Modal Loss Research Articles

Unidirectional Negative Refraction at an Exceptional Point of Acoustic PT-Symmetric Systems

We demonstrate a method to realize unidirectional negative refraction in an acoustic parity-time (PT)-symmetric system, which is composed of a pair of metasurfaces sandwiching an air gap. The pair of metasurfaces possesses loss and gain modulations. The unidirectional negative refraction, which is strictly limited to the case of incident wave imposing on the loss end of the metasurface, is demonstrated at the exception point (EP) in this PT-symmetric system, while the incidence from the other side leads to strong reflection. Based on rigorous calculations, we explicitly show the underlying mechanism of this model to achieve unidirectional wave scatterings around the EP in the parametric space. In addition, the perfect imaging of a point source in the three-dimensional space, as a signature of negative refraction, is simulated to provide a verification of our work. We envision that this work may sharpen the understanding of PT-symmetric structures and inspire more acoustic functional devices.

Excitation Threshold Reduction Techniques for Organic Semiconductor Lasers: A Review

Organic semiconductor lasers have shown great application potential in various fields, such as low-cost sensing, high-performance lighting and display, and lab-on-a-chip devices. Since the introduction of organic lasers in the 1960s, research on semiconductor laser devices has expanded to include various materials and structures. The organic laser has attracted much attention due to its wide range of emission spectrum and simple synthesis and processing. Researchers constantly pursue the goal of using organic semiconductors to fabricate low-threshold thin-film organic laser devices while retaining the characteristics of a wide luminescence spectrum of organic materials, simple and portable structure, and low cost. However, organic semiconductor lasers face challenges due to material stability under optical pumping and large optical losses under electrical pumping, making commercialization difficult. Many scholars have put great efforts into enhancing the performance of materials and optimizing the structure to minimize the threshold of organic semiconductor lasers. Herein, based on the basic principles of organic lasers, the main factors affecting the excitation threshold are summarized. A comprehensive analysis of the relevant factors and threshold conditions is performed, considering both positive and negative aspects of modal gain and modal loss that need to be addressed. We expect to provide a wide range of ideas for reducing the threshold of organic lasers and offer theoretical guidance for the practical industrial production of organic lasers.

Broadband mode-division (de)multiplexer using nanorod-assisted multimode subwavelength gratings

—Subwavelength grating (SWG) allows flexible tuning of the dispersion of waveguides, enabling wider bandwidths for directional coupler-based devices. However, its light field constraint capability for high-order modes is so weak that the mode scattering loss from sidewalls is high, which impedes low-loss transmission and efficient energy coupling from adjacent waveguides. In this paper, we propose nanorod-assisted multimode subwavelength grating (NSWG) waveguides that support stable high-order modes transmission like strip waveguides, and maintain refractive index tunability similar to SWGs. We fully analyze the mode transmission and coupling characteristics of NSWG waveguides for modes TE1, TE2, and TE3. Results show that such waveguides can hold high transmission fidelity and low losses for high-order modes (<0.33 dB/cm, 0.58 dB/cm, and 1.0 dB/cm for TE1, TE2, and TE3). Arbitrary high-order modes among NSWGs can also be excited efficiently by evanescent field coupling. As proofs of concept, we also demonstrate a three-channel mode-division multiplexer with broad bandwidth (136 nm/135 nm/100 nm), low insertion loss (0.29 dB/0.85 dB/0.62 dB), and crosstalk (-21.4 dB/-20.4 dB/-18.0 dB) for TE0/TE1/TE2. And the device has a compact footprint of 5 × 61 μm. At 1.55 μm, all three modes satisfy IL < 0.19 dB and CT < -20.0 dB. The proposed waveguide structures may provide an alternative building block for broadband and compact multimode on-chip directional coupler-based devices.

Multi-rings modulation and hole-assisted fiber under single-mode operation

In this paper, we propose a novel air-hole-assisted step refractive index fiber. Air holes located in the cladding can ensure the single-mode operation of the proposed fiber. The triangular refractive index ring structure further enlarges large mode field area. Bending performance of the proposed fiber is analyzed by the finite element method with perfectly matched layers, and the influence rule of each structure parameter on bending performance is summarized. Numerical results show that the proposed fiber can achieve stable single-mode operation over a wide wavelength range from 1.5 μm to 1.9 μm at a tight bending scale of 10 cm. Effective mode field area can reach 1002.73 μm2, and the bending loss of fundamental mode (FM) is only 0.000827 dB/m at the working wavelength of 1.55 μm; meanwhile, the loss ratio between higher-order mode (Homs) and fundamental mode is 7240. The proposed fiber is more stable compared to photonic crystal fibers. The change in bending angle does not affect the single-mode operation, and this novel fiber has potential applications in high-power lasers.

Enhancing single-mode guidance using avoided crossings in anti-resonant hollow-core fibers with five nested cladding tubes

We numerically analyze the nested anti-resonant nodeless fibers (NANFs) with five tubes to determine the best single-mode guidance and low confinement loss for the fundamental mode simultaneously. We derive the empirical relation between structural parameters that lead to avoided crossings between the core higher order modes (HOMs) and the cladding tube modes in a 5-tube nested design to achieve robust single-mode guidance. We report a low confinement loss of 1.17×10−3 dB/km and 0.23 dB/km, and huge HOM extinction ratio (HOMER) of the order of 105 and 104 at wavelengths of 650 nm and at 1550 nm respectively. We also investigate the fabrication tolerances of the confinement loss and HOMER to the changes in design parameters. These 5-tube NANFs exhibit a broad loss spectrum of 200 nm with loss of fundamental mode ¡ 0.003 dB/km and HOMER of 16000 centered around 650 nm. NANFs find applications in quantum communications, sensing and telecommunications and this work can be helpful in design optimization of such fibers.

Inverse design of asymmetric Y-junctions for ultra-compact, broadband, and low crosstalk mode (de)multiplexers.

Asymmetric Y-junctions, compared with mode coupling-based devices, possess considerably smaller wavelength dependence and thus are more promising for ultra-broadband mode (de)multiplexing in integrated optics. However, these devices also feature relatively high mode crosstalk and insertion loss. Here, we show that the mode crosstalk and loss of an asymmetric Y-junction can be significantly reduced by optimizing the waveguide shape of the Y-junction using an adjoint-based inverse design. Based on such inverse-designed asymmetric Y-junctions, we realize ultra-compact, broadband, and low crosstalk silicon photonic TE00 & TE1 and TE0 & TE2 mode (de)multiplexers with sizes of only 4.5 × 1.2 µm2 and 6 × 1.4 µm2, respectively. From simulations it is shown that the TE0 & TE1 and TE0 & TE2 mode (de)multiplexers contain wide bandwidths of 160 nm (1460-1620 nm) and 140 nm (1460-1600 nm), respectively, over which the mode crosstalks are below about -20 dB, and the losses are <0.41 dB and <0.88 dB, respectively. The experimental results show that in the corresponding TE0 & TE1 and TE0 & TE2 mode division multiplexing systems, the crosstalks are less than -15.5 dB and -15 dB over the spectral ranges of 1453-1580 nm and 1460-1566 nm, respectively, and the losses are <1.7 dB at 1520 nm and <8.24 dB over the entire measured wavelength range.

Translatorial voice through modal stance: A corpus-based study of modality shifts in Chinese-to-English translation of research article abstracts

Research on translation shifts, especially divergence in modality, that occur to English translations of Chinese research article abstracts is rare. The article aims to explore the linguistic changes in modality use, a linguistic device for expressing authors’ evaluative judgement, in the abstracts of Chinese research articles and their English translations. By adopting a three-step analytical coding procedure from Systemic Functional Linguistics, the study analyses the cross-linguistic modality shifts in modal value, type and orientation in English translations of Chinese research article abstracts (RAAs) from both quantitative and qualitative perspectives. It is found that 1) quantitatively, a considerable number of modality shifts have been observed in translated RAAs on value, followed by type and orientation, in which the distributions present a stronger and more objectivised stance resulting from a loss of modality on low-value implicit proposition; 2) qualitatively, Chinese-specific lexical, syntactic, and textual factors are associated with modality shifts, including lexical ambiguity, “run-on” sentences, and implicit information structure. The findings provide evidence of how original authorial modal stance is influenced by translators and integrated into academic discourse. The study also paves the way for future investigations of possible variables (e.g., the speech-written mode) in modality shifts.

Numerical Analysis of Dual-Wavelength Tungsten-Tellurite Fiber Raman Lasers with Controllable Mode Switching

Fiber laser sources in the spectral range near 1.7–1.8 μm are in highly demand for a lot of applications. We propose and theoretically investigate a dual-wavelength switchable Raman tungsten-tellurite fiber laser in the 1.7–1.8 µm range which can produce two stable modes at frequencies separated by ~7 THz with a pump at 1.55 µm. The Raman waves shifted by 19.8 THz (mode 1) and 27.5 THz (mode 2) from the pump frequency can be generated near two different maxima of the Raman gain spectrum (gain is higher at 19.8 THz and twice lower at 27.5 THz). We numerically simulate two-mode Raman lasing with allowance for energy transfer from the pump wave to modes 1 and 2, and from mode 1 to mode 2 due to inelastic Raman scattering. Diagrams of generation regimes depending on system parameters are constructed. We demonstrate controlled switching between two modes by changing the pump power. For the same intracavity losses for both Raman modes at relatively low pump powers, only mode 1 is generated. At medium pump power, generation occurs simultaneously in both modes. At relatively high pump power, only mode 2 is generated near the weaker maximum. This effect seems surprising, but a rigorous explanation with allowance for the nonlinear interaction between mode 1 and mode 2 is found. When losses for one of the modes change, switching of the generated regimes is also predicted.

High-Power, High-Purity HG0n Hermite–Gaussian Laser Beam Generation in Cascaded Large Aspect Ratio Slabs

High-power, high-purity, nanosecond (ns) one-dimensional HG0n laser beams are proposed and demonstrated by using Nd:YAG cascaded slabs with a large aspect ratio. The HG0n laser beams are generated by adjusting the pump distribution, the intracavity apertures, and the tilt angle of the output coupler (OC). By controlling the gain and loss of HG0n modes of different orders, the high-purity, one-dimensional, high-order HG0n laser beams with orders 1 to 9 (HG01 to HG09) are produced, and their beam quality factors M2 align well with the theoretical predictions. Meanwhile, the large aspect ratio slab provides an ideal amplifier for the strip-shaped HG0n laser beams, and further power scaling is achieved by seeding the generated HG0n laser beams into an amplifier with an identical slab module. For the HG09 mode, the average power is amplified from 289 mW to 4.73 W with 294 ns pulse width, corresponding to a peak power of 32 kW. Moreover, above 5 W average power is achieved for all HG01 to HG08 modes. Hopefully, this scheme provides a solution for high-power and high-purity HG0n laser beam generation based on the slab-shaped configuration.

Damping performance and its influencing factors of wind turbine towers with constrained layer damping treatment

Constrained layer damping (CLD) is proposed as an energy dissipation measure for vibration control of offshore monopile wind turbine towers (OMWTT). The damping performance and influencing factors of OMWTT with CLD treatment are studied in this paper. First, a theory for calculating the first-order modal loss factor provided by CLD for composite structures is derived using the modal strain energy (MSE) method; second, the error of the theoretical formulation is analyzed using the finite element method; and finally, the influence of each parameter of the CLD on the first-order modal loss factor is discussed. The modal loss factor’s temperature dependence is described with the theory’s aid. The modal damping calculation theory proposed in this paper can rapidly determine the optimal design of CLD, considering the influence of multiple parameters and a complex environment, which could be helpful in the efficient vibration reduction of OMWTT.

Design of Single-Mode Single-Polarization Large-Mode-Area Multicore Fibers.

In laser science and industry, considerable effort is directed toward designing fibers for fiber laser and fiber amplifier applications, each of which offers a particular advantage over the others. Evanescently coupled multicore fibers, however, have been studied less extensively due to the relatively small mode area in the single-mode regime. Here, by proposing a new structure with stress-applying parts in a 37-core fiber and optimizing this structure through a comprehensive framework, we present 21 solutions characterized by large-mode-area and high beam quality in the single-mode, single-polarization regime. Different fiber designs are optimal for different output parameters. In one design, the mode area can significantly increase to above 880 μm2, which is comparable with that of photonic-crystal fibers. Moreover, besides the single-mode operation, the beam quality factor (M2 factor) of the fundamental mode is considered an output parameter in the bent state and is improved up to 1.05 in another design. A comprehensive tolerance analysis is then performed to assess the performance of the designs under deviations from normal conditions. Moreover, in spite of the shifts in the loss of modes, the proposed high beam quality LMA fibers maintain single-polarization, single-mode operation across a wide range of core pitches, bending orientation angles, and bending radius deviations. Our results highlight the potential of multicore fibers for the efficient operation of fiber lasers and amplifiers.

HMGAN

Wearable Human Activity Recognition (WHAR) is an important research field of ubiquitous and mobile computing. Deep WHAR models suffer from the overfitting problem caused by the lack of a large amount and variety of labeled data, which is usually addressed by generating data to enlarge the training set, i.e., Data Augmentation (DA). Generative Adversarial Networks (GANs) have shown their excellent data generation ability, and the generalization ability of a classification model can be improved by GAN-based DA. However, existing GANs cannot make full use of the important modality information and fail to balance modality details and global consistency, which cannot meet the requirements of deep multi-modal WHAR. In this paper, a hierarchical multi-modal GAN model (HMGAN) is proposed for WHAR. HMGAN consists of multiple modal generators, one hierarchical discriminator, and one auxiliary classifier. Multiple modal generators can learn the complex multi-modal data distributions of sensor data. Hierarchical discriminator can provide discrimination outputs for both low-level modal discrimination losses and high-level overall discrimination loss to draw a balance between modality details and global consistency. Experiments on five public WHAR datasets demonstrate that HMGAN achieves the state-of-the-art performance for WHAR, outperforming the best baseline by an average of 3.4%, 3.8%, and 3.5% in accuracy, macro F1 score, and weighted F1 score, respectively.

Full stage networks with auxiliary focal loss and multi-attention module for submarine garbage object detection

Submarine garbage is constantly destroying the marine ecological environment and polluting the ocean. It is critical to use detection methods to quickly locate and identify submarine garbage. The background of submarine garbage images is much more complex than that of natural scene images, with object deformation and missing contours putting higher demands on the detection network. To solve the problem of low accuracy under complex backgrounds, full stage networks with auxiliary focal loss and multi-attention module are proposed for submarine garbage object detection based on YOLO. To maximize the gradient combination, a hierarchical fusion feature mechanism and a segmentation and merging strategy are used in this paper to optimize the difference in gradient combination to obtain full-stage features. Then the criss-cross attention module is used to precisely extract multi-scale features of small object dense regions while removing noise information from complex backgrounds. Finally, the auxiliary focal loss function addresses the issue of unbalanced positive and negative samples, focusing on the learning of difficult samples while improving overall detection precision. Based on comparative experiments and ablation experiments, the FSA networks achieved state-of-the-art performance, and is applicable to the real-time object detection of submarine garbage in complex backgrounds.

Machine Translation of Electrical Terminology Constraints

In practical applications, the accuracy of domain terminology translation is an important criterion for the performance evaluation of domain machine translation models. Aiming at the problem of phrase mismatch and improper translation caused by word-by-word translation of English terminology phrases, this paper constructs a dictionary of terminology phrases in the field of electrical engineering and proposes three schemes to integrate the dictionary knowledge into the translation model. Scheme 1 replaces the terminology phrases of the source language. Scheme 2 uses the residual connection at the encoder end after the terminology phrase is replaced. Scheme 3 uses a segmentation method of combining character segmentation and terminology segmentation for the target language and uses an additional loss module in the training process. The results show that all three schemes are superior to the baseline model in two aspects: BLEU value and correct translation rate of terminology words. In the test set, the highest accuracy of terminology words was 48.3% higher than that of the baseline model. The BLEU value is up to 3.6 higher than the baseline model. The phenomenon is also analyzed and discussed in this paper.

Mode mixing and losses in misaligned microcavities.

We present a study on the optical losses of Fabry-Pérot cavities subject to realistic transverse mirror misalignment. We consider mirrors of the two most prevalent surface forms: idealised spherical depressions, and Gaussian profiles generated by laser ablation. We first describe the mode mixing phenomena seen in the spherical mirror case and compare to the frequently-used clipping model, observing close agreement in the predicted diffraction loss, but with the addition of protective mode mixing at transverse degeneracies. We then discuss the Gaussian mirror case, detailing how the varying surface curvature across the mirror leads to complex variations in round trip loss and mode profile. In light of the severe mode distortion and strongly elevated loss predicted for many cavity lengths and transverse alignments when using Gaussian mirrors, we suggest that the consequences of mirror surface profile are carefully considered when designing cavity experiments.

Effects of Motion-Relevant Knowledge From Unlabeled Video to Human-Object Interaction Detection.

The existing works on human-object interaction (HOI) detection usually rely on expensive large-scale labeled image datasets. However, in real scenes, labeled data may be insufficient, and some rare HOI categories have few samples. This poses great challenges for deep-learning-based HOI detection models. Existing works tackle it by introducing compositional learning or word embedding but still need large-scale labeled data or extremely rely on the well-learned knowledge. In contrast, the freely available unlabeled videos contain rich motion-relevant information that can help infer rare HOIs. In this article, we creatively propose a multitask learning (MTL) perspective to assist in HOI detection with the aid of motion-relevant knowledge learning on unlabeled videos. Specifically, we design the appearance reconstruction loss (ARL) and sequential motion mining module in a self-supervised manner to learn more generalizable motion representations for promoting the detection of rare HOIs. Moreover, to better transfer motion-related knowledge from unlabeled videos to HOI images, a domain discriminator is introduced to decrease the domain gap between two domains. Extensive experiments on the HICO-DET dataset with rare categories and the V-COCO dataset with minimum supervision demonstrate the effectiveness of motion-aware knowledge implied in unlabeled videos for HOI detection.

Topological optimization design on constrained layer damping treatment for vibration suppression of thin-walled structures via improved BESO method

In this paper, an improved bi-directional evolutionary structural optimization (BESO) technique is proposed, to investigate topological optimization design on constrained layer damping (CLD) treatment for vibration suppression of experiment rack backplane. First, the optimization model for vibration reduction of CLD treated backplane is established, with volume fraction of CLD material as constraint condition, and the maximization of weighted value of the first four order modal loss factors of the structure as objective function. An improved evolutionary rate considering the CLD element sensitivity coefficient of variation of each iteration is presented to increase optimization efficiency. Then, the BESO with improved evolutionary rate is utilized to search the optimal CLD layout of the experiment rack backplane. CLD layouts of three optimization schemes with different volume fraction of CLD material are obtained, and are highly consistent with the stress distribution of the backplane viscoelastic layer, which verifies the rationality of the optimization results. Visualization of iterative optimization process, including elements removal/addition and dynamic change of CLD elements sensitivity in each iteration, is demonstrated, respectively. Further, the effect of improved evolutionary rate on optimization process is discussed. Compared with the conventional BESO method, the improved BESO technique can not only obtain a more reasonable optimized CLD layout, but also improve the optimization efficiency and accuracy. Finally, the validity of the optimized CLD layout is analyzed from three following aspects: natural frequency, modal loss factor and vibration response under order excitation. Numerical and experimental results show that the proposed BESO technique is effective and efficient in the application of CLD material, and the small amount of CLD material can be retained under the premise of ensuring damping.

Realistic feature perception for face frontalization with dual-mode face transformation

Despite face frontalization developing rapidly, limited image pairs and low image quality are still obstacles to face frontalization. Although single-view-based face frontalization does not have the constraints of image pairs, it reduces the exploration of unknown data. We alleviate the face distortion problem based on single-view face frontalization from two aspects. One is that we designed a dual-mode face transformation model for model training based on the loss of facial features from different angles. These two training modes cope with the question of face frontalization in different distortions. Specifically, we apply the 3D head model to the data transformation by fitting the 2D image to the 3D model. Rotating the 3D model at the designed angle to realize the face transformation. We judge the distortion extent with Euler Angle and produce the two training modes corresponding to the dual-mode face transformation. On the other hand, we construct a perceptual loss module to preserve the detailed information of the front view. This module combines content and identity loss to generate the photorealistic frontal image. In qualitative and quantitative experiments, we conduct experiments to illustrate the advantages of the proposed model on controlled and field datasets. Further, we apply full-reference and no-reference image quality assessment methods to estimate the quality of images generated by different face frontalization models.

Damping characteristics and sound transmission loss of finite composite plates with frequency dependent constrained viscoelastic interlayer

This paper proposes a sublaminate layer-wise finite element modeling method to accurately evaluate the damping and sound insulation characteristics of composite constrained layer damping plates. It incorporates the frequency-dependent property of viscoelastic interlayer into fully coupled structure-acoustic interaction equations, and employs the generalized high-order displacement hypothesis with tailored polynomial series expansion. The method is applicable to various boundary conditions and material layer combinations. Validation is conducted using numerical and experimental data. Parametric study identifies optimal thickness and stiffness for maximizing modal loss factor. Additionally, sound insulation performance is examined in narrow frequency bands, highlighting the damping enhancement of viscoelastic interlayer.

Enhancement of longitudinal mode characteristic of quantum cascade laser by high-order tilted asymmetric surface grating

The high-order tilted asymmetric surface grating (TASG) is firstly investigated and applied to quantum cascade lasers (QCLs) here. Due to the tilted grating of 16°in the middle of ridge, the longitudinal mode loss difference of TASG is about 4% higher than that of the uniform grating (UNG), which results in better single mode selectivity in reflectance spectrum. Further experimental results indicate that the TASG distributed feedback (DFB) QCL maintains more stable single longitudinal mode performance with increasing the current and temperature because that TASG have a larger longitudinal mode loss difference. While the UNG QCL with the equivalent coupling coefficient exhibits mode jumping. Hence, the drift coefficient of 0.311 nm/K of TASG QCL is 60% lower than that of the UNG device and the maximum side-mode suppression ratio (SMSR) achieves 30 dB. The characteristic temperature of TASG QCL device is 36K, which is higher than 32K of UNG device. This study showcases the feasibility and superiority of the TASG in mid-infrared active devices.