dB Crosstalk Research Articles

1 × N All‐Logic Optical Switch Based on Polymer Platform Using Multimode Interferometer

The compact and broadband optical switch with a large port count is demanded with the increasing communication capacity. In this article, a universal method for modeling the 1 × N switch using multimode interferometer (MMI) through transmission matrixes is proposed. Herein, the reasons for the narrowing of the operating bandwidth switch are analyzed. As a proof of concept, a wide bandwidth 1 × 4 switch, which has an insertion loss lower than 23.7 dB, and a cross talk better than −10 dB at 1550 nm are simulated, designed, and fabricated. The cross talk throughout the C band is lower than −8.5 dB. According to the experimental result, the 1 × 4 switch with four‐equal‐length modulating arms shows a 32 nm bandwidth for −10 dB cross talk which is 13 times larger than traditional switch. The switch realizes a multi‐port logic optical switch by modulation. The 1 × N switch based on the generalized Mach–Zehnder interferometer (GMZI) structure reduce the footprint significantly compared with the 1 × N switch consisting of a 1 × 2 switch cascade. It is believed that 1 × N switch based on GMZI structures is a promising solution to increase integration density.

Microlens arrays for multichannel laser-to-waveguide coupling

An optical multichannel coupling system for coupling laser arrays to waveguide arrays is developed. Based on a microlens array, the system enables coupling of nine individual optical channels, with one aspheric microlens per channel at a lateral channel pitch of 100 µm. The design process criteria for the proposed microlenses, with 97 µm diameter and working distances from laser to lens and lens to waveguide of 150 µm and 275 µm, respectively, are described. The microlens array is fabricated on a 4mm×2mm×0.41mm fused silica chip and contains an orthogonal grid with 32×16 microlenses, of which a row of nine adjacent microlenses is used for coupling. Uniform coupling over all channels can be achieved, as well as specific coupling for each channel individually with less than −13.5dB crosstalk. The coupling system is designed for optical neural stimulators.

Multilayer stacked crystalline silicon switch with nanosecond-order switching time.

To realize compact and denser photonic integrated circuits, three-dimensional integration has been widely accepted and researched. In this article, we demonstrate the operation of a 3D integrated silicon photonic platform fabricated through wafer bonding. Benefiting from the wafer bonding process, the material of all layers is c-Si, which ensures that the mobility is high enough to achieve a nanosecond response via the p-i-n diode shifter. Optical components, including multimode interferences (MMIs), waveguide crossing, and Mach-Zehnder interferometer (MZI)-based switch, are fabricated in different layers and exhibit great performance. The interlayer coupler and crossing achieve a 0.98 dB coupling loss and <-43.58 dB cross talk, while the crossing fabricated in the same layer shows <-36.00 dB cross talk. A nanosecond-order switch response is measured in different layers.

Low-loss C-band all-fiber FIFO device by novel bridge fiber

Multi-core fibers (MCF) technology is the feasible path to extend the capacity of optical fiber communication systems. However, there is a lack of low-loss and low-crosstalk multi-core fan-in and fan-out (FIFO) devices for MCF communication in the C-band. In this paper, we proposed all-fiber FIFO devices with seven trench-assisted bridge fibers (TABFs). The special structure of the TABFs could facilitate the matching of single-core fiber with MCF during the tapering progress. By using the TABFs, the average taper loss was less than 0.06 dB. The novel method can reduce the insertion loss of FIFO devices. The experiments indicated that the fabricated FIFO devices exhibited lower than 0.50 dB insertion loss and less than −55.2 dB crosstalk in the C-band. This work provides novel TABFs and an efficient way to fabricate low-loss low-crosstalk FIFO devices for MCF communication systems.

Design and simulate the demultiplexer using photonic crystal ring resonator for DWDM system

In this study, a new four channel de-multiplexer with a ring resonator design is proposed. The bus waveguide and drop waveguide that make up the Ring Resonator are ring-shaped. In the proposed four channel demultiplexer design, one bus waveguide and four drop waveguides were built using a photonic crystal ring resonator. To improve output efficiency, the proposed demultiplexer was built with distinct inner radius values for each channel. With the resonance wavelengths for each channel in the range of 1552.4 nm, 1553.2 nm, 1554.1 nm, and 1555.4 nm, the suggested demultiplexer average quality factor was 7870.90, and its average transmission efficiency was 98.67%. The demultiplexer was created with a 0.8 nm narrow channel spacing with a − 15 dB to − 25 dB crosstalk range. The proposed ring resonator structure is made of silicon, which has a refractive index of 3.47, a center wavelength ranges of 1550 nm, and a lattice constant that varies with the radius range of 540 nm. To examine the performance, one can simulate the suggested demultiplexer structure using the FDTD (Finite-Difference Time-Domain) approach. The proposed work 198.7 µm2 footprint is appropriate for DWDM applications.

Temperature-insensitive and low-loss single-mode silicon waveguide crossing covering all optical communication bands enabled by curved anisotropic metamaterial

Abstract We propose two designs of low-loss and temperature-insensitive single-mode waveguide crossing on silicon-on-insulator (SOI) platform with 415-nm operation bandwidth covering all optical communication bands. Both designs are enabled by subwavelength grating (SWG) modeled as an anisotropic metamaterial. The initial design applies straight SWG as the lateral cladding of the waveguide crossing to minimize the refractive index contrast and reduce the insertion loss (IL), but needs a relatively long taper. An improved design is then proposed where the curved SWG is introduced to replace the straight SWG to decrease the taper length and improve the performance. The waveguide crossing with the improved design achieves a calculated maximum IL of 0.229 dB and maximum crosstalk of −35.6 dB over a 415-nm wavelength range from 1260 nm to 1675 nm. The proposed devices are fabricated and characterized. Measured results of the improved design show a maximum IL of 0.264 dB and maximum crosstalk of −30.9 dB over a 230-nm wavelength range including O-, C-, and L-bands, which accord well with the simulation. Low temperature sensitivity has also been demonstrated in both simulations and experiments.

Spatial and Polarization Division Multiplexing Harnessing On‐Chip Optical Beam Forming

AbstractOn‐chip spatial and polarization multiplexing has emerged as a powerful strategy to boost the data transmission capacity of integrated optical transceivers. State‐of‐the‐art multiplexers require accurate control of the relative phase or the spatial distribution among different guided optical modes, seriously compromising the optical transmission bandwidth and performance of the devices. To overcome this limitation, a new approach based on the coupling between guided modes in integrated waveguides and optical beams free‐propagating on the chip plane is proposed. The engineering of the evanescent coupling between the guided modes and free‐propagating beams allows spatial and polarization multiplexing with state‐of‐the‐art performance. A two‐polarization multiplexed link and a three‐mode multiplexed link using standard 220‐nm‐thick silicon‐on‐insulator technology have been developed. The two‐polarization link shows a measured −35 dB crosstalk bandwidth of 180 nm, while the three‐mode link exhibits a −20 dB crosstalk bandwidth of 195 nm. These links are used to demonstrate error‐free operation (bit‐error‐rate &lt;10−9) in multiplexing and demultiplexing of two and three non‐return‐to‐zero signals at 40 Gbps each, with power penalties below 0.08 and 1.5 dB for the two‐polarization and three‐mode links, respectively. The approach demonstrated for two polarizations and three modes is transferable to future implementation of more complex multiplexing schemes.

Ultra-compact silicon multimode waveguide bends based on special curves for dual polarizations

Multimode waveguide bends (MWBs) with very compact sizes are the key building blocks in the applications of different mode-division multiplexing (MDM) systems. To further increase the transmission capacity, the silicon MWBs for dual polarizations are of particular interest considering the very distinct mode behaviors under different polarizations in the silicon waveguides. Few silicon MWBs suitable for both polarizations have been studied. In this paper, we analyze several dual-polarization-MWBs based on different bending curve functions. These special curve-based silicon MWBs have the advantages of easy fabrication and low loss compared with other structures based on subwavelength structures, such as gratings. A comparison is made between the free-form curve (FFC), Bezier curve, and Euler curve, which are used in the bending region instead of a conventional arc. The transmission spectra of the first three TE and TM modes in the silicon multimode waveguide with a core thickness of 340 nm are investigated. The simulation results indicate that, with the premise of having the same effective radius, which is only 10 μm in this paper, the six-mode MWB based on the FFC has the optimal performances, including an extremely low loss <0.052 dB and low crosstalk below −25.97 dB for all six modes in the wide band of 1500 to 1600 nm. The MWBs based on the Bezier and Euler curve have degraded performances in terms of the loss and crosstalk. The results of this paper provide an efficient design method of the polarization insensitive silicon MWBs, which may leverage research for establishing complicated optical transmission systems that incorporate both the MDM and polarization-DM technology.

Artificial Gauge Field Enabled Low‐Crosstalk, Broadband, Half‐Wavelength Pitched Waveguide Arrays

AbstractDense waveguide arrays with half‐wavelength pitch, low‐crosstalk, broadband, and flexible routing capability are essential for integrated photonics. However, achieving such performance is challenging due to the relatively weaker confinement of dielectric waveguides and the increased interactions among densely packed waveguides. Here, leveraging the artificial gauge field mechanism, half‐wavelength‐pitched dense waveguide arrays, consisting of 64 waveguides, in silicon with −30 dB crosstalk suppression from 1480 to 1550 nm are demonstrated. The waveguide array features negligible insertion loss for 90° bending. This approach enables flexibly routing a large‐scale dense waveguide array that significantly reduces on‐chip estate, leading to a high‐density photonic integrated circuit, and may open up opportunities for important device performance improvement, such as half‐wavelength‐pitch optical phased array and ultra‐dense space‐division multiplexing.

Few-mode optical parametric amplification in a multiple quasi-phase-matched thin-film lithium niobate waveguide

Mode-division multiplexing (MDM) technology, an effective approach to boost the transmission capacity, has seen momentous progress in the past decade. Wideband, low-noise, and power-efficient optical amplifiers for the MDM systems are highly desired. We propose and theoretically demonstrate a broadband few-mode optical parametric amplifier (OPA) using a multimode thin-film periodically poled lithium niobate (PPLN) waveguide with multiple quasi-phase matching (QPM). The few-mode OPA relies on intramodal and intermodal difference-frequency generation (DFG) processes, obtaining strong parametric amplification of MDM signals using a single-frequency fundamental-mode pump. Differential modal gain (DMG) is optimized by adjusting the effective length of parametric interaction for each mode. In addition, parasitic DFG processes to cause nonlinear modal crosstalk is suppressed due to the large phase mismatch ensured by the multiple QPM design. The results show that amplification of signals carrying three modes with >11.2-dB gain per mode over 41 nm bandwidth covering extended C band can be achieved with a 600-mW pump, while the DMG is less than 1.1 dB and nonlinear modal crosstalk is negligible.

Low-insertion-loss femtosecond laser-inscribed three-dimensional high-density mux/demux devices

Recently, transmitting diverse signals in different cores of a multicore fiber (MCF) has greatly improved the communication capacity of a single fiber. In such an MCF-based communication system, mux/demux devices with broad bandwidth are of great significance. In this work, we design and fabricate a 19-channel mux/demux device based on femtosecond laser direct writing. The fabricated mux/demux device possesses an average insertion loss of 0.88 dB and intercore crosstalk of no more than − 29.1 dB. Moreover, the fabricated mux/demux device features a broad bandwidth across the C+L band. Such a mux/demux device enables low-loss 19-core fiber (de)multiplexing over the whole C+L band, showing a convincing potential value in wavelength-space division multiplexing applications. In addition, a 19-core fiber fan-in/fan-out system is also established based on a pair of mux/demux devices in this work.

Polarization and Wavelength Performance Improvement in Large-Scale Silicon Photonics Switches

We review our recent results in multi-port strictly non-blocking optical switches based on silicon photonics. Silicon photonics switches are one of the most important devices to realize future datacenters capable of processing a large amount of data with high efficiency. This is because silicon photonics enables fast (ns - μs) switching, dense integration with high uniformity, low power consumption, and mass production leading to low cost. Thus far, we demonstrated switching, all-paths transmission, and fully-loaded operation of up to 32 input ports × 32 output ports silicon photonics switches. The remaining challenges for practical applications are polarization insensitivity and broadband operation. Regarding the polarization-insensitivity, we presented the non-duplicated polarization-diversity 32 × 32 switch with SiN over path waveguides and demonstrated polarization-insensitive operation in limited paths. Regarding the broadband operation, we achieved ∼90 nm bandwidth for −30 dB crosstalk with polarization-insensitivity in the 8 × 8 switch. We review these, and then discuss how to achieve a large port count (>32 × 32), polarization-insensitivity, and broadband operation simultaneously. Moreover, to broaden the range of applications, integration with wavelength filters and expanding operating wavelengths are important. We demonstrated hybrid integration with the silica platform that is suitable for wavelength filters and the 8 × 8 switch operating in the O-band.

Polarization-insensitive 40-channel 100-GHz spacing fold-back planar echelle grating Mux/Demux for photonic integrated wavelength-selective switches.

We present and numerically demonstrate a novel polarization-insensitive (PI) 40-channel 100-GHz spacing fold-back planar echelle grating (PEG) multiplexer/demultiplexer (Mux/Demux) to realize the compact 1 × 2 crossing-less photonic integrated wavelength-selective switch (PIC-WSS). The PI operation is achieved by a polarization splitter to feed the TE and TM mode into the PEG via two waveguides with different incidental angles so that the two diffracted modes combine at the same output waveguide. By optimizing the design of different input/output angle combinations and sharing the same blazed angle, a single compact PI PEG with fold-back configuration can simultaneously work as twice the Demux and Mux required. The footprint of the single fold-back PI PEG is only ∼40 mm2. The numerical results show that the 40-channel 100-GHz spaced PI fold-back PEG has < 2.4 dB insertion loss, < -60 dB cross talk, zero polarization-dependent wavelength shift (PDWS), 0.3 dB polarization-dependent loss (PDL), < 0.5 dB loss variation and < 0.01 nm wavelength shift between Mux and Demux.

A silicon TM mode multiplexer based on three waveguide directional coupler with subwavelength grating

In this work, a silicon TM mode multiplexer based on three waveguide directional coupler constructed by subwavelength gratings (SWG) is proposed. The working bandwidth is greatly increased by using the SWG waveguides and a mode block based on SWG is added to reduce the crosstalk. FDTD simulation results show that the insertion loss and crosstalk at 1550 nm are 0.7 dB and −35.9 dB respectively. Large working bandwidth of 270 nm (1400 nm∼1670 nm) can be achieved under the conditions of less than 3 dB insertion loss and −15 dB crosstalk. In addition, the fabrication tolerance of the proposed device is also investigated.

Fundamental analyses of fabrication-tolerant high-performance silicon mode (de)multiplexer.

Mode-division multiplexing (MDM) has been extensively exploited to expand the capacity of chip-scale optical interconnects, whereas high-volume manufacturing of on-chip mode (de)multiplexers is still full of challenges. In this paper, we analyze the fabrication errors (sidewall, width, height) of silicon mode (de)multiplexer and present two cases of fabrication-tolerant high-performance silicon mode (de)multiplexer. The presented mode (de)multiplexer could eliminate the mode mismatch and mode hybridization caused by the fabrication errors. When sidewall errors are 0°, 5°, width errors are 0, ± 20, ± 30 nm, and height errors are 0, ± 10 nm, the designed fabrication-tolerant high-performance silicon mode (de)multiplexer can achieve low excess loss less than 2.5 dB and low inter-mode crosstalk less than -15 dB over a broad bandwidth from 1500 to 1600 nm. The obtained results provide an important guideline for designing fabrication-tolerant photonic integrated circuits on silicon platform, which could be applied to the prospective high-volume manufacturing and large-scale integration.

Inverse design of multimode silicon waveguide bends by topology optimization

Compact, multimode, silicon waveguide bends are essential building blocks for high-density, integrated-optics, mode-division-multiplexing (MDM) circuits. However, sharp, multimode, silicon waveguide bends can typically introduce considerable insertion losses and inter-mode crosstalk. Significant efforts have been made to realize compact and high performance multimode silicon waveguide bends, but most of those designs either require complicated fabrication processes or are not really compact. We show that the transmission performances of a regular, ultra-sharp, multimode, silicon waveguide bend can be drastically improved by directly optimizing its topology. The optimization can be efficiently accomplished by using an adjoint-based inverse design. Through this design strategy, we realize a three-mode, silicon waveguide bend that is ultra-compact, with a small radius of 5.5 μm, while having ultra-low average insertion losses of 0.023, 0.046, and 0.068 dB and small average mode crosstalk of −26, −27, and −27 dB for the input modes of TE0 − 2 (TE = transverse electric), respectively, in the wavelength range 1500 to 1600 nm. Also, the waveguide bend can be fabricated through a single electron-beam lithography step. The significant advantages in size and performance of the current waveguide bend show the great potential of the proposed inverse design strategy for designing multimode waveguide bends for MDM systems.

On‐Chip Metamaterial Enabled Wavelength (De)Multiplexer

AbstractWavelength‐division multiplexing (WDM) technology can offer considerable parallelism for large‐capacity data communications. While several configurations have been demonstrated to realize on‐chip WDM systems, their practical applications might be hindered by large footprints or compromised performances. Recently, metamaterial‐assisted silicon photonics is emerging for on‐chip light manipulation by subwavelength‐scale control of optical wavefronts. They can reach more compact footprints and broadband functionalities beyond the classical waveguide‐based architectures. Herein, wavelength (de)multiplexers are experimentally demonstrated in the subwavelength‐structured metamaterials regime with highly compact footprints. Two‐dimensional metamaterials composed of quasi‐periodic dielectric perturbation arrays patterned on a multimode waveguide are proposed to stimulate multiple high‐order waveguide modes at different wavelengths, which are then coupled to different WDM channels by cascading mode (de)multiplexers. The four‐channel wavelength (de)multiplexer is demonstrated in a box‐like spectrum with a compact footprint of 2.5 × 250 µm2, with the measured losses less than 2 dB and channel crosstalk less than –14.3 dB. By varying the patterned metamaterial structures, the proposed devices also have the merits of flexible operating wavelengths and bandwidths. The concept features large scalability, compactness, and competitive performance, which can offer versatile on‐chip light manipulation and significantly improve the integration density for various on‐chip WDM optical systems.

Ultra-Compact Mode (De)Multiplexer and Polarization Beam Splitter Based on Tapered Bent Asymmetric Directional Couplers

Wepropose and experimentally demonstrate the ultra-compact mode (de)multiplexers and polarization beam splitter (PBS) based on tapered bent asymmetric directional couplers. The strong coupling strength introduced by tapered bent waveguides significantly shortens the coupling length. As a result, high coupling and conversion efficiencies over a broad bandwidth can be obtained with small footprint. The coupling length of the fabricated PBS and mode (de)multiplexers are less than 4.9 μm and 8.7 μm, respectively. From 1534-1600 nm, the measured insertion loss (IL) is less than 1.4 and 1.55 dB for the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> -TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> mode (de)multiplexers, respectively, and the crosstalk (CT) is less than −10 dB. The fabricated PBS shows low insertion loss of 0.55 dB and decent crosstalk of < −14 dB for both polarizations over an 85 nm wavelength range from 1505-1590 nm. Meanwhile, the larger fabrication tolerance ensures that the proposed devices can be readily fabricated with the standard complementary metal-oxide-semiconductor (CMOS) process.

Compact Wavelength Selective Crossbar Switch with Cascaded First Order Micro-Ring Resonators

We demonstrate a compact 4×4 wavelength selective switch with 50% fewer electrical signal pads as compared with our previous generation. We report loss and crosstalk for different paths of the switch. We measure median loss of 5.32 dB and worst case crosstalk of −35 dB. The microring resonators tune by more than one free spectral range, which is an improvement over our previous generation of switches. This switch can support 8 channels at 400 GHz spacing. We conclude that it is possible to drive both microring resonators with the same voltage and separate control is not required if the fabrication variation reduces in the future.

A 50nV/√Hz 2‐channel time multiplexed bio‐chopper amplifier with current‐attenuated feedback resistor

This paper presents 2-channel time-multiplexed chopper instrument amplifier for better gain accuracy and mismatch. Mostly, the current-attenuated feedback resistor with a duty-cycled resistor is adopted for boosting the RC time constant. It can achieve the high pass corner frequencies between 10 Hz and 320 Hz by a 3-bit digitally controlled resistor bank of the attenuator. As a result, the instrument amplifier achieves 0.6% gain accuracy between two time-multiplexed channels, 100 dB CMRR, -40 dB crosstalk, and noise densities of 50nV/√Hz. It is implemented in a 180-nm CMOS technology. It occupies a 0.6 mm*0.6 mm chip area and consumes 1.5μA current consumption from a 1.8 V supply for each channel.