pass polarizer for silicon-based slot waveguides is numerically proposed based on a directional coupler where an asymmetrical slot waveguide (ASW) and hybrid plasmonic waveguide (HPW) are involved. The input section is linearly tapered to an ASW to markedly enlarge the modal birefringence. Beneficially, only the TM mode can be coupled into the adjacent HPW and attenuated while the injected TE mode just passes through the slot waveguide without coupling by properly choosing the dimensions.

Compact polarization rotator for silicon-based cross-slot waveguides using subwavelength gratings.

A compact and broadband polarization rotator (PR) for silicon-based cross-slot waveguides using subwavelength gratings (SWGs) is proposed and analyzed. To significantly break the symmetry of the waveguide structure, the diagonal regular Si wires of the cross-slot waveguides are replaced with the full etching SWGs. Moreover, the special properties of the SWGs-whose effective index is adjustable-can effectively enhance the modal birefringence between the two lowest-order hybrid modes, resulting in a more compact device.

Design and numerical study of a compact, broadband and low-loss TE-pass polarizer using transparent conducting oxides.

A compact, broadband, and low-loss TE-pass polarizer using transparent conducting oxides (TCOs) embedded in the center of the strip waveguide and deposited on its top is proposed and analyzed in detail. With the tunable permittivity of TCO, epsilon-near-zero (ENZ) of its real part and significant increase of its imaginary part can be achieved around the wavelength of 1.55 μm under a certain electron concentration.

Ultracompact and high efficient silicon-based polarization splitter-rotator using a partially-etched subwavelength grating coupler.

On-chip polarization manipulation is pivotal for silicon-on-insulator material platform to realize polarization-transparent circuits and polarization-division-multiplexing transmissions, where polarization splitters and rotators are fundamental components. In this work, we propose an ultracompact and high efficient silicon-based polarization splitter-rotator (PSR) using a partially-etched subwavelength grating (SWG) coupler. The proposed PSR consists of a taper-integrated SWG coupler combined with a partially-etched waveguide between the input and output strip waveguides to make the input transverse-electric (TE) mode couple and convert to the output transverse-magnetic (TM) mode at the cross port while the input TM mode confine well in the strip waveguide during propagation and directly output from the bar port with nearly neglected coupling.

Compact and high extinction ratio polarization beam splitter using subwavelength grating couplers.

A compact and high extinction ratio polarization beam splitter using subwavelength grating (SWG) couplers is proposed and characterized, where the SWG couplers are located between the two input/output strip waveguides, including SWG-based transitions combined at both ends. The TM mode can be confined well in the strip waveguide and transmits along it with nearly neglected coupling, while the TE mode undergoes a strong coupling and is transferred to the adjacent waveguide with the help of SWG couplers due to dissimilar modal characteristics and cutoff conditions between these two polarizations. To further enhance the performance, an additional tapered waveguide is added in the lateral end of the input SWG-based transition.

Design of a compact and integrated TM-rotated/TE-through polarization beam splitter for silicon-based slot waveguides.

A compact and integrated TM-rotated/TE-through polarization beam splitter for silicon-based slot waveguides is proposed and characterized. For the input TM mode, it is first transferred into the cross strip waveguide using a tapered directional coupler (DC), and then efficiently rotated to the corresponding TE mode using an L-shaped bending polarization rotator (PR). Finally, the TE mode for slot waveguide at the output end is obtained with the help of a strip-to-slot mode converter.

Design of a compact polarization demultiplexer for silicon-based slot waveguides.

A compact polarization demultiplexer (P-DEMUX) composed of a microring resonator in strip waveguides and two bus channels in slot waveguides is proposed and characterized. The modal indices of the TM modes for slot and strip waveguides have comparable values, while those for the TE modes show large differences. With these unique properties, the input TE mode can directly output from the through port with nearly neglected coupling, while the input TM mode can efficiently output from the drop port at the resonant wavelength.

Compact polarization rotator for silicon-based slot waveguide structures.

A compact polarization rotator (PR) for silicon-based slot waveguides is proposed, where the slot region including the upper claddings is filled with liquid crystals (LCs). With the anisotropic features of the LCs, the transverse field components of eigenmodes have almost identical amplitudes, leading to a high modal hybridness. As a result, the TE (TM) polarization can be rotated efficiently to the TM (TE) polarization within a short length.

Design of a compact silicon-based slot-waveguide crossing.

A design scheme for silicon-based slot-waveguide crossing using a slot-to-strip mode converter (at each port) and a strip-multimode-waveguide (SMW) crossing is proposed. The guided modes of the input slot-waveguide are first efficiently transformed into that of the single-mode strip waveguide by using the mode converter, and then enter into the SMW, where fields converge at the center of the intersection due to the multimode interference effect. Consequently, the size of the input beam is much smaller than the width of the SMW at the crossing center, leading to the significant reduction of the crosstalk (CT) and radiation loss.

Vector analysis of bending waveguides by using a modified finite-difference method in a local cylindrical coordinate system.

A vector mode solver for bending waveguides by using a modified finite-difference (FD) method is developed in a local cylindrical coordinate system, where the perfectly matched layer absorbing boundary conditions are incorporated. Utilizing Taylor series expansion technique and continuity condition of the longitudinal field components, a standard matrix eigenvalue equation without the averaged index approximation approach for dealing with the discrete points neighboring the dielectric interfaces is obtained. Complex effective indexes and field distributions of leaky modes for a typical rib bending waveguide and a silicon wire bend are presented, and solutions accord well with those from the film mode matching method, which shows the validity and utility of the established method.