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Optical waveguides are far more than mere connecting elements in integrated optical systems and circuits. Benefiting from their high optical confinement and miniaturized footprints, waveguide structures established based on crystalline materials, particularly, are opening exciting possibilities and opportunities in photonic chips by facilitating their on-chip integration with different functionalities and highly compact photonic circuits. Femtosecond-laser-direct writing (FsLDW), as a true three-dimensional (3D) micromachining and microfabrication technology, allows rapid prototyping of on-demand waveguide geometries inside transparent materials via localized material modification. The success of FsLDW lies not only in its unsurpassed aptitude for realizing 3D devices but also in its remarkable material-independence that enables cross-platform solutions. This review emphasizes FsLDW fabrication of waveguide structures with 3D layouts in dielectric crystals. Their functionalities as passive and active photonic devices are also demonstrated and discussed.

Globally, there is active development of photonic sensors incorporating multidisciplinary research. The ultimate objective is to develop small, low-cost, sensitive, selective, quick, durable, remote-controllable sensors that are resistant to electromagnetic interference. Different photonic sensor designs and advances in photonic frameworks have shown the possibility to realize these capabilities. In this review paper, the latest developments in the field of optical waveguide and fiber-based sensors which can serve for environmental monitoring are discussed. Several important topics such as toxic gas, water quality, indoor environment, and natural disaster monitoring are reviewed.

In this paper, the first demonstration of simultaneous UV written Bragg gratings and waveguides in germanium and boron‐doped planar silica is presented using a 5th harmonic solid‐state nanosecond laser operating at 213 nm wavelength. The fabrication of high‐quality gratings by using a high peak power density, yielding sufficient uniformity and without any surface damage is demonstrated. The photosensitivity of the doped silica layer is investigated by measuring the local effective refractive index of the optical modes. The written gratings are used to measure grating refractive index modulation, grating detuning bandwidth and the waveguide propagation loss with a minimum value of 0.28 ± 0.07 dB cm−1. This paper shows that this new generation of pulsed UV lasers is a promising alternative for conventional longer wavelength CW laser sources used in small spot direct grating writing in doped silica.

We employ seven optical waveguides based on two-dimensional linear photonic crystal (LPhC) to realize an all optical comparator (AOCMP). The proposed 1-bit AOCMP has a very simple structure with a footprint of 356 μm 2 which is composed of 31 × 33 cubic lattice of silicon rods immersed in air. This LPhC comparator comprises two input ports and two output ports. The functionality of the proposed 1-bit AOCMP is based on constructive and destructive interference phenomenon of optical beams and phase shift keying technique. The finite difference time domain (FDTD) procedure based on Yee’s Algorithm is used to compute the propagation of optical waves in this structure. The FDTD simulation results of suggested 1-bit AOCMP indicate that the minimum and maximum values of the normalized power at ON and OFF states for output ports are 62% and 10%, respectively. Also, the ON–OFF contrast ratio, bit rate, rise and fall times ( T r and T f ) of the suggested design are about 7.92 dB, 2.22 Tb/S, 0.15 ps and 0.05 ps, respectively.

In the domains of biomedical applications, wearable devices, and soft robotics, recent advancements have underscored the potential of soft, stretchable, and biocompatible devices. The design of optical soft devices has emerged as an ideal candidate for many applications owing to their high flexibility and immunity to electromagnetic interference. In this review, recent advances in soft optical waveguides, including advanced material selection, fabrication strategies, and characterization, are discussed. Herein, a comprehensive summary of the soft‐waveguide sensing strategies and actuation approaches are provided. Furthermore, the extensive applications of soft optical waveguides in the fields of biomedicine, wearable devices, and soft robotics are explored. Lastly, the challenges and opportunities for the future of soft optical waveguides, including multimodal sensing, algorithm optimization, and manufacturing scalability, are discussed. Soft optical waveguides are increasingly gaining prominence in the fields of biomedicine, wearable devices, and soft robotics. This review focuses on recent advances in soft waveguides, including materials, fabrication, characterization, and sensing strategies. Furthermore, it delves into different applications, challenges in multimodal sensing, algorithm optimization, and scalable manufacturing.

High-index contrast lithium niobate waveguides, fabricated by the High Vacuum Vapor-phase Proton Exchange (HiVac-VPE) technique, are very promising for increasing both the optical nonlinear and electro-optical efficiencies of photonic integrated devices. So as to play this role effectively, it is mandatory to know the crystallographic phase composition of waveguides and the position of protonated layers for appropriate tailoring and optimization based on the intended applications. In addition, the estimation of structural disorder and electro-optical properties of the waveguides are also of high interest. Benefiting from Raman spectroscopy, IR reflection, IR absorption, and UV-VIS absorption, the HxLi1−xNbO3 phase compositions, as well as the structural disorder in waveguides, were determined. Based on experimental data on the shift of the fundamental absorption edge, we have quantitatively estimated the electro-optic coefficient r13 in as-exchanged waveguides. The electro-optical properties of the waveguides have been found to be depending on the phase composition. The obtained results allow for reconsidering the proton exchange fabricating process of photonic nonlinear devices and electro-optic modulators based on high-index contrast channel waveguides on the LiNbO3 platform.

In contemporary science and technology, photonic sensors are essential. They may be made to be extremely resistant to some physical parameters while also being extremely sensitive to other physical variables. Most photonic sensors may be incorporated on chips and operate with CMOS technology, making them suitable for use as extremely sensitive, compact, and affordable sensors. Photonic sensors can detect electromagnetic (EM) wave changes and convert them into an electric signal due to the photoelectric effect. Depending on the requirements, scientists have found ways to develop photonic sensors based on several interesting platforms. In this work, we extensively review the most generally utilized photonic sensors for detecting vital environmental parameters and personal health care. These sensing systems include optical waveguides, optical fibers, plasmonics, metasurfaces, and photonic crystals. Various aspects of light are used to investigate the transmission or reflection spectra of photonic sensors. In general, resonant cavity or grating-based sensor configurations that work on wavelength interrogation methods are preferred, so these sensor types are mostly presented. We believe that this paper will provide insight into the novel types of available photonic sensors.

An efficient modes analyses technique for isotropic or anisotropic material filled 2D metallic waveguides with an arbitrary contour using the multifilament current method (MFCM) is presented. The ideal PEC boundary of a 2D waveguide is replaced by a shell with a high conductivity and electrical small thickness. The thin lossy shell not only can well approximate the boundary condition of PEC waveguide wall therefore without altering the initial waveguide modes, but also can let the external excitation penetrate through to excite the inside modes, resulting in a high internal field intensity at the frequency of each mode. In this case, the modes are revealed by the peaks of field intensity responses, and the spurious modes which existed in traditional source‐free modes determination techniques can be completely avoided. Based on this idea, a generalized impedance boundary condition (GIBC) is formulated to represent the lossy waveguide wall and further utilized in the MFCM for simulating the internal field intensity over frequency. Three different configurations of a 2D waveguide are considered. The computed modes are compared with that obtained from commercial software, and an excellent agreement is achieved, yet an competitive advantage on simulation performances is observed by using the proposed technique.

In this paper, a temperature sensor based on a polymer–silica heterogeneous integrated Mach–Zehnder interferometer (MZI) structure is proposed. The MZI structure consists of a polymer waveguide arm and a doped silica waveguide arm. Due to the opposite thermal optical coefficients of polymers and silica, the hybrid integrated MZI structure enhances the temperature sensing characteristics. The direct coupling method and side coupling method are introduced to reduce the coupling loss of the device. The simulation results show that the side coupling structure has lower coupling loss and greater manufacturing tolerance compared to the direct coupling structure. The side coupling loss for PMMA material-based devices, NOA material-based devices, and SU-8 material-based devices is 0.104 dB, 0.294 dB, and 0.618 dB, respectively. The sensitivity (S) values of the three hybrid devices are −6.85 nm/K, −6.48 nm/K, and −2.30 nm/K, which are an order of magnitude higher than those of an all-polymer waveguide temperature sensor. We calculated the temperature responsivity (RT) (FSR→∞) of the three devices as 13.16 × 10−5 K, 32.20 × 10−5 K, and 20.20 × 10−5 K, suggesting that high thermo-optic coefficient polymer materials and the hybrid integration method have a promising application in the field of on-chip temperature sensing.

A metallic nanocylinder array was previously proposed for the propagation of localized light generated in nanosized objects. However, the plasmon resonant wavelength for each cylinder shifts significantly with the propagation of light. In this study, a metallic nanoelliptic cylinder waveguide is investigated to control the plasmon resonant wavelength shift. The finite‐difference frequency‐domain (FDFD) electromagnetic solver is used to reduce the computational time. The relationship between the cross‐section of the cylinder and the plasmon resonance for the metal is described with the hydrodynamic Drude model. The effectiveness of the proposed waveguide in controlling the resonant wavelength shifts is confirmed by examining the plasmon propagation for 30 nanocylinders.