Single-mode Research Articles

Improved performance of a fiber-optic hydrogen sensor based on a controllable optical heating technology.

A novel, to the best of our knowledge, and compact fiber-optic hydrogen sensor based on light intensity demodulation and controllable optical heating technology is proposed and experimentally investigated. This system employs three photodetectors for optic signal transformation. The first PD is used to receive a little fraction of the amplified spontaneous emission (ASE) for calibration, and the second PD is utilized to detect optic signal reflected by a single mode fiber deposited with WO3-Pd2Pt-Pt composite film. The last PD is utilized to receive the optical power reflected by the short fiber Bragg grating (SFBG) with a central wavelength located in a steep wavelength range (the intensity decreases approximately linearly with the increase of the wavelength) of the ASE light source. A 980 nm laser and proportion integration differentiation (PID) controller were employed to ensure the hydrogen sensitive film working at an operating temperature of 60°C. This sensing system can display a quick response time of 0.4 s toward 10,000 ppm hydrogen in air. In addition, the detection limit of 5 ppm in air can be achieved with this sensing system. The stability of this sensor can be greatly enhanced with a controllable optical heating system, which can greatly promote its potential application in various fields.

High-efficiency self-focusing metamaterial grating coupler in silicon nitride with amorphous silicon overlay

Efficient fiber-chip coupling interfaces are critically important for integrated photonics. Since surface gratings diffract optical signals vertically out of the chip, these couplers can be placed anywhere in the circuit allowing for wafer-scale testing. While state-of-the-art grating couplers have been developed for silicon-on-insulator (SOI) waveguides, the moderate index contrast of silicon nitride (SiN) presents an outstanding challenge for implementing efficient surface grating couplers on this platform. Due to the reduced grating strength, a longer structure is required to radiate the light from the chip which produces a diffracted field that is too wide to couple into the fiber. In this work, we present a novel grating coupler architecture for silicon nitride photonic integrated circuits that utilizes an amorphous silicon (α-Si) overlay. The high refractive index of the α-Si overlay breaks the coupler’s vertical symmetry which increases the directionality. We implement subwavelength metamaterial apodization to optimize the overlap of the diffracted field with the optical fiber Gaussian mode profile. Furthermore, the phase of the diffracted beam is engineered to focalize the field into an SMF-28 optical fiber placed 55 µm above the surface of the chip. The coupler was designed using rigorous three-dimensional (3D) finite-difference time-domain (FDTD) simulations supported by genetic algorithm optimization. Our grating coupler has a footprint of 26.8 × 32.7 µm2 and operates in the O-band centered at 1.31 μm. It achieves a high directionality of 85% and a field overlap of 90% with a target fiber mode size of 9.2 µm at the focal plane. Our simulations predict a peak coupling efficiency of − 1.3 dB with a 1-dB bandwidth of 31 nm. The α-Si/SiN grating architecture presented in this work enables the development of compact and efficient optical interfaces for SiN integrated photonics circuits with applications including optical communications, sensing, and quantum photonics.

High-power and narrow-linewidth nanosecond pulsed intracavity crystalline Raman laser operating at 1.7 µm

We report on a high-power and narrow-linewidth nanosecond pulsed intracavity crystalline Raman laser at 1.7 µm. Driven by an acousto-optically Q-switched 1314 nm two-crystal Nd:YLF laser, the highly efficient cascaded YVO4 Raman laser at 1715nm was obtained within the well-designed L-shaped resonator. Thanks to the absence of spatial hole burning in the stimulated Raman scattering process, significant spectral purification of second-Stokes radiation was observed by incorporating a fused silica etalon in the high-Q fundamental cavity. Under the repetition rate of 4 kHz, the highest average output power for single longitudinal mode operation was up to 2.2 W with the aid of precision vibration isolation and precision temperature controlling, corresponding to the pulse duration of ∼2.8 ns and the spectral linewidth of ∼330 MHz. Further increasing the launched pump power, the second-Stokes laser tended toward be always multimode, and the maximum average output power amounted to 4.8 W with the peak power of ∼0.8 MW and the spectral linewidth of ∼0.08 nm. The second-Stokes emission was near diffraction limited with M2 < 1.4 across the whole pump power range.

Humidity Sensing Using a Multimode Fiber Ring Laser with Thermal Compensation

We propose a multimode fiber laser sensor utilizing PI-SMF (polyimide-coated single mode fiber) for low-error relative humidity (RH) measurement, which is temperature compensated based on FBG. The PI-SMF in the laser cavity is used as a sensing element, and its length varies with humidity and temperature by volume-variation induced strain, which leads to frequency shift of the longitudinal mode beat frequency signal (BFS). When the 2000 MHz BFS is selected as the sensing signal, a RH sensitivity of −2.68 kHz/%RH and a temperature sensitivity of −14.05 kHz/°C are achieved. The peak shift of the FBG-based laser emission spectrum is only sensitive to temperature rather than RH with a temperature sensitivity of 9.95 pm/°C, which is used as the temperature compensation for RH measurements. By monitoring the response of the BFS and the laser wavelength, the cross-sensitivity effect of RH and temperature is overcome, and low-error RH measurement in the temperature range of 20 to 65 °C is realized with errors within ±0.67 %RH (25 to 85 %RH). The scheme does not require the design and production of complex structures and hygroscopic material coating processes, owning the advantages of simple structure, easy operation and high accuracy, and is expected to be practically applied in food safety and environmental monitoring.

Mining Multimodal Travel Mobilities with Big Ridership Data: Comparative Analysis of Subways and Taxis

Understanding traveler mobility in cities is significant for urban planning and traffic management. However, most traditional studies have focused on travel mobility in a single traffic mode. Only limited studies have focused on the travel mobility associated with multimodal transportation. Subways are considered a green travel mode with large capacity, while taxis are an energy-consuming travel mode that provides a personalized service. Exploring the relationship between subway mobility and taxi mobility is conducive to building a sustainable multimodal transportation system, such as one with mobility as a service (MaaS). In this study, we propose a framework for comparatively analyzing the travel mobilities associated with subways and taxis. Firstly, we divided taxi trips into three groups: competitive, cooperative, and complementary. Voronoi diagrams based on subway stations were introduced to divide regions. An entropy index was adopted to measure the mix of taxi trips. Secondly, subway and taxi trip networks were constructed based on the divided regions. The framework was tested based on the automatic fare collection (AFC) data and global positioning system (GPS) data of a subway in Beijing, China. The results showed that the proportions of taxi competition, taxi cooperation, and taxi complements were 9.1%, 35.6%, and 55.3%, respectively. The entropy was large in the central city and small in the suburbs. Moreover, it was found that the subway trip network was connected more closely than the taxi network. However, the unbalanced condition of taxis is more serious than that of the subway.

Conductive dual-network hydrogel-based multifunctional triboelectric nanogenerator for temperature and pressure distribution sensing

Conductive hydrogels are gaining attention in flexible electronics due to their high electrical conductivity and stretchable mechanical properties. Utilizing conductive hydrogels as electrodes in triboelectric nanogenerators (TENG) offers a promising avenue for developing versatile, flexible devices. However, the preparation of a multifunctional hydrogel-based TENG remains a challenging task. Here, a dual-network hydrogel TENG is denoted as DNH-TENG. It comprises poly(3,4-ethylenedioxythiophene): poly (styrene sulfonate) (PEDOT: PSS)/poly (vinyl alcohol) (PVA)/carboxylated multi-walled carbon nanotube (MWCNT-COOH) and offers the advantages of straightforward fabrication, versatile applications, and efficient output performance. Preparing a double network by combining a conductive network with a soft network improves the tensile properties but reduces its conductivity significantly. Further doping MWCNT-COOH with the dual-network leads to a conjugation effect and hydrogen bonding. This enhancement fortifies the hydrogel structure and augments the electrode's conductivity and mechanical properties. In single electrode mode, the DNH-TENG exhibits a short-circuit current of 16.2 μA, a transfer charge of 97.3 nC, and an open-circuit voltage of 270.5 V. Furthermore, DNH-TENG exhibits an impressive stretchability, allowing it to be extended to 566%. Using DNH-TENG's exceptional stretchability and ultrahigh sensitivity to mechanical and temperature stimuli, we designed DNH-TENG-based sensor arrays to showcase temperature and pressure distribution monitoring applications.

A multiparameter sensing spectroscopic system for remote plastic sorting with a single laser shot: a first information report

The spectroscopic methods have been widely applied for plastic characterization and sorting to assist the recycling process. However, most of the reported works have been carried out in conventional laboratory conditions. In this study, we discuss development and application of a single-shot standoff multimodal LIBS-Raman spectroscopy system for plastic identification by modifying existing optical models for such analyses. By utilizing a unique co-axial back-collection optical geometry in standoff signal collection and a direct signal coupling scheme, the system was able to sense LIBS-Raman signals from plastics in single laser shot mode within 20 ms, with a non-gated CCD detector remotely. We examine the application of the system in characterizing four commodity plastic types of polyethylene terephthalate (PET), polypropylene (PP), polyethylene (PE), and polystyrene (PS)) based on atomic emissions (LIBS) and molecular structure (Raman Spectroscopy) by analyzing the respective standoff spectral data recorded from a distance of 4 m. The capability of the system to classify plastics based on standoff data is evaluated by using principal component analysis (PCA), which shows good segregation of the plastic classes depending on the intrinsic spectral features.

Personalized driving behavior oriented autonomous vehicle control for typical traffic situations

autonomous driving systems not only provide services for human drivers, but also need to consider the personalized driving requirements of human beings. In current road traffic environments, the driving behaviors of drivers differ significantly. This suggests that the various needs of different drivers cannot be met by a single behavior mode in an autonomous driving decision-making system. This paper looks at the personalized characteristics of various drivers and considers the implications of their differences. First, a Proportional Integral Differential (PID) feedback channel is introduced in a traditional Model Predictive Control (MPC) to improve the performance of the controller, and comprehensively considering the collision risk, motion hysteresis and rule constraints, referring to the MPC idea, a collision avoidance method based on Q-ABSAS optimization is proposed. Then based on the Chance Constrained Programming, the control constraint is combined with driver personalization to reflect a variety of driving personality characteristics. Finally, the proposed method is tested using Hardware-in-the-loop (HIL) experiments. The experiment results demonstrate that the proposed method can successfully implement vehicle tracking control and make the vehicle's state of movement match the driver's expectations, which can increase driver comfort and driving safety.

Mixed bound states in the continuum: Disclosing BIC’s content via bulk normal modes

We present a nonperturbative theory of bound states in the continuum (BICs) based on the analytical theory of infinite-grating eigenmodes in the case of a planar grating waveguide, that is, 1D photonic crystal slab. We describe a mixed BIC mainly formed by a coherent mixture of two photonic-crystal normal modes in a vicinity of their avoided crossing where both of them have a nonzero coupling with a radiation-loss channel. We reveal the universal properties of the mixed BIC associated with the spatial parity symmetry breaking and self-similar spectral behavior of the bulk normal modes near Γ-point. We show that the mechanism of the mixed-BIC formation constitutes a special kind of the generic Friedrich–Wintgen mechanism. A cutoff from the radiation-loss channel and extremely high-Q narrow resonance are achieved due to the destructive interference of the two crossing normal modes. On the contrary, a conventional symmetry-protected BIC is formed mostly by a single photonic-crystal normal mode which has vanishing coupling Fourier coefficient with every available radiation-loss channel. Implementation of the mixed BICs in various photonic-crystals structures could lead to interesting applications.

How – and How Much? An Analysis of Major Conflict Lines Regarding the Transformation of German Animal Farming

Based on the concept of a 'future workshop', this paper contributes to the understanding of major conflict lines and future pathways for animal farming in Germany. Participants in the future workshop were tasked with outlining their ideal vision of the future of animal farming, focusing on both its mode and quantity. We found three different types of visions differing in their motivation, farming methods, and the scale of envisioned animal farming: ‘Higher Standards’ (V1), ‘System Change and Reduction’ (V2), ‘Alternatives to Animal Farming’ (V3). V1 seeks to secure the quantities of supply and demand of animal products in Germany with minor adjustments towards improved animal welfare and investments in resource-efficiency and climate friendliness. V2 advocates for comprehensive social and environmental systemic changes throughout the agri-food sector, with substantially fewer animals involved. V3 aims to replace animal farming, with plant-based and other alternatives, whenever and wherever possible. Our findings suggest that there is little ground for a societal consensus on a single future mode and scale of animal farming. Stakeholder dialogues and policy initiatives should therefore allow for different pathways while focusing on compromises in the near future. Short-term solutions may be acceptable to supporters of all three visions, however, achieving this requires reframing the debate from ‘securing the future of animal farming’ to ‘guiding the future of animal farming’, acknowledging the potential – albeit partly – replacement of animal farming by suitable alternatives.

New energy levels of atomic holmium discovered by laser spectroscopy in the near infrared spectral range

A combination of optogalvanic spectroscopy and laser-induced fluorescence spectroscopy with a continuous wave tunable single mode Ti-Sa laser was applied to find new energy levels of atomic holmium. Free and excited holmium atoms were produced in a liquid nitrogen-cooled hollow-cathode lamp. In this study, we examined 35 spectral lines of Ho I in the near-infrared wavelength range from 698 nm to 833 nm. Altogether, 16 new energy levels have been discovered, 5 levels of even parity and 11 levels of odd parity. Another four energy levels have been discovered but have already been published by another research group. Data on the hyperfine structure for the new levels are presented. Additionally, hyperfine structure data of three known levels of even parity were determined for the first time. The existence of the new energy levels was confirmed by analyzing 60 lines of previously measured Fourier transform spectra to precisely determine the energy of the levels.

Investigating changes in the electric field profiles of transverse modes in fibres due to bending

AbstractA model is presented for calculating the electric field profiles of transverse modes in fibres. Using this model, the electric field propagation of transverse modes was calculated and compared in both bent and straight Yb and Ho fibres. Our simulations revealed that the field intensities of the modes did not consistently increase or decrease by bending as expected, but rather fluctuated as the bending radii of the fibres changed from 1 to 100 cm. Additionally, it was observed that at certain bending radii, the modes of the fibres became deformed and transitioned into higher modes. Furthermore, the extent to which the modes stretched towards cladding due to bending was calculated. To provide a more comprehensive analysis of the results and investigate the effects of the physical parameters of the fibres on the bent fibre modes, simulation results were presented for two different V numbers and core radii.

Geometric-phase-based phase-knife mask for stellar nulling and coronagraphy

Exoplanets can be detected very close to stars using single-mode cross-aperture nulling interferometry, a photonic technique that relies on the inability of an anti-symmetric stellar point-spread function to couple to the symmetric mode of a single-mode fiber. We prepared an asymmetric field distribution from a laboratory point source using a flat geometric-phase-based pupil-plane phase-knife mask comprised of a planar liquid crystal polymer layer with orthogonal optical axes on opposite sides of a linear pupil bisector. Our mask yielded an on-axis laboratory point-source rejection (i.e., an interferometric “null depth”) of 2.2 × 10−5. Potential mask modifications to better reject starlight are described that incorporate additional phase regions to spatially broaden the rejection area, and additional layers to spectrally broaden the rejection. Also discussed is a topological correspondence between the spatial configurations of separated-aperture nullers, cross-aperture nullers and full-aperture phase coronagraphs.

Direct investigations of interactions between nucleolins and aptamers on pancreatic cancer and normal cells by atomic force microscopy

Nucleolin is overexpressed on the surface of pancreatic cancer cells and are regarded as the remarkable therapeutic target. Aptamers are capable of binding the external domain of nucleolin on the cell surface with high affinity and specificity. But nucleolin has not been localized on pancreatic cancer cells at very high spatial resolution, and the interactions between nucleolin and aptamers have not been investigated at very high force resolution level. In this work, nucleolin was localized on pancreatic cancer and normal cells by aptamers (9FU-AS1411-NH2, AS1411-NH2 and CRONH2) in Single Molecule Recognition Imaging mode of Atomic Force Microscopy. There are plenty of nucleolin on the surfaces of pancreatic cancer cells (area percentage about 5 %), while there are little nucleolin on the surfaces of normal cells. The interactions between three types of aptamers and nucleolins on the surfaces of pancreatic cancer cells were investigated by Single Molecule Force Spectroscopy. The unbinding forces of nucleolins-(9FU-AS1411-NH2) are larger than nucleolins-(AS1411-NH2). The dissociation activation energy on nucleolin-(9FU-AS1411-NH2) is higher than nucleolin-(AS1411-NH2), which indicates that the former complex is more stable and harder to dissociate than the later complex. There are no unbinding forces between nucleolin and CRONH2. All these demonstrate that nucleolin was localized on pancreatic cancer and normal cells at single molecule level quantitatively, and the interactions (unbinding forces and kinetics) between nucleolin and aptamers were studied at picoNewton level. The approaches and results of this work will pave new ways in the investigations of nucleolin and aptamers, and will also be useful in the studies on other proteins and their corresponding aptamers.

First principles investigation of ammonia borane decomposition for understanding h-BN growth on the copper surface

Ammonia borane (AB) is a highly promising precursor for the large-scale growth hexagonal boron nitride (h-BN) via chemical vapor deposition (CVD) methods. However, the unclear decomposition mechanism of precursor molecules on the Cu surface poses a great challenge for understanding the CVD growth process of h-BN. In this work, we addressed this issue by conducting first-principles calculations to investigate the decomposition mechanism of AB on the Cu surface. Our calculations revealed that AB preferentially decomposed into ammonia and borane. Further analysis demonstrates that borane exhibits a higher dehydrogenation rate than ammonia, suggesting a single atom self-assembly growth mode of h-BN under B-rich conditions. Our calculations revealed the formation of BNHx-related byproducts during h-BN growth using AB as a precursor, resulting from the intricate decomposition of AB on the Cu surface under CVD conditions. Our findings have significant implications for comprehending and controlling the growth of h-BN in CVD process.

Numerical investigation on dynamic characteristics of oil-water annular flow-induced vibration in curved pipes

To improve the stability of water-lubricated transportation and reliability of pipes in service, the fluid–structure interaction numerical model has been established to investigate the dynamic characteristics of oil-water annular flow-induced vibration in curved pipes and the effect of main parameters in this study. The results illustrate that a single vibration mode of curved pipes is excited by oil-water annular flow, and there exists no multi-modality due to the limited pulsation of pressure and interface of two-phase flow. The change of velocities and oil-water ratios leads to change of flow pattern, making the dynamic response severer. When the oil-water ratio is larger than 2.035, its effect is greater than velocity. The physical properties of oil have a significant effect on the dynamic response. The root-mean-square dimensionless displacement AY,RMS/D and maximum dimensionless displacement AY,MAX/D of fuel oil-water annular flow-induced vibration decrease to 75.6% and 76.5% respectively, which means that the increase of dynamic viscosity reduces the fluid force and suppresses the vibration. AY,RMS/D and AY,MAX/D increases by 1.62 times when the bending angle θ increases from 30° to 90°, indicating that the increase of bending angles also leads to the severer dynamic response.

Q-switched temperature fiber sensing device based on graded index and D-shaped multimode fiber

In the paper, a Q-switched temperature sensing device is proposed and verified, which consists of a modulator based on a large core multimode fiber and a graded index multimode optical fiber (LMF-GIMF), and a sensing unit formed by sandwiching a D-shaped multimode fiber (MMF) between two single mode fibers (SMF). Utilizing the modulator to transform continuous light into pulsed laser, a stable Er3+-doped Q-switched laser with a repetition frequency of 23.04 kHz and a modulation depth of 7.47 % has been achieved. Under its pumping, the SMF-MMF-SMF sensing structure performed thermal detection with a sensitivity of −81.5 pm/℃ as temperature varies in the range of 25 °C to 65 °C. The temperature sensing device based on Q-switching can be used as an independent pulse laser to realize the switch between the light source and the sensing device. At the same time, the sensing device can provide real-time feedback according to the change of the central wavelength, which allows for immediate adjustments in the operational state, enhancing the flexibility and responsiveness. In addition, the characteristics of the sensor based on Q-switched laser can reduce thermal damage and extends the service life of the sensor. As far as we know, this is the first report on Q-switched temperature sensing based on the LMF-GIMF modulator. The proposed sensing device has the advantages of high signal-to-noise ratio, low operating threshold, minimal thermal damage, and serves as an independent Q-switched laser source, which can be applied to pulse laser measurement with high precision and time resolution in the future.

Energizing geriatric healthcare: A triboelectric energy harvester with self-powered morse code generator and IoT-Enabled remote sensing tactile patch

Self-powered tactile sensors unleash a diverse array of distinctive interaction techniques and other possibilities in the contemporary world of small-scale electronics. In many circumstances of information exchange, the touch sensor is a fundamental and crucial human-machine interface (HMI). Rapid advancements in portable, wearable, and Internet of Things technologies demand a tactile sensing system to be thin, flexible, effective, self-powered, and secure. Triboelectric nanogenerator (TENG) based touch sensors are the future of flexible and self-powered sensors. Triboelectric nanogenerator based sensors exhibit excellent material compatibility and can produce substantial electrical signals even when frequencies are low and forces are moderate; this motivates researchers to develop self-powered triboelectric sensors. Here, we have proposed a skin-conforming and flexible tactile sensor with a micro-patterned dielectric surface, driven by a stencil-printed activated carbon electrode (Printed carbon PC-TENG). The device performance has been enhanced through the optimization of its output, achieved by analysing its structure using Finite Element Analysis in conjunction with the electrostatic module of COMSOL Multiphysics. The fabricated device produces a voltage in an open circuit measuring 242V along with a power density of 14.69 μW/cm2 in single electrode mode. Further, it can be employed as an energy harvester, demonstrated through energizing LEDs, LCD, Wristwatches, and digital ring counter. The proposed TENG can also serve as an international Morse code generator and self-powered sensor for IoT-enabled remote-sensing finger patches, providing medical assistance to geriatric and bedridden patients.

Research of multimodal transport potential of major cities in China based on the satellite remote sensing

Abstract. By two or more modes of transportation to connect with each other and complete the transport process, multimodal transport could significantly improve transport efficiency, reduce costs, while promote energy conservation and emission reduction, compared with the traditional single mode of transport. Therefore, the multimodal transport development reflects the level of national or regional economic development to some extent. In this study, multimodal transport potential index was built to evaluate the possibility of multimodal transport. Remote sensing images from satellites were used to extract roads, railway stations and airports. A method was proposed to derive the road-rail transport potential index and road-air transport potential index, and then for the analysis of multimodal transport potential in 36 cities. Results show that there is a big gap among 36 cities on the development potential of road-rail and road-air transport. Compared with the road-rail transport potential index, the distribution of road-air transport potential index is more consistent among and within the seven regions. Index values of megacity and super cities are much higher than type I and II big cities. Moreover, in the 36 cities, there is a positive correlation between the multimodal transport potential index and GDP. The research results could be used to quantitatively evaluate the development of transport infrastructure, consequently provide more solid and reliable basic information for the planning, design and scheme adjustment of multimodal transport, thus comprehensively improve the efficiency and quality of multimodal transport, serve the territorial spatial planning.

Multimode Stimuli-Responsive Room-Temperature Phosphorescence Achieved by Doping Butterfly-like Fluorogens into Crystalline Small-Molecular Hosts.

Materials with stimuli-responsive purely organic room-temperature phosphorescence (RTP) exempt from exquisite molecular design and complex preparation are highly desirable but still relatively rare. Moreover, most of them work in a single switching mode. Herein, we employ a versatile host-guest-doped strategy to facilely construct efficient RTP systems with multimode stimuli-responsiveness without ingenious molecular design. By conveniently doping butterfly-like guests, namely, N,N'-diphenyl-dihydrodibenzo[a,c]phenazines (DPACs), featured with vibration-induced emission into the small-molecular hosts via various methods, RTP systems with finely tunable photophysical properties are readily obtained. Through systematic mechanistic studies and with the aid of a series of control experiments, we unveil the critical role of the host crystallinity in achieving efficient RTP. By virtue of the inherent environmental sensitivity of both RTP and fluorescence of the DPACs, our systems exhibit multiple-stimuli-responsiveness with the luminescence not only switching between the fluorescence and phosphorescence but also continuously changing in the fluorescence color. Advanced dynamic anticounterfeiting and multilevel information encryption is thereby realized.