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

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.

96-Channel on-chip reconfigurable optical add-drop multiplexer for multidimensional multiplexing systems

The multi-dimensional multiplexing technology is very promising for further increasing the link capacity of optical interconnects. A 96-channel silicon-based on-chip reconfigurable optical add-drop multiplexer (ROADM) is proposed and demonstrated for the first time to satisfy the demands in hybrid mode/polarization/wavelengthdivision-multiplexing systems. The present ROADM consists of a six-channel mode/polarization de-multiplexer, a 6 × 16 array of microring-resonator (MRR)-based wavelength-selective switches, and a six-channel mode/polarization multiplexer. With such a ROADM, one can add/drop optical signals to/from any channels of the multimode bus waveguide arbitrarily. For the designed and fabricated ROADM chip, there are more than 1000 elements integrated monolithically, including 96 MRRs, 576 waveguide crossings, 192 grating couplers, 96 micro-heaters, 112 pads, six polarization-splitter-rotators (PSRs), four asymmetric adiabatic couplers and four asymmetric directional couplers. For any channel added/dropped with the fabricated ROADM, the on-chip excess loss is about 5-20 dB, the inter-mode crosstalk is <-12 dB, and the inter-wavelength crosstalk is <-24 dB. The system experiments are demonstrated by using 10-GBaud quadrature phase shift keying (QPSK) signals, showing that the observed optical signal noise ratio (OSNR) power penalties induced by the ROADM are less than 2 dB at a BER of 3.8 × 10-3.

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.

Fast-Response Liquid Crystals for 6G Optical Communications

We report two high birefringence and low viscosity nematic mixtures for phase-only liquid-crystal-on-silicon spatial light modulators. The measured response time (on + off) of a test cell with 2π phase change at 1550 nm, 5 V operation voltage, and 40 °C is faster than 10 ms. To improve the photostability, a distributed Bragg reflector is designed to cutoff the harmful ultraviolet and blue wavelengths. These materials are promising candidates for future 6G optical communications.

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.

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.

Bottom-metal-printed thermo-optic waveguide switches based on low-loss fluorinated polycarbonate materials

In this work, thermo-optic (TO) waveguide switches are designed and fabricated based on the bottom-metal-printed technique. Low-loss fluorinated polycarbonate (AF-Z-PC MA) and polymethyl methacrylate (PMMA) are used as core and cladding materials, respectively. The thermal stability and optical absorption characteristics of AF-Z-PC MA are analyzed. The optical and thermal field distributions of the TO switch are simulated. The insertion loss and extinction ratio of the device are found to be 4.5 dB and 19.8 dB, respectively, at a wavelength of 1550 nm. The on-off time of the switching chip is 80 µs. The electrical power consumption is approximately 8.8 mW. The proposed low-loss fluorinated polymer TO waveguide switch realized by bottom-metal-printed fabrication technology is suitable for large-scale integrated photonic circuit systems.

Silicon Mode-Selective Switch via Horizontal Metal-Oxide-Semiconductor Capacitor Incorporated With ENZ-ITO

A silicon mode-selective switch (MSS) is proposed by using a horizontal metal-oxide-semiconductor (MOS) capacitor incorporated with the epsilon-near-zero (ENZ) indium-tin-oxide (ITO). The carrier concentration of the double accumulation-layers in ITO can be adjusted via the applied gate-voltage to achieve the desired switching state. The MOS-type mode of the central MOS-capacitor based triple-waveguide coupler is introduced and optimised by using the full-vectorial finite element method to switch the "OFF" and "ON" states. The thickness of the accumulation layer and the optimal design are studied by using the 3D full-vectorial eigenmode expansion method. The optimised quasi-TE₀ and quasi-TE₁ modes based MSSes are with the extinction ratios of 28.52 dB (19.05 dB), 37.29 dB (17.8 dB), and 37.29 dB (23.7 dB), at "OFF" ("ON") states for the accumulation-layer thicknesses of 1.5, 5.0, and 10.0 nm, respectively. The operation speed can achieve to be 6.3 GHz, 6.2 GHz, and 6.2 GHz for these three accumulation-layer thicknesses, respectively. The performance of the proposed MSS with a 2.5 V gate-voltage is also studied for preventing the oxide breakdown. The proposed MSS can be applied in the mode-division-multiplexing networks for signal switching and exchanging.

Silicon-based wavelength division multiplexer using asymmetric grating-assisted couplers

A wavelength division multiplexer (WDM) based on asymmetric grating-assisted couplers is proposed, which can flexibly adjust the bandwidth by changing the corrugation width of the grating. The simulation results show that, compared with asymmetric uniform grating-assisted couplers, asymmetric unilateral amplitude apodization grating-assisted couplers and asymmetric bilateral amplitude apodization grating-assisted couplers can effectively suppress the sidelobes. The experimental results show that the insertion loss of each wavelength channel is between 0.23dB and 0.58dB, and the sidelobe suppress ratio of both unilateral amplitude apodization grating-assisted couplers and bilateral amplitude apodization grating-assisted couplers is larger than 10dB, which reduces channel crosstalk and proves the feasibility of the wavelength division multiplexers.

Flat-top optical resonance in a single-ring resonator based on manipulation of fast- and slow-light effects

Optical single-ring resonance inherently generates Lorentzian-shape magnitude and group-delay responses, leading to critical performance limitation in photonics, cavity quantum electrodynamics, cavity optomechanics, and atomic and optical physics. Here, we propose a new type of microresonator that stimulates flat-top resonance in a single-ring cavity. By manipulating the fast and slow light effects in the microresonator, the flat-top group delay can be tuned with an ignorable magnitude variation. In addition, the bandpass response can be switched to a notch, which can enable function-reconfigurable photonic integrated circuits (PICs) without a physical change in the architecture. Our demonstration provides the possibility of developing microresonator-based PICs with unprecedented high flexibility and capacity.

Crossing-free on-chip 2 × 2 polarization-transparent switch with signals regrouping function

We propose and demonstrate an on-chip 2×2 polarization-transparent switch for simultaneously handling two-group polarization-division multiplexing (PDM) signals. By introducing the polarization-transparent power splitter/combiner (PPS/PPC), waveguide crossings such as those in conventional PDM switches can be avoided and, thus, the insertion losses and complexity of the device can be reduced. Furthermore, each input polarization tributary can be independently switched and routed thanks to the output selectivity of the PPS/PPC. Thus, not only a basic switch between the dual-group PDM signals, but also a precise four-channel-signal regrouping can be achieved, enabling a complete and non-redundant switching functionality. A polarization extinction ratio larger than 15 dB with reasonable insertion losses is experimentally observed. Clear and open eye diagrams can be obtained with less than 1 dB power penalties for all the measured paths.

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.