Compact silicon 10-mode multi/demultiplexer for hybrid mode- and polarisation-division multiplexing system

Broadband and low-cross talk mode multiplexer based on subwavelength-assisted phase matching

Integrated mode multiplexers are key components in photonic networks and are crucial to achieving higher data density. Asymmetric directional couplers represent one of the most common multiplexing architectures, owing to their low losses and scalability, but further bandwidth enhancements are sought after. In this Letter, we demonstrate a multiplexer/demultiplexer link based on asymmetric directional couplers assisted by subwavelength grating metamaterials. Through subwavelength-assisted phase matching, low losses and cross talk are achieved on a small form factor over a broad bandwidth. Experimental measurements show insertion losses under 0.8 dB and cross talk under -19.8 dB in a 170 nm bandwidth (1500 nm to 1670 nm) for a full multiplexer/demultiplexer link.

Design and experimental demonstration of a silicon five-mode (de)multiplexer based on multi-phase matching condition

A compact 5-mode (de)multiplexer [(De)MUX] is proposed and experimentally demonstrated based on the principle of multi-phase matching. The proposed device comprises a cascaded asymmetric directional coupler (ADC) based on 3-mode phase-matching, a polarization beam combiner, and a taper waveguide connecting them. The multiple modes in the access waveguides are matched to different modes in the same bus waveguide, which eliminates the need for additional taper structures and results in a total coupling length of only 18.9 µm. Experimental results exhibit that the insertion losses of the five modes are below 3.4 dB, and the mode crosstalks are below -15 dB at the central wavelength. The 3-dB bandwidths of TM0, TM1, TE0, TE1, and TE2 modes are greater than 100 nm, 46 nm, 100 nm, 28 nm, and 37 nm, respectively. The proposed device can serve as a key functional component in highly integrated on-chip mode-division multiplexing systems.

SOI-based compact mode- and polarization-division multiplexer for on-chip optical interconnects: design and simulation

A compact seven-channel hybrid mode- and polarization-division multiplexer based on a silicon-on-insulator (SOI) waveguide platform is proposed for on-chip optical interconnects. Three independent signals are coupled into the TE₀, TM₀, and TE₁ mode of a three moded waveguide using a taper, a polarization beam splitter, and an asymmetric directional coupler in the first coupler section. In the adiabatically connected second and third sections, two collocated asymmetric directional couplers are used to couple two TE modes (TE₂, TE₃) and two TM modes (TM₁, TM₂) to reduce the footprint of the device. A coupling length of ∼44.29µm is achieved with low insertion loss <0.09dB and crosstalk better than -30dB at the designed wavelength of 1.55 µm.

Design of a grating-assisted silicon hybrid mode-, polarization-, and wavelength-division (De)multiplexer for on-chip optical interconnects

This paper presents a 12-channel hybrid mode-, polarization-, and wavelength-division (de)multiplexer. The proposed device is designed on a silicon-on-insulator platform. The device structure constitutes three coupling sections with a bus waveguide at the middle and two single-mode waveguides on each side. The periodic grating structures are present in between the waveguides for contra-directional coupling of the three TE modes and three TM modes of the bus waveguide to respective fundamental modes of the single-mode waveguides at 1550 and 1560 nm. The device is simulated using the 3D finite-difference time-domain technique, and the resulting insertion loss, cross talk, and return loss are <1.56dB, <-21.74dB, and >12.22dB, respectively. Moreover, the period of the grating structures is varied to perform the fabrication tolerance study.

Three-dimensional polymer mode (de)multiplexer with cascaded tapered couplers for on-board optical interconnects

A three-dimensional polymer mode (de)multiplexer based on mode evolution is proposed. The proposed configuration is mainly composed of cascaded two tapered couplers where waveguides with different heights are inversely tapered to achieve mode conversions of the E₁₁,E₂₁, and E₃₁ modes. The dimensional parameters and characteristics are analyzed by the beam propagation method. This mode (de)multiplexer exhibits the coupling ratio greater than 0.97, excess loss lower than 0.15 dB and extinction ratio higher than 20 dB for TE and TM polarizations over the wavelength range 1530∼1625nm (the C+L band). This design has weak wavelength dependence and polarization dependence, which is promising to be applied to broadband on-board mode multiplexing.

Topology design of digital metamaterials for ultra-compact integrated photonic devices based on mode manipulation

Precise manipulation of mode order in silicon waveguides plays a fundamental role in the on-chip all-optical interconnections and is still a tough task in design when the functional region is confined to a subwavelength footprint. In this paper, digital metamaterials consisting of silicon and air pixels are topologically designed by an efficient method combining 2D finite element method for optical simulations, density method for material description and method of moving asymptotes for optimization. Only around 150 iterations are required for searching satisfactory solutions. Six high-quality and efficient conversions between four TE-polarized modes are achieved in a functional region with footprint 0.645λ ² (center wavelength λ = 1550 nm). Based on asymmetric mode conversion, a reciprocal optical diode with high contrast ratio is further obtained with the optimization starting from TE0-to-TE1 mode converter. Moreover, we successfully design a 1 × 2 demultiplexer with footprint 1.0λ ² and demonstrate a simple mode division multiplexing system with satisfactory performances. Finally, by changing the refractive index to an equivalent value, quasi-3D designs are obtained and the functionalities are validated in 3D simulations for both free-standing and SOI configurations.

Ultra-compact reconfigurable device for mode conversion and dual-mode DPSK demodulation via inverse design

The mode multiplexing/de-multiplexing devices are key components for mode-division multiplexing (MDM) technology. Here, we propose an ultra-compact and reconfigurable mode-conversion device via inverse design, which can selectively implement multichannel mode conversion controlled by input phase shifts (Δφ). The device can transform input TE₀ (TE₁) mode to TE₄ (TE₃) mode at Δφ=0, or from TE₀ (TE₁) to TE₁ (TE₂) at Δφ=π spanning the wavelength range of 1525-1565 nm. We further demonstrate an integrated monolithic module based on the mode conversions to directly demodulate the dual-mode difference phase shift keying (DPSK) signal which significantly reduces the device size and benefits for future dense integrations in MDM systems.

On-chip silicon photonic controllable 2 × 2 four-mode waveguide switch

Multimode optical switch is a key component of mode division multiplexing in modern high-speed optical signal processing. In this paper, we introduce for the first time a novel 2 × 2 multimode switch design and demonstrate in the proof-of-concept. The device composes of four Y-multijunctions and 2 × 2 multimode interference coupler using silicon-on-insulator material with four controllable phase shifters. The shifters operate using thermo-optic effects utilizing Ti heaters enabling simultaneous switching of the optical signal between the output ports on four quasi-transverse electric modes with the electric power consumption is in order of 22.5 mW and the switching time is 5.4 µs. The multimode switch exhibits a low insertion loss and a low crosstalk below - 3 dB and - 19 dB, respectively, in 50 nm bandwidth in the third telecom window from 1525 to 1575 nm. With a compact footprint of 10 µm × 960 µm, this device exhibits a relatively large width tolerance of ± 20 nm and a height tolerance of ± 10 nm. Furthermore, the conceptual principle of the proposed multimode switch can be reconfigurable and scalable in multifunctional on-chip mode-division multiplexing optical interconnects.

Mode hybridization analysis in thin film lithium niobate strip multimode waveguides

Mode hybridization phenomenon in air-cladded X-cut Y-propagating and Z-propagating thin film lithium niobate strip multimode waveguides is numerically studied and a mathematical relation between structural parameters leading to hybrid modes is formulated. Dependence of hybrid modes on waveguide dimensions, sidewall angles and wavelength is also analyzed. The results obtained are used to design lithium niobate on insulator (LNOI) taper for converting fundamental TM mode to higher order TE mode, and an optimum length for achieving a high conversion efficiency of 99.5% is evaluated. Birefringent Y-propagating LN and isotropic Z-propagating LN tapers are compared in terms of length, figures of merit, and fabrication tolerance. Tapers exhibit a broad bandwidth of 200 nm with an extinction ratio less than − 18 dB. The results of mode hybridization analysis are useful in design optimization of adiabatic tapers, tunable time delays, optical interconnects, mode converters and demultiplexers for mode division multiplexing (MDM) applications.

Silicon Photonic Mode-Division Reconfigurable Optical Add/Drop Multiplexers with Mode-Selective Integrated MEMS Switches

Mode-division multiplexing (MDM) is an attractive solution for future on-chip networks to enhance the optical transmission capacity with a single laser source. A mode-division reconfigurable optical add/drop multiplexer (ROADM) is one of the key components to construct flexible and complex on-chip optical networks for MDM systems. In this paper, we report on a novel scheme of mode-division ROADM with mode-selective silicon photonic MEMS (micro-electromechanical system) switches. With this ROADM device, data carried by any mode-channels can be rerouted or switched at an MDM network node, i.e., any mode could be added/dropped to/from the multimode bus waveguide flexibly and selectively. Particularly, the design and simulation of adiabatic vertical couplers for three quasi-TE modes (TE0, TE1, and TE2 modes) based on effective index analysis and mode overlap calculation method are reported. The calculated insertion losses are less than 0.08 dB, 0.19 dB, and 0.03 dB for the TE0 mode, TE1 mode, and TE2 mode couplers, respectively, over a wavelength range of 75 nm (1515–1590 nm). The crosstalks are below −20 dB over the bandwidth. The proposed device is promising for future on-chip optical networks with flexible functionality and large-scale integration.

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.

Design and experimental demonstration of a silicon multi-dimensional (de)multiplexer for wavelength-, mode- and polarization-division (de)multiplexing

Leveraging the physical dimensions of an optical carrier (e.g., wavelength, mode, or polarization) allows significant scaling of the transmission capacity for optical communications. Here we propose a scheme for implementing on-chip silicon (de)multiplexers with simultaneous wavelength-, mode-, and polarization-division (de)multiplexing capability. The device is constructed by using cascaded subwavelength grating (SWG)-based contra-directional couplers. To verify the feasibility of the proposed structure, we perform a proof-of-concept experiment of an 8-channel (de)multiplexer with two wavelengths, two modes, and two polarizations. The insertion losses are lower than 6.6 dB and the crosstalk values are below -18.7dB at around 1540 nm and 1550 nm for all the eight channels.