Experimental realization of an O-band compact polarization splitter and rotator

Compact and broadband silicon polarization splitter-rotator using adiabaticity engineering

We propose and demonstrate a short and broadband silicon mode-conversion polarization splitter-rotator (PSR) consisting of a mode-conversion taper and an adiabatic coupler-based mode sorter both optimized by adiabaticity engineering (AE). AE is used to optimize the distribution of adiabaticity parameter over the length of the PSR, providing shortcut to adiabaticity at a shorter device length. The total length of the PSR is 85 µm. The design is compatible with standard silicon photonics platforms and requires only one patterning step. Fabricated PSR has a polarization cross talk of less than -20 dB over the entire O-band for the TE polarization and a polarization cross talk of less than -15 dB from 1267 to 1348 nm for the TM polarization. Overall, the PSR shows low polarization cross talk (-15 dB) over a bandwidth of 81 nm in the O-band. Cross-wafer measurements show that the PSR has good fabrication tolerance.

Broadband integrated polarization splitter and rotator using subwavelength grating claddings

We present a broadband integrated photonic polarization splitter and rotator (PSR) using adiabatically tapered coupled waveguides with subwavelength grating (SWG) claddings. The PSR adiabatically rotates and splits the fundamental transverse-magnetic (TM₀) input to the fundamental transverse-electric (TE₀) mode in the coupler waveguide, while passing the TE₀ input through the same waveguide. The SWGs work as an anisotropic metamaterial and facilitate modal conversions, making the PSR efficient and broadband. We rigorously present our design approaches in each section and show the SWG effect by comparing with and without the SWG claddings. The coupling coefficients in each segment explicitly show a stronger coupling effect when the SWGs are included, confirmed by the coupled-mode theory simulations. The full numerical simulation shows that the SWG-PSR operates at 1500-1750 nm (≈250 nm) wavelengths with an extinction ratio larger than 20 dB, confirmed by the experiment for the 1490-1590 nm range. The insertion losses are below 1.3 dB. Since our PSR is designed based on adiabatical mode evolution, the proposed PSR is expected to be tolerant to fabrication variations and should be broadly applicable to polarization management in photonic integrated circuits.

Ultra-compact, efficient and high-polarization-extinction-ratio polarization beam splitters based on photonic anisotropic metamaterials

Anisotropic metameterials (AM) provide a new avenue for a next-generation silicon platform to design ultra-compact, densely integrated optical components, thus functional devices based on AM are drawing increasing attention recently. Here, we propose a novel efficient polarization beam splitter (PBS) with high polarization extinction ratio based on AM. An ultra-compact coupling region of 2.5 × 14 µm² is achieved by tailoring the AM structures, which can efficiently suppress the TE mode coupling, and enhance the TM mode coupling in the directional couplers simultaneously. The insertion loss is simulated to be as low as <0.2 dB within a bandwidth of 70 nm for both modes, and the polarization extinction ratio is as high as 46 dB and 33 dB for TE and TM modes, respectively. We also experimentally demonstrate the proposed PBS, with low insertion loss of 1 dB , high extinction ratio of >20 dB and wide operational bandwidth of >80 nm.

Silicon Integrated Nanophotonic Devices for On-Chip Multi-Mode Interconnects

Mode-division multiplexing (MDM) technology has drawn tremendous attention for its ability to expand the link capacity within a single-wavelength carrier, paving the way for large-scale on-chip data communications. In the MDM system, the signals are carried by a series of higher-order modes in a multi-mode bus waveguide. Hence, it is essential to develop on-chip mode-handling devices. Silicon-on-insulator (SOI) has been considered as a promising platform to realize MDM since it provides an ultra-high-index contrast and mature fabrication processes. In this paper, we review the recent progresses on silicon integrated nanophotonic devices for MDM applications. We firstly discuss the working principles and device configurations of mode (de)multiplexers. In the second section, we summarize the multi-mode routing devices, including multi-mode bends, multi-mode crossings and multi-mode splitters. The inverse-designed multi-mode devices are then discussed in the third section. We also provide a discussion about the emerging reconfigurable MDM devices in the fourth section. Finally, we offer our outlook of the development prospects for on-chip multi-mode photonics.

Progress of infrared guided-wave nanophotonic sensors and devices

Nanophotonics, manipulating light-matter interactions at the nanoscale, is an appealing technology for diversified biochemical and physical sensing applications. Guided-wave nanophotonics paves the way to miniaturize the sensors and realize on-chip integration of various photonic components, so as to realize chip-scale sensing systems for the future realization of the Internet of Things which requires the deployment of numerous sensor nodes. Starting from the popular CMOS-compatible silicon nanophotonics in the infrared, many infrared guided-wave nanophotonic sensors have been developed, showing the advantages of high sensitivity, low limit of detection, low crosstalk, strong detection multiplexing capability, immunity to electromagnetic interference, small footprint and low cost. In this review, we provide an overview of the recent progress of research on infrared guided-wave nanophotonic sensors. The sensor configurations, sensing mechanisms, sensing performances, performance improvement strategies, and system integrations are described. Future development directions are also proposed to overcome current technological obstacles toward industrialization.

Ultracompact integrated polarizers using bent asymmetric coupled waveguides

A new type of integrated polarizer based on bent coupled optical waveguides is proposed. The simplicity of the device geometry permits its realization using only basic standard fabrication steps. A deep-etched waveguide silicon nitride TE-pass implementation is numerically investigated. The characteristics of the underlying physical mechanism bring in flexibility to the design of broadband devices robust against fabrication tolerances. The possibility of incorporating almost design-transparent polarizers in photonic integrated circuits is discussed.

Polarization-independent directional coupler and polarization beam splitter based on asymmetric cross-slot waveguides

The polarization dependence of a directional coupler (DC) based on asymmetric cross-slot waveguides is investigated. Due to structural birefringence, the coupling behaviors of the quasi-TE and quasi-TM modes in the DC vary with the waveguide geometry. A polarization-independent directional coupler (PIDC) and polarization beam splitter (PBS) are proposed by tailoring the ratio of the coupling length for quasi-TE and quasi-TM modes. The simulated results show that the coupling lengths of the designed PIDC and PBS are 8 and 28.25 μm, respectively. Both the PIDC and PBS show an insertion loss (IL) <0.7  dB on a bandwidth over 100 nm. The extinction ratios are ∼20  dB for PIDC and ∼14  dB for PBS. The fabrication-error tolerance of the practical devices is also discussed. In this study, we employ a commercial software tool for finite difference eigenmode and three-dimensional finite difference time domain methods to perform the numerical simulations.

Ultracompact, high extinction ratio polarization beam splitter-rotator based on hybrid plasmonic-dielectric directional coupling

A polarization beam splitter-rotator based on asymmetric directional coupling is proposed. An ultrashort cross-polarization coupling length of 7.7 μm is achieved by manipulating the optical field distribution via the plasmonic effect, which is the shortest one reported so far, to the best of our knowledge. At the wavelength of 1.55 μm, the extinction ratios are as high as 50.9 dB and 28.2 dB for fundamental transverse magnetic (TM₀) and transverse electric (TE₀) polarizations, respectively, and the corresponding insertion losses are 1.545 dB and 0.037 dB. In addition, the TM₀-to-TE₀ polarization conversion efficiency is higher than 95% within a bandwidth of 70 nm.