Monolithically integrated reconfigurable add-drop multiplexer for mode-division-multiplexing systems

Arrayed electro-optic modulators for novel WDM multiplexing

In this paper, a novel silicon-on-chip integrated 4 × 1 wavelength division multiplexing (WDM) multiplexer has been developed. This is the first time that the multiplexer design incorporates arrayed electro-optical modulators with crosstalk cancellation. The design utilizes two types of electro-optic modulators in each channel. The first modulator, based on 1D-PhCNBC, extracts the desired wavelengths from the WDM spectrum. The second modulator, based on coupled hybrid plasmonics, acts as a switch to eliminate crosstalk of the desired optic wavelength signal at the multiplexer output. By combining the advantages of electro-optical modulators and crosstalk cancellation techniques, we anticipate that our proposed design contributes to the advancement of WDM multiplexing technology and facilitates the implementation of efficient and compact optical communication systems. Additionally, this synergy enables enhanced performance, reduced signal interference, and improved signal quality, leading to more reliable and high-speed data transmission in optical networks. The functionality of the device is theoretically simulated using 3D-FDTD (Finite-Difference Time-Domain) method.

Silicon photonic broadband polarization-insensitive switch based on polarization-mode diversity conversion

We present a 2 × 2 polarization-insensitive switch on a 220-nm silicon-on-insulator platform, employing a balanced Mach-Zehnder interferometer (MZI) structure. This design incorporates polarization-insensitive adiabatic couplers, polarization rotators based on mode hybridization and evolution, and thermo-optic mode-insensitive phase shifters with wide waveguides. The switch exhibits broadband polarization-insensitive characteristics, with extinction ratios larger than 15 dB, insertion losses less than 2.3 dB, and polarization-dependent losses less than 1 dB for wavelengths ranging from 1500 nm to 1600 nm. The power consumption required for simultaneously switching the fundamental transverse electric (TE0) and transverse magnetic (TM0) polarized modes is 29.1 mW. These results highlight the potential of the switch as a building block for on-chip polarization-division-multiplexed optical interconnects.

Ultracompact programmable inverse-designed nanophotonic devices based on digital subwavelength structures

Inverse design is a powerful approach to achieve ultracompact nanophotonic devices. Here, we propose an ultracompact programmable near-infrared nanophotonic device platform to dynamically implement inverse-designed near-infrared devices with different functions by programming the state of the phase-change material filled in each pixel. By tuning PCM block by block, the subwavelength condition for inverse-designed ultracompact devices is satisfied with large tuning pixel size. Based on the inverse-design device platform with a footprint of 6.4µm×8µm, we design and theoretically demonstrate four power splitters with different split ratios and one mode multiplexer working in the near-infrared band. The average excess losses for the power splitters with ratios of 0:1,1:1, 2:1, and 3:1 are less than 0.82, 0.65, 0.82, and 1.03 dB over a wavelength span of 100 nm, respectively. Meanwhile, the insertion losses of the mode multiplexer are 1.4 and 2.5 dB for T E 0 and T E 1 mode, respectively, and the average crosstalk is less than -20 and -19d B, respectively. The five different devices could be configured online in a nonvolatile way by heating phase change materials with an off-chip laser, which may significantly enhance the flexibility of on-chip optical interconnections.

Guided-mode based arbitrary signal switching through an inverse-designed ultra-compact mode switching device

The application of the inverse design method and free-form geometrical optimization in photonic devices endows them with highly tunable functionality and an ultra-compact footprint. In this paper, we implemented this platform to silicon photonic guided-mode manipulation and demonstrated a guided mode-based signal switching architecture. The passive signal switching mechanism is utilized so that no power consumption is needed for routing state maintenance. To solve the explosive increasing design cost in such mechanism when the switching scale is expanded, we illustrate that only a small number of mode switching devices need to be designed as the switching basis. In theory, arbitrary signal routing states can be constructed by cascading some selected basis. The required switching devices can be decreased from factorial N to N - 1 for the N channels switching. For proof of concept, we design and experimentally demonstrate the three-mode cases and the cascade method to combine any three mode-based switching devices. Experiments show that the insertion losses of TE₀ - TE₁ mode switching unit (U1), TE₁ - TE₂ mode switching units (U2), and TE₀ - TE₂ mode switching unit (U3) are less than 2.8 dB, 3.1 dB, and 2.3 dB, respectively. The demonstrated architecture has both arbitrary signal switching capability and ultra-compact footprint, which is promising in the application of mode-division multiplexing communication systems.

Ultra-compact multimode waveguide bend with shallowly etched grooves

In this work, an ultra-sharp multimode waveguide bend (MWB) based on gradient shallowly etched grooves is proposed and demonstrated. With a bending radius of only 5.6 μm, our shallowly-etched-groove multimode waveguide bend (SMWB) can enable low excess loss and low-crosstalk propagation with the four lowest-order TE mode-channels, simultaneously. In the simulation, the excess losses of the proposed 90°- SMWB for TE₀-TE₃ are all below 0.46 dB and the inter-mode crosstalks are lower than -18 dB in 1500 nm-1600 nm. Furthermore, the measured results of the fabricated 90°- SMWB show that the excess losses for TE₀-TE₃ are less than 1 dB and the inter-mode crosstalks are all below -14 dB in 1510 nm-1580 nm. Such a proposed device thus provides a promising solution for ultra-compact MWBs in multimode silicon photonics.

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.

Edge Couplers in Silicon Photonic Integrated Circuits: A Review

Silicon photonics has drawn increasing attention in the past few decades and is a promising key technology for future daily applications due to its various merits including ultra-low cost, high integration density owing to the high refractive index of silicon, and compatibility with current semiconductor fabrication process. Optical interconnects is an important issue in silicon photonic integrated circuits for transmitting light, and fiber-to-chip optical interconnects is vital in application scenarios such as data centers and optical transmission systems. There are mainly two categories of fiber-to-chip optical coupling: off-plane coupling and in-plane coupling. Grating couplers work under the former category, while edge couplers function as in-plane coupling. In this paper, we mainly focus on edge couplers in silicon photonic integrated circuits. We deliver an introduction to the research background, operation mechanisms, and design principles of silicon photonic edge couplers. The state-of-the-art of edge couplers is reviewed according to the different structural configurations of the device, while identifying the performance, fabrication feasibility, and applications. In addition, a brief comparison between edge couplers and grating couplers is conducted. Packaging issues are also discussed, and several prospective techniques for further improvements of edge couplers are proposed.

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.

Broadband optical switch for multiple spatial modes based on a silicon densely packed waveguide array

A broadband thermo-optic 2×2 Mach-Zehnder switch that can control two spatial modes simultaneously on a silicon chip is demonstrated in this Letter. A broadband multimode 3 dB coupler is first realized based on symmetrically coupled waveguides with a sub-wavelength structure. The length of the coupler is only 24.2 μm. By employing such a multimode 3 dB coupler together with symmetrical delay arms, an optical switching functionality is realized experimentally with excess losses less than 1.3 dB and crosstalks less than -15  dB over a 60 nm bandwidth for the two spatial modes. An arbitrary splitting ratio at two output ports for the two modes simultaneously is also demonstrated when applying different currents on the heater.

On-chip reconfigurable and scalable optical mode multiplexer/demultiplexer based on three-waveguide-coupling structure

Reconfigurable optical mode multiplexers/demultiplexers have attracted increasing attention in the academic community, because they enable convenient construction of flexible and complex on-chip optical networks. Here, we propose and demonstrate a scheme of reconfigurable and scalable optical mode multiplexer/demultiplexer with large operation bandwidth, based on three-waveguide-coupling structures. As proof of concept, a reconfigurable device that can multiplex input signals to the fundamental and first-order quasi-transverse electric mode is fully fabricated and demonstrated successfully. Static response spectra show that the optical crosstalk at the output ports of the device are less than -14.3 dB and -13.7 dB over the entire C band (> 40 nm), respectively. The dynamic performance with data transmission speeds of 40 Gbps for each multiplexing channel are also demonstrated successfully. The presented device is believed to be a potential candidate for future on-chip optical network with large-scale integration, flexible functionality, and low cost.

Ultra-broadband and compact graphene-on-silicon integrated waveguide mode filters

Increasing bandwidth demands in optical communications necessitates the introduction of mode-division multiplexing (MDM) on top of the existing wavelength-division multiplexing (WDM) systems. Simultaneous management of both multiplexing systems will be a complex task, and there is the possibility of signal degradation through modal crosstalk. Here, we propose graphene-on-silicon (GOS) integrated waveguide mode filters to suppress the propagation of spurious waveguide modes at the telecommunications wavelength. Graphene’s high fabrication tolerance potentially enables surgical tailoring and deployment at targeted segments on the waveguide to absorb the undesired TE₀ or TE₁ modes. The proposed GOS waveguide mode filters can potentially improve the performance and reduce the device footprint of MDM systems.

Very sharp adiabatic bends based on an inverse design

Very sharp 90°micro-bends for adiabatic optical wave propagation in multimode waveguides are demonstrated by an inverse design method, and the devices are fabricated on a silicon-on-insulator platform. The compact bending structures are based on digital waveguide metastructures with footprints as small as 2.6  μm×2.6  μm. For waveguides with widths of 2 μm and bending radii of only 1 μm, the TE₀₀ mode is able to turn around the sharp corner with 1 dB loss and >20  dB suppression ratio to higher-order modes. The transmission spectra of the devices are measured with ∼1  dB loss over a 40 nm bandwidth which are consistent with the numerical simulations.

Polarization-insensitive 2 × 2 thermo-optic Mach-Zehnder switch on silicon

A polarization-insensitive 2×2 thermo-optic Mach-Zehnder switch (MZS) on silicon is proposed and demonstrated experimentally by utilizing silicon-on-insulator (SOI) nanophotonic waveguides with a 340-nm-thick silicon core layer. The present MZS consists of two 2×2 3 dB multimode interference (MMI) couplers, which are designed to be polarization-insensitive by choosing the core width optimally. Meanwhile, the MZS arms are designed with square SOI nanophotonic waveguides with a cross section of 340  nm×340  nm in order to achieve polarization-insensitive phase shift. The fabricated silicon MZS has an excess loss of 1∼4  dB and an extinction ratio of >20  dB in the C-band (1530∼1565  nm) for both TM and TE polarizations.

Ultra-broadband multimode 3dB optical power splitter using an adiabatic coupler and a Y-branch

As an essential component of mode division multiplexing (MDM) system, a multimode 3dB power splitter with low loss, high power balance, and low mode crosstalk is highly desired. In this paper, we propose an ultra-broadband on-chip multimode 3dB optical power splitter using an adiabatic coupler and an S-bend based Y-branch. As an example, a splitter for the four-lowest modes of a rib waveguide on silicon on insulator (SOI) platform is designed. Simulation results show that the device exhibits < 0.12dB insertion losses, within ± 0.38dB power imbalances, and < -18.5dB mode crosstalks for the four-lowest modes within a large operating wavelength range of 165 nm (from 1400 nm to 1565 nm). The fabrication tolerance of gap size at the output end of the adiabatic coupler is also analyzed.

Reconfigurable broadband mode (de)multiplexer based on an integrated thermally induced long-period grating and asymmetric Y-junction

We propose a reconfigurable broadband mode (de)multiplexer based on a thermally induced long-period grating integrated with an asymmetric Y-junction. Either of the two spatial modes of a two-mode waveguide launched into the grating end of the device can be switched into either of the two output ports of the Y-junction by controlling the electric power applied to the electrode heater that induces the grating. Our typical device fabricated with polymer material that has a length of ∼14  mm shows a mode selectivity higher than 12 dB over the C+L band at a switching power of 198 mW. The device could find applications in reconfigurable mode-division-multiplexing systems.

Polarization-insensitive broadband 2 × 2 3 dB power splitter based on silicon-bent directional couplers

We present and demonstrate a broadband and polarization-insensitive 2×2 3 dB power splitters based on silicon-bent directional couplers (DCs). The working bandwidth can be from 1520 to 1630 nm for both TE and TM polarizations, with an excess loss less than 1 dB. The total size for the fabricated device can be as small as 50 μm, and the measurement results show good agreement with the simulations.

Ultra-compact bent multimode silicon waveguide with ultralow inter-mode crosstalk

We propose and experimentally demonstrate a sharply bent multimode silicon waveguide by introducing a pair of mode converters. The principle of the multimode bend is based on the eigenmode conversion between straight and bent waveguides, and the particle swarm optimization is adopted to engineer the geometry. The device is fabricated on the standard silicon-on-insulator platform using one lithography/etching, and no additional steps are required. The measured results show that the insertion loss is <0.2  dB, and the inter-mode crosstalk is <-22  dB with a bend radius of 5 μm, from 1500 to 1600 nm. A comparable performance can be achieved using a conventional scheme with 40 μm radius.

On-chip reconfigurable optical add-drop multiplexer for hybrid wavelength/mode-division-multiplexing systems

A silicon-based on-chip reconfigurable optical add-drop multiplexer (ROADM) is presented for hybrid wavelength-division-multiplexing-mode-division-multiplexing systems. The present ROADM consists of a four-channel mode demultiplexer, four wavelength-selective thermo-optic switches based on microring resonators, and a four-channel mode multiplexer. With the present ROADM, one can add/drop one of wavelength channels of any mode to/from the multimode bus waveguide successfully with an excess loss of 2-5 dB and an extinction ratio of ∼20  dB over a wavelength range of 1525-1555 nm.