MDPI - Publisher of Open Access Journals

10 pages, 2234 KiB

Open AccessArticle

Development and Characterization of an Asymmetric MZI Temperature Sensor Using Polymer Waveguides for Extended Temperature Measurement Scopes

by Fumin Liu, Xue Zhang, Tianyue Wang and Guanghao Huang

Photonics 2025, 12(5), 491; https://doi.org/10.3390/photonics12050491 - 15 May 2025

Viewed by 393

Abstract

To meet diverse industrial needs, temperature sensors with a wide measurement range have become a key element. In this paper, we propose an asymmetric Mach–Zehnder interferometer (MZI) temperature sensor based on polymer optical waveguides. Experimental results show that the output interference signal exhibits [...] Read more.

To meet diverse industrial needs, temperature sensors with a wide measurement range have become a key element. In this paper, we propose an asymmetric Mach–Zehnder interferometer (MZI) temperature sensor based on polymer optical waveguides. Experimental results show that the output interference signal exhibits periodic changes with temperature variations. The device exhibits a temperature measurement range of 120 °C and a sensitivity of 0.27 rad/°C. This study provides an effective new approach for developing high-performance, low-cost temperature sensors suitable for an extended temperature measurement range. Full article

► Show Figures

Figure 1

11 pages, 3727 KiB

Open AccessEditor’s ChoiceArticle

Dynamically Tunable Singular States Through Air-Slit Control in Asymmetric Resonant Metamaterials

by Yeong Hwan Ko and Robert Magnusson

Photonics 2025, 12(5), 403; https://doi.org/10.3390/photonics12050403 - 22 Apr 2025

Viewed by 305

Abstract

This study presents a novel method for dynamically tuning singular states in one-dimensional (1D) photonic lattices (PLs) using air-slit-based structural modifications. Singular states, arising from symmetry-breaking-induced resonance radiation, generate diverse spectral features through interactions between resonance modes and background radiation. By strategically incorporating [...] Read more.

This study presents a novel method for dynamically tuning singular states in one-dimensional (1D) photonic lattices (PLs) using air-slit-based structural modifications. Singular states, arising from symmetry-breaking-induced resonance radiation, generate diverse spectral features through interactions between resonance modes and background radiation. By strategically incorporating air slits to break symmetry in 1D PLs, we demonstrated effective control of resonance positions, enabling dual functionalities including narrowband band pass and notch filtering. These singular states originate from asymmetric guided-mode resonances (aGMRs), which can be interpreted by analytical modeling of the equivalent slab waveguide. Moreover, the introduction of multiple air slits significantly enhances spectral tunability by inducing multiple folding behaviors in the resonance bands. This approach allows for effective manipulation of optical properties through simple adjustments of air-slit displacements. This work provides great potential for designing multifunctional photonic devices with advanced metamaterial technologies. Full article

(This article belongs to the Special Issue Optical Metasurfaces: Applications and Trends)

► Show Figures

Figure 1

13 pages, 3845 KiB

Open AccessArticle

Ultra-Compact Multimode Micro-Racetrack Resonator Based on Cubic Spline Curves

by Zhen Li, Chuang Cheng, Xin Fu and Lin Yang

Photonics 2025, 12(4), 326; https://doi.org/10.3390/photonics12040326 - 31 Mar 2025

Viewed by 452

Abstract

Micro-racetrack resonators have become one of the key components for realizing signal processing, generation, and integration in microwave photonics, owing to their high Q factor, compact footprint, and tunability. However, most of the reported micro-racetrack resonators are confined to the single-mode regime. In [...] Read more.

Micro-racetrack resonators have become one of the key components for realizing signal processing, generation, and integration in microwave photonics, owing to their high Q factor, compact footprint, and tunability. However, most of the reported micro-racetrack resonators are confined to the single-mode regime. In this paper, we designed an ultra-compact multimode micro-racetrack resonator (MMRR) based on shape-optimized multimode waveguide bends (MWBs). Cubic spline curves were used to represent the MWB boundary and adjoint methods were utilized for inverse optimization, achieving an effective radius of 8 μm. Asymmetric directional couplers (ADCs) were designed to independently couple three modes into a multimode micro-racetrack, according to phase-matching conditions and transmission analysis. The MMRR was successfully fabricated on a commercial platform using a 193 nm dry lithography process. The device exhibited high loaded Q factors of 2.3 × 10⁵, 4.1 × 10⁴, and 2.9 × 10⁴, and large free spectral ranges (FSRs) of 5.4, 4.7, and 4.2 nm for

{TE}_{0}

,

{TE}_{1}

, and

{TE}_{2}

modes, with about a 19 × 55 μm² footprint. Full article

(This article belongs to the Special Issue Recent Advancement in Microwave Photonics)

► Show Figures

Figure 1

16 pages, 5587 KiB

Open AccessArticle

Flat Emission Silicon Nitride Grating Couplers for Lidar Optical Antennas

by Thenia Prousalidi, Georgios Syriopoulos, Evrydiki Kyriazi, Roel Botter, Charalampos Zervos, Giannis Poulopoulos and Dimitrios Apostolopoulos

Photonics 2025, 12(3), 214; https://doi.org/10.3390/photonics12030214 - 28 Feb 2025

Viewed by 753

Abstract

Light detection and ranging (Lidar) is a key enabling technology for autonomous vehicles and drones. Its emerging implementations are based on photonic integrated circuits (PICs) and optical phased arrays (OPAs). In this work, we introduce a novel approach to the design of OPA [...] Read more.

Light detection and ranging (Lidar) is a key enabling technology for autonomous vehicles and drones. Its emerging implementations are based on photonic integrated circuits (PICs) and optical phased arrays (OPAs). In this work, we introduce a novel approach to the design of OPA Lidar antennas based on Si₃N₄ grating couplers. The well-established TriPleX platform and the asymmetric double stripe waveguide geometry with full etching are employed, ensuring low complexity and simple fabrication combined with the low-loss advantages of the platform. The design study aims to optimize the performance of the grating coupler-based radiators as well as the OPA, thus enhancing the overall capabilities of Si₃N₄-based Lidar. Uniform and non-uniform grating structures are considered, achieving θ and φ angle divergences of 0.9° and 32° and 0.54° and 25.41°, respectively. Also, wavelength sensitivity of 7°/100 nm is achieved. Lastly, the fundamental OPA parameters are investigated, and 35 dBi of peak directivity is achieved for an eight-element OPA. Full article

► Show Figures

Figure 1

15 pages, 3122 KiB

Open AccessArticle

Strain-Compensated Quantum Well Asymmetric Waveguide Edge-Emitting Laser Operating at 730 nm

by Lutai Fan, Lijie Cao, Peng Jia, Qian Liu, Baiheng Liu, Haofei Chen, Yongyi Chen, Li Qin, Lei Liang, Yuxin Lei, Cheng Qiu, Yue Song, Yubing Wang, Yongqiang Ning and Lijun Wang

Sensors 2025, 25(4), 1173; https://doi.org/10.3390/s25041173 - 14 Feb 2025

Viewed by 693

Abstract

Semiconductor lasers operating at the 730 nm peak wavelength have diverse applications, including biomedical diagnostics, agricultural lighting, and high-precision sensing. However, quantum well (QW) materials, commonly employed at this wavelength, often fail to simultaneously meet the dual requirements of lattice matching and bandgap [...] Read more.

Semiconductor lasers operating at the 730 nm peak wavelength have diverse applications, including biomedical diagnostics, agricultural lighting, and high-precision sensing. However, quantum well (QW) materials, commonly employed at this wavelength, often fail to simultaneously meet the dual requirements of lattice matching and bandgap alignment. In this study, GaAsP/AlGaInP large strain compensation QW with lattice mismatches of −7.533‰ and 1.112‰ was developed. Strain compensation was utilized to address the lattice mismatch while ensuring lasing action at 730 nm. Based on this, the impact of waveguide design, particularly graded and asymmetric waveguides, on the power output was explored. Additionally, the relationship between the doping profile of the device and lasing efficiency was investigated. The completed 100 μm wide semiconductor edge-emitting laser (EEL) achieved 730 nm continuous wave laser with 1 W output power at 2 A current. This study proposes an approach to enhance the lasing power and optoelectronic conversion efficiency of lasers and provide valuable solutions for their practical applications. Full article

(This article belongs to the Section Optical Sensors)

► Show Figures

Figure 1

14 pages, 4595 KiB

Open AccessArticle

High Sensitivity Design for Silicon-On-Insulator-Based Asymmetric Loop-Terminated Mach–Zehnder Interferometer

by Muhammad A. Butt

Materials 2025, 18(4), 798; https://doi.org/10.3390/ma18040798 - 11 Feb 2025

Cited by 2 | Viewed by 851

Abstract

This work presents a novel design for an asymmetric loop-terminated Mach–Zehnder interferometer (a-LT-MZI) based on a silicon-on-insulator (SOI) platform, tailored for refractive index (RI) sensing applications. A significant advantage of incorporating the Sagnac loop into the MZI configuration is its ability to reduce [...] Read more.

This work presents a novel design for an asymmetric loop-terminated Mach–Zehnder interferometer (a-LT-MZI) based on a silicon-on-insulator (SOI) platform, tailored for refractive index (RI) sensing applications. A significant advantage of incorporating the Sagnac loop into the MZI configuration is its ability to reduce the interferometer’s effective length by half, offering a more compact design. This makes it ideal for integration into miniaturized optical devices, enabling space-efficient configurations without compromising precision or performance. The proposed device, featuring a pathlength difference (∆L) of 24.35 µm demonstrates a sensitivity of 261 nm/RIU, which is further enhanced to 510 nm/RIU by incorporating a subwavelength (SWG) waveguide in the asymmetric sensing arm. This modification boosts light–matter interaction, resulting in a larger shift in the interference fringes and significantly improving the sensor’s performance. Full article

► Show Figures

Figure 1

12 pages, 4233 KiB

Open AccessArticle

Theoretical Investigation of Terahertz Spoof Surface-Plasmon-Polariton Devices Based on Ring Resonators

by Can Liu, Shenghao Gu, Mingming Sun, Ya Liu, Ying Zhang and Jiaguang Han

Photonics 2025, 12(1), 70; https://doi.org/10.3390/photonics12010070 - 15 Jan 2025

Viewed by 1009

Abstract

Terahertz is one of the most promising technologies for high-speed communication and large-scale data transmission. As a classical optical component, ring resonators are extensively utilized in the design of band-pass and frequency-selective devices across various wavebands, owing to their unique characteristics, including optical [...] Read more.

Terahertz is one of the most promising technologies for high-speed communication and large-scale data transmission. As a classical optical component, ring resonators are extensively utilized in the design of band-pass and frequency-selective devices across various wavebands, owing to their unique characteristics, including optical comb generation, compactness, and low manufacturing cost. While substantial progress has been made in the study of ring resonators, their application in terahertz surface wave systems remains less than fully optimized. This paper presents several spoof surface plasmon polariton-based devices, which were realized using ring resonators at terahertz frequencies. The influence of both the radius of the ring resonator and the width of the waveguide coupling gap on the coupling coefficient are investigated. The band-stop filters based on the cascaded ring resonator exhibit a 0.005 THz broader frequency bandwidth compared to the single-ring resonator filter and achieve a minimum stopband attenuation of 28 dB. The add–drop multiplexers based on the asymmetric ring resonator enable selective surface wave outputs at different ports by rotating the ring resonator. The devices designed in this study offer valuable insights for the development of on-chip terahertz components. Full article

► Show Figures

Figure 1

11 pages, 6470 KiB

Open AccessArticle

Multi-Structure-Based Refractive Index Sensor and Its Application in Temperature Sensing

by Zhaokun Yan, Shubin Yan, Ziheng Xu, Changxing Chen, Yuhao Cao, Xiaoran Yan, Chong Wang and Taiquan Wu

Sensors 2025, 25(2), 412; https://doi.org/10.3390/s25020412 - 12 Jan 2025

Cited by 4 | Viewed by 865

Abstract

In this paper, a new sensor structure is designed, which consists of a metal–insulator–metal (MIM) waveguide and a circular protrusion and a rectangular triangular cavity (CPRTC). The characterization of nanoscale sensors is considered using an approximate numerical method (finite element method). The simulation [...] Read more.

In this paper, a new sensor structure is designed, which consists of a metal–insulator–metal (MIM) waveguide and a circular protrusion and a rectangular triangular cavity (CPRTC). The characterization of nanoscale sensors is considered using an approximate numerical method (finite element method). The simulation results show that the sharp asymmetric resonance generated by the interaction between the discrete narrow-band mode and the continuous wideband mode is called Fano resonance. The performance of the sensor is considerably influenced by CPRTC. The sensor structure has attained a sensitivity of 3060 nm/RIU and a figure of merit (FOM) of 53.68. In addition, the application of this structure to temperature sensors is also investigated; its sensitivity is 1.493 nm/°C. The structure also has potential for other nanosensors. Full article

(This article belongs to the Section Nanosensors)

► Show Figures

Figure 1

14 pages, 8579 KiB

Open AccessArticle

Fano and Electromagnetically Induced Transparency Resonances in Dual Side-Coupled Photonic Crystal Nanobeam Cavities

by Yong Zhao, Yuxuan Chen and Lijun Hao

Materials 2024, 17(24), 6213; https://doi.org/10.3390/ma17246213 - 19 Dec 2024

Viewed by 875

Abstract

We propose two types of structures to achieve the control of Fano and electromagnetically induced transparency (EIT) line shapes, in which dual one-dimensional (1D) photonic crystal nanobeam cavities (PCNCs) are side-coupled to a bus waveguide with different gaps. For the proposed type Ⅰ [...] Read more.

We propose two types of structures to achieve the control of Fano and electromagnetically induced transparency (EIT) line shapes, in which dual one-dimensional (1D) photonic crystal nanobeam cavities (PCNCs) are side-coupled to a bus waveguide with different gaps. For the proposed type Ⅰ and type Ⅱ systems, the phase differences between the nanobeam periodic structures of the two cavities are π and 0, respectively. The whole structures are theoretically analyzed via the coupled mode theory and numerically demonstrated using the three-dimensional finite-difference time-domain (3D FDTD) method. The simulation results show that the proposed structure can achieve several kinds of spectra, including Fano, EIT and asymmetric EIT line shapes, which is dependent on the width of the bus waveguide. Compared to the previously proposed Fano resonator with 1D PCNCs, the proposed structures have the advantages of high transmission at the resonant peak, low insertion loss at non-resonant wavelengths, a wide free spectral range (FSR) and a high roll-off rate. Therefore, we believe the proposed structure can find broad applications in optical switches, modulators and sensors. Full article

(This article belongs to the Special Issue Preparation, Characterization and Application of Photonic Materials and Devices)

► Show Figures

Figure 1

15 pages, 6575 KiB

Open AccessArticle

Tunable Characteristics of Optical Frequency Combs from InGaAs/GaAs Two-Section Mode-Locked Lasers

by Dengqun Weng, Yanbo Liang, Zhongliang Qiao, Xiang Li, Jia Xu Brian Sia, Zaijin Li, Lin Li, Hao Chen, Zhibin Zhao, Yi Qu, Guojun Liu, Chongyang Liu and Hong Wang

Sensors 2024, 24(24), 7905; https://doi.org/10.3390/s24247905 - 11 Dec 2024

Viewed by 1004

Abstract

We observed tunable characteristics of optical frequency combs (OFCs) generated from InGaAs/GaAs double quantum wells (DQWs) asymmetric waveguide two-section mode-locked lasers (TS-MLLs). This involves an asymmetric waveguide mode-locked semiconductor laser (AWML-SL) operating at a center wavelength of net modal gain of approximately 1.06 [...] Read more.

We observed tunable characteristics of optical frequency combs (OFCs) generated from InGaAs/GaAs double quantum wells (DQWs) asymmetric waveguide two-section mode-locked lasers (TS-MLLs). This involves an asymmetric waveguide mode-locked semiconductor laser (AWML-SL) operating at a center wavelength of net modal gain of approximately 1.06 µm, which indicates a stable pulse shape, with the power-current(P-I) characteristic curve revealing a small difference between forward and reverse drive currents in the gain region. Under different operating conditions, the laser exhibits the characteristics of OFCs. And the pulse interval in the timing and the peak interval in the frequency domain show a periodic alternating change trend with the increase in the gain current. This tunable characteristic is reported for the first time. The study demonstrates the feasibility of generating tunable optical combs using a monolithic integrated two-section mode-locked semiconductor laser (MI-TS-MLL). This has important reference value for the application of OFCs generated from MI-TS-MLLs or integrated optical chips. Full article

(This article belongs to the Section Optical Sensors)

► Show Figures

Figure 1

32 pages, 6740 KiB

Open AccessEditor’s ChoiceReview

Magnetohydrodynamic Waves in Asymmetric Waveguides and Their Applications in Solar Physics—A Review

by Robertus Erdélyi and Noémi Kinga Zsámberger

Symmetry 2024, 16(9), 1228; https://doi.org/10.3390/sym16091228 - 18 Sep 2024

Cited by 2 | Viewed by 1204

Abstract

The solar atmosphere is a complex, coupled, highly dynamic plasma environment, which shows rich structuring due to the presence of gravitational and magnetic fields. Several features of the Sun’s atmosphere can serve as guiding media for magnetohydrodynamic (MHD) waves. At the same time, [...] Read more.

The solar atmosphere is a complex, coupled, highly dynamic plasma environment, which shows rich structuring due to the presence of gravitational and magnetic fields. Several features of the Sun’s atmosphere can serve as guiding media for magnetohydrodynamic (MHD) waves. At the same time, these waveguides may contain flows of various magnitudes, which can then destabilise the waveguides themselves. MHD waves were found to be ubiquitously present in the solar atmosphere, thanks to the continuous improvement in the spatial, temporal, and spectral resolution of both space-born and ground-based observatories. These detections, coupled with recent theoretical advancements, have been used to obtain diagnostic information about the solar plasma and the magnetic fields that permeate it, by applying the powerful concept of solar magneto-seismology (SMS). The inclusion of asymmetric shear flows in the MHD waveguide models used may considerably affect the seismological results obtained. Further, they also influence the threshold for the onset of the Kelvin–Helmholtz instability, which, at high enough relative flow speeds, can lead to energy dissipation and contribute to the heating of the solar atmosphere—one of the long-standing and most intensely studied questions in solar physics. Full article

(This article belongs to the Special Issue Symmetry in Magnetohydrodynamic Flows and Their Applications)

► Show Figures

Figure 1

13 pages, 14855 KiB

Open AccessArticle

Transmission Characteristics Analysis of a Twin-Waveguide Cavity

by Chanchan Luo, Ruiying Zhang, Ben Zhang, Bisheng Qin, Yanshuang Zhao, Bocang Qiu, Bohan Liu and Xiaoming Zhao

Photonics 2024, 11(8), 777; https://doi.org/10.3390/photonics11080777 - 21 Aug 2024

Viewed by 968

Abstract

The transmission spectrum of a twin-waveguide cavity is systematically analyzed based on coupled mode theory, using the transfer matrix method (TMM). The results show that the traveling-wave transmission spectra of the twin-waveguide cavity is entirely determined by the coherent coupling effect involving the [...] Read more.

The transmission spectrum of a twin-waveguide cavity is systematically analyzed based on coupled mode theory, using the transfer matrix method (TMM). The results show that the traveling-wave transmission spectra of the twin-waveguide cavity is entirely determined by the coherent coupling effect involving the parameters of the effective refractive indices of the upper and lower waveguides, the coupling length Lc, and the ratio of the cavity length L to the coupling length (L/Lc). Filters with single, double, or triple-notch filtering could be obtained by choosing an appropriate L/Lc value. When the facet reflection is taken into consideration, the traveling-wave transmission spectrum is modified by the Fabry––Perot (FP) resonance, making it a standing-wave transmission spectrum. As a result, resonance splitting has been observed in the transmission spectrum of twin-waveguide resonators with high facet reflectivity. Further analysis shows that such an abnormal resonance phenomenon can be attributed to the destructive interference between the two FP resonance modes of the upper and lower waveguide through coherent coupling. In addition, narrow bandwidth amplification has also been observed through asymmetric facet reflections. Undoubtedly, all these unique spectral characteristics should be beneficial to the twin-waveguide cavity, achieving many more functions and being widely used in photonic integration circuits (PICs). Full article

(This article belongs to the Special Issue Advances in Semiconductor Photonic Integrated Circuits)

► Show Figures

Figure 1

8 pages, 4038 KiB

Open AccessCommunication

PLC-Based Polymer/Silica Hybrid Inverted Ridge LP₁₁ Mode Rotator

by Jiaqi Liang, Daming Zhang, Xinyu Lv, Guoyan Zeng, Pai Cheng, Yuexin Yin, Xiaoqiang Sun and Fei Wang

Micromachines 2024, 15(6), 792; https://doi.org/10.3390/mi15060792 - 16 Jun 2024

Viewed by 1290

Abstract

The mode rotator is an important component in a PLC-based mode-division multiplexing (MDM) system, which is used to implement high-order modes with vertical intensity peaks, such as LP_11b mode conversions from LP_11a in PLC chips. In this paper, an LP₁₁ [...] Read more.

The mode rotator is an important component in a PLC-based mode-division multiplexing (MDM) system, which is used to implement high-order modes with vertical intensity peaks, such as LP_11b mode conversions from LP_11a in PLC chips. In this paper, an LP₁₁ mode rotator based on a polymer/silica hybrid inverted ridge waveguide is demonstrated. The proposed mode rotator is composed of an asymmetrical waveguide with a trench. According to the simulation results, the broadband conversion efficiency between the LP_11a and LP_11b modes is greater than 98.5%, covering the C-band after optimization. The highest mode conversion efficiency (MCE) is 99.2% at 1550 nm. The large fabrication tolerance of the proposed rotator enables its wide application in on-chip MDM systems. Full article

(This article belongs to the Section E：Engineering and Technology)

► Show Figures

Figure 1

11 pages, 875 KiB

Open AccessArticle

Silicon Nitride Bent Asymmetric Coupled Waveguides with Partial Euler Bends

by Pedro Chamorro-Posada

Photonics 2024, 11(3), 218; https://doi.org/10.3390/photonics11030218 - 28 Feb 2024

Cited by 4 | Viewed by 2490

Abstract

Waveguide geometries combining bent asymmetric coupled structures and adiabatic transitions shaped as partial Euler bends are put forward and theoretically analyzed in this work. Designs aiming to reduce the radiation loss, with application in curved waveguide sections and Q-enhanced microresonators, and polarization selection [...] Read more.

Waveguide geometries combining bent asymmetric coupled structures and adiabatic transitions shaped as partial Euler bends are put forward and theoretically analyzed in this work. Designs aiming to reduce the radiation loss, with application in curved waveguide sections and Q-enhanced microresonators, and polarization selection geometries, both for the silicon nitride platform, are studied using highly accurate numerical techniques. Full article

(This article belongs to the Special Issue Silicon Photonics Devices and Integrated Circuits)

► Show Figures

Figure 1

12 pages, 5141 KiB

Open AccessArticle

Studies on Dual Helmholtz Resonators and Asymmetric Waveguides for Ventilated Soundproofing

by Inkyuk Han, Inho Lee and Gwanho Yoon

Sensors 2024, 24(5), 1432; https://doi.org/10.3390/s24051432 - 22 Feb 2024

Cited by 2 | Viewed by 2076

Abstract

Achieving the simultaneity of ventilation and soundproofing is a significant challenge in applied acoustics. Ventilated soundproofing relies on the interplay between local resonance and nonlocal coupling of acoustic waves within a sub-wavelength structure. However, previously studied structures possess limited types of fundamental resonators [...] Read more.

Achieving the simultaneity of ventilation and soundproofing is a significant challenge in applied acoustics. Ventilated soundproofing relies on the interplay between local resonance and nonlocal coupling of acoustic waves within a sub-wavelength structure. However, previously studied structures possess limited types of fundamental resonators and lack modifications from the basic arrangement. These constraints often force the specified position of each attenuation peak and low absorption performance. Here, we suggest the in-duct-type sound barrier with dual Helmholtz resonators, which are positioned around the symmetry-breaking waveguides. The numerical simulations for curated dimensions and scattered fields show the aperiodic migrations and effective amplifications of the two absorptive domains. Collaborating with the subsequent reflective domains, the designed structure holds two effective attenuation bands under the first Fabry–Pérot resonance frequency. This study would serve as a valuable example for understanding the local and non-local behaviors of sub-wavelength resonating structures. Additionally, it could be applied in selective noise absorption and reflection more flexibly. Full article

(This article belongs to the Section Intelligent Sensors)

► Show Figures

Figure 1

Search Results (76)

Further Information

Guidelines

MDPI Initiatives

Follow MDPI

Saved Queries

Search Filter Reset All

Years

Feature Papers

Subjects

Journals

Article Types

Countries / Regions

Search Results (76)

Further Information

Guidelines

MDPI Initiatives

Follow MDPI