Compact fabrication-tolerant subwavelength-grating-based two-mode division (de)multiplexer

On-chip mode division (de)multiplexer for multi-band operation

We propose an on-chip mode division (de)multiplexer based on asymmetric directional couplers (ADCs) for multi-band operation. In mode-coupling process, the minimum coupling length is wavelength-dependent. The longer the wavelength is, the shorter the minimum coupling length is. A light of longer wavelength can be coupled back and forth multiple times with a total coupling length which equals to the minimum coupling length of a shorter wavelength light, thus realizing multi-band transmission at different wavelengths. As a proof-of-concept experiment, a four-mode (de)multiplexer for joint operation in the C- and O-Bands is designed and experimentally demonstrated. For the four modes (TE₀, TE₁, TE₂ and TE₃), the measured insertion losses (ILs) and crosstalk (CT) of the (de)multiplexer are < 4.7 dB and < -10.1 dB respectively from 1290 nm to 1360 nm, and they are < 3.5 dB and < -11.8 dB respectively from 1510 nm to 1580 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.

Compact, high-performance, and fabrication friendly two-mode division multiplexer based on a silicon bent directional coupler

In response to the increasing demands of the capacity enhancement of optical communication, a compact and high-performance silicon mode division multiplexer is proposed that multiplexes the fundamental and first-order transverse magnetic modes. The device structure is based on an asymmetric bent directional coupler with an ultrasmall coupling length of 3.67 µm. Utilizing single-layer silicon waveguides with the same heights allows the proposed device to be fabricated using a single-step CMOS-compatible fabrication process, which provides a cost-effective design in comparison with the previously reported structures. The three-dimensional finite-difference time-domain simulation results confirm that the device has a low loss of 0.87 dB, low crosstalk of ${-}{21.8}\;{\rm dB}$-21.8dB, and high mode conversion efficiency of 98.3% at the communication wavelength of 1.55 µm. Furthermore, the device shows a broad bandwidth of about 110 nm, completely covering the C and L bands with crosstalk less than ${-}{10}\;{\rm dB}$-10dB. Moreover, it is shown that the proposed mode (de)multiplexer is fabrication tolerant for the coupling gap variation of ${-}{40}\;{\rm nm} \lt \Delta {g} \lt {23}\;{\rm nm}$-40nm<Δg<23nm and the waveguide width variation of ${-}{25}\;{\rm nm} \lt \Delta {W} \lt {25}\;{\rm nm}$-25nm<ΔW<25nm for a low loss of ${ \lt }- {1.67}\;{\rm dB}$<-1.67dB and low crosstalk of ${ \lt }- {10}\;{\rm dB}$<-10dB.

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. Moreover, to effectively minimize the loss, tapered waveguides are used, and two right angles are cut at both corners of the used MMI coupler. Results show that a footprint of ${2}.{2}\;\unicode{x00B5} {\rm m} \times {3}.{8}\;\unicode{x00B5} {\rm m}$2.2µm×3.8µm for the MMI region is achieved with an insertion loss of 0.07 dB for both TE and TM modes (polarization dependent loss ${\sim\;{0}\;{\rm dB} }$∼0dB) and a reflection loss of $ - {28}.{29}\;{\rm dB}$-28.29dB ($ - {31}.{25}\;{\rm dB}$-31.25dB) for TE mode at the wavelength of 1.55 µm. Insertion loss below 0.3 dB is obtained over the bandwidth of 200 nm, covering the C-band. In addition, fabrication tolerances to the structural parameters are analyzed, and the injected light propagating through the power splitter is also presented.

Ultracompact and ultrabroadband mode division multiplexer based on an Au nanocube array assisted directional coupler

An ultracompact and ultrabroadband two-mode (de)multiplexer based on an asymmetric directional coupler for mode division multiplexing is proposed. The device structure consists of a pair of silicon waveguides with an array of plasmonic Au nanocubes sandwiched in the coupling region. The coupling region length of the directional coupler is decreased to 1 µm for coupling of the fundamental transverse magnetic (TM) mode to the first order mode by excitation of the surface plasmon polaritons. This is the shortest length reported for multiplexing of the TM modes until now, to the best of our knowledge. The proposed mode (de)multiplexer has a low loss of 0.72 dB and low crosstalk of -28.3dB at the communication wavelength of 1.55 µm. Also, the 3D finite-difference time-domain simulation results show that a broad bandwidth of 190 nm is realized with crosstalk less than -10dB and the insertion loss lower than 1.29 dB. Furthermore, impact of the fabrication tolerances on the performance of the proposed (de)multiplexer is studied in detail.

On-chip silicon dual-mode multiplexer via a subwavelength grating-based directional coupler and a mode blocker

An on-chip silicon dual-mode multiplexer (MUX) is proposed via a subwavelength grating (SWG)-based directional coupler and a mode blocker. The proposed mode MUX is optimized by utilizing the three-dimensional full-vectorial finite-difference time-domain-based band structure and propagation calculations. The simulated results indicate that an ultralow mode cross talk of $ - {40}\,\,{\rm dB}$-40dB at the operating wavelength of 1550 nm can be achieved by introducing a ${{\rm TE}_0}$TE₀ mode blocker. Benefiting from the triple SWG coupler, the mode-coupling strength can be significantly enhanced inside the proposed mode MUX, which can achieve a compact coupling length of 8.75 µm, a low insertion loss of 0.39 dB at 1550 nm, an ultrabroad bandwidth of 310 nm for the mode CT $ \lt - {15.0}\,\,{\rm dB}$<-15.0dB, and insensitivity to fabrication errors.

Compact and broadband data exchange for mode-division-multiplexed networks

Recently, mode-division multiplexing (MDM) technology has been used to enable high-bandwidth data transmission. Information swapping or data exchange is an essential function in such high-capacity networks. This paper presents a small, broadband on-chip optical data exchange between two multiplexed modes on the silicon-on-insulator platform. The device consists of a three-waveguide coupler in which the middle waveguide has a hybrid plasmonic structure and the two other ones are silicon waveguides. The total length and the width of the proposed structure are 15.25 μm and 1.0885 μm, respectively. The 3D finite-difference-time-domain simulation results show that the insertion loss is less than 2 dB for a broad bandwidth of 100 nm, over which the crosstalk is lower than -14.7  dB and -21.9  dB for TM0 to TM1 and TM1 to TM0 conversion, respectively.