Single-mode Research Articles

One millijoule mid-IR nanosecond pulses at ∼2.78 µm in a robust single transverse mode from a coiled 46 µm core low-NA Er:ZBLAN fiber amplifier

We demonstrate high-energy nanosecond pulse amplification at ∼2.78 µm in a robust single transverse mode from a coiled large core low-numerical aperture (NA) Er:ZBLAN fiber amplifier. The highest energy achieved is 1.01 mJ from a 46 µm core Er:ZBLAN fiber, seeded with a preamplifier in a single-mode fiber 30 ns long pulses from a ring-cavity Q-switched Er:ZBLAN fiber laser. Near-diffraction-limited beams with M2 = 1.09 were obtained using coiling-induced high-order mode filtering in a custom-fabricated 46 µm diameter and ∼0.064 NA core fiber. This constitutes, to the best of our knowledge, the highest-energy nanosecond duration pulses in a single transverse mode from an Er:ZBLAN fiber laser system to date.

Designing a Lithium-Ion Cell for Studies of a Single Degradation Mechanism Over a Wide Temperature Range

Abstract 18650-sized cylindrical cells containing single crystal Li[Ni0.6Mn0.4Co0.0]O2, Li[Ni0.6Mn0.35Co0.05]O2, and Li[Ni0.6Mn0.3Co0.1]O2 positive electrodes along with artificial graphite negative electrodes were constructed to be balanced at 4.05 V. These cells were designed so that they would have only the single degradation mode of lithium inventory loss due to the solid-electrolyte interphase layer growth. Cells were cycled both at C/3 and C/20 over a wide temperature range from 20 to 100°C in order to accelerate degradation processes at higher temperatures and more rapidly predict low-temperature behaviour. A low upper cutoff voltage of 4.0 V was selected to avoid electrolyte oxidation, and an electrolyte composition incorporating pure lithium bis(fluorosulfonyl)imide salt was chosen based on the temperature and voltage range of operation. A thorough post-cycling analysis was performed to verify the elimination of all degradation modes except inventory loss and minor impedance growth, which enabled the application of a simple square root time model to make accurate lifetime predictions. In addition, the capacity retention of these cells at elevated temperature is incredible, with the best cells retaining 87% capacity after 1400 C/3 cycles (one year) continuously at 85°C.

High-power (500 mW) narrow-linewidth (21 kHz) low-RIN (−168 dB/Hz) distributed feedback laser

A high-power, narrow-linewidth, low-relative-intensity-noise (RIN) distributed feedback (DFB) laser is demonstrated. The laser employs four strained AlGaInAs quantum wells and three diluted waveguides to reduce internal loss, and enhance heat dissipation performance to reach a high power and low noise operation. At a test temperature of 15 °C, the DFB laser with a cavity length of 1.5 mm exhibited single mode output power of 517 mW in pulsed mode and 424 mW in continuous-wave (CW) mode. with far-field full width at half maximum (FWHM) divergence angles of 10°×20°. In addition, the DFB laser achieved an intrinsic linewidth of 21 kHz, and an ultra-low RIN of less than −168 dB/Hz.

Sagnac Interference-Based Contact-Type Fiber-Optic Vibration Sensor

The observation and evaluation of vibration signals is crucial for enhancing engineering quality and ensuring the safe operation of equipment. This paper proposes a fiber-optic vibration sensor based on the Sagnac interference principle. The polarization-maintaining fiber (PMF) is spliced between two single mode fibers (SMFs) to form the SMF-PMF-SMF (SPS) fiber structure. The Sagnac interferometer consists of an SPS fiber structure connected to a 3 dB coupler. Due to the principle of the elastic-optical effect, the interferometric spectrum of the PMF-based Sagnac interferometric structure changes when the PMF is subjected to stress, enabling vibration to be measured. The experimental results show that the relative measurement error of the fiber-optic vibration sensor for healthy and faulty bearings is less than 1.8%, which verifies the effectiveness and accuracy of the sensor. The sensor offers benefits of excellent anti-vibration fatigue characteristics, simple production, small size, light weight, and has a wide range of applications in mechanical engineering, fault detection, safety and security, and other fields.

Multi-branch convolutional neural network with cross-attention mechanism for emotion recognition

Research on emotion recognition is an interesting area because of its wide-ranging applications in education, marketing, and medical fields. This study proposes a multi-branch convolutional neural network model based on cross-attention mechanism (MCNN-CA) for accurate recognition of different emotions. The proposed model provides automated extraction of relevant features from multimodal data and fusion of feature maps from diverse sources as modules for the subsequent emotion recognition. In the feature extraction stage, various convolutional neural networks were designed to extract critical information from multiple dimensional features. The feature fusion module was used to enhance the inter-correlation between features based on channel-efficient attention mechanism. This innovation proves effective in fusing distinctive features within a single mode and across different modes. The model was assessed based on EEG emotion recognition experiments on the SEED and SEED-IV datasets. Furthermore, the efficiency of the proposed model was evaluated via multimodal emotion experiments using EEG and text data from the ZuCo dataset. Comparative analysis alongside contemporary studies shows that our model excels in terms of accuracy, precision, recall, and F1-score.

Real-time monitoring of thin film thickness and surface roughness using a single mode optical fiber

Real-time monitoring of thin film thickness and surface roughness using a single mode optical fiber

High-precision 1.5 µm band tunable single-frequency fluoro-sulfo-phosphate fiber laser based on a self-injection locking scheme

In this paper, a high-precision 1.5 µm band tunable single-frequency Er3+/Yb3+ co-doped fluoro-sulfo-phosphate (EYFSP) fiber laser based on a self-injection locking scheme is experimentally demonstrated. A self-developed 4 cm long EYFSP fiber with a high gain coefficient (3.2–4.7 dB/cm) at the C-band serves as the gain medium for laser output. The single longitudinal mode (SLM) operation is realized via a 15 cm long commercial unpumped gain fiber and a self-injection locking scheme, while flexible wavelength tuning is controlled by a fiber Fabry–Perot tunable filter (FFP-TF). The laser wavelength is tuned from 1528.08 to 1555.55 nm, with a tuning precision of up to 10–18 pm and a tuning speed of 30 nm/s. Throughout the entire tuning range of 27.47 nm, the measured laser linewidth is less than 1.3 kHz, the relative intensity noise remains below −140dB/Hz at frequencies above 3 MHz, and the power fluctuation is as low as 1% over a measurement period of 2 h. These results demonstrate that the high-precision 1.5 µm band tunable single-frequency EYFSP fiber laser is a promising candidate for future coherent optical communication systems and optical fiber sensing.

Remote bonding of fiber Bragg grating sensors for lamb wave measurements: A review

Abstract This article reviews the state-of-the-art in remote bonding of fiber Bragg grating sensors, primarily for Lamb wave measurements in structures. The presence of damage in a structure modifies Lamb waves through reflection and scattering, as well as the potential conversion between Lamb modes. While FBG sensors have been applied to capture ultrasonic waves for the past 30 years, the mounting configuration called remote bonding has recently come to attention as an alternative method to capture guided waves from structures. In this case, the FBG is not located at the bond, but instead at a remote location along the optical fiber. The Lamb waves are converted into propagating acoustic waves along the fiber, which are measured by the FBG. This article presents a discussion of the primary benefits of remote bonding, including the higher sensitivity to low-amplitude Lamb waves, the fact that the FBG can situated in a less harsh environment than the sensing region, and the insensitivity to quasi-static strain. The properties of ultrasonic modes in optical fibers and their conversion from Lamb waves is first reviewed. Strategies to detect these waves with FBGs and the associated instrumentation is also presented. Recent examples from the literature utilizing remote bonded FBGs are then presented. Finally, acoustic couplers to transfer the ultrasonic modes from one or mode optical fibers to another are also reviewed.

A Broadband Mode Converter Antenna for Terahertz Communications

The rise of artificial intelligence (AI) necessitates ultra-fast computing, with on-chip terahertz (THz) communication emerging as a key enabler. It offers high bandwidth, low power consumption, dense interconnects, support for multi-core architectures, and 3D circuit integration. However, transitioning between different waveguides remains a major challenge in THz systems. In this paper, we propose a THz band mode converter that converts from a rectangular waveguide (RWG) (WR-0.43) in TE10 mode to a substrate-integrated waveguide (SIW) in TE20 mode. The converter comprises a tapered waveguide, a widened waveguide, a zigzag antenna, and an aperture coupling slot. The zigzag antenna effectively captures the electromagnetic (EM) energy from the RWG, which is then coupled to the aperture slot. This coupling generates a quasi-slotline mode for the electric field (E-field) along the longitudinal side of the aperture, exhibiting odd symmetry akin to the SIW’s TE20 mode. Consequently, the TE20 mode is excited in the symmetrical plane of the SIW and propagates transversely. Our work details the mode transition principle through simulations of the EM field distribution and model optimization. A back-to-back RWG TE10-to-TE10 mode converter is designed, demonstrating an insertion loss of approximately 5 dB over the wide frequency range band of 2.15–2.36 THz, showing a return loss of 10 dB. An on-chip antenna is proposed which is fed by a single higher-order mode of the SIW, achieving a maximum gain of 4.49 dB. Furthermore, a balun based on the proposed converter is designed, confirming the presence of the TE20 mode in the SIW. The proposed mode converter demonstrates its feasibility for integration into a THz-band high-speed circuit due to its efficient mode conversion and compact planar design.

Simultaneous measurement of multimode squeezing through multimode phase-sensitive amplification

Multimode squeezed light is an increasingly popular tool in photonic quantum technologies, including sensing, imaging, and computation. Meanwhile, the existing methods of its characterization are technically complicated, which reduces the level of squeezing, and mostly deal with a single mode at a time. Here, for the first time, to the best of our knowledge, we employ optical parametric amplification to characterize multiple squeezing eigenmodes simultaneously. We retrieve the shapes and squeezing degrees of all modes at once through direct detection followed by modal decomposition. This method is tolerant to inefficient detection and does not require a local oscillator. For a spectrally and spatially multimode squeezed vacuum, we characterize eight strongest spatial modes, obtaining squeezing and anti-squeezing values of up to −5.2 ± 0.2 dB and 8.6 ± 0.3 dB, respectively, despite the 50% detection loss. This work, being the first exploration of an optical parametric amplifier’s multimode capability for squeezing detection, paves the way for the real-time detection of multimode squeezing.

Atomic fluorescence collection into planar photonic devices

Fluorescence collection from individual emitters plays a key role in state detection and remote entanglement generation, fundamental functionalities in many quantum platforms. Planar photonics have been demonstrated for robust and scalable addressing of trapped-ion systems, motivating consideration of similar elements for the complementary challenge of photon collection. Here, using an argument from the reciprocity principle, we show that far-field photon collection efficiency can be simply expressed in terms of the fields associated with the collection optic at the emitter position alone. We calculate collection efficiencies into ideal paraxial and fully vectorial focused Gaussian modes parameterized in terms of focal waist, and further quantify the modest enhancements possible with more general beam profiles, establishing design requirements for efficient collection. Toward practical implementation, we design, fabricate, and characterize two diffractive collection elements operating at λ = 397 nm; a forward emitting design is predicted to offer 0.25% collection efficiency into a single waveguide mode, while a more efficient reverse-emitting design offers 1.14% collection efficiency, albeit with more demanding fabrication requirements. Close agreement between simulated and measured emission for both designs indicates practicality of these collection efficiencies, and we indicate avenues to improved devices approaching the limits predicted for ideal beams. We point out a particularly simple integrated waveguide configuration for polarization-based remote entanglement generation enabled by integrated collection.

Single soliton microcomb combined with optical phased array for parallel FMCW LiDAR

The frequency-modulated continuous-wave (FMCW) technology combined with optical phased array (OPA) is promising for the all-solid-state light detection and ranging (LiDAR). We propose and experimentally demonstrate a silicon integrated OPA combined with an optical frequency microcomb for parallel LiDAR system. For realizing the parallel wavelengths emission consistent with Rayleigh criterion, the wide waveguide beyond single mode region combined with the bound state in the continuum (BIC) effect is harnessed to obtain an ultra-long optical grating antenna array. The single soliton comb, generating about multiple distinct wavelength channels and combined with the high performance integrated OPA, is also demonstrated for coherent three-dimensional (3D) imaging by utilizing FMCW method. The modulation bandwidth of parallel modulation of the microcomb is beyond the modulation region of single soliton microcomb. The result paves the way for developing all-solid-state and ultrahigh-frame-rate coherent LiDAR systems.

Modal decomposition in multimode optical fibres by solving the challenges posed by phase ambiguity and large datasets

Mode decomposition (MD), which can obtain the amplitude and phase of each propagating mode in multimode optical fibres, provides important information and unleashes enormous potential applications in the fields of optical communication and optical sensing. Phase ambiguity and large datasets are two big challenges for mode decomposition based on deep-learning numerical methods. In this paper, the influence of phase ambiguity is eliminated by combining near-field and far-field intensity patterns. By designing a two-step hybrid process, the accuracy of mode decomposition is improved significantly. Compared with previous reported methods, our results demonstrate that even for the complicated 10-mode case, the average correlation coefficient (CC) between the reconstructed and the true intensity pattern can be optimized to over 0.99 using much smaller datasets with 10,000 stacked images.

Thermodynamic Analysis of Pumped Thermal Energy Storage System Combined Cold, Heat, and Power Generation

Aiming at problems such as the low efficiency of renewable energy conversion and the single energy flow mode, this paper proposes a heat pump energy storage system combining cold, heat and power generation to achieve the purpose of diversified utilization of renewable energy. The system is suitable for buildings requiring cooling, heating/domestic hot water production and electricity. This paper mainly uses MATLAB for numerical calculations, selects several key cycle parameters to calculate and analyze the thermodynamic performance of the system, and uses the genetic algorithm and TOPSIS decision method to carry out fine design of the system working conditions. Through the multi-objective optimization calculation and the optimal solution, the system can achieve a total energy efficiency of 2.39 and a high thermal economic performance, indicating that the system can achieve the goals of cooling, heating water and power supply and providing ideas for the application of the multiple energy storage system.

Tristate Switching of Terahertz Metasurfaces Enabled by Transferable VO2

AbstractAchieving dynamic switching among absorption (A), reflection (R), and transmission (T) states is not only essential for advancing the understanding of light‐metasurface interactions but also holds significant potential for practical applications, such as selective electromagnetic shielding and smart windows. However, at terahertz and higher frequencies, implementing active elements in multilayer configurations presents challenges that are not as straightforward as those encountered in the microwave range. In this work, it is demonstrated that tristate ART tuning can be realized in a single‐layer, free‐standing metasurface by switching between a dual dipolar mode (electric dipole and magnetic dipole) and a single dipole mode (electric dipole). By transferring a flexible vanadium dioxide (VO2) thin film onto the free‐standing dielectric Huygens’ metasurface, ART modulation is achieved, transitioning from a near‐unity transmission state to a near‐perfect absorption state, and finally to a high‐reflection state with the reflection up to 0.65 during the insulator‐to‐metal transition induced by heating onto the phase‐change material. The results may lead to new approaches in designing reconfigurable metasurfaces based on phase‐change materials for wavefront control and electromagnetic shielding applications.

Analysis of crosstalk drift and optimization of spatial light modulator based mode-selective multiplexers for multimode fibers.

Adaptive mode-selective multiplexers offer the potential to control the modal content within multimode fibers for space division multiplexing (SDM). To such an end, spatial light modulators allow programmable control over the phase, amplitude, and polarization of optical wavefronts. One of the major challenges is to precisely match the manipulated beam to the waveguide modes in the multimode fiber. Achieving precise alignment of optical components within the free space system is crucial for accurate mode multiplexing while active alignment may be necessary to overcome environment induced drift. In this paper, we investigate, through theory, simulations and experiments, the impact of misalignment errors in a free space telescopic Fourier system, including phase mask and lenses misposition and angular misalignment in fiber collimation. Mode multiplexing is achieved using phase holograms in the Fourier domain while mode demultiplexing is achieved through off-axis holography and Fourier domain processing. Furthermore, we analyze the crosstalk drift with time in SDM transmission over a 45 mode OM3 fiber. System stability is experimentally evaluated over a 9-hour transmission period.

Parallel six-mode-selective converter based on photonic crystal fiber without holes in the cladding.

Mode-selective converters (MSCs) play an indispensable role in mode division multiplexing (MDM) systems, and the commonly used cascaded waveguide-based MSCs not only have a relatively large size but also increase the insertion loss and mode crosstalk during the conversion process. In this paper, a parallel six-mode-selective converter (6-MSC) is proposed to enhance the integration of the device, which consists of a photonic crystal fiber (PCF) and six step-index fibers (SIFs). Here, a PCF without any holes in the cladding is proposed. Based on this structure, the distance between the PCF core and the SIF core can be arbitrarily adjusted, which greatly improves the design freedom of multi-core optical fiber devices. Simulation results show that at a wavelength of 1.55 µm, the proposed 6-MSC can achieve parallel conversion from the fundamental mode in each SIF to the corresponding modes (L P01x, L P01y, L P11a x, L P11a y, L P11b x and L P11b y) in the PCF at a device length of 800 µm. The maximum coupling efficiency is 96.22% and the crosstalk is below 3.45%. Moreover, the coupling efficiencies of all six mode pairs exceed 80% within the wavelength range of 1.538∼1.560 µm, and the 3-dB coupling efficiency bandwidth covers the entire C-band. To the best of our knowledge, this is the first report of parallel conversion between six orthogonal fiber modes and fundamental modes based on PCF. The proposed 6-MSC holds promising prospects for future integrated high-capacity MDM systems.

Single mode, distributed feedback interband cascade lasers grown on Si for gas sensing

A single mode distributed feedback (DFB) interband cascade laser (ICL) grown on a (001) Si substrate has been developed. The designed DFB ICL with a grating on top of the ridge emits at a wavelength near 3.4 μm, suited for methane gas sensing, and operates in continuous wave up to 35 °C, with a maximum output power of 4 mW/facet at 15 °C and a side mode suppression ratio of 20 dB in the whole operating range. Methane detection has been demonstrated by integrating the DFB ICL on Si with a quartz-enhanced photoacoustic spectroscopy setup, a step forward in the development of integrated photonic gas sensors on silicon platforms.

Chip-scale integrated optical gyroscope based on a multi-mode co-detection technique

Gyroscopes are crucial components of inertial navigation systems, with ongoing development emphasizing miniaturization and enhanced accuracy. The recent advances in chip-scale optical gyroscopes utilizing integrated optics have attracted considerable attention, demonstrating significant advantages in achieving tactical-grade accuracy. In this paper, a new, to our knowledge, integrated optical gyroscope scheme based on the multi-mode co-detection technology is proposed, which takes the high-Q microcavity as its core sensitive element and uses the multi-mode characteristics of the microcavity to achieve the measurement of rotational angular velocity. This detection scheme breaks the tradition of optical gyroscopes based on a single mode within the sensitive ring to detect the angular rotation rate, which not only greatly simplifies the optical and electrical system of the optical gyroscope, but also has a higher detection accuracy. The gyroscope based on this detection scheme has successfully detected the Earth’s rotation on a 9.2 mm diameter microcavity with a bias instability as low as 1 deg/h, which is the best performance among the chip-scale integrated optical gyroscopes known to us. Moreover, its high dynamic range and highly simplified and reciprocal system architecture greatly enhance the feasibility of practical applications. It is anticipated that these developments will have a profound impact on the field of inertial navigation.

Direct analysis of engineered iron nanotubes and platinum nanorods: A challenge for single particle inductively coupled plasma mass spectrometry.

The use of inductively coupled plasma mass spectrometry in single particle mode (SP-ICP-MS) for the characterization of micro and nanostructured materials is a growing field of research. In this work, the possibility of expanding the boundaries to anisotropic structures including solid Pt-nanorods and hollowed Fe2O3-nanotubes is presented. The obtained structures are evaluated by scanning electron microscopy (SEM), high-resolution electron microscopy (HR-TEM) and SP-ICP-MS techniques. Solid Pt-nanorods (191±18nm in diameter) showed important heterogeneity in their length, ranging 42-72nm, due to sample preparation difficulties. The analysis by SP-ICP-MS confirmed the presence of two different populations of Pt/nanorods at 19±4fg and 41±5fg, respectively, yielding a mean value of 23±12fgPt/rod and a length range of 38-67nm, in agreement with TEM measurements. In the case of the two different sized double-walled Fe2O3-nanotubes of 900nm and 1800nm in length, the SP-ICP-MS measurements provided results of 16±10 and 25±4fg Fe/nanotube, respectively. Out of this data, the layer thickness of the Fe2O3 nanotube wall was calculated, reporting values ranging between 20±6 and 17±4nm, respectively, in good agreement with the TEM estimations (18±4nm). Considering the complexity of the highly anisotropic nano and micro-structures analyzed, SP-ICP-MS technique can be seen as a novel tool to evaluate the fabrication process of non-spherical nanomaterials as well as the sample preparation strategies, complementary to microscopic techniques and with higher sample throughput.