Low Crosstalk Research Articles

Miniaturized silicon-based capacitive six-axis force/torque sensor with large range, high sensitivity, and low crosstalk

Miniaturized six-axis force/torque sensors have potential applications in robotic tactile sensing, minimally invasive surgery, and other narrow operating spaces, where currently available commercial sensors cannot meet the requirements because of their large size. In this study, a silicon-based capacitive six-axis force/torque sensing chip with a small size of 9.3 × 9.3 × 0.98 mm was designed, fabricated, and tested. A sandwich decoupling structure with a symmetrical layered arrangement of S-shaped beams, comb capacitors, and parallel capacitors was employed. A decoupling theory considering eccentricity and nonlinear effects was derived to realize low axial crosstalk. The proposed S-shaped beams achieved a large measurement range through stress optimization. The results of a coupled multiphysics field finite-element simulation agreed well with those of theoretical analyses. The test results show that the proposed sensing chip can detect six-axis force/torque separately, with all crosstalk errors less than 2.59%FS. Its force and torque measurement ranges can reach as much as 2.5 N and 12.5 N·mm, respectively. The sensing chip also has high sensitivities of 0.52 pF/N and 0.27 pF/(N·mm) for force and torque detection, respectively.

Efficient mode coupling/(de)multiplexing between a few-mode fiber and a silicon photonic chip

Mode-division multiplexing (MDM) has attracted much attention due to its ability to further increase the transmission capacity of optical interconnects. While further developments of MDM optical interconnects are hindered by the coupling of few-mode fibers (FMFs) and silicon photonic chips, a high-efficiency, broadband, and scalable multimode FMF-chip interface is still eagerly desired. To address this challenge, a novel scheme for efficient multimode coupling is proposed by introducing a silica planar lightwave circuit as an intermediate. The core idea is to couple and demultiplex higher-order modes by leveraging the superiorities of silica optical waveguides for manipulating LP modes, facilitated through tailoring the mode conversion related to different mode symmetric properties. The demultiplexed modes are consequently butt-coupled to the silicon photonic chip in single-mode manner, thus being available for fulfilling further data transmitting/receiving/routing directly. As a proof of concept, a six-channel FMF-chip coupler working with the LP01-x/y, LP11a-x/y, and LP11b-x/y modes is designed with low coupling losses of 0.77–1.39 dB and low intermode crosstalk of <−27.2 dB in a broad bandwidth (>150 nm). Minimum coupling losses of 1.36–2.48 dB are experimentally demonstrated. It is the first demonstration for the integrated multimode FMF-chip coupler enabling the simultaneous coupling of six mode-channels, to the best of our knowledge. We believe that this work has the great potential for developing energy-efficient and low-cost chip-to-chip MDM interconnections in the future.

Sub-millimeter scale 3D integration strategy enables ultrahigh-density and ultralow-crosstalk flexible tactile sensor array for robotic e-skin application

The rapid development of intelligent robots and wearable electronics brings great demand for flexible tactile sensor arrays with high sensitivity, fast response and high spatial resolution. However, simultaneously achieving high sensing performance and low crosstalk remains a fundamental challenge as sensor unit size is further reduced and array density increases. This work proposes a novel design and assembly strategy based on a mechanical microlattice structure, combined with in-situ laser direct writing and batch transfer techniques, to address the challenges from sub-millimeter active material array fabrication to 3D tactile sensor array integration. Additionally, the one-step preparation of high-consistency microcones on flexible substrates during laser direct writing enables the sensor to achieve a high sensitivity of 2.68 kPa−1 and a rapid response time of 50 ms. Both finite element simulation and experiments validate that the meticulously engineered mechanical microlattice structure, which incorporates a multi-material Young’s modulus gradient design, effectively concentrates external pressure on the pressed sensor units while shielding unpressed units, significantly mitigating the crosstalk and facilitating an ultrahigh resolution of ∼ 1 mm. Finally, the multipoint precision detection capability of the fabricated ultrahigh-density (over 113 units/cm2) tactile sensor array is verified as robotic e-skin for object grasping and pattern recognition.

Low crosstalk, low Pπ silicon optical switch based on highly balanced couplers and folded phase shifters

Low crosstalk, low Pπ silicon optical switch based on highly balanced couplers and folded phase shifters

Enhanced three-channel dual-polarization silicon photonic balanced receiver for long-range FMCW laser ranging systems

We present an improved three-channel dual-polarization silicon photonic balanced receiver specifically designed for frequency-modulated continuous-wave (FMCW) ranging systems. By leveraging silicon photonics technology, we have integrated previously discrete functional devices onto a single chip, significantly reducing system volume and enhancing integration. To address the limitations of existing multi-channel balanced receivers, we performed a systematic analysis and optimization of the receiver’s components. Our optimizations included enhancing photodetector responsivity, minimizing dark current, refining layout and wire bonding, optimizing packaging coupling processes, and improving the bandwidth and noise characteristics of the receiving link. As a result, we developed a germanium-silicon photodetector with a high responsivity of ∼ 1.09 A/W, minimal dark current of ∼ 4 nA, and bandwidth of 28 GHz. Furthermore, we realized a 3-channel dual-polarization balanced receiver chip using 130 nm CMOS technology, achieving low loss and crosstalk through layout optimization and effective packaging. The receiver was validated through an FMCW ranging system setup, demonstrating a ranging capacity exceeding 180 m across all three channels, outperforming previous works. Our receiver satisfies the potential demands of long-range and high-resolution FMCW ranging, particularly relevant for automotive LiDAR applications.

Highly-efficient (>70%) and Wide-spectral (400–1700 nm) sub-micron-thick InGaAs photodiodes for future high-resolution image sensors

This paper demonstrates the novel approach of sub-micron-thick InGaAs broadband photodetectors (PDs) designed for high-resolution imaging from the visible to short-wavelength infrared (SWIR) spectrum. Conventional approaches encounter challenges such as low resolution and crosstalk issues caused by a thick absorption layer (AL). Therefore, we propose a guided-mode resonance (GMR) structure to enhance the quantum efficiency (QE) of the InGaAs PDs in the SWIR region with only sub-micron-thick AL. The TiOx/Au-based GMR structure compensates for the reduced AL thickness, achieving a remarkably high QE (>70%) from 400 to 1700 nm with only a 0.98 μm AL InGaAs PD (defined as 1 μm AL PD). This represents a reduction in thickness by at least 2.5 times compared to previous results while maintaining a high QE. Furthermore, the rapid transit time is highly expected to result in decreased electrical crosstalk. The effectiveness of the GMR structure is evident in its ability to sustain QE even with a reduced AL thickness, simultaneously enhancing the transit time. This breakthrough offers a viable solution for high-resolution and low-noise broadband image sensors.

Femtosecond laser-inscribed in-fiber Mach-Zehnder interferometer for ultra-sensitive small-angle torsion measurement

We propose and demonstrate a novel in-fiber Mach-Zehnder interferometer (MZI), using a femtosecond (FS) laser to inscribe two parallel straight waveguides along both sides of the core-cladding boundary of an optical fiber. Both simulations and experiments demonstrated that constructing a second straight waveguide (WG2) on the opposite side of the fiber core significantly enhances the mode coupling efficiency in fiber, resulting in ultra-sensitive small twist-angle torsion measurement by intensity interrogation. By finely adjusting the waveguide spacing between the WG2 and the fiber core during the FS fabrication process, a large tuning range of ∼ 200 nm for the designable interference wavelength with maximal fringe contrast has been achieved. Through comparative experiments, the proposed in-fiber MZI performs ultra-sensitivity with 17417.92 dB/(rad/mm) over small twist angles from −10° to 0° for the sample “WG1-4/WG2-4” and 19480.57 dB/(rad/mm) over small twist angles from 0° to 10° for the sample “WG1-4/WG2-6”. Besides, the device is insensitive to temperature and strain. The exhibited advantages of high sensitivity, high reproducibility, and low crosstalks promote the proposed in-fiber MZI to be applied in future industrial engineering monitoring and intelligent health monitoring.

Liquid Crystal Display with High Transmittance and Excellent Image Quality

High transmittance and excellent image quality are two crucial requirements for thin-film transistor liquid crystal displays (TFT-LCDs). This work presents a novel pixel structure utilizing a vertical alignment liquid crystal (VA-LC) to improve both transmittance and image quality. The new pixel structure, called TCL-pixel, features a unique transparent conducting layer (TCL) that constructs an electric field shielding layer and a large transparent storage capacitor, resulting in good stability and a large aperture ratio. The 55" 8K product incorporating the TCL-pixel achieves high transmittance (3.87%) and a high contrast ratio (5100:1), attributed to the large aperture ratio. Additionally, it exhibits negligible vertical cross talk (0.7%), low color cross talk (3‰), low horizontal cross talk (JND 2.3), and enhanced variable refresh rate performance, confirming its good stability. Furthermore, the TCL-pixel demonstrates an improved viewing angle (CESI 0.03：65°) due to the multi-domain alignment produced by the transparent conducting layer.

High-performance and scalable polarization splitter-rotator based on photonic crystal nanobeam reflection.

The polarization splitter-rotator (PSR) is a key device for polarization processing in polarization diversity systems, which has wide applications in achieving polarization independence and mixed multiplexing. However, it remains a significant challenge to simultaneously achieve a better balance in bandwidth, crosstalk (CT), polarization extinction ratio (PER), and compact footprint of the PSR. In this article, a photonic crystal nanobeam (PCN) structure is introduced to PSR for large bandwidth and compact size, with a device length of only 104 µm. Additionally, to achieve lower CT, a bridge waveguide is introduced for primary filtering. Simulation results show that the insertion loss (IL) is less than 0.55 dB, CT less than -35 dB, and PER greater than 35 dB within a bandwidth exceeding 110 nm, while maintaining a large process tolerance. Furthermore, the proposed PSR design breaks through the limitations of traditional schemes by extending its functionality effectively. To further improve integration, a novel approach to PSR using mode hybridization followed by spatial beam splitting is proposed. By controlling the phase-matching condition of various modes in different waveguides, the designed spatial beam splitting achieves lower CT and better compactness. Simulation results verify that the IL of the improved scheme is less than 1 dB, CT less than -24 dB, and PER greater than 22 dB within an 85 nm bandwidth, while reducing the overall length to less than 20 µm.

Sensitivity-enhanced optical fiber probe-type humidity sensor based on CaAlg film

Optical fiber probe-type humidity sensors based on calcium alginate (CaAlg) hydrogel films are proposed. The sensor is based on the Michelson interferometer (MI) and constructed by splicing a tapered multimode fiber (MMF) in the middle of two single-mode fibers (SMFs). One of the 1.5 cm long SMFs is coated with CaAlg hydrogel film and can be regarded as the sensor probe. The sensitivity of the sensor is 0.31015 nm/%RH. The high sensitivity, low temperature crosstalk, compact probe structure, and stability of the sensor allow it to be used for humidity monitoring in a variety of complex environments.

Temperature-compensated optical fiber sensor for urea detection based on the femtosecond laser-inscribed process

This study demonstrates a low crosstalk temperature-compensated fiber-based urea sensor based on the hybridization of surface plasmon resonance (SPR) and Mach-Zehnder interferometer (MZI). In this sensing structure, an inscribed waveguide and a semi-open cavity are processed by a femtosecond laser in an optical fiber to excite an SPR signal sensitive to refractive index (RI) and an MZI signal highly sensitive to temperature. Based on the temperature detected by the MZI, the influence of temperature on the SPR signal’s RI detection can be calculated and compensated for using a sensitivity matrix. Based on this precise RI detection of the SPR signal, we have achieved selective urea detection by bio-functionalizing the sensor surface using the urease self-assembly method. The sensitivity and limit of detection for urea are 7.98 (nm/mmol/L) and 0.13 mmol/L, respectively, which are better than those in previous studies. Compared to previous temperature compensation studies, this SPR + MZI sensing method using femtosecond laser processing significantly improves the temperature sensitivity of the MZI signal, which reduces crosstalk and ensures a sufficient working range for the SPR signal. Therefore, this sensor is crucial for advancing high-precision temperature compensation detection, high-sensitivity biological detection, and the future application of the femtosecond laser technology.

Highly sensitive threaded LPFG strain sensor

Long Period Fiber Grating (LPFG) strain sensors have been used to monitor the structural health of bridges and buildings, and there is a need to further improve their sensing sensitivity. This article proposed a threaded LPFG strain sensor, which used a CO2 laser to etch a long groove at the top of the cladding of a single-mode fiber. The fiber in the long groove area was heated and twisted to fabricate a long period fiber grating (TH-LPFG) strain sensor with a threaded groove on the cladding. When strain is applied, the strain is concentrated in the threaded area on the cladding, increasing and decreasing the effective refractive index changes of the cladding mode and core mode respectively, achieving enhanced strain sensing. The experimental outcomes demonstrate that the TH-LPFG strain sensor achieves a sensitivity of 44.9 pm/µε to strain, while its cross-sensitivity to temperature is minimal, at just 1.28 µε/°C. TH-LPFG provides a new solution for enhancing the sensitivity of LPFG strain sensing, with low production cost and low temperature crosstalk. It can be used in the field of highly sensitive strain monitoring of building structures.

4H-SiC ultraviolet photodetector array with vertical MSM configuration

Abstract The increasing demand for ultraviolet (UV) imaging in extreme conditions, such as high temperatures and strong radiation, has spurred advancements in UV photodetector arrays. Traditional metal-semiconductor- metal (MSM) 4H-SiC UV photodetector array, with their planar structure and M×N electrode connections, face challenges in circuit design. Our research utilizes a vertical design for building the photodetector array, reducing the connections to just M+N, thereby simplifying the circuit design and signal processing. Utilizing semi-insulating 4H-SiC wafer and TiN electrodes, we developed an 8×8 vertical MSM photodetector array. Tested under a 365 nm light source at 10.5 mW/cm2 and a 5V bias, the array demonstrated low dark currents, high contrasts under illumination, and a 100% operational yield. With average photo current and dark currents of 1.56×10-8 A and 2.94×10-13 A, respectively, the average photo-to-dark current ratio (PDCR) exceeded 5×104 Our design effectively minimized sneak path currents and achieved a low crosstalk rate of 0.46%, enabling the capture of clear, high-contrast images. This marks a significant advancement in the application of MSM 4H-SiC UV photodetectors for imaging in extreme conditions.

Non-volatile and Secure Optical Storage Medium with Multilevel Information Encryption.

Non-volatile photomemory based on photomodulated luminescent materials offers unique advantages over voltage-driven memory, including low residual crosstalk and high storage speed. However, conventional materials have thus far been volatile and insecure for data storage because of low trap depth and single-level storage channels. Therefore, the development of a novel non-volatile multilevel storage medium for data encryption remains a challenge. Herein, a robust, non-volatile, multilevel optical storage medium is reported, based on a photomodulated Ba3MgSi2O8:Eu3+, which combined the merits of light-induced valence (Eu3+ → Eu2+) and photochromic phenomena using optical stimulation effects, accompanied by larger luminescent and color contrasts (>90%). These two unique features provided dual-level storage channels in a single host, significantly improving the data storage security. Notably, dual-level optical signals could be written and erased simultaneously by alternating 265 and 365 nm light stimuli. Theoretical calculations indicated that robust color centers induced by intrinsic interstitial Mg and vacancy defects with suitable trap depths enable excellent reversibility and long-term storage capability. By relying on different luminescent readout mechanisms, the encrypted dual-level information can be accurately decrypted by separately probing the Eu2+ and Eu3+ signals, thus ensuring information security. This study proposes a novel approach for constructing multilevel information storage channels for information security.

Double-Layer Metasurface Integrated with Micro-LED for Naked-Eye 3D Display.

Naked-eye 3D micro-LED display combines the characteristics of 3D display with the advantages of micro-LED. However, the 3D micro-LED display is still at the conceptual stage, limited by its intrinsic emission properties of large divergence angle and non-coherence, as well as difficulties in achieving large viewing angles with high luminous efficiency. In this work, we propose a double-layer metasurface film integrating functions of collimation with multiple deflections, constituting a micro-LED naked-eye 3D display system. The system is characterized through numerical simulations using the 3D finite-difference time-domain method. The simulation results show that the double-layer metasurface film restricts 90% of the emitted light of the micro-LED to the vicinity of the 0° angle, improving its spatial coherence. Subsequently, a large-angle, low-crosstalk outgoing from -45° to 45° is achieved, while providing a deflection efficiency of over 80% and a pixel density of up to 605. We believe this design provides a feasible approach for realizing naked-eye 3D micro-LED displays with a large field of view, low crosstalk, and high resolution.

Ultra-compact silicon nitride dual-polarization mode converter for visible light demonstrated on a thinned 320 nm silicon wafer

A dual-polarization higher-order mode converter is proposed for visible light, which operates within the (400–700) nm range, to enhance data capacity in on-chip visible light communication systems. The proposed structure is optimized at wavelengths of 410 nm, 560 nm, and 660 nm to cover the entire visible spectrum. These three optimized designs, in particular, have an intrinsic property wherein they can be optimized to simultaneously convert both polarized fundamental (TE0) and fundamental (TM0) modes to the second higher-order (TE2) and (TM2) modes, providing significant advantages for hybrid multiplexing systems like PDM-MDM and PDM-WDM schemes. The mode converters are constructed on a silicon nitride waveguide. This waveguide is etched and filled with silicon dioxide material to create two dielectric substrips, followed by an additional rectangle shape etched using the same process into the propagating waveguide. This alteration enhances insertion loss and diminishes crosstalk to the fundamental mode. The devices achieve a broad operating bandwidth of approximately 100 nm while maintaining a compact footprint of only 1µm×1.754µm for the entire device at the center wavelength of 560 nm. Upon optimizing the suggested structure, a TE0-to-TE2 mode converter with a modal conversion efficiency of 94% is designed at the wavelength of 410 nm. The insertion loss is 0.6025 dB, and the crosstalk with the transverse electric fundamental mode TE0 is 35 dB. The reported devices feature a straightforward structure with low insertion loss and minimal crosstalk.

Terahertz Metal-Graphene Composite Plasmon Waveguides Combining Strong Field Confinement with Low Cross-Talk: A FEM Study

Terahertz Metal-Graphene Composite Plasmon Waveguides Combining Strong Field Confinement with Low Cross-Talk: A FEM Study

Sodium-based plasmonic waveguides with high confinement factors and ultra-low gain thresholds.

The noble metal-based hybrid plasmon mode features low loss and strong field localization, making it widely applicable in the field of nanophotonic devices. However, due to the high loss of noble metals, the gain threshold is unacceptably high, usually larger than 0.1 µm-1. Here we present a hybrid plasmonic waveguide consisting of a SiO2 layer coated Na nanowire and a hexagonal semiconductor nanowire. Based on the high performance of the proposed waveguide, the Purcell factor exceeding 120 and a confinement factor above 90% are achieved, leading to an ultra-low gain threshold of 0.02117 µm-1. In addition, the proposed waveguide exhibits an extremely low cross talk, making it highly suitable for applications in compact photonic integrated devices. The proposed waveguide may contribute to the development of low-threshold nano-lasers and promote other applications in nanophotonics.

Deep learning and genetic algorithm driven accelerated design for frequency-multiplexed complex-amplitude coding meta-hologram.

Frequency-multiplexed metasurfaces represent a significant innovation in breaking the functional limitations of traditional metasurfaces, showing immense potential in multi-channel communication. However, existing frequency-multiplexed metasurfaces primarily focus on pure phase and linear polarization modulation, neglecting the modulation for complex amplitude and circularly polarized waves. Additionally, crosstalk suppression between dual-frequency channels often requires meticulous tuning of the meta-atom structure. Therefore, manually designing a set of meta-atoms that satisfies both complex amplitude modulation and low crosstalk at dual frequencies is extremely challenging and time-consuming. Here, we utilize the method of deep learning and genetic algorithm to design a kind of meta-atom capable of bi-spectral 2-bit amplitude and arbitrary phase modulation, which greatly reduces the design difficulty and achieves excellent low-crosstalk performance. This method can be easily generalized to the design of other complex meta-atoms to improve the design efficiency. Furthermore, we propose a frequency-multiplexed complex-amplitude coding meta-hologram for modulating left-handed circularly polarized (LCP) waves. When illuminated with LCP light, it can reconstruct two distinct holographic images at two different frequencies in the near field with high quality. The independent modulation capability of the metasurface for multiple degrees of freedom of frequency, amplitude and phase gives it broad application prospects in multi-channel communication, data storage and perfect holography.

Multi-plane multiplicative-noise-multiplexing holography with low crosstalk assisted by temporal multiplexing.

Multi-plane holography has attracted increasing interest for reconstructing depth information. However, achieving multi-plane holography with high capacity and low crosstalk is always highly desired. Here, we proposed and demonstrated a novel multi-plane holography based on multiplicative noise multiplexing and temporal multiplexing. By utilizing the orthogonality inherent between multiplicative noise phases, the proposed holography enables the image reconstruction with simultaneous combination of multi-plane and multiplexing dimensions. The integration of temporal multiplexing contributes to further improve the signal-to-noise ratio (SNR). Both simulation and experimental results have demonstrated that, by introducing the noise engineering, this holography can enhance the information capacity and significantly lower the inter-plane and inter-channel crosstalk. This promising holography has the potential in the fields of ultrahigh-capacity 3D display, information storage, and information encryption.